United States
Environmental Protection
Agency
Technology Transfer
Design
Manual
Office of Water Program
Operations
Washington DC 20460
Office of Research and
Development
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Onsite Wastewater
Treatment and
Disposal Systems
-------
EPA 625/1-80-012
DESIGN MANUAL
ON SITE WASTBAATER TREATMENT
AND DISPOSAL SYSTEMS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Water Program Operations
Office of Research and Development
Municipal Environmental Research Laboratory
October 1980
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NOTICE
The mention of trade names or commercial products in this publication is
for illustration purposes and does not constitute endorsement or
recommendation for use by the U.S. Environmental Protection Agency.
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FOREWORD
Rural and suburban communities are confronted with problems that are
unique to their size and population density, and are often unable to
superimpose solutions typically applicable to larger urban areas. A
good example of such problems is the provision of wastewater services.
In the past, priorities for water pollution control focused on the
cities, since waste generation from these areas was most evident. In
such high-density development, the traditional sanitary engineering
approach was to construct a network of sewers to convey wastewater to a
central location for treatment and disposal to surface waters. Since a
large number of users existed per unit length of sewer line, the costs
of construction and operation could be divided among many people, thus
keeping the financial burden on each user relatively low.
Within the past several decades, migration of the population from cities
to suburban and rural areas has been significant. With this shift came
the problems of providing utility services to the residents.
Unfortunately, in many cases, solutions to wastewater problems in urban
areas have been applied to rural communities. With the advent of
federal programs that provide grants for construction of wastewater
facilities, sewers and centralized treatment plants were constructed in
these low-density rural settings. In many cases the cost of operating
and maintaining such facilities impose severe economic burdens on the
communities.
Although wastewater treatment and disposal systems serving single homes
have been used for many years, they have often been considered an
inadequate or temporary solution until sewers could be constructed.
However, research has demonstrated that such systems, if constructed and
maintained properly, can provide a reliable and efficient means of
wastewater treatment and disposal at relatively low cost.
This document provides technical information on onsite wastewater
treatment and disposal systems. It does not contain standards for those
systems, nor does it contain rules or regulations pertaining to onsite
systems.
The intended audience for this manual includes those involved in the
design, construction, operation, maintenance, and regulation of onsite
wastewater systems.
iii
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ACKNOWLEDGMENTS
There were three groups of participants involved in the preparation of
this manual: (1) the contractor-authors, (2) the contract supervisors,
and (3) the technical reviewers. The manual was written by personnel
from SCS Engineers and Rural Systems Engineering (RSE). Contract
supervision was provided by U.S. Environmental Protection Agency (EPA)
personnel from the Municipal Construction Division in Washington, D.C.,
and from the Municipal Environmental Research Laboratory in Cincinnati,
Ohio. The technical reviewers were experts in certain areas of onsite
waste treatment and disposal, and included professors, health officials,
consultants, and government officials. Each provided technical review
of a section or sections of the report. The membership of each group is
listed below.
CONTTRACTOR-^UTHORS:
Direction: Curtis J. Schmidt, SCS
William C. Boyle, RSE
Senior Authors: Ernest V, Clements, Project Manager, SCS
Richard J. Otis, RSE
Contributing Authors: David H. Bauer, SCS
Robert L. Siegrist, E. Jerry Tyler,
David E. Stewart, James C. Converse, RSE
CONTRACT SUPERVISORS:
Project Officers: Robert M. Southworth, OWPO, EPA, Washington, D.C.
Robert P. G. Bowker, MERL, EPA,. Cincinnati, Ohio
Reviewers: James Kretssl, MERL, EPA, Cincinnati, Ohio
Denis Lussier, CERI, EPA, Cincinnati, Ohio
Sherwood Reed, CRREL, COE, Hanover, N.H.
TECHNICAL REVIEWERS:
1. Michael Hansel - Minnesota Pollution Control Agency
2. Roger Machmeier - University of Minnesota
3. Jack Abney - Parrott, Ely & Hurt, Inc., Lexington, Kentucky
4. William Mellen - Lake County Health Department, Illinois
5. Rein Laak - University of Connecticut
6. Gary Plews - Washington Department of Social & Health Services
7. B. L. Carlile - North Carolina State University
8. John Clayton - Fairfax County Health Department, Virginia
9. William Sharps - Pennsylvania State University
10. Elmer Jones - U.S. Department of Agriculture
11. Edwin Bennett - University of Colorado
12. Harry Pence - Virginia Polytechnic Institute
13. Briar Cook - U.S. Department of Agriculture, Forest Service
14. Marek Brandes - Ontario Ministry of the Environment (Retired)
15. Michael Nines - Illinois State Department of Public Health
16. John Fancy - John Fancy, Inc., Waldoboro, Maine
IV
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CONTENTS
Chapter Page
FOREWORD tii
ACWOMEDGBVENTS, v
CONTENTS v i i
LIST CF FIGURES ix
LIST CF TABLES xv
1 INTRODUCTION
1.1 Background 1
1.2 Purpose 2
1.3 Scope 3
2 STRATEGY FOR ONSITE SYSTEM DESIGN
2.1 Introduction 4
2-2 Onsite System Design Strategy 4
3 SITE EVALUATION PROCEDURES
3.1 Introduction 13
3.2 Disposal Alternatives 13
3.3 Site Evaluation Strategy 17
3.4 References 48
4 \AASTE\AATER CHARACTERISTICS
4.1 Introduction 50
4.2 Residential Wastewater Characteristics 50
4.3 Nonresidential Wastewater Characteristics 57
4.4 Predicting Wastewater Characteristics 65
4.5 References 68
5 WASTEWATER MODIFICATION
5.1 Introducti on 70
5.2 Water Conservation and Wastewater n o/v Reduction 71
5.3 Pollutant Mass Reduction 84
5.4 Onsite Containment - Holding Tanks 88
5.5 Reliability 88
5.6 Impacts on Onsite Treatment and Disposal
Practices 92
5.7 References 95
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CONTENTS (continued)
Chapter Pa9e
6 ONSITE TREATMENT METHODS
6.1 Introduction 97
6.2 Septic Tanks 98
6.3 Intermittent Sand Filters 113
6.4 Aerobic Treatment Units 140
6.5 Disinfection 161
6.6 Nutrient Removal 184
6.7 Waste Segregation and Recycle Systems 197
6,8 References 199
7 DISPOSAL METHODS
7.1 Introduction 206
7.2 Subsurface Soil Absorption 207
7.3 Evaporation Systems 300
7.4 Outfall to Surface Waters 316
7.5 References 316
8 APPURTENANCES
8.1 Introduction 321
8.2 Grease Traps 321
8.3 Dosing Chambers 327
8.4 Flow Diversion Methods for Alternating Beds 335
8.5 References 337
9 RESIDUALS DISPOSAL
9.1 Introduction 338
9.2 Resi dual s Characteristics 338
9.3 Resi duals Handl ing Option 343
9.4 Ultimate Disposal of Septage 343
9.5 References 351
10 MANAGEMENT CF ONSITE SYSTEMS
10.1 Introduction 353
10.2 Theory of Management 354
10.3 Types of ManagementEnti ties 355
10.4 Management Program Functions 353
10,5 References 366
APPENDIX- Soil Properties and Soil-Water Relationships 36/
GLOSSARY 382
VI
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FIGURES
Number
2-1 Onsite Wastewater Management Options 5
2-2 Onsite System Design Strategy 7
3-1 Potential Evaporation Versus Mean Annual
Precipitation 16
3-2 Example of a Portion of a Soil Map as Published
i n a Detailed Soil Survey (Actual Size) 20
3-3 Translation of Typical Soil Mapping Unit Symbol 20
3-4 Plot Plan Showing Soil Series Boundaries from
Soil Survey Report 23
3-5 Plot Plan Showing Surface Features 25
3-6 Landscape Positions 27
3-7 Methods of Expressing Land Slopes - 27
3-8 Preparation of Soil Sample for Field Determination
of Soil Texture 30
3-9 1 Texture Determination by Hand: Physical
Appearance of Various Soil Textures 32
3-10 Comparison of Ribbons and Casts of Sandy Loam and
Clay (Ribbons Above, Casts Below) 33
3-11 Example Procedure for Collecting Soil Pit
Observation Information 34
3-12 Types of Soil Structure 36
3-13 Typical Observation Well for Determining Soil
Saturation 38
3-14 Construction of a Percometer 42
3-15 Percolation Test Data Form 43
3-16 Compilation of Soils and Site Information
(Information Includes Topographic, Soil Survey,
Onsite SI ope and Soil P i t Observations) 45
vn
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FIGURES (continued)
Number Page
4-1 Frequency Distribution for Average Daily
Residential Water Use/Waste Flows 53
4-2 Peak Discharge Versus Fi xture Units Present 64
4-3 Strategy for Predicting Wastewater Characteristics 67
5-1 Example Strategies for Management of Segregated
Human Wastes 89
5-2 Example Strategies for Management of Residential
Graywater 89
5-3 Fl ow Reduction Effects on Pollutant Concentrations 93
6-1 ' Typical Septic Tank Outlet Structures to Mnimize
Suspended Solids in Discharge 105
6-2 Septic Tank Sam and Sludge Clear Spaces 107
6-3 Typical Two-Compartment Septi C Tank 108
6-4 Four Precast Reinforced Concrete Septic Tanks
Combined into One Unit for Large Flow
Application 114
6-5 Typical Buried Intermittent Filter Installation 129
6-6 Typical Free Access Intermittent Filter 131
6-7 Typical Recirculati ng Intermittent Filter System 134
6-8 Recircul ation Tank 134
6-9 By-Pass Alternatives for Recirculati ng Filters 135
6-10 Aerobic and Anaerobic Decomposition Products 142
6-11 Examples of Extended Aeration Package Plant
Configurations 144
6-12 Examples of Fixed Film Package Plant Configurations 156
6-13 Stack Feed Chi orinator 171
vi n
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FIGURES (continued)
Number Page
5-14 Iodine Saturator 172
&-15 Sample Contact Chamber 174
5-16 Typical UV Disinfection Unit 177
6-17 Typical UV Steril izing Chamber 178
6-18 Qnsite Peni tn~ fi cati on Systems 190
f-l Typical Trench System 209
I-I Typical Bed System 210
7-3 Alternating Trench System with Diversion VaUe 218
/-4 Provision of a Reserve Area Between Trenches of
the Initial System on a Sloping Site 220
7-5 Trench System Installed to Overcome a Shallow Water
Table or Restrictive Layer 222
7-6 Typical Inspection Pipe 228
'-7 Backhoe Bucket with Removable Raker Teeth 228
'-8 Methods of Soi 1 fcbsorpti on.Field.Rehabilitation 232
7-9 Seepage Pit Cross Section 236
7-10 Typical Mound Systems 240
7-11 Jetaii sd Schematic of a Mound System 241
1-12 Proper Orientation of a Mound System on a Complex
Si ope ?46
7-13 Mound 31mensi ans ?47
?-14 Tiered Mound System Z57
7-15 :urtai n 3rai ti to Intercept Laterally ^lovl ig Perched
Water Table Caused by a Shallow, Impermeable Layer 261
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FIGURES (continued)
Number Page
7-16 Vertical Drain to Intercept Laterally Moving
Perched Water Table Caused by a Shallow,
Thin, Impermeable Layer 261
7.-17 Underdrains Used to Lower Water Table 262
7-18 Typical Electro-Osmosis System 270
7-19 Single LI ne Distribution Network 273
7-20 Drop Box Distribution Network 274
7-21 Closed Loop Distribution Network 276
7-22 Di stri buti on Box Network 277
7-23 Relief Line Distribution Network 279
7-24 Central Manifold Distribution Network 280
7-25 End Manifold Distribution Network 281
7-26 Lateral Detail - Tee to Tee Construction 282
7-27 Lateral Detail - Staggered Tees or Cross
Construction 283
7-28 Required Lateral Pipe Diameters for Various Hole
Diameters, Hole Spacings, and Lateral Lengths
(for Plastic Pipe Only) 285
7-29 Recommended Mam fol d Diameters for Various Manifold
Lengths, Number of Laterals, and Lateral
Discharge Rates (for Plastic Pipe Only) 286
7-30 Nomograph for Determining the Minimum Dose Volume
for a Given Lateral Diameter, Lateral Length,
and Number of Laterals 287
7-31 Distribution Network for Example 7-2 289
7-32 Distribution Network for Example 7-3 294
7-33 Schematic of a Leaching Chamber 298
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FIGURES (continued)
Number Page
7-34 Use of Metal Holders for the Laying of Flexible
Plastic Pipe 299
7-35 Cross Section of Typical ET Bed 302
7-36 Curve for Establishing Permanent Home Loading Rate
for Boulder, Colorado Based on Winter Data,
1976-1977 307
7-37 Typical Evaporation/Infiltration Lagoon for
Small Installations 312
8-1 Double-Compartment Grease Trap 326-
8-2 Typical Dosing Chamber with Pump 329
8-3 Level Control Switches 331
8-4 Typical Dosing Chamber with Siphon 333
8-5 Typical Diversion Valve 335
8-6 Top View of Diversion Box Utilizing a Treated
Wood Gate 336
8-7 Section View of Diversion Box Utilizing
Adjustable Ells 336
A-1 Names and Size Limits of Practical-Size Classes
According to Six Systems 368
A-2 Textural Triangle Defining Twelve Textural Classes
of the USDA (Illustrated for a Sample Containing
37% Sand, 451 Silt, and 18% Clay) 370
A-3 Schematic Diagram of a Landscape and Different
Soils Possible 375
A-4 Upward Movement by Capillarity in Glass Tubes as
Compared with Soils 377
A-5 Soil Moisture Retention for Four Different Soil
Textures 378
XI
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FIGURES (continued)
Number Page
A-6 Hydraulic Conductivity (K) Versus Soil
Moisture Retention 379
A-7 Schematic Representation of Water Movement Through
a Soil with Crusts of Different Resistances 381
XI
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TABLES
Number
2-1 Selection of Disposal Methods Under Various
Site Constraints 9
3-1 Suggested Site Evaluation Procedure 18
3-2 Soil Limitations Ratings Used by SCS for Septic
Tank /Soil Absorptian Fields 22
3-3 Soil Survey Report Information for Parcel i n
Figure 3-4- 24
3-4 Textural Properties of Mineral Soils 31
3-5 Grades of Soil Structure 36
3-6 Description of Soil Mottles 37
3-7 Estimated Hydraulic Characteristics of Soil 39
3-8 Falling Head Percolation Test Procedure 41
4-1 Summary of Average Daily Residential Wastewater
Fl ovs 52
4-2 Residential Water Use by Activity 54
4-3 Characteristics of Typical Residential Wastewater 56
4-4 Pollutant Contributions of Major Residential
Wastewater Fractions (gm/cap/day) 58
4-5 Pollutant Concentrations of Major Residential
Wastewater Fractions (mg/1 ) 58
4-6 Typical Wastewater Flows from Commercial Sources 60
4-7 Typical Wastewater Flows from Institutional
Sources 61
4-8 Typical Wastewater Flows from Recreational Sources 62
4-9 Fixture-Units per Fixture 63
5-1 Example Wastewater Fl ow Reducti on Methods 72
xm
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TABLES [continued)
Number Page
5-2 Wastewater Flow Reduction - Water Carriage
Toilets and Systems 74
5-3 Wastewater Flow Reduction - Non-Water Carriage
Toi1ets 77
5-4 Wastewater Flow Reduction - Bathing Devices
and Systems 79
5-5 Wastewater Flow Reduction - Miscellaneous Devices
and Systems 82
5-6 Wastewater Flow Reduction - Wastewater Recycle and
Reuse Systems 85
5-7 Example Pollutant Mass Reduction Methods 87
5-8 Additional Considerations in the Design, Installation
and Operation of Holding Tanks 90
5-9 Potential Impacts of Wastewater Modification on
Onsite Disposal Practices 94
6-1 Summary of Effluent Data from Various Septic Tank
Studi es 100
6-2 Typical Septic Tank Liquid Volume Requirements 103
6-3 Location of Top and Bottom of Outlet Tee or Baffle 107
6-4 Performance of Buried Intermittent Filters -
Septic Tank Effluent 121
6-5 Performance of Free Access Intermittent Filters 122
6-6 Performance of Recirculating Intermittent Filters 123
6-7 Design Criteria for Buried Intermittent Filters 124
5-8 Design Criteria for Free Access Intermittent Filters 126
6-9 Design Criteria for Recirculating Intermittent Filters 128
6-10 Operation and Maintenance Requirements for Buried
Intermittent Filters 138
XIV
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TABLES (continued)
Number
6-11 Operation and Maintenance Requirements for Free
Access Intermittent Filters 138
6-12 Operation and Maintenance Requirements for
Reel restating Intermittent Filters 139
6-13 Summary of Effluent Data from Various Aerobic Unit
Field Studies 146
6-14 Typical Operating Parameters for Onsite Extended
Aeration Systems 151
6-15 Suggested Maintenance for Onsite Extended Aeration
Package Plants 153
6-16 Operational Problems - Extended Aeration Package
Plants 154
6-17 Typical Operating Parameters for Onsite Fixed Film
Systems 158
6-18 Suggested Maintenance for Onsite Fixed Film Package
Plants 160
6-19 Operational Problems - Fixed Film Package Plants 162
6-20 Selected Potential Disinfectants for Onsite
Application 163
6-21 Halogen Properties 164
6-22 Chlorine Demand of Selected Domestic Wastewaters 165
6-23 Performance of Halogens and Ozone at 25°C 167
6-24 Halogen Dosage Design Guidelines 168
6-25 UV Dosage for Selected Organisms 180
6-26 Potential Onsite Nitrogen Control Options 186
6-27 Potential Onsite Phosphorus Removal Options 194
6-28 Phosphorus Adsorption Estimates for Selected
Natural Materials 198
xv
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TABLES (continued)
Number Page
7-1 Site Criteria for Trench and Bed Systems 212
7-2 Recommended Rates of Wastewater Application for
Trench and Bed Bottom Areas • 214
7-3 Typical Dimensions for Trenches and Beds 221
7-4 Dosi ng Frequencies for Various Soil Textures 224
7-5 Methods of Wastewater Application for Various
System Designs and Soil Permeabilities 225
7-6 Si dewall Areas of Circular Seepage Pits (ft2) 237
7-7 Site Criteria for Mound Systems 242
7-8 Commonly Used Fill Materials and their Design
Infiltration Rates 245
7-9 Dimensions for Mound Systems 248
7-10 Infiltration Rates for Determining Mound Basal
Area 249
7-11 Drainage Methods for Various Site Characteristics 266
7-12 Distribution Networks for Various System Designs
and Application Methods 271
7-13 Discharge Rates for Various Sized Holes a t Various
Pressures (gpro) 284
7-14 Friction Loss in Schedule 40 Plastic Pipe, C=150
(ft/100 ft) 291
7-15 Pipe Materials for Nonpressuri zed Distributi on
Networks 297
7-16 Sample Water Balance for Evaporation Lagoon Design 314
8-1 Recommended Ratings for Commercial Grease Traps 324
9-1 Resi duals Generated from Ons 1 te Wastewater Systems 339
xvi
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TABLES (continued)
Number Page
9-2 Characteristics of Domestic Septage 340
9-3 Indicator Organism and Pathogen Concentrations
i n Domestic Septage 342
9-4 Land Disposal Alternatives for Septage 345
9-5 Independent Septage Treatment Facilities 348
9-6 Septage Treatment at Wastewater Treatment Plants 350
10-1 Site Evaluation and System Design Functions 359
10-2 Installation Functions 362
10-3 Operation and Maintenance Functions 364
10-4 Rehabilitation Functions 365
A-1 U.S. Department of Agriculture Size Limits for
Soil Separates 367
A-2 Types and Classes of Soil Structure 372
xvn
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CHAPTER 1
INTRODUCTION
1.1 Background
Approximately 18 million housing units, or 25% of all housing units in
the United States, dispose of their wastewater using onsite wastewater
treatment and disposal systems. These systems include a variety of com-
ponents and configurations, the most common being the septic tank/soil
absorption system. The number of onsite systems is increasing, with
about one-half million new systems being installed each year.
The first onsite treatment and disposal systems were constructed by
homeowners themselves or by local entrepreneurs in accordance with de-
sign criteria furnished by federal or state health departments. Usu-
ally, a septic tank followed by a soil absorption field was installed.
Trenches in the soil absorption system were dug wide enough to accommo-
date open-jointed drain tile laid directly on the exposed trench bottom.
Some health departments suggested that deeper and wider trenches be used
in "dense" soils and that the bottom of those trenches be covered with
coarse aggregate before the drain tile was laid. The purposes of the
aggregate were to provide a porous media through which the septic tank
effluent could flow and to provide storage of the liquid until it could
infiltrate into the surrounding soil.
It has been estimated that only 32% of the total land area in the United
States has soils suitable for onsite systems which utilize the soil for
final treatment and disposal of wastewater. In areas where there is
pressure for development, onsite systems have often been installed on
land that is not suitable for conventional soil absorption systems.
Cases of contaminated wells attributed to inadequately treated septic
tank effluent, and nutrient enrichment of lakes from near-shore develop-
ment are examples of what may occur when a soil absorption system is
installed in an area with unsuitable soil or geological conditions.
Alarmed by the potential health hazards of improperly functioning sys-
tems, public health officials have continually sought methods to improve
the design and performance of onsite systems.
Unfortunately, the great increases in population have exacerbated the
problems associated with onsite systems. The luxury of vast amounts of
land for homesites is gone; instead, denser housing in rural areas is
more common.
1
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In many areas, onsite systems have been plagued by poor public accep-
tance; feelings that those systems were second rate, temporary, or fail-
ure prone. This perspective contributed to poorly designed, poorly con-
structed, and inadequately maintained onsite systems.
Recently, the situation has begun to change. Federal, state, and local
governments have refocused their attention on rural wastewater disposal
and, more particularly, on wastewater systems affordable by the rural
population. Onsite systems are now gaining desired recognition as a
viable wastewater management alternative that can provide excellent,
reliable service at a reasonable cost, while still preserving environ-
mental quality. Federal and many state and local governments have
initiated public education programs dealing with the technical and
administrative aspects of onsite systems and other less costly waste-
water handling alternatives for rural areas.
In this time of population movements to rural and semi rural areas, high
costs of centralized sewage collection and treatment, and new funding
incentives for cost and energy saving technologies, those involved with
rural wastewater management need more information on the planning, de-
sign, construction, and management of onsite systems. This process de-
sign manual provides primarily technical guidance on the design, con-
struction, and maintenance of such systems.
1.2 Purpose
This document provides information on generic types of onsite wastewater
treatment and disposal systems. It contains neither standards for those
systems nor rules and regulations pertaining to onsite systems. The de-
sign information presented herein is intended as technical guidance re-
flective of sound, professional practice. The intended audience for the
manual includes those involved in the design, construction, operation,
maintenance, and regulation of onsite systems.
Technologies discussed in this manual were selected because of past
operating experience and/or because of the availability of information
and performance data on those processes. Because a particular waste-
water handling option is not discussed in this manual does not mean that
it is not acceptable. All available technologies should be considered
when planning wastewater management systems for rural and suburban com-
munities.
Groundwater and surface water pollution are major environmental consid-
erations when onsite systems are used. All wastewater treatment and
disposal systems must be designed, constructed, operated, and maintained
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to prevent degradation of both groundwater and surface water quality.
For onsite systems designed and constructed using Environmental Protec-
tion Agency funds, all applicable regulations must be complied with,
including requirements for disposal to groundwaters (40 R^ 6190, Feb-
ruary 11, 1976).
This manual is only a guide. Before an onsite system is designed and
constructed, appropriate local or state authorities should be contacted
to determine the local design requirements for a particular system.
1.3 Scope
This manual includes:
1. A strategy for selecting an onsite system
2. A procedure for conducting a site evaluation
3. A summary of wastewater characteristics
4. A discussion of waste load modification
5. A presentation of generic onsite wastewater treatment methods
6. A presentation of generic onsite wastewater disposal methods
7. A discussion of appurtenances for onsite systems
8. An overview of residuals characteristics and treatment/disposal
alternatives
9. A discussion of management of onsite systems
The emphasis of this manual is on systems for single dwellings and small
clusters of up to 10 to 12 housing units. Additional factors must be
considered for clusters of systems serving more than 10 to 12 housing
units. A brief discussion of onsite systems for multi-home units and
commercial/institutional establishments is also presented, when the
system designs differ significantly from those for single dwellings.
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CHAPTER 2
STRATEGY FOR ONSITE SYSTEM DESIGN
2.1 Introduction
A wide variety of onsite system designs exist from which to select the
most appropriate for a given site. The primary criterion for selection
of one design over another is protection of the public health while pre-
venting environmental degradation. Secondary criteria are cost and ease
of operating and maintaining the system. The fate of any residuals re-
sulting from the treatment and disposal system must be considered in the
selection process.
Figure 2-1 summarizes wastewater management options for onsite systems.
Because of the wide variety, selection of the system that prevents pub-
lic health hazards and maintains environmental quality at the least cost
is a difficult task. The purpose of this chapter is to present a stra-
tegy for selecting the optimum onsite system for a particular environ-
ment. At each step, the reader is referred to the appropriate chapters
in the manual for site evaluation, and subsequent system design, con-
struction, operation and maintenance, and residuals disposal.
2.2 Onsite System Design Strategy
Traditionally, subsurface soil absorption has been used almost exclu-
sively for onsite disposal of wastewater because of its ability to meet
the public health and environmental criteria without the necessity for
complex design or high cost. A properly designed, constructed, and
maintained subsurface absorption system performs reliably over a long
period of time with little attention. This is because of the large
natural capacity of the soil to assimilate the wastewater pollutants.
Unfortunately, much of the land area in the United States does not have
soils suited for conventional subsurface soil absorption fields. If
soil absorption cannot be utilized, wastewater also may be safely dis-
posed of into surface waters or evaporated into the atmosphere. How-
ever, more complex systems may be required to reliably meet the public
health and environmental criteria where these disposal methods are used.
Not only are complex systems often more costly to construct, but they
are also more difficult and costly to maintain. Therefore, the onsite
system selection strategy described here is based on the assumption that
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FIGURE 2-1
ONSITE WASTEWATER MANAGEMENT OPTIONS
WASTEWATER
MODIFICATION (Ch. 5}
- Flow Reduction
- Pollutant Mass
Reduction
WASTEWATER
FLOW
(Ch.4)
1
RESIDUALS
DISPOSAL
(Ch. 9)
1
HOLDING'
TANK
1
PRETRE/
(Ch
- Septit
- Aerob
EVAPORATION
(Ch. 7)
ET
ETA
Evaporation
Lagoon
E/l Lagoon
FURTHER TREATMENT
(Ch. 6)
- Aerobic Unit
- Granular Filter
- Nutrient Removal
- Disinfection
SUBSURFACE SOIL
ABSORPTION (Ch. 7)
- Trenches
- Beds
- Pits
- Mounds
- Fill Systems
- Artificially Drained
Systems
SURFACE
DISCHARGE
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subsurface soil absorption is the preferred onsite disposal option be-
cause of its greater reliability with a minimum of attention.Where the
site characteristicsare unsuitablefor conventionalsubsurface soil
absorption systems, other subsurface soil absorption systems may be
possible. Though these other systems may be more costly to construct
than systems employing surface water discharge or evaporation, their
reliable performance under a minimum of supervision may make them the
preferred alternative. Figure 2-2 illustrates the onsite system design
strategy discussed in this chapter.
2.2.1 Preliminary System Screening
The first step in the design of an onsite system is the selection of the
most appropriate components to make up the system. Since the site char-
acteristics constrain the method of disposal more than other components-,
the disposal component must be selected first. Selection of wastewater
modification and treatment components follow. To select the disposal
method properly, a detailed site evaluation is required. However, the
site characteristics that must be evaluated may vary with the disposal
method. Since it is not economical nor practical to evaluate a site for
every conceivable system design, the purpose of this first step is to
eliminate the disposal options with the least potential so that the de-
tailed site evaluation can concentrate on the most promising options.
To effectively screen the disposal options, the wastewater to be treated
and disposed must be characterized, and an initial site investigation
made.
2.2.1.1 Wastewater Characterization
The estimated daily wastewater volume and any short- or long-term
variations in flow affect the size of many of the system components. In
addition, the concentrations of various constituents can affect the
treatment and disposal options chosen. Characteristics are presented in
Chapter 4 for wastewater from residential dwellings as well as from
commercial operations.
2.2.1.2 Initial Site Evaluation
All useful information about the site should be collected. This may be
accomplished by client contact, a review of available published resource
information and records, and an initial site visit. Client contact and
a review of published maps and reports should provide information re-
garding the soils, geology, topography, climate, and other physical
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FIGURE 2-2
ONSITE SYSTEM DESIGN STRATEGY
Waste
Characterization
(Ch. 4)
>
t
Initial
Site
Evaluation
(Sec. 3.3.1. 3.3.2)
>
t
Preliminary
Screening of
Disposal Options
(Table 2-1)
>
t
Detailed
Site
Evaluation
(Ch. 3)
Design System
• Treatment (Ch. 5,6,8)
• Disposal (Ch. 7)
• Residuals (Ch. 9)
S
k
Selection of
Treatment
Component(s)
(Figure 2-1)
;
k
Selection of
Disposal Option
J
k
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features of the site (See 3.3.1 and 3.3.2). An initial site visit
should also be made, and should include a visual survey of the area and
preliminary field testing, if required, with a hand auger (See 3.3.3).
From this site visit, general site features such as relative soil perme-
ability, depth and nature of bedrock, depth to water table, slope, lot
size, and landscape position should be identified. Sources of informa-
tion and evaluation procedures for site evaluation are detailed in
Chapter 3.
2.2.1.3 Preliminary Screening of Disposal Options
From the wastewater characteristics and site information gathered in
this step, a preliminary screening of the disposal options can be made
using Table 2-1. This table indicates the onsite disposal options that
potentially may work for the given site constraints. The potentially
feasible disposal options are identified by noting which ones perform
effectively under all the given site constraints. Note that with suffi-
cient treatment and presence of receiving waters, surface water dis-
charge is always a potential disposal option.
As an example, suppose a site for a single-family home has the following
general characteristics:
1. Very rapidly permeable soil
2. Deep bedrock
3. Shallow water table
4. Five to 15 percent slope
5. Large lot
6. Low evaporation potential
From Table 2-1, the disposal options most applicable to the example site
constraints are:
1. Mounds
2. Fills
3. Surface water discharge
The design sections in Chapter 7 would be consulted at this point to
determine the specific characteristics to be evaluated at the site in
order to select the most feasible disposal options.
-------
TABLE 2-1
SELECTION OF DISPOSAL METHODS UNDER VARIOUS SITE CONSTRAINTS
Method
Trenches
Beds
Pits
Mounds
Fill Systems
Sand-Lined
Trenches or
Beds
Artificially
Drained
Systems
Evaporation
Infiltration
Lagoons
Evaporation
Lagoons
(lined)4-6
ET Beds
or Trenches
(lined) 4-s
ETA Beds
or Trenches4
Site Constraints
Soil Permeability
Very
Rapid
X
X
X
X
X
Rapid-
Moderate
X
X
X
X
X'
X
X
X
X
X
X
Slow-
Very Slow
X2
X
X1
X2
X6
X
X
X
Depth to Bedrock
Shallow
and
Porous
X
X
X
X
Shallow
and
Nonporous
X
X
X
X
Deep
X
X
X
X
X
X
X
X
X
X
X
Depth to
Water table
Shallow
X
X
X
X
X
Deep
X
X
X
X
X
X
X
X
X
X
Slope
0-5%
X
X
X
X
X
X
X
X
X
X
X
5-15%
X
X
X
X
X3
X
X6
X
15%
X
X
X
X3
X3
X
Small
Lot
Size
X"
X
X
X4
X'
X
1 Only where surface soil can be stripped to expose sand or sandy loam material.
2 Construct only during dry soil conditions. Use trench configuration only.
3 Trenches only.
4 Flow reduction suggested.
5 High Evaporation potential required.
6 Recommended for south-facing slopes only.
X means system can function effectively
with that constraint.
-------
2.2.2 System Selection
With the potentially feasible disposal options in mind, a detailed site
evaluation is performed. The information collected is used to identify
the system options that meet the public health and environmental cri-
teria. If more than one system is feasible, final selection is based on
results of a cost effective analysis. Local codes should be consulted
to determine which onsite treatment and disposal methods are permitted
in the area.
2.2.2.1 Detailed Site Evaluation
A careful, detailed site evaluation is needed to provide sufficient
information to select the most appropriate treatment and disposal system
from the potentially feasible system options. The evaluation should be
performed in a systematic manner so as to insure that the information
collected is useful and in sufficient detail. A site evaluation proce-
dure is suggested in Chapter 3, including descriptions of the tests and
observations to be made. This procedure is based on the assumption that
subsurface soil absorption is the preferred method of disposal. If sub-
surface absorption cannot be used, techniques are explained for evaluat-
ing the suitability of a site for surface water discharge or evapora-
tion.
2.2.2.2 Selection of Most Appropriate System
The disposal option selected after the detailed site evaluation dictates
the quality of the wastewater required prior to disposal. If suitable
soils exist onsite to employ one of the subsurface soil absorption meth-
ods of disposal, the quality of the wastewater applied need not be high
due to the assimilative capacity of the soil. Where suitable soils do
not exist onsite, other methods of disposal that require a higher qual-
ity of wastewater may be necessary. These wastewater quality require-
ments are established during the site evaluation (Chapter 3). Waste-
water reduction and treatment options are selected to meet the required
wastewater quality.
Altering the characteristics of the wastewater generated can have a
major impact on the design of the treatment and disposal system.
Alteration can be beneficial in reducing the size or complexity of the
system. Chapter 5 describes a variety of wastewater reduction options.
10
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Chapter 6 provides detailed information regarding the design,
construction and operation of various treatment options. Selection of
the most appropriate treatment option is based on performance and cost.
Various onsite systems may be synthesized from the data presented in
Chapters 5 and 6. As an example of the synthesis of treatment and
disposal systems following the detailed site evaluation, assume that all
three disposal options selected in 2.2.1.3 proved to be feasible.
Examination of the first two disposal options indicates that only
minimal pretreatment may be required. Thus, two systems might be:
1. Septic tank - mounds
2. Septic tank - fill
If groundwater quality is a constraint, however, it may be necessary to
develop other systems. Thus, if nitrogen discharges from the disposal
system to the groundwater must be controlled, the two treatment-disposal
systems may be revised to include the following:
1. Septic tank - mound - denitrifi cation
2. In-house toilet segregation/graywater - septic tank - fill
Note that a variety of other systems may be developed as well. The
other disposal option listed in 2.2.1.3 is surface water discharge.
Several treatment options exist if the wastewater is disposed of by
discharge to surface waters. Filtration and disinfection may be
required as part of those treatment options, depending on the water
quality requirements of the appropriate regulatory agency.
Residuals produced from the treatment processes also require safe
disposal. This must be considered in the selection of the treatment and
disposal system. Chapter 9 provides information regarding the
character, required treatment, and methods of ultimate disposal of
various residuals produced.
2.2.3 System Design
Once all the components are selected, design of the system follows.
Chapters 5, 6, 7, 8, and 9 should be consulted for design information.
11
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2.2.4 Onsite System Management
Past experience has shown that onsite management districts have many
benefits, including improved site selection, system design, construc-
tion, and operation and maintenance. Management districts also facili-
tate the use of more complex systems or larger systems servicing a clus-
ter of several homes. These districts can take many forms with varying
powers. Chapter 10 provides an overview of management options for on-
site systems.
12
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CHAPTER 3
SITE EVALUATION PROCEDURES
3.1 Introduction
The environment into which the wastewater is discharged can be a valu-
able part of an onsite wastewater and disposal system. If utilized
properly, it can provide excellent treatment at little cost. However,
if stressed beyond its assimilative capacity, the system will fail.
Therefore, careful site evaluation is a vital part of onsite system
design.
3.2 Disposal Options
In general, facilities designed to discharge partially treated waste-
water to the soil for ultimate disposal are the most reliable and least
costly onsite systems. This is because little pretreatment of the
wastewater is necessary before application to the soil. The soil has a
very large capacity to transform and recycle most pollutants found in
domestic wastewaters. While the assimilative capacity of some surface
waters also may be great, the quality of the wastewater to be discharged
into them is usually specified by local water quality regulatory
agencies.
To achieve the specified quality may require a more costly treatment
system. On the other hand, evaporation of wastewater into the atmo-
sphere requires little wastewater pretreatment, but this method of dis-
posal is severely limited by local climatic conditions. Therefore, the
soil should be carefully evaluated prior to the investigation of other
receiving environments.
3.2.1 Wastewater Treatment and Disposal by Soil
Soil is the weathered and unconsolidated outer layer of the earth's
surface. It is a complex arrangement of primary mineral and organic
particles that differ in composition, size, shape, and arrangement.
Pores or voids between the particles transmit and retain air and water.
Since it is through these pores that the wastewater must pass to be
absorbed and treated, their characteristics are important. These are
13
-------
determined largely by the physical properties of the soil. Descriptions
of some of the more important physical properties appear in Appendix A.
The soil is capable of treating organic materials, inorganic substances,
and pathogens in wastewater by acting as a filter, exchanger, adsorber,
and a surface on which many chemical and biochemical processes may
occur. The combination of these processes acting on the wastewater as
it passes through the soil produces a water of acceptable quality for
discharge into the groundwater under the proper conditions.
Physical entrapment of particulate matter in the wastewater may be
responsible for much of the treatment provided by soil. This process
performs best when the soil is unsaturated. If saturated soil condi-
tions prevail, the wastewater flows through the larger pores and re-
ceives minimal treatment. However, if the soil is kept unsaturated -by
restricting the wastewater flow into the soil, filtration is enhanced
because the wastewater is forced to flow through the smaller pores of
the soil.
Because most soil particles and organic matter are negatively charged,
they attract and hold positively charged wastewater components and repel
those of like charge. The total charge on the surfaces of the soil sys-
tem is called the cation exchange capacity, and is a good measure of the
soil's ability to retain wastewater components. The charged sites in
the soil are able to sorb bacteria, viruses, ammonium, nitrogen, and
phosphorus, the principal wastewater constituents of concern. The
retention of bacteria and viruses allows time for their die-off or
destruction by other processes, such as predation by other soil micro-
organisms (1)(2). Ammonium ions can be adsorbed onto clay particles.
Where anaerobic conditions prevail, the ammonium ions may be retained on
the particles. If oxygen is present, bacteria can quickly nitrify the
ammonium to nitrate which is soluble and is easily leached to the
groundwater. Phosphorus, on the other hand, is quickly chemisorbed onto
mineral surfaces of the soil, and as the concentration of phosphorus
increases with time, precipitates may form with the iron, aluminum, or
calcium naturally present in most soils. Therefore, the movement of
phosphorus through most soils is very slow (1)(2).
Numerous studies have shown that 2 ft to 4 ft (0.6 to 1.2 m) of
unsaturated soil is sufficient to remove bacteria and viruses to
acceptable levels and nearly all phosphorus (1)(2). The needed depth is
determined by the permeability of the soil. Soils with rapid
permeabilities may require greater unsaturated depths below the
infiltrative surface than soils with slow permeabilitiers.
14
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3.2.2 Wastewater Treatment and Disposal by Evaporation
Wastewater can be returned directly to the hydrologic cycle by evapora-
tion. This has appeal in onsite wastewater disposal because it can be
used in some areas where site conditions preclude soil absorption or in
areas where surface water or groundwater contamination is a concern.
The wastewater can be confined and the water removed to concentrate the
pollutants within the system. Little or no treatment is required prior
to evaporation. However, climatic conditions restrict the application
of this method.
Evaporation can take place from a free water surface, bare soil, or
plant canopies. Evaporation from plants is called transpiration. Since
it is often difficult to separate these two processes on partially bare
soil surfaces, they are considered as a single process called evapo-
transpiration (ET).
If evaporation is to occur continuously, three conditions must be met
(3). First, there must be a continuous supply of heat to meet the la-
tent heat requirements of water (approximately 590 cal/gm of water evap-
orated at 15 C). Second, the vapor pressure in the atmosphere over the
evaporative surface must remain lower than the vapor pressure at the
surface. This vapor pressure gradient is necessary to remove the mois-
ture either by diffusion, convection, or both. Third, there must be a
continuous supply of water to the evaporative surface. The first two
conditions are strongly influenced by meteorological factors such as air
temperature, humidity, wind velocity, and solar radiation, while the
third can be controlled by design.
Successful use of evaporation for wastewater disposal requires that
evaporation exceed the total water input to the system. Rates of evap-
oration decrease dramatically during the cold winter months. In the
case of evaporative lagoons or evapotranspiration beds, input from pre-
cipitation must also be included. Therefore, application of evaporation
for wastewater disposal is largely restricted to areas where evaporation
rates exceed precipitation rates. These areas occur primarily in the
southwestern United States (see Figure 3-1). In other areas, evapora-
tion can be used to augment percolation into the soil.
Transpiration by plants can be used to augment evaporation in soil-cov-
ered systems (5)(6). Plants can transpire at high rates, but only dur-
ing daylight hours of the growing season. During such periods, evapo-
transpiration rates may exceed ten times the rates measured in Class A
evaporation pans (7)(8)(9). However, overall monthly evaporation rates
exceed measured evapotranspiration rates. Ratios of evapotranspiration
to evaporation (as measured from Class A pans) are estimated to be 0.75
15
-------
FIGURE 3-1
POTENTIAL EVAPORATION VERSUS MEAN ANNUAL PRECIPITATION (4)
(inches)
CTl
+ 30
+ 50
+ 50
+ 3 0
, Potential Evapotranspiration more than
mean annual precipitation
Potential Evapotranspiration less than
mean annual precipitation
-------
to 0.8 (6)(7). Therefore, if covered disposal systems are to be used,
they must be larger than systems with a free water surface.
3.2.3 Wastewater Treatment and Disposal in Surface Waters
Surface waters may be used for the disposal of treated wastewaters if
permitted by the local regulatory agency. The capacity of surface
waters to assimilate wastewater pollutants varies with the size and type
of the body of water. In some cases, because of the potential for human
contact as well as the concern for maintaining the quality of lakes,
streams, and wetlands, the use of such waters for disposal are limited.
Where they can be used, the minimum quality of the wastewater effluent
to be discharged is specified by the appropriate water quality agency.
3.3 Site Evaluation Strategy
The objective of a site investigation is to evaluate the characteristics
of the area for their potential to treat and dispose of wastewater. A
good site evaluation is one that provides sufficient information to se-
lect the most appropriate treatment and disposal system from a broad
range of feasible options. This requires that the site evaluation begin
with all options in mind, eliminating infeasible options only as
collected site data indicate (see Chapter 2). At the completion of the
investigation, final selection of a system from those feasible options
is based on costs, aesthetics, and personal preference.
A site evaluation should be done in a systematic manner to ensure the
information collected is useful and is sufficient in detail. A sug-
gested procedure is outlined in Table 3-1 and discussed in the following
section. This procedure, which can be used to evaluate the feasibility
of sites for single dwellings or small clusters of dwellings (up to 10
to 12), is based on the assumption that subsurface soil disposal is the
most appropriate method of wastewater disposal. Therefore, the suit-
ability of the soils and other site characteristics for subsurface
disposal are evaluated first. If found to be unsuitable, then the
site's suitability for other disposal options is evaluated.
17
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TABLE 3-1
SUGGESTED SITE EVALUATION PROCEDURE
Step
Client Contact
Preliminary Evaluation
Field Testing
Other Site
Characteristics
Organization of Field
Information
Data Collected
Location and description of lot
Type of use
Volume and characteristics of
wastewater
Available resource information
(soil maps, geology, etc.)
Records of onsite systems in
surrounding area
Topography and landscape features
Soil profile characteristics
Hydraulic conductivity
If needed, site suitability for
evaporation or discharge to
surface waters should be
evaluated
Compilation of all data into
useable form
3.3.1 Client Contact
Before performing any onsite testing, it is important to gather informa-
tion about the site that will be useful in evaluating its potential for
treating and disposing of wastewater. This begins with the party devel-
oping the lot. The location of the lot and the intended development
should be established. The volume and character of the generated waste-
water should be estimated. Any wastewater constituents that may pose
potential problems in treatment and disposal, such as strong organic
wastewaters, large quantities of greases, fats or oils, hazardous and
toxic substances, etc., should be identified. This information helps to
focus the site evaluation on the important site characteristics.
18
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3.3.2 Preliminary Evaluation
The next step is to gather any available resource information about the
site. This includes soils, geology, topography, etc., that may be pub-
lished on maps or in reports. Local records of soil tests, system de-
signs, and reported problems with onsite systems installed in the sur-
rounding area should also be reviewed. This information may lack accu-
racy, but it can be useful in identifying potential problems or particu-
lar features to investigate. A plot plan of the lot and the land im-
mediately adjacent to it should be drawn to a scale large enough so that
the information gathered in this and later steps can be displayed on the
drawing. The proposed layout of all buildings and other manmade fea-
tures should also be sketched in.
3.3.2.1 Soil Surveys
Soil surveys are usually found at the local USDA Soil Conservation Ser-
vice (SCS) office. Also, some areas of the country have been mapped by
a state agency and these maps may be located at the appropriate state
office. In counties now being mapped, advance field sheets with inter-
pretive tables often can be obtained from the SCS.
Modern soil survey reports are a collection of aerial photographs of the
mapping area, usually a county, on which the distribution and kind of
soils are indicated. Interpretations about the potential uses of each
soil for farming, woodland, recreation, engineeering uses, and other
nonfarm uses are provided. Detailed descriptions of each soil series
found in the area are also given. The maps are usually drawn to a scale
of 4 in. to 1 mile. An example of a portion of a soil map is shown in
Figure 3-2.
The map symbols for each mapping unit give the name of the soil series,
slope, and degree of erosion (10). The soil series name is given a two-
letter symbol, the first in upper case, the second in lower case. Slope
is indicated by an upper case letter from A to F. A slopes are flat or
nearly flat and F slopes are steep. The specific slope range that each
letter represents differs from survey to survey. The degree of erosion,
if indicated, is given a number representing an erosion class. The
classes usually range from 1 to 3, representing slightly eroded to se-
verely eroded phases. The legend for the map symbols is found immedi-
ately preceding and following the map sheets in the modern published
surveys. An example translation of a map symbol from Figure 3-2 is
given in Figure 3-3.
19
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FIGURE 3-2
EXAMPLE OF A PORTION OF A SOIL MAP AS PUBLISHED
IN A DETAILED SOIL SURVEY (ACTUAL SIZE)
3 Acres
100' x 100'
Soil Absorption
Area
FIGURE 3-3
TRANSLATION OF TYPICAL SOIL MAPPING UNIT SYMBOL
Dn C 2
Soil Series
Erosion Class
(Moderately Eroded)
Slope Class
(In This Survey 2-6%)
20
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Interpretations about potential uses of each soil series are listed in
tables within the text of the report. The soil's suitabiliy for subsur-
face soil absorption systems and lagoons are specifically indicated.
Engineering properties are also listed, often including depth to bed-
rock, seasonal high water table, percolation rate, shrink-swell poten-
tial, drainage potential, etc. Flooding hazard and other important fac-
tors are discussed for each mapping unit with the profile descriptions.
While the soil surveys offer good preliminary information about an area,
it is not complete nor a substitute for a field study. Because of the
scale used, the mapping units cannot represent areas smaller than 2 to 3
acres (8,100 to 12,100 nr). Thus, there may be inclusions of soils with
significantly different character within mapping units that cannot be
indicated. For typical building lots, the map loses accuracy. There-
fore, these maps cannot be substituted for onsite testing in most cases.
Limitations ratings used by SCS for septic tank-soil absorption systems
are based on conventional trench or bed designs, and thus do not indi-
cate the soil's suitability for other designs. Table 3-2 lists the
criteria used in making the limitation ratings. They are based on a
soil absorption system with the bottom surface located 2 ft (0.6 m)
below the soil surface. In many cases, the limitations can be overcome
through proper design. Therefore, the interpretations should be used
only as a guide.
The information provided by the soil survey should be transferred to the
site drawing along with other important information. An example for a
parcel is shown in Figure 3-4. Information for each of the soil sites
shown on Figure 3-4 is presented in Table 3-3.
3.3.2.2 U.S. Geological Survey Quadrangles
Quadrangles published by the U.S. Geological Survey may be useful in
estimating slope, topography, local depressions or wet areas, rock out-
crops, and regional drainage patterns and water table elevations. These
maps are usually drawn to a scale of 1:24,000 (7.5 minute series) or
1:62,500 (15 minute series). However, because of their scale, they are
of limited value for evaluating small parcels.
21
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TABLE 3-2
SOIL LIMITATIONS RATINGS USED BY SCS
FOR SEPTIC TANK/SOIL ABSORPTION FIELDS
[Modified after (10)]
Property
USDA Texture
Floodi ng
Depth to Bedrock,
in.
Depth to Cemented
Pan, in.
Depth to High
Water Table, ft
below ground
Permeability,
in./hr
24-60 in. layer
layers <24 in.
Slope, percent
Fraction >3 in.,
percent by wt
Limits
siignt ModerateSevere
None,
Protected
>72
>72
>6
Rare
40-72
40-72
4-6
Ice
Common
Restrictive
Feature
Permafrost
Floods
<40 Depth to Rock
<40 Depth to
Cemented Pan
<4 Ponding,
Wetness
0-6.0
0-8
<25
0.6-2.0
8-15
25-50
<0.6
>6.0
>15
>50
Slow Perc. Rate
Poor Filter
Slope
Large Stones
3.3.2.3 Local Records
Soil test reports and records of reported failure of onsite systems from
the surrounding area may be a source of valuable information. The soil
test reports can provide an indication of soil types and variability.
Performance of systems may be determined from the reported failures.
These records are usually available from the local regulatory agency.
22
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FIGURE 3-4
PLOT PLAN SHOWING SOIL SERIES BOUNDARIES
FROM SOIL SURVEY REPORT
Drainage
Way
Soil
Boundary
Drainage
Way
Adjacent Lot
Property
Line
23
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TABLE 3-3
SOIL SURVEY REPORT INFORMATION
FOR PARCEL IN FIGURE 3-4
Soil
Absorption Depth to
Map Soil Limitation Flood High Water Depth to Permeability
Symbol Sen'es Slope Rating Hazard Tab! e Bedrock Depth Perm.
* ftft in. in./hr
DnC2 Dodge 2-6 Moderate No >5 5-10 0-40 0.63-2.0
40-60 2.0-6^3
TrB Troxel 2-6 Severe Yes 3-5 >10 0-60 0.63-20
PnB Piano 2-6 Moderate No 3-5 >10 0-41 0.63-2.0
41-60 2.0-e!3
3.3.3 Field Testing
Field testing begins with a visual survey of the parcel to locate poten-
tial sites for subsurface soil absorption. Soil borings are made in the
potential sites to observe the soil characteristics. Percolation tests
may be conducted in those soils that appear to be well suited. If no
potential sites can be found from either the visual survey, soil bor-
ings, or percolation tests, then other means of disposal should be
investigated.
3.3.3.1 Visual Survey
A visual survey is made to locate the areas on the lot with the greatest
potential for subsurface soil absorption. The location of any depres-
sions gullies, steep slopes, rocks or rock outcrops, or other obvious
land and surface features are noted and marked on the plot plan. Vege-
tation types are also noted that may indicate wetness or shallow soils.
Locations and distances from a permanent benchmark to lot lines, wells,
surface waters, buildings, and other features or structures are also
marked on the plot plan (see Figure 3-5). If a suitable area cannot be
24
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FIGURE 3-5
PLOT PLAN SHOWING SURFACE FEATURES
1100
o
CM
CO
,1110
B.M.-
I
400 Ft.
'30 Ft.
°T
'Well!
Ft.
25
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found for a subsurface soil absorption system based on this information
other disposal options must be considered (see Chapter 2). The remain-
der of the field testing can be altered accordingly.
3.3.3.2 Landscape Position
The landscape position and landform for each suitable area should be
noted. Figure 3-6 can be used as a guide for identifying landscape
positions. This information is useful in estimating surface and subsur-
face drainage patterns. For example, hilltops and sideslopes can be
expected to have good surface and subsurface drainage, while depressions
and foots!opes are more likely to be poorly drained.
3.3.3.3 Slope
The type and degree of slope of the area should be determined. The type
of slope indicates what surface drainage problems may be expected. For
example, concave slopes cause surface runoff to converge, while convex
slopes disperse the runoff (see Figure 3-6).
Some treatment and disposal systems are limited by slopes. Therefore,
slope measurement is important. Land slopes can be expressed in several
ways (see Figure 3-7):
1. PERCENT OF GRADE - The feet of vertical rise or fall in 100 ft
horizontal distance.
2. SLOPE - The ratio of vertical rise or fall to horizontal
distance.
3. ANGLE - The degrees and minutes from horizontal.
4. TOPOGRAPHIC ARC - The feet of vertical rise or fall in 66 ft
(20 m) horizontal distance.
Land slopes are usually determined by measuring the slope of a line
parallel to the ground with an Abney Level either at eye height or at
some other fixed height above the ground. If an ordinary hand level is
used, then slopes are determined by horizontal line of sight which give
changes in elevation for specific horizontal distances. A hand level is
limited in use because it is best suited for slope determinations up
grade only, but has the advantage that only one person is needed for
mapping slopes. Three methods of slope determinations are discussed
below.
26
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FIGURE 3-6
LANDSCAPE POSITIONS
Depression
/
Ridge Line
Hill Top
Convex
Slope
Sideslope
Foot Slope
Concave
Slope
FIGURE 3-7
METHODS OF EXPRESSING LAND SLOPES (10)
Horizontal
Percent of Grade-20
Slope-1:5
Angle-11° 19'
Topographic Arc- 13.2
27
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Instrument Supported - Abney Level: For accurate slope determinations,
notch two sticks or cut forked sticks so they will hold the level 5 ft
(1.5 m) above the ground. Rest the level in the notch or fork and sight
to the notch or fork of the other stick held by another person at a
point on the slope. The land slope is read directly in percent on the
Abney Level.
Abney Level: On level ground, sight the person working with you to
determine the point of intersection of your line of sight on him when
the instrument is in position for use as a hand level (zero level posi-
tion). When he is on the slope, sight the same point on the person
assisting you and read the slope directly.
Hand Level: Height of eye must be determined. Then sight the point of
interception with the ground surface and determine, by tape measurement
or pacing, the ground surface distance between the sighting point and
the point of intercept. To calculate land slope in percent, divide your
height of eye by the ground surface distance and multiply by 100.
Using one of the above procedures or other surveying methods, slopes at
selected sites can be determined so that topography can be mapped. The
number of sites needed will depend on the complexity of slopes. Slope
determinations should be made at each apparent change in slope at known
locations so steep slope areas can be accurately drawn. Experience will
be required for proficiency and accuracy in mapping. Steep slope areas
in natural topography have irregular form and curved boundaries.
Uniform boundaries having straight lines and angular corners indicate
man-altered conditions. For large areas it may be necessary to draw
contour lines so that slopes at different points in the plot can be
determined.
3.3.3.4 Soil Borings
Observation and evaluation of soil characteristics can best be deter-
mined from a pit dug by a backhoe or other excavating equipment. How-
ever, an experienced soil tester can do a satisfactory job by using a
hand auger or probe. Both methods are suggested. Hand tools can be
used to determine soil variability over the area and pits used to de-
scribe the various soil types found.
Soil pits should be prepared at the perimeter of the expected soil
absorption area. Pits prepared within the absorption area often settle
after the system has been installed and may disrupt the distribution
network. If hand augers are used, the holes may be made within the
28
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absorption area. Sufficient borings or pits should be made to ade-
quately describe the soils in the area, and should be deep enough to
assure that a sufficient depth of unsaturated soil exists below the
proposed bottom elevation of the absorption area. Variable soil
conditions may require many pits.
Since in some cases subtle differences in color need to be recognized,
it is often advantageous to prepare the soil pit so the sun will be
shining on the face during the observation period. Natural light will
give true color interpretations. Artificial lighting should not be
used.
3.3.3.5 Soil Texture
Texture is one of the most important physical properties of soil because
of its close relationship to pore size, pore size distribution, and pore
continuity. It refers to the relative proportion of the various sizes
of solid particles in the soil that are smaller than 2 mm in diameter.
The soil texture is determined in the field by rubbing a moist sample
between the thumb and forefinger. A water bottle is useful for moistur-
izing the sample. The grittiness, "silkiness," or stickiness can be
interpreted as being caused by the soil separates of sand, silt, and
clay. It is extremely helpful to work with some known samples to gain
experience with field texturing.
While laboratory analysis of soil texture is done routinely by many lab-
oratories, field texturing can give as good information as laboratory
data and therefore expenditures of time and money for laboratory analy-
ses are not necessary. To determine the soil texture, moisten a sample
of soil about one-half to one inch in diameter. There should be just
enough moisture so that the consistency is like putty. Too much mois-
ture results in a sticky material, which is hard to work. Press and
squeeze the sample between the thumb and forefinger. Gradually press
the thumb forward to try to form a ribbon from the soil (see Figure
3-8). By using this procedure, the texture of the soil can be easily
described.
Table 3-4 and Figures 3-9 and 3-10 describe the feeling and appearance
of the various soil textures for a general soil classification.
29
-------
FIGURE 3-8
PREPARATION OF SOIL SAMPLE FOR FIELD
DETERMINATION OF SOIL TEXTURE
(A) Moistening Sample
(B) Forming Cast
(C) Ribboning
30
-------
TABLE 3-4
TEXTURAL PROPERTIES OF MINERAL SOILS
Feeling and Appearance
Sand
Sandy Loam
Loam
Silt Loam
Clay Loam
Clay
Dry Soil
Loose, single grains which
feel gritty. Squeezed in
the hand, the soil mass
falls apart when the
pressure is released.
Aggregates easily crushed;
very faint velvety feeling
initially but with continued
rubbing the gritty feeling
of sand soon dominates.
Aggregates are crushed under
moderate pressure; clods can
be quite firm. When pulver-
ized, loam has velvety feel
that becomes gritty with
continued rubbing. Casts
bear careful handling.
Aggregates are firm but may
be crushed under moderate
pressure. Clods are firm to
hard. Smooth, flour-like
feel domi nates when soi1 i s
pulverized.
Very firm aggregates and
hard clods that strongly
resist crushing by hand.
When pulverized, the soil
takes on a somewhat gritty
feeling due to the harshness
of the very small aggregates
which persist.
Aggregates are hard; clods
are extremely hard and
strongly resist crushing by
hand. When pulverized, it
has a grit-like texture due
to the harshness of numerous
very small aggregates which
persist.
31
Moist Soil
Squeezed in the hand, it
forms a cast which crumbles
when touched. Does not form
a ribbon between thumb and
forefinger.
Forms a cast which bears
careful handling without
breaking. Does not form a
ribbon between thumb and
forefinger.
Cast can be handled quite
freely without breaking.
Very slight tendency to
ribbon between thumb and
forefinger. Rubbed surface
is rough.
Cast can be freely handled
without breaking. Slight
tendency to ribbon between
thumb and forefinger. Rubbed
surface has a broken or
rippled appearance.
Cast can bear much handling
without breaking. Pinched
between the thumb and
forefinger, it forms a ribbon
whose surface tends to feel
slightly gritty when dampened
and rubbed. Soil is plastic,
sticky and puddles easily.
Casts can bear considerable
handling without breaking.
Forms a flexible ribbon
between thumb and forefinger
and retains its plasticity
when elongated. Rubbed
surface has a very smooth,
satin feeling. Sticky when
wet and easily puddled.
-------
FIGURE 3-9
SOIL TEXTURE DETERMINATION BY HAND: PHYSICAL
APPEARANCE OF VARIOUS SOIL TEXTURES
Dry
Moist
Sandy
Loam
Weak Aggregates
No Ribbon; Non-Plastic Cast
Silt
Loam
Clay
Firm Aggregates
Hard Aggregates
Very Slight Ribboning
Tendency; Moderately
Plastic Cast
Ribbons Easily; Plastic Cast
32
-------
FIGURE 3-10
COMPARISON OF RIBBONS AND CASTS OF SANDY LOAM
AND CLAY (RIBBONS ABOVE, CASTS BELOW)
If the soil sample ribbons (loam, clay loam, or clay), it may be desir-
able to determine if sand or silt predominate. If there is a gritty
feel and a lack of smooth talc-like feel, then sand very likely predomi-
nates. If there is a lack of a gritty feel but a smooth talc-like feel,
then silt predominates. If there is not a predominance of either the
smooth or gritty feel, then the sample should not be called anything
other than a clay, clay loam, or loam. If a sample feels quite smooth
with little or no grit in it, and will not form a ribbon, the sample
would be called silt loam.
Beginning at the top or bottom of the pit sidewall, obvious changes in
texture with depth are noted. Boundaries that can be seen are marked.
The texture of each layer or horizon is determined and the demarcations
of boundaries changed as appropriate. When the textures have been
determined for each layer, the depth, thickness, and texture of each
layer is recorded (see Figure 3-11).
3.3.3.6 Soil Structure
Soil structure has a significant influence on the soil's acceptance and
transmission of water. Soil structure refers to the aggregation of soil
particles into clusters of particles, called peds, that are separated by
surfaces of weakness. These surfaces of weakness open planar pores
between the peds that are often seen as cracks in the soil. These pla-
nar pores can greatly modify the influence of soil texture on water
movement. Well-structured soils with large voids between peds will
transmit water more rapidly than structureless soils of the same tex-
ture, particularly if the soil has become dry before the water is
added. Fine-textured, massive soils (soils' with little structure) have
very slow percolation rates.
33
-------
FIGURE 3-11
EXAMPLE PROCEDURE FOR COLLECTING
SOIL PIT OBSERVATION INFORMATION
Depth
(Ft.)
0
2
4
6
8
10
12
14
Texture Structure Color Soil Saturation
Silt
Loam
Silty
Clay Loan
Clay Loam
Sandy
Loam
1 1
Granular
Platy
Blocky
Platy
Massive
Brown
None
34
-------
If a detailed analysis of the soil structure is necessary, the sidewall
of the soil pit should be carefully examined, using a pick or similar
device to expose the natural cleavages and planes of weakness. Cracks
in the face of the soil profile are indications of breaks between soil
peds. The shapes created by the cracks should be compared to the shapes
shown in Figure 3-12. If cracks are not visible, a sample of soil
should be carefully picked out and, by hand, carefully separated into
the structural units until any further breakdown can only be achieved by
fracturing.
Since the structure can significantly alter the hydraulic characteris-
tics of soils, more detailed descriptions of soil structure are some-
times desirable. Size and grade of durability of the structural units
provide useful information to estimate hydraulic conductivities. De-
scriptions of types and classes of soil structure used by SCS are given
in Appendix A. Grade descriptions are given in Table 3-5. The type,
size, and grade of each horizon or zone is recorded in Figure 3-11.
3.3.3.7 Soil Color
The color and color patterns in soil are good indicators of the drainage
characteristics of the soil. Soil properties, location in the land-
scape, and climate all influence water movement in the soil. These
factors cause some soils to be saturated or seasonally saturated,
affecting their ability to absorb and treat wastewater. Interpretation
of soil color aids in identifying these conditions.
Color may be described by estimating the true color for each horizon or
by comparing the soil with the colors in a soil color book. In either
case, it is particularly important to note the colors or color patterns.
Pick up some soil and, without crushing, observe the color. It is
important to have good sunlight and know the moisture status of the
sample. If ped faces are dry, some water applied from a mist bottle
will allow observation of moist colors.
Though it is often adequate to speak of soil colors in general terms,
there is a standard method of describing colors using Munsell color
notation. This notation is used in soil survey reports and soil de-
scription. Hue is the dominant spectral color and refers to the light-
ness or darkness of the color between black and white. Chroma is the
relative purity of strength of the color, and ranges from gray to a
bright color of that hue. Numbers are given to each of the variables
and a verbal description is also given. For example, 10YR 3/2 corre-
sponds to a color hue of 10YR value of 3 and chroma 2. This is a very
dark grayish brown.
35
-------
FIGURE 3-12
TYPES OF SOIL STRUCTURE
Prismatic
Columnar
Subangular
Angular Blocky
Blocky
Granular
TABLE 3-5
GRADES OF SOIL STRUCTURE
Grade
Structureless
Weak
Moderate
Strong
Characteristics
No observable aggregation.
Poorly formed and difficult to see.
Will not retain shape on handling.
Evident but not distinct in undisturbed
soil.
Moderately durable on handling.
Visually distinct in undisturbed soil.
Durable on handling.
36
-------
If a soil color book 1s used to determine soil colors, hold the soil and
book so the sun shines over your shoulder. Match the soil color with
the color chip in the book. Record the hue, chroma and value, and the
color name.
Mottling in soils is described by the color of the soil matrix and the
color or colors, size, and number of the mottles. Each color may be
given a Munsell designation and name. However, it is often sufficient
to say the soil is mottled. A classification of mottles used by the
USDA is shown in Table 3-6. Some examples of soil mottling are shown on
the inside back cover of this manual.
TABLE 3-6
DESCRIPTION OF SOIL MOTTLES (10)
Character Class Limit
Abundance Few <2% of exposed face
Common 2-20% of exposed face
Many >20% of exposed face
Size Fine <5mm longest dimension
Medium 5-15mm longest dimension
Coarse >15mm longest dimension
Contrast Faint Recognized only by close observation
Distinct Readily seen but not striking
Prominent Obvious and striking
3.3.3.8 Seasonally Saturated Soils
Seasonally saturated soils can usually be detected by soil borings made
during the wet season or by the presence of mottled soils (see 3.3.3.7).
For large cluster systems or for developments where each dwelling is
served by an onsite system, the use of observation wells may be justi-
fied. They are constructed as shown in Figure 3-13. The well should be
placed in, but not extended through, the horizon that is to be moni-
tored. More than one well in each horizon that may become seasonally
saturated is desirable. The wells are monitored over a normal wet sea-
son by observing the presence and duration of water in the well. If
water remains in the well for several days, the water level elevation is
measured and assumed to be the elevation of the seasonally saturated
soil horizon.
37
-------
FIGURE 3-13
TYPICAL OBSERVATION WELL FOR
DETERMINING SOIL SATURATION
Excavated
(Tamped in
1 "-4'
Solid
1
Soil He
to be Me
^
J^S
t~«—
Soil Material _
when placing)
' Diameter
Wall Pipe
1
)rizon
mitored
r
1
|
, ,«
. ff.'
• *
'oV
• *.*
-
•;»•. :
1
i
g
V/X
.o'.1
•;<
»" .
• > t
••
^f
12" Puddled Clay
.7 or Equal Parts of
Soil and Cement
Mixture
V
,. ^/2"-3/4" Gravel
l!?'"
•> \i.
1" Min.
3.3.3.9 Other Selected Soil Characteristics
Soil bulk density is related to porosity and the movement of water.
High bulk density is an indication of low porosity and restricted flow
of water. Relative bulk densities of different soil horizons can be
detected in the field by pushing a knife or other instrument into each
horizon. If one horizon offers considerably more resistance to penetra-
tion than the others, its bulk density is probably higher. However, in
some cases, cementing agents between soil grains or peds may be the
cause of resistance to penetration.
Swelling clays, particularly montmorillonite clays, can seal off soil
pores when wet. They can be detected during field texturing of the soil
by their tendency to be more sticky and plastic when wet.
38
-------
3.3.3.10 Hydraulic Conductivity
Several methods of measuring the hydraulic conductivity of soils have
been developed (1)(11). The most commonly used test is the percolation
test. When run properly, the test can give an approximate measure of
the soil's saturated hydraulic conductivity. However, the percolation
of wastewater through soil below soil disposal systems usually occurs
through unsaturated soils. Therefore, empirical factors must be used to
estimate unsaturated conductivities. The unsaturated hydraulic conduc-
tivities can vary dramatically from the saturated hydraulic conductivity
with changes in soil characteristics and moisture content (see Appendix
A).
The percolation test is often criticized because of its variability and
failure to measure the hydraulic conductivity accurately. Percolation
tests conducted in the same soils can vary by 90% or more (1)(11)(12)
(13)(14). Reasons for the large variability are attributed to the pro-
cedure used, the soil moisture conditions at the time of the test, and
the individual performing the test. Despite these shortcomings, the
percolation test can be useful if used together with the soil borings
data. The test can be used to rank the relative hydraulic conductivity
of the soil. Estimated percolation rates for various soil textures are
given in Table 3-7.
TABLE 3-7
ESTIMATED HYDRAULIC CHARACTERISTICS OF SOIL (15)
Soil Texture Permeabi1ity Percolation
in./hr min/in.
Sand >6.0 <10
Sandy loams
Porous silt loams 0.2-6.0 10-45
Silty clay loams
Clays, compact
Silt loams <0.2 >45
Silty clay loams
39
-------
If test results agree with this table, the test and boring data are
probably correct and can be used in design. If not, either the test was
run improperly or soil structure or clay mineralogy have a significant
effect on the hydraulic conductivity. For example, if the texture of a
soil is
-------
TABLE 3-8
FALLING HEAD PERCOLATION TEST PROCEDURE
1. Number and Location of Tests
Commonly a minimum of three percolation tests are performed within the area proposed
for an absorption system. They are spaced uniformly throughout the area. If soil
conditions are highly variable, more tests may be required.
2. Preparation of Test Hole
The diameter of each test hole is 6 in., dug or bored to the proposed depths at the
absorption systems or to the most limiting soil horizon. To expose a natural soil
surface, the sides of the hole are scratched with a sharp pointed instrument and the
loose material is removed from the bottom of the test hole. Two inches of 1/2 to 3/4
in. gravel are placed in the hole to protect the bottom from scouring action when the
water is added.
3. Soaking Period
The hole is carefully filled with at least 12 in. of clear water. This depth of
water should be maintained for at least 4 hr and preferably overnight if clay soils
are present. A funnel with an attached hose or similar device may be used to prevent
water from washing down the sides of the hole. Automatic siphons or float valves may
be employed to automatically maintain the water level during the soaking period. It
is extremely important that the soil be allowed to soak for a sufficiently long
period of time to allow the soil to swell if accurate results are to be obtained.
In sandy soils with little or no clay, soaking is not necessary. If, after filling
the hole twice with 12 in. of water, the water seeps completely away in less than ten
minutes, the test can proceed immediately.
4. Measurement of the Percolation Rate
Except for sandy soils, percolation rate measurements are made 15 hr but no more than
30 hr after the soaking period began. Any soil that sloughed into the hole during
the soaking period is removed and the water level is adjusted to 6 in. above the
gravel (or 8 in. above the bottom of the hole). At no time during the test is the
water level allowed to rise more than 6 in. above the gravel.
Immediately after adjustment, the water level is measured from a fixed reference
point to the nearest 1/16 in. at 30 min intervals. The test is continued until two
successive water level drops do not vary by more than 1/16 in. At least three
measurements are made.
After each measurement, the water level is readjusted to the 6 in. level. The last
water level drop is used to calculate the percolation rate.
In sandy soils or soils in which the first 6 in. of water added after the soaking
period seeps away in less than 30 min, water level measurements are made at 10 min
intervals for a 1 hr period. The last water level drop is used to calculate the
percolation rate.
5. Calculation of the Percolation Rate
The percolation rate is calculated for each test hole by dividing the time interval
used between measurements by the magnitude of the last water level drop. This
calculation results in a percolation rate in terms of min/in. To determine the
percolation rate for the area, the rates obtained from each hole are averaged. (If
tests in the area vary by more than 20 min/in., variations in soil type are
indicated. Under these circumstances, percolation rates should not be averaged.)
Example: If the last measured drop in water level after 30 min is 5/8 in., the
percolation rate = (30 min)/(5/8 in.) = 48 min/in.
