EPA 625/l-81-013a
PROCESS DESIGN MANUAL
FOR
LAND TREATMENT OF
MUNICIPAL WASTEWATER
SUPPLEMENT
ON
RAPID INFILTRATION
AND
OVERLAND FLOW
U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
OFFICE OF RESEARCH AND DEVELOPMENT
October 1984
Published by.
U. S. Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, Ohio 45268
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FOREWORD
This document is intended as a supplement to the 1981
Technology Transfer Process Design Manual for Land Treatment of
Municipal Wastewater, (EPA 625/1-81-013). Throughout this
document, the 1981 Process Design Manual will be referred to as
the Manual. Part I in this text covers design of rapid
infiltration systems and Part II discusses overland flow
systems. A substantial amount of new information on both _ the
concepts and their performance has been developed since
1980-81.
Part I on rapid infiltration provides additional guidance and
detail on planning, design, construction, and operation of
rapid infiltration systems to avoid problems which have been
observed at a few recently constructed systems. The basic
criteria in the 1981 Manual are still valid and are not
repeated in this text.
Part II on overland-flow supplements the 1981 design basis with
information from operating municipal facilities, as well as
additional results from research studies completed in the
period December 1980 to March 1984. This information
reinforces and strengthens the basis for this rapidly
developing technology.
11
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ACKNOWLEDGEMENTS FOR PART I
Preparation of Part I oil rapid infiltration systems has
involved the participation of a number of individuals. There
were a team of. authors