41
-------
FIGURE 3-14
CONSTRUCTION OF A PERCOMETER
Thin Steel
Rod
ru*—- Yard Stick
Eyelets
Cork
Float
Cross Arm
Support
6"-9"
Diameter
2" Layer
of Gravel
When making percolation
tests, mark lines here at
regular time intervals.
Guide Line^
• Measuring Stick
Batter Board or
/— Other Fixed
r . Reference Point
6"-9" Diameter
«- 2" Layer of Gravel
(a) Floating Indicator
(b) Fixed Indicator
-------
FIGURE 3-15
PERCOLATION TEST DATA FORM (17)
Percolation test
Location
ia
Test hole number
Depth to bottom of hole <3 £ inches. Diameter of hole.
Depth, inches Soil texture
0 ~ V
inches.
Percolation test by.
Date of test
Time
t];30
lo:°°
io:*°
10 '^
!l;*D
U-°°
13-3°
I • oo
I-.30
Time
Interval,
minutes
—
3.0
Ao
3o
3o
i-o
30
2.0
3D
Measure-
ment,
inches
w-
+3
Y3
Y3'/f
t3'/,(.
*?'/?
^3L
^3^
^H,
Drop in
water level,
inches
—
/
/
VY
lf/l(o
%
'V/i,
"At,
"k
Percola-
tion rate,
minutes
per inch
W
Remarks
Percolation rate = T'T minutes per inch.
43
-------
Establishing evaporation data at a speci'fic location can be a more
difficult problem. Measurements of Class A pan evaporation rates are
reported for all of the states by NOAA in the publication,
"Climatological Data," U.S. Department of Commerce, available in
depository libraries for government documents at major universities in
each state. Pan evaporation measurements are made at a few (5 to 30)
weather stations in each state. Data for the winter months are often
omitted because this method cannot be used under freezing weather
conditions. The critical period of the year for design of systems for
permanent homes is in the winter. Obtaining representative winter
evaporation data is probably the most difficult part of design analysis.
Application of evaporation systems is most favorable in the warm, dry
climates of the southwestern United States. For these areas, pan
evaporation data are available for the complete year. The analysis of
evaporative potential for cooler, semi-arid regions, such as eastern
Washington and Oregon, Utah, Colorado, and similar areas, requires that
winter data be established by means other than pan evaporation
measurements, since these data are generally not available.
One method for establishing representative winter evaporation data is to
take measurements on buried lysimeters. Another method is to use empir-
ical formulations such as the Penman formula (18). The Penman formula
has been shown to give results comparable to measured winter values
(5).
3.3.4.2 Site Evaluation for Surface Water Discharge
For surface water disposal to be a viable option, access to a suitable
surface body of water must be available. Onsite investigations must
locate the body of water, identify it, and determine the means by which
access can be gained. Since discharges to surface waters are usually
regulated, the local water quality agency must be contacted to learn if
discharge of wastewater into that body of water is permitted and, if so,
what effluent standards must be met.
3.3.5 Organizing the Site Information
As the site information is collected, it is organized so that it can be
easily used to check site suitability for any of the various systems
discussed in this manual. One such method of organization is shown in
Figure 3-16. In this example, two soil observations have been made.
The number of soil observations varies. It is important that all perti-
nent site information be presented in a clear fashion to provide suffi-
cient information to the designer of the system without making addi-
tional site visits.
44
-------
FIGURE 3-16
COMPILATION OF SOILS AND SITE INFORMATION
(INFORMATION INCLUDES TOPOGRAPHIC, SOIL SURVEY,
ONSITE SLOPE AND SOIL PIT OBSERVATIONS)
Name
Address
Site evaluator
Address
Waste water quantity
-^'. 5
o
Well
^ Soil Pit
O Percolation Tests (If Determined)
— Set Back
Contour
— Soil Boundary
.— Drainage Way
45
-------
FIGURE 3-16 (continued)
Name:
Soil Pit No.
Depth
iTexture
Structure
Color
Soil Saturation
8
10
12
14
Silt
Loam
Silty
Clay
Loam
Clay Loam
Sandy
Loam
Granular
Platy
Blocky
Platy
Brown
None
Massive
Soil Map Unit - DnC2
Slope - 6%
Landscape Position - Side Slope
Landscape Type - Plane to Concave
46
-------
FIGURE 3-16 (continued)
Name :
Soil Pit No.
0
Texture
Structure
Color
Soil Saturation
8
10
12
14
Silt
Loam
Silty
Clay
Loam
Silt
Loam
Blocky
Granular
Blocky
Massive
Brown
Black
Brown
Brown and
Grey and Red
Mottles
Soil Map Unit - TrB
Slope - 4%
Landscape Position - Footslope
Landscape Type - Concave
47
-------
3.4 References
1. Small Scale Waste Management Project, University of Wisconsin,
Madison. Management of Small Waste Flows. EPA 600/2-78-173, NTIS
Report No. PB 286 560, September 1978. 804 pp.
2. Tyler, E. J., R. Laak, E. McCoy, and S. S. Sandhu. The Soil as a
Treatment System. In: Proceedings of the Second National Home
Sewage Treatment Symposium, Chicago, Illinois, December 1977.
American Society of Agricultural Engineers, St. Joseph, Michigan,
1978. pp. 22-37.
3. Hillel, D. I. Soil and Water: Physical Principles and Processes.
Academic Press, New York, 1971. 302 pp.
4. Flach, K. W. Land Resources. In: Recyclying Municipal Sludges
and Effluents on Land. Champafgn, University of Illinois, July
1973.
5. Bennett, E. R., and K. D. Linstedt. Sewage Disposal by Evaporation-
Transpiration. EPA 600/2-78-163, NTIS Report No. PB 288 588,
September 1978. 196 pp.
6. Pickett, E. M. Evapotranspiration and Individual Lagoons. In:
Proceedings of Northwest Onsite Wastewater Disposal Short Course,
University of Washington, Seattle, December 1976. pp. 108-118.
7. Pruitt, W. 0. Empirical Method for Estimating Evapotranspiration
Using Primarily Evaporation Pans. In: Evapotranspiration and Its
Role in Water Resources ManagementfTonference Proceedings, Ameri-
can Society of Agricultural Engineers, St. Joseph, Michigan, 1966.
pp. 57-61.
8. Beck, A. F. Evapotranspiration Pond Design. Environ. Eng. Div.,
Am. Soc. Civil Eng., K)5_ 411-415, 1979.
9. Bernhart, A. P. Treatment and Disposal of Wastewater From Homes by
Soil Infiltration and Evapotranspiration. 2nd ed. University of
Toronto Press, Toronto, Canada, 1973.
10. Soil Conservation Service. Soil Survey Manual. USDA Handbook 18,
U.S. Government Printing Office, Washington, D.C., 1951. 503 pp.
11. Studies on Household Sewage Disposal Systems. Environmental Health
Center, Cincinnati, Ohio, 1949-3 pts.
12. Bouma, J. Evaluation of the Field Percolation Test and an Alter-
native Procedure to Test Soil Potential for Disposal of Septic Tank
Effluent. Soil Sci. Soc. Amer. Proc. 35:871-875, 1971.
48
-------
13. Winneberger, J. T. Correlation of Three Techniques for Determining
Soil Permeability. Environ. Health, 37:108-118, 1974.
14. Healy, K. A., and R. Laak. Factors Affecting the Percolation Test.
J. Water Pollut. Control Fed., 45:1508-1516, 1973.
15. Bouma, J. Unsaturated Flow During Soil Treatment of Septic Tank
Effluent. J. Environ. Eng., Am. Soc. Civil Eng., 101:967-983,
1975.
16. Onsite Wastewater Management. National Environmental Health Asso-
ciation, Denver, Colorado, 1979.
17. Machmeier, R. E. How to Run a Percolation Test. Extension Folder
261, University of Minnesota, St. Paul, 1977.
18. Penman, H. L. Estimating Evaporation. Trans. Amer. Geophys.
Union, 37:43-46, 1956.
49
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CHAPTER 4
WASTEWATER CHARACTERISTICS
4.1 Introduction
The effective management of any wastewater flow requires a reasonably
accurate knowledge of its characteristics. This is particularly true
for wastewater flows from rural residential dwellings, commercial estab-
lishments and other facilities where individual water-using activities
create an intermittent flow of wastewater that can vary widely in volume
and degree of pollution. Detailed characterization data regarding these
flows are necessary not only to facilitate the effective design of
wastewater treatment and disposal systems, but also to enable the de-
velopment and application of water conservation and waste load reduction
strategies.
For existing developments, characterization of the actual wastewaters to
be encountered may often times be accomplished. However, for many exis-
ting developments, and for almost any new development, wastewater char-
acteristics must be predicted. The purpose of this chapter is to
provide a basis for characterizing the wastewater from rural develop-
ments. A detailed discussion of the characteristics of residential
wastewaters is presented first, followed by a limited discussion of the
characteristics of the wastewaters generated by nonresidential estab-
lishments, including those of a commercial, institutional and recrea-
tional nature. Finally, a general procedure for predicting wastewater
characteristics for a given residential dwelling or nonresidential
establishment is given.
4.2 Residential Wastewater Characteristics
Residential dwellings exist in a variety of forms, including single- and
multi-family households, condominium homes, apartment houses and
cottages or resort residences. In all cases, occupancy can occur on a
seasonal or year-round basis. The wastewater discharged from these
dwellings is comprised of a number of individual wastewaters, generated
through water-using activities employing a variety of plumbing fixtures
and appliances. The characteristics of the wastewater can be influenced
by several factors. Primary influences are the characteristics of the
plumbing fixtures and appliances present as well as their frequency of
use. Additionally, the characteristics of the residing family in terms
of number of family members, age levels, and mobility are important as
50
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is the overall socioeconomic status of the family. The characteristics
of the dwelling itself, including seasonal or yearly occupancy,
geographic location, and method of water supply and wastewater disposal,
appear as additional, but lesser, influences.
4.2.1 Wastewater Flow
4.2.1.1 Average Dally Flow
The average daily wastewater flow from a typical residential dwelling is
approximately 45 gal/capita/day (gpcd) (170 liters/capita/day [Ipcd])
(Table 4-1). While the average daily flow experienced at one residence
compared to that of another can vary considerably, it is typically no
greater than 60 gpcd (227 Ipcd) and seldom exceeds 75 gpcd (284 Ipcd)
(Figure 4-1).
4.2.1.2 Individual Activity Flows
The individual wastewater generating activities within a residence are
the building blocks that serve to produce the total residential waste-
water discharge. The average characteristics of several major residen-
tial water-using activities are presented in Table 4-2. A water-using
activity that falls under the category of miscellaneous in this table,
but deserves additional comment, is water-softener backwash/regeneration
flows. Water softener regeneration typically occurs once or twice a
week, discharging about 30-88 gal (114 to 333 1) per regeneration cycle
(11). On a daily per capita basis, water softener flows have been shown
to average about 5 gpcd (19 Ipcd), ranging from 2.3 to 15.7 gpcd (8.7 to
59.4 Ipcd) (7).
4.2.1.3 Wastewater Flow Variations
The intermittent occurrence of individual wastewater-generating activi-
ties creates large variations in the wastewater flow rate from a resi-
dence.
a. Minimum and Maximum Daily Flows
The daily wastewater flow from a specific residential dwelling is typ-
ically within 1Q% and 300% of the average daily flow at that dwelling,
with the vast majority within 50 and 150% of the average day. At the
51
-------
TABLE 4-1
SUMMARY OF AVERAGE DAILY RESIDENTIAL WASTEWATER FLOWS
en
PO
Studv
Linaweaver, et al. (1)
Anderson and Watson (2)
Watson, et al. (3)
Cohen and Wallman (4)
Laak (5)
Bennett and Linstedt (6)
Siegrist, et al. (7)
Otis (8)
Duffy, et al. (9)
No
Resi
. of
dences
22
18
3
8
5
5
11
21
16
Duration
of
Study
months
-
4
2-12
6
24
0.5
1
12
12
Wastewater Flow
Study
Average
gpcd
49
44
53
52
41.4
44.5
42.6
36
42.3
Range of Individual
Residence Averages
gpcd
36 - 66
18 - 69
25-65
37.8 - 101.6
26.3 - 65.4
31.8 - 82.5
25.4 - 56.9
8-71
-
Weighted Average
44
-------
FIGURE 4-1
FREQUENCY DISTRIBUTION FOR AVERAGE DAILY
RESIDENTIAL WATER USE/WASTE FLOWS
o
Q.
O)
90
80
70
.-? 60
CD
V-i
I
'(/>
0)
40
30
20
10
0
I I
I I
I I
I
I I I I I
I I
I I
1 2 5 10 20304050607080 90 95 9899
Flow Values Less than or Equal to Stated Flow Value (%)
Note: Based on the average daily flow measured
for each of the 71 residences studied in (2) (3) (4)
(5) (6) (7) (8).
53
-------
TABLE 4-2
RESIDENTIAL WATER USE BY ACTIVITY*
Acti vi ty
Toilet Flush
Bathi ng
Clotheswashing
Dishwashing
Garbage Grinding
Miscellaneous
Total
Gal /use
4.3
4.0 - 5.0
24.5
21.4 - 27.2
37.4
33.5 - 40.0
8.8
7.0 - 12.5
2.0
2.0 - 2.1
-
"
Uses/cap/day
3.5
2.3 - 4.1
0.43
0.32 - 0.50
0.29
0.25 - 0.31
0.35
0.15 - 0.50
0.58
0.4 - 0.75
-
""
gpcdb
16.2
9.2 - 20.0
9.2
6.3 - 12.5
10.0
7.4 - 11.6
3.2
1.1 - 4.9
1.2
0.8 - 1.5
6.6
5.7 - 8.0
45.6
41.4 - 52.0
a Mean and ranges of results reported in (4)(5)(6)(7)(10).
b gpcd may not equal gal/use multiplied by uses/cap/day due to
difference in the number of study averages used to compute the
mean and ranges shown.
54
-------
extreme, however, minimum and maximum daily flows of 0% and 900% of the
average daily flow may be encountered (2)(3)(12).
b. Minimum and Maximum Hourly Flows
Minimum hourly flows of zero are typical. Maximum hourly flows are more
difficult to quantify accurately. Based on typical fixture and appli-
ance usage characteristics, as well as an analysis of residential water
usage demands, maximum hourly flows of 100 gal/hr (380 1/hr) can occur
(2)(13). Hourly flows in excess of this can occur due to plumbing fix-
ture and appliance misuse or malfunction (e.g., faucet left on or worn
toilet tank flapper).
c. Instantaneous Peak Flows
The peak flow rate from a residential dwelling is a function of the
characteristics of the fixtures and appliances present and their posi-
tion in the overall plumbing system layout. The peak discharge rate
from a given fixture/appliance is typically around 5 gal/minute {gpm)
(0.3 liters/sec), with the exception of the tank-type water closet which
discharges at a peak flow of up to 25 gpm (1.6 I/sec). The use of
several fixtures/appliances simultaneously can increase the total flow
rate from the isolated fixtures/appliances. However, attenuation occur-
ring in the residential drainage network tends to decrease the peak flow
rates in the sewer exiting the residence.
Although field data are limited, peak discharge rates from a single-
family dwelling of 5 to 10 gpm (0.3 to 0.6 I/sec) can be expected. For
multi-family units, peak rates in excess of these values commonly occur.
A crude estimate of the peak flow in these cases can be obtained using
the fixture-unit method described in Section 4.3.1.2.
4.2.2 Wastewater Quality
4.2.2.1 Average Daily Flow
The characteristics of typical residential wastewater are outlined in
Table 4-3, including daily mass loadings and pollutant concentrations.
The wastewater characterized is typical of residential dwellings
equipped with standard water-using fixtures and appliances (excluding
garbage disposals) that collectively generate approximately 45 gpcd (170
Ipcd).
55
-------
TABLE 4-3
CHARACTERISTICS OF TYPICAL RESIDENTIAL WASTEWATER9
Parameter Mass Loading Concentration
gm/cap/day mg/1
Total Solids 115 - 170 680 - 1000
Volatile Solids 65 - 85 380 - 500
Suspended Solids 35 - 50 200 - 290
Volatile Suspended Solids 25 - 40 150 - 240
BODs 35 - 50 200 - 290
Chemical Oxygen Demand 115 - 125 680 - 730
Total Nitrogen 6-17 35 - 100
Ammonia 1-3 6-18
Nitrites and Nitrates <1 <1
Total Phosphorus
Phosphate
Total Coliformsb
Fecal Coliformsb
3-5 18 -
1-4 6 -
ioio -
108 -
29
24
1012
ioio
a For typical residential dwellings equipped with standard water-using
fixtures and appliances (excluding garbage disposals) generating
approximately 45 gpcd (170 Ipcd). Based on the results presented in
b Concentrations presented in organisms per liter.
56
-------
4.2.2.2 Individual Activity Contributions
Residential water-using activities contribute varying amounts of pollu-
tants to the total wastewater flow. The individual activities may be
grouped into three major wastewater fractions: (1) garbage disposal
wastes, (2) toilet wastes, and (3) sink, basin, and appliance waste-
waters. A summary of the average contribution of several key pollutants
in each of these three fractions is presented in Tables 4-4 and 4-5.
With regard to the microbiological characteristics of the individual
waste fractions, studies have demonstrated that the wastewater from
sinks, basins, and appliances can contain significant concentrations of
indicator organisms as total and fecal coliforms (14)(15)(16)(17).
Traditionally, high concentrations of these organisms have been used to
assess the contamination of a water or wastewater by pathogenic organ-
isms. One assumes, therefore, that these wastewaters possess some po-
tential for harboring pathogens.
4.2.2.3 Wastewater Quality Variations
Since individual water-using activities occur intermittently and contri-
bute varying quantities of pollutants, the strength of the wastewater
generated from a residence fluctuates with time. Accurate quantifica-
tion of these fluctuations is impossible. An estimate of the type of
fluctuations possible can be derived from the pollutant concentration
information presented in Table 4-5 considering that the activities
included occur intermittently.
4.3 Nonresidential Wastewater Characteristics
The rural population, as well as the transient population moving through
the rural areas, is served by a wide variety of isolated commercial
establishments and facilities. For many establishments, the wastewater-
generating sources are sufficiently similar to those in a residential
dwelling that residential wastewater characteristics can be applied.
For other establishments, however, the wastewater characteristics can be
considerably different from those of a typical residence.
Providing characteristic wastewater loadings for "typical" non-residen-
tial establishments is a very complex task due to several factors.
First, there is a relatively large number of diverse establishment cate-
gories (e.g., bars, restaurants, drive-in theaters, etc.). The inclu-
sion of potentially diverse establishments within the same category
produces a potential for large variations in waste-generating sources
57
-------
TABLE 4-4
POLLUTANT CONTRIBUTIONS OF MAJOR RESIDENTIAL
WASTEWATER FRACTIONS3 (gm/cap/day)
Parameter
BOD5
Suspended
Sol i ds
Ni trogen
Phosphorus
Garbage
Disposal
18.0
10.9 - 30.9
26.5
15.8 - 43.6
0.6
0.2 - 0.9
0.1
0.1 - 0.1
Toi 1 et
16.7
6.9 - 23.6
27.0
12.5 - 36.5
8.7
4.1 - 16.8
1.2
0.6 - 1.6
Basins,
Sinks,
Appliances
28.5
24.5 - 38.8
17.2
10.8 - 22.6
1.9
1.1 - 2.0
2.8
2.2 - 3.4
Approximate
Total
63.2
70.7
11.2
4.0
a Means and ranges of results reported in (5)(6)(7)(10)(14)
TABLE 4-5-
POLLUTANT CONCENTRATIONS OF MAJOR RESIDENTIAL
WASTEWATER FRACTIONS* (mg/1)
Parameter
BOD5
Suspended
Sol i ds
Ni trogen
Phosphorus
Garbage
Di sposal
2380
3500
79
13
Toilet
280
450
140
20
Basins, Sinks,
Appliances
260
160
17
26
Combi ned
Wastewater
360
400
63
23
a Based on the average results presented in Table 4-4 and the
following wastewater flows: Garbage disposal - 2 gpcd (8 Ipcd);
toilet - 16 gpcd (61 Ipcd); basins, sinks and appliances - 29 gpcd
(110 Ipcd); total - 47 gpcd (178 Ipcd).
58
-------
and the resultant wastewater characteristics. Further, many intangible
Influences such as location, popularity, and price may produce substan-
tial wastewater variations between otherwise similar establishments.
Finally, there is considerable difficulty in presenting characterization
data in units of measurement that are easy to apply, yet predictively
accurate. (For example, at a restaurant, wastewater flow in gal/seat is
easy to apply to estimate total flow, but is less accurate than if
gal/meal served were used.)
In this section, limited characterization data for nonresidential estab-
lishments, including commercial establishments, institutional facili-
ties, and recreational areas, are presented. These data are meant to
serve only as a guide, and as such should be applied cautiously. Wher-
ever possible, characterization data for the particular establishment in
question, or a similar one in the vicinity, should be obtained.
4.3.1 Wastewater Flow
4.3.1.1 Average Daily Flow
Typical daily flows from a variety of commercial, institutional, and
recreational establishments are presented in Tables 4-6 to 4-8.
4.3.1.2 Wastewater Flow Variation
The wastewater flows from nonresidential establishments are subject to
wide fluctuations with time. While difficult to quantify accurately, an
estimate of the magnitude of the fluctuations, including minimum and
maximum flows on an hourly and daily basis, can be made if consideration
is given to the characteristics of the water-using fixtures and appli-
ances, and to the operational characteristics of the establishment
(hours of operation, patronage fluctuations, etc.).
Peak wastewater flows can be estimated utilizing the fixture-unit method
(19)(20). As originally developed, this method was based on the premise
that under normal usage, a given type of fixture had an average flow
rate and duration of use (21)(22). One fixture unit was arbitrarily set
equal to a flow rate of 7.5 gpm (0.5 I/sec), and various fixtures were
assigned a certain number of fixture units based upon their particular
characteristics (Table 4-9). Based on probability studies, relation-
ships were developed between peak water use and the total number of fix-
ture units present (Figure 4-2).
59
-------
TABLE 4-6
TYPICAL WASTEWATER FLOWS FROM COMMERCIAL SOURCES (18)
Source
Airport
Automobile Service Station
Bar
Hotel
Industrial Building
(excluding industry and
cafeteria)
Laundry (self-service)
Motel
Motel with Kitchen
Office
Restaurant
Rooming House
Store, Department
Shopping Center
Wastewater Flow
Unit
Passenger
Vehicle Served
Empl oyee
Customer
Employee
Guest
Empl oyee
Empl oyee
Machine
Wash
Person
Person
Empl oyee
Meal
Resident
Toilet room
Empl oyee
Parking Space
Employee
Ran
2.1 -
7.9 -
9.2 -
1.3 -
10.6 -
39.6 -
7.9 -
7.9 -
475 -
47.5 -
23.8 -
50.2 -
7.9 -
2.1 -
23.8 -
423 -
7.9 -
0.5 -
7.9 -
ge
gpd/uni
4.0
13.2
15.8
5.3
15.8
58.0
13.2
17.2
686
52.8
39.6
58.1
17.2
4.0
50.1
634
13.2
2.1
13.2
Typical
t
2.6
10.6
13.2
2.1
13.2
50.1
10.6
14.5
580
50.1
31.7
52.8
14.5
2.6
39.6
528
10.6
1.1
10.6
60
-------
TABLE 4-7
TYPICAL WASTEWATER FLOWS FROM INSTITUTIONAL SOURCES (18)
Source
Hospital, Medical
Hospital.Mental
Prison
Rest Home
School, Day:
With Cafeteria, Gym,
Showers
With Cafeteria Only
Without Cafeteria, Gym,
Showers
School, Boarding
Unit
Bed
Employee
Bed
Empl oyee
Inmate
Employee
Resi dent
Employee
Wastewater Flow
RangeTypical
gpd/unit
132
5.3
79.3
5.3
79.3
5.3
52.8
5.3
251
15.9
172
15.9
159
15.9
119
15.9
172
10.6
106
10.6
119
10.6
92.5
10.6
Student
Student
Student
Student
15.9 -
10.6 -
5.3 -
52.8 -
30.4
21.1
17.2
106
21.1
15.9
10.6
74.0
61
-------
TABLE 4-8
TYPICAL WASTEWATER FLOWS FROM RECREATIONAL SOURCES (18)
Wastewater Flow
Source
Apartment, Resort
Cabin, Resort
Cafeteria
Campground (developed)
Cocktail Lounge
Coffee Shop
Country Club
Day Camp (no meals)
Dining Hall
Dormitory, Bunkhouse
Hotel , resort
Laundromat
Store Resort
Swimming Pool
Theater
Visitor Center
Unit
Person
Person
Customer
Empl oyee
Person
Seat
Customer
Empl oyee
Member Present
Empl oyee
Person
Meal Served
Person
Person
Machine
Customer
Empl oyee
Customer
Empl oyee
Seat
Vi si tor
Range
gpd/un
52.8 -
34.3 -
1.1 -
7.9 -
21.1 -
13.2 -
4.0 -
7.9 -
66.0 -
10.6 -
10.6 -
4.0 -
19.8 -
39.6 -
476 -
1.3 -
7.9 -
5.3 -
7.9 -
2.6 -
4.0 -
74
50.2
2.6
13.2
39.6
26.4
7.9
13.2
132
15.9
15.9
13.2
46.2
63.4
687
5.3
13.2
13.2
13.2
4.0
7.9
Typical
n't
58.1
42.3
1.6
10.6
31.7
19.8
5.3
10.6
106
13.2
13.2
7.9
39.6
52.8
581
2.6
10.6
10.6
10.6
2.6
5.3
62
-------
TABLE 4-9
FIXTURE-UNITS PER FIXTURE (19)
Fixture Type Fixture-Units
One bathroom group consisting of tank-operated water
closet, lavatory, and bathtub or shower stall 6
Bathtub (with or without overhead shower) 2
Bi det 3
Combination sink-and-tray 3
Combination sink-and-tray with food-disposal unit 4
Dental unit or cuspidor 1
Dental lavatory 1
Drinking fountain 1/2
Dishwasher, domestic 2
Floor drains 1
Kitchen sink, domestic 2
Kitchen sink, domestic, with food waste grinder 3
Lavatory 1
Lavatory 2
Lavatory, barber, beauty parlor 2
Lavatory, surgeon's 2
Laundry tray (1 or 2 compartments) 2
Shower stall, domestic 2
Showers (group) per head 3
Sinks
Surgeon's 3
Flushing rim (with valve) 8
Service (trap standard) 3
Service (P trap) 2
Pot, scullery, etc. 4
Urinal, pedestal, syphon jet, blowout 8
Urinal, wall lip 4
Urinal stall, washout 4
Urinal trough (each 2-ft section) 2
Wash sink (circular or multiple) each set of faucets 2
Water closet, tank-operated 4
Water closet, valve-operated 8
63
-------
FIGURE 4-2
PEAK DISCHARGE VERSUS FIXTURE UNITS PRESENT (22)
450
CTi
Q_
co
0)
D_
CD
_Q
O
400
350
300
o
u- 250
200
150
100
50
0
I I I I I i^ I I I|I
Note: Curves show probable amount of time indicated peak flow will
be exceeded during a period of concentrated fixture use.
System in which flush
valves predominate
System in which flush
tanks predominate
0
_L
I
I
I
6 8 10 12 14 16
Fixture Units on System (Hundreds)
18
20 22
-------
4.3.2 Wastewater Quality
The qualitative characteristics of the wastewaters generated by non-
residential establishments can vary significantly between different
types of establishments due to the extreme variation which can exist in
the waste generating sources present. Consideration of the waste-gen-
erating sources present at a particular establishment can give a general
idea of the character of the wastewater, and serve to indicate if the
wastewater will contain any problem constituents, such as high grease
levels from a restaurant or lint fibers in a laundromat wastewater.
If the waste-generating sources present at a particular establishment
are similar to those typical of a residential dwelling, an approximation
of the pollutant mass loadings and concentrations of the wastewater pro-
duced may be derived using the residential wastewater quality data pre-
sented in Tables 4-3 to 4-5. For establishments where the waste-gener-
ating sources appear significantly different from those in a residential
dwelling, or where more refined characterization data are desired, a de-
tailed review of the pertinent literature, as well as actual wastewater
sampling at the particular or a similar establishment, should be conduc-
ted.
4.4 Predicting Wastewater Characteristics
4.4.1 General Considerations
4.4.1.1 Parameter Design Units
In characterizing wastewaters, quantitative and qualitative character-
istics are often expressed in terms of other parameters. These para-
meter design units, as they may be called, vary considerably depending
on the type of establishment considered. For residential dwellings,
daily flow values and pollutant contributions are expressed on a per
person (capita) basis. Applying per capita data to predict total resi-
dential wastewater characteristics requires that a second parameter be
considered, namely, the number of persons residing in the residence.
Residential occupancy typically ranges from 1.0 to 1.5 persons per bed-
room. Although it provides for a conservative estimate, the current
practice is to assume that maximum occupancy is two persons per bedroom.
For nonresidential establishments, wastewater characteristics are
expressed in terms of a variety of units. Although per capita units are
employed, a physical characteristic of the establishment, such as per
seat, per car stall, or per square foot, is more commonly used.
65
-------
4.4.1.2 Factors of Safety
To account for the potential variability in the wastewater character-
istics at a particular dwelling or establishment, versus that of the
average, conservative predictions or factors of safety are typically
utilized. These factors of safety can be applied indirectly, through
choice of the design wastewater characteristics and the occupancy pat-
terns, as well as directly through an overall factor. For example, if
an average daily flow of 75 gpcd (284 Ipcd) and an occupancy of two
persons per bedroom were selected, the flow prediction for a three-
bedroom home would include a factor of safety of approximately 3 when
compared to average conditions (i.e., 45 gpcd [170 Ipcd] and 1 .person
per bedroom). If a direct factor of safety were also applied (e.g.,
1.25), the total factor of safety would increase to approximately 3.75.
Great care must be exercised in predicting wastewater characteristics so
as not to accumulate multiple factors of safety which would yield an
extremely overconservative estimate.
4.4.2 Strategy for Predicting Wastewater Characteristics
Predicting wastewater characteristics from rural developments can be a
complex task. Following a logical step-by-step procedure can help
simplify the characterization process and render the estimated waste-
water characteristics more accurate. A flow chart detailing a procedure
for predicting wastewater characteristics is presented in Figure 4-3.
66
-------
FIGURE 4-3
STRATEGY FOR PREDICTING WASTEWATER CHARACTERISTICS
Determine Primary Function of Facility
and Classify It Accordingly
(e.g., Single-Family Home, Restaurant...)
Identify Intended Application of Wastewater
Characterization Data
Identify Wastewater Characterization
Data Needed (e.g., Q, BODs...)
Determine Physical Characteristics of Facility
• Wastewater Generating Fixtures and Appliances
• Parameter Design Units (e.g.. Bedrooms,
Seating Spaces...)
• Occupancy or Operation Patterns (e.g.,
Seasonal Home. Hours of Operation...)
Obtain Characterization Data from Literature
• Tables and Text of This Chapter
• Reference and Bibliography Attached
to This Chapter
• Other Sources
Gather Existing Measured Characterization Data
Applicable to Facility
• Water Meter Records
• Holding Tank Pumpage Records
• Other
Evaluate Available Data
Select Data Judged Most Accurate
Determine if Needed Data has been Obtained
Calculate Waste Load Characteristics
(e.g., 45 GPCD x 2 CAP/Bedroom x
2 Bedrooms = 180 GPD)
Conduct Characterization
Field Studies at Facility
in Question or a Very
Similar One
Apply Overall Factor of Safety as Required
by Intended Application of Data
Estimate Wastewater
Characteristics
67
-------
4.5 References
1. Linaweaver, F. P., Jr., J. C. Geyer, and J. B. Wolff. A Study of
Residential Water Use. Department of Environmental Studies, Johns
Hopkins University, Baltimore, Maryland, 1967. 105 pp.
2. Anderson, J. S., and K. S. Watson. Patterns of Household Usage.
J. Am. Water Works Assoc., 59:1228-1237, 1967.
3. Watson, K. S., R. P. Farrell, and J. S. Anderson. The Contribution
from the Individual Home to the Sewer System. J. Water Pollut.
Control Fed., 39:2039-2054, 1967.
4. Cohen, S., and H. Wallman. Demonstration Of Waste Flow Reduction
From Households. EPA 670/2-74-071, NTIS Report No. PB 236 904,
1974. Ill pp.
5. Laak, R. Relative Pollution Strengths of Undiluted Waste Materials
Discharged in Households and the Dilution Waters Used for Each.
Manual of Grey Water Treatment Practice - Part II, Monogram Indus-
tries, Inc., Santa Monica, California, 1975.
6. Bennett, E. R., and E. K. Linstedt. Individual Home Wastewater
Characterization and Treatment. Completion Report Series No. 66,
Environmental Resources Center, Colorado State University, Fort
Collins, 1975. 145 pp.
7. Siegrist, R. L. , M. Witt, and W. C. Boyle. Characteristics of
Rural Household Wastewater. J. Env. Eng. Div., Am. Soc. Civil
Eng., 102:553-548, 1976.
8. Otis, R. J. An Alternative Public Wastewater Facility for a Small
Rural Community. Small Scale Waste Management Project, University
of Wisconsin, Madison, 1978.
9. Duffy, C. P., et al . Technical Performance of the Wisconsin Mound
System for On-Site Wastewater Disposal - An Interim Evaluation.
Presented in Preliminary Environmental Report for Three Alternative
Systems (Mounds) for On-site Individual Wastewater Disposal in
Wisconsin. Wisconsin Department of Health and Social Services,
December 1978.
10. Ligman, K., N. Hutzler, and W. C. Boyle. Household Wastewater
Characterization. J. Environ. Eng. Div., Am. Soc. Civil Eng.,
150:201-213, 1974.
11. Weickart, R. F. Effects of Backwash Water and Regeneration Wastes
From Household Water Conditioning Equipment on Private Sewage Dis-
posal Systems. Water Qua! ity Association, Lombard, Illinois, 1976.
68
-------
12. Witt, M. Water Use in Rural Homes. M.S. study. University of
Wisconsin-Madison, 1974.
13. Jones, E. E., Jr. Domestic Water Use in Individual Homes and
Hydraulic Loading of and Discharge from Septic Tanks. In: Pro-
ceedings of the National Home Sewage Disposal Symposium,~~C"hicago,
December 1974. American Society of Agricultural Engineers, St.
Joseph, Michigan, pp. 89-103.
14. Olsson, E., L. Karlgren, and V. Tullander. Household Wastewater.
National Swedish Institute for Building Research, Stockholm, Swe-
den, 1968.
15. Hypes, W. D., C. E. Batten, and J. R. Wilkins. The Chemical/
Physical and Microbiological Characteristics of Typical Bath and
Laundry Wastewaters. NASA TN D-7566, Langley Research Center,
Langley Station, Virginia, 1974. 31 pp.
16. Small Scale Waste Management Project, University of Wisconsin,
Madison. Management of Small Waste Flows. EPA 600/2-78-173, NTIS
Report No. PB 286 560, September 1978. 804 pp.
17. Brandes, M. Characteristics of Effluents From Separate Septic
Tanks Treating Grey and Black Waters From the Same House. J. Water
Pollut. Control Fed., 50:2547-2559, 1978.
18. Metcalf and Eddy, Inc. Wastewater Engineering: Treatment/
Disposal/Reuse. 2nd ed. McGraw-Hill, New York, 1979. 938 pp.
19. Design and Construction of Sanitary and Storm Sewers. Manual of
Practice No. 9. Water Pollution Control Federation, Washington,
D.C., 1976. 369 pp.
20. Uniform Plumbing Code. International Association of Plumbing and
Mechanical Officials, Los Angeles, California, 1976.
21. Hunter, R. B. Method of Estimating Loads in Plumbing Systems.
Building Materials and Structures Report BMS65, National Bureau of
Standards, Washington, D.C., 1940. 23 pp.
22. Hunter, R. B. Water Distribution Systems for Buildings. Building
Materials and Structures Report BMS79, National Bureau of Stan-
dards, Washington, D.C., 1941.
69
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CHAPTER 5
WASTEWATER MODIFICATION
5.1 Introduction
The characteristics of the influent wastewater can have a major impact
on most any onsite treatment and disposal/reuse system. To enhance con-
ventional strategies, and to encourage new ones, methods can be used to
modify the typical characteristics of the influent wastewater.
Methods for wastewater modification have been developed as part of
three, basic interrelated strategies: water conservation and wastewater
flow reduction, pollutant mass reduction, and onsite containment for
offsite disposal. Each strategy attempts to reduce the flow volume or
to decrease the mass of key pollutants such as oxygen-demanding sub-
stances, suspended solids, nutrients, and pathogenic organisms in the
influent wastewater to the onsite disposal system.
Although the primary thrust of this chapter is directed toward resi-
dential dwellings, many of the concepts and techniques presented have
equal or even greater application to nonresidential establishments.
Good practice dictates that water conservation/flow reduction be
employed to the maximum extent possible in a dwelling served by an
onsite wastewater system.
At the onset, there are several general considerations regarding waste-
water modification. First, there are a number of methods available,
including a wide variety of devices, fixtures, appliances, and systems.
Further, the number of methods and the diversity of their characteris-
tics is ever growing. In many cases, the methods involve equipment
manufactured by one or more companies as proprietary products. In this
chapter, only generic types of these products are considered. Also,
many methods and system components are presently in various stages of
development and/or application; therefore, only preliminary or projected
operation and performance information may be available. Finally, the
characteristics of many of the methods discussed may result in their
nonconfonuance with existing local plumbing codes.
70
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5.2 Water Conservation and Wastewater Flow Reduction
An extensive array of techniques and devices are available to reduce the
average water use and concomitant wastewater flows generated by indivi-
dual water-using activities and, in turn, the total effluent from the
residence or establishment. The diversity of present wastewater flow
reduction methods is illustrated in Table 5-1. As shown, the methods
may be divided into three major groups: (1) elimination of nonfunc-
tional water use, (2) water-saving devices, fixtures, and appliances;
and (3) wastewater recycle/reuse systems.
5.2.1 Elimination of Nonfunctional Water Use
Wasteful water use habits can occur with most water-using activities. A
few illustrative examples include using a toilet flush to dispose of a
cigarette butt, allowing the water to run while brushing teeth or shav-
ing, or operating a clotheswasher or dishwasher with only a partial
load. Obviously, the potential for wastewater flow reductions through
elimination of these types of wasteful use vary tremendously between
homes, from minor to significant reductions, depending on existing
habits.
5.2.1.1 Improved Plumbing and Appliance Maintenance
Unseen or apparently insignificant leaks from household fixtures and ap-
pliances can waste large volumes of water and generate similar quanti-
ties of wastewater. Most notable in this regard are leaking toilets and
dripping faucets. For example, a steadily dripping faucet can waste up
to several hundred gallons per day.
5.2.1.2 Maintain Nonexcessive Water Supply Pressure
The water flow rate through sink and basin faucets, showerheads, and
similar fixtures is highly dependent on the water pressure in the water
supply line. For most residential uses, a pressure of 40 psi (2.8 kg/
cm2) is adequate. Pressure in excess of this can result in unnecessary
water use and wastewater generation. To illustrate, the flow rate
through a typical faucet opened fully is about 40% higher at a supply
pressure of 80 psi (5.6 kg/cm2) versus that at 40 psi (2.8 kg/cm2).
71
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TABLE 5-1
EXAMPLE WASTEWATER FLOW REDUCTION METHODS
I. Elimination of Nonfunctional Water Use
A. Improved water use habits
B. Improved plumbing and appliance maintenance
C. Nonexcessive water supply pressure
II. Water-Saving Devices, Fixtures, and Appliances
A. Toilet
1. Water carriage toilets
a. Toilet tank inserts
b. Dual-flush toilets
c. Water-saving toilets
d. Very low-volume flush toilets
(1) Wash-down flush
(2) Mechanically assisted
o Pressurized tank
o Compressed air
o Vacuum
o Grinder
2. Non-water carriage toilets
a. Pit privies
b. Composting toilets
c. Incinerator toilets
d. Oil-carriage toilets
B. Bathing devices, fixtures, and appliances
1. Shower flow controls
2. Reduced-flow showerheads
3. On/Off showerhead valves
4. Mixing valves
5. Air-assisted low-flow shower system
C. Clotheswashing devices, fixtures, and appliances
1. Front-loading washer
2. Adjustable cycle settings
3. Washwater recycle feature
D. Miscellaneous
1. Faucet inserts
2. Faucet aerators
3. Reduced-flow faucet fixtures
4. Mixing valves
5. Hot water pipe insulation
6. Pressure-reducing valves
III. Wastewater Recycle/Reuse Systems
A. Bath/Laundry wastewater recycle for toilet flushing
B. Toilet wastewater recycle for toilet flushing
C. Combined wastewater recycle for toilet flushing
D. Combined wastewater recycle for several uses
72
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5.2.2 Water-Saving Devices, Fixtures, and Appliances
The quantity of water traditionally used by a given water-using fixture
or appliance is often considerably greater than actually needed.
Certain tasks may even be accomplished without the use of water. As
presented in Table 4-2, over 70% of a
wastewater flow volume is collectively
bathing, and clotheswashing. Thus,
wastewater flow reductions should be
activities.
without
typical residential dwelling's
generated by toilet flushing,
efforts to accomplish major
directed toward these three
5.2.2.1 Toilet Devices and Systems
Each flush of a conventional water-carriage toilet uses between 4 and 7
gal (15 and 26 1) of water depending on the model and water supply
pressure. On the average, a typical flush generates approximately 4.3
gal (16 1) of wastewater. When coupled with 3.5 uses/cap/day, a daily
wastewater flow of approximatley 16 gpcd (61 Ipcd) results (Table 4-2).
A variety of devices have been developed for use with a conventional
flush toilet to reduce the volume of water used in flushing.
Additionally, alternatives to the conventional water-carriage toilet are
available, certain of which use little or no water to transport human
wastewater products. Tables 5-2 and 5-3 present a summary of a variety
of toilet devices and systems. Additional details regarding the
non-water carriage toilets may be found elsewhere (1)(2)(3)(4)(5).
5.2.2.2 Bathing Devices and Systems
Although great variation exists in the quantity of wastewater generated
by a bath or shower, typical values include approximatley 25 gal (95 1)
per occurrence coupled with a 0.4 use/capita/day frequency to yield a
daily per capita flow of about 10 gal (38 1) (Table 4-2). The majority
of devices available to reduce bathing wastewater flow volumes are
concentrated around the activity of showering, with their objective
being to reduce normal 4- to 10-gal/min (0.25 to 0.63 I/sec) showering
flow rate. Several flow reduction devices and systems for showering are
characterized in Table 5-4. The amount of total wastewater flow
reduction accomplished with these devices is highly dependent on
individual user habits. Reductions vary from a negative value to as
much as 12% of the total wastewater volume.
73
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TABLE 5-2
WASTEWATER FLOW REDUCTION - WATER CARRIAGE TOILETS AND SYSTEMS
Generic Type
Description
Develop-
ment
Stage3
Application
Considerations
Operation and
Maintenance
Water Use
Per Event
gal
Total Flow
Reduction*1
gpcd
Toilet with
Tank Inserts
Displacement devices
placed into storage
tank of conventional
toilets to reduce
volume but not height
of stored water.
Varieties: Plastic
bottles, flexible
panels, drums or
plastic bags.
4-5 Device must be
compatible with
existing toilet and
not interfere with
flush mechanism.
Installation by
owner.
Post-installation and
periodic inspections
to insure proper
positioning.
3.3-3.8
1.8-3.5
4-8
Dual Flush
Toi 1 ets
Devices made for use
with conventional
flush toilets; enable
user to select from
two or more flush
volumes based on
solid or liquid waste
materials.
Varieties: Many
Device must be
compatible with
existing toilet and
not interfere with
flush mechanism.
Installation by
owner.
Post-installation and
periodic inspections
to insure proper
positioning and
functioning.
2.5-4.3
3.0-7.0
6-15
Water-Saving
Toilets
Variation of
conventional flush
toilet fixture;
similar in appearance
and operation.
Redesigned flushing
rim and priming jet
to initiate siphon
flush in smaller
trapway with less
water.
Varieties: Many
manufacturers but
units similar.
Interchangeable with
conventi onal fi xture.
Requires pressurized
water supply.
Essentially the same
as for a conventional
unit.
3.0-3.5
2.8-4.6
6-10
-------
TABLE 5-2 (continued)
Generic Type
Descri pti on
Develop-
ment
Stage3
Application
Considerations
Operation and
Maintenance
Water Use
Per Event
gal
Total Flow
Reductionb
gpcd
Washdown Flush
Toilets
Flushing uses only
water, but
substantially less
due to washdown
flush.
Varieties: Few.
3-4 Rough-In for unit may
be nonstandard.
Drain line slope and
lateral run
restrictions.
Requires pressurized
water supply.
Similar to
conventional toilet,
but more frequent
cleaning possible.
0.8-1.6
9.4-12.2
21-27
Pressurized
Tank
Specially designed
toilet tank to
pressurize air
contained in toilet
tank. Upon flushing,
the compressed air
propels water into
bowl at increased
vel oci ty.
Varieties: Few.
Compatible with most
any conventional
toilet unit.
Increased noise
1evel.
Water supply pressure
of 35 to 120 psi.
Similar to
conventi onal
fixture.
2.0-2.5
6.3-8.0
14-18
toi1et
Compressed
Air-Assisted
Flush Toilets
Similar in appearance
and user operation to
conventional toilet;
specially designed to
utilize compressed
air to ai d in
flushing.
Varieties: Few
3-4 Interchangeable with
rough-in for
conventional fixture.
Requires source of
compressed air;
bottled or air
compressor.
If air compressor,
need power source.
Periodic maintenance
of compressed air
source.
Power use - 0.002 KwH
per use.
0.5
13.3
30
-------
TABLE 5-2 (continued)
Generic Type
Description
Develop-
ment
Stage3
Appl i cati on
Considerations
Operation and
Maintenance
Water Use
Per Event
gal
Total Flow
Reduction'5
gpcd
Vacuum-
Assisted Flush
Toilets
Similar in appearance
and user operation to
conventional toilet;
specially designed
fixture is connected
to vacuum system
which assists a small
volume of water in
flushing.
Varieties: Several.
Application largely
for multi-unit toilet
installations
Above floor, rear
discharge.
Drain pipe may be
horizontal or
inclined.
Requires vacuum pump.
Requires power
source.
Periodic maintenance
of vacuum pump.
Power use = 0.002 KwH
per use.
0.3
14
31
a 1 = Prototype developed and under evaluation.
2 = Development complete, commercial production initiated, not locally available.
3 = Fully developed, limited use, not locally available, mail order purchase likely.
4 = Fully developed, limited use, locally available from plumbing supply houses or hardware stores.
5 = Fully developed, widespread use, locally available from plumbing supply houses or hardware stores.
Compared to conventional toilet usage (4.3 gal/flush, 3.5 uses/cap/day, and a total daily flow of 45 gpcd)
-------
TABLE 5-3
WASTEWATER FLOW REDUCTION - NON-WATER CARRIAGE TOILETS
Generic Typea
Pit Privy
Description
Hand-dug hole in the
ground covered with a
squatting plate or
stool /seat with an
enclosing house.
May be sealed vault
rather than dug hole.
Develop-
ment
Application
Considerations
Requires same site
conditions as for
wastewater disposal
(see Chapter 8),
unless sealed vault.
Handles only toilet
wastes
Outdoor installation.
May be constructed by
user.
Operation and
Maintenance
When full, cover with
2 ft of soil and
construct new pit.
Composti ng
Privy
Similar to pit privy
except organic matter
is added after each
use. When pit is
full it is allowed to
compost for a period
of about 12 months
prior to removal and
use as soil
amendment.
Can be constructed
independent of site
conditions if sealed
vault.
Handles only toilet
waste and garbage.
May be constructed by
user.
Outdoor installation.
Residuals disposal.
Addition of organic
matter after each
use.
Removal and
disposal /reuse of
composted material.
Composti ng-
Small
Small self-contained
units accept toilet
wastes only and
utilize the addition
of heat in
combination with
aerobic biological
activity to stabilize
human excreta.
Varieties: Several.
3-4 Installation requires
4-in. diameter roof
vent.
Handles only toilet
waste.
Set usage capacity.
Power required.
Residuals disposal.
Removal and disposal
of composted material
quarterly.
Power use =2.5
KwH/day.
Heat loss through
vent.
Composti ng-
Large
Larger units 'with a
separated
decomposition
chamber. Accept
toilet wastes and
other organic matter,
and over a long time
period stabilize
excreta through
biological activity.
Varieties: Several
3-4 Installation requires
6- to 12-i n. diameter
roof vent and space
beneath floor for
decomposition
chamber.
Handles toilet waste
and some kitchen
waste. »
Set usage capacity.
May be difficult to
retrofit.
Residuals disposal.
Periodic addition of
organic matter.
Removal of composted
material at 6 to 24
month intervals.
Power use = 0.3 to
1.2 KwH/day.
Heat loss through
vent.
77
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TABLE 5-3 (continued)
Generic Type3
Incinerator
Description
Small self-contained
units which
volatilize the
organic components of
human waste and
evaporate the
liquids.
Varieties: Several.
Develop-
ment
Stageb
Application
Considerations
Installation requires
4-in. diameter roof
vent.
Handles only toilet
waste.
Power or fuel
required.
Increased noi se
level.
Residuals disposal.
Operation and
Maintenance
Weekly removal of
ash.
Semiannual cleaning
and adjustment of
burni ng assembly
and/or heating
elements.
Power use = 1.2 KwH
or 0.3 Ib LP gas per
use.
Oil Recycle
Systems use a mineral
oil to transport
human excreta from a
fixture (similar in
appearance and use to
conventional ) to a
storage tank. Oil is
purified and reused
for flushing.
Varieties: few.
Requires separate
plumbing for toilet
fixture.
May be difficult to
retrofit.
Handles only toilet
wastes
Residuals disposal.
Yearly removal and
disposal of excreta
in storage tank.
Yearly maintenance of
oil purification
system by skilled
technician.
Power use = 0.01
KwH/use.
a None of these devices uses any water; therefore, the amount of flow reduction is equal to the
amount of conventional toilet use: 16.2 gpcd or 36% of normal daily flow (45 gpcd). Significant
quantities of pollutants {including N, BODs, SS, P and pathogens) are therefore removed from
wastewater stream.
b 1 = Prototype developed and under evaluation.
2 = Development complete; commercial production initiated, but distribution may be restricted;
mail order purchase.
3 = Fully developed; limited use, not locally available; mail order purchase likely.
4 = Fully developed; limited use, available form local plumbing supply houses or hardware stores.
5 = Fully developed; widespread use, available from local plumbing supply houses or hardware stores.
78
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TABLE 5-4
WASTEWATER FLOW REDUCTION - BATHING DEVICES AND SYSTEMS
Generi c Type3
Shower Flow
Control
Inserts and
Restrictors
Description
Reduce flow rate by
reducing the diameter
of supply line ahead
of shower head.
Varieties: Many.
Develop-
ment
Stageb
Application
Consi derations
Compatible with most
existing showerheads.
Installed by user.
Water Use
gal /mi n
1.5-3.0
Reduced-Fl ow
Showerheads
Fixtures similar to
conventi onal, except
restrict flow rate.
Varieties: Many
manufacturers, but
units similar.
4-5 Can match to most
plumbing fixture
appearance schemes.
Compatible with most
conventional
pi umbi ng.
1.5-3.0
ON/OFF
Showerhead
Valve
Small valve device
placed in the supply
line ahead of
showerhead, allows
shower fl ow to be
turned ON/OFF without
readjustment of
volume or
temperature.
Compatible with most
conventional plumbing
and fixtures.
May be installed by
user.
Thermostat-
ically
Controlled
Mi xi ng Val ve
Specifically designed
valve controls
temperature of total
flow according to
predetermi ned
setting. Valve may
be turned ON/OFF
without readjustment.
May be difficult to
retrofit.
79
-------
TABLE 5-4 (continued)
Generic Type3
Description
Devel op-
ment
Stageb
Application
Considerations
Water Use
gal /min
Pressure- Specifically designed
Balanced valve maintains
Mixing constant temperature
Valve of total flow
regardless of
pressure changes.
Single control allows
temperature to be
preset.
Compatible with most
conventi onal pi umbi ng
and fixtures.
Air-Assisted
Low-Flow
Shower
System
Specifically designed
system uses
compressed air to
atomize water flow
and provide shower
sensation.
May be impossible to 0.5
retrofi t.
Shower location < 50
feet of water heater.
Requires compressed
air source.
Power source
required.
Maintenance of air
compressor.
Power use = 0.01
KwH/use.
a No reduction in pollutant mass; slight increase in pollutant concentration.
b 1 = Prototype developed and under evaluation.
2 = Development complete; commercial production initiated, but distribution may be
restricted; mail order purchase.
3 = Fully developed; limited use, not locally available; mail order purchase
likely.
4 = Fully developed; limited use, available from local plumbing supply houses or
hardware stores.
5 = Fully developed; widespread use, available from local plumbing supply houses or
hardware stores.
80
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5.2.2.3 Clotheswashing Devices and Systems
The operation of conventional clotheswashers consumes varying quantities
of water depending on the manufacturer and model of the washer and the
cycle selected. For most, water usage is 23 to 53 gal (87 to 201 1} per
usage. Based on home water use monitoring, an average water use/waste-
water flow volume of approximately 37 gal (140 1) per use has been iden-
tified, with the clotheswasher contributing about 10.0 gpcd (38 Ipcd) or
22% of the total daily water use/wastewater flow (Table 4-2). Practical
methods to reduce these quantities are somewhat limited. Eliminating
wasteful water use habits, such as washing with only a partial load, is
one method. Front-loading model automatic washers can reduce water used
for a comparable load of clothes by up to 40%. In addition, wastewater
flow reductions may be accomplished through use of a clotheswasher with
either adjustable cycle settings for various load sizes or a wash water
recycle feature.
The wash water recycle feature is included as an optional cycle setting
on several commercially made washers. Selection of the recycle feature
when washing provides for storage of the wash water from the wash cycle
in a nearby laundry sink or a reservoir in the bottom of the machine,
for subsequent use as the wash water for the next load. The rinse
cycles remain unchanged. Since the wash cycle comprises about 45% of
the total water use per operation, if the wash water is recycled once,
about 17 gal (64 1) will be saved, if twice, 34 gal (129 1) , and so
forth. Actual water savings and wastewater flow reductions are highly
dependent on the user's cycle selection.
5.2.2.4 Miscellaneous Devices and Systems
There are a number of additional devices, fixtures, and appliances
available to help reduce wastewater flow volumes. These are directed
primarily toward reducing the water flow rate through sink and basin
faucets. Table 5-5 presents a summary of several of these additional
flow reduction devices. Experience with these devices indicates that
wastewater volume can be reduced by 1 to 2 gpcd (4 to 8 Ipcd) when used
for all sink and basin faucets.
5.2.3 Wastewater Recycle and Reuse Systems
Wastewater recycle and reuse systems collect and process the entire
wastewater flow or the fractions produced by certain activities with
storage for subsequent reuse. The performance requirements of any
wastewater recycle system are established by the intended reuse
activities. To simplify the performance requirements, most recycle
81
-------
TABLE 5-5
WASTEWATER FLOW REDUCTION - MISCELLANEOUS DEVICES AND SYSTEMS
Generic Type3
Description
Develop-
ment
Stageb
Application
Considerations
Faucet
Inserts
Devi ce whi ch i nserts
into faucet valve or
supply line and
restricts flow rate
with a fixed or
pressure
compensating
orifice.
Varieties: Many.
Compatible with most
plumbing.
Installation simple.
Faucet
Aerators
Devices attached to
faucet outlet which
entrain air into
water flow.
Varieties: Many.
Compatible with most
plumbing.
Installation simple.
Periodic cleaning of
aerator screens.
Reduced-Flow
Faucet
Fi xtures
Similar to
conventional unit,
but restrict flow
rate with a fixed
pressure
compensati ng
orifice.
Varieties: Many.
or
Compatible with most
plumbing.
Installation
identical to
conventional.
82
-------
TABLE 5-5 (continued)
Generic Type3
Desert pti on
Develop-
ment
Stageb
Application
Consi derati ons
Mi xi ng
Valves
Specifically
designed valve units
which allow flow and
temperature to be
set with a single
control.
Varieties: Many.
Compatible with most
plumbing.
Installation
identical to
conventional.
Hot Water
Pipe
Insulation
Hot water piping is
wrapped with
insulation to reduce
heat loss from hot
water standing in
pipe between uses.
Varieties: Many.
May be difficult to
retrofit.
3 No reduction in pollutant mass; insignificant increase in pollutant
concentration.
b 1 = Prototypes developed and under evaluation.
2 = Development complete; commercial production initiated, but
distribution restriced.
3 = Fully developed, limited use, not locally available; mail order
purchase likely.
4 = Fully developed, limited use, locally available from plumbing
supply houses or hardware stores.
5 = Fully developed, widespread use, locally available from plumbing
supply houses or hardware stores.
83
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systems process only the wastewaters discharged from bathing, laundry,
and bathroom sink usage, and restrict the use of the recycled water to
flushing water-carriage toilets and possibly lawn irrigation. At the
other extreme, systems are under development that process the entire
wastewater flow and recycle it as a potable water source.
The flow sheets proposed for residential recycle systems are numerous
and varied, and typically employ various combinations of the unit pro-
cesses described in Chapter 6, complemented by specially designed con-
trol networks. In Table 5-6, several generic units are characterized
according to their general recycle flow sheet.
5.3 Pollutant Mass Reduction
A second strategy for wastewater modification is directed toward
decreasing the mass of potential pollutants at the source. This may
involve the complete elimiriation of the pollutant mass contributed by a
given activity or the isolation of the pollutant mass in a concentrated
wastewater stream. In Table 5-7, several methods for pollutant mass
reduction are outlined.
5.3.1 Improved User Habits
Unnecessary quantities of many pollutants enter the wastewater stream
when materials, which could be readily disposed of in a solid waste
form, are added to the wastewater stream. A few examples include
flushing disposable diapers or sanitary napkins down the toilet, or
using hot water and detergents to remove quantities of solidified grease
and food debris from pots and pans to enable their discharge down the
sink drain.
5.3.2 Cleaning Agent Selection
The use of certain cleansing agents can contribute significant quanti-
ties of pollutants. In particular, cleaning activities, such as
clotheswashing and dishwashing, can account for over 70% of the phos-
phorus in residential wastewater (Table 4-4). Detergents are readily
available that contain a low amount of phosphorus compared to other
detergents.
84
-------
TABLE 5-6
WASTEWATER FLOW REDUCTION - WASTEWATER RECYCLE AND REUSE SYSTEMS
Flow Sheet Description
Develop-
ment
Stagea
Application
Considerations
Operation and Maintenance
Total Flow
Reducti onb
gpcd?
Wastewater Quality
Impacts
co
01
Recycle bath and laundry
for toilet flushing
Requires separate toilet
supply and drain line.
May be difficult to
retrofit to multi-story
building.
Requries wastewater
disposal system for
toilet and kitchen sink
wastes.
Periodic replenishment of
chemicals, cleaning of
filters and storage
tanks.
Residuals disposal.
Power use.
16 36 Sizable removals of
pollutants, primarily P.
Recycle portion of total
wastewater stream for
toilet flushing
Requires separate toilet
supply line.
May be difficult to
retrofit to multi-story
building.
Requires disposal system
for unused recycle water.
Cleaning/replacement of
filters and other
treatment and storage
components.
Residuals disposal.
Periodic replenishment of
chemicals.
16 36 Significant removals of
pollutants.
-------
TABLE 5-6 (continued)
Flow Sheet Description
Develop-
ment
Stage8
Application
Considerations
Operation and Maintenance
Total Flow
Reduction^
gpcd£
Wastewater Quality
Impacts
Recycle toilet
wastewaters for flushing
water carriage toilets
CO
CTt
Requires separate toilet
pi umbi ng network.
Utilizes low-flush
toilets.
Requires system for
nontoilet wastewaters.
May be difficult to
retrofi t.
Application restricted to
high use on multi-unit
installations.
Cleaning/replacement of
filters and other
treatment components.
Resi duals disposal.
Power use.
16 36 Significant removals of
pollutants.
Recycle total wastewater
stream for all water uses
1-2 Requires major variance
from State/local health
codes for potable reuse.
Difficult to retrofit.
All maintenance by
skilled personnel.
Routine service check.
Periodic pump out and
residuals disposal.
Power use.
Comprehensive monitoring
program required.
45 100 No wastewater generated
for onsite disposal.
1 = Prototype developed and under evaluation.
2 = Development complete; commercial production initiated, but distribution may be restricted.
3 = Fully developed; limited use, not locally available, mail order purchase likely.
4 = Fully developed; limited use, locally available from plumbing supply houses and hardware stores.
5 = Fully developed; widespread use, locally available from plumbing supply houses and hardware stores.
Based on the normal waste flow information presented in Table 4-2.
-------
TABLE 5-7
EXAMPLE POLLUTANT MASS REDUCTION METHODS
I. Improved User Habits
II. Cleaning Agent Selection
III. Elimination of Garbage Disposal Appliance
IV. Segregated Toilet Systems
A. Non-Water Carriage Toilets
B. Very Low-Volume Flush Toilets/Hoi ding Tank
C. Closed Loop Wastewater Recycle Systems
5.3.3 Elimination of the Garbage Disposal Appliance
The use of a garbage disposal contributes substantial quantities of
and suspended solids to the wastewater load (Table 4-4). As a result,
it has been shown that the use of a garbage disposal may increase the
rate of sludge and scum accumulation and produce a higher failure rate
for conventional disposal systems under otherwise comparable conditions
(6). For these reasons, as well as the fact that most waste handled by
a garbage disposal could be handled as solid wastes, the elimination of
this appliance is advisable.
5.3.4 Segregated Toilet Systems
Several toilet systems can be used to provide for segregation and
separate handling of human excreta (often referred to as blackwater)
and, in some cases, garbage wastes. Removal of human excreta from the
wastewater stream serves to eliminate significant quantities of
pollutants, 'particularly suspended solids, nitrogen, and pathogenic
organisms (Table 4-4).
A number of potential strategies for management of segregated human
excreta are presented in Figure 5-1. A discussion of the toilet systems
themselves is presented in the wastewater flow reduction section of this
chapter, while details regarding the other unit processes in the flow
sheet may be found in Chapter 6.
87
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Wastewaters generated by fixtures other than toilets are often referred
to collectively as "graywater." Characterization studies have demon-
strated that typical graywater contains appreciable quantities of
organic matter, suspended solids, phosphorus and grease in a daily flow
volume of 29 gpcd (110 Ipcd) (7)(8){9)(10)(11)(12)(13)(14)(15) (see
Table 4-4). Its temperature as it leaves the residence is in the range
of 31° C, with a pH slightly on the alkaline side. The organic materi-
als in graywater appear to degrade at a rate not significantly different
from those in combined residential wastewater (15). Microbiological
studies have demonstrated that significant concentrations of indicator
organisms as total and fecal coliforms are typically found in graywater
(7)(11)(12)(13)(14)(15). One should assume, therefore, that graywater
harbors pathogens.
Although residential graywater does contain pollutants and must be prop-
erly managed, graywater may be simpler to manage than total residential
wastewater due to a reduced flow volume. While diverse strategies have
been proposed for graywater management (Figure 5-2), rigorous field
evaluations have not been conducted in most cases. Until further field
data become available, it is recommended that graywater treatment and
disposal/reuse systems be designed as for typical residential wastewater
(as described in Chapter 6). Design allowances should be made only for
the reductions in flow volume, as compared to typical residential
wastewater.
5.4 Onsite Containment - Holding Tanks
Wastewaters may be contained on site using holding tanks, and then
transported off site for subsequent treatment and disposal. In many
respects, the design, installation, and operation of a holding tank is
similar to that for a septic tank (as described in Chapter 6). Several
additional considerations do exist, as indicated in Table 5-8. A dis-
cussion regarding the disposal of the pumpage from holding tanks is pre-
sented in Chapter 9 of this manual.
5.5 Reliability
An important aspect of wastewater modification concerns the reliability
of a given method to yield a projected modification at a specific
dwelling or establishment over the long term. This is of particular
importance when designing an onsite wastewater disposal system based on
modified wastewater characteristics.
88
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FIGURE 5-1
EXAMPLE STRATEGIES FOR MANAGEMENT OF SEGREGATED HUMAN WASTES
T T
Privy Compost Toi
i '
Disinfection
i '
Soil Amendme
Human Wastes
let Very Low-Volume
Flush Toilet
I
\
Treatment
i •
nt Onsite
Disposal
*
T
Closed Loop Incinerator
Recycle Toilet Toilet
Holding Tank
i r
Disinfection
1 r
Land Sewage
Disposal Treatment
'
Refuse
FIGURE 5-2
EXAMPLE STRATEGIES FOR MANAGEMENT OF RESIDENTIAL GRAYWATER
Sediments
*
*
Soil Absorption
Alternatives
tion
1
Further
Treatment
i
Reuse
|
Surface
Water
Discharge
89
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TABLE 5-8
ADDITIONAL CONSIDERATIONS IN THE DESIGN,
INSTALLATION, AND OPERATION OF HOLDING TANKS
Item Consideration
Sizing Liquid holding capacity >7 days
wastewater flow generation. Minimum
capacity = 1,000 gallons.
Discharge There should be no discharge.
Alarm System High water alarm positioned to allow at
least 3 days storage after activation.
Accessibility Frequent pumping is likely; therefore
holding tank(s) must be readily accessible
to pumping vehicle.
Flotation Large tanks may be subject to severe
flotation forces in high groundwater areas
when pumped.
Cost Frequent pumping and residuals disposal
results in very high operating costs.
Assessing the reliability of wastewater modification methods is a com-
plex task which includes considerations of a technological, sociologi-
cal, economic, and institutional nature. Major factors affecting
reliability include:
1. The actual wastewater characteristics prior to modification
compared to the average.
2. User awareness and influence on method performance.
3. Installation.
4. Method performance.
5. User circumvention or removal.
90
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In most situations, projections of the impact of a wastewater modifica-
tion method must necessarily be made, assuming the wastewater charac-
teristics prior to modification are reasonably typical. If the actual
wastewater characteristics deviate significantly from that of the aver-
age, a projected modification may be inaccurate.
The prospective user should be fully aware of the characteristics of a
method considered for use prior to its application. Users who become
aware of the characteristics of a method only after it has been put into
use are more likely to be dissatisfied and attempt to circumvent or
otherwise alter the method and negate the wastewater modification
expected.
In general, passive wastewater modification methods or devices not sig-
nificantly affected by user habits tend to be more reliable than those
which are subject to user habits and require a preconceived active role
by the users. For example, a low-flush toilet is a passive device,
while a flow-reducing shower head is an active one.
Installation of any devices or systems should be made by qualified
personnel. In many situations, a post-installation inspection is
recommended to ensure proper functioning of the device or system.
Method performance is extremely important in assessing the reliability
of the projected modification. Accurate performance data are necessary
to estimate the magnitude of the reduction, and to predict the likeli-
hood that the method will receive long-term user acceptance. Accurate
performance data can only be obtained through the results of field test-
ing and evaluations. Since many methods and system components are pres-
ently in various stages of development, only preliminary or projected
operation and performance data may be available. This preliminary or
projected data should be considered cautiously.
The continued employment of a wastewater modification method can be
encouraged through several management actions. First, the user(s)
should be made fully aware of the potential consequences if they should
discontinue employing the modification method (e.g., system failure,
water pollution, rejuvenation costs, etc.). Also, the appropriate man-
agement authority can approve only those methods whose characteristics
and merits indicate a potential for long-term user acceptance. Further,
installation of a device or system can be made in such a manner as to
discourage disconnection or replacement. Finally, periodic inspections
by a local inspector within the framework of a sanitary district or the
like may serve to identify plumbing alterations; corrective orders could
then be issued.
91
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To help ensure that a projected modification will actually be realized
at a given site, efforts can be expended to accomplish the following
tasks:
1. Confirm that the actual wastewater characteristics prior to
modification are typical.
2. Make the prospective user(s) of the modification method fully
aware of the characteristics of the method, including its oper-
ation, maintenance, and costs.
3. Determine if the projected performance of a given method has
been confirmed through actual field evaluations.
4. Ensure that any devices or systems are installed properly by
competent personnel.
5. Prevent user removal or circumvention of devices, systems, or
methods.
5.6 Impacts on Onsite Treatment and Disposal Practices
5.6.1 Modified Wastewater Characteristics
Reducing the household wastewater flow volume without reducing the mass
of pollutants contributed will increase the concentration of pollutants
in the wastewater stream. The increase in concentrations will likely be
insignificant for most flow reduction devices with the exception of
those producing flow reductions of 20% or more. The increase in pollu-
tant concentrations in any case may be estimated utilizing Figure 5-3.
5.6.2 Wastewater Treatment and Disposal Practices
In Table 5-9, a brief summary of several potential impacts that
wastewater modification may have on onsite disposal practices is
presented. It must be emphasized that the benefits derived from
wastewater modification are potentially significant. Wastewater
modification methods, particularly wastewater flow reduction, should be
considered an integral part of any onsite wastewater disposal system.
92
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FIGURE 5-3
FLOW REDUCTION EFFECTS ON POLLUTANT CONCENTRATIONS
CD
O
CD
Q.
0)
(/>
CO
CD
u.
O
c
O
4-1
CD
L_
4— f
C
CD
O
C
O
CJ
•I-'
CO
O
CL
0 10 20 30 40
Wastewater Flow Reduction, percent of total daily flow
(Assumes Pollutant Contributions the Same
Under the Reduced Flow Volume)
93
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TABLE 5-9
POTENTIAL IMPACTS OF WASTEWATER MODIFICATION
ON ONSITE DISPOSAL PRACTICES
Disposal System
Type
Modification Practice
Flow
Reduction
Pollutant
Reduction
Potential Impact
All Disposal
Surface Di sposal
Evapotranspiration
Onsite Containment
X
X
X
X
X
X
X
X
X
X
X
May extend service life of
functioning system, but
cannot quantify.
Reduce water resource
contamination.
Simplify site constraints.
Reduce frequency of septic
tank pumping.
Reduce sizing of
infiltrative area.
Remedy hydraulically
overloaded system.
Reduce component 0 and M
costs.
Reduce sizing of components.
Eliminate need for certain
components (e.g., nitrogen
removal).
Remedy hydraulically
overloaded system.
Remedy a hydraulically
overloaded system.
Reduce sizing of ET area.
Reduce frequency of pumping.
Reduce sizing of containment
basin.
94
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5.7 References
1. Wagner, E. G., and J. N. Lanoix. Excreta Disposal for Rural Areas
and Small Communities. WHO Monograph 39, World Health Organiza-
tion, Geneva, Switzerland, 1958. 187 pp.
2. Stoner, C. H. Goodbye to the Flush Toilet. Rodale Press, Emmaus,
Pennsylvania, 1977.
3. Rybczynski, W., and A. Ortega. Stop the Five Gallon Flush. Mini-
mum Coast Housing Group, School of Architecture, McGill University,
Montreal, Canada, 1975.
4. Van Der Ryn, S. Compost Privy. Technical Bulletin No. 1, Faral-
lones Institute, Occidental, California, 1974.
5. Rybezynski, W., C. Polprasert, and M. McGarry. Low-Cost Technical
Options for Sanitation. Report IDRC-102e, International Develop-
ment Research Center, Ottawa, Canada, 1978.
6. Bendixen, T. W., R. E. Thomas, A. A. McMahan, and J. B. Coulter.
Effect of Food Waste Grinders on Septic Tank Systems. Robert A.
Taft Sanitary Engineering Center, Cincinnati, Ohio, 1961. 119 pp.
7. Siegrist, R. L., M. Witt, and W. C. Boyle. Characteristics of
Rural Housing Wastewater. J. Environ. Eng. Div., Am. Soc. Civil
Eng., 102:553-548, 1976.
8. Laak, R. Relative Pollution Strengths of Undiluted Waste Materials
Discharged in Households and the Dilution of Waters Used for Each.
Manual of Grey Water Treatment Practice - Part II. Monogram Indus-
tries, Inc., Santa Monica, California, 1975.
9. Bennett, E. R., and E. K. Linstedt. Individual Home Wastewater
Characterization and Treatment. Completion Report Series No. 66,
, Environmental Resources Center, Colorado State University, Fort
Collins, 1975. 145 pp.
10. Ligman, K., N. Hutzler, and W. E. Boyle. Household Wastewater
Characterization. J. Environ. Eng. Div., Am. Soc. Civil Eng.,
150:201-213, 1974.
11. Olsson, E., L. Karlgren, and V. Tullander. Household Wastewater.
National Swedish Institute for Building Research, Stockholm, Swe-
den, 1968.
12. Hypes, W. D., C. E. Batten, and J. R. Wilkins. The Chemical/
Physical and Microbiological Characteristics of Bath and Laundry
Wastewaters. NASA TN D-7566, Langley Research Center, Langley
Center, Virginia, 1974. 31 pp.
95
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13. Small Scale Waste Management Project, University of Wisconsin,
Madison. Management of Small Waste Flows. EPA-600/2-78-173, NTIS
Report No. PB 286 560, September 1978. 804 pp.
14. Brandes, M. Characteristics of Effluents from Separate Septic
Tanks Treating Grey and Black Waters From the Same House. J. Water
Pollut. Control Fed., 50:2547-2559, 1978.
15. Siegrist, R. L. Management of Residential Grey Water. Proceedings
of the Second Pacific Northwest Onsite Wastwater Disposal Short
Course, University of Washington, Seattle, March 1978.
96
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CHAPTER 6
ONSITE TREATMENT METHODS
6.1 Introduction
This chapter presents information on the component of an onsite system
that provides "treatment" of the wastewater, as opposed to its
"disposal" (disposal options for treated wastewater are covered in
Chapter 7). Treatment options included in this discussion are:
1. Septic tanks
2. Intermittent sand filters
3. Aerobic treatment units
4. Disinfection units
5. Nutrient removal systems
6. Wastewater segregation and recycle systems
Detailed design, O&M, performance, and construction data are provided
for the first four components above. A more general description of
nutrient removal is provided as these systems are not yet in general
use, and often involve in-house changes in product use and plumbing. A
brief mention of wastewater segregation and recycle options is included,
since these also function as treatment options.
Options providing a combined treatment/disposal function, i.e., soil
absorption systems, are discussed in Chapter 7.
6.1.1 Purpose
The purpose of the treatment component is to transform the raw household
wastewater into an effluent suited to the disposal component, such that
the wastewater can be disposed of in conformance with public health and
environmental regulations. For example, in a subsurface soil absorption
system, the pretreatment unit (e.g., septic tank) should remove nearly
all settleable solids and floatable grease and scum so that a reasonably
clear liquid is discharged into the soil absorption field. This allows
the field to operate more efficiently. Likewise, for a surface
discharge system, the treatment unit should produce an effluent that
will meet applicable surface discharge standards.
97
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6.1.2 Residuals
No treatment process is capable of continuous operation without experi-
encing some type of residuals buildup. Removal and disposal of these
residuals is a very important and often neglected part of overall system
O&M.
Residuals handling is discussed in detail under each individual compo-
nent in Chapters 6 and 7. Final disposal of residuals is covered in
Chapter 9.
6.2 Septic Tanks
6.2.1 Introduction
The septic tank is the most widely used onsite wastewater treatment
option in the United States. Currently, about 25% of the new homes
being constructed in this country use septic tanks for treatment prior
to disposal of home wastewater.
This section provides detailed information on the septic tank, its sit-
ing considerations, performance, design, construction procedures, and
operation and maintenance. The discussion centers on tanks for single-
family homes; tanks for larger flows are discussed where they differ
from the single-family model.
6.2.2 Description
Septic tanks are buried, watertight receptacles designed and constructed
to receive wastewater from a home, to separate solids from the liquid,
to provide limited digestion of organic matter, to store solids, and to
allow the clarified liquid to discharge for further treatment and dis-
posal. Settleable solids and partially decomposed sludge settle to the
bottom of the tank and accumulate. A scum of lightweight material
(including fats and greases) rises to the top. The partially clarified
liquid is allowed to flow through an outlet structure just below the
floating scum layer. Proper use of baffles, tees, and ells protects
against scum outflow. Clarified liquid can be disposed of to soil
absorption systems, soil mounds, lagoons, or other disposal systems.
Leakage from septic tanks is often considered a minor factor; however,
if tank leakage causes the level of the scum layer to drop below the
outlet baffle, excessive scum discharges can occur. In the extreme
case, the sludge layer will dry and compact, and normal tank cleaning
98
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practices will not remove it (1). Another problem, if the tank is not
watertight, is infiltration into the tank which can cause overloading of
the tank and subsequent treatment and disposal components.
6.2.3 Application
Septic tanks are normally the first component of an onsite system. They
must be followed by polishing treatment and/or disposal units. In most
instances, septic tank effluent is discharged to a soil absorption field
where the wastewater percolates down through the soil. In areas where
soils are not suitable for percolation, septic tank effluent can be
discharged to mounds or ET beds for treatment and disposal, or to
filters or lagoons for further treatment.
Septic tanks are also amenable to chemical addition for nutrient
removal, as discussed later in this manual.
Local regulatory agencies may require that the septic tank be located
specified distances from home, water well, and water lines to reduce any
risk of disease-causing agents from the septic tank reaching the potable
water supply. A number of minimum separation distances have been
developed for protecting water supplies and homes from septic tank
disposal systems, but these are largely arbitrary and depend to a great
degree on the soil conditions. Many state and local building codes
feature suggested separation distances that should be adhered to in the
absence of any extenuating circumstances.
6.2.4 Performance
Table 6-1 summarizes septic tank effluent quality. In addition to the
tabulated results, bacterial concentrations in the effluent are not
significantly changed since septic tanks cannot be relied upon to remove
disease-causing microorganisms. Oil and grease removal is typically 70
to 80%, producing an effluent of about 20-25 mg/1. Phosphorus removal
is slight, at about 15%, providing an effluent quality of about 20 mg/1
total P.
Brandes (7) studied the quality of effluents from septic tanks treating
graywater and blackwater. He found that without increasing the volume
of the septic tank, the efficiency of the blackwater (toilet wastewater)
treatment was improved by discharging the household graywater to a
separate treatment disposal system.
99
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TABLE 6-1
SUMMARY OF EFFLUENT DATA FROM VARIOUS SEPTIQ TANK STUDIES
o
o
Parameter
BOD5
Mean, mg/1
Range, mg/1
No. of Samples
COD
Mean, mg/1
Range, mg/1
No. of Samples
Suspended Solids
Mean, mg/1
Range, mg/1
No. of Samples
Total Nitrogen
Mean, mg/1
Range, mg/1
No. of Samples
Ref. (2)
7 Sites
138
7-480
150
327
25-780
152
49
10-695
148
45
9-125
99
Ref. (3)
10 Tanks
1383
64-256
44
--
—
—
1553
43-485
55
—
—
Source
Ref. (4) Ref. (5)
19 Sites 4 Sites
140 240&
70.385
51 21
—
—
--
101 95b
48-340
51 18
36
—
51
Ref. (6)
1 Tank
120
30-280
50
200
71 -360
50
39
8-270
47
--
—
a Calculated from the average values from 10 tanks, 6 series of tests.
b Calculated on the basis of a log-normal distribution of data.
-------
Factors affecting septic tank performance include geometry, hydraulic
loading, inlet and outlet arrangements, number of compartments,
temperature, and operation and maintenance practices. If a tank is
hydraulically overloaded, retention time may become too short and solids
may not settle or float properly.
A single-compartment tank will give acceptable performance. However,
multi-compartment tanks perform somewhat better than single-compartment
tanks of the same total capacity, because they provide better protection
against solids carry-over into discharge pipes during-periods of surges
or upset due to rapid digestion.
Improper design and placement of baffles can create turbulence in the
tank, seriously impairing settling efficiency. In addition, poor
baffles or outlet devices may promote scum or sludge entry to discharge
pipes. Obviously, improper operation and maintenance will impair
performance. Flushing problem wastes (paper towels, bones, fats,
diapers, etc.) into the system can clog piping. Failure to pump out
accumulated solids will eventually lead to problems with solids
discharge in the effluent.
6.2.5 Design
6.2.5.1 General
Septic tanks for single-family homes are usually purchased "off the
shelf," ready for installation, and are normally designed in accordance
with local codes.
The tank must be designed to ensure removal of almost all settleable
solids. To accomplish this, the tank must provide:
1. Liquid volume sufficient for a 24-hr fluid retention time at
maximum sludge depth and scum accumulation (8).
2. Inlet and outlet devices to prevent the discharge of sludge or
scum in the effluent.
3. Sufficient sludge storage space to prevent the discharge of
sludge or scum in the effluent.
4. Venting provisions to allow for the escape of accumulated
methane and hydrogen sulfide gases.
101
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6.2.5.2 Criteria
The first step in selecting a tank volume is to determine the average
volume of wastewater produced per day. Ideally, this is done by meter-
ing wastewater flows for a given period; but that is seldom feasible,
particularly if a septic tank system is being selected for a building
still under construction.
In the past, the design capacity of most septic tanks was based on the
number of bedrooms per home and the average number of persons per bed-
room. Chapter 4 showed that the average wastewater contribution is
about 45 gpcd (170 Ipcd) (2). As a safety factor, a value of 75 gpcd
(284 Ipcd) can be coupled with a potential maximum dwelling density of
two persons per bedroom, yielding a theoretical design flow of 150
gal/bedroom/day (570 1/bedroom/day). A theoretical tank volume of 2 to
3 times the design daily flow is common, resulting in a total tank
design capacity of 300 to 450 gal per bedroom (1,140 to 1,700 1 per
bedroom).
While not ideal, most state and local codes rely on some version of this
method by assigning required septic tank capacities solely by the number
of bedrooms (see Table 6-2). Unfortunately, hourly and daily flows from
the home can vary greatly. During high flow periods, higher solids con-
centrations are discharged from the septic tank. Well-designed, two-
compartment tanks reduce the effect of peak hour loads.
Another key factor in the design and performance of septic tanks is the
relationship between surface area, surge storage, discharge rate, and
exit velocity. These parameters affect the hydraulic efficiency and
sludge retention capacity of the tank.
Tanks with greater surface area and shallower depth are preferred,
because increased liquid surface area increases surge storage capacity;
a given inflowing volume creates a smaller rise in water depth and a
slower discharge rate and exit velocity. These surges of flow through
the tank are dampened as surface area increases. This allows a longer
time for separation of sludge and scum that are mixed by turbulence re-
sulting from the influent surge (8).
In addition to increasing the surface area, there are two other means of
reducing the exit velocity and reducing the opportunity for solids and
scum to escape through the outlet. These are: 1) increase the size of
the outlet riser; and 2) reduce the size of the final discharge pipe.
The use of a 6-in. (15-cm) outlet riser instead of a 4-in. (10-cm)
102
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o
OJ
TABLE 6-2
TYPICAL SEPTIC TANK LIQUID VOLUME REQUIREMENTS
Minimum, gal
1-2 bedrooms, gal
3 bedrooms, gal
4 bedrooms, gal
5 bedrooms, gal
Additional bedrooms (ea), gal
Federal
Housing
Authon' ty
750
750
900
1,000
1,250
250
U.S. Public
Health
Servi ce
750
750
900
1,000
1,250
250
Uni form
PI umbl ng
Code
750
750
1,000
1,200
1,500
150
Range of State
Requirements (9)
500 - 1,000
500 - 1,000
900 - 1,500
1,000 - 2,000
1,100 - 2,000
-------
outlet riser will reduce the exit velocity from 0.025 ft/sec to 0.011
ft/sec (0.76 cm/sec to 0.34 cm/sec) a reduction of 56% (8).
Use of garbage grinders increases both the settleable and floatable sol-
ids in the wastewater and their accumulation rates in the septic tank.
U.S. Public Health Service (USPHS) studies indicate that the increase in
the sludge and scum accumulation rate is about 37% (10). This means
either more frequent pumping or a larger tank to keep the pumping fre-
quency down. A common expedient is to add 250 gal (946 1) to the tank
size when garbage grinders are used, although this volume is arbitrary.
It is generally a good idea to avoid the use of garbage grinders with
onsite systems.
6.2.5.3 Inlet and Outlet Devices
The flow out of a septic tank should carry only minimal concentrations
of settleable solids. Higher concentrations can occur if:
1. The inlet turbulence in a single-compartment tank causes mixing
of the sludge with the wastewater in the clear space.
2. The rise velocity of the water in the vertical leg of the out-
let tee resuspends previously captured solids.
3. The rising gases produced by anaerobic digestion interfere with
particle-settling and resuspend previously captured solids,
which then are lost in the effluent.
The inlet to a septic tank should be designed to dissipate the energy of
the incoming water, to minimize turbulence, and to prevent short-cir-
cuiting. The inlet should preferably be either a sanitary tee or baf-
fle. The baffle should be small enough so that it is flushed out each
time, and yet keeps floating solids from blocking the inlet. The invert
radius in a tee helps dissipate energy in the transition from horizontal
to vertical flow, and prevents dripping that, at the proper frequency,
can amplify water surface oscillations and increase intercompartmental
mixing. The vertical leg of the inlet tee should extend below the
liquid surface. This minimizes induced turbulence by dissipating as
much energy in the inlet as possible.
The outlet structure's ability to retain sludge and scum in either the
first or second compartment is a major factor in overall task
performance. The outlet of a septic tank can be a tee, a baffle, or
some special structure (see Figure 6-1). The outlet must have the
proper submergence and height above liquid level such that the sludge
104
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FIGURE 6-1
TYPICAL SEPTIC TANK OUTLET STRUCTURES TO
MINIMIZE SUSPENDED SOLIDS IN DISCHAR6E(11)
Liquid Level
Tank Outlet Pipe
Gas Deflection /
Baffle
Liquid Level
Outlet Scum
Baffle—*
Liquid Level
Gas Deflection
Device
Gas Deflection
Configuration
105
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and scum clear spaces balance, and proper venting of sludge gases is
provided (see Figure 6-2). Although the Manual of Septic Tank Prac-
tices recommends an outlet submergence equal to 40% of the liquid depth,
other studies have shown that shallower submergence decreases solids
discharges and allows for greater sludge accumulation, and thus for less
frequent pumping (8). Table 6-3 summarizes the results of these
studies.
As shown in Figure 6-1, various types of gas deflection baffles and
wedges have been developed to prevent gas-disturbed sludge from entering
the rising leg of the outlet.
6.2.5.4 Compartmentation
Recent trends in septic tank design favor multiple, rather than single,
compartmented tanks. When a tank is properly divided into compartments,
BOD and SS removal are improved. Figure 6-3 shows a typical two-
compartment tank.
The benefits of compartmentation are due largely to hydraulic isolation,
and to the reduction or elimination of intercompartmental mixing. Mix-
ing can occur by two means: water oscillation and true turbulence.
Oscillatory mixing can be minimized by making compartments unequal in
size (commonly the second compartment is 1/3 to 1/2 the size of the
first), reducing flow-through area, and using an ell to connect
compartments (1).
In the first compartment, some mixing of sludge and scum with the liquid
always occurs due to induced turbulence from entering wastewater and the
digestive process. The second compartment receives the clarified efflu-
ent from the first compartment. Most of the time it receives this hy-
draulic load at a lower rate and with less turbulence than does the
first compartment, and, thus, better conditions exist for settling low-
density solids. These conditions lead to longer working periods before
pump-out of solids is necessary and improve overall performance.
6.2.5.5 Access and Inspection
In order to provide access and a means to inspect the inside of the sep-
tic tank, manholes should be provided. Manholes are usually placed over
both the inlet and the outlet to permit cleaning behind the baffles.
The manhole cover should extend above the actual septic tank to a height
not more than 6 in. (15 cm) below the finished grade. The actual cover
can extend to the ground surface if a proper seal is provided to prevent
106
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FIGURE 6-2
SEPTIC TANK SCUM AND SLUDGE CLEAR SPACES (8)
Liquid
Level"
j . Sc
Scum
Scum Clear
Space
Total Clear Space Sludge Clear
Space
•Outlet
TABLE 6-3
LOCATION OF TOP AND BOTTOM OF OUTLET TEE OR BAFFLE (12)
Tank Receiving Sewage
Tank Receiving Sewage
and Garbage
Total
Liquid
Tank
Capacity
gal
500
750
1,000
Projection3
Above
Liquid Level
12
12
12
Penetration3
Below
Liquid Level
22
24
26
Projection3
Above
Liquid Level
18
18
Penetration3
Below
Liquid Level
38
41
3 Percentage of liquid depth. See Figure 6-2 for diagram.
107
-------
FIGURE 6-3
TYPICAL TWO-COMPARTMENT SEPTIC TANK
_
__ _
l~ 1>
! i
_
• — 1 r
= 14=
—i * *
\ i
l|
— \ I'
Access iQi ~ ]~F
Manholes ' .1*
^ -ty
( '
i ;
_j
. .. -_ _,.. ,
~i
i — i
^sT^
^__ !
i_
b
Plan
Sanitary
Tee
Inlet
-
A*
., V Ul 11 r
Liquid Level
^
(r
"
~.
*
~l
JJ
•Outlet
Longitudinal Section
108
-------
the escape of odors and accidental entry into the tank. In addition,
small inspection pipes can be placed over the inlet and outlet to allow
inspection without having to remove the manhole.
6.2.5.6 Materials
The most commonly used construction material for septic tanks is con-
crete. Virtually all individual-home septic tanks are precast for easy
installation in the field. The walls have a thickness of 3 to 4 in. (8
to 10 cm), and the tank is sealed for watertightness after installation
with two coats of bituminous coating. Care must be taken to seal around
the inlet and discharge pipes with a bonding compound that will adhere
both to concrete and to the inlet and outlet pipe.
Steel is another type of material that has been used for septic tanks.
The steel must be treated so as to be able to resist corrosion and
decay. Such protection includes bituminous coating or other
corrosion-resistant treatment. However, despite a corrosion-resistant
coating, tanks deteriorate at the liquid level. Past history indicates
that steel tanks have a short operational life (less than 10 years) due
to corrosion (3).
Other materials include polyethylene and fiberglass. Plastic and
fiberglass tanks are very light, easily transported, and resistant to
corrosion and decay. While these tanks have not had a good history,
some manufacturers are now producing an excellent tank with increased
strength. This minimizes the chance of damage during installation or
when heavy machinery moves over it after burial.
6.2.6 Installation Procedures
The most important requirement of installation is that the tank be
placed on a level grade and at a depth that provides adequate gravity
flow from the home and matches the invert elevation of the house sewer.
The tank should be placed on undisturbed soil so that settling does not
occur. If the excavation is dug too deep, it should be backfilled to
the proper elevation with sand to provide an adequate bedding for the
tank. Tank performance can be impaired if a level position is not main-
tained, because inlet and outlet structures will not function properly.
109
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Other considerations include:
1. Cast iron inlet and outlet structures should be used in dis-
turbed soil areas where tank settling may occur.
2. Flotation collars should be used in areas with high groundwater
potential.
3. The tank should be placed so that the manhole is slightly below
grade to prevent accidental entry.
4. The tank should be placed in an area with easy access to alle-
viate pump-out problems.
5. During installation, any damage to the watertight coating
should be repaired. After installation, the tank should b.e
tested for watertightness by filling with water.
6. Care should be taken with installation in areas with large
rocks to prevent undue localized stresses.
7. Baffles, tees, and elbows should be made of durable and
corrosion-proof materials. Fiberglass or acid-resistant
concrete baffle materials are most suitable. Vitrified clay
tile, plastic, and cast iron are best for tees and ells.
6.2.7 Operation and Maintenance
One of the major advantages of the septic tank is that it has no moving
parts and, therefore, needs very little routine maintenance. A well-
designed and maintained concrete, fiberglass, or plastic tank should
last for 50 years. Because of corrosion problems, steel tanks can be
expected to last no more than 10 years. One cause of septic tank prob-
lems involves a failure to pump out the sludge solids when required. As
the sludge depth increases, the effective liquid volume and detention
time decrease. As this occurs, sludge scouring increases, treatment
efficiency falls off, and more solids escape through the outlet. The
only way to prevent this is by periodic pumping of the tank.
Tanks should be inspected at intervals of no more than every 2 years to
determine the rates of scum and sludge accumulation. If inspection
programs are not carried out, a pump-out frequency of once every 3 to 5
years is reasonable. Once the characteristic sludge accumulation rate
is known, inspection frequency can be adjusted accordingly. The inlet
and outlet structures and key joints should be inspected for damage
after each tank pump-out.
110
-------
Actual inspection of sludge and scum accumulations is the only way to
determine definitely when a given tank needs to be pumped. When a tank
is inspected, the depth of sludge and scum should be measured in the
vicinity of the outlet baffle. The tank should be cleaned whenever:
(1) the bottom of the scum layer is within 3 in. of the bottom of the
outlet device; or (2) the sludge level is within 8 in. of the bottom of
the outlet device. The efficiency of suspended solids removal may start
to decrease before these conditions are reached.
Scum can be measured with a stick to which a weighted flap has been
hinged, or with any device that can be used to feel the bottom of the
scum mat. The stick is forced through the mat, the hinged flap falls
into a horizontal position, and the stick is raised until resistance
from the bottom of the scum is felt. With the same tool, the distance
to the bottom of the outlet device can be determined.
A long stick wrapped with rough, white toweling and lowered to the bot-
tom of the tank will show the depth of sludge and the liquid depth of
the tank. The stick should be lowered behind the outlet device to avoid
scum particles. After several minutes, the sludge layer can be distin-
guished by sludge particles clinging to the toweling.
Other methods for measuring sludge include connecting a small pump to a
clear plastic line and lowering the line until the pump starts to draw
high solids concentrations.
Following is a list of considerations pertaining to septic tank opera-
tion and maintenance:
1. Climbing into septic tanks can be very dangerous, as the tanks
are full of toxic gases. When using the manhole, take every
precaution possible, i.e., do not lower an individual into the
tank without a proper air supply, and safety rope tied around
chest or waist.
2. The manhole, not the inspection pipe, should be used for pump-
ing so as to minimize the risk of harm to the inlet and outlet
baffles.
3. Leaving solids in the septic tank to aid in starting the system
is not necessary.
4. When pumped, the septic tank must not be disinfected, washed,
or scrubbed.
Ill
-------
5. Special chemicals are not needed to start activity in a septic
tank.
6. Special additives are not needed to improve or assist tank
operation once it is under way. No chemical additives are
needed to "clean" septic tanks. Such compounds may cause
sludge bulking and decreased sludge digestion. However, ordi-
nary amounts of bleaches, lyes, caustics, soaps, detergents,
and drain cleaners do not harm the system. Other preparations,
some of which claim to eliminate the need for septic tank pump-
ing, are not necessary for proper operation and are of ques-
tionable value.
7. Materials not readily decomposed (e.g., sanitary napkins, cof-
fee grounds, cooking fats, bones, wet-strength towels, dispos-
able diapers, facial tissues, cigarette butts) should never be
flushed into a septic tank. They will not degrade in the tank,
and can clog inlets, outlets, and the disposal systems.
6.2.8 Considerations for Multi-Home and Commercial Wastewater
6.2.8.1 General
In some instances, a septic tank can serve several homes, or a commer-
cial/institutional user such as a school, store, laundry, or restaurant.
Whereas septic tanks for single-family homes must handle highly variable
flows (i.e., approximately 45% of the total household flow occurs in the
peak four hours), commercial systems must also be able to treat continu-
ous wastewater flows for 8-16 hours a day as well as peak loadings. In
addition, commercial wastewaters may present special problems that need
to be handled prior to discharge to the septic tank (i.e., grease re-
moval for restaurant wastewaters, and lint removal for laundry waste-
water).
As explained previously, septic tanks of two compartments give better
results than single-compartment tanks. Although single-compartment
tanks are acceptable for small household installations, tanks with two
compartments should be provided for the larger institutional systems.
Tanks with more than two compartments are not used frequently.
Multiple-compartment tanks for commercial/institutional flows should
have the same design features as single-family home tanks discussed
above. These include: compartments separated by walls with ports or
slits at proper elevations, proper venting, access to all compartments,
and proper inlet and outlet design and submergence.
112
-------
The effect of a multiple-compartment tank can be accomplished by using
two or more tanks in series. A better construction arrangement, par-
ticularly for medium or large installations, is to connect special tank
sections together into a unit having single end-walls and two compart-
ments. A unit of four precast tank sections forming two compartments is
shown in Figure 6-4.
6.2.8.2 Design
Larger tanks for commercial/institutional flows or for clusters of homes
must be sized for the intended flow. Whenever possible with existing
facilities, the flow should be metered to obtain accurate readings on
average daily flows and flow peaks. For housing clusters, if the total
flow cannot be measured, the individually metered or estimated flows
(based on the expected population and the generation rate of 45 gal/cap/
day (170 1/cap/day) from each house must be summed to determine the de-
sign flow. For commercial/institutional applications, consult Chapter
4. For flows between 750 and 1,500 gal per day (2,840 to 5,680 1 per
day), the capacity of the tank is normally equal to 1-1/2 days waste-
water flow. For flows between 1,500 and 15,000 gpd (5,680 to 56,800
Ipd), the minimum effective tank capacity can be calculated at 1,125 gal
(4,260 1) plus 75% of the daily flow; or
V = 1,125 + 0.75Q
where:
V = net volume of the tank (gal)
Q = daily wastewater flow (gal)
If garbage grinders are used, additional volume or extra sludge storage
may be desired to minimize the frequency of pumping (10).
6.3 Intermittent Sand Filters
6.3.1 Introduction
Intermittent sand filtration may be defined as the intermittent
application of wastewater to a bed of granular material which is
underdrained to collect and discharge the final effluent. One of the
oldest methods of wastewater treatment known, intermittent sand
filtration, if properly designed, operated, and constructed, will
produce effluents of very high quality. Currently, many intermittent
sand filters are used throughout the United States to treat wastewater
from small commercial and institutional developments and from individual
113
-------
FIGURE 6-4
FOUR PRECAST REINFORCED CONCRETE SEPTIC TANKS COMBINED
INTO ONE UNIT FOR LARGE FLOW APPLICATION (10)
T
I
J_
Plan
12" Dia. Inspection Hole-
Influent
Section
Vent
Joints to be Grouted
Effluent
-------
homes. The use of intermittent sand filters for upgrading stabilization
ponds has also become popular (13).
Intermittent sand filtration is well suited to onsite wastewater
treatment and disposal. The process is highly efficient, yet requires a
minimum of operation and maintenance. Normally, it would be used to
polish effluents from septic tank or aerobic treatment processes and
would be followed by disinfection (as required) prior to reuse or
disposal to land or surface waters.
6.3.2 Description
Intermittent sand filters are beds of granular materials 24 to 36 in.
(61 to 91 cm) deep and underlain by graded gravel and collecting tile.
Wastewater is applied intermittently to the surface of the bed through
distribution pipes or troughs. Uniform distribution is normally
obtained by dosing so as to flood the entire surface of the bed.
Filters may be designed to provide free access (open filters), or may be
buried in the ground (buried filters). A relatively new concept in
filtration employs recirculation of filter effluent (recirculating
filters).
The mechanisms of purification attained by intermittent sand filters are
complex and not well understood even today. Filters provide physical
straining and sedimentation of solid materials within the media grains.
Chemical sorption also plays a role in the removal of some materials.
However, successful treatment of wastewaters is dependent upon the
biochemical transformations occurring within the filter. Without the
assimilation of filtered and sorbed materials by biological growth
within the filter, the process would fail to operate properly. There is
a broad range of trophic levels operating within the filter, from the
bacteria to annelid worms.
Since filters entrap, sorb, and assimilate materials in the wastewater,
it is not surprising to find that the interstices between the grains may
fill, and the filter may eventually clog. Clogging may be caused by
physical, chemical, and biological factors. Physical clogging is
normally caused by the accumulation of stable solid materials within or
on the surface of the sand. It is dependent on grain size and porosity
of the filter media, and on wastewater suspended solids characteristics.
The precipitation, coagulation, and adsorption of a vareity of materials
in wastewater may also contribute to the clogging problem in some filter
operations (14). Biological clogging is due primarily to an improper
115
-------
balance of the intricate biological population within the filter. Toxic
components in the wastewater, high organic loading, absence of dissolved
oxygen, and decrease in filter temperatures are the most likely causes
of microbial imbalances. Accumulation of biological slimes and a
decrease in the rate of decomposition of entrapped wastewater contami-
nants within the filter accelerates filter clogging. All forms of pore
clogging likely occur simultaneously throughout the filter bed. The
dominant clogging mechanism is dependent upon wastewater characteris-
tics, method and rate of wastewater application, characteristics of the
filtering media, and filter environmental conditions.
6.3.3 Application
Intermittent sand filtration is well adapted to onsite disposal. Its
size is limited by land availability. The process is applicable to
single homes and clusters of dwellings. The wastewater applied to the
intermittent filters should be pretreated at least by sedimentation.
Septic tanks should be required as a minimum. Additional pretreatment
by aerobic biological processes normally results in higher acceptable
rates of wastewater application and longer filter runs. Although exten-
sive field experience is lacking to date, the application of pretreated
graywaters to intermittent sand filters may be advantageously employed.
There is some evidence that higher loading rates and longer filter runs
can be achieved with pretreated graywaters.
Site constraints should not limit the application of intermittent sand
filters, although odors from open filters receiving septic tank effluent
may require isolation of the process from dwellings. Filters are often
partially (or completely) buried in the ground, but may be constructed
above ground when dictated by shallow bedrock or high water tables.
Covered filters are required in areas with extended periods of subfreez-
ing weather. Excessive long-term rainfall and runoff on submerged fil-
ter systems may be detrimental to performance, requiring appropriate
measures to divert these sources away from the system.
6.3.4 Factors Affecting Performance
The degree of stabilization attained by an intermittent sand filter is
dependent upon: (1) the type and biodegradability of wastewater applied
to the filter, (2) the environmental conditions within the filter, and
(3) the design characteristics of the filter.
Reaeration and temperature are two of the most important environmental
conditions that affect the degree of wastewater purification through an
intermittent sand filter. Availability of oxygen within the pores
116
-------
allows for the aerobic decomposition of the wastewater. Temperature
directly affects the rate of microbial growth, chemical reactions,
adsorption mechanisms, and other factors that contribute to the
stabilization of wastewater within the sand media.
Proper selection of process design variables also affects the degree of
purification of wastewater by intermittent filters. A brief discussion
of those variables is presented below.
6.3.4.1 Media Size and Distribution
The successful use of a granular material as a filtering media is depen-
dent upon the proper choice of size and uniformity of the grains. Fil-
ter media size and uniformity are expressed in terms of "effective size"
and "uniformity coefficient." The effective size is the size of the
grain, in millimeters, such that 10% by weight are smaller. The unifor-
mity coefficient is the ratio of the grain size that has 60% by weight
finer than itself to the size which has 10% finer than itself. The ef-
fective size of the granular media affects the quantity of wastewater
that may be filtered, the rate of filtration, the penetration depth of
particulate matter, and the quality of the filter effluent. Granular
media that is too coarse lowers the retention time of the applied waste-
water through the filter to a point where adequate biological decomposi-
tion is not attained. Too fine a media limits the quantity of waste-
water that may be successfully filtered, and will lead to early filter
clogging. This is due to the low hydraulic capacity and the existence
of capillary saturation, characteristic of fine materials. Metcalf and
Eddy (15) and Boyce (16) recommended that not more than 1% of the media
should be finer than 0.13 mm. Many suggested values for the effective
size and uniformity coefficient exist in the literature (10)(17)(18)(19)
(20). Recommended filter media effective sizes range from a minimum of
0.25 mm up to approximately 1.5 mm. Uniformity coefficients (UC) for
intermittent filter media normally should be less than 4.0.
Granular media other than sand that have been used include anthracite,
garnet, ilmenite, activated carbon, and mineral tailings. The media
selected should be durable and insoluble in water. Total organic matter
should be less than 1%, and total acid soluble matter should not exceed
3%. Any clay, loam, limestone, or organic material may increase the
initial adsorption capacity of the sand, but may lead to a serious
clogging condition as the filter ages.
Shapes of individual media grains include round, oval, and angular con-
figurations. Purification of wastewater infiltrating through granular
media is dependent upon the adsorption and oxidation of organic matter
in the wastewater. To a limiting extent, this is dependent on the shape
117
-------
of the grain; however, it is more dependent on the size distribution of
the grains, which is characterized by the UC.
The arrangement or placement of different sizes of grains throughout the
filter bed is also an important design consideration. A homogeneous bed
of one effective size media does not occur often due to construction
practices and variations in local materials. In a bed having fine media
layers placed above coarse layers, the downward attraction of wastewater
is not as great due to the lower amount of cohesion of the water in the
larger pores (21). The coarse media will not draw the water out of the
fine media, thereby causing the bottom layers of the fine material to
remain saturated with water. This saturated zone acts as a water seal,
limits oxidation, promotes clogging, and reduces the action of the fil-
ter to a mere straining mechanism. The use of media with a UC of less
than 4.0 minimizes this problem.
The media arrangement of coarse over fine appears theoretically to be
the most favorable, but it may be difficult to operate such a filter due
to internal clogging throughout the filter.
6.3.4.2 Hydraulic Loading Rate
The hydraulic loading rate may be defined as the volume of liquid ap-
plied to the surface area of the sand filter over a designated length of
time. Hydraulic loading is normally expressed as gpd/ft , or cm/day.
Values of recommended loading rates for intermittent sand filtration
vary throughout the literature and depend upon the effective size of
sand and the type of wastewater. They normally range from 0.75 to 15
gpd/ft2 (0.3 to 0.6 m3/mz/d).
6.3.4.3 Organic Loading Rate
The organic loading rate may be defined as the amount of soluble and
insoluble organic matter applied per unit volume of filter bed over a
designated length of time. Organic loading rates are not often reported
in the literature. However, early investigators found that the perfor-
mance of intermittent sand filters was dependent upon the accumulation
of stable organic material in the filter bed (14)(21). To account for
this, suggested hydraulic loading rates today are often given for a
particular type of wastewater. Allowable loading rates increase with
the degree of pretreatment. A strict relationship establishing an
organic loading rate, however, has not yet been clearly defined in the
literature.
118
-------
6.3.4.4 Depth of Media
Depths of intermittent sand filters were initially designed to be 4 to
10.feet; however, it was soon realized at the Lawrence Experimental
Station (21) that most of the purification of wastewater occurred within
the top 9 to 12 in. (23 to 30 cm) of the bed. Additional bed depth did
not improve the wastewater purification to any significant degree. Most
media depths used today range from 24 to 42 in. (62 to 107 cm). The use
of shallow filter beds helps to keep the cost of installation low.
Deeper beds tend to produce a more constant effluent quality, are not
affected as severely by rainfall or snow melt (22), and permit the
removal of more media before media replacement becomes necessary.
6.3.4.5 Dosing Techniques and Frequency
Dosing techniques refer to methods of application of wastewater to the
intermittent sand filter. Dosing of intermittent filters is critical to
the performance of the process. The system must be designed to insure
uniform distribution of wastewater throughout the filter cross-section.
Sufficient resting must also be provided between dosages to obtain
aerobic conditions. In small filters, wastewater is applied in doses
large enough to entirely flood the filter surface with at least 3 in. (8
cm) of water, thereby insuring adequate distribution. Dosing frequency
is dependent upon media size, but should be greater with smaller doses
for coarser media.
Dosing methods that have been used include ridge and furrow application,
drain tile distribution, surface flooding, and spray distribution meth-
ods. Early sand filters for municipal wastewater were surface units
that normally employed ridge and furrow or spray distribution methods.
Intermittent filters in use today are often built below the ground sur-
face and employ tile distribution.
The frequency of dosing intermittent sand filters is open to
considerable design judgement. Most of the earlier studies used a
dosing frequency of I/day. The Florida studies investigatged multiple
dosings and concluded that the BOD removal efficiency of filters with
media effective size greater than 0.45 mm is appreciably increased when
the frequency of loading is increased beyond twice per day (23). This
multiple dosing concept is successfully used in recirculating sand
filter systems in Illinois (24), which employ a dosing frequency of once
every 30 min.
119
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6.3.4.6 Maintenance Techniques
Various techniques to maintain the filter bed may be employed when the
bed becomes clogged. Some of these include: (1) resting the bed for a
period of time, (2) raking the surface layer and thus breaking the in-
hibiting crust, or (3) removing the top surface media and replacing it
with clean media. The effectiveness of each technique has not been
clearly established in the literature.
6.3.5 Filter Performance
A summary of the performance of selected intermittent sand filters
treating household wastwaters appears in Table 6-4, 6-5, and 6-6. These
tables illustrate that intermittent filters produce high-quality
effluents with respect to BODs and suspended solids. Normally,
nitrogen is transformed almost completely to the nitrate form provided
the filter remains aerobic. Rates of nitrification may decrease in
winter months as temperatures fall. Little or no denitrification should
occur in properly operated intermittent filters.
Total and ortho-phosphate concentrations can be reduced up to approxi-
mately 50% in clean sand; but the exchange capacity of most of the sand
as well as phosphorus removal after maturation is low. Use of calcare-
ous sand or other high-aluminum or iron materials intermixed within the
sand may produce significant phosphorus removal. Chowdhry (28) and
Brandes, et al. (23), reported phosphorus removals of up to 90% when
additions of 4% "red mud" (high in A1203 and Fe203) were made to a medi-
um sand. Intermittent filters are capable of reducing total and fecal
coliforms by 2 to 4 logs, producing effluent values ranging from 100 to
3,000 per 100 ml and 1,000 to 100,000/100 ml for fecal and total coli-
forms, respectively (2)(19)(28).
6.3.6 Design Criteria
6.3.6.1 Buried Filters
Table 6-7 summarizes design criteria for subsurface intermittent sand
filters.
Hydraulic loading of these filters is normally equal to or less than 1.0
gpd/ft2 (0.04 m3/m2/d) for full-time residences. This value is
similar to loading rates for absorption systems in sandy soils after
120
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TABLE 6-4
PERFORMANCE OF BURIED INTERMITTENT FILTERS - SEPTIC TANK EFFLUENT
Filter Characteristics
Effluent Characteristics
IV3
Effecti Me
Size
mm
0.24
0.30
0.60
1.0
2.5
0.17
0.23 - 0.36
Urn" f ormi ty
Coefficient
3.9
4.1
2.7
2.1
1.2
11.8
2.6 - 6.1
Hydraulic
Loading
gpd/ft2
1
1
1
1
1
0.2
1.15
Depth
in.
30
30
30
30
30
39
24
BOD
mg/1
2,0
4.7
3.8
4.3
8.9
1.8
4
SS
mg/1
4.4
3.9
4.3
4.9
12.9
11.0
12
NHjN
mg/1
0.3
3.8
3.1
3.7
6.7
1.0
0.7
N03N
mg/1
25
23
27
24
18
32
17
Reference
25
25
25
25
25
22
19
-------
TABLE 6-5
PERFORMANCE OF FREE ACCESS INTERMITTENT FILTERS
Filter Characteristics
Effluent Quality
ro
ro
Source
Effective Unif.
Size Coeff.
mm
Septic Tank
Septic Tank
Trick. Filter
Trick. Filter
Primary
Primary
Primary
Primary
Septic Tank
Extended Aer.
La goon (Summer)
Lagoon (Winter)
0.23
0
0
0
0
0
1
1
0
0
0
0
-0.26
.41
.27
.41
.25
.25
.04
.04
.45 3.0
.19 3.3
.19 9.7
.19 9.7
Hydraulic
Loading
gpd/ft2
4.5
2.3
11.4
14.0
2.75
4.7
-
14
5
3.8
9.1
9.1
Depth
in.
60
60
60
60
30
30
30
30
30
30
36
36
Dose
Freq.
per
day
-
-
-
-
1
2
2
24
3-6
3-6
1
1
BOD
mg/1
233
113
173
183
6
3
28
4
8
3
2
9.4
SS
mg/1
-
-
-
-
6
8
36
9
4
9
3
9.6
NHjN
mg/1
8
3
2
2
5
2
10
3
3
0.3
0.5
4.6
N03M
mq/1
32
46
29
33
19
22
13
17
25
34
4.0
1.0
Filter
Run
months
6 - 9b
6 - 9b
6b
12b
4.5
36
>54
>54
3
12
1
4
Ref.
21
21
21
21
23
23
23
23
2
2
13
13
a Estimated from "oxygen consumed."
b Weekly raking 3 inches deep.
-------
TABLE 6-6
PERFORMANCE OF RECIRCULATING INTERMITTENT FILTERS9
Filter Characteristics
Effluent Quality
ro
CO
Effective
Size
ran
0.6 - 1.0
0.3 - 1.5
1.2
Unif.
Coeff.
2.5
3.5
2.0
Hydraulic
Loading
gpd/ft2
3.0 - 5.0b
3. Ob
Depth
in.
36
24
36
Recir culation
Ratio
r/Q
4:1
3:1 - 5:1
4:1
Dose BOD
mg/1
5-10 min every 4
30 min
20 min every 15.8C
2-3 hr
5 mi n every 4
30 min
SS NHsN Mtnce. Ref.
mg/1 rnq/1
5 - Weed/Rake 24
as Req'd
10. DC 8.4C Rake 26
Weekly
3 - Weed 27
as Req'd
a Septic tank effluent.
b Based on forward flow.
c Average for 12 installations (household flow to 6,500 gpd plant).
-------
TABLE 6-7
DESIGN CRITERIA FOR BURIED INTERMITTENT FILTERS
Item
Design Criteria
Pretreatment
Hydraulic Loading
All year
Seasonal
Media
Material
Effective size
Unif. Coeff.
Depth
Underdrai ns
Material
Slope
Beddi ng
Venti ng
Di stri buti on
Material
Beddi ng
Venti ng
Dosi ng
Minimum level - sedimentation (septic
tank or equivalent)
<1.0 gpd/ft2
<2.0 gpd/ft2
Washed durable granular material (less
than 1 percent organic matter by weight)
0.50 to 1.00 mm
<4.0 (<3.5 preferable)
24 to 36 inches
Open joint or perforated pipe
0.5 to 1.0 percent
Washed durable gravel or crushed stone
(1/4 to 1-1/2 in.)
Upstream end
Open joint or perforated pipe
Washed durable gravel or stone (3/4 to
2-1/2 in.)
Downstream end
Flood filter; frequency greater than
2 per day
124
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equilibrium conditions are obtained. When filters are designed for
facilities with seasonal occupation, hydraulic loading may be increased
to 2.0 gpd/ft2 (0.08 m3/m2/d) since sufficient time will be
available for drying and restoring the infiltrative surface of the bed.
The effective size of media for subsurface filters ranges from 0.35 to
1.0 mm with a UC less than 4.0, and preferably less than 3.5. Finer
media will tend to clog more readily, whereas coarser media may result
in poorer distribution and will normally produce a lower effluent
quality.
Distribution and underdrains are normally perforated or open-joint pipe
with a minimum 4-in. (10-cm) diameter. The distribution and underdrain
lines are surrounded by at least 8 in. of washed durable gravel or
crushed stone. For distribution lines, the gravel or stone is usually
smaller than 2-1/2 in. (6 cm) but larger than 3/4 in. (2 cm), whereas
the size range of the gravel or stone for the underdrains is between 1-
1/2 to 1/4 in. (3.8 to 0.6 cm). Slopes of underdrain pipe range from
0.5 to 1%. With dosing, there would be no requirement for slopes on
distribution piping.
Proper dosing to the filter is critical to its successful performance.
The dosing system is designed to flood the entire filter during the dos-
ing cycle. A dosing frequency of greater than two times per day is rec-
ommended. Details on design and construction of dosing chamber facili-
ties appear in Chapter 8.
6.3.6.2 Free Access Filters (Non-Recirculating)
Design criteria for free access filters are presented in Table 6-8.
Hydraulic loading to these filters depends upon media size and waste-
water characteristics. Septic tank effluent may be applied at rates up
to 5 gpd/ft2 (0.2 nr/nr/d), whereas a higher quality pretreated waste-
water may be applied at rates as high as 10 gal/d ftz (40 cm/d). Selec-
tion of hydraulic loading will also be influenced by desired filter run
times (see Section 6.3.8). Higher acceptable loadings on these filters
as compared to subsurface filters relates primarily to the accessibility
of the filter surface for maintenance.
Media characteristics and underdrain systems for free access filters are
similar to those for subsurface filters. Distribution is often provided
through pipelines and directed on splash plates located at the center or
corners of the sand surface. Occasionally, troughs or spray nozzles are
125
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TABLE 6-8
DESIGN CRITERIA FOR FREE ACCESS INTERMITTENT FILTERS
Item
Design Criteria
Pretreatment
Hydraulic Loading
Septic tank feed
Aerobic feed
Medi a
Material
Effective size
Unif. Coeff.
Depth
Underdrains
Material
Slope
Beddi ng
Venting
Distribution
Dosi ng
Number
Septic tank fe,ed
Aerobic feed
Minimum level - sedimentation (septic
tank or equivalent)
2.0 to 5.0 gpd/ft2
5.0 to 10.0 gpd/ft2
Washed durable granular material (less
than 1 percent organic matter by weight)
0.35 to 1.00 mm
<4.0 (<3.5 preferable)
24 to 36 inches
Open joint or perforated pipe
0.5 to 1.0 percent
Washed durable gravel or crushed stone
(1/4 to 1-1/2 in.)
Upstream end
Troughs on surface; splash plates at center
or corners; sprinkler distribution
Flood filter to 2 inches; frequency greater
than 2 per day
Dual filters, each sized for design flow
Single filter
126
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employed as well, and ridge and furrow application has been successful
during winter operation in severe climatic conditions. Dosing of the
filter should provide for flooding the bed to a depth of approximately
2 in. Dosing frequency is usually greater than two times per day. For
coarser media (greater than 0.5 mm), a dosing frequency greater than 4
times per day is desirable. Design criteria for dosing chambers, pumps,
and siphons are found in Chapter 8.
The properties of the wastewater applied affect the clogging character-
istics of the filter and, therefore, the methods of filter maintenance.
Dual filters, each designed to carry the design flow rate, may be desir-
able when treating septic tank effluent to allow sufficient resting
after clogging (see Section 6.3.8).
6.3.6.3 Recirculating Filters
Proposed design criteria for recirculating intermittent sand filters are
presented in Table 6-9 (24)(26). These free access filters employ a
recirculation (dosing) tank between the pretreatment unit and filter
with provision for return of filtered effluent to the recirculation
tank.
Hydraulic loading ranges from 3 to 5 gpd/ft2 (0.12 to 0.20 m3/m2/d)
depending on media size. Media size range is from 0.3 to 1.5 mm, the
coarser sizes being recommended (23)(26). Underdrain and distribution
arrangements are similar to those for free access filters. Recircula-
tion is critical to effective operation, and a 3:1 to 5:1 recirculation
ratio (Recycle: Forward Flow) is preferable. Pumps should be set by
timer to dose approximately 5 to 10 min per 30 min. Longer dosing
cycles may be desirable for larger installations - 20 min every 2 to 3
hr. Dosing should be at a rate high enough to insure flooding of the
surface to greater than 2 in. (5 cm). Recirculation chambers are nor-
mally sized at 1/4 to 1/2 the volume of the septic tank.
6.3.7 Construction Features
6.3.7.1 Buried Filters
A typical plan and profile of a buried intermittent sand filter are
depicted in Figure 6-5. The filter is placed within the ground with a
natural topsoil cover in excess of 10 in. (25 cm) over the crown of the
distribution pipes. The filter must be carefully constructed after
excavation and the granular fill settled by flooding. Distribution and
underdrain lines should be constructed of an acceptable material with a
minimum diameter of 4 in. (10 cm). The tile is normally laid with open
127
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TABLE 6-9
DESIGN CRITERIA FOR RECIRCULATING INTERMITTENT FILTERS
Item
Design Criteria
Pretreatment
Hydraulic Loading
Media
Material
Effective size
Unif. Coeff.
Depth
Underdrai ns
Material
Slope
Beddi ng
Venti ng
Distribution
Recirculation Ratio
Dosing
Recirculation Tank
Minimum level - sedimentation (septic
tank or equivalent)
3.0 to 5.0 gpd/ft2 (forward flow)
Washed durable granular material (less
than 1 percent organic matter by weight)
0.3 to 1.5 mm
<4.0 (<3.5 preferable)
24 to 36 inches
Open joint or perforated pipe
0.5 to 1.0 percent
Washed durable gravel or crushed stone
(1/4 to 1-1/2 in.)
Upstream end
Troughs on surface; splash plates at center
or corners; sprinkler distribution
3:1 to 5:1 (5:1 preferable).
Flood filter to approx. 2 inches; pump 5
to 10 min per 30 min; empty recirculation
tank in less than 20 min
Volume equivalent to at least one day's raw
wastewater flow
128
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FIGURE 6-5
TYPICAL BURIED INTERMITTENT FILTER INSTALLATION
Distribution
House Sewer?
^tjaDischarge
Inspection Manhole
and Disinfection
Contact Tank
(If Required)
Profile
Top Soil Fill
Drainage
Marsh Hay or
Drainage Fabric
XXX
vxxxxxxxxxVxxx!
xxxxxxxxxxxxxx
xxxxxxxxxxxxxx
xxxxxxxxxxxxxx
xxxxxxxxxxxxxx
xxxxxxxxxxxxxx
..,,XXXXXyVXX
XXXXXXXXXXjiX
XXXXX XXX XX^'X
xxxxxxxxxxxxx
xxxxxxxxxxxxx
xxxxxxxxxxx>^
xxxxxxxxxi? ""^
xxxxxxxx^r-
XXXXXXXX/jfl
xxx
•xxxxxxxxxxxxxxxxxxxxxxxxxxx.
'XXXXXXXXXXXXXXXXXXXXXXXXXXX.
'XXXXXXXXXXXXXXXXXXXXXXXXXXX
'XXXXXXXXXXXXXXXXXXXXXXXXXXX,
L-*^xxxxxxxxxxxxxxxxxxxxxxx -
Z^vxxxxxxxxxxxxxxxxxxxx
Vxxxxxxxxxxxxxxxxxxx
k.Vxxxxxxxxxxxxxxxxxx'
XXX -
XXXXXXXXX, - , ™
xxxxxxxxxxxxx.
xxxxxxxxxxxx,
• -xxxxxxxxxx,
•xxxxxxxxx.
vxxxxxxxx-
• -xxxxxxx.
'XXXXXXX,
• x/./x-
_.._
Graded Gravel 3/4" to 21/2"
,,,,
////// S//S///S
XXXXXXX>XXXXXX,
Perforated or Open
Joint Distributors
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxx
xxxxxxxxxxxxxx?
xxxxxxxxxxxxxx xxxx ............ .
XXXXXXXXXXXXXXXXXXXXXXXXXXX XXX, .
' XXXXX XXXX XX XXXXXX/XXXXXXXXXXVX
'''-
?5«^
^1
xxxx
xxxxx •
XXXXX
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
XXXXX
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
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xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
in.
>8 in.
24-36 in.
XXXXXXXXXXXXXXXXXXXXXXXXXXX
'XXXXXXXXXXXXXXXXXXXXXXXXXXX
'XXXXXXXXXXXXXXXXXXXXXXXXXXX
X'^^/^/xxxxxxxxxxxxxxxxxxxxx
v\xxxxxxxxxxxxxxxxxxxxx/xxxx
> 8 in.
Graded Gravel 1/4" to 11/2"
Perforated or Open
Joint Pipe, Tarpaper
Over Open Joints
Section A-A
129
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joints with sections spaced not less than 1/4 in. (0.6 cm) or greater
than 1/2 in. (1.3 cm) apart. If continuous pipeline is used, conven-
tional perforated pipe will provide adequate distribution and collection
of wastewater within the filter.
The underdrain lines are laid to grade (0.5 to 1%) and one line is pro-
vided for each 12 ft (3.6 m) of trench width. Underdrains are provided
with a vent pipe at the upstream end extending to the ground surface.
The bedding material for underdrain lines is usually a minimum of 10 in.
(25 cm) washed graded gravel or stone with sizes ranging from 1/4 to
1-1/2 in. (0.6 to 3.8 cm). The gravel or stone may be overlain with a
minimum of 3 in. (8 cm) of washed pea gravel (1/4- to 3/8-in. [0.6 to
1.0 cm] stone) interfacing with the filter media.
The distribution lines should be level and are normally spaced at 3-ft
(0.9 m) centers. Distribution lines should be vented at the downstream
end with vertical risers to the ground surface. Approximately 10 in.
(25 cm) of graded gravel (3/4- to 2-1/2-in. [1.9- to 6.3-cm] size) is
usually employed for bedding of distribution lines. Marsh hay, washed
pea gravel, or drainage fabric should be placed between the bedding
material and the natural topsoil.
The finished grade over the filter should be mounded so as to provide
drainage of rainfall away from the filter bed. A grade of approximately
3 to 5%, depending upon topsoil characteristics, would be sufficient.
Any washed, durable granular material that is low in organic matter may
be used for filter medium. Mixtures of sand, slag, coal, or other
materials have been used to enhance the removal of selected pollutants
and to extend filter life. Care must be taken, however, to insure that
the media does not stratify with fine layers over coarse.
Design and construction of the dosing chamber and pump or siphon em-
ployed for proper application of wastewater to the filter are described
in Chapter 8 of this manual.
6.3.7.2 Free Access Filters
The plan and profile of a typical free access filter appear in Figure
6-6. These filters are often built within the natural soil, but may
also be constructed completely above the ground surface. They are
usually surrounded by sidewalls, often of masonry construction, to pre-
vent earth from washing into the filter media and to confine the flow of
wastewater. Where severe climates are encountered, filter walls should
130
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FIGURE 6-6
TYPICAL FREE ACCESS INTERMITTENT FILTER
Splash Plate
•——OT—"™
t j
Plan
Discharge
From
Pretreatment
//^\^
Graded
Gravel
to 11/2"
Insulated Cover
Splash
Plate
fl
Vent
Pipe
**•:
A 4 «
4» ' *
»!••
*:/.«•
-:.•••i
1 4 » »
' » 't.
•«••*•
« *< » •
» ft »
» ' .' - « *
ft 4
«
Distribution
Pipe
0
I
>\; .v.v.:-...:.'.'—::::^
* S •.»»•••••«»•. »•,... • -, K *•
t\ -•.•.":/-•/;/.-.;::;-..»»
1 • ;.V-V. Media »/.« ."^V
;.:--.v,v.-.-.-..-//.-.-.--.-/--/.--
^':vi-\V:X-:::.v:-:v/.^'>
5*7" " ^—-~r " - n J~ - - ~-----
^v^>^\
24"-36"
Collection Pipe
Profile
Perforated or Open Joint
131
-------
be insulated if exposed directly to the air. The floor of the filter is
often constructed of poured concrete or other masonry, but may consist
of the natural compacted soil. It is usually sloped to a slight grade
so that effluent can be collected into open joint or perforated
underdrains.
Free access filters may be covered to protect against severe weather
conditions, and to avoid encroachment of weeds or animals. The cover
also serves to reduce odor conditions. Covers may be constructed of
treated wooden planks, galvanized metal, or other suitable material.
Screens or hardware cloth mounted on wooden frames may also serve to
protect filter surfaces. Where weather conditions dictate, covers
should be insulated. A space of 12 to 24 in. (30 to 61 cm) should be
allowed between the insulated cover and sand surface.
The underdrain lines should be constructed of an acceptable material
with a minimum diameter of 4 in. (10 cm). The tile is normally laid so
that joints are spaced not less than 1/4 in. (0.6 cm) or greater than
1/2 in. (1.3 cm) apart. Conventional perforated pipe may also be em-
ployed for distribution and collection. The underdrain lines may be
laid directly on the filter floor, which should be slightly pitched to
carry filtered effluent to the drain line. In shallow filters, the
drain line may be laid within a shallow trench within the filter floor.
Drain lines are normally spaced at 12-ft (3.6-m) centers and sloped at
approximately 0.5 to 1% grade to discharge. The upstream end of each
drain line should be vented with a vertical vent pipe above the filter
surface, but within the covered space.
The bedding material for underdrain lines should be a minimum of 10 in.
(25 cm) of washed graded gravel or stone with sizes ranging from 1/4 to
1-1/2 in. (0.6 to 3.8 cm). The gravel or stone may be overlain with a
minimum of 3 in. (8 cm) of washed pea gravel interfacing with the filter
media.
Distribution to the filter may be by means of troughs laid on the sur-
face, pipelines discharging to splash plates located at the center or
corners of the filter, or spray distributors. Care must be taken to
insure that lines discharging directly to the filter surface do not
erode the sand surface. The use of curbs around the splash plates or
large stones placed around the periphery of the plates will reduce
scour. A layer of washed pea gravel placed over the filter media may
also be employed to avoid surface erosion. This practice will create
maintenance difficulties; however, when it is time to rake or remove a
portion of the media surface.
132
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Filter media employed in free access filters may be any washed, durable
granular material free of organic matter. As indicated previously for
buried filters, mixtures of sand, slag, coal, or other materials may be
employed, but with caution.
The design and construction features of the dosing chamber and pumps, or
siphon systems for these filters, are described in Chapter 8.
6.3.7.3 Recirculating Filters
A profile of a typical recirculating intermittent sand filter is pre-
sented in Figure 6-7. Recirculating filters are normally constructed
with free access to the filter surface. The elements of filter con-
struction are identical to those for the free access filter (Section
6.3.6.3 above). A schematic of a recirculation tank is presented in
Figure 6-8.
The basic difference between the recirculating filter and the free ac-
cess filter is the recirculation chamber (dosing chamber) which incorpo-
rates a pump to recycle filter effluent. The recirculation tank re-
ceives the overflow from a septic tank, as well as a portion of sand
filter effluent. A pump, controlled by a time clock mechanism, pumps
the wastewater mixture to the filter surface. The recirculation tank is
of equivalent strength and material to the septic tank. It is normally
1/4 to 1/2 the size of the septic tank (or a volmw equivalent to at
least one day's volume of raw wastewater flow). The tank must be ac-
cessible for maintenance of pumps, timers, and control valves. Covers
should be provided and insulated as required by climatic conditions.
Recirculation ratios may be controlled by a variety of methods. These
include splitter boxes, moveable gates, check valves, and a unique
"float valve" arrangement (Figure 6-9). The "float valve" incorporates
a simple tee and a rubber ball suspended in a wire basket. The ball
will float up and close off the inverted tee when the water level rises.
Recirculation ratios are normally established between 3:1 to 5:1 (26).
Recirculation pumps are normally submersible pumps rated for 1/3 horse-
power. They should be sized to empty the recirculation tank in less
than 20 min. The recirculation pump should be controlled by a time
clock to operate between 5 to 10 min every 30 min (26), and should be
equipped with a float shut-off and high water override. Details on pump
and control specifications may be found in Chapter 8.
133
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FIGURE 6-7
TYPICAL RECIRCULATING INTERMITTENT FILTER SYSTEM
Raw Waste
Pretreatment
Unit
To Discharge
Float Valve t {
i^,*44»jifc.'ijji ,y
Recirculation
Tank
Pump
Free Access
Sand Filter
FIGURE 6-8
RECIRCULATION TANK
From
Pretreatment
Discharge
Liquid Level
Float
Valve
fir
fl
From Filter
To Filter
Pump
134
-------
FIGURE 6-9
BY-PASS ALTERNATIVES FOR RECIRCULATING FILTERS
Discharge
Filter ^
Return
Sanitary Tee
Filter
Return
Open Basket
Floating Ball
t
Discharge
n
Moveable Gate
u
To Filter
To Filter —
Discharge
::JD
:.--0
r
-(
r
^
7
M
^Adjustable
A. Filtel"
Return
Weirs
135
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6.3.8 Operation and Maintenance
6.3.8.1 General
Intermittent sand filters require relatively little operational control
or maintenance. Once wastewater is applied to the filter, it takes from
a few days to two weeks before the sand has matured (2)(28). BOD and SS
concentrations in the effluent will normally drop rapidly after
maturation. Depending upon media size, rate of application, and ambient
temperature, nitrification may take from 2 weeks up to 6 months to
develop. Winter start-up should be avoided since the biological growth
on the filter media may not properly develop (14).
As discussed above, clogging of the filter eventually occurs as the pore
space between the media grains begins to fill with inert and biological
materials. Once hydraulic conductivity falls below the average hydrau-
lic loading, permanent ponding occurs. Although effluent quality may
not initially suffer, anaerobic conditions within the filter result in
further rapid clogging and a cessation of nitrification. Application of
wastewater to the filter should be discontinued when continuous ponding
occurs at levels in excess of 12 in. (30 cm) above the sand surface. A
high water alarm located 12 in. (30 cm) above the sand surface serves to
notify the owner of a ponded condition.
Since buried filters cannot be easily serviced, the media size is
normally large and hydraulic application rates are low (usually less
than 2 in./d [5 cm/d]). Proper pretreatment maintenance is- of paramount
importance. Free access filters, on the other hand, may be designed
with finer media and at higher application rates. Experience indicates
that intermittent sand filters receiving septic tank influent will clog
in approximately 30 and 150 days for effective sizes of 0.2 mm and 0.6
mm, respectively (2). Aerobically treated effluent can be applied at
the same rates for up to 12 months if suspended solids are under 50 mg/1
(2)(23). Results with recrrculated filters using coarse media (1.0 -
1.5 mm) indicate filter runs in excess of one year (27).
6.3.8.2 Maintenance of Media
Maintenance of the media includes both routine maintenance procedures
and media regeneration upon clogging. These procedures apply to free
access filters only. The effectiveness of routine raking of the media
surface has not been clearly established, although employed in several
studies (2)(14)(21)(24). Filters open to the air require weed removal
as well. Cold weather maintenance of media may require different meth-
ods of wastewater application, including ridge and furrow and continuous
136
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flooding. These methods are designed to eliminate ice sheet develop-
ment. Use of insulated covers permits trouble-free winter operation in
areas with ambient temperatures as low as -40° F (2).
Eventually, filter clogging requires media regeneration. Raking of the
surface will not in itself eliminate the need for more extensive reha-
bilitation (2)(14). The removal of the top layer of sand, as well as
replacement with clean sand when sand depths are depleted to less than
24 to 30 in. (61 to 76 cm), appears to be very effective for filters
clogged primarily by a surface mat. This includes filters receiving
aerobically treated effluent (2). In-depth clogging, however, often
prevails in many intermittent filters requiring oxidation of the clog-
ging materials. Resting of the media for a period of time has proven to
be very effective in restoring filter hydraulic conductivity (2). Hy-
drogen peroxide treatment may also prove to be effective, although in-
sufficient data are available on long-term application of this oxidizing
agent.
6.3.8.3 Other Maintenance Requirements
The successful operation of filters is dependent on proper maintenance
of the pretreatment processes. The accumulation of scum, grease, and
solid materials on the filter surface due to inadequate pretreatment
results in premature filter failure. This is especially critical for
buried filters. Grease traps, septic tanks, and other pretreatment
processes should be routinely maintained in accordance with requirements
listed in other sections of this manual.
Dosing chambers, pumps, and siphons should receive periodic maintenance
checks as recommended in Chapter 8. If electronic sensing devices are
employed to warn owners of filter ponding, these devices should also be
periodically checked as well.
6.3.8.4 Summary
The maintenance and operational requirements for buried, free access and
recirculating filters are summarized in Tables 6-10, 6-11, and 6-12.
Routine maintenance requirements have not been well documented for in-
termittent filtration onsite, but visits should be made four times per
year to check filters and their appurtenances. Based on a meager data
base, unskilled manpower requirements for buried filter systems would be
less than 2 man days per year for examination of dosing chamber and
appurtenances and septic tank. Free access filters may require from 2
to 4 man days per year for media maintenance and replacement and examin-
ation of dosing chamber, septic tank, and appurtenances. Additional
137
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TABLE 6-10
OPERATION AND MAINTENANCE REQUIREMENTS
FOR BURIED INTERMITTENT FILTERS
Item 0/M Requirement
Pretreatment Depends upon process
Dosing Chamber
Pumps and controls Check every 3 months
Tinier sequence Check and adjust every 3 months
Appurtenances Check every 3 months
Filter Media None
TABLE 6-11
OPERATION AND MAINTENANCE REQUIREMENTS
FOR FREE ACCESS INTERMITTENT FILTERS
Item 0/M Requirement
Pretreatment Depends upon process
Dosing Chamber
Pumps and controls Check every 3 months
Timer sequence Check and adjust every 3 months
Appurtenances Check every 3 months
Filter Media
Raking Every 3 months, 3 in. deep
Replacement
Septic tank feed Replace when ponded more than 12 in. deep;
replace top 2 to 3 in. sand; rest while
alternate unit in operation (60 days)
Aerobic feed Replace when ponded more than 12 in. deep;
replace top 2 to 3 in. sand; return to
service
Other Weed as required
Maintain distribution device as required
Protect against ice sheeting
Check high water alarm
138
-------
time would be required by analytical technicians for effluent quality
analysis as required. Power requirements would be variable, depending
upon the dosing method employed, but should be less than 0.1 kWh/day.
The volume of waste media from intermittent filters may amount to
approximately 0.25 ft3/ft2 (0.08 m3/mz) of surface area each time media
must be removed.
TABLE 6-12
OPERATION AND MAINTENANCE REQUIREMENTS
FOR RECIRCULATING INTERMITTENT FILTERS
Item 0/M Requirement
Pretreatment Depends upon process
Dosing Chamber
Pumps and controls Check every 3 months
Timer sequence Check and adjust every 3 months
Appurtenances Check every 3 months
Filter Media
Raking Every 3 months, 3 in. deep
Replacement Skim sand when heavy incrustations occur;
add new sand when sand depth falls
below 24 in.
Other Weed as required
Maintain distribution device as required
Protect against ice sheeting
6.3.9 Considerations for Multi-Home and Commercial Wastewaters
6.3.9.1 Applicability
Intermittent filtration processes have been successfully employed in
larger scale installations to achieve high levels of treatment of waste-
water.
139
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6.3.9.2 Design Criteria
Intermittent filters for larger installations may be designed in accor-
dance with similar criteria used for onsite systems (20). Submerged
filters should be avoided. The biggest difficulty with larger flow
units is adequate wastewater distribution. Troughs, ridge and furrow
spray distributors, and multiple pipe apron systems may be used. Si-
phons or pumps should be employed to achieve from 2 to 4 doses per day.
Filter flooding to approximately 2 in. (5 cm) should be achieved per
dose.
Multiple beds are desirable instead of one large filter unit. Allowance
should be made for 60-day resting periods for filters receiving septic
tank effluent.
6.3.9.3 Construction Features
Construction features for large intermittent sand filters are similar to
those of smaller units. Distribution and collection systems are normal-
ly more elaborate. Covering is desirable in very cold climates.
6.3.9.4 Operation and Maintenance
Day-to-day operation and maintenance of larger filter systems are mini-
mal. Sand surfaces should be raked and leveled on a weekly basis. Dis-
tribution troughs should be kept level; pumps or siphons and controls
must be periodically maintained. Unskilled manpower requirements of 10
to 15 man-hours per week may be expected for larger installations. Pow-
er requirements depend on dosing systems employed.
6.4 Aerobic Treatment Units
6.4.1 Introduction
Biological wastewater treatment processes are employed to transform dis-
solved and colloidal pollutants into gases, cell material, and metabolic
end products. These processes may occur in the presence or absence of
oxygen. In the absence of oxygen (anaerobic process), wastewater mate-
rials may be hydrolyzed and the resultant products fermented to produce
a variety of alcohols, organic acids, other reduced end products, syn-
thesized cell mass, and gases including carbon dioxide, hydrogen, and
methane. Further treatment of the effluents from anaerobic processes is
140
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normally required in order to achieve an acceptable quality for surface
discharge. On the other hand, aerobic processes will generate high-
quality effluents containing a variety of oxidized end products, carbon
dioxide, and metabolized biomass. Figure 6-10 summarizes the basic dif-
ferences in these processes.
Biological wastewater treatment is normally carried out in an open cul-
ture whereby a great variety of microorganisms exist symbiotically. The
system is, therefore, very versatile in carrying out a variety of bio-
chemical reactions in response to variations in input pollutants as well
as other environmental factors.
An important feature of biological processes is the synthesis and sub-
sequent separation of microbial cells from the treated liquid. In con-
ventional aerobic processes, new biological growth may be expected to
range from 30 to 60% of the dry weight of organic matter added to the
system. As the residence time of microbial cells in the system in-
creases, the net cell synthesis decreases, but never reaches zero due to
the presence of a certain amount of inert material in the influent
wastewater as well as nondegradable solids synthesized by the microbes.
It is necessary, therefore, to waste these solids as they build up
within the system. Yet, it is equally as important to maintain within
the system an active population of microbes to carry out the desired
biochemical reactions.
Aerobic biological treatment processes can be employed onsite to remove
substantial amounts of BOD and suspended solids that are not removed by
simple sedimentation. A secondary feature of the process is nitrifica-
tion of ammonia in the waste (under appropriate conditions) and the
significant reduction of pathogenic organisms.
Despite their advantageous treatment capabilities, aerobic units for on-
site treatment are susceptible to upsets. Without regular supervision
and maintenance, the aerobic unit may produce low-quality effluents. To
avoid the problems associated with operation and maintenance, some manu-
facturers have incorporated various features into the design of these
package units in order to reduce the need for frequent surveillance.
At least two process schemes are commercially available today for onsite
application. These are: (1) suspended growth and (2) fixed growth.
Each system has its own unique operational characteristics and design
features, but all provide oxygen transfer to the wastewater, intimate
contact between the microbes and the waste, and solids separation and
removal.
141
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FIGURE 6-10
AEROBIC AND ANAEROBIC DECOMPOSITION PRODUCTS
CO
CELLS
NONDEGRADABLE
(ORGANICS, INORGANICS)
NONDEGRADABLE
CELL MASS
142
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Anaerobic biological treatment processes may also be employed for onsite
wastewater treatment. Septic tanks provide anaerobic treatment as dis-
cussed more fully in Section 6.2. Septic tank designs, however, do not
normally incorporate considerations to optimize anaerobic decomposition.
Anoxic denitrification of nitrified wastewaters may also be practiced
onsite. Details of this process are described in Section 6.6. Anaero-
bic packed beds have been proposed for onsite treatment of wastewaters
(29), but there has been no long-term field experience with these
processes.
6.4.2 Suspended Growth Systems - Extended Aeration
6.4.2.1 Description
Extended aeration is a modification of the activated sludge process
whereby a high concentration of microorganisms are maintained in an
aeration tank, followed by separation and recycle of all or a portion of
the biomass back to the aeration tank. There are a variety of proprie-
tary extended aeration package plants available on the market today for
onsite application. Figure 6-11 depicts two typical package extended
aeration systems. The process may be operated in a batch or continuous
flow mode, and oxygen is supplied by either diffused or mechanical
aeration. Positive biomass return to the aeration tank is normally
employed, but wasting of excess solids varies widely between manufac-
tured units.
6.4.2.2 Applicability
Extended aeration processes are necessarily more complex than septic
tanks, and require regular operation and maintenance. The plants may be
buried or housed onsite, but must be readily accessible. The aeration
system requires power, and some noise and odor may be associated with
it. There are no significant physical site conditions that limit its
application, although local codes may require certain set-back dis-
tances. The process is temperature-dependent, and should be insulated
and covered as climate dictates.
6.4.2.3 Factors Affecting Performance
In extended aeration package plants, long hydraulic and solids retention
times (SRT) are maintained to ensure a high degree of treatment at mini-
mum operational control, to hedge against hydraulic or organic overload
to the system, and to reduce net sludge production (20). Since wasting
of accumulated solids is often not routinely practiced in many of these
143
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FIGURE 6-11
EXAMPLES OF EXTENDED AERATION PACKAGE PLANT CONFIGURATIONS
Batch - Extended Aeration
Influent
Pump Shut-off
Elevation
Blower
High Water
Alarm
• , !
« « i
o « A
.Effluent
•'r) >
Diffuser
-Pump
Influent
Mechanical or
Diffused Aeration
Settling
Chamber
Sludge
Flow-Th'rough Extended Aeration
144
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units, SRT increases to a point where the clarifier can no longer handle
the solids, which will be uncontrollably wasted in the effluent. Treat-
ment performance (including nitrification) normally improves with in-
creasing hydraulic retention time and SRT to a point where excessive
solids build-up will result in high suspended solids washout. This is
one of the biggest operational problems with extended aeration units,
and is often the reason for reports of poor performance.
Dissolved oxygen concentrations in the aeration tank should exceed
2 mg/1 in order to insure a high degree of treatment and a good settling
sludge. Normally, onsite extended aeration plants supply an excess of
dissolved oxygen due to minimum size restrictions on blower motors or
mechanical drives. An important element of most aeration systems is the
mixing provided by the aeration process. Most package units provide
sufficient mixing to ensure good suspension of solids and mass transfer
of nutrients and oxygen to the microbes.
Wastewater characteristics may also influence performance of the pro-
cess. Excess amounts of certain cleaning agents, greases, floating
matter, and other detritus can cause process upsets and equipment
malfunctions. Process efficiency may be affected by temperature,
generally improving with increasing temperature.
The clarifier is an important part of the process. If the biomass can-
not be properly separated from the treated effluent, the process has
failed. Clarifier performance depends upon the settleability of the
biomass, the hydraulic overflow rate, and the solids loading rate. Hy-
draulic surges can result in serious clarifier malfunctions. As men-
tioned previously, high solids loadings caused by accumulation of mixed
liquor solids result in eventual solids carryover. Excessively long re-
tention times for settled sludges in the clarifier may result in gasi-
fication and flotation of these sludges. Scum and floatable material
not properly removed from the clarifier surface will greatly impair
effluent quality as well.
The field performance of onsite extended aeration package systems is
summarized in Table 6-13. Results presented in this summary indicate
that performance is variable due to the wide diversity of factors that
can adversely affect extended aeration systems. Shock loads, sludge
bulking, homeowner abuse or neglect, and mechanical malfunctions are
among the most common reasons for poor performance. In general, the
uncontrolled loss of solids from the system is the major cause of
effluent deterioration.
Generally, extended aeration plants produce a high degree of nitrifica-
tion since hydraulic and solids retention times are high. Reductions of
145
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TABLE 6-13
SUMMARY OF EFFLUENT DATA FROM VARIOUS AEROBIC UNIT FIELD STUDIES
Source
Parameter
BOD5
Mean, mg/1
Range, mg/1
No. of Samples
Suspended Solids
Mean, mg/1
Range, mg/1
No. of Sampl es
Ref. (2)
37
<1-208
112
39
3-252
117
Ref. (30)
37
1-235
167
62
1-510
167
Ref. (31)
47
10-280
86
94
18-692
74
Ref. (32)
92
-
146
94
-
146
Ref. (33)
144
10-824
393
122
17-768
251
Ref. (34)
31
9-80
10
49
5-164
10
Ref. (35)
36
3-170
124
57
4-366
132
en
-------
phosphorus are normally less than 25%. The removal of indicator bac-
teria in onsite extended aeration processes is highly variable and not
well documented. Reported values of fecal coliforms appear to be about
2 orders of magnitude lower in extended aeration effluents than in
septic tank effluents (2).
6.4.2.4 Design
A discussion of some of the important features of onsite extended aera-
tion package plants in light of current operational experience is pre-
sented below.
a. Configuration
Most extended aeration package plants designed for individual home ap-
plication range in capacity from 600 to 1,500 gal (2,270 to 5,680 1),
which includes the aeration compartment, settling chamber, and in some
units, a pretreatment compartment. Based upon average flows from house-
holds, this volume will provide total hydraulic retention times of
several days.
b. Pretreatment
Some aerobic units provide a pretreatment step to remove gross solids
(grease, trash, garbage grindings, etc.). Pretreatment devices include
trash traps, septic tanks, comminutors, and aerated surge chambers. The
use of a trash trap or septic tank preceding the extended aeration pro-
cess reduces problems with floating'debris in the final clarifier, clog-
ging of flow lines, and plugging of pumps.
c. Flow Mode
Aerobic package plants may be designed as continuous flow or batch flow
systems. The simplest continuous flow units provide no flow equaliza-
tion and depend upon aeration tank volume and/or baffles to reduce the
impact of hydraulic surges. Some units employ more sophisticated flow
dampening devices, including air lift or float-controlled mechanical
pumps to transfer the wastewater from aeration tank to clarifier. Still
other units provide multiple-chambered tanks to attenuate flow. The
batch (fill and draw) flow system eliminates the problem of hydraulic
variation. This unit collects and treats the wastewater over a period
of time (usually one day), then discharges the settled effluent by pump-
ing at the end of the cycle.
147
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d. Method of Aeration
Oxygen is transferred to the mixed liquor by means of diffused air,
sparged turbine, or surface entrainment devices. When diffused air sys-
tems are employed, low head blowers or compressors are used to force the
air through the diffusers placed on the bottom of the tank. The sparged
turbine employs both a diffused air source and external mixing, usually
by means of a submerged flat-bladed turbine. The sparged turbine is
more complex than the simple diffused air system. There are a variety
of mechanical aeration devices employed in package plants to aerate and
mix the wastewater. Air is entrained and circulated within the mixed
liquor through violent agitation from mixing or pumping action.
Oxygen transfer efficiencies for these small package plants are normally
low (0.2 to 1.0 Ib Oo/hp hr) (3.4 to 16.9 kg 02/MJ) as compared with
large-scale systems due primarily to the high power inputs to the
smaller units (constrained by minimum motor sizes for these relatively
small aeration tanks) (2). Normally, there is sufficient oxygen trans-
ferred to produce high oxygen levels. In an attempt to reduce power
requirements or to enhance nitrogen removal, some units employ cycled
aeration periods. Care must be taken to avoid the development of poor
settling biomass when cycled aeration is used.
Mixing of the aeration tank contents is also an important consideration
in the design of oxygen transfer devices. Rule of thumb requirements
for mixing in aeration tanks range from 0.5 to 1 hp/1,000 ft3 (13 to
26 kw/1,000 nr) depending upon reactor geometry. Commercially available
package units jire reported to deliver mixing inputs rangi-ng from 0.2 to
3 hp/1,000 ft3 (5 to 79 kw/1,000 m3) (2). Deposition problems may
develop in those units with the lower mixing intensities.
e. Biomass Separation
The clarifier is critical to the successful performance of the extended
aeration process. A majority of the commercially available package
plants provide simple gravity separation. Weir and baffle designs have
not been given much attention in package units. Weir lengths af at
least 12 in. (30 cm) are preferred (10,000 gpd/ft at 7 gpm) (127 nr/d/m
at 0.4 I/sec) and sludge deflection baffles should be included as a part
of the outlet design. The use of gas deflection barriers is a simple
way to keep floating solids away from the weir area.
Upflow clarifier devices have also been employed to improve separation.
Hydraulic surges must be avoided in these systems. Filtration devices
148
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have also been employed in some units. While filters may produce high-
quality effluent, they are very susceptible to both internal and
external clogging. The behavior of clarifiers is dependent upon biomass
settling properties, solids loading rate, and hydraulic overflow rates.
Design peak hydraulic overflow rates should be less than 800 gpd ft2 (32
nr/d/m2); and at average flow design values normally range from 200 to
400 gpd/ftz (8 to 16 nr/d/nr). Solids loading rates are usually less
than 30 lb/ftz/d (145 kg/mz/d) based upon average flow and less than 50
lb/ftz/d (242 kg/mz/d) based upon peak flows.
f. Biomass Return
Once separated from the treated wastewater, the biomass must be returned
to the aeration tank or be wasted. Air lift pumps, draft tubes working
off the aerator, and gravity return methods are normally used. Batch
units and plants that employ filters do not require sludge return.
Rapid removal of solids from the clarifier is desirable to avoid deni-
trification and subsequent floatation of solids. Positive sludge return
should be employed in package plants since the use of gravity return
systems has generally proved ineffective (2)(20).
Removal of floating solids from clarifiers has normally been ignored in
most onsite package plant designs. Since this material results in
serious deterioration of the effluent, efforts should be made to provide
for positive removal of this residue. Reliance on the owner to remove
floating scum is unrealistic.
g. Biomass Wasting
Most onsite package plants do not provide for routine wasting of solids
from the unit. Some systems, however, do employ an additional chamber
for aerobic digestion of wasted sludge. Wasting is normally a manual
operation whereby the operator checks mixed liquor solids and wasted
sludge when mixed liquor concentrations exceed a selected value. In
general, wasting should be provided once every 8 to 12 months (2)(35).
h. Controls and Alarms
Most aerobic units are supplied with some type of alarm and control sys-
tem to detect mechanical breakdown and to control the operation of elec-
trical components. They do not normally include devices to detect
effluent quality or biomass deterioration. Since the control systems
149
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contain electrical components, they are subject to corrosion. All elec-
trical components should be waterproofed and regularly serviced to
ensure their continued operation.
6.4.2.5 Additional Construction Features
Typical onsite extended aeration package plants are constructed of
noncorrosive materials, including reinforced plastics and fiberglass,
coated steel, and reinforced concrete. The unit may be buried provided
that there is easy access to all mechanical parts and electrical control
systems, as well as appurtenances requiring maintenance such as weirs,
air lift pump lines, etc. Units may also be installed above ground, but
should be properly housed to protect against severe climatic conditions.
Installation of the units should be in accordance with specifications of
the manufacturers.
Appurtenances for the plant should be constructed of corrosion-free
materials including polyethylene plastics. Air diffuser support legs
are normally constructed from galvanized iron or equivalent.
Large-diameter air lift units should be employed to avoid clogging
problems. Mechanical units should be properly waterproofed and/or
housed from the elements.
Since blowers, pumps, and other prime movers are abused by severe envi-
ronment, receive little attention, and are often subject to continuous
operation, they should be designed for heavy duty use. They should be
easily accessible for routine maintenance and tied into an effective
alarm system.
6.4.2.6 Operation and Maintenance
a. General Plant Operation
Typical operating parameters for onsite extended aeration systems are
presented in Table 6-14. The activated sludge process can be operated
by controlling only a few parameters - the aeration tank dissolved oxy-
gen, the return sludge rate, and the sludge wasting rate. For onsite
package plants, these control techniques are normally fixed by mechani-
cal limitations so that very little operational control is required.
Dissolved oxygen is normally high and cannot be practically controlled
except by "on or off" operation. Experimentation with the process may
dictate a desirable cycling arrangement employing a simple time clock
150
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TABLE 6-14
TYPICAL OPERATING PARAMETERS FOR ONSITE EXTENDED AERATION SYSTEMS
Parameter9
MLSS, mg/1
F/M, Ib BOD/d/lb MLSS
Solids Retention Time, days
Hydraulic Retention Time, days
Dissolved Oxygen, mg/1
Mixing, hp/1,000 ft3
Clarifier Overflow Rate, gpd/ft2
Clarifier Solids Loading, Ib/d/ft2
Clarifier Weir Loading, gpd/ft2
Sludge Wasting, months
Average
2,000-6,000
0.05 - 0.1
20-100
2-5
>2.0
0.5-1.0
200-400
20-30
10,000-30,000
8-12
Maximum
8,000
800
50
30,000
a Pretreatment: Trash trap or septic tank.
Sludge Return and Scum Removal: Positive.
151
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control that results in power savings and may also achieve some nitrogen
removal (Section 6.6).
The return sludge rate is normally fixed by pumping capacity and pipe
arrangements. Return sludge pumping rates often range from 50 to 200%
of forward flow. They should be high enough to reduce sludge retention
times in the clarifier to a minimum (less than 1 hr), yet low enough to
discourage pumping of excessive amounts of water with the sludge. Time
clock controls may be used to regulate return pumping.
Sludge wasting is manually accomplished in most package plants. Through
experience, the operator knows when mixed liquor solids concentrations
become excessive, resulting in excessive clarifier loading. Usually 8-
to 12-month intervals between wasting is satisfactory, but this varies
with plant design and wastewater characteristic. Wasting is normally
accomplished by pumping mixed liquor directly from the aeration tank.
Wasting of approximately 75% of the aeration tank volume is usually
satisfactory. Wasted sludge must be handled properly (see Chapter 9).
b. Start-Up
Prior to actual start-up, a dry checkout should be performed to insure
proper installation. Seeding of the plant with bacterial cultures is
not required as they will develop within a 6- to 12-week period. Ini-
tially, large amounts of white foam may develop, but will subside as
mixed liquor solids increase. During start-up, it is advisable to re-
turn sludge at a high rate. Intensive surveillance- by qualified
maintenance personnel is desirable during the first month of start-up.
c. Routine Operation and Maintenance
Table 6-15 itemizes suggested routine maintenance performance for onsite
extended aeration package plants. The process is labor-intensive and
requires semi-skilled personnel. Based upon field experience with these
units, 12 to 48 man-hr per yr plus analytical services are required to
insure reasonable performance. Power requirements are variable, but
range between 2.5 to 10 kWh/day.
d. Operational Problems
Table 6-16 outlines an abbreviated listing of operational problems and
suggested remedies for them. A detailed discussion of these problems
152
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TABLE 6-15
SUGGESTED MAINTENANCE FOR ONSITE
EXTENDED AERATION PACKAGE PLANTS3
Item
Suggested Maintenance
Aeration Tank
Aeration System
Diffused air
Mechanical
Clarifier
Trash Trap
Controls
Sludge Wasting
Analytical
Check for foaming and uneven air distribution.
Check air filters, seals, oil level, back pressure;
perform manufacturer's required maintenance.
Check for vibration and overheating; check oil
level, seals; perform manufacturer's required
maintenance.
Check for floating scum; check effluent appearance;
clean weirs; hose down sidewalls and appurtenance;
check sludge return flow rate and adjust time
sequence if required; locate sludge blanket;
service mechanical equipment as required by
manufacturer.
Check for accumualted solids; hose down sidewalls.
Check out functions of all controls and alarms;
check electrical control box.
Pump waste solids as required.
Measure aeration tank grab sample for DO, MLSS, pH,
settleability, temperature; measure final effluent
composite sample for BOD, SS, pH (N and P if
required).
a Maintenance activities should be performed about once per month.
153
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TABLE 6-16
OPERATIONAL PROBLEMS—EXTENDED AERATION PACKAGE PLANTS
Observation
Cause
Remedy
Excessive local
turbulence in
aeration tank
White thick billowy
foam on aeration
tank
Thick scummy dark
tan foam on
aeration tank
Dark brown/black foam
and mixed liquor in
aeration tank
Billowing sludge
washout in
clarifier
Clumps of rising
sludge in clarifier
Fine dispersed floe
over weir, turbid
effluent
Diffuser plugging
Pipe breakage
Excessive aeration
Insufficient MLSS
High MLSS
Anaerobic conditions
Aerator failure
Hydraulic or solids
overload
Bulking sludge
De nitrification
Septic conditions
in clarifier
Turbulence in
aeration tank
Sludge age too
high
Remove and clean
Replace as required
Throttle blower
Avoid wasting solids
Waste solids
Check aeration system,
aeration tank D.O.
Waste sludge; check
flow to unit
See reference (37)
Increase sludge return
rate to decrease
sludge retention time
in clarifier
Increase return rate
Reduce power input
Waste sludge
154
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for larger, centralized systems can be found in the "Manual of Practice
- Operation of Wastewater Treatment Plants" (36) and "Process Control
Manual for Aerobic Biological Wastewater Treatment Facilities" (37).
Major mechanical maintenance problems for onsite treatment units are
with blower or mechanical aerator failure, pump and pipe clogging,
electrical motor failure, corrosion and/or failure of controls, and
electrical malfunctions (35). Careful attention to a maintenance
schedule will reduce these problems to a minimum, and will also
alleviate operational problems due to the biological process upset.
Emphasis should be placed on adequate maintenance checks during the
first 2 or 3 months of operation.
6.4.2.7 Considerations for Multi-Home Application
The extended aeration process may be well suited for multiple-home or
commercial applications. The same requirements listed for single onsite
systems generally apply to the larger scale systems (20)(36)(37)(38).
However, larger package plant systems may be more complex and require a
greater degree of operator attention.
6.4.3 Fixed Film Systems
6.4.3.1 Description
Fixed film systems employ an inert media to which microorganisms may
become attached. The wastewater comes in contact with this fixed film
of microorganisms either by pumping the water past the media or by mov-
ing the media past the wastewater to be treated. Oxygen may be supplied
by natural ventilation or by mechanical or diffused aeration within the
wastewater. Fixed film reactors are normally constructed as packed tow-
ers or as rotating plates. Figure 6-12 depicts three types of onsite
fixed film systems - the trickling filter (gravity flow of wastewater
downward), the upflow filter (wastewater pumped upward through the
media), and the rotating biological contractor.
The trickling filter has been used to treat wastewater for many years.
Modern filters today consist of towers of media constructed from a vari-
ety of plastics, stone, or redwood laths into a number of shapes (honey-
comb blocks, rings, cylinders, etc.). Wastewater is distributed over
the surface of the media and collected at the bottom through an undrain
system. Oxygen is normally transferred by natural drafting, although
some units employ blowers. Treated effluent is settled prior to being
discharged or partially recycled back through the filter.
155
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FIGURE 6-12
EXAMPLES OF FIXED FILM PACKAGE PLANT CONFIGURATIONS
Motor
ROTATING
BIOLOGICAL CONTACTOR
UPFLOW FILTER
Effluent
TRICKLING F1LTFR
Influent Distributor
J'
*"•»"**"*» ' »"
Vr Under Drain 'v/a
Fixed
Media
r
h Pump
TJ __
I v I
— j"»-tt
V
^ ^
I^^^^^^illil^iCS'udge 1
Clarifier
Timer
Control
Valve
Influent
Septic
Tank
•Effluent to Clarifier or Septic Tank
156
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In an upflow filter, wastewater flows through the media and is
subsequently collected at an overflow weir. Oxygen may be transferred
to the biomass by means of diffusers located at the bottom of the tower
or by surface entrainment devices at the top. One of the commercially
available units of this type (built primarily for shipboard use) does
not require effluent sedimentation prior to discharge (Figure 6-12).
Circulation of wastewater through this particular unit promotes the
shear of biomass from the media and subsequent carriage to the tank
bottom.
The rotating biological contactor (RBC) employs a series of rotating
discs mounted on a horizontal shaft. The partially submerged discs
rotate at rates of 1 to 2 rpm through the wastewater. Oxygen is
transferred to the biomass as the disc rotates from the air to the water
phase. Recirculation of effluent is not normally practiced.
6.4.3.2 Applicability
There has been little long-term field experience with onsite fixed film
systems. Generally, they are less complex than extended aeration sys-
tems and should require less attention; if designed properly they should
produce an effluent of equivalent quality.
There are no significant physical site constraints that should limit
their application, although local codes may require certain set-back
distances. The process is more temperature sensitive than extended
aeration and should be insulated as required. Rotating biological
contactors should also be protected from sunlight to avoid excessive
growth of algae which may overgrow the plate surfaces.
6.4.3.3 Factors Affecting Performance
Limited data are currently available on long-term performance of onsite
fixed film systems. Detailed description of process variables that
affect fixed film process perfomrance appear in the "Manual of Practice
for Wastewater Treatment Plant Design" (20). Low loaded filters should
also achieve substantial nitrification, as well as good BOD and SS
reductions.
6.4.3.4 Design
Onsite fixed film systems include a variety of proprietary devices.
Design guidelines are, therefore, difficult to prescribe. Table 6-17
157
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presents suggested design ranges for two generic fixed film systems, the
RBC and the fixed media processes.
TABLE 6-17
TYPICAL OPERATING PARAMETERS FOR ONSITE FIXED FILM SYSTEMS
Parameter9 Fixed Media RBC
Hydraulic Loading, gpd/ft2 25-100 0.75-1.0
Organic Loading, Ib BOD/d/1000 ft3 5-20 1.0-1.5
Dissolved Oxygen, mg/1 >2.0 >2.0
Overflow Rate, gpd/ft2 600-800 600-800
Weir Loading, gpd/ft2 10,000-20,000 10,000-20,000
Sludge Wasting, months 8-12 8-12
a Pretreatment: Settling or screening.
Recirculation: Not required.
All fixed film systems should be preceded by settling and/or screening
to remove materials that will otherwise cause process malfunction.
Hydraulic loadings are normally constrained by biological reaction rates
and mass transfer.
Organic loading is primarily dictated by oxygen transfer within the bio-
logical film. Excessive organic loads may cause anaerobic conditions
resulting in odor and poor performance. Dissolved oxygen in the liquid
should be at least 2 mg/1. Recirculation is not normally practiced in
package fixed film systems since it adds to the degree of complexity and
is energy and maintenance intensive. However, recirculation may be
desirable in certain applications where minimum wetting rates are
required for optimal performance.
The production of biomass on fixed film systems is similar to that for
extended aeration. Very often, accumulated sludge is directed back to
the septic tank for storage and partial digestion.
158
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6.4.3.5 Construction Features
Very few commercially produced fixed film systems are currently avail-
able for onsite application. Figure 6-12 illustrates several flow ar-
rangements that have been employed. Specific construction details are
dependent on system characteristics. In general, synthetic packing or
attachment media are preferred over naturally occurring materials be-
cause they are lighter, more durable, and provide better void volume -
surface area characteristics. All fixed film systems should be covered
and insulated as required against severe weather. Units may be in-
stalled at or below grade depending upon site topography and other adja-
cent treatment processes. Access to all moving parts and controls is
required, and proper venting of the unit is paramount, especially if
natural ventilation is being used to supply oxygen. Underdrains, where
required, should be accessible and designed to provide sufficient air
space during maximum hydraulic loads. Clarification equipment should
include positive sludge and scum handling. All pumps, blowers, and
aeration devices, if required, should be rugged, corrosion-resistant,
and built for continuous duty.
6.4.3.6 Operation and Maintenance
a. General Process Operation
Fixed film systems for onsite application normally require very minimal
operation. Rotating biological contactors are installed at fixed rota-
tional speed and submergence. Flow to these units is normally fixed
through the use of an integrated pumping system. Sludge wasting is nor-
mally controlled by a timer setting. Through experience, the operator
may determine when clarifier sludge should be discharged in order to
avoid sludge flotation (denitrification) or excessive build-up.
Where aeration is provided, it is normally designed for continuous duty.
On-off cycling of aeration equipment may be practiced for energy conser-
vation if shown not to cause a deterioration of effluent quality.
b. Routine Operation and Maintenance
Table 6-18 itemizes suggested routine maintenance performance for onsite
fixed film systems. The process is less labor-intensive than extended
aeration systems and requires semi-skilled personnel. Based upon very
limited field experience with these units, 8 to 12 man-hr per yr plus
159
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TABLE 6-18
SUGGESTED MAINTENANCE FOR ONSITE
FIXED FILM PACKAGE PLANTS*
Item
Suggested Maintenance
Media Tank
Aeration System
RBC Unit
Clarifier
Trash Trap
Controls
Analytical
Check media for debris accumulation, ponding,
and excessive biomass - clean as required;
check underdrains - clean as required; hose
down sidewalls and appurtenances; check
effluent distribution and pumping - clean as
required.
See Table 6-15
Lubricate motors and bearings; replace seals as
required by manufacturer.
See Table 6-15
See Table 6-15
See Table 6-15
Measure final effluent composite sample for
BOD, SS, pH (N and P if required).
a Maintenance activities should be performed about once per month.
160
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analytical services are required to ensure adequate performance. Power
requirements depend upon the device employed, but may range from 1 to
4 kWh/day.
c. Operational Problems
Table 6-19 outlines an abbreviated list of potential operational
problems and suggested remedies for onsite fixed film systems. A
detailed discussion of these may be found in the "Manual of Practice -
Operation of Wastewater Treatment Plants" (36) and "Process Control
Manual for Aerobic Biological Wastewater Treatment Facilities" (37).
6.4.3.7 Considerations for Multi-Home Applications
Fixed film systems may be well suited for multiple-home or commercial
applications. The same requirements for single-home onsite systems
apply to the large-scale systems (20)(29)(37)(38). However, larger
systems may be more complex and require a greater degree of operator
attention.
6.5 Disinfection
6.5.1 Introduction
Disinfection of wastewaters is employed to destroy pathogenic organisms
in the wastewater stream. Since disposal of wastewater to surface water
may result in potential contacts between individuals and the wastewater,
disinfection processes to reduce the risk of infection should be
considered.
There are a number of important waterborne pathogens found in the United
States (39)(40)(41)(42). Within this group of pathogens, the protozoan
cyst is generally most resistant to disinfection processes, followed by
the virus and, the vegetative bacteria (43). The design of the disin-
fection process must necessarily provide effective control of the most
resistant pathogen likely to be present in the wastewater treated. Up-
stream processes may effectively reduce some of these pathogens, but
data are scant on the magnitude of this reduction for most pathogens.
Currently, the effectiveness of disinfection is measured by the use of
indicator bacteria (total or fecal coliform) or disinfectant residual
where applicable. Unfortunately, neither method guarantees complete
destruction of the pathogen, and conservative values are often selected
to hedge against this risk.
161
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TABLE 6-19
OPERATIONAL PROBLEMS—FIXED FILM PACKAGE PLANTS
Observation
Cause
Remedy
Filter Ponding
Filter Flies
Odors
Freezing
Excessive Biomass
Accumulation
Poor Clarification
Media too fine
Organic overload
Debri s
Poor wastewater
distribution
Poor ventilation/
aeration
Improper
insultati on
Organic overload
Low pH; anaerobic
conditions
Denitrifi cation
in clarifier
Hydraulic overload
Replace media
Flush surface with high pressure
stream; increase recycle rate;
dose with chlorine (10-20 mg/1
for 4 hours)
Remove debris; provide
pretreatment
Provide complete wetting of
media; increase recycle rate;
chlorinate (5 mg/1 for 6 hours
at 1 to 2 week intervals)
Check underdrains; maintain
aeration equipment, if
employed; insure adequate
ventilation; increase recycle
Check and provide sufficient
insulation
Increase recycle; flush surface
with high pressure stream;
dose with chlorine; increase
surface area (RBC)
Check venting; preaerate
wastewater
Remove sludge more often
Reduce recycle; provide flow
buffering
162
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Table 6-20 presents a listing of potential disinfectants for onsite
application. Selection of the best disinfectant is dependent upon the
characteristics of the disinfectant, the characteristics of the
wastewater and the treatment processes preceding disinfection. The most
important disinfectants for onsite application are chlorine, iodine,
ozone, and ultraviolet light, since more is known about these
disinfectants and equipment is available for their application.
TABLE 6-20
SELECTED POTENTIAL DISINFECTANTS FOR ONSITE APPLICATION
Disinfectant
Formula
Form Used
Equipment
Sodium Hypochlorite NaOCl
Cal ci um Hypochl ori te Ca (OC1);
Elemental Iodine 12
Ozone 03
Ultraviolet Light
Li qui d
Tablet
Crystals
Gas
Electromagnetic
Radiation
Metering Pump
Tablet Contactor
Crystal/Liquid
Contactor
Generator, Gas/
Liquid Contactor
Thin Film
Radiation
Contactor
Disinfection processes for onsite disposal must necessarily be simple
and safe to operate, reliable, and economical. They normally are the
terminal process in the treatment flow sheet.
6.5.2 The Halogens - Chlorine and Iodine
6.5.2.1 Description
Chlorine and iodine are powerful oxidizing agents capable of oxidizing
organic matter, including organisms, at rapid rates in relatively low
concentrations. Some of the characteristics of these halogens appear in
Table 6-21 (20)(44)(45).
163
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TABLE 6-21
HALOGEN PROPERTIES (27)
Sodi urn
Hypochlorite
Form
Liquid
Calcium Tablet
Hypochlorite (115 gm)
Iodine
Crystal s
Commercial
Strength
Available
12 - 15
70
100
Sped fi c
Gravity
1.14 - 1.17
4.93
Handling
Materials
Ceramic, Glass,
Plastic,
Rubber
Glass, Wood,
Fiberglass,
Rubber
Fiberglass, Some
Plastics
Characteristics
Deteriorates rapidly at
high temperatures, in
sunlight, and at high
concentrations.
Deteriorates at
3-5%/year
Stable in water;
solubility:
10° - 200 mg/1
20° - 290 mg/1
30° - 400 mq/1
Chlorine may be added to wastewater as a gas, Clp- However, because the
gas can represent a safety hazard and is highly corrosive, chlorine
would normally be administered as a solid or liquid for onsite applica-
tions. Addition of either sodium or calcium hypochlorite to wastewater
results in an increase in pH and produces the chlorine compounds hypo-
chlorous acid, HOC!, and hypochlorite ion, OC1", which are designated as
"free" chlorine. In wastewaters containing reduced compounds such as
sulfide, ferrous iron, organic matter, and ammonia, the free chlorine
rapidly reacts in nonspecific side reactions with the reduced compounds,
producing chloramines, a variety of chloro-organics, and chloride. Free
chlorine is the most powerful disinfectant, while chloride has virtually
no disinfectant capabilities. The other chloro-compounds, often called
combined chlorine, demonstrate disinfectant properties that range from
moderate to weak. Measurement of "chlorine residual" detects all of
these forms except chloride. The difference between the chlorine dose
and the residual, called "chlorine demand," represents the consumption
of chlorine by reduced materials in the wastewater (Table 6-22). Thus,
in disinfection system design, it is the chlorine residual (free and
combined) that is of importance in destroying pathogens.
164
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TABLE 6-22
CHLORINE DEMAND OF SELECTED DOMESTIC WASTEWATERS3
Chlorine
Wastewater Demand
Raw fresh wastewater 8-15
Septic tank effluent 30 - 45
Package biological treatment plant effluent 10 - 25
Sand-filtered effluent 1-5
a Estimated concentration of chlorine consumed in nonspecific
side reactions with 15-minute contact time.
Iodine is normally used in the elemental crystalline form, Io, for water
and wastewater disinfection. Iodine hydrolyzes in water to form the hy-
poiodus forms, HIO and 10", and iodate, I03. Normally, the predominant
disinfectant species in water are I2, HIO, and 10", as little I03 will
be found at normal wastewater pH values (less than pH 8.0). Iodine does
not appear to react very rapidly with organic compounds or ammonia in
wastewaters. As with chlorine, however, most wastewaters will exhibit
an iodine demand due to nonspecific side reactions. The reduced form of
iodine, iodide, which is not an effective disinfectant, is not detected
by iodine residual analyses.
6.5.2.2 Applicability
The halogens are probably the most practical disinfectants for use in
onsite wastewater treatment applications. They are effective against
waterborne pathogens, reliable, easy to apply, and are readily avail-
able.
165
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The use of chlorine as a disinfectant may result in the production of
chlorinated by-products which may be toxic to aquatic life. No toxic
by-products have been identified for iodine at this time.
6.5.2.3 Performance
The performance of halogen disinfectants is dependent upon halogen re-
sidual concentration and contact time, wastewater characteristics, na-
ture of the specific pathogen, and wastewater temperature (20). Waste-
water characteristics may effect the selection of the halogen as well as
the required dosage due to the nonspecific side reactions that occur
(halogen demand). Chlorine demands for various wastewaters are pre-
sented in Table 6-22. The demand of wastewaters for iodine is less
clear. Some investigators have reported iodine demands 7 to 10 times
higher than those for chlorine in wastewaters (46)(47) while others
indicate that iodine should be relatively inert to reduced compounds
when compared to chlorine (48). Design of halogen systems is normally
based upon dose-contact relationships since the goal of disinfection is
to achieve a desired level of pathogen destruction in a reasonable
length of time with the least amount of disinfectant. Because of the
nonspecific side reactions that occur, it is important to distinguish
between halogen dose and halogen residual after a given contact period
in evaluating the disinfection process.
Table 6-23 provides a summary of halogen residual-contact time informa-
tion for a variety of organisms (43). These are average values taken
from a number of studies and should be used with caution. Relationships
developed between disinfectant residual, contact time, and efficiency
are empirical. They may be linear for certain organisms, but are often
more complex. Thus, it is not necessarily true that doubling the con-
tact time will halve the halogens residual requirements for destruction
of certain pathogens. In the absence of sufficient data to make these
judgements, conservative values are normally employed for residual-dose
requirements.
The enteric bacteria are more sensitive to the halogens than cysts or
virus. Thus, the use of indicator organisms to judge effective disin-
fection must be cautiously employed.
Temperature effects also vary with pathogen and halogen, and the general
rule of thumb indicates that there should be a two to threefold decrease
in rate of kill for every 10° C decrease in temperature within the
limits of 5 to 30° C.
166
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TABLE 6-23
PERFORMANCE OF HALOGENS AND OZONE AT 25°C [After (43)]
Necessary Residual After 10 Min. to
Achieve 99.999% Destruction (tng/1)
Amoebi c Enteric Enteri c
Halogen Cysts Bacteria Virus
HOC1 (Predominates 3.5 0.02 0.4
@ pH <7.5)
OCL- (Predominates 40 1.5 100
6 pH >7.5)
NH2Cla 20 4 20
12 (Predominates 3.5 0.2 15
(? pH <7.0)
HIO/IO- (Predominates 7 0.05 0.5
6 8.0>pH>7.0)
03 0.3->1.8 0.2-0.3 0.2-0.3
a NHCl2:NH2Cl Efficiency = 3.5:1
6.5.2.4 Design Criteria
The design of disinfection processes requires the determination of the
wastewater characteristics, wastewater temperature, pathogen to be de-
stroyed, and disinfectant to be employed (20). From this information,
the required residual-concentration relationship may be developed and
disinfectant dose may be calculated.
Wastewater characteristics dictate both halogen demand and the species
of the disinfectant that predominates. In effluents from sand filters,
chlorine demands would be low and, depending upon pH, hypochlorous acid
or hypochlorite would prevail if chlorine is used. (The effluent would
be almost completely nitrified, leaving little ammonia available for
reaction). At pH values below 7.5, the more potent free chlorine form,
167
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HOC!, would predominate. It is clear from Table 6-23 that pH plays an
important role in the effectiveness of chlorine disinfection against
virus and cysts (10 to 300 fold differences).
The effect of temperature is often ignored except to ensure that conser-
vatively long contact times are selected for disinfection. Temperature
corrections are necessary for estimating iodine doses if a saturator is
employed, since the solubility of iodine in water decreases dramatically
with decreased temperature.
Design of onsite wastewater disinfection systems must result in conser-
vative dose-contact time values, since careful control of the process is
not feasible. Guidelines for chlorine and iodine disinfection for on-
site applications are presented in Table 6-24. These values are guide-
lines only, and more definitive analysis may be warranted in specific
cases.
TABLE 6-24
HALOGEN DOSAGE DESIGN GUIDELINES
Dose3
Septic Tank Package Biological Sand Filter
Disinfectant Eff1uent Process Effluent Effluent
mg/1 imjTI mg/1
Chiori ne
pH 6 35-50 15-30 2-10
pH 7 40-55 20-35 10-20
pH 8 50-65 30-45 20-35
Iodineb
pH 6-8 300-400 90-150 10-50
a Contact time = 1 hour at average flow and 20°C; increase
contact time to 2 hours at 10°C and 8 hours at 5°C for similar
efficiency.
b Based upon very small data base, assuming iodine demand from
3 to 7 times that of chlorine.
168
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The sizing of halogen feed systems is dependent upon the form of the
halogen used and the method of distribution. Sample calculations are
presented below.
Sample calculations:
Estimate of sodium hypochlorite dose - liquid feed
Halogen: NaOCl - trade strength 15% (150 g/1)
Dose required: 20 mg/1 available chlorine
Wastewater flow: 200 gpd average
1. Available chlorine =
(150 g/1) x (3.785 1/gal) x (1.0 lb/453.6 g) = 1.25 Ib/gal
2. Dose required =
(20 mg/1) x (3.785 1/gal) x (1 lb/453.6 g) x (10"3 g/mg)
= 1.67 x 10"4 Ib/gal
3. Dose required =
(1.67 x 10~4 Ib/gal) x (200 gal/d) = 3.34 x 10"2 Ib/d
4. NaOCl dose =
(3.34 x 10"2 Ib/d) + (1.25 Ib/gal) = 0.027 gal/day
Estimate of halogen design - tablet feed
Halogen: Ca(OCL)o tablet - 115 g; commercial strength 70%
Dose Required: 20 mg/1 available chlorine
Wastewater Flow: 200 gpd (750 1/d)
1. Available chlorine in tablet = 0.7 x 115(g) = 80.5 g/tablet
2. Dose required = 20 (mg/1) x 750 (1/d) = 15 g/d
3. Tablet consumption = QQ g^g/tablet) = °'19 tablets/day
or: 5.4 days/tablet
169
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6.5.2.5 Construction Features
a. Feed Systems
There are basically three types of halogen feed systems commercially
available for onsite application: stack or tablet feed systems, liquid
feed systems, and saturators. Tablet feed devices for Ca(OCl)2
tablets (Figure 6-13) are constructed of durable, corrosion-free plastic
or fiberglass, and are designed for in-line installation. Wastewater
flows past the tablets of Ca(OCl)2, dissolving them in proportion to
flow rate (depth of immersion). Tablets are added as requried upon
manual inspection of the unit. One commercial device provides 29-115
g/tablet per tube which would require refilling in approximately 155
days (5.4 days/tablet x 29).
Halogens may also be fed to the wastewater by an aspirator feeder or a
suction feeder. The aspirator feeder operates on a simple hydraulic
principle that employs the use of the vacuum created when water flows
either through a venturi tube or perpendicular to a nozzle. The vacuum
created draws the disinfection solution from a container into the disin-
fection unit, where it is mixed with wastewater passing through the
unit. The mixture is then injected into the main wastewater stream.
Suction feeders operate by pulling the disinfection solution from a
container by suction into the disinfection unit. The suction may be
created by either a pump or a siphon.
The storage reservoir containing the halogen should provide ample volume
for several weeks of operation. A 1-gal (4-1) storage tank would hold
sufficient 15% sodium hypochlorite solution for approximately 37 days
before refill (see sample computation). A 2-gal (8-1) holding tank
would supply 50 days of 10% sodium hypochlorite. A 15% sodium
hypochlorite solution would deteriorate to one-half its original
strength in 100 days at 25°C (49). The deterioration rate of sodium
hypochlorite decreases with decreased strength; therefore, a 10%
solution would decrease to one-half strength in about 220 days.
If liquid halogen is dispersed in this fashion, care must be taken to
select materials of construction that are corrosion-resistant. This
includes storage tanks, piping, and appurtenances as well as the pump.
Iodine is best applied to wastewater by means of a saturator whereby
crystals of iodine are dissolved in carriage water subsequent to being
pumped to a contact chamber (Figure 6-14). Saturators may be con-
structed or purchased commercially. The saturator consists of a tank of
fiberglass or other durable plastic containing a supporting base medium
170
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FIGURE 6-13
STACK FEED CHLORINATOR
Feed Tubes
Water Inlet
Water Outlet
^3(001)2
Tablets
171
-------
FIGURE 6-14
IODINE SATURATOR
Wastewater
Tank
I
Iodine Solution
Iodine Crystals
172
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and iodine crystals. Pretreated wastewater is split, and one stream is
fed to the saturator. The dissolution of iodine is dependent upon water
temperature, ranging from 200 to 400 mg/1 (Table 6-21). The iodine
solution from the saturator is subsequently blended with the wastewater
stream, which is discharged to a contact chamber. Depending upon
saturator size and dosage requirements, replenishment of iodine every 1
to 2 yr may be required (assumes a dosage of 50 mg/1 for 750 I/day using
a 0.2-cu-ft saturator).
b. Contact Basin
Successful disinfection depends upon the proper mixing and contact of
the disinfectant with the wastewater. If good mixing is achieved, a
contact time of 1 hour should be sufficient to achieve most onsite dis-
infection objectives when using doses presented in Table 6-24. Where
flows are low (e.g., under 1,000 gal per day) (3,785 1 per day), contact
basins may be plastic, fiberglass, or a length of concrete pipe placed
vertically and outfitted with a concrete base (Figure 6-15). A 48-in.
(122-cm) diameter concrete section would theoretically provide 6 hr of
wastewater detention for an average flow of 200 gal per day (757 1 per
day) if the water depth were only approximately 6 in. (15 cm). A 36-in.
(91-cm) diameter pipe section provides 6 hr detention at approximately
12 in. (30 cm) of water depth for the same flow. Therefore, substanti-
ally longer theoretical detention times than necessary for ideal mixing
conditions are provided using 36- or 48-in. (91- or 122-cm) diameter
pipe. This oversizing may be practically justified for onsite
applications with low flows, since good mixing may be difficult to
achieve.
Contact basins should be baffled in order to prevent serious short-
circuiting within the basin. One sample baffling arrangement is illus-
trated in Figure 6-15.
6.5.2.6 Operation and Maintenance
The disinfection system should be designed to minimize operation and
maintenance requirements, yet insure reliable treatment. Routine
operation and maintenance of premixed liquid solution feed equipment
consists of replacing chemicals, adjusting feed rates, and maintaining
the mechanical components. Tablet feed chlorination devices should
require less frequent attention, although recent experience indicates
that caking of hypochlorite tablets occurs due to the moisture in the
chamber. Caking may result in insufficient dosing of chlorine, but may
also produce excessive dosage due to cake deterioration and subsequent
spillage into the wastewater stream. Dissolution of chlorine may also
be erratic, requiring routine adjustment of tablet and liquid elevation
173
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(experience with some units indicates that dissolution rates actually
increase with decreased flow rates). Routine maintenance of iodine
saturators includes replacing chemicals, occasionally adjusting feed
rates, redistributing iodine crystals within the saturator, and
maintaining mechanical components.
FIGURE 6-15
SAMPLE CONTACT CHAMBER
Wastewater
with
Disinfectant
J
Concrete Pipe Section
Wooden Baffles
Process control is best achieved by periodic analysis of halogen resid-
uals in the contact chamber. The halogen residuals can be measured by
unskilled persons using a color comparator. Periodic bacteriological
analyses of treated effluents provide actual proof of efficiency.
Skilled technicians are required to sample and analyze for indicator
organisms or pathogens.
174
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It is estimated that tablet feed chlorinators could be operated with
approximately 6 unskilled man-hr per yr including monthly chlorine re-
sidual analyses. Iodine saturator systems and halogen liquid feed sys-
tems may require 6 to 10 semi-skilled man-hr per yr. Electrical power
consumption would be highly variable depending upon other process pump-
ing requirements, as well as the use of metering pumps and controls.
Chemical requirements will vary, but are estimated fo be about 5 to
25 I b (2 to 11 kg) of iodine and 5 to 15 I b (2 to 7 kg) of available
chlorine per yr for a family of four.
6.5.2.7 Other Considerations
In making a final decision on halogen disinfection, other considerations
must be included in addition to cost, system effectiveness, and relia-
bility. Without a dechlorination step, chlorine disinfection may be
ruled out administratively. Currently, there is no evidence that iodine
or its compounds are toxic to aquatic life.
6.5.3 Ultraviolet Irradiation
6.5.3.1 Description
The germicidal properties of ultraviolet (UV) irradiation have been rec-
ognized for many years (50)(51). UV irradiation has been used for the
disinfection of water supplies here and abroad, and currently finds
widest application for small water systems for homes, commercial estab-
lishments, aboard ship, and in some industrial water purification sys-
tems. The use of UV irradiation for wastewater disinfection has only
recently been seriously studied (52)(53).
Ultraviolet is germicidal in the wave length range of 2,300 to 3,000 A,
its greatest efficiency being at 2,540 A. Currently, high-intensity,
low-pressure mercury vapor lamps emit a major percentage of their energy
at this wave length, making them most efficient for use. The primary
mode of action of UV is the denaturation of nucleic acids, making it
especially effective against virus.
In order to be effective, UV energy must reach the organism to be
destroyed. Unfortunately, UV energy is rapidly absorbed in water and by
a variety of organic and inorganic molecules in water. Thus, the trans-
mittance or absorbance properties of the water to be treated are criti-
cal to successful UV disinfection. To achieve disinfection, the water
to be treated is normally exposed inathinfilmtotheUV source. This
may be accomplished by enclosing the UV lamps within a chamber, and
175
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directing flow through and around the lamps. It may also be accom-
plished by exposing a thin film of water flowing over a surface or weir
to a bank of lamps suspended above and/or below the water surface.
The lamps are encased within a clear, high transmittance, fused quartz
glass sleeve in order to protect them. This also insulates the lamps so
as to maintain an optimum lamp temperature (usually about 105° F or
41° C). To ensure maintenance of a very high transmittance through the
quartz glass enclosure, wipers are usually provided with this equip-
ment. Figures 6-16 and 6-17 depict a typical UV disinfection lamp
arrangement currently being used. There are a number of commercially
available units that may be applicable to onsite wastewater applica-
tions.
6.5.3.2 Applicability
Site conditions should not restrict the use of UV irradiation processes
for onsite application, although a power source is required. The unit
must be housed to protect it from excessive heat, freezing, and dust.
Wastewater characteristics limit the applicability of UV equipment since
energy transmission is dependent upon the absorbance of the water to be
treated. Therefore, only well-treated wastewater can be disinfected
with UV.
6.5.3.3 Factors Affecting Performance
The effectiveness of UV disinfection is dependent upon UV power, contact
time, liquid film thickness, wastewater absorbance, process configura-
tion, input voltage, and temperature (50)(51)(52).
The UV power output for a lamp is dependent upon the input voltage, lamp
temperature, and lamp characteristics. Typically, UV output may vary
from as low as 68% of rated capacity at 90 volts to 102% at 120 volts.
Lamp temperatures below and above about 104° F (40° C) also results in
decreased output. The use of quartz glass enclosures normally ensures
maintenance of optimum temperature within the lamp.
Since disinfection by UV requires that the UV energy reaches the organ-
isms, a measure of wastewater absorbance is crucial to proper design.
Transmissability is calculated as an exponential function of depth of
penetration and the absorption coefficient of the wastewater:
T = e-ad
176
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FIGURE 6-16
TYPICAL UV DISINFECTION UNIT
[ Outlet
n
I Flow Control Valve
Mounting Brackets for Easy
Installation Vertical or Horizontal
Removeable
Flanged head
c
Drain Plug
Q-
Warning
Sterlizing Chamber
(see Figure 6-17)
Wipers
Fail-Safe Monitor
Solenoid Shut Off Valve
Extension
Inlet
-------
FIGURE 6-17
TYPICAL UV STERILIZING CHAMBER
Ultraviolet light rays are emitted from
high intensity ultraviolet lamps and
pass through the quartz sleeves.
Baffles force the water to travel
tangetially through the chamber in a
spinning motion around quartz sleeves.
Sterlizing Chamber
Quartz Sleeve
Typical sterilizers employ one to
twelve lamps per sterilizing chamber.
178
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where T is the fraction transmitted, a is the absorption coefficient in
cm"1 at 2,537 A, and d is the depth in cm. Typically, a very high-
quality distilled water will have an absorption coefficient of 0.008,
where tap water would normally vary from 0.18 to 0.20. Wastewaters pol-
ished with sand filtration should produce absorption coefficients of
from 0.13 to 0.20, whereas septic tank effluents may be as high as 0.5.
Currently, rule of thumb requirements for UV application indicate that
turbidities should be less than 10 JTU and color less than 15 mg/1. (1
Jackson Turbidity Unit [JTU] is about equivalent to 1 Formazin Turbidity
Unit [FTU]).
The relationship between UV power and contact time is still uncertain.
Empirical relationships have been used to express the performance of UV
equipment. Currently^ the empirical term, microwatt seconds per square
centimeter (mw sec/cm2), is used (50)(51).
The required contact time for a given exposure is dictated by the waste-
water absorbance, film thickness, and the pathogen to be destroyed.
Typical values of UV dosage for selected organisms appear in Table 6-25.
This tabulation indicates that a wide spectrum of organisms are about
equally sensitive to UV irradiation. There are exceptions to this, how-
ever; Bacillus spores require dosages in excess of 220,000 mw sec/cm2,
and protozoan as high as 300,000 mw sec/cm .
One characteristic trait of UV disinfection of water has been the photo-
reactivation of treated organisms within the wastewater. Exposure of
the wastewater to sunlight following UV disinfection has produced as
much as 1.5 log increase in organisms concentration. This phenomena
does not always occur, however, and recent field tests indicate that
photoreactivation may not be of significant concern (52).
6.5.3.4 Design
There has been little long-term experience with wastewater UV disinfec-
tion (2)(52)(53). Therefore, firm design criteria are not available.
One may draw upon water supply disinfection criteria, however, for con-
servative design (50)(51).
179
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TABLE 6-25
UV DOSAGE FOR SELECTED ORGANISMS
Organi sm Dosage for 99% Kill at a = 0.0
nw sec/cm^
Shi gel la 4,000
Salmonella 6,000
Poliovirus 6,000
IH Viral Form 8,000
E. Coli 7,000
Protozoan 180-300,000a
Fecal Coliform 23,000b
a For 99.9% inactivation.
b Field studies corrected to a = 0; 99.96% inactivation.
Wastewater should be pretreated to a quality such that turbidity is less
than 10 JTU and color is less than 15 mg/1. Intermittent sand filtered
effluent quality will generally not exceed these limits when properly
managed. It would be desirable to provide measurement of UV transmit-
tance in the wastewater on a continuous basis to ensure that sufficient
UV power reaches the organism to be treated. Dosage values should be
conservative until more data are available. Therefore, a desired mini-
mum UV dose of .16,000 mw sec/cnr or kw sec/nr should be applied at all
points throughout the disinfection chamber. A maximum depth of penetra-
tion should be limited to about 2 in. (5 cm) to allow for variation in
wastewater absorption. The UV lamps should be enclosed in a quartz
glass sleeve and appropriate-automatic cleaning devices should be pro-
vided. A UV intensity meter should be installed at a point of greatest
water depth from the UV tubes, and an alarm provided to alert the owner
when values fall below on acceptable level.
180
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6.5.3.5 Construction Features
Commercially available UV units sold primarily for water supply disin-
fection are applicable for onsite wastewater disinfection. Most of
these units are self-contained and employ high-intensity UV irradiation
over a thin film of water for short contact times.
The self-contained unit should be installed following the last treatment
process in the treatment sequence, and should be protected from dust,
excessive heat, and freezing. It should be accessible for maintenance
and control. As described in the previous section, the unit should be
equipped with a cleaning device (manual or automatic) and an intensity
meter that is properly calibrated. Flow to the unit should be main-
tained relatively constant. This is often achieved by means of a pres-
sure compensated flow control valve.
Some larger UV modules are available that consist of a series of lamps
encased within quartz glass enclosures. The module may be placed within
the flow stream such that all water passes through the module. UV lamps
positioned over discharge weirs, and therefore out of the water, are
also available. These systems are not as efficient as flow through
units since only a fraction of the lamp arc intercepts the water. Con-
trol of the water film over the weir plate (V-notch or sharp crested) is
difficult to maintain unless upstream flows are carefully regulated.
Cleaning and metering devices are required for both of these systems.
Depending on upstream processes and the UV unit employed, the UV system
may be operated on a continuous flow or intermittent basis. For small
flows, self-contained tubular units and intermittent flow would be
employed. Influent to the unit could be pumped to the UV system from a
holding tank. In order to obtain full-life expectancy of the UV lamps,
they should be operated continuously regardless of flow arrangement.
Where UV modules are employed, continuous flow through the contact
chamber may be more practical.
6.5.3.6 Operation and Maintenance
Routine operational requirements include quartz glass enclosure clean-
ing, lamp replacement, and UV intensity meter reading. Since UV disin-
fection does not produce a residual, the only monitoring required would
be periodic bacterial analyses by skilled technicians. Periodic mainte-
nance of pumping equipment and controls, and cleaning of quartz jackets
during lamp replacement, would also be required.
181
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Cleaning of quartz glass enclosures 1s of paramount Importance since UV
transmittance is severely impaired by the accumulation of slimes on the
enclosures. Cleaning is required at least 3 to 4 times per year at a
minimum, and more often for systems employing intermittent flow. If
automatic wipers are employed, the frequency of manual cleaning may be
reduced to twice per year. Expected lives of lamps are variable, nor-
mally ranging from 7,000 to 12,500 hours. It is good practice, however,
to replace lamps every 10 months, or when metered UV intensity falls be-
low acceptable values. A complete cleaning of quartz glass enclosures
with alcohol is required during lamp replacement. Based on limited
operational experience, it is estimated that 10 to 12 man-hr per yr are
required to maintain the UV system. Power requirements for the UV
system for design flow rates up to 4 gpm (0.25 I/ sec) are approximately
1.5 kWh/day.
6.5.4 Ozonation
6.5.4.1 Description
Ozone, Oo, a pale blue gas with pungent odor, is a powerful oxidizing
agent. It is only slightly soluble in water, depending upon tempera-
ture, and is highly unstable. Because of its instability, ozone must be
generated on site.
Ozone is produced by the dissociation of molecular oxygen into atomic
oxygen with subsequent formation of Oo. It is produced commercially by
passing an oxygen-containing feed gas between electrodes separated by an
insulating material (54)(55). In the presence of a high-voltage, high-
frequency discharge, ozone is generated from oxygen in the electrode
gap.
Ozone is a powerful disinfectant against virus, protozoan cysts, and
vegetative bacteria (54)(55)(56)(57). It is normally sparged into the
water to be treated by means of a variety of mixing and contact devices.
Because of its great reactivity, ozone will interact with a variety of
materials in the water, resulting in an ozone demand. The short half-
life of ozone also results in the rapid disappearance of an ozone resid-
ual in the treated water.
6.5.4.2 Applicability
Ozone is currently used to disinfect water supplies in the United States
and Europe, and is considered an excellent candidate as an alternate
wastewater disinfectant (54)(55)(56)(57). The major drawback to its
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widespread use to date has been the expense of generation. There is no
documented long-term field experience with ozone disinfection onsite.
Since ozone is a highly corrosive and toxic gas, its generation and use
onsite must be carefully monitored and controlled. The generator
requires an appropriate power source, and must be properly housed to
protect it from the elements. Wastewater characteristics will have an
impact on ozone disinfectant efficiency and must be considered in the
evaluation of this process.
Data on the effectiveness of ozone residuals against pathogens are
scant. Employing the same criteria as used for halogens, ozone appears
to be more effective against virus and amoebic cysts than the halogens
(Table 6-23).
The literature indicates that ozone action is not appreciably affected
by pH variations between 5.0 and 8.0 (58). Turbidity above values of 5
JTU has a pronounced effect upon ozone dosage requirements, however
(47)(58). Limited field experience indicates that ozone requirements
may approximately double with a doubling of turbidity to achieve
comparable destruction of organisms (47).
Currently, with very limited operating data, prescribed ozone applied
dosages recommended for wastewater disinfection vary from 5 to 15 mg/1
depending upon contactor efficiencies and pathogen to be destroyed.
6.5.4.3 Construction Features
The ozone disinfection system consists of the ozone gas generation
equipment, a contactor, appropriate pumping capacity to the contactor
and controls. There are two basic types of generating equipment. The
tube-type unit is an air-cooled system whereby ozone is generated
between steel electrode plates faced with ceramic. Oxygen-containing
feed gas may be pure oxygen, oxygen-enriched air, or air. The gas is
cleaned, usually through cartride-type inpingement filters, and com-
pressed to about 10 psi. The compressed gas is subsequently cooled and
then dried prior to being reacted in the ozone contacting chamber. Dry-
ing is essential to prevent serious corrosion problems within the gener-
ator.
The generated ozone-enriched air is intimately mixed with wastewater in
a contacting device. Ozone contactors include simple bubble diffusers
in an open tank, packed columns, and positive pressure injection (PPI)
devices. Detention times within these systems range from 8-15 min in
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the bubble diffuser units to 10-30 sec in packed columns and PPI systems
(59)(60). Limited data are currently available on long-term use of
these contactor devices for onsite systems. There are relatively few
field-tested, small-capacity systems commercially available.
6.5.4.4 Operation and Maintenance
The ozone disinfection system is a complex series of mechanical and
electrical units, requiring substantial maintenance, and is susceptible
to a variety of malfunctions. Since data on long-term experience are
relatively unavailable, it is not possible to assess maintenance re-
quirements on air cleaning equipment, compressors, cooling and drying
equipment, and contactors. It is estimated that 8 to 10 kWh/lb of ozone
generated will be required (54). Monitoring requirements are similar to
those for UV disinfection, including occasional bacterial analyses and
routine ozone monitoring.
6.6 Nutrient Removal
6.6.1 Introduction
6.6.1.1 Objectives
Nitrogen and phosphorus may have to be removed from wastewaters under
certain circumstances. Both are plant nutrients and may cause undesir-
able growths of plants in lakes and impoundments. Nitrogen may also
create problems as a toxicant to fish (free ammonia), as well as to ani-
mals and humans (nitrates). In addition, the presence of reduced nitro-
gen may create a significant oxygen demand in surface waters.
Nitrogen may be found in domestic wastewaters as organic nitrogen, as
ammonium, or in the oxidized form as nitrite and nitrate. The usual
forms of phosphorus in domestic wastewater include orthophosphate, poly-
phosphate, pyrophosphate, and organic phosphate. Sources of wastewater
nitrogen and phosphorus from the home are presented in Table 4-4.
The removal or transformation of nitrogen and phosphorus in wastewaters
has been the subject of intensive research and demonstration over the
past 15 to 20 yr. Excellent reviews of the status of these treatment
processes can be found in the literature (61)(62). As discussed in
Chapter 7, the soil may also serve to remove and/or transform the
nitrogen and phosphorus in wastewaters percolating through them.
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The treatment objective for nitrogen and phosphorus in wastewater is
dependent upon the ultimate means of disposal. Surface water quality
objectives may require limitations of total phosphate, organic and
ammonia nitrogen, and/or total nitrogen. Subsurface water quality
objectives are less well developed, but may restrict nitrate-nitrogen
and/or total phosphate.
6.6.1.2 Application of Nutrient Removal Processes to Onsite
Treatment
There are a number of nutrient removal processes applicable to onsite
wastewater treatment, but there are very little data on long-term field
applications of these systems. In-house wastewater management through
segregation and household product selection appears to be the most
practical and cost-effective method for nitrogen and phosphorus control
onsite. Septic tanks may remove a portion of these nutrients as flot-
able and settleable solids. Other applicable chemical, physical, or
biological processes may also be employed to achieve a given level of
nutrient removal. Although these supplemental processes may be very
effective in removing nutrients, they are normally complex and energy
and labor intensive.
Since the state-of-the-art application of onsite nutrient removal is
limited, the discussion that follows is brief. Processes that may be
successful for onsite application are described. Acceptable design,
construction, and operation data are presented where they are available.
6.6.2 Nitrogen Removal
6.6.2.1 Description
Table 6-26 outlines the potential onsite nitrogen control options. In
many instances, these options also achieve other treatment objectives as
well, and should be evaluated as to their overall performance. The
removal or transformation of nitrogen within the soil absorption system
is described fully in Chapter 7.
6.6.2.2 In-House Segregation
Chapter 4 provides a detailed description of the household wastewater
characteristics and sources of these wastewaters. Between 78 and 90% of
the nitrogen in the wastewater discharged from the home is from toilets.
Separation of toilet wastewaters would result in average nitrogen levels
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TABLE 6-26
POTENTIAL ONSITE NITROGEN CONTROL OPTIONS*
Option
Description
Effectiveness
Comments
Onsite
Techno! ogy
Status
oo
cr>
In-House
Segregation
Biological
Ni tri f i cati on
Biological
Denitrification
Ion Exchange
Separate toilet
wastes from other
wastewater
Granular Filters
Aerobic package
plants
Anaerobi c
processes
f ol 1 owi ng
nitrification
Cationic
exchange-NH4
Anionic
exchange-N03
78-90% removal of N
in blackwater
>90% conversion to
ni trate
85-95% conversion to
nitrate
80-95% removal of N
>99% removal of
or
Management of residue
requi red
Achieves high level of
BOD and solids removal
May achieve good levels
of BOD and solids
removal; labor/energy
i ntensi ve; resi due
management
Requires carbon source;
labor intensive; high
capital cost
Good
Good
Good
Tentati ve
Very high operation costs Tentative
a Not including the soil absorption system—see Chapter 7.
-------
of about 0.004 Ib/cap/day (1.9 mg/cap/day) or 17 mg/1 as N in the re-
maining graywater (Tables 4-4 and 4-5). Chapter 5 describes the process
features, the performance, and the operation and maintenance of low-
water carriage and waterless toilet systems. The resultant residuals
from toilet segregation, whether they be ash, compost, chemical sludge,
or blackwater, must be considered in this treatment strategy. A dis-
cussion of residuals disposal is presented in Chapter 9.
The success of this method of nitrogen removal is dependent upon appro-
priate management of the in-house segregation fixtures and the disposal
of the residues from them. These devices must be considered a part of
the treatment system when developing appropriate authority for institu-
tional control.
6.6.2.3 Biological Processes
Nitrogen undergoes a variety of biochemical transformations depending
upon its form and the environmental conditions (61). Organic nitrogen
in domestic wastewaters readily undergoes decomposition to ammonia in
either aerobic or anaerobic conditions. In an aerobic environment, a
select group of bacteria oxidize ammonia to nitrite and ultimately
nitrate. Nitrates may be reduced by a variety of organisms to various
nitrogen gas under anaerobic conditions. Depending upon the treatment
objectives, one or several of these processes may be employed to achieve
the desired end product.
a. Applicability
A number of biological processes for nitrogen conversion are applicable
to onsite treatment. Domestic wastewater characteristics should not
limit application of these processes, provided the nitrogen is in the
appropriate form for conversion. Since biological processes are tem-
perature-sensitive, such systems should be covered and insulated in cold
climates. Covering also contains odors, should problems occur.
b. Process Performance
Although data are sketchy, about 2 to 10% of the total nitrogen from the
home may be removed in the septic tank with septage (63)(64). Approxi-
mately 65 to 75% of the total nitrogen in septic tank effluents is in
the ammonia-nitrogen form, indicating a significant level of decomposi-
tion of organic nitrogen (2).
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Nitrification of septic tank effluents occurs readily within intermit-
tent sand filters (see Section 6.3). Field experience indicates that
intermittent sand filters loaded up to 5 gpd/ft2 (0.02 orr/nr/d), and
properly maintained to avoid excessive ponding (and concomitant anaero-
bic conditions), converts up to 99% of the influent ammonia to nitrate-
nitrogen (2). Aerobic biological package plants also provide a high
degree of nitrification, provided solids retention times are long and
sufficient oxygen is available (see Section 6.6).
The biological denitrification (nitrates to nitrogen gases) of waste-
water follows a nitrification step (61). There has been little experi-
ence with long-term field performance of onsite denitrification pro-
cesses. Ideally, total nitrogen removal in excess of 90% should be
achievable, if the system is properly operated and maintained (61).
c. Design and Construction Features
Septic Tanks: There are no septic tank design requirements specifically
established to enhance high levels of nitrogen removal. Designs that
provide excellent solid-liquid separation ensure lower concentrations of
nitrogen associated with suspended solids.
Nitrification: Biological nitrification is achieved by a select group
of aerobic microorganisms referred to as nitrifiers (61). These organ-
isms are relatively slow-growing and more sensitive to environmental
conditions than the broad range of microorganisms found in biological
wastewater treatment processes. The rate of growth of n-itrifiers (and
thus the rate of nitrification) is dependent upon a number of parame-
ters, including temperature, dissolved oxygen, pH, and certain toxi-
cants. The design and operating parameter used to reflect the growth
rates of nitrifiers is the solids retention time (SRT). Details of the
impact of temperature, dissolved oxygen, pH, and toxicants on design SRT
values for nitrification systems are outlined in reference (61). In
brief, biological nitrification systems are designed with SRT values in
excess of 10 days; dissolved oxygen concentrations should be in excess
of 2.0 mg/1; and pH values should range between 6.5 and 8.5. Toxicants
known to be troublesome are discussed in reference (61).
Details of the design and construction of intermittent sand filters and
aerobic package plants are found in Sections 6.3 and 6.4. In general,
designs normally employed for onsite application of these processes to
remove BOD and solids are sufficient to encourage nitrification as well.
Denitrification: Biological denitrification is carried out under anoxic
conditions in the presence of facultative, heterotrophic microorganisms
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which convert nitrate to nitrogen gases (61). Numerous microorganisms
are capable of carrying out this process, provided there is an organic
carbon source available. These organisms are less sensitive to environ-
mental conditions than the nitrifiers, but the process is temperature-
dependent. Process design and operational details for conventional de-
nitrification processes are discussed in reference (61).
Design and operational experience with onsite biological denitrification
systems is limited at this time (2)(65)(66). Several systems have been
suggested for onsite application, two of which are shown in Figure 6-
18. One employs a packed bed containing approximately 3/8-in. (1-cm)
stone that receives, on a batch basis, effluent from a nitrification
process. The nitrified wastewater flows to a dosing tank, where it is
held until a predetermined volume is obtained. Methanol (or other or-
ganic carbon source) is then added to provide a C:N ratio of approxi-
mately 3:1. After approximately 15 min, the wastewater is pumped up
through the anoxic packed stone bed. Effluent flows from the top of the
bed. Liquid retention times in the packed bed (based on void volume)
varying from 12 to 24 hours have been employed with good results (2).
Pumping may be provided by a 1/3-hp submersible pump actuated by a
switch float within the sump. A small chemical feed pump controlled by
a timer switch may be used to feed the organic carbon source to the
sump. A 30% methanol solution may be used as the carbon source. Other
organic carbon sources include septic tank effluent, graywater, and
molasses. Metering of organic carbon source to the nitrified wastewater
requires substantial control to ensure a proper C:N ratio. Insufficient
carbon results in decreased denitrification rates, whereas excess carbon
contributes to the final effluent BOD (61). The use of an easily ob-
tainable, slowly decomposable, solid carbon source could also be con-
sidered. Peat, forest litter, straw, and paper mill sludges, for exam-
ple, could be incorporated as a portion of the upflow filter. Control
of the denitrification process using these solid carbon sources would be
difficult.
Another onsite nitrification-denitrification system that has been field
tested employs a soil leach field (66) (Figure 6-18). Septic tank ef-
fluent is distributed to a standard soil absorption field. An imperme-
able shield of fiberglass is placed approximately 5 ft (1.5 m) below the
distribution line. The location of this collector should be deep enough
to ensure complete nitrification within the overlying unsaturated soil.
The nitrified wastewater is collected on the sloped fiberglass shield,
and directed to a 24-in. (61-cm) deep bed of pea gravel contained within
a plastic liner (denitrifying reactor). The gravel bed is sized deep
enough to provide a hydraulic detention time of approximately 10 days
(based on void volume). Methanol or other energy source is metered to
the gravel bed through a series of distributors. The gravel bed is
vented with vertical pipes to allow escape of nitrogen gas evolved in
the process. Short-term experience with this system has been good.
Total nitrogen concentrations of less than 1 mg/l-N were achievable in
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FIGURE 6-18
ONSITE DENITRIFICATION SYSTEMS
Effluent
a a .
-------
effluent samples during summer months. Higher values (5-10 mg/l-N) were
observed during the colder winter months.
Although no studies have been reported in the literature for onsite
applications, intermittent or cycled extended aeration processes are
potentially promising (61)(67) for the nitrification-denitrification of
wastewater. This process makes use of existing proprietary extended
aeration package plants where aeration is cycled to provide both aerobic
and anoxic environments. In this mode of operation, sufficient solids
retention time (SRT) is provided to insure nitrification, and a suffi-
cient period of anoxic holding is provided to insure denitrification.
The biomass serves as the energy source for denitrification. The cycle
times vary dependent on temperature and wastewater characteristics. A
typical cycle, using a SRT of 20 days, aerates 180 min and holds anoxic
for 90 min (67). Nitrogen removals in excess of 50% are attainable with
this system (2)(67). Operation of the cyclic aeration system requires
substantial supervision for a period of time until proper sequences have
been selected.
d. Operation and Maintenance
Nitrification Systems: Operation and maintenance requirements to
achieve nitrification in either intermittent sand filters or aerobic
package units are not significantly different from those discussed in
Sections 6.3 and 6.4. In both systems, the process must be maintained
in an aerobic condition at all times to ensure effective nitrification.
Denitrification Systems; Operation and maintenance requirements for
denitrification systems are normally complex and require semi-skilled
labor for proper performance. In addition to routine maintenance of
pumping systems, mixers, and timer controls, the addition and balance of
a carbon source is required.
Routine analyses of nitrogen compounds and biological solids is also im-
portant. Rough estimates for semi-skilled labor for maintenance of an
onsite denitrification system varies from 15 to 30 man-hr per yr. If
methanol is used as a carbon source, it is estimated that from 33 to 55
Ib/yr (15 to 25 kg/yr) are required for a family of four. Power re-
quirements for methanol feed and pumping are about 15 to 25 kWh/yr.
6.6.2.4 Ion Exchange
Ion exchange is a process whereby ions of a given species are displaced
from an insoluble exchange material by ions of a different species in
191
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solution. It can be used to remove either ammonium or nitrate nitrogen
from wastewaters. This process has been employed in full-scale water
and wastewater treatment plants for several years (61)(67)(68), but
there is no long-term experience with the process for nitrogen removal
in onsite applications.
Nitrogen removal by ion exchange has potential for onsite application,
since it is very effective and is simple to operate. Unfortunately, per-
iodic replacement of the exchange media is expensive and regeneration of
the media onsite does not appear to be practical at this time. Site
conditions and climatological factors should not limit its application.
a. Ammonia Removal
Ammonia removal may be achieved by employing the naturally occurring ex-
change media, clinoptilolite, which has a high affinity for the ammonium
ion (61). Laboratory experience has shown that packed columns of cli-
noptilolite resin (20 x 40 mesh) will effectively remove ammonium ion
from septic tank effluent without serious clogging problems (2). Regen-
eration with 5% NaCl was successful over numerous trials. Breakthrough
exchange capacity of this resin was found to be about 0.4 meq NH4 /gram
in hard water at application rates of 10 bed volumes per hr. (This
value will vary, increasing with decreased hardness.) Very large quan-
tities of resin are required to treat household wastewaters (approxi-
mately 10 Ib per day). Treatment of segregated graywaters substantially
lower in ammonium concentration decreases the amount of resin needed.
This process employs a packed column or bed of the exchange resin fol-
lowing a septic tank. The waste is pumped from a sump to the column in
an upflow or down flow mode on a periodic basis. Once the resin has been
exhausted, it is removed and replaced by fresh material. Regeneration
occurs offsite.
Operation and maintenance of this process requires routine maintenance
of the pump and occasional monitoring of ammonium levels from the pro-
cess. Replacement of exhausted resin is dictated by wastewater charac-
teristics and bed volume. There are insufficient data at this time to
delineate labor, power, and resin requirements.
b. Nitrate Removal
Nitrate removal from water may be achieved by the use of strong and weak
base ion exchange resins (68)(69). There are very little data available
192
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on long-term performance of these nitrate removal systems for waste-
water. Numerous anions in water compete with nitrate for sites on these
resins; therefore, tests on the specific wastewater to be treated need
to be performed.
This process has potential for onsite application, where it would follow
a nitrification process such as intermittent sand filters. As with
ammonia resins, regeneration is performed off site.
There is insufficient information on nitrate exchange to provide design,
construction, operation, and maintenance data at this time.
6.6.3 Phosphorus Removal
6.6.3.1 Description
Table 6-27 outlines the most likely treatment processes available for
onsite removal of phosphorus in wastewater. In many instances, these
processes will also achieve other treatment objectives as well, and must
be evaluated as to their overall performance.
6.6.3.2 In-House Processes
Review of Chapter 4 indicates that the major sources of phosphorus in
the home are laundry, dishwashing, and toilet wastewaters. Contribu-
tions of phosphorus in the home could be reduced from approximately 4 to
2 gm/cap/day through the use of 0.5% phosphate detergents.
Segregation of toilet wastewaters (blackwater) from household waste-
waters reduces phosphorus levels to approximately 2.8 gm/cap/day in the
graywater stream. Chapter 4 describes the process features, perfor-
mance, and operation and maintenance of low-water carriage and waterless
toilet systems that would be employed for this segregation. Note that
the resultant residues from these toilet systems must be considered in
this treatment strategy. A discussion of residuals disposal appears in
Chapter 9.
As with any in-house measure to reduce pollutional loads, the success of
the process is dependent upon owner commitment and appropriate manage-
ment of the alternative plumbing equipment.
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TABLE 6-27
POTENTIAL ONSITE PHOSPHORUS REMOVAL OPTIONS
Opti on
Descrlpti on
Effectiveness
Comments
Onsi te
Technology
Status
In-House
Segregation
Chemical
Precipitation:
Iron, Calcium
and Aluminum
Salts
Laundry detergent
substi tuti on
Separate toilet
wastes from other
wastewaters
Dosing prior to
or following
septic tanks
Sorpti on
Processes:
Calcite or Iron Beds or columns
Alumi na
Beds or columns
50% P
removal
20-40% P
removal
Up to 90% P
removal
Up to 90% P
removal
90-99% P
removal
0.5% P detergents
available
Management of residues
required; achieves
significant BOD, SS
reduction
Increases quantity of
sludge; labor intensive
Replacement required
High cost for material,
labor intensive
Excellent
Good
Fair
Tentati ve
Tentati ve
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6.6.3.3 Chemical Precipitation
Phosphorus in wastewater may be rendered insoluble by a selected number
of metal salts, including aluminum, calcium, and iron (62). Although
the reactions are complex, the net result is the precipitation of an
insoluble complex that contains phosphate. Phosphorus precipitation
methods normally include the addition of the chemical, high-speed
mixing, and slow agitation followed by sedimentation.
There has been little long-term experience with phosphorus removal of
wastewaters onsite (2)(70). Precipitation of phosphates is less easily
accomplished for polyphosphates and organic phosphorus than for ortho-
phosphate. Therefore, precipitation within the septic tanks, although
simpler to manage, may not remove a significant portion of the phos-
phate, which is in the poly and organic form. Substantial hydrolysis of
these forms may occur in the septic tank, however, producing the ortho-
form. Thus, precipitation following the septic tank may achieve higher
overall removals of total phosphorus.
Performance is dependent on the point of chemical addition, chemical
dosage, wastewater characteristics, and coagulation and sedimentation
facilities. Dose-performance relationships must be obtained through
experimentation, but one should expect phosphorus removals between 75
and 90%. Improvement in this performance may be achieved if intermit-
tent sand filters follow the precipitation/sedimentation process. Side
benefits are achieved with the addition of the precipitating chemicals.
Suspended and colloidal BOD and solids will be carried down with the
precipitate, producing a higher quality effluent than would otherwise be
expected.
Chemical precipitation of wastewaters generates more sludge than do
conventional systems due to both the insoluble end product of the added
chemical and the excess suspended and colloidal matter carried down with
it. Estimates of this increased quantity are very crude at this time,
but may range from 200 to 300% by weight in excess of the sludge nor-
mally produced from a septic tank system.
a. Process Features
The chemicals most often used for phosphate precipitation are aluminum
and iron compounds. Calcium salts may also be used, but require pH ad-
justment prior to final discharge to the environment. Aluminum is gen-
erally added as alum (A^SO^'n H20). Ferric chloride and ferric sulfate
are the most commonly used iron salts.
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Anionic polyelectrolytes can be used in combination with the aluminum
and iron salts to improve settling, but may overly complicate the onsite
treatment system.
The required dosages of aluminum and iron compounds are generally re-
ported as molar ratios of trivalent metal salt to phosphate phosphorus.
Molar ratios currently used in practice today range from 1.5:1 to 4:1,
depending upon wastewater characteristics, point of addition, and de-
sired phosphorus removal (20)(62).
Adding aluminum or iron salts to the raw wastewater prior to the septic
tank has the advantage of using the existing septic tank for sedimenta-
tion (70). Aluminum or iron salts may be metered to the raw wastewater
with a chemical feed pump activated by electrical or mechanical impulse.
Mixing of the chemical with the wastewater is provided in the sewer line
to the septic tank. The quantity of metal salt added to the wastewater
is dependent upon wastewater characteristics. Since the impulse to the
feed pump may come from any of a number of household events, it is not
possible to precisely adjust metal dosage. An average dose of salt
based on estimated phosphorus discharge is most practical.
Addition of iron or aluminum salts following the septic tank may also be
considered. A batch feed system could be employed whereby a preset
chemical dose is provided when the wastewater reaches a preset volume in
a holding tank. Mixing may be provided by aeration or mechanical mixer,
followed by a period of quiescence. Additional raw wastewater flow
would be diverted to a holding tank until the precipitation-sedimenta-
tion cycle is completed. This system may be employed after the septic
tank and preceding the intermittent sand filter.
The processes briefly described above represent a few of the many chemi-
cal treatment processes that might be considered for onsite treatment.
They may be designed and constructed to fit the specific needs of the
site, or purchased as a proprietary device. Storage and holding of
chemicals must be considered in the design of these systems. Details on
chemical storage, feeding, piping, and control systems may be found
elsewhere (20)(62). Attention must be given to appropriate materials
selection, since many of the metal salts employed are corrosive in
liquid form.
b. Operation and Maintenance
Every effort should be made to select equipment that is easily operated
and maintained. Nonetheless, chemical precipitation systems require
semi-skilled labor to maintain chemical feed equipment, mixers, pumps,
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and electronic or mechanical controls. More frequent pumping of waste-
water sludge or septage is also required. A rough estimate for semi-
skilled labor is 10 to 25 man-hr per yr depending upon the complexity of
the equipment. Conservative estimates on sludge accumulation dictate
sludge or septage pumping every 0.5 to 2 years for an average home.
Chemical requirements would vary widely, but are estimated to range from
22 to 66 Ib/yr Al (10 to 30 kg/yr) or 11 to 33 Ib/yr Fe (5 to 15 kg/yr)
for a family of four.
6.6.3.4 Surface Chemical Processes
Surface chemical processes, which include ion exchange, sorption, and
crystal growth reactions, have received little application in treatment
of municipal wastewaters, but hold promise for onsite application (62).
These types of processes are easy to control and operate; the effluent
quality is not influenced by fluctuations in influent concentration; and
periods of disuse between applications should not affect subsequent per-
formance. Phosphorus removal on selected anion exchange resins has been
demonstrated, but control of the process due to sulfate competition for
resin sites has discouraged its application (71). Phosphorus removal by
sorption in columns or beds of calcite or other high-calcium, iron, or
aluminum minerals is feasible; but long-term experience with these mate-
rials has been lacking (2)(25)(72). Many of these naturally occurring
materials have limited capacity to remove phosphorus, and some investi-
gations have demonstrated the development of biological slimes that
reduce the capacity of the mineral to adsorb phosphorus. Table 6-28
lists a range of phosphorus adsorption capacities of several materials
that may be considered. The use of locally available calcium, iron, or
aluminum as naturally occurring materials, or as wastewater products
from industrial processing, may prove to be cost-effective; but trans-
port of these materials any distance normally rules out their widespread
application. Incorporation of phosphate-sorbing materials within
intermittent sand filters is discussed more fully in Section 6.3.5.
The use of alumina ^203), a plentiful and naturally occurring material
for sorption of phosphorus, has been demonstrated in laboratory studies,
but has not yet been employed in long-term field tests (75). Alumina
has a high affinity for phosphorus, and may be regenerated with sodium
hydroxide. Application of an alumina sorption process is similar to ion
exchange, whereby a column or bed would be serviced by replacement on a
routine basis. Costs for this process are high.
6.7 Wastewater Segregation and Recycle Systems
Chapter 5 discusses in detail in-house methods that may be employed to
modify the quality of the wastewater. These processes are an important
197
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component of the onsite treatment system as they remove significant
quantities of pollutants from the wastewater prior to further treatment
and/or disposal.
TABLE 6-28
PHOSPHORUS ADSORPTION ESTIMATES FOR SELECTED
NATURAL MATERIALS (73)(74)(75)a
Media Adsorption
(mg P/100 gm media
Acid Soil Outwash 10 - 35
Calcereous Soil Outwash 5-30
Sandy Soils 2-20
F-l Alumina (AleOa), 24-48 mesh 700 - 1500
a Based on maximum Langmuir isotherm values.
6.7.1 Wastewater Segregation
Among the wastewater segregation components which significantly alter
wastewater quality are the non-water carriage toilets (Table 5-3), and
the very low water flush toilets (Table 5-2) with blackwater contain-
ment. Impacts of wastewater modification on onsite disposal practices
are outlined in Table 5-9.
The graywater resulting from toilet segregation practices normally re-
quire some treatment prior to disposal (Tables 4-4 and 4-5 - "Basins,
Sinks, and Appliances"). Treatment methods for graywater are similar to
those employed for household wastewaters (Sections 6.2 to 6.6 and Figure
5-2), but performance data are lacking.
198
-------
Residuals resulting from the treatment or holding of segregated waste
streams must be considered when evaluating these alternatives. Details
of the characterization and disposal of these residuals appear in
Chapter 9.
6.7.2 Wastewater Recycle
In-house wastewater recycle systems are treatment systems employed to
remove specific pollutants from one or more wastewater streams in order
to meet a specific water use objective (for example, graywater may be
treated to a quality that is acceptable for flushing toilets, watering
lawns, etc.). These systems are summarized in Table 5-6.
The impact of recycle systems on the quality of wastewater to be ulti-
mately disposed is difficult to assess at this time owing to the absense
of long term experience with these systems. It is likely that substan-
tial pollutant mass reduction will occur in addition to flow reduction.
As with segregated systems, the disposal of residuals from these pro-
cesses must be considered in system evaluation.
6.8 References
1. Jones, E. E. Septic Tank - Configuration versus Performance. Pre-
sented at the 2nd Pacific Northwest On-Site Wastewater Disposal
Short Course, University of Washington, Seattle, March 1978.
2. Small Scale Waste Management Project, University of Wisconsin,
Madison. Management of Small Waste Flows. EPA 600/2-78-173, NTIS
Report No. PB 286 560, September 1978. 804 pp.
3. Weibel, S. R., C. P. Straub, and J. R. Thoman. Studies on House-
hold Sewage Disposal Systems, Part I. NTIS Report No. PB 217 671,
Environmental Health Center, Cincinnati, Ohio, 1949. 279 pp.
4. Salvato, J. A. Experience with Subsurface Sand Filters. Sewage
and Industrial Wastes, 27(8):909, 1955.
5. Bernhart, A. P. Wastewater from Homes. University of Toronto,
Toronto, Canada, 1967.
6. Laak, R. Wastewater Disposal Systems in Unsewered Areas. Final
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7. Brandes, M. Characteristics of Effluents from Separate Septic
Tanks Treating Gray and Black Waters from the Same House. J. Water
Pollut. Control Fed., 50:2547-2559, 1978.
8. Weibel, S. R., T. W. Bendixen, and J. B. Coulter. Studies on
Household Sewage Disposal Systems, Part III. NTIS Report No. PB
217 415, Environmental Health Center, Cincinnati, Ohio, 1954. 150
pp.
9. Plews, G. D. The Adequacy and Uniformity of Regulations for
On-Site Wastewater Disposal - A State Viewpoint. JJT; National
Conference on Less Costly Wastewater Treatment Systems for Small
Communities. EPA 600/9-79-010, NTIS Report No. PB 293 254, April
1977. pp. 20-28.
10. Manual of Septic Tank Practices. NTIS Report No. PB 216 240,
Public Health Service, Washington, D.C., 1967. 92 pp.
11. Baumann, E. R., E. E. Jones, W. M. Jakubowski, and M. C. Notting-
ham. Septic Tanks. In: Proceedings of the Second National Home
Sewage Treatment Symposium, Chicago, Illinois, December 1977.
American Society of Agricultural Engineers, St. Joseph, Michigan,
1978. pp. 38-53.
12. Weibel, S. R. Septic Tanks: Studies and Performance. Agric.
Eng., 36:188-191, 1955.
13. Harris, S. E., J. H. Reynolds, D. W. Hill, D. S. Filip, and E. J.
Middlebrooks. Intermittent Sand Filtration for Upgrading Waste
Stabilization Pond Effluents. J. Water Pollut. Control Fed. 49:83-
102, 1977.
14. Schwartz, W. A., T. W. Bendixen, and R. E. Thomas. Project Report
of Pilot Studies on the Use of Soils as Waste Treatment Media; In-
house Report. Federal Water Pollution Control Agency, Cincinnati,
Ohio, 1967.
15. Metcalf, L., and H. P. Eddy. American Sewerage Practice. 3rd ed.,
Volume III. McGraw-Hill, New York, 1935. 892 pp.
16. Boyce, E. Intermittent Sand Filters for Sewage. Munic. Cty. Eng.,
72:177-179, 1927.
17. Recommended Standards for Sewage Works. Great Lakes-Upper Missis-
sippi River Board of State Sanitary Engineers, Albany, New York,
1960. 138 pp.
18. Filtering Materials for Sewage Treatment Plants. Manual of Engi-
neering Practice No. 13, American Society of Civil Engineers, New
York, 1937. 40 pp.
200
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19. Salvato, J. A., Jr. Experience With Subsurface Sand Filters. Sew.
Ind. Wastes, 27:909-916, 1955.
20. Wastewater Treatment Plant Design. Manual of Practice No. 8, Water
Pollution Control Federation, Washington, D.C., 1977. 560 p.
21. Clark, H. W. and S. Gage. A Review of Twenty-One Years of Experi-
ments upon the Purification of Sewage at the Lawrence Experimental
Station. 40th Annual Report, State Board of Health of Massachu-
setts, Wright E. Potter, Boston, Massachusetts, 1909. 291 pp.
22. Brandes, M. Effect of Precipitation and Evapotranspiration on Fil-
tering Efficiency of Wastewater Disposal Systems. Publication No.
W70, Ontario Ministry of Environment, Toronto, Canada, May 1970.
23. Emerson, D. L., Jr. Studies on Intermittent Sand Filtration of
Sewage. Florida Engineering and Industrial Experimental Station
Bulletin No. 9, University of Florida College of Engineering,
Gainesville, 1954.
24. Mines, J., and R. E. Favreau. Recirculating Sand Filter: An
Alternative to Traditional Sewage Absorption Systems. In: Pro-
ceedings of the National Home Sewage Disposal Symposium, Chicago,
Illinois, December 1974. American Society of Agricultural Engi-
neers, St. Joseph, Michigan, 1975. pp. 130-136.
25. Brandes, M., N. A. Chowdhry, and W. W. Cheng. Experimental Study
on Removal of Pollutants From Domestic Sewage by Underdrained Soil
Filters. In: Proceedings of the National Home Sewage Disposal
Symposium, Chicago, Illinois, December 1974. American Society of
Agricultural Engineers, St. Joseph, Michigan, 1975. pp. 29-36.
26. Teske, M. G. Recirculation - An Old Established Concept Solves
Same Old Established Problems. Presented at the 51st Annual Con-
ference of the Water Pollution Control Federation, Anaheim, Cali-
fornia, 1978.
27. Bowne, W. C. Experience in Oregon With the Hines-Favreau Recircu-
lating Sand Filter. Presented at the Northwest States Conference
on Onsite Sewage Disposal, 1977.
28. Chowdhry, N. A. Underdrained Filter Systems - Whitby Experiment
Station. Ministry of the Environment Interim Report Part 2.
Toronto, Canada, 1973.
29. Kennedy, J. C. Performance of Anaerobic Filters and Septic Tanks
Applied to the Treatment of Residential Wastewater. M.S. Thesis,
University of Washington, Seattle, 1979.
201
-------
30. Bernhart, A. P. Wastewater From Homes. University of Toronto,
Toronto, Canada, 1967.
31. Voell, A. T. and R. A. Vance. Home Aerobic Wastewater Treatment
Systems - Experience in a Rural County. Presented at the Ohio Home
Sewage Disposal Conference, Ohio State University, Columbus, 1974.
32. Tipton, D. W. Experiences of a County Health Department with Indi-
vidual Aerobic Sewage Treatment Systems. Jefferson County Health
Department, Lakewood, Colorado, 1975.
33. Brewer, W. S., J. Lucas, And G. Prascak. An Evaluation of the
Performance of Household Aerobic Sewage Treatment Units. Journal
of Environmental Health, 41:82-85, 1978.
34. Glasser, M. B. Garrett County Home Aeration Wastewater Treatment
Project. Bureau of Sanitary Engineering, Maryland State Department
of Health and Mental Hygiene, Baltimore, 1974.
35. Hutzler, N. J., L. E. Waldorf, and J. Fancy. Performance of Aero-
bic Treatment Units. In: Proceedings of the Second National Home
Sewage Treatment Symposium, Chicago, Illinois, December 1977.
American Society of Agricultural Engineers, St. Joseph, Michigan,
1978. pp. 149-163.
36. Operation of Wastewater Treatment Plants. Manual of Practice No.
11, Water Pollution Control Federation, Washington, D.C., 1976.
547 pp.
37. Tsugita, R. A., D. C. W. Decoite, and L, Russell. Process Control
Manual for Aerobic Biological Wastewater Treatment Facilities.
EPA 430/9-77-006, NTIS Report No. PB 279 474, James M. Montgomery
Inc., 1977. 335 pp.
38. Process Design Manual, Wastewater Treatment Facilities for Sewered
Small Communities. EPA-625/1-77-009, U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1977.
39. Craun, C. F. Waterborne Disease - A Status Report Emphasizing Out-
breaks in Ground Water Systems. Ground Water, 17:183-191, 1979.
40. Craun, C. F. Disease Outbreaks Caused by Drinking Water. J. Water
Pollut. Control Fed., 50:1362-1375, 1978.
41. Jakubowski, W., and J. C. Hoff, eds. Waterborne Transmission of
Giardiasis. EPA 600/9-79-001, NTIS Report No. PB 299 265, U. S.
Environmental Protection Agency, Health Effects Research Labora-
tory, Cincinnati, Ohio, June 1979. 306 pp.
42. Berg, G., ed. Transmission of Viruses by the Water Route. Wiley,
New York, 1967. 502 pp.
202
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43. Chang, S. L. Modern Concept of Disinfection. J. Sanit. Eng. Div.,
Am. Soc. Civil Eng., 97:689-707, 1971.
44. White, G. C. Handbook of Chlorination. Van Nostrand Reinhold, New
York, 1972. 751 pp.
45. Morris, J. C. Chlorination and Disinfection: State of the Art.
J. Am. Water Works Assoc., 63:769-774, 1971.
46. McKee, J. E. Report on the Disinfection of Seattle Sewerage. Cali-
fornia Institute of Technology, Pasadena, California, April 1957.
47. Budde, P. E., P. Nehm, and W. C. Boyle. Alternatives to Wastewater
Disinfection. J. Water Pollut. Control Fed., 49:2144-2156, 1977.
48. Black, A. P. Better Tools for Treatment. J. Am. Water Works
Assoc., 58:137-146, 1966.
49. Baker, R. J. Characteristics of Chlorine Compounds. J. Water
Pollut. Control Fed., 41:482-485, 1969.
50. Jepson, J. D. Disinfection of Water Supplies by Ultraviolet Irra-
diation. Water Treat. Exam., 22:175-193, 1973.
51. Huff, C. B., H. F. Smith, W. D. Boring, and N. A. Clarke. Study of
Ultraviolet Disinfection of Water and Factors in Treatment Effi-
ciency. Pub. Health Rep., 80:695,705, 1965.
52. Scheible, 0. K., G. Binkowski, and T. J. Mulligan. Full Scale
Evaluation of Ultraviolet Disinfection of a Secondary Effluent.
In: Progress in Wastewater Disinfection Technology, EPA 600/9-79-
UT8, NTIS Report No. PB 299 338, Municipal Environmental Research
Laboratoty, Cincinnati, Ohio, 1979. pp. 117-125.
53. Kreissl, J. F., and J. M. Cohen. Treatment Capability of a Physi-
cal Chemical Package Plant. Water Res., 7:895-909, 1973.
54. Rosen, H. M. Ozone Generation and Its Economical Application in
Wastewater Treatment. Water Sew. Works, 119:114-120, 1972.
55. Johansen, R. P., and D. W. Terry. Comparison of Air and Oxygen
Recycle Ozonation Systems. Presented at the Symposium on Advanced
Ozone Technology, Toronto, Canada, November 1977.
56. Majumdar, S. B., and 0. 0. Sproul. Technical and Economic Aspects
of Water and Wastewater Ozonation: A Critical Review. Water Res.,
8:253-260, 1974.
57. McCarthy, J. J., and C. H. Smith. A Review of Ozone and Its Appli-
cation to Domestic Wastewater Treatment. J. Am. Water Works
Assoc., 66:718-725, 1974.
203
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58. Poynter, S. F., J. S. Slade, and H. H. Jones. The Disinfection of
Water with Special Reference to Viruses. Water Treat. Exam.,
22:194-208, 1973.
59. Scaccia, C., and H. M. Rosen. Ozone Contacting: What is the
Answer? Presented at the Symposium on Advanced Ozone Technology,
International Ozone Institute, Toronto, Canada, November 1977.
60. Venosa, A. D., M. C. Meckes, E. J. Opatken, and J. W. Evans.
Comparative Efficiencies of Ozone Utilization and Microorganism
Reduction in Different Ozone Contactors. In: Progress in
Wastewater Disinfection Technology, EPA 600/9-79^018, NTIS Report
No. PB 299 338, MERL, Cincinnati, Ohio, 1979. pp. 141-161.
61. Process Design Manual for Nitrogen Control. EPA 625/1-75-007,
United States Environmental Protection Agency, Center for
Environmental Research Information, Cincinnati, Ohio, October 1975.
434 pp.
62. Process Design Manual for Phosphorus Removal. EPA 625/1-76-001,
United States Environmental Protection Agency, Center for
Environmental Research Information, Cincinnati, Ohio, April 1976.
63. Brandes, M. Accumulation Rate and Characteristics of Septic Tank
Sludge and Septage. J. Water Pollut. Control Fed., 50:936-943,
1978.
64. Laak, R., and F. J. Crates. Sewage Treatment by Septic Tank. In;
Proceedings of the Second Home Sewage Treatment Symposium, Chicago,
Illinois, December 1977. American Society of Agricultural Engi-
neers, St. Joseph, Michigan, 1978. pp. 54-60.
65. Sikora, L. J., and D. R. Keeney. Laboratory Studies on Stimulation
of Biological Denitrification. In: Proceedings of the National
Home Sewage Disposal Symposium, Chicago, Illinois, December 1975,
American Society of Agricultural Engineers, St. Joseph, Michigan,
1975. pp. 64-73.
66. Andreoli, A., N. Bartilucci, R, Forgione, and R. Reynolds. Nitro-
gen Demand in a Subsurface Disposal System. J. Water Pollut. Con-
trol Fed., 51:841-854, 1979.
67. Goronszy, M. C. Intermittent Operation of the Extended Aeration
Process for Small Systems. J. Water Pollut. Control Fed., 51:274,
1979.
68. Clifford, D. A., and W. J. Weber, Jr. Multicomponent Ion Exchange:
Nitrate Removal Process with Land Disposal Regenerant. Ind. Water
Eng., 15:18-26, 1978.
204
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69. Beulow, R. W., K. L. Kropp, J. Withered, and J. M. Symons. Nitrate
Removal by Anion Exchange Resins. Water Supply Research Labora-
tory, National Environmental Research Center, Cincinnati, Ohio,
1974.
70. Brandes, M. Effective Phosphorus Removal by Adding Alum to Septic
Tank. J. Water Pollut. Control Fed., 49:2285-2296, 1977.
71. Midkiff, W. S., and W. J. Weber, Jr. Operating Characteristics of
Strong Based Anion Exchange Reactor. Proc. Ind. Waste Conf.,
25:593-604, 1970.
72. Erickson, A. E., J. M. Tiedje, B. G. Ellis, and C. M. Hansen. A
Barriered Landscape Water Renovation System for Removing Phosphate
and Nitrogen from Liquid Feedlot Waste. In: Livestock Waste Man-
agement Pollution Abatement; Proceedings "of the International Sym-
posium on Livestock Wastes, St. Joseph, Michigan, 1971. pp. 232-
234.
73. Ellis, B. G., and A. E. Erickson. Movement and Transformation of
Various Phosphorus Compounds in Soils. Michigan Water Resources
Commission, Lansing, 1969.
74. Tofflemire, T. J., M. Chen, F. E. Van Alstyne, L. J. Hetling, and
D. B. Aulenbach. Phosphate Removal by Sands and Soils. Research
Unit Technical Paper 31, New York State Department of Environmental
Conservation, Albany, 1973. 92 pp.
75. Detweiler, J. C. Phosphorus Removal by Adsorption on Alumina as
Applied to Small Scale Waste Treatment. M.S. Report. University
of Wisconsin, Madison, 1978.
205
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CHAPTER 7
DISPOSAL METHODS
7.1 Introduction
Under the proper conditions, wastewater may be safely disposed of onto
the land, into surface waters, or evaporated into the atmosphere by a
variety of methods. The most commonly used methods for disposal of
wastewater from single dwellings and small clusters of dwellings may be
divided into three groups: (1) subsurface soil absorption systems, (2)
evaporation systems, and (3) treatment systems that discharge to surface
waters. Within each of these groups, there are various designs that may
be selected based upon the site factors encountered and the characteris-
tics of the wastewater. In some cases, a site limitation may be over-
come by employing flow reduction or wastewater segregation devices (see
Chapter 6). Because of the broad range of possible alternatives, the
selection of the most appropriate design can be difficult. The site
factor versus system design matrix presented in Chapter 2 should be con-
sulted to aid in this selection.
Onsite disposal methods discussed in this chapter are:
1. Subsurface soil absorption systems
- trenches and beds
- seepage pits
- mounds
- fills
- artificially drained systems
- electro-osmosis
2. Evaporation systems
- evapotranspiration and evapotranspiration-absorption
- evaporation and evaporation-percolation ponds
3. Treatment systems that .discharge to surface waters
Performance data and design, construction, operation, and maintenance
information are provided for each of these methods.
206
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7.2 Subsurface Soil Absorption
7.2.1 Introduction
Where site conditions are suitable, subsurface soil absorption is
usually the best method of wastewater disposal for single dwellings
because of its simplicity, stability, and low cost. Under the proper
conditions, the soil is an excellent treatment medium and requires
little wastewater pretreatment. Partially treated wastewater is
discharged below ground surface where it is absorbed and treated by the
soil as it percolates to the groundwater. Continuous application of
wastewater causes a clogging mat to form at the infiltrative surface,
which slows the movement of water into the soil. This can be beneficial
because it helps to maintain unsaturated soil conditions below the
clogging mat. Travel through two to four feet of unsaturated soil is
necessary to provide adequate removals of pathogenic organisms and other
pollutants from the wastewater before it reaches the groundwater.
However, it can reduce the infiltration rate of soil substantially.
Fortunately, the clogging mat seldom seals the soil completely.
Therefore, if a subsurface soil absorption system is to have a long
life, the design must be based on the infiltration rate through the
clogging mat that ultimately forms. Formation of the clogging mat
depends primarily on loading patterns, although other factors may impact
its development.
7.2.1.1 Types of Subsurface Soil Absorption Systems
Several different designs of subsurface soil absorption systems may be
used. They include trenches and beds, seepage pits, mounds, fills, and
artificially drained systems. All are covered excavations filled with
porous media with a means for introducing and distributing the waste-
water throughout the system. The distribution system discharges the
wastewater into the voids of the porous media. The voids maintain expo-
sure of the soil's infiltrative surface and provide storage for the
wastewater until it can seep away into the surrounding soil.
These systems are usually used to treat and dispose of septic tank ef-
fluent. While septic tank effluent rapidly forms a clogging mat in most
soils, the clogging mat seems to reach an equilibrium condition through
which the wastewater can flow at a reasonably constant rate, though it
varies from soil to soil (1)(2)(3)(4). Improved pretreatment of the
wastewater does not appear to reduce the intensity of clogging, except
in coarse granular soils such as sands (4)(5)(6).
207
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7.2.1.2 System Selection
The type of subsurface soil absorption system selected depends on the
site characteristics encountered. Critical site factors include soil
profile characteristics and permeability, soil depth over water tables
or bedrock, slope, and the size of the acceptable area. Where the soil
is at least moderately permeable and remains unsaturated several feet
below the system throughout the year, trenches or beds may be used.
Trenches and beds are excavations of relatively large areal extent that
usually rely on the upper soil horizons to absorb the wastewater through
the bottom and sidewalls of the excavation. Seepage pits are deep exca-
vations designed primarily for lateral absorption of the wastewater
through the sidewalls of the excavation; they are used only where the
groundwater level is well below the bottom of the pit, and where beds
and trenches are not feasible.
Where the soils are relatively impermeable or remain saturated near the
surface, other designs can be used to overcome some limitations. Mounds
may be suitable where shallow bedrock, high water table, or slowly per-
meable soil conditions exist. Mounds are beds constructed above the
natural soil surface in a suitable fill material. Fill systems are
trench or bed systems constructed in fill material brought in to replace
unsuitable soils. Fills can be used to overcome some of the same limi-
tations as mounds. Curtain or underdrain designs sometimes can be used
to artifically lower groundwater tables beneath the absorption area so
trenches or beds may be constructed. Table 2-1 presents the general
site conditions under which the various designs discussed in this manual
are best suited. For specific site criteria appropriate for each, refer
to the individual design sections in this chapter.
7.2.2 Trench and Bed Systems
7.2.2.1 Description
Trench and bed systems are the most commonly used method for onsite
wastewater treatment and disposal. Trenches are shallow, level excava-
tions, usually 1 to 5 ft (0.3 to 1.5 m) deep and 1 to 3 ft (0.3 to 0.9
m) wide. The bottom is filled with 6 in. (15 cm) or more of washed
crushed rock or gravel over which is laid a single line of perforated
distribution piping. Additional rock is placed over the pipe and the
rock covered with a suitable semi permeable barrier to prevent the back-
fill from penetrating the rock. Both the bottoms and sidewalls of the
trenches are infiltrative surfaces. Beds differ from trenches in that
they are wider than 3 ft (0.9 m) and may contain more than one line of
distribution piping (see Figures 7-1 and 7-2). Thus, the bottoms of the
beds are the principal infiltrative surfaces.
208
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FIGURE 7-1
TYPICAL TRENCH SYSTEM
Backfill
Perforated
Distribution
Pipe
Barrier
Material
3A - 2-1/2 in. Rock
Water Table or
Creviced Bedrock
209
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FIGURE 7-2
TYPICAL BED SYSTEM
Distribution
Box
,-^v.vv*<
k-Nl '. .S-« c^SkT^—-. »r--- >.?•>
Perforated
Distribution
Pipe
2-4 ft. min
Water Table or
Creviced Bedrock
J6-12 in. of
3/4-21/2 inch
dia. Rock
210
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7.2.2.2 Application
Site criteria for trench and bed systems are summarized in Table 7-1.
They are based upon factors necessary to maintain reasonable infiltra-
tion rates and adequate treatment performance over many years of con-
tinuous service. Chapter 3 should be consulted for proper site eval-
uation procedures.
The wastewater entering the trench or bed should be nearly free from
settleable solids, greases, and fats. Large quantities of these
wastewater constituents hasten the clogging of the soil (9). The
organic strength of the wastewater has not been well correlated with the
clogging mat resistance except in granular soils (4)(5). Water softener
wastes have not been found to be harmful to the system even when signi-
ficant amounts of clay are present (9)(10). However, the use of water
softeners can add a significant hydraulic load to the absorption system
and should be taken into account. The normal use of other household
chemicals and detergents have also been shown to have no ill effects on
the system (9).
7.2.2.3 Design
a. Sizing the Infiltrative Surface
The design of soil absorption systems begins at the infiltrative surface
where the wastewater enters the soil. With continued application of
wastewater, this surface clogs and the rate of wastewater infiltration
is reduced below the percolative capacity of the surrounding soil.
Therefore, the infiltrative surface must be sized on the basis of the
expected hydraulic conductivity of the clogging mat and the estimated
daily wastewater flow (see Chapter 4).
Direct measurement of the expected wastewater infiltration rate through
a mature clogging mat in a specific soil cannot be done prior to design.
However, experience with operating subsurface soil absorption systems
has shown that design loadings can sometimes be correlated with soil
texture (3).(4)(11)(12). Recommended rates of application versus soil
textures and percolation rates are presented in Table 7-2. This table
is meant only as a guide. Soil texture and measured percolation rates
will not always be correlated as indicated, due to differences in struc-
ture, clay mineral content, bulk densities, and other factors in various
areas of the country (see Chapter 3).
211
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TABLE 7-1
SITE CRITERIA FOR TRENCH AND BED SYSTEMS
Item
Criteria
Landscape Position3
SI opea
Typical Horizontal Separation
Distancesb
Water Supply Wells
Surface Waters, Springs
Escarpments, Manmade Cuts
Boundary of Property
Building Foundations
Soil
Texture
Structure
Color
Level, well drained areas, crests of
slopes, convex slopes most desirable.
Avoid depressions, bases of slopes and
concave slopes unless suitable surface
drainage is provided.
0 to 25%. Slopes in excess of 25% can
be utilized but the use of construction
machinery may be limited (7). Bed
systems are limited to 0 to 5%.
50
50
10
5
10
100 ft
100 ft
20 ft
10 ft
20 ft
Soils with sandy or loamy textures are
best suited. Gravelly a'nd cobbley
soils with open pores and slowly
permeable clay soils are less
desirable.
Strong granular, blocky or prismatic
structures are desirable. Platy or
unstructured massive soils should be
avoided.
Bright uniform colors indicate
well-drained, well-aerated soils.
Dull, gray or mottled soils indicate
continuous or seasonal saturation and
are unsuitable.
212
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TABLE 7-1 (continued)
Item Criteria
Layering Soils exhibiting layers with distinct
textural or structural changes should
be carefully evaluated to insure water
movement will not be severely
restricted.
Unsaturated Depth 2 to 4 ft of unsaturated soil should
exist between the bottom of the system
and the seasonally high water table or
bedrock (3)(4)(8).
Percolation Rate 1-60 min/in. (average of at least 3
percolation tests).0 Systems can be
constructed in soils with slower
percolation rates, but soil damage
during construction must be avoided.
a Landscape position and slope are more restrictive for beds because
of the depths of cut on the upslope side.
b Intended only as a guide. Safe distance varies from site to site,
based upon topography, soil permeability, ground water gradients,
geology, etc.
c Soils with percolation rates <1 min/in. can be used for trenches and
beds if the soil is replaced with a suitably thick (>2 ft) layer of
loamy sand or sand.
213
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TABLE 7-2
RECOMMENDED RATES OF WASTEWATER APPLICATION
FOR TRENCH AND BED BOTTOM AREAS (4)(!!){12)*
Soil Texture
Gravel , coarse sand
Coarse to medium sand
Fine sand, loamy sand
Sandy loam, loam
Loam, porous silt loam
Silty clay loam, clay loamd
Percolation Application
Rate Rateb
min/in. gpd/ft^
<1 Not suitable0
1-5 1.2
6-15 0.8
16 - 30 0.6
31 - 60 0.45
61 - 120 0.26
a May be suitable estimates for sidewall infiltration rates.
b Rates based on septic tank effluent from a domestic waste
source. A factor of safety may be desirable for wastes of
significantly different character.
c Soils with percolation rates <1 min/in. can be used if the
soil is replaced with a suitably thick (>2 ft) layer of loamy
sand or sand.
d Soils without expandable clays.
e These soils may be easily damaged during construction.
214
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Conventional trench or bed designs should not be used for rapidly perme-
able soils with percolation rates faster than 1 min/in. (0.4 min/cm)
(11). The rapidly permeable soils may not provide the necessary treat-
ment to protect the groundwater quality. This problem may be overcome
by replacing the native soil with a suitably thick (greater than 2 feet)
layer of loamy sand or sand textured soil. With the liner in place, the
design of the system can follow the design of conventional trenches and
beds using an assumed percolation rate of 6 to 15 min/in. (2.4 to 5.9
min/cm).
Conventional trench or bed designs should also be avoided in soils with
perc'olation rates slower than 60 min/in. (24 min/cm). These soils can
be easily smeared and compacted during construction, reducing the soil's
infiltration rate to as little as half the expected rate (12). Trench
systems may be used in soils with percolation rates as slow as 120
min/in (47 min/cm), but only if great care is exercised during construc-
tion. Construction should proceed only when the soil is sufficiently
dry to resist compaction and smearing during excavation. This point is
reached when it crumbles when trying to roll a sample into a wire be-
tween the palms of the hands. Trenches should be installed so that con-
struction machinery need not drive over the infiltrative surface. A 4-
to 6-in. (10- to 15-cm) sand liner in the bottom of the trench may be
used to protect the soil from compaction during placement of the aggre-
gate and to expose infiltrative surface that would otherwise be covered
by the aggregate (11)(13).
b. Geometry of the Infiltrative Surface
Sidewalls as Infiltrative Surfaces: Both the horizontal bottom area and
the vertical sidewalls of trenches and beds can act as infiltrative sur-
faces. When a gravity-fed system is first put into service, the bottom
area is the only infiltrative surface. However, after a period of
wastewater application, the bottom can become sufficiently clogged to
pond liquid above it, at which time the sidewalls become infiltrative
surfaces as well. Because the hydraulic gradients and resistances of
the clogging mats on the bottom and sidewalls are not likely to be the
same, the infiltration rates may be different. The objective in design
is to maximize the area of the surface expected to have the highest
infiltration rate while assuring adequate treatment of wastewater and
protection of the groundwater.
Because the sidewall is a vertical surface, clogging may not be as se-
vere as that which occurs at the bottom surface, due to several fac-
tors: (1) suspended solids in the wastewater may not be a significant
factor in sidewall clogging; (2) the rising and falling liquid levels in
the system allow alternative wetting and drying of the sidewall while
the bottom may remain continuously inundated; and (3) the clogging mat
215
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can slough off the sidewall. These factors tend to make the sidewall
clogging less severe than the bottom surface. However, the hydraulic
gradient across the sidewall mat is also less. At the bottom surface,
gravity, the hydrostatic pressure of the ponded water above, and the
matric potential of the soil below the mat contribute to the total
hydraulic gradient. At the.sidewall, the gravity potential is zero, and
the hydrostatic potential diminshes to zero at the liquid surface. Be-
cause the matric potential varies with changing soil moisture condi-
tions, it is difficult to predict which infiltrative surface will be
more effective.
In humid regions where percolating rainwater reduces the matric poten-
tial along the sidewall, shallow trench systems are suggested (4). The
bottom area is the principal infiltrative surface in these systems.
Shallow trenches often are best because the upper soil horizons are usu-
ally more permeable and greater evapotranspiration can occur. In dry
climates, the sidewall area may be used to a greater extent. The bottom
area may be reduced as the sidewall area is increased. Common practice
is not to give credit to the first 6 in. (15 cm) of sidewall area mea-
sured from the trench bottom, but any exposed sidewall above 6 in. (15
cm) may be used to reduce the bottom area (3) (11). The infiltration
rates given in Table 7-2 may be used for sidewall areas.
Trench versus Bed Design: Because beds usually require less total land
area and are less costly to construct, they are often installed instead
of trenches. However, trenches are generally more desirable than beds
(4)(11)(12)(13)(14). Trenches can provide up to five times more side-
wall area than do beds for identical bottom areas. Less damage is
likely to occur to the soil during construction because the excavation
equipment can straddle the trenches so it is not necessary to drive on
the infiltrative surface. On sloping sites, trenches can follow the
contours to maintain the infiltrative surfaces in the same soil horizon
and keep excavation to a minimum. Beds may be acceptable where the site
is relatively level and the soils are sands and loamy sands.
Shallow versus Deep Absorption Systems: Shallow soil absorption systems
offer several advantages over deep systems. Because of greater plant
and animal activity and less clay due to eluviation, the upper soil ho-
rizons are usually more permeable than the deeper subsoil. Also, the
plant activity helps reduce the loading on the system during the growing
season by transpiring significant amounts of liquid and removing some
nitrogen and phosphorus from the waterwater. Construction delays due to
wet soils are also reduced because the upper horizons dry more quickly.
On the other hand, deep systems have advantages. Increased depths per-
mit increased sidewall area exposure for the same amount of bottom area.
They also permit a greater depth of liquid ponding which increases the
216
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hydraulic gradient across the infiltrative surface. In some instances,
deep systems can be used to reach more permeable soil horizons when the
proximity of groundwater tables do not preclude their use.
Freezing of shallow absorption systems is not a problem if kept in con-
tinuous operation (4)(11). Carefully constructed systems with 6 to 12
in. (15 to 30 cm) of soil cover, which are in continuous operation, will
not freeze even in areas where frost penetration may be as great as 5 ft
(1.5 m) if the distribution pipe is gravel packed and header pipes insu-
lated where it is necessary for them to pass under driveways or other
areas usually cleared of snow.
Alternating Systems; Dividing the soil absorption system into more than
one field to allow alternate use of the individual fields over extended
periods of time can extend the life of the absorption system. Alterna-
ting operation of the fields permits part of the system to "rest" peri-
odically so that the infiltrative surface can be rejuvenated naturally
through biodegradation of the clogging mat (4) (11)(12)(13)(15)(16). The
"resting" field also acts as a standby unit that can be put into immedi-
ate service if a failure occurs in the other part of the system. This
provides a period of time during which the failed field can be rehabili-
tated or rebuilt without an unwanted discharge.
Alternating systems commonly consist of two fields. Each field contains
50 to 100% of the total required area for a single field. Common prac-
tice is to switch fields on a semiannual or annual schedule by means of
a diversion valve (see Figure 7-3 and Chapter 8). Though it has not yet
been proven, such operation may permit a reduction in the total system
size. In sandy soils with a shallow water table, the use of alternating
beds may increase the chance of groundwater contamination because of the
loss of treatment efficiency when the clogging mat is decomposed after
resting.
c. Layout of the System
Location: Locating the area for the soil absorption system should be
done with care. On undeveloped lots, the site should be located prior
to locating the house, well, drives, etc., to ensure the best area is
reserved. The following recommendations should be considered when
locating the soil absorption system:
1. Locate the system where the surface drainage is good. Avoid
depressions and bases of slopes and areas in the path of runoff
from roofs, patios, driveways, or other paved areas unless sur-
face drainage is provided.
217
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FIGURE 7-3
ALTERNATING TRENCH SYSTEM WITH DIVERSION VALVE
Septic
Tank
Diversion
Valve
2.
3.
In areas with severe winters, avoid areas that are kept clear
of snow. Automobiles, snowmobiles, and other vehicles should
not be allowed on the area. Compacted or cleared snow will
allow frost to penetrate the system, and compacted soil and
losf of vegetation from traffic over the system will reduce
evapotranspiration in the summer.
Preserve as many trees as possible. Trenches may be run be-
tween trees. Avoid damaging the trees during construction.
is £e>erable°nto a single trench because of the flexibility ,t offers in
wastewater application.
nn lots with insufficient area for trenches or on sites with granular
?he excUJaY iSn becomes too deep on the upslope side. In such instances,
218
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deep trenches with a greater depth of rock below the distribution pipe
to increase the sidewall area is more suitable.
Reserve Area: When planning and locating the absorption system, consid-
eration should be given to reserving a suitable area for construction of
a second system. The second system would be added if the first were to
fail or if the system required expansion due to increased wastewater
flows. Care must be used in constructing the second system so that the
original system is not damaged by the construction equipment.
The reserve area should be located to facilitate simultaneous or alter-
nating loading of both systems. If the reserve area is used because the
initial system has failed, the failing system should not be permanently
abandoned. With time, the initial system will be naturally rejuvenated
and can be used alternately with the reserve system. Reserve areas can
be provided very easily with trench systems by reserving sufficient area
between the initial trenches as shown in Figure 7-4.
Dimensions: The absorption system should be dimensioned to best fit the
lot while maintaining separation distances and avoiding excessive depths
of excavation. Commonly used dimensions are given in Table 7-3.
The depth of excavation is determined by the location of the most perme-
able soil horizon and flow restricting layers or the high water table
elevation. Unless a deep, more permeable horizon exists, the trench or
bed bottom elevation should be maintained at about 18 to 24 in. (46 to
61 cm) below the natural ground surface. To prevent freezing in cold
climates, 6 to 12 in. (15 to 30 cm) of cover should be backfilled over
the aggregate (11).
If the water table or a very slowly permeable layer is too near the
ground surface to construct the system at this depth, the system can be
raised. Very shallow trenches 6 to 12 in. (15 to 30 cm) deep can be in-
stalled and the area backfilled with additional soil (see Figure 7-5).
Adequate separation distance must be provided between the trench bottom
and the seasonally high groundwater level to prevent groundwater
contamination.
The length of the trench or bed system depends on the site characteris-
tics. The length of the distribution laterals is commonly restricted to
100 ft (30 m). This is based on the fears of root penetration, uneven
settling, or pipe breakage which could disrupt the flow down the pipe to
render the remaining downstream length useless. However, these fears
are unwarranted because the aggregate transmits the wastewater (4)(13)
(17). To assure adequate transmission and distribution of the
219
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FIGURE 7-4
PROVISION OF A RESERVE AREA BETWEEN TRENCHES
OF THE INITIAL SYSTEM ON A SLOPING SITE
From
Pretreatment
Reserve
System
Drop
Box
Trenches
Following
Contours
Primary
System
Diversion
Valve
Drop Box
220
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wastewater through the aggregate, extreme care must be taken to con-
struct the.trench bottom at the same elevation throughout its length.
The overriding considerations for determining trench or bed lengths are
the site characteristics.
Spacing between trench sidewalls could be as little as 18 in. (46 cm).
A spacing of 6 ft (1.8 m) is suggested, however, to facilitate con-
struction and to provide a reserve area between trenches.
TABLE 7-3
TYPICAL DIMENSIONS FOR TRENCHES AND BEDS
Bottom Cover
System Wi dth Lengthb Depthc Thickness Spacingd
FF~ ft FETTu fl
Trenches 1-3* 100 1.5-2.0 6 (min) 6
Beds >3 100 1.5-2.0 6 (min) 6
a Excavations generally should not be less than 1 ft wide
because the sidewall may slough and infiltrate the aggregate(lO)
b Length of lateral from distribution inlet manifold. May be
greater if site characteristics demand.
c May be deeper if a more suitable horizon exists at greater
depth and sufficient depth can be maintained between the bottom
and seasonably high water table.
d From sidewall to sidewall. Trench spacing may be decreased
because of soil flow net patterns, specifically for shallow
trenches in sandy soils.
221
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FIGURE 7-5
TRENCH SYSTEM INSTALLED TO OVERCOME A SHALLOW WATER
TABLE OR RESTRICTIVE LAYER [AFTER (11)]
Diversion for
Surface Water
12-18 in. Soil Backfill Over Area
6-12 in.
2-4
Seasonally High Water Table
or Flow Restrictive Layer
Drainfield
Trenches
ft.
Original.
Ground
—1 Surface
d. Effluent Distribution
Methods of Application; To ensure that the absorption system performs
satisfactorily over a reasonably long lifetime, the method of wastewater
application to the infiltrative surface must be compatible with the ex-
isting soil and site characteristics. Methods of wastewater application
can be grouped into three categories: (1) gravity flow; (2) dosing; and
(3) uniform application. For designs of distribution networks employing
these methods, see Section 7.2.8)
1. Gravity flow is the simplest and most commonly employed of the
distribution methods. Wastewater is allowed to flow into the
absorption system directly from the treatment unit. With time,
a clogging mat usually develops on the bottom surface of the
absorption system and continuous ponding of the wastewater
results. This may lead to more severe clogging of the soil,
reducing the infiltration rate. However, this effect may be
offset by the greater effective infiltrative area provided by
submerging the sidewalls of the system, particularly in trench
systems. The ponding also increases the hydraulic gradient
222
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across the clogging mat, which can increase the infiltration
rate (2)(18).
If adequate treatment is to be achieved in coarse granular
soils such as sands, wastewater application by gravity flow
requires that a clogging mat exist at the infiltrative surfaces
to prevent saturated conditions in the underlying soil and to
prevent groundwater contamination. The mat develops with con-
tinued application, but groundwater contamination by pathogenic
organisms and viruses can be a danger at first.
2. Dosing can be employed to provide intermittent aeration of the
infiltrative surface. In this method, periods of loading are
followed by periods of resting, with cycle frequencies ranging
from hours to days. The resting phase should be sufficiently
long to allow the system to drain and expose the infiltrative
surface to air, which encourages rapid degradation of the clog-
ging materials by aerobic bacteria.
This method of operation may increase the rate of infiltration,
as well as extend the life of the absorption system, because
the clogging mat resistance is reduced (1)(4)(6)(15)(17). In
sands or coarser textured materials, the rapid infiltration
rates can lead to bacterial and viral contamination of shallow
groundwater, expecially when first put into use (4). There-
fore, systems constructed in these soils should be dosed with
small volumes of wastewater several times a day to prevent
large saturated fronts moving through the soil. In finer tex-
tured soils, absorption, rather than treatment, is the con-
cern. Large, less frequent doses are more suitable in these
soils to provide longer aeration times between doses (4). See
Table 7-4 for suggested dosing frequencies.
3. Uniform Application means applying the wastewater in doses uni-
formly over the entire bottom area of the system to minimize
local overloading and the depth of ponding following each dose.
This is usually achieved with a pressure distribution network.
In this manner, the soil is more likely to remain unsaturated
even during initial start-up when no clogging mat is present.
The minimum depths of ponding during application permit rapid
draining and maximum exposure of the bottom surface to air
which reduces the clogging mat resistance. The sidewall is
lost as an infiltrative surface, but this may be compensated
for by the maintenance of higher infiltration rates through the
bottom surface. See Table 7-4 for suggested dosing frequen-
cies.
223
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TABLE 7-4
DOSING FREQUENCIES FOR VARIOUS SOIL TEXTURES
Soil Texture Dosing Frequency
Sand 4 Doses/Day
Sandy Loam 1 Dose/Day
Loam Frequency Not Critical3
Silt Loam 1 Dose/Daya
Silty Clay Loam
Clay Frequency Not Critical9
a Long-term resting provided by alternating fields may be
desirable.
Selection of Application Method: The selection of an appropriate method
ofwastewater applicationdepends on whether improved absorption or
improved treatment is the objective. This is determined by the soil
permeability and the geometry of the infiltrative surface. Under some
conditions, the method of application is not critical, so selection is
based on simplicity of design, operation, and cost. Methods of appli-
cation for various soil and site conditions are summarized in Table 7-
5. Where more than one may be appropriate, the methods are listed in
order of preference.
e. Porous Media
The function of the porous media placed below and around the distribu-
tion pipe is four-fold. Its primary purposes are to support the dis-
tribution pipe and to provide a media through which the wastewater can
flow from the distribution pipe to reach the bottom and sidewall infil-
tration areas. A second function is to provide storage of peak waste-
water flows. Third, the media dissipates any energy that the incoming
wastewater may have which could erode the infiltrative surface.
Finally, the media supports the sidewall of the excavation to prevent
its collapse.
224
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TABLE 7-5
METHODS OF WASTEWATER APPLICATION FOR VARIOUS SYSTEM DESIGNS
AND SOIL PERMEABILITIES9
Soil
Permeabi11ty
(Percolation Rate)
Trenches or Beds
(Fills, Drains)
On Level Site
Trenches (Drains)
On SI opi ng
Site (>5%)
Very Rapid
(<1 min/in.)
Rapi d
(1-10 min/in.)
Moderate
(11-60 min/in.)
Slow
(>60 min/in.)
Uniform Application15
Dosi ng
Uniform Application
Dosing
Gravity
Dosi ng
Gravity
Uniform Application
Not Critical
Gravity
Dosi ng
Gravity
Dosing
Gravity
Dosing
Not Critical
a Methods of application are listed in order of preference.
b Should be used in alternating field systems to ensure adequate
treatment.
225
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The depth of the porous media may vary. A minimum of 6 in. (15 cm)
below the distribution pipe invert and 2 in. (5 cm) above the crown of
the pipe is suggested. Greater depths may be used to increase the
sidewall area and to increase the hydraulic head on the infiltrative
surface.
Gravel or crushed rock is usually used as the porous media, though other
durable porous materials may be suitable. The suggested gravel or rock
size is 3/4 to 2-1/2 in. (1.8 to 6.4 cm) in diameter. Smaller sizes are
preferred because masking of the infiltrative surface by the rock is
reduced (13). The rock should be durable and resistant to slaking and
dissolution. A hardness of 3 or greater on the Moh's Scale of Hardness
is suggested. Rock that can scratch a copper penny without leaving any
residual rock meets this criterion. Crushed limestone is unsuitable
unless dolomitic. The media should be washed to remove all fines that
could clog the infiltrative surface.
To maintain the porous nature of the media, the media must be covered
with a material to prevent backfilled soil from entering the media and
filling the voids. Treated building paper was once used but has been
abandoned in favor of untreated building paper, synthetic drainage fab-
ric, marsh hay or straw. These materials do not create a vapor barrier
and permit some moisture to pass through to the soil above where it can
be removed through evapotranspiration. All these materials, except for
the drainage fabric, will eventually decay. If they decay before the
soil has stabilized, the value of the materials is lost. To ensure the
barrier is not lost prematurely, heavy duty building paper of 40 to 60
Ib (18 to 27 kg) weight or a 4 to 6 in. (10 to 15 cm) layer of marsh hay
or straw should be used. In dry sandy soils, a 4 in. (10 cm) layer of
hay or straw covered with untreated building paper is suggested to pre-
vent the backfill from filtering down into the rock.
f. Inspection Pipes
Inspection pipes located in the subsurface soil absorption system pro-
vide limited access to observe the depth of ponding, a measure of the
performance of the system, and a means of locating the subsurface field.
If used, the inspection pipes should extend from the bottom infiltrative
surface of the system up to or above final grade. The bottom should be
open and the top capped. The portion of the pipe within the gravel
should be perforated to permit a free flow of water (see Figure 7-6).
226
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7.2.2.4 Construction
A frequent cause of early failure of soil absorption systems is the use
of poor construction techniques. The following should be considered for
construction of a soil absorption system:
a. Layout
The system should be laid out to facilitate the maneuvering of construc-
tion equipment so that damage to the soil is minimized.
1. Absorption system area should be staked out and roped off
immediately after the site evaluation to keep construction
equipment and other vehicles off the area until construction of
the system begins.
2. Trenches rather than beds are preferable in soils with signifi-
cant clay content (greater than 25% by weight) because equip-
ment can straddle the trenches. This reduces the compaction
and smearing at the exposed infiltrative surface.
3. Trenches should be spaced at least 6 ft (1.8 m) apart to facil-
itate the operation of the construction equipment if there is
sufficient area.
4. To minimize sidewall compaction, trench widths should be made
larger than the bucket used for excavation. Buckets are made
to compact the sidewall to prevent caving during excavation.
If the excavation is wider than the bucket, this effect is min-
imized. An alternative is to use modified buckets with side
cutters or raker teeth (see Figure 7-7).
5. Trenches should follow the contour and be placed outside the
drip lines of trees to avoid root damage.
b. Excavation
Absorption of waste effluent by soil requires that the soil pores remain
open at the infiltrative surface. If these are sealed during construc-
tion by compaction, smearing, or puddling of the soil, the system may be
rendered useless. The tendency toward compaction, smearing, and pud-
dling depends upon the soil type, moisture content, and applied force.
227
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FIGURE 7-6
TYPICAL INSPECTION PIPE
Vent Cap,
Distribution
Pipe
^K<$i^-%-
*&f#'* GRAVE"!? o$
4" Perforated
Inspection Pipe
-Open Bottom
FIGURE 7-7
BACKHOE BUCKET WITH REMOVABLE RAKER TEETH (11)
in. Rods or Bolts
Approximately 1-1/2 in. Long
Spaced Approximately 3 in. on
Center
228
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Soils with high clay contents (greater than 25% by weight) are very
susceptible to damage, while sands are rarely affected. Careful con-
struction techniques minimize this soil damage. They include:
1. Excavation may proceed in clayey soils only when the moisture
content is below the soil's plastic limit. If a sample of soil
taken at the depth of the proposed bottom of the system forms a
"wire" instead of crumbling when attempting to roll it between
the hands, the soil is too wet.
2. A backhoe is usually used to excavate the system. Front-end
loaders or bulldozer blades should not be used because the
scraping action of the bucket or blade can smear the soil se-
verely, and the wheels or tracks compact the exposed
infiltrative surface.
3. Excavation equipment must not be driven on the bottom of the
system. If trenches are used, the equipment can straddle the
excavation. If a bed is used, the bed should be divided into
segments so the machinery can always operate from undisturbed
soil.
4. The bottom of each trench or bed must be level throughout to
ensure more uniform distribution of effluent. A level and tri-
pod are essential equipment.
5. The bottom and sidewalls of the excavation should be left with
a rough open surface. Any smeared and compacted surfaces
should be removed with care.
6. Work should be scheduled only when the infiltrative surface can
be covered in one day, because wind-blown silt or raindrop im-
pact can clog the soil.
c. Backfilling
Once the infiltrative surface is properly prepared, the backfilling
operations must be done carefully to avoid any damage to the soil.
1. The gravel or crushed rock used as the porous media is laid in
by a backhoe or front-end loader rather than dumped in by
truck. This should be done from the sides of the system rather
than driving out onto the exposed bottom. In large beds, the
gravel or rock should be pushed out ahead of a small bulldozer.
2. The distribution pipes are covered with a minimum of 2 in. (5
cm) of gravel or rock to retard root growth, to insulate
229
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against freezing and to stabilize .the pipe before backfill-
ing. Procedures for constructing the distribution network are
discussed in Section 7.2.8.
3. The gravel or rock is covered with untreated building paper,
synthetic drainage fabric, marsh hay or straw to prevent the
unconsolidated soil cover from entering the media. The media
should be covered completely. If untreated building paper is
used, the seams should overlap at least 2 in. (5 cm) and any
tears covered. If marsh hay or straw is used, it should be
spread uniformly to a depth of 4 to 6 in. (10 to 15 cm). In
bed construction, spreading a layer of hay or straw covered
with untreated building paper is good practice.
4. The backfill material should be similar to the natural soil and
no more permeable. It should be mounded above natural grade to
allow for settling and to channel runoff away from the system.
7.2.2.5 Operation and Maintenance
a. Routine Maintenance
Once installed, a subsurface soil absorption system requires little or
no attention as long as the wastewater discharged into it is nearly free
of settleable solids, greases, fats, and oils. This requires that the
pretreatment unit be maintained (see Chapter 6). To provide added in-
surance that the system will have a long, useful life, the following
actions are suggested:
1. Resting of the system by taking it out of service for a period
of time is an effective method of restoring the infiltration
rate. Resting allows the absorption field to gradually drain,
exposing the infiltrative surfaces to air. After several
months, the clogging mat is degraded through biochemical and
physical processes (1)(4)(6)(13)(15). This requires that a
second absorption system exist to allow continued disposal,
while the first is in the resting phase. The systems can be
alternated on a yearly basis by means of a diversion valve (see
Figure 7-3).
2. The plumbing fixtures in the home should be checked regularly
to repair any leaks which can add substantial amounts of water
to the system.
3. The use of special additives such as yeast, bacteria, chemi-
cals, and enzyme preparations is not necessary and is of
230
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little value for the proper function of the soil absorption
system (3)(4).
4. Periodic application of oxidizing agents, particularly hydrogen
peroxide, are being tried as a preventative maintenance proce-
dure (19). If properly applied, the agents oxidize the clog-
ging mat to restore much of the system's infiltration capacity
within a day or two. Handling of these agents is very danger-
ous, and therefore the treatment should be done by trained
individuals only. Experience with this treatment has been
insufficient to determine its long-term effectiveness in a
variety of soil types.
b. Rehabilitation
Occasionally, soil absorption systems fail, necessitating their reha-
bilitation. The causes of failure can be complex, resulting from poor
siting, poor design, poor construction, poor maintenance, hydraulic
overloading, or a combination of these. To determine the most approp-
riate method of rehabilitation, the cause of failure must be determined.
Figure 7-8 suggests ways to determine the cause of failure and corres-
ponding ways of rehabilitating the system.
The failure frequency should be determined before isolating the cause.
Failure may occur occasionally or continuously. Occasional failure man-
ifests itself with occasional seepage on the ground surface, sluggish
drains, or plumbing backups. These usually coincide with periods of
heavy rainfall or snowmelt. Continuous failure can have the same symp-
toms but on a continuous basis. However, some systems may seriously
contaminate the groundwater with no surface manifestations of failure.
These failures are detected by groundwater sampling.
Occasional Failure: The cause of occasional failure is much easier to
determine, and rehabilitation can be more simple. Since the system
functions between periods of failure, sizing and construction usually
can be eliminated as the cause. In these instances, failure is the
result of poor siting, poor maintenance, or hydraulic overloading.
Excessive water use, plumbing leaks, or foundation drain discharges are
common reasons for hydraulic overloading. These can be corrected by the
appropriate action as indicated in Figure 7-8.
The next step is to investigate the site of the absorption system.
Occasional failure usually is due to poor drainage or seasonally high
water table conditions. The surface grading and landscape position
should be checked for poor surface drainage conditions. Local soil
conditions should also be investigated by borings for seasonally high
231
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FIGURE 7-8
METHODS OF SOIL ABSORPTION FIELD REHABILITATION
Failure Noted
oo
ro
Determine Failure Frequency
Periodic
Determine Cause
^—-"T^
*--^ \
Siting Overloading
Check Check:
• Landscape * Plumbing
Within 1
—- - — -
Year
Continuous
1
Determine Age of System When
Failure First Occurred
^ ^^
^^ ^^—
Determine Cause
- — ^^^ Siting/
~~ — •— ^^_ Design/
Maintenance Construction
Check: Check:
• Ma.ntenance . So,l Type (Tex-
Position Record ture Hydraulic
• Excessive Water
Use Conductivity)
* Surface Grading
• Condition of • Unsati
rated
• Foundation Dram Treatment Depth of Soil
. .. !!• L. Discharge
• Depth to High 3
Groundwater
* Neighboring Systems
Conditions
1
Corrective Actions Corrective Act
Unit
_^-—
„—-
Overloading
Check:
• Plumbing
• System Loading
* Waste Character-
istics
—
After 1 Year
Determine Cause
.--— "
Maintenance
Check:
• Maintenance Record
• Condition of Tank
• Condition of Field
• System Age
* Landscape
Position
• Building Lateral
' System Size
(Tank and Field)
ons Corrective Actions
Corrective Actions
Corrective Actions
* Distribution of
• Draining • Repair Plumbing • Pumping Effluent
• Regrading/Filling • Flow Reduction • Repair
• Holding Tank (when • Eliminate Clear * Infiltra
lion Rate
necessary) Water Discharges
• Reconstruction
• Neighboring Sys-
tern
Corrects
• Dosin
s Conditions
e Actions
g
• Repair Plumbing
• Flow Reduction
• Eliminate Clear
Water Discharges
. Waste Segregation
• Improved Pretreatment
• Pumping
m Hydrogen Peroxide
• Resting
• Addition
• Reconstruct
Conventional
Alternate
Holding Tank
Community Facility
-------
water tables (see Chapter 3). Corrective actions include improving
surface drainage by regrading or filling low areas. High water table
conditions may be corrected in some instances by installing drains (see
Section 7.2.6).
Lack of maintenance of the treatment unit preceding the soil absorption
field may also be a cause of occasional failure. The unit may be a
point of infiltration and inflow during wet periods. The unit should be
pumped and leaks repaired.
Continuous Failure: The causes of continuous failure are more difficult
to determine.However, learning the age of the system when failure
first occurred is very useful in isolating the cause.
If failure occurred within the first year of operation, the cause is
probably due to poor siting, design, or construction. It is useful to
check the performance of neighboring systems installed in similar
soils. If they have similar loading rates and are working well, the
failing system should be checked for proper sizing. A small system can
be enlarged by adding new infiltration areas. In some instances, the
sizing may be adequate but the distribution of the wastewater is poor
due to improper construction. Providing dosing may correct this problem
(see Section 7.2.8). Damage to the soil during construction may also
cause failure, in which case the infiltrative area is less effective.
Reconstruction or an addition is necessary. Alternate systems should be
considered if the site is poor. This includes investigating the fea-
sibility of a cluster or community system if surrounding systems are
experiencing similar problems.
If the system had many years of useful service before failure occurred,
hydraulic overloading or poor maintenance is usually the cause. The
first step is to find out as much about the system as possible. A
sketch of the system showing the size, configuration, and location
should be made. A soil profile description should also be obtained.
These items may be on file at the local regulatory agency but their ac-
curacy should be confirmed by an onsite visit. If the system provided
several years of useful service, evidences of overloading should be
investigated first. Wastewater volume and characteristics (solids,
greases, fats, oil) should be determined. Overloading may be corrected
by repairing plumbing, installing flow reduction fixtures (see Chapter
5), and eliminating any discharges from foundation drains. If the vol-
ume reductions are insufficient for acceptance by the existing infiltra-
tive surface, then additional infiltrative areas must be constructed.
Systems serving commercial buildings may fail because of the wastewater
characteristics. High solids concentrations or large amounts of fats,
233
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oils, and greases, can cause failure. This is. particularly true of sys-
tems serving restaurants and laundromats. These failures can be correc-
ted by segregating the wastewaters to eliminate the troublesome waste-
waters (see Chapter 5), or by improving pretreatment (see Chapter 6).
Lack of proper maintenance of the treatment unit may have resulted in
excessive clogging due to poor solids removal by the unit. This can be
determined by checking the maintenance record and the condition of the
unit. If this appears to be the problem, the unit should be pumped and
repaired, or replaced if necessary. The infiltrative surface of the ab-
sorption field should also be checked. If siting, design, or mainten-
ance do not appear to be the cause of failure, excessive clogging is
probably the problem. In such cases, the infiltrative surface can some-
times be rejuvenated by oxidizing the clogging mat (4)(9)(13)(16). This
can be done by allowing the system to drain and rest for several months
(4). To permit resting, a new system must be constructed with means
provided for switching back and forth. Alternatively, the septic tank
could be operated as a holding tank until the clogging mat has been
oxidized. However, this involves frequent pumping, which may be costly.
Another method, still in the experimental stage, is the use of the
chemical oxidant, hydrogen peroxide (16). Because it is new, it is not
known if it will work well in all soils. Extreme care should be used in
its application because it is a strong oxidizing agent. Only individu-
als trained in its use should perform the treatment.
7.2.2.6 Considerations for Multi-Home and Commercial
Wastewaters
Design of trench and bed soil absorption systems for small institutions,
commercial establishments, and clusters of dwellings generally follows
the same design principles as for single dwellings. In cluster systems
serving more than about five homes, however, peak flow estimates can be
reduced because of flow attenuation, but contributions from infiltration
through the collection system must be included. Peak flow estimates
should be based on the total number of people to be served (see Chapter
4). Rates of infiltration will vary with the type of collection sewer
used (19M20).
With commercial flows, the character of the wastewater is an important
consideration. Proper pretreatment is necessary if the character is
significantly different than domestic wastewater.
Flexibility in operation should also be incorporated into systems serv-
ing larger flows since a failure can create a significant problem. Al-
ternating bed systems should be considered. A three-field system can be
constructed in which each field contains 50% of the required absorption
234
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area (19). This design allows flexibility in operation. Two beds are
always in operation, providing 100% of the needed infiltrative surface.
The third field is alternated in service on a semi annual or annual
schedule. Thus, each field is in service for one or two years and
"rested" for 6 months to one year to rejuvenate. The third field also
acts as a standby unit in case one field fails. The idle field can be
put into service immediately while a failed field is rehabilitated.
Larger systems should utilize some dosing or uniform application to
assure proper performance.
7.2.3 Seepage Pits
7.2.3.1 Description
Seepage pits or dry wells are deep excavations used for subsurface dis-
posal of pretreated wastewater. Covered porous-walled chambers are
placed in the excavation and surrounded by gravel or crushed rock (see
Figure 7-9). Wastewater enters the chamber where it is stored until it
seeps out through the chamber wall and infiltrates the sidewall of the
excavation.
Seepage pits are generally discouraged by many local regulatory agencies
in favor of trench or bed systems. However, seepage pits have been
shown to be an acceptable method of disposal for small wastewater flows
(21). Seepage pits are used where land area is too limited for trench
or bed systems; and either the groundwater level is deep at all times,
or the upper 3 to 4 ft (0.9 to 1.2 m) of the soil profile is underlain
by a more permeable unsaturated soil material of great depth.
7.2.3.2 Site Considerations
The suggested site criteria for seepage pits are similar to those for
trench and bed systems summarized in Table 7-1 except that soils with
percolation rates slower than 30 min/in. (12 min/cm) are generally ex-
cluded. In addition, since the excavation sidewall is used as the
infiltrative surface, percolation tests are run in each soil layer en-
counted. Maintaining sufficient separation between the bottom of the
seepage pit and the high water table is a particularly important con-^"
sideration for protection of groundwater quality.
235
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FIGURE 7-9
SEEPAGE PIT CROSS SECTION
4" Inspection Pipe
Influent,
Reinforced Concrete Cover
Extended to Solid Earth
Effluent
Brick, Block, Ring, or
.Precast Chamber
with Open Joints
6" to 12" of 3/4-21/2"
Clean Rock
Water Table
or Impervious Layer
4' min. Unsaturated Soil
i
7.2.3.3 Design
a. Sizing the Infiltrative Surface
Since the dominant infiltration surface of a seepage pit is the side-
wall, the depth and diameter of the pit is determined from the perco-
lation rate and thickness of each soil layer exposed by the excavation.
A weighted average of the percolation test results (sum of thickness
times percolation rate of each layer divided by the total thickness) is
used. Soil layers with percolation rates slower than 30 min/in. (12
min/cm) are excluded from this computation (3).
The weighted percolation rate is used to determine the required sidewall
area. Infiltration rates presented previously in Table 7-2 are used
with the estimated daily wastewater flow to compute the necessary
sidewall area.
236
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Table 7-6 can be used to determine the necessary seepage pit sidewall
area for various effective depths below the seepage pit inlet.
TABLE 7-6
SIDEWALL AREAS OF CIRCULAR SEEPAGE PITS (ft2)3
Seepage15
Pit
Pi ameter
ft
1
2
3
4
5
6
7
8
9
10
11
12
Thickness of Effective Layers Below Inlet (ft)
10
3.1
6.3
9.4
12.6
15.7
18.8
22.0
25.1
28.3
31.4
34.6
37.7
6
13
19
25
31
38
44
50
57
63
69
75
9
19
28
38
47
57
66
75
85
94
104
113
13
25
38
50
63
75
88
101
113
126
138
151
16
31
47
63
79
94
110
126
141
157
173
188
19
38
57
75
94
113
132
151
170
188
207
226
22
44
66
88
110
132
154
176
198
220
242
264
25
50
75
101
126
151
176
201
226
251
276
302
28
57
85
113
141
170
198
226
254
283
311
339
31
63
94
126
157
188
220
251
283
314
346
377
a Areas for greater depths can be found by adding columns. For
example, the area of a 5 ft diameter pit, 15 ft deep is equal to
157 + 79, or 236 ft.
b Diameter of excavation.
b. System Layout
Seepage pits may be any diameter or depth provided they are structurally
sound and can be constructed without seriously damaging the soil. Typi-
cally, seepage pits are 6 to 12 ft (1.8 to 3.6 m) in diameter and 10 to
20 ft (3 to 6 m) deep but pits 18 in. (0.5 m) in diameter and 40 ft (12
m) deep have been constructed (22). When more than one pit is required,
experience has shown that a separation distance from sidewall to side-
wall equal to 3 times the diameter of the largest pit should be main-
tained (3).
237
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The same guidelines used in locating trenches and beds are used to lo-
cate seepage pits. Area should be reserved for additional pits if
necessary.
7.2.3.4 Construction
Pits may be dug with conventional excavating equipment or with power
augers. Particular care must be exercised to ensure that the soils are
not too wet before starting construction. If powered bucket augers are
used, the pits should be reamed to a larger diameter than the bucket to
minimize compaction and smearing of the soil. Power screw augers should
only be used in granular soils because smearing of the sidewall is dif-
ficult to prevent with such equipment.
To maximize wastewater storage, porous walled chambers without bottoms
are usually used. Precast concrete seepage chambers may be used or the
chambers may be constructed out of clay or concrete brick, block or
rings. The rings must have notches in them to provide for seepage.
Brick or block are laid without mortar, with staggered open joints.
Hollow block may be laid on its side but a 4-in. (10-cm) wall thickness
should be maintained. Large-diameter perforated pipe standing on end
can be used in small diameter pits. Six to 12 in. (15 to 30 cm) of
clean gravel or 3/4 to 2-1/2 in. (1.8 to 6.4 cm) crushed rock is placed
at the bottom of the excavation prior to placement or construction of
the chamber. This provides a firm foundation for the chamber and
prevents bottom soil from being removed if the pit is pumped.
The chamber is constructed one to two feet smaller in diameter than the
excavation. The annular space left between the wall of the chamber and
the excavation is filled with clean gravel or crushed rock to the top of
the chamber.
Covers of suitable strength to support the soil cover and any antici-
pated loads are placed over the chamber and extend at least 12 in.
beyond the excavation. Access to the pit for inspection purposes can be
provided by a manhole. If a manhole is used, it should be covered with
6 to 12 in. (15 to 30 cm) of soil. An inspection pipe can extend to
ground surface. A noncorrosive, watertight cap should be used with the
inspection pipe.
233
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7.2.3.5 Maintenance
A well-designed and constructed seepage pit requires no routine mainte-
nance. However, failure ocassionally occurs. Pumping and resting is
the only reasonable rehabilitation technique available.
7.2.4 Mound Systems
7.2.4.1 Description
The mound system was originally developed in North Dakota in the late
1940's where it became known as the NODAK disposal system (23). The
mound was designed to overcome problems with slowly permeable soils and
high water tables in rural areas. The absorption bed was constructed in
coarse gravel placed over the original soil after the topsoil was re-
moved. Monitoring of these systems revealed that inadequate treatment
occured before the groundwater was reached, and seepage often occurred
during wet periods of the year. Successful modifications of the design
were made to overcome these limitations (4). Mound systems are now used
under a variety of conditions.
A mound system is a soil absorption system that is elevated above the
natural soil surface in a suitable fill material. The purpose of the
design is to overcome site restrictions that prohibit the use of conven-
tional soil absorption systems (4)(24). Such restrictions are: (1)
slowly permeable soils, (2) shallow permeable soils over creviced or
porous bedrock, and (3) permeable soils with high water tables. In
slowly permeable soils, the mound serves to improve absorption of the
effluent by utilizing the more permeable topsoil and eliminating con-
struction in the wetter and more slowly permeable subsoil, where smear-
ing and compaction are often unavoidable. In permeable soils with
insufficient depth to groundwater or creviced or porous bedrock, the
fill material in the mound provides the necessary treatment of the
wastewater (see Figure 7-10).
The mound system consists of: (Da suitable fill material, (2) an ab-
sorption area, (3) a distribution network, (4) a cap, and (5) top soil
(see Figure 7-11). The effluent is pumped or siphoned into the absorp-
tion area through a distribution network located in the upper part of
the coarse aggregate, it passes through the aggregate and infiltrates
the fill material. Treatment of the wastewater occurs as it passes
through the fill material and the unsaturated zone of the natural soil.
The cap, usually a finer textured material than the fill, provides frost
protection, sheds precipitation, and retains moisture for a good vegeta-
tive cover. The topsoil provides a growth medium for the vegetation.
239
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FIGURE 7-10
TYPICAL MOUND SYSTEMS
Strayv, Hay or Fabric
Fill
Topsoil
Distribution Lateral
Absorption Bed
Layer of Top Soil
Strata or Impermeable Soil Layer ;-:^~r=r ~s^
(a) Cross Section of a Mound System for Slowly Permeable
Soil on a Sloping Site.
Straw, Hay or Fabric
Fill
Topsoil
Distribution Lateral
Absorption Bed
Plowed Layer of Top Soil
"
^
Water Table or Creviced Bedrock |
(b) Cross Section of a Mound System for a Permeable Soil,
with High Groundwater or Shallow Creviced Bedrock
240
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FIGURE 7-11
DETAILED SCHEMATIC OF A MOUND SYSTEM
Layer of Straw
or Marsh Hay
Sandy Loam
11/4" Perforated
Laterals
Inlet Pi
:.*>$& Surface Water
" - 21/2" '
Clean Rock
QG
7.2.4.2 Application
a. Site Considerations
Site criteria for mound systems are summarized in Table 7-7. These
criteria reflect current practice. Slope limitations for mounds are
more restrictive than for conventional systems, particularly for mounds
used on sites with slowly permeable soils. The fill material and na-
tural soil interface can represent an abrupt textural change that re-
stricts downward percolation, increasing the chance for surface seepage
from the base of the mound.
241
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TABLE 7-7
SITE CRITERIA FOR MOUND SYTSTEMS
Item
Cri ten' a
Landscape Position
Slope
Typical Horizontal Separation
Distances from Edge of Basal Area
Water Supply Wells
Surface Waters, Springs
Escarpments
Boundary of Property
Building Foundations
Soil
Profile Description
Unsaturated Depth
Well drained areas, level or
sloping. Crests of slopes or
convex slopes most desirable.
Avoid depressions, bases of slopes
and concave slopes unless suitable
drainage is provided.
0 to 6% for soils with percolation
rates slower than 60 min/in.a
0 to 12% for soils with percolation
rates faster than 60 min/in.a
50
50
10
5
10
to
to
to
to
to
100 ft
100 ft
20 ft
10 ft
20 ft
(30 ft when located upslope from a
building in slowly permeable
soils).
Soils with a well developed and
relatively undisturbed A horizon
(topsoil) are preferable. Old
filled areas should be carefully
investigated for abrupt textural
changes that would affect water
movement. Newly filled areas
should be avoided until proper
settlement occurs.
20 to 24 in. of unsaturated soil
should exist between the original
soil surface and seasonally
saturated horizons or pervious or
creviced bedrock.
242
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TABLE 7-7 (continued)
Depth to Impermeable Barrier 3 to 5 ftb
Percolation Rate 0 to 120 min/in. measured at 12 to
20 in.c
a These are present limits used in Wisconsin established to coincide
with slope classes used by the Soil Conservation Service in soil
mapping. Mounds have been sited on slopes greater than these, but
experience is limited (25).
b Acceptable depth is site dependent.
c Tests are run at 20 in. unless water table 1s at 20 in., in which
case test is run at 16 in. In shallow soils over pervious or creviced
bedrock, tests are run at 12 in.
243
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The acceptable depth to an impermeable layer or rock strata is site spe-
cific. Sufficient depth must be available to channel the percolating
wastewater away from the mound (see Figure 7-10). If not, the soil
beneath the mound and the fill material may become saturated, resulting
in seepage of effluent on the ground surface. The suggested depths to
an impermeable layer given in Table 7-7 may be adjusted in accordance
with the site characteristics. Soil permeability, climate, slope, and
mound layout determine the necessary depth. Slowly permeable soils
require a greater depth to remove the liquid than do permeable soils.
Frost penetration reduces the effective depth and therefore a greater
depth is required in areas with severe winters. Level sites require a
greater depth because the hydraulic gradients in the lateral direction
are less than on sloping sites. Finally, mound systems extended along
the contour of a sloping site require less depth than a square mound.
Not enough research information is available to give specific depths for
these various conditions. Until further information is available,
mounds on slowly permeable soils should be made as long as possible,
with the restricting layer at least 3 ft (0.9 m) below the natural soil.
b. Influent Wastewater Characteristics
The wastewater entering the mound system should be nearly free from set-
tlcable solids, greases, and fats. Septic tanks are commonly used for
pretreatment and have proved to be satisfactory. Water softener wastes
are not harmful to the system nor is the use of common household chemi-
cals and detergents (9)(10).
7.2.4.3 Design
a. Fill Selection
The mound design must begin with the selection of a suitable fill mater-
ial because its infiltrative capacity determines the required absorption
bed area. Medium texture sands, sandy loams, soil mixtures, bottom ash,
strip mine spoil and slags are used or are being tested (24). To keep
costs of construction to a minimum, the fill should be selected from
locally available materials. Very permeable materials should be
avoided, however, because their treatment capacity is less and there is
a greater risk of surface seepage from the base of the mound when used
over the more slowly permeable soils. Commonly used fill materials and
their respective design infiltration rates are presented in Table 7-8.
244
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TABLE 7-8
COMMONLY USED FILL MATERIALS AND THEIR
DESIGN INFILTRATION RATES (24)
Fill Material
Characteristics3
Design
Infiltration
Rate
gpd/ft2
Medium Sand
Sandy Loam
Sand/Sandy Loam
Mi xture
Bottom Ash
>25%
<30-35%
0.25-2.0 mm
0.05-0.25 mm
0.002-0.05 mm
5-15% Clay Content
88-93%
7-12%
Sand
Fi ner Grai ned
Material
1.2
0.6
1.2
1.2
a Percent by weight.
b. Geometry of the Absorption Bed
The absorption area within the mound system can either be a bed or a
series of trenches. Beds are typically used for single homes or other
small systems because they are easier to construct. The shape of the
bed, however, depends on the permeability of the natural soil and the
slope of the site. In most instances, a rectangular bed with the long
axis parallel to the slope contour is preferred to minimize the risk of
seepage from the base of the mound. If the natural soil has a percola-
tion rate slower than 60 min/in. (24 min/cm), the bed should be made
narrow and extended along the contour as far as possible (see Figure 7-
12). In soils with percolation rates faster than 60 min/in. (24
min/cm), the bed can be square if the water table is greater than 3 ft
(0.9 m) below the natural ground surface (4)(25).
245
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FIGURE 7-12
PROPER ORIENTATION OF A MOUND SYSTEM ON A COMPLEX SLOPE
Mound Shaped to Conform to
the Contour and to Shed Runoff
Avoid Concave Slopes
to Prevent Convergence
of Flow
c. Sizing the Filled Area
The dimensions of the mound are dependent on the size and shape of the
absorption bed, the permeability of the natural soil, the slope of the
site, and the depth of fill below the bed (see Figure 7-13). Depths and
dimensions are presented in Table 7-9.
The downslope setback (I) in Figure 7-13 is dependent on the permeabili-
ty of the natural soil. The basal area of the mound must be sufficient-
ly large to absorb the wastewater before it reaches the perimeter of the
mound or surface seepage will result. On level sites, the entire basal
area (L x W) is used to determine I. However, on sloping sites, only
the area below and downslope from the absorption bed is considered [(B)
x (A + I)]. The infiltrative rates used for the natural soil to size
the downslope setback are given in Table 7-10. These rates assume that
a clogging mat forms at the fill/natural soil interface, which may not
be true. Since the percolating wastewater can and does move laterally
from this area, these values are conservative. However, for soils with
percolation rates faster than 60 min/in. (24 min/cm), the side slope
criteria determine the basal area instead of the infiltration rate of
246
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FIGURE 7-13
MOUND DIMENSIONS
Straw or Marsh Hay
Medium Sand Fill
Perforated
f4V:«raag!tegvhK:tMCTai
'Subsoil Plowed Surface ^3A-2V2 in. Rock
(A) Cross Section
W
X
1
Distribution
Laterals "==:
;
Bed of
3A"-2V2" Rock-"-*
V
A
*
\
., |
___^--*.
' — ~-_^
1 ^
1 i '
i
1
I
,
Slope
•• >
'
i
^v
J
/
B
K
J
*
L
(B) Plan View
247
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TABLE 7-9
DIMENSIONS FOR MOUND SYSTEMS3 (25)
Item
Dimension
Mound Height
Fill Depth (D), ft
Absorption Bed Depth (F), in.
Cap at Edge of Bed (G), ft
Cap at Center of Bed (H), ft
Mound Perimeter
Downs!ope Setback (I)
Upslope Setback (J), ft
Side Slope Setback (K), ft
Si de SI opes
1 (inin)b
9 (min)
1C
1.5C
Depends on Soil Permeability
10d
No Steeper Than 3:1
a Letters refer to lettered dimensions in Figure 7-13.
b On sloping sites, this depth will increase downs!ope to maintain a
level bed. In shallow soils where groundwater contamination is a
concern, the fill depth should be increased to 2 ft.
c A 4-6 in. depth of quality topsoil is included. This depth can be
decreased by 6 in. in areas with mild winters. If depths less than
1 ft are used, erosion after construction must be avoided so
sufficient soil covers the porous media.
d Based on 3:1 side slopes. On sloping sites, (J) will be less if
3:1 side slope is maintained.
248
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the top soil. It is only in the more slowly permeable soils where addi-
tional basal area is required, and a conservative design may be appro-
priate for these situations.
TABLE 7-10
INFILTRATION RATES FOR DETERMINING MOUND BASAL AREA (4)
Natural Soil Texture
Percolation
Rate
mi n/i n.
Infiltration
Rate
Sand, Sandy Loam
Loams, Silt Loams
Silt Loams, Silty Clay Loams
Clay Loams, Clay
0-30
31-45
46-60
61-120
1.2
0.75
0.5
0.25
d. Effluent Distribution
Although both gravity and pressure distribution networks have been used
in mound systems, pressure distribution networks are superior (4)(24)
(25). With pressure distribution, the effluent is spread more uniformly
over the entire absorption area to minimize saturated flow through the
fill and short circuiting, thus assuring good treatment and absorp-
tion. Approximately four doses per day is suggested (25). The design
of pressure distributed networks is found in Section 7.2.8.
e. Porous Media
The porous media placed in the absorption bed of the mound is the same
as described in Section 7.2.2.3.
f. Inspection Pipes
Inspection pipes are not necessary, but can be useful in observing pond-
ing depths in the absorption bed (see Figure 7-6) of the mound.
249
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Example 7.1: Calculation of Mound Dimensions and Pumping Requirements
Design a mound for a 3 bedroom house with the following site conditions.
Letter notations used in Figure 7-13 are used in this example.
Natural Soil Texture: Clay loam
Percolation Rate at 20 in depth: 110 min/in.
Depth to Seasonally High Water Table: 20 in.
Slope: 6%
No bedrock or impermeable layers
Step 1: Select the Site. The mound site should be selected prior to
locating the house and the road when possible. Consider all
criteria listed in Table 7-7 for possible mound locations on
the lot. Consider the difficulties in construction of the
mound at the various locations. Evaluate all criteria, then
pick the best site.
Step 2: Select Suitable Fill Material. It may be necessary to make a
subjective judgement on the quality of fill material versus
transportation costs. The ideal fill material may not be
readily available and thus selection of lesser quality fill may
be practical. If finer, the loading rates used to design the
absorption bed may have to be reduced. Assume a medium texture
sand for this example. The design infiltration rate is 1.2
gpd/ft^ (Table 7-8).
Step 3: Estimate Design Flow. Peak flow is estimated from the size of
the building.In this instance, 150 gpd/bedroom is assumed
(see Chapter 4).
Step 4: Size Absorption Bed.
Absorption Bed Area = 45° gpd , =375 ft2
1.2 gpd/fr
Step 5: Calculate Absorption Bed Dimension. The bed must parallel the
site contour.Since the natural soil is slowly permeable, it
is desirable to run the bed along the contour as far as possi-
ble. In this example, assume sufficient area exists for a 65-
ft length bed.
O7C ff£
Bed Width (A) = Jir I; = 5.8 ft or 6 ft
oo it
250
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Bed Dimensions: A = 6 ft
B = 65 ft
Step 6: Calculate Mound Dimensions.
a. Mound Height
Fill Depth (D) = 1 ft (Table 7-9)
Fill Depth (E) = D + [(Slope) x (A)]
= 1 ft + [(0.06) x (6)]
= 1.4 ft (This is only approximate. Criti-
cal factor is construction of level bed
bottom.)
Bed Depth (F) = 9 in. (min) (Table 7-9). (A minimum of 6
in. must be below the inverts of the dis-
tribution laterals.)
Cap at Edge of Bed (G) = 1 ft (min) (Table 7-9)
Cap at Center of Bed (H) = 1-1/2 ft (min) (Table 7-9)
b. Mound Perimeter
Downslope Setback (I): The area below and downslope of the
absorption bed and sloping sites must be sufficiently large
to absorb the peak wastewater flow. Select the proper
natural soil infiltration rate from Table 7-10. In this
case, the natural soil infiltration rate is 0.25 gpd/ft .
Upslope Setback (J) = (mound height at upslope edge of bed)
x (3:1 slope)
= [(D) + (E) + (G)] x (3)
= (1.0 + 0.75 + 1.0) x (3)
= (2.75) x (3)
= 8.25 ft (This will be less because
of natural ground slope, use 8 ft.)
Side Slope Setback (K) = (mound height at bed center) x (3:1
slope)
(D) + (E) ^ lc\ ^ /in x (3)
251
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= M.£-Id + 0.75 + 1.5 | x (3)
= (3.5) x (3)
= 10.5 ft, or 11 ft
Basal Area Required = (B) x [(I) + (A)]
, 450 gpd = Ij800ft2
0.25 gpd/fr
(I) MA) =
(I)= If|°°- (A)
1,800 (•
= --
= 21.7 ft, or 22 ft
Check to see that the downslope setback (I) is great
enough so as not to exceed a 3:1 slope:
(mound height at downslope edge of bed) x (3:1 slope)
= C(E) + (F) + (G)] x (3)
= (1.4 + 0.75 + 1.0) x (3)
= 9.5 ft
Since the distance needed to maintain a 3:1 slope is
less than the distance needed to provide sufficient
basal area, (I) = 22 ft
Mound Length (L) = (B) + 2(K)
=65+2 (11)
= 87 ft
Mound Width (W) = (J) + (A) + (I)
=8+6+22
= 36 ft
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Step 7: Design Effluent Distribution Network. See Section 7.2.8(f).
7.2.4.4 Construction
a. Site Preparation
Good construction techniques are essential if the mound is to function
properly. The following techniques should be considered:
Step 1: Rope off the site to prevent damage to the area during other
construction activity on the lot. Vehicular traffic over the
area should be prohibited to avoid soil compaction.
Step 2: Stake out the mound perimeter and bed in the proper orienta-
tion. Reference stakes set some distance from the mound peri-
meter are also required in case the corner stakes are dis-
turbed.
Step 3: Cut and remove any excessive vegetation. Trees should be cut
at ground surface and the stumps left in place.
Step 4: Measure the average ground elevation along the upslope edge of
the bed to determine the bottom elevation of the bed.
Step 5: Install the delivery pipe from the dosing chamber to the
mound. Lay the pipe below the frost line or slope it uniformly
back to the dosing chamber so it may drain after dosing. Back
fill and compact the soil around the pipe.
Step 6: Plow the area within the mound perimeter. Use a two bottom or
larger moldboard plow, plowing 7 to 8 in. (18 to 20 cm) deep
parallel to the contour. Single bottom plows should not be us-
ed, as the trace wheel runs in every furrow, compacting the
soil. Each furrow should be thrown upslope. A chisel plow may
be used in place of a moldboard plow. Roughening the surface
with backhoe teeth may be satisfactory, especially in wooded
sites with stumps. Rototilling is not recommended because of
the damage it does to the soil structure. However, rototilling
may be used in granular soils, such as sands.
Plowing should not be done when the soil is too wet. Smearing
and compaction of the soil will occur. If a sample of the soil
taken from the plow depth forms a wire when rolled between the
palms, the soil is too wet. If it crumbles, plowing may
proceed.
?53
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b. Fill Placement
Step 1: Place the fill material on the upslope edges of the plowed
area. Keep trucks off the plowed area. Minimize traffic on
the downslope side.
Step 2: Move the fill material into place using a small track type
tractor with a blade. Always keep a minimum of 6 in. of mate-
rial beneath the tracks of the tractor to minimize compaction
of the natural soil. The fill material should be worked in
this manner until the height of the fill reaches the elevation
of the top of the absorption bed.
Step 3: With the blade of the tractor, form the absorption bed. Hand
level the bottom of the bed, checking it for the proper eleva-
tion. Shape the sides to the desired slope.
c. Distribution Network Placement
Step 1: Carefully place the coarse aggregate in the bed. Do not create
ruts in the bottom of the bed. Level the aggregate to a
minimum depth of 6 in. (15 cm).
Step 2: Assemble the distribution network on the aggregate. The mani-
fold should be placed so it will drain between doses, either
out the laterals or back into the pump chamber. The laterals
should be laid level.
Step 3: Place additional aggregate to a depth of at least 2 in. (5 cm)
over the crown of the pipe.
Step 4: Place a suitable backfill barrier over the aggregate.
d. Covering
Step 1: Place a finer textured soil material such as clay or silt loam
over the top of the bed to a minimum depth of 6 in. (15 cm).
Step 2: Place 6 in. (15 cm) of good quality topsoil over the entire
mound surface.
Step 3: Plant grass over the entire mound using grasses adapted to the
area. Shrubs can be planted around the base and up the side-
slopes. Shrubs should be somewhat moisture tolerant since the
downslope perimeter may become moist during early spring and
late fall. Plantings on top of the mound should be drought
254
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tolerant, as the upper portion of the mound can become dry
during the summer.
7.2.4.5 Operation and Maintenance
a. Routine Maintenance
A properly designed and constructed mound should operate satisfactorily
with virtually no regular maintenance.
b. Rehabilitation
Three failure conditions may occur within the mound. They are (1) se-
vere clogging at the bottom of the absorption area, (2) severe clogging
at the fill material and natural soil interface, and (3) plugging of the
distribution network. Usually these failures can be easily corrected.
If severe clogging occurs at the bottom of the absorption bed, its cause
should first be determined. If it is due to failure to maintain the
pretreatment unit, hydrogen peroxide to oxidize the accumulated organics
at the infiltrative surface could be used. The chemical can be applied
directly to the bed or through the dosing chamber. Because of the dan-
ger in handling this strong oxidant, this treatment should be done by
professionals.
If the clogging is due to overloading or unusual wastewater charac-
teristics, efforts should be made to reduce the wastewater volume or
strength. It may be necessary to enlarge the mound. The mound cap
should be removed and the aggregate in the absorption bed stripped out.
The area down si ope of the mound should be plowed and additional fill
added to enlarge the mound to the proper size. The absorption bed can
then be reconstructed.
Severe clogging at the fill and natural soil interface will cause sur-
face seepage at the base of the mound. This area should be permitted to
dry and the downslope area plowed. Additional fill can then be added.
If this does not correct the problem, the site may have to be abandoned.
Partial plugging of the distribution piping may be detected by extremely
long dosing times. The ends of the distribution laterals should be ex-
posed and the pump activated to flush out any solid material. If neces-
sary, the pipe can be rodded.
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7.2.4.6 Considerations for Multi-Home and Commercial
Wastewaters
Designs of the mound system for larger flows follow the same design
principles as for smaller flows. In cluster systems serving more than
five homes, however, peak flow estimates can be reduced because of flow
attenuation, but contributions from infiltration through the collection
system must be included. Peak flow estimates should be based on the to-
tal number of people to be served (see Chapter 4). Rates of infiltra-
tion vary with the type of collection sewer used (19)(20).
With commercial flows, the character of the wastewater is an important
consideration. Proper pretreatment is necessary if the character is
significantly different than domestic wastewater.
Modifications to the design of the mound system may be desirable for
larger flows on sloping sites or in slowly permeable soils. In both
instances, the absorption area should be broken up into a series of
trenches or smaller beds. This is beneficial on sloping sites because
the beds can be tiered to reduce the amount of fill required (see Figure
7-14). Depths of fill material below beds should not exceed 4 to 5 ft
(1.3 to 1.7 m) because differential settling will cause the bed to set-
tle unevenly. If the system is tiered, each trench or bed must be dosed
individually. This can be done by automatic valving or alternating
pumps or siphons.
In sites with slowly permeable soils, breaking the absorption area into
smaller trenches or beds helps distribute the effluent over much wider
areas. Spacing of the beds or trenches should be sufficient so that the
wastewater contributed from one trench or bed is absorbed by the natural
soil before it reaches the lower trench or bed (see Table 7-10). The
beds or trenches should be as long as the site allows. A long bed, bro-
ken into several shorter systems, each served by a pump or siphon, is
preferred over two or more short parallel beds, especially in soils
where the effluent moves downslope.
Flexibility in operation should also be incorporated into systems serv-
ing larger flows, since a failure can create a significant problem.
Alternating bed systems should be considered. A three-bed system is
suggested where each bed contains 50% of the required absorption area
(19). Two beds are always in operation, providing 100% of the needed
infiltrative surface. The third bed is alternated into service on a
yearly schedule. Thus, each field is in service for two years and
"rested" for one year to rejuvenate. The third bed also acts as a
standby unit in case one bed fails. The idle fields can be put into
service immediately while the failed bed is rehabilitated.
256
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FIGURE 7-14
TIERED MOUND SYSTEM
Gravel-Filled
Absorption Bed
7.2.5 Fill Systems
7.2.5.1 Description
Fill systems may be used on sites with slowly permeable soils overlying
sands and sandy loams where construction of a conventional system below
the tight soil horizons may be ruled out. If the depth from the top
surface of the underlying sand or sandy loam to the seasonally high
water table or bedrock is inadequate to construct a trench or bed sys-
tem, the slowly permeable soil may be stripped away and replaced with a
sandy fill material to provide 2 to 4 ft (0.6 to 1.2 m) of unsaturated
soil. A trench or bed system may then be constructed within the fill.
Mound systems would also be suitable designs for these conditions and
may be less expensive to construct, but fill systems offer some advan-
tages. If the soils overlying the sands or sandy loams are very slowly
permeable, the size of a fill system may be smaller than that of a mound
permitting their installation in smaller areas. Also, fill systems usu-
ally have less vertical relief above the natural grade than do mounds.
This may be desirable for landscaping purposes.
257
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7.2.5.2 Application
a. Site Considerations
The use of fills is restricted to sites where unsuitable surface soils
may be stripped away without damaging the underlying soils. Therefore,
fills are limited to sites where the underlying soils are sands or sandy
loams and the seasonally high water table or bedrock surface is not
within 1 ft (0.3 m) of the sand or sandy loam surface. If the depth to
the seasonally high water table or bedrock is greater than 3 to 5 ft
(0.9 to 1.5 m) from the sandy or sandy loam surface, a fill system is
not necessary. A deep trench or bed system can be constructed directly
in the more permeable underlying area.
Once the fill is placed, the site must meet all the site and soil crite-
ria required for trench or bed systems (see Table 7-1).
b. Influent Wastewater Characteristics
The influent wastewater must be free of settleable solids, fats, and
grease. Water softener wastes are not harmful, nor is the normal use of
household chemicals and detergents.
7.2.5.3 Design
Since fill systems differ from trench and bed systems only in that they
are constructed in a filled area, the design of fill systems is identi-
cal to trenches and beds. The only unique features are the sizing of
the area to be filled and the fill selection. Uniform distribution of
the wastewater over the infiltrative surface through a pressurized net-
work is suggested to maintain groundwater quality (11).
a. Sizing of the Filled Area
A minimum separation distance of 5 ft (1.5 m) between the sidewalls of
the absorption trenches or bed, and the edge of the filled area should
be maintained. This allows for sidewall absorption and lateral movement
of the wastewater.
258
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If a perched water table condition occurs in the surface soils that are
to be moved, provisions should be made to prevent this water from flow-
ing into the filled area. Curtain drains, perimeter drains or barrier
trenches may be necessary upslope or around the filled area to remove
this water (see Section 7.2.6).
b. Fill Selection
The fill material should be similar in texture to the underlying sand or
loamy sand. The backfill material used to cover the system should be
finer textured to shed surface runoff. It may be the original soil that
was removed.
7.2.5.4 Construction
Care should be exercised in removing the unsuitable soil prior to fill-
ing to prevent excessive disturbance of the sandy soil below. The
machinery should always operate from unexcavated areas. The top few
inches of the sand or sandy loam soil should be removed to ensure that
all the unsuitable soil is stripped.
The exposed surface should be harrowed or otherwise broken up to a depth
of 6 in. (15 cm) prior to filling. This eliminates a distinct interface
forming between the fill and the natural soil that would disrupt liquid
movement.
Once the fill has been placed, construction of the absorption system can
proceed as for trenches or beds in sands. However, if the fill depth is
greater than 4 ft (1.2 m), the fill should be allowed to settle before
construction begins. This may require a year to settle naturally. To
avoid this delay, the fill can be spread in shallow lifts and each me-
chanically compacted. This must be done carefully, however, so that
layers of differing density are not created. The fill should be com-
pacted to a density similar to the underlying natural soil.
7.2.5.5 Operation and Maintenance
The operation and maintenance of fill systems are identical to trenches
and beds constructed in sands. The fill system lends itself very well
to treatment with chemical oxidants or reconstruction in the same area.
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7.2.6 Artificially Drained Systems
7.2.6.1 Description
High water tables that limit the use of trenches, beds or seepage pits
can sometimes be artificially lowered to permit the use of these dispo-
sal methods. Vertical drains, curtain drains and underdrains are com-
monly used subsurface drainage techniques. Soil and site conditions
determine which method is selected.
Curtain drains and vertical drains are used to lower perched water
tables. These methods are most effective where the perched water is
moving laterally under the soil absorption site. The drains are placed
upstream of the absorption area to intercept the groundwater as it flows
into the area.
Curtain drains are trench excavations in which perforated drainage pipe
is placed. These are placed around the upslope perimeter of the soil
absorption site to intercept the groundwater moving into the area (see
Figure 7-15). If the site has sufficient slope, the drains are brought
to the surface downsi ope to allow free drainage. On level sites, pumps
must be used to remove the collected water. If the restrictive layer
that creates the water table is thin and overlies permeable soil, verti-
cal drains may be used. These are trench excavations made through the
restrictive layer into the more permeable soil below and backfilled with
porous material (see Figure 7-16). Thus, water moving toward the ex-
cavation is able to drain into the underlying soil. Vertical drains are
susceptible to sealing by fine sediment transported by the water.
Underdrains are used where water tables exist 4 to 5 ft (1.2 to 1.5 m)
below the surface in permeable soils. The drains are similar to curtain
drains in construction, but several drains may be necessary to lower the
water table sufficiently (see Figure 7-17). Depth and spacing of the
drains are determined by the soil and water table characteristics.
7.2.6.2 Site Considerations
Successful design of artificially drained systems depends upon the cor-
rect diagnosis of the drainage problem. The source of the groundwater
and its flow characteristics must be determined to select the proper
method of drainage. Particular attention must be given to soil strati-
fication and groundwater gradients.
260
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FIGURE 7-15
CURTAIN DRAIN TO INTERCEPT LATERALLY MOVING PERCHED WATER
TABLE CAUSED BY A SHALLOW, IMPERMEABLE LAYER
Curtain
Drain
Material
Perched
Water
Table
Gravel Filled
Above High
Water Table
-^.Drainage Pipe
^Absorption
Trenches
Impermeable Layer
FIGURE 7-16
VERTICAL DRAIN TO INTERCEPT LATERALLY MOVING PERCHED WATER TABLE
CAUSED BY A SHALLOW, THIN, IMPERMEABLE LAYER
l?gpK^^^^
$&$ Fl11 ' ^^^^,
*kfe# i\/i-,+—;-,i I y
Material
i
Gravel Filled
Above High
Water Table
Permeable
Soil
Impermeable Layer
Absorption Bed
Backfill
261
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FIGURE 7-17
UNDERDRAINS USED TO LOWER WATER TABLE
Underdrains
Fill
Material
Absorption
Trenches
1 I
Gravel Filled
Above High
Water Table
Drainage Pipe
a. Subsurface Drainage Problems
There is an unlimited variety of subsurface drainage problems but the
most common ones can be grouped into four general types (26). These
are: (1) free water tables, (2) water tables over artesian aquifers,
(3) perched water tables, and (4) lateral groundwater flow problems.
Free water tables typically are large, slow moving bodies of water fed
by surface waters, precipitation, and subsurface percolation from other
areas. In the lower elevations of the drainage basin, the groundwater
is discharged into streams, on the ground surface in low areas, or by
escape into other aquifers. The groundwater elevation fluctuates
seasonally. The slope of a free water table surface is usually quite
gentle.
262
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Where the soil is permeable, underdrains can be used to lower the water
table sufficiently to permit the installation of trench or bed disposal
systems. In fine textured soils of slow permeability, however, subsur-
face drainage is impractical.
An artesian aquifer is a groundwater body confined by an impervious
layer over the aquifer. Its pressure surface ( the elevation to which
it would rise in a well tapping the aquifer) is higher than the local
water table, and may even rise above the ground surface. Pressure in
the aquifer is caused by the weight of a continuous body of water that
is higher than the local water table. Leaks at holes or weak points in
the confining layer create an upward flow, with the hydraulic head
decreasing in the upward direction. The groundwater moves in the
direction of the decreasing gradient and escapes as seepage at the
ground surface or moves laterally into other aquifers.
Areas with this problem are impractical to drain. The water removed is
continually replenished from the aquifer. This requires relatively deep
and closely spaced drains and pumped discharges. Onsite disposal op-
tions other than soil absorption systems should be investigated in areas
with shallow artesian aquifers.
In stratified soils, a water table may develop that is separated from
the free water table by a slowly permeable layer, i.e., a perched water
table. This occurs when surface sources of water saturate the soil
above the layer due to slow natural drainage. Methods employed to drain
perched water tables depend upon the particular site conditions. Verti-
cal drains, curtain drains or underdrains may be used.
Lateral groundwater flow problems are characterized by horizontal
groundwater movement across the area. This flow pattern is usually
created by soil stratification or other natural barriers to flow.
The depth, orientation and inclination of the strata or barriers deter-
mine the drainage method used and its location. Curtain drains or vert-
ical drains are usually employed to intercept the water upstream of the
area to be drained.
b. Site Evaluation
Soils with high water tables that may be practical to drain to make a
site suitable for a trench or bed system are ones having (1) shallow
perched water tables, (2) lateral groundwater flow, or (3) free water
tables in coarse textured soils. Soils that are saturated for prolonged
263
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periods, particularly on level sites, are not practical to drain. Other
disposal methods should be investigated for such sites.
Because each of these drainage problems require different solutions, it
is important that the site evaluation be done in sufficient detail to
differentiate between them. Where the need for subsurface drainage is
anticipated, topographic surveys, soil profile descriptions and estima-
tion of the seasonally high groundwater elevations and gradients should
be emphasized. Evaluation of these site characteristics must be done in
addition to other characteristics that are evaluated for subsurface dis-
posal (see Chapter 3).
Topographic Surveys: Topographic maps of the site with 1 to 2 ft (0.3
to 0.6 m) contour intervals are useful as base maps on which water and
soils information can be referenced. Water table elevations, seep areas
and areas with vegetation indicative of seasonal or prolonged high water
tables should be locaf >n the map. Elevations of ridges, knolls, rock
outcrops and natural drainage ways should also be noted. This informa-
tion is useful in establishing the source of the groundwater, its direc-
tion of flow, and the placement of the drainage system.
Soil Profile Descriptions: The soil profile must be carefully examined
toidentify the type of drainage problem and the extent of seasonal
water table fluctuations. Soil stratification and soil color are used
to make these determinations.
Soil stratification or layering may or may not be readily visible. Soil
texture, density, color, zones of saturation and root penetration aid in
identifying layers of varying hydraulic conductivity (see Chapter 3).
The thickness and slope of each layer should be described. Deep uniform
soils indicate that the drainage problem must be handled as a free water
table problem. Stratified soils indicate a perched or lateral flow
groundwater problem.
The soil color helps to identify zones of periodic and continous satura-
tion. Soil mottling occurs when the soil is periodically saturated, and
gleyed soil indicates continuous saturation (see Chapter 3). The high-
est elevation of the mottling provides an estimate of the seasonally
high water table, while the top of the gleyed zone indicates the season-
ally low water table elevation. It is particularly important to estab-
lish the extent of the seasonal fluctuations to determine if drainage is
practical. If the seasonally low water table is above the elevation to
which the soil must be drained to make the site acceptable, drainage
must be provided throughout the year. If pumps are used to remove the
water, costs may be excessive and other alternatives should be
investigated.
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Groundwater. Elevation and Gradients: To accurately determine ground-
water elevations and gradients, observation wells or piezometers are
used. Observation wells are used to observe groundwater flutuations
throughout a year or more. If several are strategically placed about
the area, the local gradient can also be established by measuring the
water surface elevation in each well. Piezometers differ from obser-
vation wells in that they are constructed so that there is no leakage
around the pipe. The water surface elevation measured establishes the
hydrostatic pressure at the bottom of the well. If placed at several
depths, they can be used to establish whether artesian conditions exist.
For construction of piezometers and interpretation of results, see USDA,
"Drainage of Agricultural Land" (26).
The measured or estimated water table elevations for a specific time
period are plotted on the topographic map. By drawing the contours of
the water table surface from these plots, the direction of groundwater
movement is determined, since movement is perpendicular to the ground-
water contours. This helps locate the source of the water and how to
best place the drainage network.
7.2.6.3 Design
a. Selection of Drainage Method
In designing a subsurface drainage system, the site characteristics are
evaluated to determine which method of drainage is most appropriate.
Table 7-11 presents the drainage method for various site
characteristics. In general, shallow, lateral flow problems are the
easiest drainage problems to correct for subsurface wastewater disposal.
Since the use of underdrains for onsite disposal systems has been very
limited, other acceptable disposal methods not requiring drains should
first be considered.
b. Curtain Drains
Curtain drains are placed some distance upslope from the proposed soil
absorption system to intercept the groundwater, and around either end of
the system to prevent intrusion. On sites with sufficient slope, the
drain is extended downslope until it surfaces, to provide free drainage.
The drain is placed slightly into the restrictive layer to ensure that
all the groundwater is intercepted. A separation distance from the soil
absorption system is required to prevent insufficiently treated waste-
water from entering the drain. This distance depends on the soil perme-
ability and depth of drain below the bottom of the absorption system;
however, a separation distance of 10 ft is commonly used.
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TABLE 7-11
DRAINAGE METHODS FOR VARIOUS SITE CHARACTERISTICS
Site Characteristics
Drainage
Problem
Drainage Method
Saturated or mottled soils above a
restrictive layer with water source
located at a higher elevation; site
usually sloping
Saturated or mottled soils above a
restrictive layer; soil below
restrictive layer is unsaturated;
site is level or only gently sloping
Deep uniform soils mottled or
saturated
Saturated soils above and below
restrictive layer with hydraulic
gradients increasing with depth
Lateral flow
Perched water
table
Free water
table
Artesian-fed
water table
Curtain drain
Vertical drain3
Underdrai nb
Vertical drain3
Underdrai
Avoi d
3 Use only where restrictive layer is thin and underlying soil is
reasonably permeable.
b Soils with more than 70% clay are difficult to drain and should be
avoided.
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The size of the drain is dependent upon the soil permeability, the size
of the area drained, and the gradient of the pipe. Silt traps are some-
times provided in the drain to improve the quality of the discharged
drainage. These units may require infrequent cleaning to maintain their
effectiveness.
c. Vertical Drains
Vertical drains may be used to intercept a laterally flowing perched
water table. Separation distances between the drain and the bottom of
the soil absorption system are the same as for curtain drains to main-
tain an unsaturated zone under the absorption system.
The size and placement of the drain depends upon the relative permeabil-
ities of the saturated soil and the soil below the restrictive layer,
and the size of the area to be drained. The infiltration surface of the
vertical drain (sidewalls and bottom area) must be sized to absorb all
the water it receives. The width and depth of the drain below the re-
strictive layer is calculated by assuming an infiltration rate for the
underlying soil. If clay and silt are transported by the groundwater,
the infiltration rate will be less than the saturated conductivity of
the soil. Clogging of the vertical drain by silt can be a significant
problem. Unfortunately, experience with these drains in wastewater
disposal is lacking.
d. Underdrains
Underdrains must be located to lower the water table to provide the
necessary depth of unsaturated soil below the infiltrative surface of
the soil absorption system, and to prevent poorly treated effluent from
entering the drain. Sometimes, a network of drains is required through-
out the area where the soil absorption system is located. The depth and
spacing of the drains is determined by the soil permeability, the size
of the area to be drained, and other factors. Where necessary, however,
see USDA Drainage of Agricultural Land (26) for design procedures.
7.2.6.4 Construction
a. Curtain Drains and Underdrains
To maximize infiltration of the groundwater into the pipe, a coarse,
porous material such as gravel, crushed rock, etc., should be placed
under and above the pipe. The porous material is extended above the
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high water table elevation. To prevent silt from entering the pipe
while the disturbed area is stabilizing, the tops of the joints or per-
forations should be covered with waterproof building paper or the pipe
jacketed with mesh. Natural soil material is used for the remainder of
the backfill (27).
The outlet must be protected from small animals. The outlet may be
covered with a porous material such as rock or gravel. Various com-
mercial outlet protection devices are also available (26).
b. Vertical Drains
Vertical drains are dug to the desired depth and width, and are back-
filled with a coarse, porous media such as coarse sand, 1/4- to 1/2-in
(0.6 to 1.3 cm) gravel, or similar material, to a level above the high
perched water table elevation. Natural soil materials are used for the
remainder of the backfill.
7.2.6.5 Maintenance
A well-designed and constructed drainage system requires little mainte-
nance. The outlets should be inspected routinely to see that free
drainage is maintained. If a silt trap is used, it should be inspected
annually to determine the need for cleaning.
7.2.7 Electro-Osmosis
7.2.7.1 Description
Electro-osmosis is a technique used to drain and stabilize slowly perme-
able soils during excavation. A direct current is passed through the
soil, which draws the free water in the soil pores to the cathode
(28). The water collects at the cathode and is pumped out. Steel well
points serve as cathodes, and steel rods driven between wells are used
as anodes. Common practice is to install the electrodes approximately
15 ft (4.6 m) apart, and apply a 30- to 180-volt potential. Current
flow is 20 to 30 amps (28).
Recently, a similar technique has been applied to onsite wastewater dis-
posal in soils with percolation rates slower than 60 min/in. (24
min/cm). A galvanic cell is constructed out of natural materials, which
requires no external power source. This cell is capable of generating a
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0.7- to 1.3-volt potential (29). Conventional absorption trenches are
constructed and a mineral rock-filled anode is installed immediately
adjacent to the trench. Coke-filled cathodes with graphite cores are
installed some distance from the trench (see Figure 7-18). The water
that moves to the cathode is claimed to be removed by evapotranspiration
(30). These systems have been used successfully in California, Iowa,
Minnesota, and Wyoming (29).
7.2.7.2 Site Considerations
Electro-osmosis systems were developed to enhance wastewater absorption
in slowly permeable soils. They are used in soils with percolation
rates slower than 60 min/in. (24 min/cm). Criteria for soil absorption
trench or bed are presented in Table 7-1.
7.2.7.3 Design and Construction
The electro-osmosis system is patented. Design and construction of sys-
tems are done by licensees.
7.2.7.4 Operation and Maintenance
Once installed, no routine maintenance of the electrodes has been re-
ported. Maintenance techniques for the. soil absorption trench are
presented in Section 7.2.2.5.
7.2.8 Effluent Distribution Network for Subsurface
Soil Absorption Systems
Several different distribution networks are used in subsurface soil
absorption systems. They include single line, closed loop, distribution
box, relief line, drop box, and pressure networks. The objective of
each is to apply the pretreated wastewater over the infiltrative
surface.
The choice of one network over another depends on the type of system
proposed and the method of wastewater application desired. Networks for
the various types of systems versus the method of wastewater application
are given in Table 7-12. Where more than one network is suitable, they
are listed in order of preference.
269
-------
FIGURE 7-18
TYPICAL ELECTRO-OSMOSIS SYSTEM (30)
Anodes
Anodes
From
Anode /
V
Septic Tank g^H
u_ - —
Absorption
/f8"
/
Trenches
Wide Dike
a Anode
\
i
Q.
CO
Cathode
4" Graphite Cores
Cathode
4" Vents
Plan View
A*-1
Slope
.Cathode (+)
Absorption
Trench
Anode (-),
Distribution
Coke
Mineral
Rock
Section A-A
270
-------
TABLE 7-12
DISTRIBUTION NETWORKS FOR VARIOUS SYSTEM DESIGNS AND APPLICATION METHODS*
Method
of
Application Single Trench
Mul ti-Trench
(Fills, Drains)
On Level Site
Multi-Trench
(Drains)
On Sloping Site
Beds
(Fills, Drains) Mounds
ro
Gravity
Dosing
Single line
Single line
Pressure
Uniform Pressure
Application
Drop box
Closed loop
Distribution box
Closed loop
Pressure
Distribution box
Pressure
Drop box
Relief line
Distribution boxb
Distribution box
Pressure0
Closed loop Not applicable
Distribution box
Closed loop
Pressure
Distribution box
Pressure
Not applicable
Pressure
a Distribution networks are listed in order of preference.
b Use limited by degree of slope (see Section 7.2.8.1 d)
c Because of the complexity of a pressure network on a sloping site, drop boxes or relief lines are
suggested.
-------
7.2.8.1 Design
a. Single Line
Single-line distribution networks are trenches loaded by gravity or
dosing. The distribution line is a 3- to 4-in. (8- to 10-cm) diameter
perforated pipe laid level in the center of the gravel-filled excavation
(see Figure 7-19). The pipe is usually laid such that the holes are at
or near the invert of the pipe. Where the length of single lines
exceeds 100 ft (30 m), it is preferable to locate the wastewater inlet
toward the center of the line.
b. Drop Box
Drop box networks are typically used for continuously ponded multi-
trench systems on level to maximum sloping lots. It is a network that
serially loads each trench to its full hydraulic capacity.
A drop box is a small, circular or square box with a removable cover.
It has an inlet, one or two distribution lateral outlets, and an over-
flow. The lateral outlet inverts are located at or near the bottom of
the box, all of the same diameter pipe. The overflow invert can be the
same elevation as the crown of the lateral outlet, or up to 2 in. above
it, to cause the full depth of the trench to flood. The inlet invert of
the drop box may be at the same elevation as the overflow invert or a
few inches above. An elevation difference of 1 to 2 in. (3 to 5 cm)
between trenches is all that is needed to install a drop box network.
The boxes may be buried, but it is suggested that the covers be left
exposed for periodic inspection and maintenance (see Figure 7-20).
Drop boxes are installed at the wastewater inlet of each trench. The
inlets may be located anywhere along the trench length. A solid wall
pipe connects the overflow from the higher box to the inlet of the lower
box. The first box in the network receives all the effluent from the
pretreatment tank and distributes it into the first trench. When the
first trench fills, the box overflows into the next trench. In this
manner, each trench in the system is used successively to its full capa-
city. Thus, only the portion of the system required to absorb the
wastewater is used. During periods of high flow or low absorptive capa-
city of the soil, more trenches will be used. When flows are low or
during the hot dry summer months, the lower trenches may not be needed,
so they may drain and dry out, automatically resting more trenches,
which rejuvenates their infiltrative surfaces (11).
272
-------
FIGURE 7-19
SINGLE LINE DISTRIBUTION NETWORK
\A
P
Z2 C
/atertight
ipes and
Joints
*5 ft.H
Min 1
— * B A
, \
—-R
Pretreatment
Unit
:•• K^M^i^fTTT^Tr??^.'..i....... . in.,..
IT-ITI '•••- * 'f? Perforated Pipe'^:;;^--'^S:>
' ^-1 ft ^"—-~;;r—_'.. J".'.".'; ;^_"" " """ •.'" ~—^1
^"Level or 2 in. to 4 in.
per 100 ft. Slope
Section A-A
Overfill to Allow
for Settlement
Distribution Pipe
Crushed Stone
or Gravel
I 6 in. min
Section B-B
273
-------
FIGURE 7-20
DROP BOX DISTRIBUTION NETWORK ([AFTER (22)]
Inlet From
Pretreatment
or Previous
Drop Box
Outletto
Trench
Overflow
to Next
Drop Box
Outlet to
Trench
Plan
- Inlet
Outlet to
Trench
Distribution \\\
Pipes
Pretreatment
Unit Water-Tight
Pipes
Plan
Extra Trenches
j Can be
._!.Li_ Added If
Necessary
Covers May be Exposed at
Surface if Insulated in
Cold Climates
Drop Boxes
Section A-A
274
-------
The liquid level in the trenches is established by the elevation of the
overflow invert leading to the succeeding drop box. If the elevation of
this invert is near the top of the rock in the trench, the entire trench
sidewall will be utilized, maximum hydrostatic head will be developed to
force the liquid into the surrounding soil, and evapotranspiration by
plants during the growing season will be maximized by providing a supply
of liquid to the overlying soil.
The drop box design has several advantages over single lines, closed
loop, and distribution box networks for continuously ponded systems. It
may be used on steeply sloping sites without surface seepage occurring
unless the entire system is overloaded. If the system becomes over-
loaded, additional trenches can be added easily without abandoning or
disturbing the existing system. Drop box networks also permit unneeded
absorption trenches to rest and rejuvenate. The lower trenches are
rested automatically when flows are low or infiltration capacity is
high. The upper trenches may be rested when necessary by plugging the
drop box lateral outlets.
c. Closed Loop
In absorption systems where the entire infiltrative surface is at one
elevation, such as in beds or multi-trench systems on level or nearly
level sites, closed loop networks may be used. The distribution pipe is
laid level over the gravel filled excavation and the ends connected to-
gether with additional pipe with ell or tee fittings. In beds, the
parallel lines are usually laid with 3 to 6 ft (0.9 to 1.8 m) spacings.
A tee, cross, or distribution box may be used at the inlet to the closed
system (See Figure 7-21).
d. Distribution Box
Distribution box networks may be used in multi-trench systems or beds
with independent distribution laterals. They are suitable for all gra-
vity-flow systems.
The distribution laterals in the network extend from a common watertight
box called the distribution box. The box may be round or rectangular,
with a single inlet, and an outlet for each distribution lateral. It
has an exposed, removable cover. Its purpose is to divide the incoming
wastewater equally between each lateral. To achieve this objective, the
outlet inverts must be at exactly the same elevation. The inlet invert
should be about 1 in. above the outlet inverts. Where dosing is em-
ployed or where the slope of the inlet pipe imparts a significant velo-
city to the wastewater flow, a baffle should be placed in front of the
inlet to prevent short-circuiting.
275
-------
FIGURE 7-21
CLOSED LOOP DISTRIBUTION NETWORK
Watertight Pipe & Joints,
Tight Joints
at All Ends
«—^— Perforated
/ Distribution Pipe
' ^ I H I fl I TTT . I I" I I . <
Pretreatment lv ' » * *' « » « •» « < < » ••> »
Unit
Distribution box networks are suggested only for absorption systems
located on level or gently sloping sites, where the system can be in-
stalled so that the ground surface elevation above the lowest trench is
above the box outlets (11). This is because it is difficult to prevent
the distribution box from settling (11)(17)(31). If the box were to
settle unevenly so that the lowest trench received a greater share of
the effluent, wastewater would seep onto the ground surface unless the
distribution lateral of the lowest trench were at a high enough eleva-
tion to back up the wastewater into the box, where it could flow into a
different lateral. Therefore, to utilize the full capacity of each
trench in the system, distribution box networks should be used only
where each trench can back up into the distribution box (see Figure 7-
22). On steeply sloping sites, other networks should be used, unless
great care is used to construct the distribution box on a stable
footing. If used for dividing flow between independent trenches, any
combination of trenches can be rested by plugging the appropriate
outlets.
e. Relief Line
Relief line networks may be used in place of drop box networks in
continuously ponded multi-trench systems on sites up to the maximum
permissible slopes. The network provides serial distribution as in drop
276
-------
FIGURE 7-22
DISTRIBUTION BOX NETWORK
Distribution Box
Firmly Supported
in Level Position
All Pipe Inverts
at Same Elevation
Central Feed
Perforated
Distribution
Pipe
n
Distribution i
Box \ j
T u'^ ii -4*. <—
\ \\
n|
I fa ^••ff /X "^
\ \\
\ \
i \
I J
Water-Tight-^* ^/
Pipes Distributic
Pipes
^
h
in
End Feed
Ground Surface at Lowest
Trench Higher Than Outlets
of Distribution Box.
Trenches'
•Distribution
Box
Section A-A
277
-------
box networks (see Figure 7-23). However, .the design makes it more
difficult to add trenches to the system and it does not permit the owner
to manually rest the upper trenches.
The network uses overflow or relief lines between trenches in place of
drop boxes. The invert of the overflow section should be located near
the top of the porous media to use the maximum capacity of the trench,
but it should be lower than the septic tank outlet invert. The invert
of the overflow from the first absorption trench should be at least 4
in. lower than the invert of the pretreatment unit outlet. Relief lines
may be located anywhere along the length of the trench, but successive
trenches should be separated 5 to 10 ft (1.3 to 3.0 m) to prevent short-
circuiting.
f. Pressure Distribution
If uniform distribution of wastewater over the entire infiltrative sur-
face is required, pressure distribution networks are suggested. These
networks may also be used in systems that are dosed since the mode of
the network operation is intermittent.
To achieve uniform distribution, the volume of water passing out each
hole in the network during a dosing cycle must be nearly equal. To
achieve this, the pressure in each segment of pipe must also be nearly
equal. This is accomplished by balancing the head losses through proper
sizing of the pipe diameter, hole diameter and hole spacing. Thus,
approximately 75 to 85% of the total headloss incurred is across the
hole in the lateral, while the remaining 15 to 25% is incurred in the
network delivering the liquid to each hole. The networks usually con-
sist of 1- to 3-in. (3- to 8-cm) diameter laterals, connected by a cen-
tral or end manifold of larger diameter. The laterals are perforated at
their inverts with 1/4 to 1/2 in. (0.6 to 1.3 cm) diameter holes. The
spacing between holes is 2 to 10 ft (0.6 to 3.0 m) (see Figures 7-24 to
27).
Pumps are used to pressurize the network, although siphons may be used
if the dosing chamber is located at a higher elevation than the lateral
inverts. The active dosing volume is about 10 times the total lateral
pipe volume. This ensures more uniform distribution since the laterals,
drained after each dose, must fill before the network can become prop-
erly pressurized. (See Section 8.3 for dosing chamber design.)
278
-------
FIGURE 7-23
RELIEF LINE DISTRIBUTION NETWORK
Distribution Pipe
rFlow
V
From Pretreatment Unit
A
Absorption
Trenches
Follow Contours
L
JEnds Capped
Relief
Line
L*-A Distribution Pipe and
•Relief Trencn to be Level
Line
Distribution Pipe
Relief
Line
Section A-A
279
-------
«* CO
cvi >-<
1 t I
OH
00
I—I
O
O
O
280
-------
FIGURE ?-
r
End Cap
From
Dosing
Chamber
Distribution
Lateral
281
-------
FIGURE 7-26
LATERAL DETAIL - TEE TO TEE CONSTRUCTION
Reducing Coupling
ro
CO
ro
Manifold
Holes Spaced
2 ft to 10 ft
.Lateral
i-Cap
~Q<-Hole in Cap Near
Crown for Air
Venting in
Larger Systems
-------
FIGURE 7-27
LATERAL DETAIL - STAGGERED TEES OR CROSS CONSTRUCTION
.Staggered Tees or Cross,
ro
CO
GO
Lateral
Cap
d ?
N Holes Spaced
2 Ft. to 10 Ft.
w Hole in Cap
Near Crown for
Air Venting in
Larger Systems
-------
To simplify the design of small pressure distribution networks, Table 7-
13, and Figures 7-28, 7-29, and 7-30, may be used. Examples 7-2 and 7-3
illustrate their use. Other design methods may be equally suitable,
however.
TABLE 7-13
DISCHARGE RATES FOR VARIOUS SIZED HOLES
AT VARIOUS PRESSURES (gpm)
Pressure Hole Diameter (in.)
ftpsT
1 0.43
2 0.87
3 1.30
4 1.73
5 2.17
1/4
0.74
1.04
1.28
1.47
1.65
5/16
1.15
1.63
1.99
2.30
2.57
3/8
1.66
2.34
2.87
3.31
3.71
7/16
2.26
3.19
3.91
4.51
5.04
1/2
2.95
4.17
5.10
5.89
6.59
Example 7-2: Design of a Pressure Distribution Network for a Trench
Absorption Field'
Design a pressure network for an absorption field consisting of five
trenches, each 3 ft wide by 40 ft long, and spaced 9 ft apart center to
center.
Step 1: Select lateral length. Two layouts are suitable for this
system: central manifold (Figure 7-24) or end manifold (Figure
7-25). For a central manifold design, ten 20-ft laterals are
used; for an end manifold design, five 40-ft laterals are
required. The end manifold design is used in this example.
Step 2: Select hole diameter and hale spacing for laterals. For this
example, 1/4-in. diameter holes spaced every 30 in. are used,
although other combinations could be used.
284
-------
FIGURE 7-28
REQUIRED LATERAL PIPE DIAMETERS FOR VARIOUS HOLE DIAMETERS, HOLE SPACINGS, AND LATERAL LENGTHS'
(FOR PLASTIC PIPE ONLY)
ro
CO
en
.c
O)
c
-£
CD —
w
0)
CD
10
b
25-
30-
OK
OO
45
RO
LATERAL DIAMETER (IN)
Hole Diameter
1/4
in)
Hole Spacing (ft)
2
(\
V
2"
3
H-E
11
[)
>^
IV
4
5
1"
Exampl
/ // "
/4
S^
6
7
e7-3
Example
7-2
Hole Diameter (in)
5/16
Hole Spacing (ft)
2
\w
2
3"
3
1!
"
4
5
1"
/ "
1V4
6
rt
1Y2"
7
Hole Diameter
3/8
Hole Spacing
2
1J4-
.3"
3
2"
4
5
r
11/
1
^"
(ft)
6
in)
7
1/4"
Hole Diameter ( n)
7/16
Hole Spacing (ft)
2
3
r/r
3
r/,"
ir
4
5
1
6
7
;r
1 1/4"
m-
2"
v/
it
Hole Diameter (in)
1/2
Hole Spacing (ft)
2
i3
4
Vh"
1»"
2
3"
567
1"
1 V*"
, 11/2"
a Computed for plastic pipe only. The Hazen-Williams equation was used to compute headlosses
through each pipe segment (Hazen-Williams C= 150). The -orifice equation for sharp-edged orifices
(discharge coefficient = 0.6) was used to compute the discharge rates through each orifice.
The maximum lateral length for a given hole and spacing was defined as that length at which the
difference between the rates of discharge from the distal end and the supply end orifice reached
10 percent of the distal end orifice discharge rate.
-------
FIGURE 7-29
RECOMMENDED MANIFOLD DIAMETERS FOR VARIOUS MANIFOLD LENGTHS, NUMBER OF LATERALS,
AND LATERAL DISCHARGE RATES (FOR PLASTIC PIPE ONLY)
MANIFOLD DIAMETER (IN)
ro
co
Flow
Late
(gp
-a'
Central Manifol
per
:ral •
m)
5
10
15
20
25
Manifold Length (ft)
5
4
-
;!
3
2
6
3
*
CM
^
rn
3
10
4 6
1 'a"
f 2
8
i-
••
10
^
« — Exam
3"
r
2 3
4 1 5
15
4
*
2
6
*
»
a|io|i2
2"
ale 7-3
3"
,,P
2|3|4 | 5
it
6
20
6
8
10|12
2"
'S.
3J4
56
...
14
r
7
25
30
35
40
Number of Laterals with Central Manifold
6J8
2"
jJ
IOJ12|l4
si 4|5l 6 1 7
6| 8
2"
3
10J12|14
r
3I4J5
Number
-------
FIGURE 7-30
NOMOGRAPH FOR DETERMINING THE MINIMUM DOSE VOLUME FOR A GIVEN LATERAL DIAMETER,
LATERAL LENGTH, AND NUMBER OF LATERALS
• 4,000 r 1
• 3,500 I
• 3,000
- 2,500
1 2,000
"5 EXAMPLE 7-3 — i
05 V,
•1.500 ^ 7
.3 /
0 EXAMPLE 7-2 /
- 1 ,000 ^ . . — y
•900 ^ /X \
•800 W
• 700 Q
-600 ^
3
• 500 ^
1 450 H
•400 ^
350
s
300 x^x v
^200 ^
150 /
F20 / V
• / "
- / x
-10 < .' ^''
UJ xX
• t? '
• 5^5
/. U-
'4 0
•3 DC
J UJ
CO
•2 ^
3
z
• 1
/
•100
MO
\
\ -15 *:
•3 \ -c
\ -20 1-
\ "25 Z
•^ --^^ ^-30 a
6 , ^^'!«N<
05 50 LU s.^^
•6 'uJ < X "*^^
,7 1 ^ ^N
-I \
•8 ^ \
9 S X\
•10IE
-15
9O
3
U5
01
O
c
UJ
QC
UJ
1V.
• 1
287
-------
Step 3: Sel ec t 1 ateral di ameter • For 1/4-in. hole diameter, 30-in.
hole spacing, and 40-ft length, Figure 7-28 indicates either a
1-1/4-in. or 1-1/2-in. diameter lateral could be used. The 1-
1/2-in. diameter is selected for this example.
Step 4: Calculate lateral discharge rate. By maintaining higher
pressures in the lateral, small variations in elevation along
the length of the lateral and between laterals do not signifi-
cantly affect the rates of discharge from each hole. This
reduces construction costs, but increases pump size. For this
example, a 2-ft head is to be maintained in the lateral. For a
1/4-in. hole at 2 ft of head, Table 7-13 shows the hole dis-
charge rate to be 1.04 gpm.
of hoHs/Utera! .
= 16
Lateral discharge rate = (16 holes/lateral) x (1.04 gpm/hole)
= 16.6 gpm/1 ateral
Step 5: Select manifold size. There are to be five laterals spaced 9
ft apart. A manifold length of 36 ft is therefore required.
For five laterals and 16.6 gpm/lateral, Figure 7-29 indicates
that a 3-in. diameter manifold is required.
Step 6: Determine minimum dose volume (Figure 7-30).
With: lateral diameter = 1-1/2 in.
lateral length = 40 ft
number of laterals = 5
Then: pipe volume = 3.7 gal
Minimum dose volume = approx. 200 gal
The final dose volume may be larger than this minimum depending
on the desired number of doses per day (see Table 7-4).
See Figure 7-31 for completed network design.
Step 7: Determine minimum discharge rate.
Minimum discharge rate = (5 laterals) x (16.6 gpm/lateral)
= 83 gpm
288
-------
FIGURE 7-31
DISTRIBUTION NETWORK FOR EXAMPLE 7-2
3ft
CD
40ft
9ft.
9ft
9ft.
9ft
3 in. Manifold
From Dosing
Chamber
Hole Spacing
30 in.
y2 in. Laterals
-------
Step 8: Select proper pump or siphon.
For a pump system, the total pumping head of the network must
be calculated. This is equal to the elevation difference
between the pump and the distribution lateral inverts, plus
friction loss in the pipe that delivers the wastewater from the
pump to the network at the required rate, plus the desired
pressure to be maintained in the network (the velocity head is
neglected). A pump is then selected that is able to discharge
the minimum rate (83 gpm) at the calculated pumping head.
For a siphon system, the siphon discharge pipe must be elevated
above the lateral inverts at a distance equal to the friction
losses and velocity head in the pipe that delivers the waste-
water from the siphon to the network at the required rate, plus
the desired pressure to be maintained in the network.
For this example, assume the dosing tank is located 25 ft from
the network inlet, and the difference in elevation between the
pump and the inverts of the distribution laterals is 5 ft.
a. Pump (assume 3-in. diameter delivery pipe)
1. Friction loss in 3-in. pipe at 83 gpm (from Table 7-14)
= 1.38 + - (1.73 - 1.38)
= 1.49 ft/100 ft
Friction loss in 25 ft
= (1.49 ft/100 ft) x (25 ft)
= 0.4 ft
2. Elevation Head = 5.0 ft
3. Pressure to be maintained = 2.0
Total pumping head = 7.4 ft
Therefore, a pump capable of delivering at least 83 gpm against
7.4 ft of head is required.
b. Siphon (assume 4-in. diameter delivery pipe)
1. Friction loss in 4-in. pipe at 83 gpm (from Table 7-14)
= 0.37 + -j| (0.46 - 0.37)
= 0.4 ft/100 ft
290
-------
TABLE 7-14
FRICTION LOSS IN SCHEDULE 40 PLASTIC PIPE, C = 150
(ft/100 ft)
Flow 1
gpm
1 0.07
2 0.28
3 0.60
4 1.01
5 1.52
6 2.14
7 2.89
8 3.63
9 4.57
10 5.50
11
12
13
14
15
16
17
18
19
20
25
30
35
40
45
50
60
70
80
90
100
150
200
250
300
350
400
450
500
600
700
800
900
1000
1-1/4
0.07
0.16
0.25
0.39
0.55
0.76
0.97
1.21
1.46
1.77
2.09
2.42
2.74
3.06
3.49
3.93
4.37
4.81
5.23
1-1/2
0.07
0.12
0.18
0.25
0.36
0.46
0.58
0.70
0.84
1.01
1.17
1.33
1.45
1.65
1.86
2.07
2.28
2.46
3.75
5.22
_2
0.07
0.10
0.14
0.17
0.21
0.25
0.30
0.35
0.39
0.44
0.50
0.56
0.62
0.68
0.74
1.10
1.54
2.05
2.62
3.27
3.98
_3 _4 _6 8
0.07
0.08
0.09
0.10
0.11
0.12
0.16
0.23
0.30 0.07
0.39 0.09
0.48 0.12
0.58 0.16
0.81 0.21
1.08 0.28
1.38 0.37
1.73 0.46
2.09 0.55 0.07
1.17 0.16
0.28 0.07
0.41 0.11
0.58 0.16
0.78 0.20
0.99 0.26
1.22 0.32
0.38
0.54
0.72
1£
0.07
0.09
0.11
0.14
0.18
0.24
0.32
0.38
0.46
291
-------
Friction loss in 25 ft
= (0.4 ft/100 ft) x (25 ft)
= 0.10 ft
2. Velocity head in delivery pipe
Discharge rate = 83 gpm = 0.185 ft /sec
Area =(1/4)7T ()2 = 0.087 ft2
Velocity = 0-185 ft3/sec = 2>13 ft/$ec
0.087 ftT
Velocity head . (Velocity)2
_ ([2.13] ft/sec)2
2(32.2 ft/sec2)
= 0.07 ft
3. Pressure to be maintained
= 2.0 ft
Total 2.2 ft
Minimum elevation of the siphon discharge invert above the
lateral inverts must be 2.2 ft.
In summary, the final network design consists of five 40-ft laterals 1-
1/2 in. in diameter connected with a 36-ft end manifold 3-in. in dia-
meter, with the inlet from the dosing chamber at one end of the mani-
fold. The inverts of the laterals are perforated with 1/4-in. holes
spaced every 30 in.
Example 7-3: Design of a Pressure Distribution Network for a Mound
Design a pressure distribution network for the mound designed in Example
7-1.
292
-------
Step 1: Select 1ateral 1ength. A central manifold (Figure 7-24) design
is used in this example.
Lateral length = 65,ft - 0.5 ft (for manifold)
= 32 ft
Step 2: Select hole diameter and hole spacing for laterals. For this
example, 1/4-in. diameter holes spaced every 30 in. are used,
although other combinations could be used.
Step 3: Select lateral diameter. For 1/4-in. hole diameter, 30-in.
hole spacing, and 32-ft lateral length, Figure 7-28 indicates
that either a 1-1/4-in. or 1-1/2-in. diameter lateral could be
used. The 1-1/4-in. diameter is selected for this example.
Step 4: Calculate lateral discharge rate. A 2-ft head is to be
maintained in the lateral.
For 1/4-in. hole at 2 ft of head, Table 7-13 shows the hole
discharge rate to be 1.04 gpm.
of holes per lateral = A g i
= 13
Lateral discharge rate = (13 holes/lateral) x (1.04 gpm/hole)
= 13.5 gpm/lateral
Step 5: Select manifold size. There are to be four laterals (two on
either side of the center manifold) spaced 3 ft apart. A
manifold length of less than 5 ft is required (see Figure 7-
32).
For four laterals, 13.5 gpm/lateral, and manifold length less
than 5 ft, Figure 7-29 indicates that a 1-1/2-in. diameter
manifold is required.
293
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FIGURE 7-32
DISTRIBUTION NETWORK FOR EXAMPLE 7-3
1-1/2 in.
Manifold
Hole Spacing
30 in.
1-1/4 in
Laterals
294
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Step 6: Determine minimum dose volume (Figure 7-30).
With: lateral diameter = 1-1/4 in.
lateral length = 32 ft
number of laterals = 4
Then: pipe volume = 2 gal
Minimum dose volume = <100 gal
From Table 7-4, for a medium texture sand, 4 doses/day are
desirable. Therefore, the dose volume is:
= 112 gal/dose
Step 7: Determine minimum discharge rate.
Minimum discharge rate = (4 laterals) x (13.5 gpm/lateral)
= 54 gpm
Step 8: Sel ec t proper pump . For this example, assume the dosing tank
is located 75 ft from the network inlet, the difference in
elevation between the pump and the inverts of the distribution
laterals is 7 ft, and a 3-in. diameter delivery pipe is to be
used.
Friction loss in 3-in. pipe at 54 gpm (from Table 7-14)
= 0.58 + -- (0.81 - 0.58)
= 0.67 ft/100 ft
Friction loss in 75 ft
= (0.67 ft/100 ft) x (75 ft)
= 0.5 ft
Elevation head = 7.0 ft
Pressure to be maintained = 2.0 ft
Total pumping head = 9.5 ft
Therefore, a pump capable of delivering at least 54 gpm against
9.5 ft of head is required.
295
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In summary, the final network design consists of four 32-ft laterals 1-
1/4 in. in diameter (two on each side of a 3-in. diameter center mani-
fold. The inverts of the laterals are perforated with 1/4-in. holes
spaced every 30 in.
g. Other Distribution Networks
Several other distribution network designs are occasionally used. Among
these are the inverted network and leaching chambers. While users of
these networks claim they are superior to conventional networks, compre-
hensive evaluations of their performance have not been made.
Inverted Network: This network uses perforated pipe with the holes lo-
cated in the crown rather than near the invert (32). This arrangement
is designed to provide more uniform distribution of wastewater over a
large area, and to prolong the life of the field by collecting any set-
tleable solids passing out of the septic tank in the bottom of the pipe.
Water-tight sumps are located at both ends of each inverted line to fa-
cilitate periodic removal of the accumulated solids.
Leaching Chambers: In place of perforated pipe and gravel for distri-
bution and storage of the wastewater, this method employs open bottom
chambers. The chambers interlock to form an underground cavern over the
soils' infiltrative surface. The wastewater is discharged into the cav-
ern through a central weir, trough, or splash plate and allowed to flow
over the infiltrative surface in any direction. Access holes in the
roof of the chamber allow visual inspection of the soil surface and
maintenance as necessary. A large number of these systems have been in-
stalled in the northeastern United States (see Figure 7-33).
7.2.8.2 Materials
Three to 4-in. (8- to 10-cm) diameter pipe or tile is typically used for
nonpressurized networks. Either perforated pipe or 1-ft (30 cm) lengths
of suitable drain tile may be used. The perforated pipe commonly has
one or more rows of 3/8- to 3/4-in. (1.0- to 2.0-cm) diameter holes.
Hole spacing is .not critical. Table 7-15 can be used as a guide for
acceptable materials for nonpressurized networks.
Plastic pipe is used for pressure distribution networks because of the
ease of drilling and assembly. Either PVC Schedule 40 (ASTM D 2665) or
ABS (ASTM 2661) pipe may be used.
296
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TABLE 7-15
PIPE MATERIALS FOR NONPRESSURIZED DISTRIBUTION NETWORKS
Type of Material
Specification
Class
Clay Drain Tile
Clay Pipe
Standard and Extra-
Strength Perforated
Bituminized Fiber Pipe
Homogeneous
Perforated
Laminated-Wall
Perforated
Concrete Pipe
Perforated Concrete
Plastic
Acryloni tri1e-
Butadiene-
Styrene (ABS)
Poly vi nyl
Chloride (PVC)
ASTM C-4
ASTM C-211
ASTM D-2312
ASTM D-2313
ASTM C-44 (Type 1
or Type 2)
ASTM D-2751&
ASTM D-2729b
D-3033b D-3034b
Standard Drain Tile
Standard
ASTM C-143
Styrene-Rubber
Plastic (SR)
Polyethylene (PE)
o Straight Wall
o Corrugated (Flexible)
ASTM D-2852b
D-3298b
ASTM D-1248b
ASTM F-405-76b
a Must be of quality to withstand sulfuric acid.
b These specifications are material specifications only. They do not
give the location or shape of perforations.
297
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FIGURE 7-33
SCHEMATIC OF A LEACHING CHAMBER
Fresh Air
Vent-
-»;•-• .... At^ *au- _-- «-£.
•*K.--"*J
x-^' j,," -2k~- ":;" •*• —. jit-."" - ^"--a-~ .sffc ,.~^'-y- ^--.•>!*-- •r_A""~^L"".^
- ~"'^ --..•~'^" 7~~i. -?
-• -«M'I. -"• _jjft. "^ _ -
.-.f--.-;;-^*::
'^.- j$£- _~3^.—*_-
. '--••" -*.. _ — J,
..-•a- -- — *«.—_;=
.*;:'Jt-~-"..*.--
ife- -_fc-^-,5C- t
.-;*-. ~^-i."-*
«/"
7.2.8.3 Construction
a. Gravity Network Pipe Placement
To insure a free flow of wastewater, the distribution pipe should be
laid level or on a grade of 1 in. to 2 in. per 100 ft (8.5 to 16.9
cm/100 m). To maintain a level or uniform slope, several construction
techniques can be employed. In each case a tripod level or transit is
used to obtain the proper grade elevations. Hand levels are not
adequate.
298
-------
The rock is placed in the excavation to the elevation of the pipe in-
vert. The rock must be leveled by hand to establish the proper grade.
Once the pipe is laid, more rock is carefully placed over the top of the
pipe. Care must also be taken when flexible corrugated plastic pipe is
used, because the pipe tends to "float" up as rock is placed over the
top of the pipe. One method is to employ special holders which can be
removed once all the rock is in place (see Figure 7-34).
FIGURE 7-34
USE OF METAL HOLDERS FOR THE
LAYING OF FLEXIBLE PLASTIC PIPE
Metal Holder
(Removed After
Rock Placement)
4" Flexible
Perforated
Plastic Pipe
b. Pressure Network Pipe Placement
Pressure distribution networks are usually fabricated at the construc-
tion site. This may include drilling holes in distribution laterals.
The holes must be drilled on a straight line along the length of the
pipe. This can be accomplished best by using 1-in. by 1-in. angle iron
as a straight-edge to mark the pipe. The holes are then drilled at the
proper spacing. Care must be used to drill the holes perpendicular to
the pipe and not at an angle. All burrs left around the holes inside
the pipe should be removed. This can be done by sliding a smaller
diameter pipe or rod down the pipe to knock the burrs off.
Solvent weld joints are used to assemble the network. The laterals are
attached to the manifold such that the perforations lie at the bottom of
the pipe.
299
-------
Since the network is pressurized, small elevation differences along the
length of the lateral do not affect the uniform distribution signifi-
cantly. However, these variations should be held within 2 to 3 in. (5
to 8 cm). The rock is placed in the absorption area first, to the ele-
vation of the distribution laterals. The rock should be leveled by
hand, maintaining the same elevation throughout the system, before lay-
ing the pipe. After the pipe is laid, additional rock is placed over
the pipe.
c. Distribution Boxes
If used, distribution boxes should be installed level and placed in an
area where the soil is stable and remains reasonably dry. To protect
the box from frost heaving, a 6-in. (15-cm) layer of 1/2- to 2-1/2-in.
(1.2- to 6.4-cm) rock should be placed below and around the sides of the
box. Solid wall pipe should be used to connect the box with the distri-
bution laterals. Separate connections should be made for each lateral.
To insure a more equal division of flow, the slope of each connecting
pipe should be identical for at least 5 to 10 ft (1.3 to 3.0 m) beyond
the box.
7.3 Evaporation Systems
7.3.1 Introduction
Two basic types of onsite evaporation systems are in use today:
1. Evapotranspiration beds (with and without infiltration)
2. Lagoons (with and without infiltration)
The advantages of these systems are that they utilize the natural energy
of the sun and, optionally, the natural purification capabilities of
soil to dispose of the wastewater. They must, however, be located in
favorable climates. In some water-short areas where consumptive water
use is forbidden (e.g., Colorado), they may not be allowed.
Mechanical evaporators are in the experimental stage, and are not com-
mercially available. For this reason, they are not included in this
discussion.
300
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7.3.2 Evapotranspiration and Evapotranspiration/Absorption Beds
7.3.2.1 Introduction
Evapotranspiration (ET) beds can be used to dispose of wastewater to the
atmosphere so that no discharge to surface or groundwater is required.
Evapotranspiration/absorption (ETA) is a modification of the ET concept
in which discharges to both the atmosphere and to the groundwater are
incorporated. Both ET and ETA have been utilized for onsite wastewater
disposal to the extent that several thousand of these systems are in use
in the United States (33).
7.3.2.2 Description
Onsite ET disposal normally consists of a sand bed with an impermeable
liner and wastewater distribution piping (see Figure 7-35). The surface
of the sand bed may be planted with vegetation. Wastewater entering the
bed is normally pretreated to remove settleable and floatable solids.
An ET bed functions by raising the wastewater to the upper portion of
the bed by capillary action in the sand, and then evaporating it to the
atmosphere. In addition, vegetation transports water from the root zone
to the leaves, where it is transpired. In ETA systems, the impervious
liner is omitted, and a portion of the wastewater is disposed of by
seepage into the soil.
Various theoretical approaches are used to describe the evaporation
process. This suggests that there may be some uncertainty associated
with a precise quantitative description of the process. However, cur-
rent practice is to limit the uncertainties by basing designs on a
correlation between available pan evaporation data and observed ET
rates, thereby minimizing assumptions and eliminating the need to aver-
age long-term climatic data. References (33)(34)(35) and (36) provide a
more detailed discussion of the correlation method.
7.3.2.3 Application
Onsite systems utilizing ET disposal are primarily used where geological
limitations prevent the use of subsurface disposal, and where discharge
to surface waters is not permitted or feasible. The geological condi-
tions that tend to favor the use of ET systems include very shallow soil
mantle, high groundwater, relatively impermeable soils, or fractured
bedrock. ETA systems are generally used where slowly permeable soils
are encountered.
301
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FIGURE 7-35
CROSS SECTION OF TYPICAL ET BED
Slope
Impervious
Liner
Topsoil (Varies 0-4")
'Filter Cloth or Equivalent
Rock
Perforated'
Pipe (4")
Although ET systems may be used where the application of subsurface dis-
posal systems is limited, they are not without limitations. As with
other disposal methods that require area-intensive construction, the use
of ET systems can be constrained by limited land availability and site
topography. Based on experience to date with ET disposal for year-round
single-family homes, approximately 4,000 to 6,000 ft2 (370 to 560 nr) of
available land is typically required. The maximum slope at which an ET
system is applicable has not been established, but use on slopes greater
than 15% may be possible if terracing, serial distribution, and other
appropriate design features are incorporated.
By far the most significant constraint on the use of ET systems is cli-
matic conditions. The evaporation rate is controlled primarily by cli-
matic factors such as precipitation, wind speed, humidity, solar radia-
tion, and temperature. Recent studies indicate that essentially all of
the precipitation that falls on an ET bed infiltrates into the bed and
becomes part of the hydraulic load that requires evaporation (33)(34)
(37). Provisions for long-term storage of effluent and precipitation in
ET systems during periods of negative net evaporation, and for subse-
quent evaporation during periods of positive net evaporation, are expen-
sive. Thus, the year-around use of nondischarging ET systems appears to
be feasible only in the arid and semiarid portions of the western and
southwestern United States where evaporation exceeds precipitation dur-
ing every month of operation, so that long-term storage capacity is not
302
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required. ET systems for summer homes may be feasible in the more tem-
perate parts of the country. For ETA systems, the range of applicabil-
ity is less well defined, but the soils must be capable of accepting all
of the influent wastewater if net evaporation is zero for any signifi-
cant periods of the year.
In addition to climate and site conditions, the characteristics of
wastewater discharged to an onsite disposal system may affect its appli-
cation. For ET disposal, pretreatment to remove settleable and float-
able solids is necessary to prevent physical clogging of the wastewater
distribution piping. The relative advantages of aerobic versus septic
tank pretreatment for ET and ETA systems have been discussed in the
literature (33)(35)(37)(38). Although each method has been supported by
some researchers, reports of well-documented, controlled studies indi-
cate that septic tank pretreatment is adequate (33)(34)(37).
7.3.2.4 Factors Affecting Performance
The following factors affect the performance of ET and ETA systems:
1. Cl imate
2. Hydraulic loading
3. Sand capillary rise characteristics
4. Depth of free water surface in the bed
5. Cover soil and vegetation
6. Construction techniques
7. Salt accumulation (ET only)
8. Soil permeability (ETA only)
As noted previously, climate has a significant effect on the application
and performance of ET and ETA systems. Solar radiation, temperature,
humidity, wind speed, and precipitation all influence performance. Since
these parameters fluctuate from day to day, season to season, and year
to year, evaporation rates also vary substantially. To insure adequate
overall performance, these fluctuations must be considered in the
design.
The hydraulic loading rate of an ET bed affects performance. Too high a
loading rate results in discharge from the bed; too low a loading rate
results in a lower gravity (standing) water level in the bed and ineffi-
cient utilization. Several researchers noted decreased evaporation
rates with decreased water levels (33)(34)(35). This problem can be
overcome by sectional construction in level areas to maximize the water
level in a portion of the bed, and by serial distribution for sloping
sites.
303
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The capillary rise characteristic of the sand used to fill the ET bed is
important since this mechanism is responsible for transporting the water
to the surface of the bed. Thus, the sand needs to have a capillary
rise potential at least as great as the depth of the bed, and yet should
not be so fine that it becomes clogged by solids in the applied
wastewater (33).
Significant seasonal fluctuations in the free water surface are normal,
necessitating the use of vegetation that is tolerant to moisture ex-
tremes. A variety of vegetation, including grasses, alfalfa, broad-leaf
trees, and evergreens, have been reported to increase the average annual
evaporation rate from an ET bed to above that for bare soil (35). How-
ever, grasses and alfalfa also result in nearly identical or reduced
evaporation rates as compared to bare soil in the winter and the spring
when evaporation rates are normally at a minimum (33)(34). Similarly,
top soil has been reported to reduce evaporation rates. Certain ever-
green shrubs, on the other hand, have been shown to produce slightly
greater evaporation rates than bare soil throughout the year (33).
Thus, there are conflicting views on the benefits of cover soil and
vegetation.
Although ET system performance is generally affected less by construc-
tion techniques than most subsurface disposal methods, some aspects of
ET construction can affect performance. Insuring the integrity of the
impermeable liner and selecting the sand to provide for maximum capil-
lary rise properties are typically the most important considerations.
For ETA systems, the effects of construction techniques are similar to
those discussed previously with reference to subsurface disposal systems
in slowly permeable soils.
Salt accumulation in ET disposal systems occurs as wastewater is evapor-
ated. Salt accumulation is particularly pronounced at the surface of
the bed during dry periods, although it is redistributed throughout the
bed by rainfall. Experience to date indicates that salt accumulation
does not interfere with the operation of nonvegetated ET systems (39)
(40). For ET systems with surface vegetation, salt accumulation may
adversely affect performance after a long period of use, although obser-
vations of ET systems that have been in operation for 5 years indicate
no significant problems (33). In order to minimize potential future
problems associated with salt accumulation, the ET or ETA piping system
may be designed to permit flushing of the bed.
Since ETA systems utilize seepage into the soil as well as evaporation
for wastewater disposal, soil permeability is also a factor in the per-
formance of these systems. Discussion of this factor relative to sub-
surface disposal systems (Section 7.2) applies here.
304
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Data that quantitatively describe performance are not available for ET
or ETA disposal. However, the technical feasibility of nondischarging
ET disposal has been demonstrated under experimental conditions (33)
(34). In addition, observations of functioning ET systems indicate that
adequate performance can be achieved at least in semi arid and arid
areas. The performance of ETA systems depends primarily on the rela-
tionship between climate and soil characteristics, and has not been
quantified. However, the technical feasibility of such systems is well
accepted.
7.3.2.5 Design
ET and ETA systems must be designed so that they are acceptable in per-
formance and operation. Requirements for acceptability vary. On one
hand, acceptable performance can be defined for an ET system as zero
discharge for a specified duration such as 10 years, based on the wea-
ther data for a similar period. Alternatively, occasional seepage or
surface overflow during periods of heavy rainfall or snowmelt may be al-
lowed. In addition, physical appearance requirements for specific types
of vegetation and/or a firm bed surface for normal yard use (necessita-
ting a maximum gravity water level approximately 10 in. [25 cm] below
the surface) may also be incorporated in the criteria.
Appropriate acceptance criteria vary with location. For example, occa-
sional discharge may be acceptable in low-density rural areas, whereas
completely nondischarging systems are more appropriate in higher density
suburban areas. Thus, acceptance criteria are usually defined by local
health officials to reflect local conditions (33).
Since the size (and thus the cost) of ET and ETA systems are dependent
on the design hydraulic loading rate, any reduction in flow to those
systems is beneficial. Therefore, flow reduction devices and techniques
should be considered an integral part of an ET or ETA system.
The design hydraulic loading rate is the principal design feature affec-
ted by the acceptance criteria. Where a total evaporation system is
required, the loading rate must be low enough to prevent the bed from
filling completely. Some discrepancy in acceptable loading rates has
been reported. Although reports of system designs based on higher load-
ing rates have been presented in the literature (35)(37), other data
obtained under controlled conditions indicate that pan evaporation must
exceed precipitation in all months of a wet year (based on at least 10
years of data) if a total, year-round evaporation system is used. Under
these conditions, loading rates between 0.03 and 0.08 gpd/ft^ (1.2 and
3.3 1/nr/day) were found to be appropriate in western states (Colorado
and Arizona) (33)(34).
305
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The hydraulic loading rate is determined by an analysis of the monthly
net ET ([pan evaporation x a local factor] minus precipitation) expe-
rienced in the wettest year of a 10-year period. Ten years of data
should be analyzed, as very infrequent but large precipitation events
may be experienced over the life of the system that would result in very
infrequent discharge. Where occasional discharge from an ET system is
acceptable, loading rates may be determined on a less restrictive basis,
such as minimum monthly net ET in a dry year. If the unit is used for
seasonal application, then only those months of occupancy will consti-
tute the basis for design.
The loading rate for ETA systems is determined in the same manner, ex-
cept that an additional factor to account for seepage in the soil is
included. Thus, the loading rate for an ETA system is generally greater
than the loading rate for an ET system in the same climate. The avail-
able data indicate that ETA systems can be used with a wider range of
climatic conditions. For example, if soil can accept 0.2 gpd/ft (8.1
1/nr/day), and the minimum monthly net ET is zero (determined as neces-
sary according to ±he acceptance criteria), the loading rate for design
is also 0.2 gpd/ftz (8.1 l/mVday).
In addition to loading rates, the designer must also consider selection
of fill material, cover soil, and vegetation. The role of vegetation in
providing additional transpiration for ET systems is uncertain at this
time. During the growing season, the impact of vegetation could be
significant. However, during the nongrowing season, the effect of vege-
tation has not been well documented. Sand available for ET and ETA bed
construction should be tested for capillary rise height and rate before
one is selected. In general, clean and uniform sand in the size of DCQ
= 0.1 mm (50% by weight smaller than or equal to 0.1 mm) is desirable
(33).
The assumptions for a sample ET bed design are given below:
1. Four occupants of home
2. 45-gpcd design flow (no in-home water reduction)
3. Location: Boulder, Colorado
4. Critical months: December 1976 (see Figure 7-36)
5. Precipitation: 0.01 in./day (0.25 mm/day)
6. Pan evaporation: 0.07 in./day (1.7 mm/day)
An ET bed must be able to evaporate the household wastewater discharged
to it as well as any rain that falls on the bed surface. Thus, the
design of an ET system is based on the estimated flow from the home and
the difference between the precipitation rate and the evaporation rate
306
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FIGURE 7-36
CURVE FOR ESTABLISHING PERMANENT HOME LOADING RATE FOR BOULDER, COLORADO
BASED ON WINTER DATA, 1976-1977(33)
10
mm/day
0
JJ
1976
DJ
J J
1977
307
-------
during the critical months of the year. In this example, we are assum-
ing an average household flow of 4 persons x 45 gcpd, or 180 gpd
total. Past work has shown that actual evaporation from an ET system is
approximately the same as the measured pan evaporation rate in winter
(33). Summer rates are approximately 70% of the measured pan evapora-
tion rates in this area, but excessive evaporation potential more than
offsets this condition. Therefore, the design is based on pan evapora-
tion (in./day) minus precipitation (in./day). In this example,
(0.07 in./day) - (0.01 in./day) = 0.06 in./day
This equates to a rate of 0.04 gpd/ft2.
In this example, then, the required area for the ET bed is finally cal-
culated:
180 gPd , = 4,500 ft2
0.04 gpd/fr
To allow a factor of safety, the size could be increased to as much as
7,500 ft2 based on 75 gpcd. A more realistic size would be 5,000 to
6,000 ft2, which would insure no overflows. If water conservation is
practiced, direct significant savings in size and costs could be
achieved.
7.3.2.6 Construction Features
A typical ET bed installation was shown previously in Figure 7-35.
Characteristics of an ETA bed are identical except that the liner is
omitted. Limited data are available on optimum construction features
for ET and ETA disposal units. The following construction features are
desirable:
1. Synthetic liners should have a thickness of at least 10 mil; it
may be preferable to use a double thickness of liner material
so that the seams can be stagqered if seams are unavoidable.
2. Synthetic liners should be cushioned on both sides with layers
of sand at least 2 in. (5 cm) thick to prevent puncturing dur-
ing construction.
3. Surface runoff from adjacent areas should be diverted around
the system by berms or drainage swales.
308
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4. Crushed stone or gravel placed around the distribution pipes
should be 3/4 to 2-1/2 in. (2 to 6 cm).
5. Filter cloth or equivalent should be used on top of the rock or
gravel to prevent sand from settling into the aggregate, thus
reducing the void capacity.
6. Care should be exercised in assembling the perforated distri-
bution pipes (4 in. [10 cm]) to prevent pipe glues and solvents
from contacting the synthetic liner.
7. The bed surface should be sloped for positive drainage.
8. A relatively porous topsoil, such as loamy sand or sandy loam,
should be used if required to support vegetation to prevent
erosion, or to make the appearance more acceptable.
9. The bed should be located in conformance with local code re-
quirements.
10. Construction techniques described previously for subsurface
disposal systems, where soil permeability may be decreased by
poor construction practices, should be used for ETA systems
(39WOH41).
7.3.2.7 Operation and Maintenance
Routine operation and maintenance of an ET or ETA disposal unit consists
only of typical yard maintenance activities such as vegetation trimming.
Pretreatment units and appurtenances require maintenance as described in
Chapter 8. Unscheduled maintenance requirements are rare, and stem
mainly from poor operating practices such as failure to pump out septic
tank solids.
7.3.2.8 Considerations for Multi-Home and Commercial
Wastewaters
ET systems may be applicable to small housing clusters and commer-
cial/institutional establishments, but large area requirements may limit
their practicality. Adjustments in the type of pretreatment used may be
required depending on the wastewater characteristics. For example, a
grease trap is normally required prior to septic tank or aerobic treat-
ment of restaurant wastewater disposed of in an ET system.
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7.3.3 Evaporation and Evaporation/Infiltration Lagoons
7.3.3.1 Description
Lagoons have found widespread application for treatment of municipal
wastewater from small communities, and have occasionally been used for
wastewater treatment in onsite systems prior to discharge to surface
waters. A more common application in onsite systems has been for treat-
ment and subsequent disposal by evaporation, or a combination of evapo-
ration and infiltration.
A discussion of evaporation and evaporation/infiltration lagoons is pro-
vided, since thousands are currently in use across the United States.
However, performance data are very limited. The information provided in
this section is based on current practice without assurance that such
practice is optimal.
7.3,3.2 Application
In the United States, an evaporation or evaporation/infiltration lagoon
could be used in most locations that have enough available land. How-
ever, local authorities typically prefer or require the use of subsur-
face disposal systems where conditions permit. Thus, actual application
of these lagoons is generally limited to rural areas where subsurface
disposal is not possible. In addition, use of evaporation/infiltration
lagoons is not appropriate in areas where wastewater percolation might
contaminate groundwater supplies, such as in areas of shallow or crev-
iced bedrock, or high water tables. Use of both types of lagoons, espe-
cially evaporation lagoons, is favored by the large net evaporation po-
tentials found in arid regions.
Data on the impact of influent wastewater characteristics on evaporation
and evaporation/infiltration lagoons are very limited. Pretreatment is
desirable, especially if a garbage grinder discharges to the system.
7.3.3.3 Factors Affecting Performance
The major climatic factors affecting performance of evaporation and
evaporation/infiltration lagoons include sunlight, wind circulation,
310
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humidity, and the resulting net evaporation potential. Other features
that affect performance include:
1. Soil permeability (evaporation/infiltration only)--lagoon size
and soil permeability are inversely proportional
2. Salt accumulation (evaporation only)—results in decreased
evaporation rate
3. Hydraulic loading—size must accommodate peak flows
4. Inlet configuration—center inlet tends to improve mixing and
minimize odors
5. Construction techniques
7.3.3.4 Design
Lagoons can be circular or rectangular. The maximum wastewater depth is
normally 3 to 5 ft (0.9 to 1.5 m) with a freeboard of 2 or 3 ft, (0.6 to
0.9 m), although depths greater than 8 ft (2.4 m) have also been used
(42)(43)(44)(45)(46)(47). The minimum wastewater depth is generally 2
ft (0.6 m). This may necessitate the addition of fresh water during
high-evaporation summer months. Figure 7-37 shows the dimensional re-
quirements for a typical onsite lagoon. The size ranges from 3 to 24
ftVgpcd (0.07 to 0.57 nr/lpcd), depending primarily on the type of
lagoon (evaporation or evaporation/infiltration), soil permeability,
climate, and loc'al regulations.
Lagoon design is usually based on locally available evaporation and pre-
cipitation data, soil percolation rates (evaporation/infiltration only),
and an assumed wastewater flow. Since runoff is excluded by the con-
tainment berms, evaporation lagoons need only provide adequate surface
area to evaporate the incident precipitation and the influent waste-
water. Calculations may be made initially on an annual basis, but must
then be checked to insure that adequate volume is provided for storage
during periods when liquid inputs exceed evaporation. A brief design
example is outlined below.
Assumptions:
1. Four occupants of home
2. 45-gpcd wastewater flow
3. Annual precipitation: 15.3 in.
4. Annual evaporation: 46.7 in.
311
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FIGURE 7-37
TYPICAL EVAPORATION/INFILTRATION LAGOON FOR SMALL INSTALLATIONS
Inlet -*£>:
Min
3:1 Slope
Min. Freeboard
-iz. Winter
- 3 Operating Depth,
Inlet
312
-------
Design flow:.
4 persons x 45 gpcd = 180 gpd
Net evaporation per year:
46.7 in. - 15.3 in. = 31.4 in.
(31.4 in.)(144 in.2) = 4,522 in.3 of water/ft2 water surface
(4,522 in.3)(ft3/l,728 in.3)(7.48 gal/ft3) = 19.6 gal of water/ft2
water surface
Lagoon area required:
(180 gpd){365 days)/(19.6 gal/ft2) = 3,352 ft2
This can be provided by a round lagoon, 65.3-ft diameter.
At this point, we need to ensure that the lagoon will have adequate
storage capacity to allow accumulation of water to a depth of no more
than 4 or 5 ft in low-evaporation months (usually winter), and to allow
sufficient surface area for evaporation of the accumulated water plus
new influent flows during the months when evaporation rates exceed the
monthly wastewater flow (usually summer). This is done by comparing the
wastewater flow against the evaporation rate for each months, and by
performing a water balance (i.e., calculating the gain or loss in gal-
lons for each months). Table 7-16 shows such a balance.
From October through April, the lagoon will gain 35,443 gal of volume.
This is equivalent to a gain of 1.4 ft:
(35,443 gal)( L-^)(? 48\a1) = 1.4 ft
3,352 fr /tW 9al
Beginning with a 2-ft minimum depth, the depth of the lagoon varies from
2 ft to 3.4 ft.
Some sources indicate that BOD loadings should also be considered in
lagoon sizing for odor control. Loadings in the range of 0.25 to 0.8
#BOD/day/l,000 ft* (1.2 to 3.9 kg/day/1000 m*) have been recommended,
but supporting data for onsite systems are not available (43)(45)(46).
If infiltration is permitted and feasible considering local soils, the
size of the lagoon can be reduced by the amount of water lost through
percolation.
313
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TABLE 7-16
SAMPLE WATER BALANCE FOR EVAPORATION LAGOON DESIGN
Preci pi tati ona Net"
Month
October
November
December
January
February
March
oo April
£ May
June
July
August
September
Influent
gal
5580
5400
5580
5580
5040
5580
5400
5580
5400
5580
5580
5400
SBTTKJ
Preci p .
in.
1.1
1.4
1.8
1.6
1.6
1.4
1.4
1.2
1.2
0.8
0.8
1.0
T573~
Evap.
in.
2.2
1.8
1.7
0.7
0.9
1.2
3.1
5.3
6.1
9.4
9.0
5.3
"5577
-Evaporation
in.
-1.1
-0.4
0.1
0.9
0.7
0.2
-1.7
-4.1
-4.9
-8.6
-8.2
-4.3
gal
- 2299
- 836
209
1881
1463
418
- 3553
- 8569
-10241
-17974
-17138
- 8987
Flow
gai
3281
4564
5789
7461
6503
5998
1847
- 2989
- 4841
-12394
-11558
- 3587
Cum.
Vol ume
gal
3281
7845
13634
21095
27598
33596
35443
32454
27613
15219
3661
74
a [Precip. - Evap. (gal)] = [Precip. - Evap. (in.)] x (3352 ft2) x (7.48 gal/ft3) x (1/12)
= 2090 x [Precip. - Evap. (in.)]
b Net Flow = (Influent) + (Precip. - Evap.)
-------
Other design features which are frequently incorporated include fencing,
center inlet, specific berm slopes, and buffer zones. Five- or 6-ft
(1.5- to 1.8-m) high fencing is preferred to limit animal and human in-
trusion. Submerged center inlets are recommended to facilitate mixing,
to provide even solids deposition, and to minimize odors. Interior berm
slopes, steep enough to minimize rooted aquatic plant growth in the
lagoon, but resistant to erosion, are desirable. Slopes sufficient to
accomplish this objective have been reported to be between 3:1 and 2:1,
depending primarily on height and soil characteristics. Buffer zones
are normally controlled by local regulations, but typically range from
100 to 300 ft (30 to 91 m).
7.3.3.5 Construction Features
To prevent seepage through the berm in unlined lagoons, a good interface
between the berm and the native soil is necessary. In areas where the
use of subsurface disposal systems is restricted due to slowly permeable
soils, B-horizon soils are frequently appropriate for berm construction.
Excavation of the topsoil prior to berm placement (so that the base of
the berm rests on the less permeable subsoils) reduces the incidence of
seepage, as does compaction of the berm material during placement. For
evaporation lagoons, care during construction to insure placement of a
leak-free liner reduces the need for impermeable berm material and asso-
ciated construction precautions.
7.3.3.6 Operation and Maintenance
Start-up of a lagoon system requires filling the lagoon from a conven-
ient freshwater source to a depth of at least 2 ft (0.6 m). This ini-
tial filling helps to prevent rooted plant growth and septic odors.
Solids removal is required periodically for evaporation lagoons. Data
are not available to indicate the exact frequency of solids removal
required, but intervals of several years between pump-outs can be anti-
cipated.
The reported need for chemical addition to control odors, insects,
rooted plants, and microbial growth varies on a case-by-case basis with
climate, lagoon location and configuration, and loading rate. Mainte-
nance of a minimum 2-ft (0.6-m) wastewater depth in the lagoon, and fre-
quent trimming of vegetation on the berm and in the vicinity of the la-
goon, are suggested. No other maintenance is required.
315
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7.3.3.7 Seasonal, Multifamily, and Commercial Applications
Use of evaporation and evaporation/infiltration lagoons for summer homes
would result in somewhat reduced area requirements per gallon of waste-
water handled, since storage would not need to be provided during the
winter months. Otherwise, application of these systems to seasonal
dwellings is comparable to year-round residences.
Evaporation and evaporation/infiltration lagoons are also applicable to
multifamily and commercial applications, although additional pretreat-
ment may be required depending on the wastewater characteristics.
7.4 Outfall to Surface Waters
Direct discharge of onsite treatment system effluent is a disposal op-
tion if an appropriate receiving water is available and if the regula-
tory agencies permit such a discharge. The level of treatment required
varies, depending on local regulations, stream water quality require-
ments, and other site-specific conditions. In general, onsite treatment
system effluent disposed by surface discharge must at least meet secon-
dary treatment standards for publicly owned treatment works. Depending
on site-specific conditions, more stringent BOD and SS discharge re-
quirements and/or limitations on N and P discharges may be applicable.
The performance, operation, and maintenance requirements, and the en-
vironmental acceptability of the surface discharge depend predominantly
on the preceding treatment system. Operation and maintenance associated
specifically with the surface discharge pipe are minimal in a gravity
situation. If the effluent must be pumped, then routine pump mainte-
nance will be required.
Discharge pipes should be made of corrosion- and crush-resistant materi-
als such as cast iron or rigid plastic pipe. For single-family systems,
the pipe should range from 2 to 4 in. (5 to 10 cm) in diameter, should
be buried, and should be moderately sloped (between 0.5 and 3%). Steep
slopes may cause washout at the discharge point.
7.5 References
1. Bendixen, T. W., M. Berk, J. P. Sheehy, and S. R. VJeibel. Studies
on Household Sewage Disposal Systems, Part II. NTIS Report No. PB
216 128, Environmental Health Center, Cincinnati, Ohio, 1950. 96
pp.
316
-------
2. Bouma, J. Unsaturated Flow During Soil Treatment of Septic Tank
Effluent. J. Environ. Eng. Div., Am. Soc. Civil Eng., 101:996-983,
1975.
3. Manual of Septic Tank Practice. Publication No. 526, Public Health
Service, Washington, D.C., 1967. 92 pp.
4. Small Scale Waste Management Project, University of Wisconsin,
Madison. Management of Small Waste Flows. EPA 600/2-78-173, NTIS
Report No. PB 286 560, September 1978. 804 pp.
5. Laak, R. Pollutant Loads From Plumbing Fixtures and Pretreatment
to Control Soil Clogging. J. Environ. Health, 39:48-50, 1976.
6. Winneberger, J. H., L. Francis, S. A. Klein, and P. H. McGauhey.
Biological Aspects of Failure of Septic Tank Percolation Systems;
Final Report. Sanitary Engineering Research Laboratory, University
of California, Berkeley, 1960.
7. Winneberger, J. T., and J. W. Klock. Current and Recommended Prac-
tices for Subsurface Waste Water Disposal Systems in Arizona. En-
gineering Research Center Report No. ERC-R-73014, College of Engi-
neering Service, Arizona State University, Tempe, 1973.
8. Subsurface Wastewater Disposal Regulations. Plumbing Code, Part
II. Department of Human Services, Division of Health Engineering,
Augusta, Maine, 1978.
9. Weibel, S. R., T. W. Bendixen, and J. B. Coulter. Studies on
Household Sewage Disposal Systems, Part III. NTIS Report No. PB
217 415, Environmental Health Center, Cincinnati, Ohio, 1954. 150
pp.
10. Corey, R. B., E. J. Tyler, and M. V. Olotu. Effects of Water Soft-
ener Use on the Permeability of Septic Tank Seepage Fields. In:
Proceedings of the Second National Home Sewage Treatment Symposium,
Chicago, Illinois, December 1977. American Society of Agricultural
Engineers, St. Joseph, Michigan, 1978. pp. 226-235.
11. Machmeier, R. E. Town and Country Sewage Treatment. Bulletin 304,
University of Minnesota, St. Paul, Agricultural Extension Service,
1979.
12. Otis, R. J., G. D. Plews, and D. H. Patterson. Design of Conven-
tional Soil Absorption Trenches and Beds. In: Proceedings of the
Second National Home Sewage Treatment Symposium, Chicago, Illinois,
December 1977. American Society of Agricultural Engineers, St.
Joseph, Michigan, 1978. pp. 86-99.
317
-------
13. McGauhey, P. H., and J. T. Winneberger. Final Report on a Study of
Methods of Preventing Failure of Septic Tank Percolation Systems.
SERL Report No. 65-17, Sanitary Engineering Research Laboratory,
University of California, Berkeley, 1965. 33 pp.
14. Bendixen, T. W., J. B. Coulter, and G. M. Edwards. Study of Seep-
age Beds. Robert A. Taft Sanitary Engineering Center, Cincinnati,
Ohio, 1960.
15. Bouma, J., J. C. Converse, and F. R. Magdoff. Dosing and Resting
to Improve Soil Absorption Beds. Trans. Am. Soc. Civ. Eng.,
17:295-298, 1974.
16. Harkin, J. M., and M. D. Jawson. Clogging and Unclogging of Septic
System Seepage Beds. In: Proceedings of the Second Illinois Pri-
vate Sewage Disposal system, Champaign, Illinois, 1977. Illinois
Department of Public Health, Springfield, pp. 11-21.
17. Otis, R. J., J. C. Converse, B. L. Carlile, and J. E. Witty. Ef-
fluent Distribution. In: Proceedings of the Second National Home
Sewage Treatment Symposium, Chicago, Illinois, December 1977.
American Society of Agricultural Engineers, St. Joseph, Michigan,
1978. pp. 61-85.
18. Kropf, F. W., R. Laak, and K. A. Healey. Equilibrium Operation of
Subsurface Absorption Systems. J. Water Pollut. Control Fed.,
49:2007-2016, 1977.
19. Otis, R. J. An Alternative Public Wastewater Facil-ity for a Small
Rural Community. Small Scale Waste Management Project, University
of Wisconsin, Madison, 1978.
20. Alternatives for Small Wastewater Treatment Systems. EPA 625/4-77-
011, NTIS Report No. PB 299 608, Center for Environmental Research
Information, Cincinnati, Ohio, 1977.
21. Bendixen, T. W., R. E. Thomas, and J. B. Coulter. Report of a
Study to Develop Practical Design Criteria for Seepage Pits as a
Method for Disposal of Septic Tank Effluents. NTIS Report No. PB
216 931, Cincinnati, Ohio, 1963. 252 pp.
22. Winneberger, J. T. Sewage Disposal System for the Rio-Bravo Tennis
Ranch, Kern County, California. 1975.
23. Witz, R. L., G. L. Pratt, S. Vogel, and C. W. Moilanen. Waste Dis-
posal Systems for Rural Homes. Circular No. AE 43, North Dakota
State University Cooperative Extension Service, Fargo, 1974.
24. Converse, J. C., B. L. Carlile, and G. W. Peterson. Mounds for the
Treatment and Disposal of Septic Tank Effluent. In: Proceedings
of the Second National Home Sewage Treatment Symposium, Chicago,
318
-------
Illinois, December 1977. American Society of Agricultural Engi-
neers, St. Joseph, Michigan, 1978. pp. 100-120.
25. Converse, J. C. Design and Construction Manual for Wisconsin
Mounds. Small Scale Waste Management Project, University of Wis-
consin, Madison, 1978. 80 pp.
26. Soil Conservation Service. Drainage of Agricultural Land. Water
Information Center, Port Washington, New York, 1973. 430 pp.
27. Mellen, W. L. Identification of Soils as a Tool for the Design of
Individual Sewage Disposal Systems. Lake County Health Department,
Waukegan, Illinois, 1976. 67 pp.
28. Casagrande, L. Electro-Osmotic Stabilization of Soils. J. Boston
Soc. Civ. Eng., 39:51-82, 1952.
29. On-Site Wastewater Management. National Environmental Health Asso-
ciation, Denver, Colorado, 1979. 108 pp.
30. Electro-Osmosis, Inc., Minneapolis, Minnesota.
31. Bendixen, T. W., and J. B. Coulter. Effectiveness at the Distri-
bution Box. U.S. Public Health Service, Washington, D.C., 1958.
32. Sheldon, W. H. Septic Tank Drainage Systems. Research Report No.
10, Farm Science Agricultural Experiment Station, Michigan State
University, East Lansing, 1964.
33. Bennett, E. R. and K. D. Linstedt. Sewage Disposal by Evaporation-
Transpiration. EPA 600/2-78-163, NTIS Report NO. PB 288 588,
September 1978. 196 pp.
34. Rugen, M. A., D. A. Lewis, and I. J. Benedict. Evaporation - A
Method of Disposing of Septic Tank Effluent. Edwards Underground
Water District, San Antonio, Texas, (no date). 83 pp.
35. Bernhardt, A. P. Treatment and Disposal of Wastewater from Homes
by Soil Infiltration and Evapotranspiration. University of Toronto
Press, Toronto, Canada, 1973. 173 pp.
36. Pence, H. J. Evaluation of Evapotranspiration as a Disposal System
for Individual Household Wastes (A Seven-State Test); Draft Report.
National Science Foundation, Washington, D.C., 1979.
37. Lomax, K. M., P. N. Winn, M. C. Tatro, and L. S. Lane. Evapotran-
spiration Method of Wastewater disposal. UMCEES Ref. No. 78-40,
University of Maryland Center for Environmental and Estuarine Stud-
ies, Cambridge, 1978. 42 pp.
319
-------
38. Bernhart, A. P. Return of Effluent Nutrients to the Natural Cycle
Through Evapotranspiration and Subsoil-Infiltration of Domestic
Wastewater. In: Proceedings of the National Home Sewage Disposal
Symposium, Chicago, Illinois, December 1974. American Society of
Agricultural Engineers, St. Joseph, Michigan, 1975. pp. 175-181.
39. Land Treatment of Municipal Wastewater Effluents. EPA 625/4-76-010,
NTIS Report No. PB 259 994, Center for Environmental Research
Information, Cincinnati, Ohio, 1976.
40. Jensen, M. E., H. G. Collins, R. D. Burman, A. E. Cribbs, and A. I.
Johnson. Consumptive Use of Water and Irrigation Water Require-
ments. Irrigation Drainage Division, American Society of Civil
Engineers, New York, 1974. 215 pp.
41. Priestly, C. H. B., and R. J. Taylor. On the Assessment of Surface
Heat Flux and Evaporation Using Large-Scale Parameters. Mon.
Weather Rev., 100:81-82, 1972.
42. Witz, R. L. Twenty-Five Years with the Nodak Waste Disposal Sys-
tem. In: Proceedings of the National Home Sewage Disposal Sympo-
sium, Chicago, Illinois, December 1974. American Society of Agri-
cultural Engineers, St. Joseph, Michigan, 1975. pp. 168-174.
43. Standards for Designing a Stabilization Lagoon. North Dakota State
Department of Health, Bismarck, (No date). 3 pp.
44. Pickett, E. M. Evapotranspiration and Individual Lagoons. In:
Proceedings of Northwest Onsite Wastewater Disposal Short Course,
University of Washington, Seattle, December 1976. pp. 108-118.
45. Standards for Subsurface and Alternative Sewage and Non-Water-
Carried Waste Disposal. Oregon Administrative Rules, Chapter 340,
Division 7, May 1978. p 97.
46. Hines, M. W., E. R. Bennett, and J. A. Hoehne. Alternate Systems
for Effluent Treatment and Disposal. In: Proceedings of the Sec-
ond National Home Sewage Disposal Symposium, Chicago, Illinois,
December 1977. American Society of Agricultural Engineers, St.
Joseph, Michigan, 1978. pp. 137-148.
47. Code of Waste Disposal Regulations - Part III. Utah State Depart-
ment of Health, Sewers and Wastewater Treatment Works, Salt Lake
City, 1977. 41 pp.
320
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CHAPTER 8
APPURTENANCES
8.1 Introduction
This chapter discusses several types of equipment used in onsite waste-
water treatment/disposal systems that have general application to the
components previously presented. The following items are covered:
1. Grease traps (or grease interceptors)
2. Dosing chambers
3. Flow diversion methods
Grease traps are used to remove excessive amounts of grease that may
interfere with subsequent treatment. Dosing chambers are necessary when
raw or partially treated wastewater must be lifted or dosed in large
periodic volumes. Flow diversion valves are used when alternating use
of treatment or disposal components is employed. These components are
described as to applicability, performance, design criteria, construc-
tion features, and operation and maintenance.
8.2 Grease Traps
8.2.1 Description
In some instances, the accumulation of grease can be a problem. In cer-
tain commercial/institutional applications, grease can clog sewer lines
and inlet and outlet structures in septic tanks, resulting in restricted
flows and poor septic tank performance. The purpose of a grease trap is
simply to remove grease from the wastewater stream prior to treatment.
Grease traps are small flotation chambers where grease floats to the
water surface and is retained while the clearer water underneath is dis-
charged. There are no moving mechanical parts, and the design is simi-
lar to that of a septic tank.
321
-------
The grease traps discussed here are the large, outdoor-type units, and
should not to be confused with the small grease traps found on some
kitchen drains.
8.2.2 Application
Grease traps are very rarely used for individual homes. Their main ap-
plication is in treating kitchen wastewaters from motels, cafeterias,
restaurants, hospitals, schools, and other institutions with large vol-
umes of kitchen wastewaters.
Influents to grease traps usually contain high organic loads including
grease, oils, fats, and dissolved food particles, as well as detergents
and suspended solids. Sanitary wastewaters are not usually treated by
grease traps. Wastewaters from garbage grinders should not be dis-
charged to grease traps, as the high solids loadings can upset grease
trap performance and greatly increase both solids accumulations and the
need for frequent pumpout.
8.2.3 Factors Affecting Performance
Several factors can affect the performance of a grease trap: wastewater
temperature, solids concentrations, inlet conditions, retention time,
and maintenance practices.
By placing the grease trap close to the source of the wastewater (usual-
ly the kitchen) where the wastewater is still hot, grease separation and
skimming (if used) are facilitated. As previously mentioned, high sol-
ids concentrations can impair grease flotation and cause a solids build-
up on the bottom, which necessitates frequent pumpout. Flow control
fittings should be installed on the inlet side of smaller traps to pro-
tect against overloading or sudden surges from the sink or other fix-
tures. These surges can cause agitation in the trap, impede grease flo-
tation, and allow grease to escape through the outlet. Hydraulic load-
ing and retention time can also affect performance. High loadings and
short retention times may not allow sufficient time for grease to sepa-
rate fully, resulting in poor -removals. Maintenance practices are im-
portant, as failure to properly clean the trap and remove grease and
solids can result in excessive grease buildup that can lead to the dis-
charge of grease in the effluent.
322
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8.2.4 Design
Sizing of grease traps is based on wastewater flow and can be calculated
from the number and kind of sinks and fixtures discharging to the trap.
In addition, a grease trap should be rated on its grease retention capa-
city, which is the amount of grease (in pounds) that the trap can hold
before its average efficiency drops below 90%. Current practice is that
grease-retention capacity in pounds should equal at least twice the flow
capacity in gallons per minute. In other words, a trap rated at 20 gpm
(1.3 I/sec) should retain at least 90% of the grease discharged to it
until it holds at least 40 Ib (18 kg) of grease (1). Most manufacturers
of commercial traps rate their products in accordance with this
procedure.
Recommended minimum flow-rate capacities of traps connected to different
types of fixtures are given in Table 8-1.
Another design method has been developed through years of field experi-
ence (3). The following two equations are used for restaurants and
other types of commercial kitchens:
1. RESTAURANTS:
UD
(D) x (GL) x (ST) x (££) x (LF) = Size of Grease Interceptor, gallons3
where:
D = Number of seats in dining area
GL = Gallons of wastewater per meal, normally 5 gal
ST = Storage capacity factor -- minimum of 1.7
onsite disposal -2.5
HR = Number of hours open
LF = Loading factor — 1.25 interstate freeways
1.0 other freeways
1.0 recreational areas
0.8 main highways
0.5 other highways
2. HOSPITALS, NURSING HOMES, OTHER TYPE COMMERCIAL KITCHENS WITH
VARIED SEATING CAPACITY:
(M) x (GL) x (ST) x (2.5) x (LF) = Size of Grease Interceptor, gallons3
where:
M = Meals per day
GL = Gallons of wastewater per meal, normally 4.5
323
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TABLE 8-1
RECOMMENDED RATINGS FOR COMMERCIAL GREASE TRAPS (1)
Type of Fixture
Restaurant kitchen
sink
Si ngle-compartment
scullery sink
Double-compartment
scullery sink
2 single-compartment
sinks
2 double-compartment
sinks
Dishwashers for
restaurants:
Up to 30 gal
water capacity
Up to 50 gal
water capacity
50 to 100 gal
water capacity
Flow
Rate
gpm
15
20
25
25
35
15
25
40
Grease
Retenti on
Capacity
Rating
Ib
30
40
50
50
70
30
50
80
Recommended
Maximum Capacity
Per Fixture Connected
to Trap
gal
50.0
50.0
62.5
62.5
87.5
50.0
62.5
100.0
324
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SC =. Storage capacity factor — minimum of 1.7
onsite disposal - 2.5
LF = Loading factor — 1.25 garbage disposal &
dishwashing
1.0 without garbage disposal
0.75 without dishwashing
0.5 without dishwashing
and garbage disposal
a Minimum size grease interceptor should be 750 gal
Thus, for a restaurant with a 75-seat dining area, an 8 hr per day oper-
ation, a typical discharge of 5 gal (19 1) per meal, a storage capacity
factor of 1.7 and a loading factor of 0.8, the size of the grease inter-
ceptor is calculated as follows:
(75) x (5) x (1.7) x (|) x (0.8) = 2,040 gal (7,722 1)
Other design considerations include: facilities for insuring that both
the inlet and outlet are properly baffled; easy manhole access for
cleaning; and inaccessibility of the trap to insects and vermin.
8.2.5 Construction Features
Grease traps are generally made of pre-cast concrete, and are purchased
completely assembled. However, very large units may be field construc-
ted. Grease traps come in single- and double-compartment versions.
Figure 8-1 shows a typical pre-cast double-compartment trap (2).
Grease traps are usually buried so as to intercept the building sewer.
They must be level, located where they are easily accessible for clean-
ing, and close to the wastewater source. Where efficient removal of
grease is very important, an improved two-chamber trap has been used
which has a primary (or grease-separating) chamber and a secondary (or
grease-storage) chamber. By placing the trap as close as possible to
the source of wastewaters, where the wastewaters are still hot, the
separating grease at the surface of the first chamber can be removed by
means of an adjustable weir and conveyed to the separate secondary
chamber, where it accumulates, cools, and solidifies. This decreases
the requirement for cleaning and allows better grease separation in the
first chamber.
325
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FIGURE 8-1
DOUBLE-COMPARTMENT GREASE TRAP
Removable Slab
Gas Tight Manhole Frame
and Cover for Traffic
Duty Anticipated
Inlet —
Outlet
Top View
Tee with
Cleanout Plug
Inlet
Precast Concrete
Tank
Grade
~v
Manhole Cover
•^Concrete Pad
Outlet
Tee with
Cleanout Plug
Section
326
-------
The inlet, outlet, and baffle fittings are typically of "T" design with
a vertical extension 12 in. (30 cm) from the tank floor and reaching
well above the water line (3).
To allow for proper maintenance, manholes to finished grade should be
provided. The manhole covers should be of gas-tight construction and
should be designed to withstand expected loads.
A check of local ordinances and codes should always be made before the
grease trap is designed or purchased.
8.2.6 Operation and Maintenance
In order to be effective, grease traps must be operated properly and
cleaned regularly to prevent the escape of appreciable quantities of
grease. The frequency of cleaning at any given installation can best be
determined by experience based on observation. Generally, cleaning
should be done when 75% of the grease-retention capacity has been
reached. At restaurants, pumping frequencies range from once a week to
once every 2 or 3 months.
8.3 Dosing Chambers
8.3.1 Description
Dosing chambers are tanks that store raw or pretreated wastewater for
periodic discharge to subsequent treatment units or disposal areas.
Pumps or siphons with appropriate switches and alarms are mounted in the
tank to discharge the accumulated liquid.
8.3.2 Application
Dosing chambers are used where it is necessary to elevate the wastewater
for further treatment or disposal, where intermittent dosing of treat-
ment units (such as sand filters) or subsurface disposal fields is de-
sired, or where pressure distribution networks are used in subsurface
disposal fields. If the dosing chamber is at a lower elevation than the
discharge point, pumps must be used. If the dosing chamber is at a
higher elevation, siphons may be used, but only if the settleable and
floatable solids have been removed from the wastewater stream.
327
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8.3.3 Factors Affecting Performance
Factors that must be considered in design of dosing chambers are (1) the
dose volume, (2) the total dynamic head, (3) the desired flow rate, and
(4) the wastewater characteristics. When pumps are used, they must be
selected based on all three factors. If raw wastewater with large
solids is pumped, grinder pumps or pneumatic ejectors must be used.
Siphons are chosen on the basis of the desired flow rate and their dis-
charge invert elevations determined from the total dynamic head. Only
wastewaters free from settleable and flotable solids can be discharged
by siphons. If corrosive wastewaters such as septic tank effluent are
being discharged, all equipment must be selected to withstand the cor-
rosive atmosphere.
8.3.4 Design
8.3.4.1 Dosing Chambers with Pumps
A pumping chamber consists of a tank, pump, pump controls, and alarm
system. Figure 8-2 shows a cross section of a typical pumping chamber
used for pumping pretreated wastewater. The tank can be a separate unit
as shown, or it can have common wall construction with the pretreatment
unit.
The tank should have sufficient volume to provide the desired dosing
volume, plus a reserve volume. The reserve volume is the volume of the
tank between the high water alarm switch and the invert of the inlet
pipe. It provides storage during power outages or pump failure. A
reserve capacity equal to the estimated daily wastewater flow is typi-
cally used for residential application (4). In large flow applications,
duplex pump units can be used as an alternative to provide reserve capa-
city. No reserve capacity is necessary when siphons are used.
Pump selection is based on the wastewater characteristics, the desired
discharge rate, and the pumping head. Raw wastewater requires a pump
with solids-handling capabilities. Grinder pumps, pneumatic ejectors,
or solids-handling centrifugal pumps are suitable for these applica-
tions. While pneumatic ejectors may be used in other applications as
well, submersible centrifugal pumps are best suited where large volumes
are to be pumped in each dose.
The pump size is determined from pump performance curves provided by the
manufacturers. Selection is based on the flow rate needed and the pump-
ing head. The specific application determines the flow rate needed.
328
-------
FIGURE 8-2
TYPICAL DOSING CHAMBER WITH PUMP
Influent
CO
ro
10
Relay in Weather Proof
Enclosure
Manhole Cover
High Water
Alarm Switch
Reserve Capacity
After Alarm Sounds
Level
Level Control Switch
Shut-Off Level
Level
Control
Switch
Hanger Pipe
for Pump Removal
Quick Disconnect
Sliding Coupler
-*• Effluent
-------
The pumping head is calculated by adding the, elevation difference be-
tween the discharge outlet and the average or low water level in the
dosing chamber to the friction losses incurred in the discharge pipe.
The velocity head can be neglected in most applications.
If the liquid pumped is to be free from suspended solids, the pump may
be set on a pedestal. This provides a quiescent zone below the pump
where any solids entering the chamber can settle, thus avoiding pump
damage or malfunction. These solids must be removed periodically.
In cold climates where the discharge pipe is not buried below the frost
line, the pipe should be drained between doses. This may be done by
sloping the discharge pipe back to the dosing chamber and eliminating
the check valve at the pump. In this manner, the pipe is able to drain
back into the dosing chamber through the pump. The dosing volume is
sized to account for this backflow. Weep holes may also be used if the
check valve is left in place.
The control system for the pumping chamber consists of a "pump off"
switch, a "pump on" switch, and a high water alarm switch. The pump off
switch is set several inches above the pump intake. The pump on switch
is set above the pump off switch to provide the proper dosing volume.
Several inches above the pump on switch, a high water alarm switch is
set to alert the owner of a pump malfunction by activating a visual
and/or audible alarm. This switch must be on a circuit separate from
the pump switches.
The switches should withstand the humid and other corrosive atmosphere
inside the tank. Pump failures can usually be traced to switch failures
resulting in pump burn out, so high quality switches are a good invest-
ment. Some types are:
Mercury: Two basic types are available. One is an on-off
switch sealed within a polyethylene float suspended from the
top of the chamber by its power cord. Two switches are neces-
sary to operate the pump (See Figure 8-3). The elevations are
adjusted individually. Differential switches are also avail-
able to turn the pump on and off with one switch, but these
lack the ability to adjust the dosing volume.
330
-------
FIGURE 8-3
LEVEL CONTROL SWITCHES
Relay
Pump-on Level
Vent Tube
in Cord
\ Pump-off Level /\ Pump-off Level
-> C—-*\ Switch
b) Pressure Diaphragm
Switch
Pump-on Level
a) Mercury Float
Pump-off Level
c) Weighted Float
2. Pressure Diaphragm: The pressure diaphragm switch is a micro-
switch mounted behind a neoprene diaphragm. The microswitch
side of the diaphragm is vented to the atmosphere by means of a
vent tube imbedded in the power cord. The other side is sub-
merged in the liquid. As the liquid level rises and falls, the
pressure on the diaphragm activates the switch (See Figure 8-
3). Thus, one switch is sufficient to operate the pump; but
the differential in liquid levels is usually limited to about 6
in, although switches with larger differentials can be pur-
chased. If used in pumping chambers, the vent tube must be
located outside the pumping chamber or the humid atmosphere in
the chamber can cause the switch to corrode.
3. Weighted Float: The switch is mounted above the water with 2
weights attached to a single cable hanging from the switch (See
Figure 8-3). When the weights are hanging free, the switch is
held open; but as the liquid level rises, the weights are
buoyed up, closing the switch when the second weight is sub-
merged. The switch is held closed by a magnet; but as the
331
-------
liquid level drops, the weights lose their buoyancy and open
the switch when the bottom weight is exposed. The dosing vol-
ume can be changed by adjusting the spacing between the floats.
All electrical contacts and. relays must be mounted outside the chamber
to protect them from corrosion. Provisions should be made to prevent
the gases from following the electrical conduits into the control box.
8.3.4.2 Dosing Chambers with Siphons
Siphons may be used in place of pumps if the point of discharge is at a
lower elevation than the outlet of the pretreatment unit. A chamber em-
ploying siphons consists of only a tank and the siphon. No mechanical
or electrical controls are necessary, since the siphon operation is
automatic. A typical siphon chamber is illustrated in Figure 8-4. Two
siphons may be placed in a tank and automatically alternate, providing a
simple method of dividing the wastewater flow between two treatment or
disposal units.
The design of the dosing chamber is determined by the siphon selected
and the head against which it must operate. The size of the siphon is
determined by the average flow rate desired. The manufacturer specifies
the "drawing depth," or the depth from the bottom of the siphon bell to
the high water level necessary to activate the siphon (See Figure 8-
4). The length and width of the chamber are determined by the dosing
volume desired.
Siphon capacity is rated when discharging into the open atmosphere.
Therefore, if the discharge is into a long pipe or pressure distribution
network, the headlosses must be calculated and the invert at the siphon
discharge set at that distance above the outlet. For high discharge
rates or where the discharge pipe is very long, the discharge pipe
should be one nomimal pipe size larger than the siphon to facilitate air
venting.
The siphons may be cast iron or fiberglass. Cast iron siphons are the
most common. Their advantage is that the bell is merely set on the
discharge pipe so they may be easily removed and inspected. They are
subject to corrosion, however. Fiberglass siphons do not corrode, but
because of their light weight, they must be bolted to the chamber floor.
332
-------
FIGURE 8-4
TYPICAL DOSING CHAMBER WITH SIPHON
Influent
CO
CO
CO
Manhole
Vent
->!
t
Drawing Depth
1
/
s
^inhr^n Roll*^^^
W/Vent r-
Siphon Leg -^
T=
f
\
1
=^
'
->
^
1
/
I
u
^v .
i
/
y
i
\
i
i
.
— - — ,.
— Vent & Overflow
(j » Effluent
-------
8.3.5 Construction
The tank must be watertight so groundwater does not infiltrate it.
Waterproofing consists of adequately sealing all joints with asphalt or
other suitable material. Coating the outside of the tank prevents
groundwater from seeping into the tank. Asphalt coating the inside and
outside of steel tanks helps retard corrosion. Application of 4-mil
plastic to the wet asphalt coating protects the coating when back-
filling.
At high water table sites, precautions should be taken so the chamber
does not float out of position due to hydrostatic pressures on a near-
empty tank. This is not normally a problem for concrete tanks, but for
the lighter-weight materials, such as fiberglass, it could present a
problem. The manhole riser pipe should be a minimum of 24 in. (61 cm)
in diameter and should extend 6 in. (15 cm) above ground level to keep
surface-water from entering the chamber.
If plastic pipe is used for the inlet or discharge, precaution should be
taken to ensure that the pipe does not break as the backfilled soil
around the tank settles. A cast iron pipe sleeve or other suitable
device can be slipped over the plastic pipe extending from the tank to
unexcavated soil to provide this protection.
8.3.6 Operation and Maintenance
Little routine maintenance of dosing chambers is required. The tank
should be inspected periodically, and any solids that accumulate on the
floor of the tank should be removed. If pumps are used, the system
should be cycled to observe operation of the switches and pump. If
siphons are used, the water level in the tank should be noted over a
period of time to determine if the siphon is operating properly. If the
siphon is working properly, the water level will fluctuate from the bot-
tom lip of the siphon bell to several inches above the bell. If the
water elevation does not change despite water addition, the siphon is
"dribbling," indicating that the vent tube on the bell requires
cleaning.
334
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8.4 Flow Diversion Methods for Alternating Beds
8.4.1 Description
Under some circumstances, it is desirable to divert the wastewater flow
from one soil absorption area to another to provide long-term alternate
resting periods (see Chapter 7). Flow diversion may be accomplished by
the use of commercially available diversion valves (Figure 8-5) or by
diversion boxes (Figures 8-6 and 8-7).
FIGURE 8-5
TYPICAL DIVERSION VALVE
Water-Tight Access Cap
Valve Direction Handle
Valve Body
335
-------
FIGURE 8-6
TOP VIEW OF DIVERSION BOX UTILIZING A TREATED WOOD GATE
LjJ Ljj LJJ
Wood Gate
FIGURE 8-7
SECTION VIEW OF DIVERSION BOX UTILIZING ADJUSTABLE ELLS
90° Ell in
zontal Po
(Open)
n
isition
^--^
-A
I Cover
Inlet
U
^n O I
Liquid Level
|
** — •
Nipp
„. —
le
— — - — .
^-90
Vertic
(C
tf
(Closed)
Open
Closed
336
-------
8.4.2 Design
Diversion boxes can be made from conventional distribution boxes. One
type of diversion box shown in Figure 8-6 uses a treated wood gate to
divert the flow to the desired outlet pipe (5).
Another, shown in Figure 8-7, uses 90° ells that can be moved from the
horizontal to the vertical position to shut off flow. Caps or plugs can
be used in place of elbows. Elbows, however, provide a freer flow of
air into the resting system. Insulated covers must be provided with
diversion boxes when installed in cold climates.
8.4.3 Construction
Construction follows manufacturers recommendations or the procedures
outlined for distribution boxes (Chapter 7).
8.4.4 Maintenance
Maintenance of diversion valves involves little more than turning the
valve at the desired frequency. Any accumulated solids in the diversion
box or valve should be removed periodically.
8.5 References
1. Manual of Septic Tank Practice. NTIS Report No. PB 216 240, Public
Health Service, Washington, D.C., 1967. 92 pp.
2. Hogan, J. R. Grease Trap Discussion. Plumb Eng., May-June 1975.
3. HYGI Design Manual. M. C. Nottingham Company, Pasadena, California,
1979.
4. Converse, J. C. Design and Construction Manual for Wisconsin
Mounds. Small Scale Waste Management Project, University of
Wisconsin, Madison, 1978. 80 p.
5. Machmeier, R. E. Home Sewage Treatment Workbook. Agricultural
Extension Service, University of Minnesota, St. Paul, 1979.
337
-------
CHAPTER 9
RESIDUALS DISPOSAL
9.1 Introduction
Proper maintenance of onsite treatment systems requires periodic dis-
posal of residual solids, sludges, or brines. In some areas, finding
environmentally sound techniques for disposal of these residuals has
been very difficult. Because of the possible presence of pathogens in
many of these wastewaters, proper handling and disposal are important
from a public health perspective. The homeowner's role in residuals
handling is to ensure that residuals from his system are removed peri-
odically at the appropriate interval so that proper system performance
is maintained.
This chapter discusses the characteristics of residuals, and describes
treatment and disposal options for septage (septic tank pumpings). The
chapter is intended to be merely an overview of residuals handling op-
tions. The reader is referred to publications that discuss particular
alternatives in greater detail.
9.2 Residuals Characteristics
Table 9-1 summarizes the residuals that may be generated by onsite
wastewater handling systems. Typical characteristics, removal frequen-
cies, and disposal modes are presented. Many of the residuals listed
may contain significant amounts of pathogenic organisms, nutrients, and
oxygen-demanding materials; thus, they require proper handling and dis-
posal to protect public health and to prevent degradation of groundwater
and surface water quality.
In general, residuals generated by onsite wastewater systems are highly
variable in character. This is due to several factors, including type
and number of fixtures, number and age of occupants, type of wastewater
treatment system, and user habits.
The wastewater removed from septic tanks, commonly referred to as sept-
age, is the most common residual generated from onsite wastewater sys-
tems. The characteristics of septage are presented in Tables 9-2 and
9-3. While information on septage characteristics and treatment
/disposal alternatives is relatively abundant, data on other residuals
listed in Table 9-1 are limited.
338
-------
TABLE 9-1
RESIDUALS GENERATED FROM ONSITE WASTEWATER SYSTEMS (1)
Resi dual
Source
Frequency of
Removal
Characteristics
Disposal3
CO
CO
Septage
Sludge
Sewage
Blackwater
Recycle
Residuals
Compost
Ash
Scum
Septic tank
Aerobic unit
Holding tank
Holding tank
Recycle systems
Compost toilet;
large
small
Incinerator toilet
Sand filters
2 to 5 yr High BOD and SS; odor,
grease, grit, hair,
pathogens
1 yr
High BOD and SS;
grease, hair, grit,
pathogens
week to months Strong septic sewage;
odor, pathogens
6 months-1 yr High BOD and SS; odor,
pathogens
6 months-1 yr Variable depending on
unit processes employed
6 months-1 yr
3 months
weekly
6 months
Relatively stable, high
organics, low pathogens
Dry, sterile, low
volume
Odor, pathogens, low
volume
Pump out by professional
hauler for off-site
disposal.
Pump out by professional
hauler for off-site
disposal.
Pump out by professional
hauler for off-site
disposal.
Pump out by professional
hauler for off-site
disposal.
Pump out by profesisonal
hauler for off-site
disposal.
Homeowner performs
onsite disposal; garden
burial.
Onsite burial by
homeowner or di sposal
with rubbish to landfill
Onsite burial by
homeowner or off-site
disposal
a Approval by state or local regulatory agency necessary.
-------
TABLE 9-2
CHARACTERISTICS OF DOMESTIC SEPTAGE
Parameter Mean Value
mg/1
Reference
Total Solids
Total Volatile Solids
Suspended Solids
Volatile Suspended Solids
BOD
COD
PH
Alkalinity (CaCOs)
TKN
NH3-N
22,400
11,600
39,500
15,180
8,170
27,600
2,350
9,500
21,120
13,060
1,770
7,650
12,600
8,600
4,790
5,890
3,150
26,160
19,500
60,580
24,940
16,268
6-7 (typical)
610
1,897
410
650
820
472
59
100
120
92
153
2
3
4
2
3
4
2
3
5
6
2
3
5
6
2
3
6
2
3
4
5
6
2,3,4
3
5
3
4
5
6
2
3
4
5
6
340
-------
TABLE 9-2 (continued)
Parameter Mean Value Reference
Total Phosphorus
Grease
Alumi num
Arsenic
Cadmium
Chromium
Copper
Iron
Mercury
Manganese
Nickel
Lead
Sel eni urn
Zinc
mg/1
190
214
172
351
3,850
9,560
48
0.16
0.1
0.2
9.1
0.6
1.1
8.7
8.3
210
160
190
0.02
0.4
5.4
4.8
0.4
<1.0
0.7
2.0
8.4
0.07
9.7
62
30
3
4
5
6
3
4
6
6
3
4
6
3
6
3
6
3
4
6
4
6
4
6
3
4
6
3
6
6
3
4
6
341
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TABLE 9-3
INDICATOR ORGANISM AND PATHOGEN CONCENTRATIONS
IN DOMESTIC SEPTAGE
Parameter Typi cal Range Reference
counts/100 ml
Total Coliform 10? - 10$ 5
Fecal Coliform 106 - 108 4,5,7
Fecal Streptococci 106 - 107 4,5,7
Ps. aeruginosa IQl - 103 4,5,7
Salmonella sp.
-------
Accumulation rates of residuals differ for the same reasons that account
for their variability in characteristics: that is, type and number of
fixtures, occupancy characteristics, type of wastewater system, user ha-
bits, etc. The figures presented in Table 9-1 for frequency of resid-
uals removal reflect typical ranges found in practice, although the
range of actual values may be greater.
9.3 Residuals Handling Options
Residuals that potentially may be disposed of onsite by the homeowner
include compost from compost toilets, ash from incinerating toilets, and
the solids mat from sand filters. Assuming proper operation of the
unit, ash from incinerating toilets is sterile and can be safely dis-
posed by mixing it with soil on the homeowner's property, or by handling
with household solid wastes. Residuals from compost toilets are rela-
tively stable, but may contain pathogenic bacteria and virus, especially
if the system has not been properly operated and maintained. Onsite
burial is approved in some states but not in others, due to the possible
health hazards of handling the waste. The same conditions hold for dis-
posal of the scum that must be periodically raked off filtration units.
Pathogens may be present in the scum layer, and approval for onsite dis-
posal varies with locale. The appropriate state or local regulatory
agency should be consulted for the requirements in a particular area.
As Table 9-1 indicates, the residues from septic tanks, aerobic treat-
ment units, holding tanks, and recirculating toilets must be periodi-
cally pumped out and disposed of by professional haulers. The home-
owner's responsibility should be to ensure that this service is provided
before residuals buildup impairs performance of the treatment unit.
9.4 Ultimate Disposal of Septage
By far the most common waste material generated from onsite systems is
septage. The following discussion provides a brief overview of tech-
niques for disposal of this waste. For a more complete description of
these processes, the reader is referred to the list of references at the
end of this chapter.
There are three basic methods for disposing of septage: disposal to
land, treatment and disposal at separate septage handling facilities,
and treatment at existing wastewater treatment plants.
343
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9.4.1 Land Disposal
Four methods can be used for disposing of septage to land: surface
spreading, subsurface disposal, trenching, and landfilling. Table 9-4
summarizes the main characteristics of these disposal techniques.
Land spreading is the most frequently used septage disposal method in
the United States. Surface spreading of septage is generally accom-
plished by the same techniques as municipal liquid wastewater sludge
spreading. This may simply involve the septage pumping truck emptying
its contents on the field while slowly driving across the site. This
technique has very low operation and maintenance requirements. A more
controlled approach is to use a holding tank to receive septage loads
when the soil is not suitable for spreading. A special vehicle (tractor
.or truck with flotation tires) can then be used to spread the septage
when weather and soil conditions permit.
Subsurface disposal techniques have gained wide acceptance as alterna-
tives for disposal of liquid sludge and, to some extent, septage. Three
basic approaches to subsurface disposal are available:
1. Incorporation using a farm tractor and tank trailer with at-
tached subsurface injection equipment.
2. Incorporation using a single, commercially available tank truck
with subsurface injection equipment.
3. Incorporation using tractor-mounted subsurface injection equip-
ment in conjunction with a central holding facility and flex-
ible "umbilical cord." Liquid sludge is continually' pumped
from the holding tank to the injection equipment.
Disposal of septage by burial in excavated trenches is another common
disposal technique. Trenches are typically 3 to 6 ft (0.9 to 1.8 m)
deep and 2 to 3 ft (0.6 to 0.9 m) wide, with dimensions varying with
site location. Space between trenches should be sufficient to allow
movement of heavy equipment. A series of trenches is usually dug by a
backhoe to allow sequential loading and maximum dewatering. Septage is
usually applied in 6- to 8-in. (15 to 20 cm) layers. When the trenches
are full, the solids can be excavated and placed in a landfill if they
have dewatered sufficiently, or the trenches can be covered with 2 ft
(0.6 m) of soil. A thorough site evaluation is essential to prevent
groundwater contamination with this disposal technique.
344
-------
TABLE 9-4
LAND DISPOSAL ALTERNATIVES FOR SEPTAGE
Alternative
Design Considerations
Advantages
Di sa dvantages
CO
-pi
en
Subsurface Septage volume /character!" sties
Disposal Climate
(1)(2)(8) Site characteristics
(9)(17) - Soil type/permeability
(19) - Depth to groundwater
or bedrock
- Aquifer size, flow
characteristics, use
- Slope
- Proximity to dwellings, etc.
- Crop and crop use
- Size of site
- Site protection
Equipment selection
Application rate
Winter storage or
contingency plan
Mom" tori ng wel 1 s
Surface Septage volume/character!sties
Spreading Application rate (N loading)
(1)(2)(8) Climate
(9)(17) Storage facilities
(19) Site characteristics (same
as subsurface disposal)
Equipment selection
Moni tori ng wel1s
Low human contact potential
Low incidence of odors
and vectors
Aesthetically more
acceptable than surface
spreading
Good soil amendment
Small labor requirement
Minimum equipment required
Benefit from fertilizer -
soil amendment value
Low cost
Simple Operation
Large land requirements
Storage may be required
during inclement weather -
wet or frozen ground
Need more equipment than for
surface spreading
Possible odor and aesthetic
nuisance
Spreading restricted by wet
or frozen soil
Storage may be required
during inclement weather
Pretreatment may be required
for deodorization and
pathogen destruction
Possible human contact or
vector attraction
-------
TABLE 9-4 (continued)
Alternative
Desi gn Consi derati ons
Advantages
Disadvantages
Trench
Disposal
(18)
GO
*.
CPi
Sanitary
Landfill
Disposal
Septage volume/characteristics
Si te characteri sti cs
- Soil type/permeability
- Depth to groundwater or
bedrock
- Aquifer size, flow
characteristics, use
- Proximity to dwellings, etc.
- Proximity to septage sources
Site protection
Equipment selection
Design life
Monitoring wells
Septage/refuse ratio
Leachate collection/treatment
Monitoring wells
Simple operation
Low labor requirement
Minimal equipment required
Low cost
Less land required than
surface or subsurface
spreading operations
No new equipment needed
Low odor and pathogen
problems due to daily
soi1 cover
Low cost
Higher potential for
groundwater contami nati on
Odors and vectors
Limited design life -
usually cannot use same
land repeatedly
Limited application due to
leachate generation
Good operating procedures
required - refuse/septage
mi xi ng
Extensi ve moni tori ng
required - leachate,
runoff, groundwater
May not be approved in some
states
-------
Sanitary landfills in the United States generally accept a multiplicity
of materials such as refuse, industrial wastes, and sometimes hazardous
or toxic wastes. All of these wastes are compiled on a daily basis at
the landfill and buried under a soil cover. The acceptance of septage
at a landfill depends chiefly on the ratio of the mixture of septage to
refuse to maintain moisture control. However, a few states do not allow
landfill disposal of septage, and some others do not recommend it be-
cause of potential runoff and leachate problems.
9.4.2 Independent Septage Treatment Facilities
In some areas of the country, facilities have been constructed exclu-
sively for handling septage. These systems vary from simple holding
lagoons to sophisticated, mechanically based plants. The latter systems
are generally more capital intensive, and may also have greater opera-
tional requirements. Such systems have been found to be cost effective
in areas of significant septic system density, such as Long Island, New
York. In rural areas, simpler, less expensive alternatives may be more
economically favorable. Of the independent facilities listed in Table
9-5, lagoons are the most common and among the least expensive indepen-
dent septage handling alternatives. All of the other independent sys-
tems have been implemented to some degree, although in most cases, not
widely.
9.4.3 Septage Handling at Wastewater Treatment Plants
Two methods exist for handling septage at wastewater treatment facili-
ties: addition to the liquid stream (near the headworks or upstream
from the plant), or addition to the solids handling train (see Table 9-
6). Both have advantages under appropriate conditions. For example,
addition to the headworks (screens, grit chamber) is desirable where the
plant employs primary clarification, since this effectively introduces
the septage solids directly into the sludge handling scheme. For ex-
tended aeration plants, however, septage addition to the wastewater flow
may have a severe impact on the aeration capacity of the system. Thus,
introducing the septage into the sludge stream may be desirable. Con-
sideration of plant aeration and solids handling capacity is necessary
to determine whether either scheme is feasible. Under either mode of
addition, solids production increases with increased septage addition.
Septage holding facilities allow controlled addition of the septage to
the wastewater treatment plant.
For additional information on the capability of wastewater treatment fa-
cilities to handle septic tank pumpings, the reader is referred to the
publications list in Section 9.5 (3)(11).
347
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TABLE 9-5
INDEPENDENT SEPTAGE TREATMENT FACILITIES
Process
Description
Design Considerations
Advantages
Disadvantaoes
Lagooning Usually anaerobic or facultative
(1)(13)(14) Inlet on bottom for odor control
(161(17) Liquid disposal by percolation
and evaporation in lagoon or by
separate infiltration bed
pH adjustment to pH 6-8 may be
necessary for odor control
Lime Collection, mixing, and reaction
Stabilization with lime to pH 12 (hold 1 hour)
to (1)(4)(5) Dewatering optional
-p> Odors eliminated, pathogens greatly
00 reduced
Chlorine Chlorine and septage mixed in
Oxidation pressurized reaction chamber
(1)(9)(15) pH 1.2 - 2.5
Chlorine dosage 700-3,000 mg/1
Aerobic Similar to aerobic digestion of
Digestion sewage sludge
(1)(9)(13) Often accomplished at existing
wastewater treatment plant
Septage volume/characteri sties
Site location
- Distance to dwellings, etc.
- Depth to groundwater or
bedrock
- Distance to surface water
Depth of liquid, surface area
Climate
Aquifer characteristics
Moni tori ng we! 1 s
Solids removal and disposal
Septaqe volume/characteristics
Septage receiving/holding
Mixing (air or mechanical)
Lime handling and feeding
Final disposal
Septage volume/characteristics
Equipment sizing
Septage receiving/holding
Dewatering facilities
Final solids disposal
Chlorine storage/safety
Septage volume/characteristics
Seotage receiving/holding
Organic loading
Solids retention time (20-30
days)
Climate (temperature)
Mixino and DO level
Fi nal di soosal
Low cost
Simple operation
Odor problems if pH not maintained
Cannot use in areas with hiqh
water table
Possible vector orohlem
Soil cloaqlng may stop percolation
Odor eliminated
Good pathogen
reduction
Low land requirement
Enhanced solids
dewatering
Stable, odor-free
sludge produced
High pathogen
destructi on
Enhanced solids
dewatering
Low land requirement
SS reduction
BOD reduction
Reduction of odor and
pathogens
May enhance solids
dewa ten" no
Low land requirement
No reduction in oraanic matter
Lime increases Quantity for
final disposal
H1nh cost for labor and lime
Unknown effects of long-term
storage
High operating costs dependent on
chlori ne cost
Neutralization may be required
Question of harmful chlorinated
organics
Underdrainage liquor ream'res
further treatment
Biolooical operation not simple
Subject to ornanic overloadina
Requires mom'tori no and lab
analysis
Can have foaming problems
-------
TABLE 9-5 (continued)
Process
Descrlpti on
Desian Considerations
Advantaoes
Disadvantaqes
CO
J5.
UD
Composting May be natural draft or forced air
(1) Septage mixed with bulking material
High temperature/pathogen
destruction
Storage/di stri buti on
Anaerobic Often accomplished in combination
Digestion with sewage sludge
(9)(11) Demonstrated on pilot-scale
Identical to sludge digestion
technology
Chemical Chemical coagulation
Treatment - Mixing and settling
(1)(9)(10) - Supernatant collection,
treatment/di sposal
- Sludge hoi di ng/dewatering/di sposal
Acidification ^SO^)
- Mixing and settling
- Additional coagulation possible
with lime
Septage volume/characteristics
Septage receiving/hoi ding
Bulking agent availability
Dewatering
Materials handling capability
Septage volume/characteri sties
Septage receiving/holding
Gri t removal
Soli ds retenti on ti me
Maintenance of digester
temperature
No toxic materials input
Final disposal
Septage volume/characteristics
Septage receiving/holding
Chemical feed equipment and
dose levels
Mixing, reaction time, settling
time
Fi nal di sposal
Provides pathogen
destruction and
stabilization
Produces soil
amendment
Operationally simple
Low energy
requi rements
Methane recovery/
utilization
possible
Stabilized product
Can handle variety of
organic wastes
Low land reauirement
High bulking agent reoiiirement if
not dewatered
Product market must be established
Mav he labor-intensive
Bioloaical process reouires close
operator control
Subject to upset bv toxics
Reouires continuous supply of
organic materials
Hinh labor reauirement
Hiqh costs
Dewateri ng
Dryi ng beds
Pressure filtration
Vacuum filtration
Drying lagoons
Centrifugation
Septage volume/characteristics
Septage receiving/holding
SS concentrations
Filterability
Pretreatment-chemi cal
conditioning
Final disposal
Reduced hauling costs
Reduces area required
for disposal
Hiqh cost for some alternatives
Hiqh operation and maintenance
requi rements
Mechanical dewaterino devices
require an enclosure
-------
TABLE 9-6
SEPTAGE TREATMENT AT WASTEWATER TREATMENT PLANTS
Process
Description
Design Considerations
Advantages
Disadvantages
oo
01
o
Liquid Stream Septage placed in storage tank at
Addition plant
(3)(6)(11)(12) Pretreatment (screening, grit
removal)
Controlled bleed into headworks to
prevent shock overload
Sludge Stream Septage placed in storage tank
Addition Fed directly into sludge stream with
(6)(11)(12) or without separate conditioning/
handling
Septage volume/characteristics
Plant capacity (aeration and
solids handling)
Receiving station
- Truck transfer
- Storage
- Pretreatment (optional)
- Controlled discharge to plant
Sludge production
O&M (power, labor, chemicals)
Septage volume/characteristics
Septage receiving /hoi dinn
Organic and solids loading on
each sludqe handling unit
Pumping and storage capacity
Additional mixing and feedina
equipment
Increase in chemical usaae
Easily implemented
Low capital cost
Public acceptance
good
Particularly
desirable at. plants
with orimary
clarification
Avoids overloading
secondary and
tertiary systems
Avoids oossihility of
final effluent
degradation
Additional sludqe Generation
May organically overload plant
Increased nw
Final disposal site and sludqe
eouipment exoansion may he
needed
Additional sludge oeneration
Final disposal site and sludne
eauipment expansion >nay he
needed
-------
9.5 References
1. Bowker, R. P. G., and S. W. Hathaway. Alternatives for the Treat-
ment and Disposal of Residuals from On-Site Wastewater Systems.
Municipal Environmental Research Laboratory, Cincinnati, Ohio,
1978.
2. Kolega, I. 0., A. W. Dewey, B. J. Cosenza, and R. L. Leonard.
Treatment and Disposal of Wastes Pumped from Septic Tanks. EPA
600/2-77-198, NTIS Report No. PB 272 656, Storrs Agricultural
Experiment Station, Connecticut, 1977. 170 pp.
3. Segal 1, B. A., C. R. Ott, and W. B. Moeller. Monitoring Septage
Addition to Wastewater Treatment Plants, Volume I: Addition to the
Liquid Stream. EPA 600/2-79-132, NTIS Report No. PB 80-143613,
1979.
4. Feige, W. A., E. T. Oppelt, and J. F. Kreissl. An Alternative
Septage Treatment Method: Lime Stabilization/Sand-Bed Dewatering.
EPA 600/2-75-036, NTIS Report No. PB 245 816, Municipal Environ-
mental Research Laboratory, Cincinnati, Ohio, 1975. 64 pp.
5. Noland, R. F., J. D. Edwards, and M. Kipp. Full Scale Demonstration
of Lime Stabilization. EPA 600/2-78-171, NTIS Report No. PB 286
937, Burgess and Niple Ltd., Columbus, Ohio, 1978. 89 pp.
6. Bennett, S. M., J. A. Heidman, and J. F. Kreissl. Feasibility of
Treating Septic Tank Waste by Activated Sludge. EPA 600/2-77-141,
NTIS Report No. PB 272 105, District of Columbia, Department of
Environmental Services, Washington, D.C., 1977. 71 pp.
7. Deninger, J. F. Chemical Disinfection Studies of Septic Tank
Sludge with Emphasis on Formaldehyde and Glutaraldehyde. M.S.
Thesis. University of Wisconsin, Madison, 1977.
8. Maine Guidelines for Septic Tank Sludge Disposal on the Land.
Miscellaneous Report 155. Life Sciences and Agriculture Experiment
Station and Cooperative Extension Service, University of Maine,
Orono, Maine Solid and Water Conservation Commission, 1974.
9. Cooper, I. A., and J. W. Rezek. Septage Treatment and Disposal.
Prepared for the EPA Technology Transfer Seminar Program on Small
Wastewater Treatment Systems, 1977. 43 pp.
10. Condren, A. J. Pilot Scale Evaluations of Septage Treatment
Alternatives. EPA 600/2-78-164, NTIS Report No. PB 288 415, Maine
Municipal Association, Augusta, Maine, 1978. 135 pp.
351
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11. Bowker, R. P. G. Treatment and Disposal of Septic Tank Sludges. A
Status Report. May 1977. In: Small Wastewater Treatment Facili-
ties. Design Seminar Handout. Environmental Protection Agency
Technology Transfer, Cincinnati, Ohio, 1978.
12. Cooper, I. A., and J. W. Rezek. Septage Disposal in Wastewater
Treatment Plants. In: Individual On-Site Wastewater Systems. Pro-
ceedings of the Thfrcl National Conference. N. McClelland, ed. Ann
Arbor Science, Ann Arbor, Michigan, 1977. pp. 147-169.
13. Jewell, J. W., J. B. Howley, and D. R. Perrin. Design Guidelines
for Septic Tank Sludge Treatment and Disposal. Prog. Water Tech-
nol., 7, 1975.
14. Guidelines for Septage Handling and Disposal. New England Inter-
state Water Pollution Control Commission, Boston, Massachusetts,
August 1976.
15. Wise, R. H., T. A. Pressley, and B. M. Austern. Partial Charac-
terization of Chlorinated Organics in Superchlorinated Septages
and Mixed Sludges. EPA 600/2-78-020, NTIS Report No. PB 281 529,
USEPA, MERL, Cincinnati, Ohio, 1978. 30 pp.
16. Brown, D. V., and R. K. White. Septage Disposal Alternatives for
Rural Areas. Research Bulletin 1096, Ohio State University,
Columbus, 1977.
17. Barlow, Gill and E. Allan Cassell. Technical Alternatives for
Septage Treatment and Disposal in Vermont. Draft. Vermont Water
Resources Research Center, University of Vermont, Burlington, 1978.
18. Walker, J. M., W. D. Burge, R. L. Chaney, E. Epstein, and J. D.
Menzies. Trench Incorporation of Sewage Sludge in Marginal Agri-
cultural Land. EPA 600/2-75-034, NTIS Report No. PB 246 561,
Agricultural Research Service, Beltsville, Maryland, 1975. 252 pp.
19. Sommers, L. E., R. C. Fehrmann, H. L. Selznick, and C. E. Pound.
Principles and Design Criteria for Sewage Sludge Application on
Land. In: Sludge Treatment and Disposal Seminar Handout, Environ-
mental Research Information Center, Cincinnati, Ohio, 1978.
352
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CHAPTER 10
MANAGEMENT OF ONSITE SYSTEMS
10.1 Introduction
Onsite systems offer a viable means for controlling public health haz-
ards, environmental degradation, and nuisances that might otherwise
arise from wastewater generated in unsewered areas. If onsite systems
are to perform successfully over a reasonable lifetime, a sound manage-
ment program with sufficient technical assistance and enforcement cap-
abilities is needed.
Management programs may take many forms. A good program, at a minimum,
performs the following functions:
1. Site evaluation validation
2. System design review
3. Construction supervision
4. Operation and maintenance certification
5. Rehabilitation assistance
6. Monitoring and enforcement
7. Public education activities
Most states perform some or all of these functions with much of the
responsibility often delegated to local units of government. These
programs are very diverse (1). At one end of the spectrum, the state
may limit its responsibility to the promulgation of minimum standards to
be adopted by local jurisdictions, which may have the right to establish
stricter standards. At the other end, the state may retain all
management functions over onsite systems.
Thus, the management programs used in various jurisdictions differ
greatly as do their effectiveness. Therefore, the following examination
of approaches and techniques that may be used to manage onsite systems
is intended to:
1. Provide a means of evaluating the existing management program.
2. Suggest techniques used to improve an existing management pro-
gram or to establish a new one.
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Some of the techniques discussed may not be .readily incorporated into
existing management programs due to different state constitutional and
statutory provisions and legal interpretations. Some techniques may
require the enactment of enabling legislation granting the management
entity necessary authority to manage onsite systems.
10.2 Theory of Management
An effective management program provides technical assistance together
with strong regulation enforcement. Both aspects are directed at major
control points.
10.2.1 Principal Control Points
There are several distinct phases in the life of an onsite system that
require control. These are:
1. Installation
2. Operation
3. Maintenance
During the "installation" phase, the management program must limit
installation to suitable sites, and assure the proper design and
construction of all onsite systems. It is during this phase that
management programs can be most effective in minimizing the potential
threat to public health and water quality.
During the "operation" phase, the management program must assure proper
operation of an onsite system through periodic monitoring. While there
are very few operational requirements for a septic tank-soil absorption
system, some of the onsite systems have more extensive requirements. A
good management program imposes controls during this phase whether the
system's operation is straightforward or elaborate.
Finally, in the "maintenance" phase, the management program must provide
for adequate maintenance of an onsite system, e.g., periodic pumping of
septic tanks. It also must detect any onsite system that fails to func-
tion properly. This may be done through systematic or random inspec-
tions. A good program takes the necessary action to assure that repair,
replacement, or abandonment of failed systems is completed.
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10.2.2 Authority Needed by Management Entities
If adequate management is to be provided at the principal control
points, management entities should have the authority to perform the
functions listed below. The optional functions become imperative if the
management entities own the onsite systems.
Suggested Functions Optional Functions
1. Site evaluation 1. Planning
2. System design 2. Legal functions
3. Installation 3. Financing
4. Operation and Maintenance 4. Public education
5. Rehabilitation
6. Monitoring
The authority to perform these functions does not need to be granted to
a single management entity. In fact, it is unlikely that one entity
will have all the program responsibility. However, the total management
program should have the combined authority to perform the necessary
functions.
In each jurisdiction, the authority of each management entity should be
examined. Statutory authority, judicial decisions, and the state
constitution must be carefully reviewed. Often existing programs may be
adapted and/or utilized to aid in management. For example, the
management entities may require that certain onsite systems be designed
by registered professional engineers even though the entities themselves
do not register engineers. In the event that additional authority is
needed, enabling statutory language will be required.
10.3 Types of Management Entities
There are several types of entities that have the authority to perform
the management functions previously described. These include:
1. State agencies
2. Local governmental/quasi-governmental units
3. Special purpose districts
4. Private institutions (profit, nonprofit)
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10.3.1 State Agencies
Except for the limitations contained in its own constitution, each state
retains complete authority to protect the general welfare of its citi-
zens, including the management of onsite systems. The state health
agency and/or agency responsible for water quality are the agencies most
likely to exercise the state's authority.
The degree of control exerted by state agencies over onsite systems
varies from state to state. Many states set design standards for onsite
systems. Those that do not set standards delegate authority to local
governments to do so. Several states retain the responsibility for
administrative/technical portions of the onsite management program.
A state management program is often considered more effective, because
local pressures to weaken onsite regulation are not thought to be as
effective at the state level. In addition, since states typically have
more resources to hire or retain experienced individuals than most local
units of government, state agencies are in a better position to take
responsibility for many of the regulatory and administrative require-
ments.
10.3.2 Local Governmental/Quasi-Governmental Units
In some states, a portion or most of the responsibility of onsite system
management is delegated by the legislature to units of local government.
In other states with strong "home rule" powers, the local unit of gov-
ernment has the authority to manage onsite systems even without being so
delegated by the state legislature. The various types of local govern-
mental units are:
Municipalities - Incorporated units of government have full
responsibility for the general welfare of its citizens; have
broad financing authority, including the authority to levy
property taxes, to incur general obligation debts, to use
revenue bonding and to impose special assessments upon bene-
fitted property; and are legal entities authorized to contract,
commence law suits, and own property.
Unincorporated Government (e.g., County) - Unincorporated gov-
ernmentalunits often have authority equal to municipalities;
however, these units may not have the authority for some onsite
program management responsibilities, i.e., ownership of onsite
systems which do not serve county institutions. Typically,
356
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these units have financial authority and legal entity status
similar to municipalities.
3. Quasi-Governmental Units - These units include regional (multi-
county)water quality boards, regional planning commissions,
local or regional health departments/boards, councils of gov-
ernment, and other agencies with the exception of special pur-
pose entities. Their authority varies with the intended pur-
pose of each unit; however, the financial authority is typi-
cally less than that of municipalities and unincorporated gov-
ernmental units.
10.3.3 Special Purpose Districts
Special purpose districts depend entirely on enabling legislation for
their authority and extent of services. These districts are independent
units of government, created to provide one or more services, such as
water and wastewater services to those within their boundaries. If per-
mitted by the enabling legislation, services may also be provided to
others outside their boundaries. The boundaries are often permitted to
cross local governmental boundaries so that services can be provided to
all those in need, despite the fact that residents of the district
reside on either side of local governmental boundaries (counties, towns,
villages, etc.).
Nearly all special purpose districts have sufficient financial authority
to impose service charges, collect fees, impose special assessments upon
property benefitted, and issue revenue and/or special assessment bonds.
In addition, some special purpose districts receive the same financing
authority enjoyed by municipalities, including the authority to levy
taxes and incur general obligation debt (i.e., general obligation bonds
backed by taxing authority). These districts are usually legal entities
that may enter into contracts, sue, and be sued.
10.3.4 Private Institutions
Private institutions do not rely on enabling legislation, but are
founded upon the right of individuals or corporations to enter into con-
tracts. However, they are often subject to review or regulation by
state public service or utility commissions.
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10.3.4.1 Private Nonprofit Institutions (Associations
and Corporations)
These entities include homeowners' associations, private cooperatives,
and nonprofit corporations that provide services for onsite systems.
The range of services may vary from merely providing maintenance to
complete ownership of the system. The freedom of the contract permits
this complete range of services; however, the association or corporation
may be regulated by the state public service or public utility laws.
10.3.4.2 Private-for-Profit Institutions
This type of entity may be a sole proprietorship, partnership, or cor-
poration that provides services for onsite systems. The homeowner or a
group of owners (homeowners' associations) typically enters into a con-
tract with this private entity for the provision of services. These
services could include maintenance and operation of the owner's onsite
system, or the private entity could own the systems and charge the
homeowner for the use of the systems. The state public service or
public utility commission may regulate the private entity.
10.4 Management Program Functions
A good management program consists of many functions that may be per-
formed by one entity only or shared among several entities. The user of
this manual is urged to review the range of functions discussed here,
and to select entities that are best able to perform those functions.
For a more complete discussion of the various functions, see References
(2) and (3).
10.4.1 Site Evaluation and System Design
In developing a management program, a choice can be made between per-
forming the site evaluation and system design functions within the
entity itself or reviewing work done in the private sector. Table 10-1
summarizes the suggested activities that should be performed for both
options.
358
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TABLE 10-1
SITE EVALUATION AND SYSTEM DESIGN FUNCTIONS
Scope of Activities
Adml ni strati ve/Techni cal
Activities
Regulatory/Enforcement
Activities
Perform all
evaluations
provide system
designs
site
and
a. Conduct site evaluations
for each lot to be
devel oped
b. Identify and evaluate
feasible (or permitted)
system designs
c. Design selected system
Review all site
evaluations and
system designs
a.
Verify site evaluation
procedures and data
collected for each lot
b.
Review and approve or
disapprove plans
a. Establish guidelines and
procedures for
identifying sites
suitable for development
b. Develop cost-effective-
ness guideli nes and
evaluation procedures
c. Establish design
standards, construction
specifications, and
performance standards
and
Issue construction permit
a. Develop guidelines and
procedures for
identifying sites
suitable for development
and
Develop training,
certification, or
licensing program for
site evaluators
b. Establish design
standards, construction
specifications, and
performance standards
and/or
Develop training
certification or
licensing program for
system designers
and
Issue construction
per mi t
359
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10.4.1.1 Standards for Site Suitability, System
Design, and Performance
A state agency with appropriate authority may establish minimum stan-
dards for site suitability, system design, and performance. This may be
preferred over each management entity establishing its own standards.
The advantages are (1) more uniformity of regulations throughout the
state (although the local management entity may choose to be more strin-
gent if it has the power to do so), and (2) more resources and experi-
enced personnel at the state level to develop appropriate standards.
10.4.1.2 Site Evaluation and System Design
It may be desirable to include site evaluation and system design activi-
ties as part of the management program. These activities could be per-
formed by any of the entities making up the management program. How-
ever, if the local management entity proposes to own and operate systems
within its jurisdiction, this would be the preferred entity to perform
these activities. Legal advice should be sought regarding liability
which may result from undertaking this activity.
As an alternative to performing site evaluations and system designs as
part of the management program, these activities could be performed by
site evaluators and system designers licensed or registered by the man-
agement entity. Licensure or registration is suggested to assure quali-
ty. However, such assurances can only be obtained if the license or
registration is subject to suspension or revocation. Random or preap-
proved site inspections by the management entity are suggested to check
compliance with established procedures and standards, particularly where
site limitations are anticipated.
10.4.1.3 Plan Approval and Construction Permits
The management process should be initiated either by submission of plans
for review and approval or by application for a permit to construct a
system. Either requirement for plan approval or permit issuance for
construction of a system provides the management entity with a conveni-
ent method of obtaining information about the site evaluation and system
design. Site suitability and design standards may be easily enforced by
refusing to approve plans or issue permits.
Plan approval or permit programs at the state level may be more desira-
ble than at the local level because of greater technical resources and
isolation from local political pressures to allow development on poorly
360
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suited sites. As an alternative to the review of all applications, the
state agency could review a random sample of the plans approved or per-
mits issued by the local management entity. The state agency would have
the authority to countermand local approval. However, it would be nec-
essary to limit the period of time that the state agency has to act on
the local action.
10.4.2 Installation
As with site evaluation and system design, the management entities could
choose to install all new systems themselves. This would be particu-
larly desirable if ownership were to be retained by the entity. If not,
the entity may choose to control installation through inspections.
Table 10-2 summarizes the suggested activities that should be performed
for both options.
10.4.2.1 Construction Inspections
A program to inspect the onsite system at each critical stage during
construction is very desirable to prevent improper construction and pre-
mature failure of the system. The inspection may be performed by any
entity involved in the total management program, but it would be most
appropriate for the entity that has responsibility for the rehabilita-
tion or abandonment of improperly functioning systems.
If the management entity does not perform the inspections, they could be
performed by licensed or registered inspectors. A state agency would be
the most likely entity to develop a program to train inspectors in pro-
per design and construction techniques for all acceptable types of sys-
tems. This would assure more uniform quality of inspections statewide.
To further assure uniformity and thoroughness of inspections, checklists
of specific items to be inspected for each type of permitted design
could be developed. The inspectors would be required to certify that
the checklist was completed after the inspector's personal inspection of
the installation, and that all entries contained on the checklist are
correct. To insure that inspections are timely, the management entity
may require the system installer to give notice as to when the construc-
tion of the system is to commence.
361
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TABLE 10-2
INSTALLATION FUNCTIONS
Scope of Activities
Admi ni strati ve/Techni cal
Activities
Regulatory/Enforcement
Activities
Perform inspection/
supervision of
construction
a. Perform construction
inspection and/or
supervision during
various phases of
construction
b. Prepare as-built drawing
Review construction
inspection/
supervi sion
a.
Revi ew certifi ed
inspection by licensed/
registered inspectors
b. Require as-built drawing
Develop guidelines and
specifications for
construction
b. Record as-built drawing
and issue system use
permit
a. Develop specifications
for construction and
checklists for inspection
and
Develop training,
certification or
licensing program for
inspectors
b. Record as-built drawing
and issue system use
permit
10.4.2.2 As-Built Drawings
It is not unusual for the system installed to be quite different from
the drawings originally approved because of changes necessary during
construction. As-built drawings become very valuable when inspection or
servicing of the system is required. Therefore, a requirement for as-
built drawings is a good practice. These plans could be indexed by
street, address, name of original owner, installer, and legal descrip-
tion.
10.4.2.3 Training and Licensing of Installers
To reduce the reliance on good construction supervision and inspections,
a program to train and license or register installers could be estab-
lished. Training would include presentation of design and construction
techniques of all approved system types. To be effective, this program
would have to be coupled with a strong enforcement program in which the
license to install systems could be suspended or revoked.
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10.4.3 Operation and Maintenance
Traditionally, the responsibility for operation and maintenance of on-
site systems has been left to the owner. This has been less than satis-
factory. As an alternative, management entities are beginning to assume
this responsibility. The program adopted may either be compulsory or
voluntary. If voluntary, the management entities perform the mainte-
nance or issue operating permits on receipt of an assurance that the
proper maintenance was performed. Table 10-3 summarizes the suggested
activities that should be performed for both options.
10.4.3.1 Standards for Operation and Maintenance
A standard for the operation and maintenance of each type of system
used, stating the procedures to be used and the frequency with which
they are to be performed, is desirable. These standards would include
those necessary to regulate the hauling and disposal of residuals gener-
ated by onsite systems as well. The state agency would be the preferred
entity to set these standards. The advantages of having the state set
the standards include more uniformity in the regulations and more re-
sources and experienced personnel to develop appropriate standards.
10.4.3.2 Operating Permits
Rather than the management entities providing services, compliance with
operation and maintenance standards could be assured through an opera-
ting permit program. The type and frequency of maintenance required for
each type of system would be established by the entity. An operating
permit allowing the owner to use the system would be renewed only if the
required maintenance is performed. The system owner would be notified
when the permit is about to expire, and told what maintenance must be
performed to obtain a renewal. The owner would be required to have the
necessary maintenance performed by an individual licensed or registered
to perform such services within a specified period of time (e.g., 60
days). This individual would sign and date one portion of the owner's
permit, thereby certifying that the service was performed.
The enabling ordinance or statutory language establishing this permit
program must indicate that it is unlawful to occupy a home served by an
onsite system unless the owner holds a valid operating permit. Thus, if
the permit were not renewed, the owner would be in violation of the ord-
inance or statute. From a legal viewpoint, enforcement of this type of
violation is straightforward.
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TABLE 10-3
OPERATION AND MAINTENANCE FUNCTIONS
Scope of Activities
Administrative/Technical
Activities
Regula tory/Enforcement
Activities
Perform necessary
operati on/
maintenance
a.
b.
Provide routine and
emergency operation/
maintenance of each
system
Determine if operation/
maintenance program is
voluntary or compulsory
Admi ni ster
operation/
maintenance
a.
Establish an operation
and maintenance program
program
b.
Determine if operation/
maintenance program is
voluntary or compulsory
c.
Develop policies for
regulating operation/
maintenance activities
a. Develop guidelines and
schedules for routine
operati on/mai ntenance
b. Establish operation/
maintenance program
and
Obtain legal authority
for right of access to
private property
a. Develop guidelines and
schedules for routine
operati on/mai ntenance
and
Impose standards for
hauling and disposal of
resi duals
b. Develop system for
notifying owner of
required operation/
maintenance
and
Issue a regularly renewed
operating permit after
certification that proper
operati on/mai ntenance
has been performed
c. Develop training,
certification, or
licensing program for
those contracting to
perform operation/
maintenance activities
364
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10.4.3.3 Li censure/Registration
To provide assurance that onsite systems are properly operated and main-
tained, licensing or registering of qualified individuals is desirable.
This could be done at the state level. If licensure/registration pro-
grams for individuals, such as plumbers, residual waste haulers, sani-
tarians, etc., already exist, and if these individuals have sufficient
knowledge of onsite systems, an additional program may not be necessary.
10.4.4 Rehabilitation
Because onsite systems are usually located on private property and below
ground, system failures are difficult to detect. If a management pro-
gram is to effectively prevent public health hazards, environmental
degradation, and nuisances, identification and correction of failures
are a necessary part of the management program. Table 10-4 summarizes
the suggested activities that should be performed.
TABLE 10-4
REHABILITATION FUNCTIONS
Administrative/Technical Regulatory/Enforcement
Scope of Activities Activitles Activities
Detect and correct a. Develop procedures for a. Develop performance
improperly identifying improperly standards
functioning systems functioning systems
(Sanitary surveys, and
presale inspections,
etc.) Obtain legal authority
for right of access to
private property
b. Rehabilitate system b. Issue order requiring
rehabilitation
or
Rehabilitate system as
part of operation/
maintenance program
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10.4.4.1 Inspections
Inspections could be performed as part of a sanitary survey of the area
or through presale inspections during real estate transactions. The
latter option may require enabling legislation. Constitutional re-
straints regarding the inspection of private property and the limita-
tions on the sale of property have to be considered prior to enacting
such legislation.
10.4.4.2 Orders and Violations
The management entity needs the authority to issue orders requiring the
repair, replacement, or abandonment of improperly functioning systems if
the systems are not owned by the entity. Various state agencies have
this authority. If the owner does not comply with the order to repair
or rehabilitate the system, the management entity could require that
copies of all violations be filed with the registrar of deeds or a
similar official. The effect of such a filing requirement would be to
give notice of the violation in the chain of title whenever an abstract
or a title insurance policy is prepared. Any potential mortgagee or
buyer would thereby be alerted to the violation.
10.5 References
1. Plews, G. D. The Adequacy and Uniformity of Regulations for On-Site
Wastewater Disposal - A State Viewpoint. In: National Conference
on Less Costly Wastewater Treatment Systems for Small Communities.
EPA 600/9-79-010, NTIS Report No. PB 293 254, April 1977. pp.
20-28.
2. Small Scale Waste Management Project, University of Wisconsin,
Madison, Management of Small Waste Flows. EPA 600/2-78-173, NTIS
Report No. PB 286 560, September 1978.
3. Interim Study Report, Management of On-Site and Small Community
Wastewater Systems. M687, U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory, Cincinnati, Ohio, 1979.
366
-------
APPENDIX A
SOIL PROPERTIES AND SOIL-WATER RELATIONSHIPS
A.I Introduction
An understanding of how water moves into and through soil is necessary
to predict the potential of soil for wastewater absorption and treat-
ment. Water moves through the voids or pore spaces within soil. There-
fore, the size, shape, and continuity of the pore spaces are very impor-
tant. These characteristics are dependent on the physical properties of
the soil and the characteristics of water as well.
A.2 Physical Properties of Soil
A.2.1 Soil Texture
Texture is one of the most important physical properties of soil because
of its close relationship to pore size, pore size distribution and pore
continuity. It refers to the relative proportion of the various sizes
of solid particles in the soil that are smaller than 2 mm in diameter.
The particles are commonly divided into three size fractions called soil
"separates." These separates are given in Figure A-l. The U.S. Depart-
ment of Agriculture (USDA) system is used in this manual (Table A-l).
TABLE A-l
U.S. DEPARTMENT OF AGRICULTURE SIZE LIMITS FOR SOIL SEPARATES
Tyler Standard
Soil Separate Size Range Sieve No.
mm
Sand 2-0.05 10-270 mesh
Very coarse sand 2-1 10-16 mesh
Coarse sand 1-0.5 16-35 mesh
Medium sand 0.5-0.25 35-60 mesh
Fine sand 0.25-0.1 60-140 mesh
Very fine sand 0.1-0.05 140-270 mesh
Silt 0.25-0.002
Clay <0.002
367
-------
FIGURE A-l
NAMES AND SIZE LIMITS OF PRACTICAL-SIZE
CLASSES ACCORDING TO SIX SYSTEMS (1)
OO
cr>
CO
SYSTEM
1. US. Bureau of
Reclamation and
Corps of Engineers (U.S
Dept. of the Army)
2. American Association
of State Highway
Officals
3. American Society for
Testing and Materials
4. Wentworth
5 U.S Department of
Agriculture
6 International Society
of Soil Science
1
M
i • •
1
1
Silt and Clay
(distinguished on the
basis of plasticity)
Colloid
Colloid
Cl"
>
3
i
V
3V
Clay
Clay
Clay
1
+•
*•
Silt
sm
Silt
Silt
Silt
1 1 1 1 1 1 f
11 1 I
'
Fine sand
Fine sand
Fine sand
Very
fine
sane
Very
fine
sand
Fine
sand
Fine
sand
Fine sand
i t i
f
,nl
v mi
Medium
sand
Coarse
sand
Medium
sand
Medium
sand JCoars
sand
Medium
sand Coar;
sand
Very
coars
sand
3
Very
coarse
sand
e
Coarse sand
i i i 1 1 1 1
1
1 '
Coarse
sand
i i in
1
Fine gravel
Fine gravel
Coarse
sand
Mediun
gravel
i 1 I
Coarse
gravel
Coarse
gravel
Gravel
Pebbles
Granules
Fine gravel
M '
Cobbles
Boulders
Cobbles
Coarse gravel
Cobbles
Gravel
i i i i i i
I
i i i
...1
0.000
0.002
0.02 02
P;ti(icle diameter, mm
20
20
200
Used in soil engineering
h
Used in geology.
USDA system used in this manual
Used in soil science.
-------
Twelve textural classes are defined by the relative proportions of the
sand, silt and clay separates. These are represented on the textural
triangle (Figure A-2). To determine the textural class of a soil hori-
zon, the percent by weight of the soil separates is needed. For exam-
ple, a sample containing 37% sand, 45% silt and 18% clay has a textural
class of loam. This is illustrated in Figure A-2.
Soil textural classes are modified if particles greater than 2 mm in
size are present. The adjectives "gravelly," "cobbly," and "stoney" are
used for particles between 2 and 75 mm, 75 and 250 mm, or 250 mm, re-
spectively, if more than 15% to 20% of the soil volume is occupied by
these fragments.
Soil permeability, aeration and drainage are closely related to the soil
texture because of their influence on pore size and pore continuity.
They are also related to the soil's ability to filter particles and
retain or adsorb pollutants from the waste stream. For example, fine
textured or clayey soils do not transmit water rapidly or drain well
because the pores are very small. They tend to retain water for long
periods of time. However, they act as better filters and can retain
more chemicals than soils of other textures. On the other hand, coarse
textured or sandy soils have large, continuous pores that can accept and
transmit large quantities of water. They retain water for only short
periods of time. The capacity to retain chemicals is generally low and
they do not filter wastewater as well as finer textured soils. Medium
textured or loamy soils have a balance between wastewater absorption and
treatment capabilities. They accept and transmit water at moderate
rates, act as good filters, and retain moderate amounts of chemical
constituents.
A.2.2 Soil Structure
Soil structure has a significant influence on. the soil's acceptance and
transmission of water. Soil structure refers to the aggregation of soil
particles into clusters of particles, called peds, that are separated by
surfaces of weakness. These surfaces of weakness open planar pores be-
tween the peds that are often seen as cracks in the soil. These planar
pores can greatly modify the influence of soil texture on water move-
ment. Well structured soils with large voids between peds will transmit
water more rapidly than structureless soils of the same texture, partic-
ularly if the soil has become dry before the water is added. Fine tex-
tured, massive soils (soils with little structure) have very slow per-
colation rates.
369
-------
FIGURE A-2
TEXTURAL TRIANGLE DEFINING TWELVE TEXTURAL CLASSES OF THE USDA
(ILLUSTRATED FOR A SAMPLE CONTAINING 37% SAND, 45% SILT, AND 18% CLAY)
100%
Clay Loam
Loam
A
30/-Sandy clay:
Loam \
V V
\ \
\~A~~A~
Silt Loam
—r r-
\ / \
-Sandy Loam
100%
sand
60 50 40
Percent Sand
by Weight
100%
silt
370
-------
The form, size and stability of the aggregates or peds depend on the
arrangement of the soil particles and the bonds between the particles.
The four major types of structures include platy, blocky, prismatic and
granular. Detailed descriptions of types and classes of soil structure
used by SCS are given in Table A-2.
Between the peds are voids which are often relatively large and continu-
ous compared to the voids or pores between the primary particles within
the peds. The type of structure determines the dominant direction of
the pores and, hence, water movement in the soil. Platy structures re-
strict vertical percolation of water because cleavage faces are horizon-
tally oriented. Often, vertical flow is so restricted that the upper
soil horizons saturate, creating a perched water table. Soils with
prismatic and columnar structure enhance vertical water flow, while
blocky and granular structures enhance flow both horizontally and
vertically.
The soil's permeability by air and water is also influenced by the fre-
quency and degree of expression of the pores created by the structual
units. These characteristics depend upon the size of the peds and their
grade or durability. Small structural units create more pores in the
soil than large structural units. Soils with strong structure have
distinct pores between peds. Soils with very weak structure, or soils
without peds or planes of weakness, are said to be structureless.
Structureless sandy soils are called single grained or granular, while
structureless clayey soils are called massive.
Structure is one soil characteristic that is easily altered or de-
stroyed. It is very dynamic, changing in response to moisture content,
chemical composition of soil solution, biological activity, and manage-
ment practices. Soils containing minerals that shrink and swell appre-
ciably, such as montmorillonite clays, show particularly dramatic
changes. When the soil peds swell upon wetting, the large pores become
smaller, and water movement through the soil is reduced. Swelling can
also result if the soil contains a high proportion of sodium salts.
Therefore, when determining the hydraulic properties of a soil for
wastewater disposal, soil moisture contents and salt concentrations
should be similar to that expected in the soil surrounding a soil dis-
posal system.
A.2.3 Soil Color
The color and color patterns in soil are good indicators of the drainage
characteristics of the soil. Soil properties, location in the land-
scape, and climate all influence water movement in the soil. These fac-
tors cause some soils to be saturated or seasonally saturated, affecting
371
-------
TABLE A-2
TYPES AND CLASSES OF SOIL STRUCTURE
TYPE (shape and arrangement of peds)
Class
Platelike, with
one dimension
(the vertical)
limited and
greatly less
than the other
two; arranged
around a hori-
zontal plane
faces mostly
horizontal
Prismlike, with two dimensions
(the horizontal) limited and con-
siderably less than the vertical; '
arranged around a vertical line;
vertical faces well defined;
vertices angular
Blocklike, polyhedronlike, or spheroids, or with three dimensions of
the same order of magnitude, arranged around a point.
Blocklike; blocks or polyhedrons Spheroids or polyhedrons having
having plane or curved surfaces plane or curved surfaces which
that are casts of the molds formed have slight or no accommodations
by the faces of the surrounding to the faces of surrounding peds
peds
Without With rounded Faces flattened, Mixed rounded Nonporous
rounded caps caps most vertices and flattened peds
sharply angular faces with many
rounded vertices
Porous peds
Platy
Prismatic
Columnar
(Angular)
Blocky*
(Subangular)
Blockyt
Granular
Crumb
Very
very
Fine
fine or
thin
or thin
Very
platy
Thin
to 2
thin
; < 1 mm
platy; 1
mm
Very fine pris-
matic;
<10 mm
Fine prismatic;
10 to 20 mm
Very fine
columnar;
<10 mm
Fine Columnar;
10 to 20 mm
Very fine angu-
lar blocky;
^ 5 mm
Fine angular
blocky; 5 to
10 mm
Very fine sub-
angular blocky;
<5 mm
Fine sub ang
blocky; 5 to
10 mm
Very fine
angular;
<1 mm
Fine
1 to
granular;
2 mm
Very fine
crumb; <
1 mm
Fine crumb;
1 to 2 mm
Medium Medium platy; Medium pris- Medium colum-Medium angu- Medium sub- Medium granu-Medium crumb;
2 to 5 mm matic, 20 to nar; 20 to 50 lar blocky; 10 angular blocky; lar; 2 to 5 mm 2 to 5 mm
50mm mm to 20 mm 10 to 20 mm
Coarse or thick Thick platy; Coarse pris- Coarse colum- Coarse angular Coarse sub- Coarse granu-
5 to 10 mm matic, 50 to nar; 50 to blocky, 20 to angular; 20 lar; 5 to 10 mm
100 mm 100 mm 50 mm to 50 mm
Very coarse or Very thick Very coarse Very coarse Very coarse Very coarse Very coarse
very thick platy; > 10 mm prismatic; columnar; angular blocky; Subangular granular;
> 100 mm > 100 mm >50mm blocky; >10 mrn
>50 mm
Source: Soil Survey Staff 1960
•(a)Somelimes called nul. (bl The word "angular" in the name can ordinarily be omitted
tSometimes called nuciform, nut, or Subangular nut Since the size connotation of these terms
is a source of great confusion to many, they are not recommended.
372
-------
their ability to absorb and treat wastewater. Interpretation of soil
color aids in identifying these conditions.
Soil colors are a result of the color of primary soil particles, coat-
ings of iron and manganese oxides, and organic matter on the particles.
Soils that are seldom or never saturated with water and are well aer-
ated, are usually uniformly red, yellow or brown in color. Soils that
are saturated for extended periods or all the time are often grey or
blue in color. Color charts have been developed for identifying the
various soil colors.
Soils that are saturated or nearly saturated during portions of the year
often have spots or streaks of different colors called mottles. Mottles
are useful to determine zones of saturated soil that may occur only dur-
ing wet periods. Mottles result from chemical and biochemical reactions
when saturated conditions, organic matter, and temperatures above 4° C
occur together in the soil. Under these conditions, the bacteria pres-
ent rapidly deplete any oxygen present while feeding on the organic mat-
ter. When the oxygen is depleted, other bacteria continue the organic
decomposition using the oxidized iron and manganese compounds, rather
than oxygen, in their metabolism. Thus, the insoluble oxidized iron and
manganese, which contribute much of the color to soil, are reduced to
soluble compounds. This causes the soil to lose its color, turning the
soil grey. When the soil drains, the soluble iron and magnesium are
carried by the water to the larger soil pores. Here they are reoxidized
when they come in contact with the oxygen introduced by the air-filled
pores, forming insoluble compounds once again. The result is the for-
mation of red, yellow and black spots near surfaces, and the loss of
color, or greying, at the sites where the iron and manganese compounds
were removed. (Examples of mottled soils are shown in Figure 3-20).
Therefore, mottles seen in unsaturated soils can be interpreted as an
indication that the soil is periodically saturated. Periodic saturation
of soil cannot always be identified by mottles, however. Some soils can
become saturated without the formation of mottles, because one of the
conditions needed for mottle formation is not present. Experience and
knowledge of moisture regimes related to landscape position and other
soil characteristics are necessary to make proper interpretations in
these situations.
Also, color spots and streaks can be present in soils for reasons other
than soil saturation. For example, soil parent materials sometimes cre-
ate a color pattern in the soil similar to mottling. However, these
patterns usually can be distinguished from true mottling. Some very
sandy soils have uniform grey colors because there are no surface coat-
ings on the sand grains. This color can mistakenly be interpreted as a
gley or a poor draining color. Direct measurement of zones of soil
373
-------
saturation may be necessary to confirm the soil moisture regimes if in-
terpretations of soil colors are not possible.
A.2.4 Soil Horizons
A soil horizon is a layer of soil approximately parallel to the soil
surface with uniform characteristics. Soil horizons are identified by
observing changes in soil properties with depth. Soil texture, struc-
ture, and color changes are some of the characteristics used to deter-
mine soil horizons.
Soil horizons are commonly given the letter designations of A, B, and C
to represent the surface soil, subsoil, and substratum, respectively.
Not all soils have all three horizons. On the other hand, many soils
show variations within each master horizon and are subdivided as Al, A2,
A3, and Bl, etc. Some example soils and their horizons are shown in
Figure A-3.
Each horizon has its own set of characteristics and therefore will re-
spond differently to applied wastewater. Also, the conditions created
at the boundary between soil horizons can significantly influence waste-
water flow and treatment through the soil. Therefore, an evaluation of
a soil must include a comparison of the physical properties of each
horizon that influences absorption and treatment of wastewater.
A.2.5 Other Selected Soil Characteristics
Bulk density and clay mineralogy are other soil characteristics that can
significantly influence water infiltration and percolation in soils.
Soil bulk density is the ratio of the mass of soil to its bulk or volume
occupied by the soil mass and pore space. There is not a direct corre-
lation between bulk density and soil permeability, since sandy soils
generally have a higher bulk density and permeability than clayey soils.
However, of soils with the same texture, those soils with the higher
bulk densities are more compact with less pore volume. Reduced porosity
reduces the hydraulic conductivity of the soil. Fragipans are examples
of horizons that have high bulk densities and reduced permeabilities.
They are very compact horizons rich in silt and/or sand but relatively
low in clay, which commonly interferes with water and root penetration.
The mineralogy of clay present in the soil can have a very significant
influence on water movement. Some clay minerals shrink and swell appre-
ciably with changes in water content. Montmorillonite is the most com-
mon of these swelling clay minerals. Even if present in small amounts,
374
-------
FIGURE A-3
SCHEMATIC DIAGRAM OF A LANDSCAPE
AND DIFFERENT SOILS POSSIBLE
DnC2
A - Surface Soil -
B - Subsoil
C - Substratum
A - Surface Soil
375
-------
the porosity of soils containing montmorillonite can vary dramatically
with varying moisture content. When dry, the clay particles shrink,
opening the cracks between peds. But when wet, the clay swells, closing
the pores.
A.3 Water in the SoiTSystem
A.3.1 Soil Moisture Potential
Soil permeability, or the capability of soil to conduct water, is not
determined by the soil porosity but, rather, the size, continuity, and
tortuosity of the pores. A clayey soil is more porous than a sandy
soil, yet the sandy soil will conduct much more water because it has
larger, more continuous pores. Under natural drainage conditions, some
pores in the soil are filled with water. The distribution of this water
depends upon the characteristics of the pores, while its movement is de-
termined by the relative energy status of the water. Water flows from
points of higher energy to points of lower energy. The energy status is
referred to as the moisture potential.
The total soil moisture potential has several components, of which the
gravitational and matric potential are the most important. The gravita-
tional potential is the result of the attraction of water toward the
center of the earth by a gravitational force and is equal to the weight
of water. The potential energy of the water at any point is determined
by the elevation of that point relative to some reference level. Thus,
the higher the water above this reference, the greater its gravitational
potential.
The matric potential is produced by the affinity of water molecules to
each other and to solid surfaces. Molecules within the body of water
are attracted to other molecules by cohesive forces, while water mole-
cules in contact with solid surfaces are more strongly attracted to the
solid surfaces by adhesive forces. The result of these forces acting
together draws water into the pores of the soil. The water tries to wet
the solid surfaces of the pores due to adhesive forces and pulls other
molecules with it due to cohesive forces. This phenomenon is referred
to as capillary rise. The rise of water is halted when the weight of
the water column is equal to the force of capillarity. Therefore, water
rises higher and is held tighter in smaller pores than in larger pores
(see Figure A-4). Upon draining, the largest pores empty first because
they have the weakest hold on the water. Therefore, in unsaturated
soils, the water is held in the finer pores because they are better able
to retain the water against the forces of gravity.
376
-------
FIGURE A-4
UPWARD MOVEMENT BY CAPILLARITY IN GLASS
TUBES AS COMPARED WITH SOILS (2)
Air Spaces
Soil Particle
Capillary Water
Adsorbed Water
The ability of the soil to draw or pull water into its pores is referred
to as its matric potential. Since the water is held against the force
of gravity, it has a pressure less than atmospheric. This negative
pressure is often referred to as soil suction or soil moisture tension.
Increasing suction or tension is associated with soil drying.
The moisture content of soils with similar moisture tensions varies with
the nature of the pores. Figure A-5 illustrates the change in moisture
content versus changes in moisture tensions. When the soil is satu-
rated, all the pores are filled with water and no capillary suction
occurs. The soil moisture tension is zero. When drainage occurs, the
tensions increase. Because the sand has many relatively large pores, it
drains abruptly at relatively low tensions, whereas the clay releases
only a small volume of water over a wide tension range because most of
it is strongly retained in very fine pores. The silt loam has more
coarse pores than does the clay, so its curve lies somewhat below that
of the clay. The sandy loam has more finer pores than the sand so its
curve lies above that of the sand.
377
-------
FIGURE A-5
SOIL MOISTURE RETENTION FOR FOUR
DIFFERENT SOIL TEXTURES (3)
20 40 60 80 100
Soil Moisture Tension (MBAR)
Soil Drying ^
A.3.2 Flow of Water in Soil
The flow of water in soil depends on the soil's ability to transmit the
water and the presence of a force to drive it. Hydraulic conductivity
is defined as the soil's ability to transmit water, and is related to
the number, size, and configuration of the pores. Soils with large,
continuous water-filled pores can transmit water easily and have a high
conductivity, while soils with small, discontinuous water-filled pores
offer a high resistance to flow, and, therefore, have low conductivity.
When the soil is saturated, all pores
tivity depends on all the soil pores.
or dries (see Figure A-5), the larger
smaller water-filled pores may transmit the water. Therefore, as seen
in Figure A-6, the hydraulic conductivity decreases for all soils as
they dry. Since clayey soils have more fine pores than sandy soils, the
hydraulic conductivity of a clay is greater than a sand beyond a soil
moisture tension of about 50 mbar.
are water-filled and the conduc-
When the soil becomes unsaturated
pores fill with air, and only the
378
-------
FIGURE A-6
HYDRAULIC CONDUCTIVITY (K) VERSUS
SOIL MOISTURE RETENTION (4)
1000-
CD
\100-
u
>-
'>
s
T3
C
O
o
10-
1.0-3
0.1-
245 r
LType I (sand)
Type II,
(sandy loam)
20 40 60 80 100
Soil Moisture Tension (MBAR)
Drying >-
Water movement in soil is governed by the total moisture potential gra-
dient and the soil's hydraulic conductivity. The direction of movement
is from a point of higher potential (gravity plus matric potential) to a
point of lower potential. When the soil is saturated, the matric poten-
tial is zero, so the water moves downward due to gravity. If the soil
is unsaturated, both the gravity and matric potentials determine the
direction of flow, which may be upward, sideward, or downward depending
on the difference in total potentials surrounding the area. The greater
the difference in potentials between two points, the more rapid the
movement. However, the volume of water moved in a given time is propor-
tional to the total potential gradient and the soils hydraulic conduc-
tivity at the given moisture content. Therefore, soils with greater
hydraulic conductivities transmit larger quantities of water at the same
potential gradient than soils with lower hydraulic conductivities.
379
-------
A.3.3 Flow of Water Through Layered Soils
Soil layers of varying hydraulic conductivities interfere with water
movement. Abrupt changes in conductivity can cause the soil to saturate
or nearly saturate above the boundary regardless of the hydraulic con-
ductivity of the underlying layer. If the upper layer has a signifi-
cantly greater hydraulic conductivity, the water ponds because the lower
layer cannot transmit the water as fast as the upper layer delivers it.
If the upper layer has a lower conductivity, the underlying layer cannot
absorb it because the finer pores in the upper layer hold the water
until the matric potential is reduced to near saturation.
Layers such as these may occur naturally in soils or as the result of
continuous wastewater application. It is common to develop a clogging
mat of lower hydraulic conductivity at the infiltrative surface of a
soil disposal system. This layer forms as a result of suspended solids
accumulation, biological activity, compaction by construction machinery,
and soil slaking (3). The clogging mat may restrict water movement to
the point where water is ponded above, and the soil below is unsatur-
ated. Water passes through the clogging mat due to the hydrostatic
pressure of the ponded water above pushing the water through, and the
soil suction of the unsaturated soil below pulling it through.
Figure A-7 illustrates three columns of similar textured soils with
clogging mats in various stages of development. Water is ponded at
equal heights above the infiltrative surface of each column.
Column A has no clogging mat so the water is able to pass through all
the pores, saturating the soil. The moisture tension in this column is
zero. Column B has a permeable clogging mat developed with moderate
size pores. Flow into the underlying soil is restricted by the clogging
mat to a rate less than the soil is able to transmit it. Therefore, the
large pores in the soil empty. With increasing intensity of the mat, as
shown in Column C, the flow rate through the soil is reduced to very low
levels. The water is forced to flow through the finest pores of the
soil, which is a very tortuous path. Flow rates through identical clog-
ging mats developed on different soils will vary with the soil's capil-
lary characteristics.
A.4 Evaluating Soil Properties
To adequately predict how soil responds to wastewater application, the
soil properties described and other site characteristics must be
identified. The procedures used to evaluate soils are described in
Chapter 3 of this manual.
380
-------
FIGURE A-7
SCHEMATIC REPRESENTATION OF WATER MOVEMENT THROUGH
A SOIL WITH CRUSTS OF DIFFERENT RESISTANCES
M Water
Clogging Layer
Qpore
Particle
<.•••: s-_-
A.5 References
1. Black, C. A. Soil Plant Relationships. 2nd ed. Wiley, New York,
1968. 799 pp.
2. Brady, N. C. The Nature and Properties of Soils. 8th ed. MacMil-
lan, New York, 1974. 655 pp.
3. Bouma, J. W., A. Ziebell, W. G. Walker, P. G. Olcott, E. McCoy, and
F. D. Hole. Soil Absorption of Septic Tank Effluent. Information
Circular 20, Wisconsin Geological and Natural History Survey, Madi-
son, 1972. 235 pp.
4. Bouma, J. Unsaturated Flow During Soil Treatment of Septic Tank
Effluent. J. Environ. Eng., Am. Soc. Civil Eng., 101:967-983, 1975.
381
-------
GLOSSARY
A horizon: The horizon formed at or near the surface, but within the
mineral soil, having properties that reflect the influence of accu-
mulating organic matter or eluviation, alone or in combination.
absorption: The process by which one substance is taken into and in-
cluded within another substance, as the absorption of water by soil
or nutrients by plants.
activated sludge process: A biological wastewater treatment process in
which a mixture of wastewater and activated sludge is agitated and
aerated. The activated sludge is subsequently separated from the
treated wastewater (mixed liquor) by sedimentation and wasted or
returned to the process as needed.
adsorption: The increased concentration of molecules or ions at a sur-
face, including exchangeable cations and anions on soil particles.
aerobic: (1) Having molecular oxygen as a part of the environment. (2)
Growing or occurring only in the presence of molecular oxygen, such
as aerobic organisms.
aggregate, soil: A group of soil particles cohering so as to behave me-
chanically as a unit.
anaerobic: (1) The absence of molecular oxygen. (2) Growing in the ab-
sence of molecular oxygen (such as anaerobic bacteria).
anaerobic contact process: An anaerobic waste treatment process in
which the microorganisms responsible for waste stabilization are
removed from the treated effluent stream by sedimentation or other
means, and held in or returned to the process to enhance the rate
of treatment.
o
angstrom (A): one hundred millionth of a centimeter.
B horizon: The horizon immediately beneath the A horizon characterized
by a higher colloid (clay or humus) content, or by a darker or
brighter color than the soil immediately above or below, the color
usually being associated with the colloidal materials. The
colloids may be of alluvial origin, as clay or humus; they may have
been formed in place (clays, including sesquioxides); or they may
have been derived from a texturally layered parent material.
382
-------
biochemical oxygen demand (BOD): Measure of the concentration of organ-
ic impurities in wastewater. The amount of oxygen required by bac-
teria while stabilizing organic matter under aerobic conditions,
expressed in mg/1, is determined entirely by the availability of
material in the wastewater to be used as biological food, and by
the amount of oxygen utilized by the microorganisms during
oxidation.
blackwater: Liquid and solid human body waste and the carriage waters
generated through toilet usage.
bulk density, soil: The mass of dry soil per unit bulk volume. The
bulk volume is determined before drying to constant weight at
105°C.
C horizon: The horizon that normally lies beneath the B horizon but may
lie beneath the A horizon, where the only significant change caused
by soil development is an increase in organic matter, which
produces an A horizon. In concept, the C horizon is unaltered or
slightly altered parent material.
calcareous soil: Soil containing sufficient calcium carbonate (often
with magnesium carbonate) to effervesce visibly when treated with
cold 0.1N hydrochloric acid.
capillary attraction: A liquid's movement over, or retention by, a
solid surface, due to the interaction of adhesive and cohesive
forces.
cation exchange: The interchange between a cation in solution and
another cation on the surface of any surface-active material, such
as clay or organic colloids.
cation-exchange capacity: The sum total of exchangeable cations that a
soil can adsorb; sometimes called total-exchange, base-exchange ca-
pacity, or cation-adsorption capacity. Expressed in milliequiva-
lents per 100 grams or per gram of soil (or of other exchanges,
such as clay).
chemical oxygen demand (COD): A measure of the oxygen equivalent of
that portion of organic matter that is susceptible to oxidation by
a strong chemical oxidizing agent.
chlorine residual: The total amount of chlorine (combined and free
available chlorine) remaining in water, sewage, or industrial
wastes at the end of a specified contact period following
c hi ori nation.
clarifiers: Settling tanks. The purpose of a clarifier is to remove
settleable solids by gravity, or colloidal solids by coagulation
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following chemical flocculation; will also remove floating oil and
scum through skimming.
clay: (1) A soil separate consisting of particles <0.002 mm in equiva-
lent diameter. (2) A textural class.
clay mineral: Naturally occurring inorganic crystalline or amorphous
material found in soils and other earthy deposits, the particles
being predominantly <0.002 mm in diameter. Largely of secondary
origin.
coarse texture: The texture exhibited by sands, loamy sands, and sandy
loams except very fine sandy loams.
coliform-group bacteria: A group of bacteria predominantly inhabiting
the intestines of man or animal, but also occasionally found else-
where. Used as an indicator of human fecal contamination.
colloids: The finely divided suspended matter which will not settle,
and the apparently dissolved matter which may be transformed into
suspended matter by contact with solid surfaces or precipitated by
chemical treatment. Substances which are soluble as judged by or-
dinary physical tests, but will not pass through a parchment mem-
brane.
columnar structure: A soil structural type with a vertical axis much
longer than the horizontal axes and a distinctly rounded upper sur-
face.
conductivity, hydraulic: As applied to soils, the ability of the soil
to transmit water in liquid form through pores.
consistence: (1) The resistance of a material to deformation or rup-
ture. (2) The degree of cohesion or adhesion of the soil mass.
Terms used for describing consistence at various soil moisture
contents are:
wet soil: Nonsticky, slightly sticky, sticky, very sticky, non-
plastic, slightly plastic, plastic, and very plastic.
moist soil: Loose, very friable, friable, firm, very firm, and
extremely firm.
dry soil: Loose, soft, slightly hard, hard, very hard, and ex-
tremely hard.
cementation: Weakly cemented, strongly cemented, and indurated.
crumb: A soft, porous, more or less rounded ped from 1 to 5 mm in dia-
meter.
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crust: A surface layer on soils, ranging in thickness from a few milli-
meters to perhaps as much as an inch, that is much more compact,
hard, and brittle when dry, than the material immediately beneath
it.
denitrification: The biochemical reduction of nitrate or nitrite to
gaseous molecular nitrogen or an oxide of nitrogen.
digestion: The biological decomposition of organic matter in sludge,
resulting in partial gasifiction, liquefaction, and mineralization.
disinfection: Killing pathogenic microbes on or in a material without
necessarily sterilizing it.
disperse: To break up compound particles, such as aggregates, into the
individual component particles.
dissolved oxygen (DO): The oxygen dissolved in water, wastewater, or
other liquid, usually expressed in milligrams per liter (mg/1),
parts per million (ppm), or percent of saturation.
dissolved solids: Theoretically, the anhydrous residues of the dis-
solved constituents in water. Actually, the term is defined by the
method used in determination.
effluent: Sewage, water, or other liquid, partially or completely
treated or in its natural state, flowing out of a reservoir, basin,
or treatment plant.
effective size: The size of grain such that 10% of the particles by
weight are smaller and 90% greater.
eutrophic: A term applied to water that has a concentration of nutri-
ents optimal, or nearly so, for plant or animal growth.
evapotranspiration: The combined loss of water from a given area, and
during a specified period of time, by evaporation from the soil
surface and by transpiration from plants.
extended aeration: A modification of the activated sludge process which
provides for aerobic sludge digestion within the aeration system.
filtrate: The liquid which has passed through a filter.
fine texture: The texture exhibited by soils having clay as a part of
their textural class name.
floodplain: Flat or nearly flat land on the floor of a river valley
that is covered by water during floods.
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floodway: A channel built to carry excess water from a stream.
food to microorganism ratio (F/M): Amount of BOD applied to the acti-
vated sludge system per day per amount of MLSS in the aeration
basin, expressed as Ib BOD/d/lb MLSS.
graywater: Wastewater generated by water-using fixtures and appliances,
excluding the toilet and possibly the garbage disposal.
hardpan: A hardened soil layer, in the lower A or in the B horizon,
caused by cementation of soil particles with organic matter or with
materials such as silica, sesquioxides, or calcium carbonate. The
hardness does not change appreciably with changes in moisture con-
tent, and pieces of the hard layer do not slake in water.
heavy soil: (Obsolete in scientific use.) A soil with a high content
of the fine separates, particularly clay, or one with a high
drawbar pull and hence difficult to cultivate.
hydraulic conductivity: See conductivity, hydraulic.
impervious: Resistant to penetration by fluids or by roots.
influent: Water, wastewater, or other liquid flowing into a reservoir,
basin, or treatment plant.
intermittent filter: A natural or artificial bed of sand or other fine-
grained material to the surface of which wastewater is applied in-
termittently in flooding doses and through which it passes; oppor-
tunity is given for filtration and the maintenance of an aerobic
condition.
ion: A charged atom, molecule, or radical, the migration of which af-
fects the transport of electricity through an electrolyte or, to a
certain extent, through a gas. An atom or molecule that has lost
or gained one or more electrons; by such ionization it becomes
electrically charged. An example is the alpha particle.
ion exchange: A chemical process involving reversible interchange of
ions between a liquid and a solid but no radical change in
structure of the solid.
leaching: The removal of materials in solution from the soil.
lysimeter: A device for measuring percolation and leaching losses from
a column of soil under controlled conditions.
manifold: A pipe fitting with numerous branches to convey fluids be-
tween a large pipe and several smaller pipes, or to permit choice
of diverting flow from one of several sources or to one of several
discharge points.
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mapping unit: A soil or combination of soils delineated on a map and,
where possible, named to show the taxonomic unit or units included.
Principally, mapping units on maps of soils depict soil types,
phases, associations, or complexes.
medium texture: The texture exhibited by very fine sandy loams, loams,
silt loams, and silts.
mineral soil: A soil consisting predominantly of, and having its pro-
perties determined by, mineral matter. Usually contains <20
percent organic matter, but may contain an organic surface layer up
to 30 cm thick.
mineralization: The conversion of an element from an organic form to an
inorganic state as a result of microbial decomposition.
mineralogy, soil: In practical use, the kinds and proportions of miner-
als present in soil.
mixed liquor suspended solids (MLSS): Suspended solids in a mixture of
activated sludge and organic matter undergoing activated sludge
treatment in the aeration tank.
montmorillonite: An aluminosilicate clay mineral with a 2:1 expanding
structure; that is, with two silicon tetrahedral layers enclosing
an aluminum octahedral layer. Considerable expansion may be caused
by water moving between silica layers of contiguous units.
mottling: Spots or blotches of different color or shades of color in-
terspersed with the dominant color.
nitrification: The biochemical oxidation of ammonium to nitrate.
organic nitrogen: Nitrogen combined in organic molecules such as pro-
teins, amino acids.
organic soil: A soil which contains a high percentage (>15 percent or
20 percent) of organic matter throughout the sol urn.
particle size: The effective diameter of a particle usually measured by
sedimentation or sieving.
particle-size distribution: The amounts of the various soil separates
in a soil sample, usually expressed as weight percentage.
pathogenic: Causing disease. "Pathogenic" is also used to designate
microbes which commonly cause infectious diseases, as opposed to
those which do so uncommonly or never.
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ped: A unit of soil structure such as an aggregate, crumb, prism,
block, or granule, formed by natural processes (In contrast with a
clod, which is formed artificially).
pedon: The smallest volume (soil body) which displays the normal range
of variation in properties of a soil. Where properties such as
horizon thickness vary little along a lateral dimension, the pedon
may occupy an area of a square yard or less. Where such a property
varies substantially along a lateral dimension, a large pedon sev-
eral square yards in area may be required to show the full range in
variation.
percolation: The flow or trickling of a liquid downward through a con-
tact or filtering medium. The liquid may or may not fill the pores
of the medium.
permeability, soil: The ease with which gases, liquids, or plant roots
penetrate or pass through soil.
pH: A term used to describe the hydrogen-ion activity of a system.
plastic soil: A soil capable of being molded or deformed continuously
and permanently, by relatively moderate pressure, into various
shapes. See consistence.
platy structure: Soil aggregates that are developed predominantly along
the horizontal axes; laminated; flaky.
settleable solids: That matter in wastewater which will not stay in
suspension during a preselected settling period, such as one hour,
but either settles to the bottom or floats to the top.
silt: (1) A soil separate consisting of particles between 0.05 and
0.002 mm in diameter. (2) A soil textural class.
single-grained: A nonstructural state normally observed in soils con-
taining a preponderance of large particles, such as sand. Because
of a lack of cohesion, the sand grains tend not to assemble in ag-
gregate form.
siphon: A closed conduit a portion of which lies above the hydraulic
grade line, resulting in a pressure less than atmospheric and re-
quiring a vacuum within the conduit to start flow. A siphon uti-
lizes atmospheric pressure to effect or increase the flow of water
through the conduit.
slope: Deviation of a plane surface from the horizontal.
soil horizon: A layer of soil or soil material approximately parallel
to the land surface and differing from adjacent genetically related
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layers in physical, chemical, and biological properties or charac-
teristics such as color, structure, texture, consistence, pH, etc.
soil map: A map showing the distribution of soil types or other soil
mapping units in relation to the prominent physical and cultural
features of the earth's surface.
soil morphology: The physical constitution, particularly the structural
properties, of a soil profile as exhibited by the kinds, thickness,
and arrangement of the horizons in the profile, and by the texture,
structure, consistence, and porosity of each horizon.
soil separates: Groups of mineral particles separated on the basis of a
range in size. The principal separates are sand, silt, and clay.
soil series: The basic unit of soil classification, and consisting of
soils which are essentially alike in all major profile characteris-
tics, although the texture of the A horizon may vary somewhat. See
soil type.
soil solution: The aqueous liquid phase of the soil and its solutes
consisting of ions dissociated from the surfaces of the soil par-
ticles and of other soluble materials.
soil structure: The combination or arrangement of individual soil par-
ticles into definable aggregates, or peds, which are characterized
and classified on the basis of size, shape, and degree of distinct-
ness.
soil suction: A measure of the force of water retention in unsaturated
soil. Soil suction is equal to a force per unit area that must be
exceeded by an externally applied suction to initiate water flow
from the soil. Soil suction is expressed in standard pressure
terms.
soil survey: The systematic examination, description, classification,
and mapping of soils in an area.
soil texture: The relative proportions of the various soil separates in
a soil.
soil type: In mapping soils, a subdivision of a soil series based on
differences in the texture of the A horizon.
soil water: A general term emphasizing the physical rather than the
chemical properties and behavior of the soil solution.
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solids: Material in the solid state.
total: The solids in water, sewage, or other liquids; includes
suspended and dissolved solids; all material remaining as residue
after water has been evaporated.
dissolved: Solids present in solution.
suspended: Solids physically suspended in water, sewage, or other
liquids. The quantity of material deposited when a quantity of
water, sewage, or liquid is filtered through an asbestos mat in a
Gooch crucible.
volatile: The quantity of solids in water, sewage, or other liquid
lost on ignition of total solids.
solids retention time (SRT): The average residence time of suspended
solids in a biological waste treatment system, equal to the total
weight of suspended solids in the system divided by the total
weight of suspended solids leaving the system per unit time
(usually per day).
subsoil: In general concept, that part of the soil below the depth of
plowing.
tensiometer: A device for measuring the negative hydraulic pressure (or
tension) of water in soil in situ; a porous, permeable ceramic cup
connected through a tube to a manometer or vacuum gauge.
tension, soil water: The expression, in positive terms, of the negative
hydraulic pressure of soil water.
textural class, soil: Soils grouped on the basis of a specified range
in texture. In the United States, 12 textural classes are recog-
nized.
texture: See soil texture.
tight soil: A compact, relatively impervious and tenacious soil (or
subsoil), which may or may not be plastic.
Total Kjeldahl Nitrogen (TKN): An analytical method for determining
total organic nitrogen and ammonia.
topsoil: (1) The layer of soil moved in cultivation. (2) The A hori-
zon. (3) The Al horizon. (4) Presumably fertile soil material
used to topdress roadbanks, gardens, and lawns.
uniformity coefficient (UC): The ratio of that size of grain that has
60% by weight finer than itself, to the size which has 10% finer
than itself.
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unsaturated flow: The movement of water in a soil which is not filled
to capacity with water.
vapor pressure: (1) The pressure exerted by a vapor in a confined
space. It is a function of the temperature. (2) The partial pres-
sure of water vapor in the atmosphere. (3) Partial pressure of any
liquid.
water table: That level in saturated soil where the hydraulic pressure
is zero.
water table, perched: The water table of a discontinuous saturated zone
in a soil.
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EXAMPLES OF SOIL MOTTLING (EXAMPLES A, B & C INDICATE
SEASONAL SOIL SATURATION, EXAMPLE D DOES NOT)
(A)
Extremely Prominent Mottling
in a Clayey Soil
(B)
Mottling in a Loamy Soil
(C)
Mottling in a Sandy Soil
(D)
Mottling Inherited
from Geologic Processes
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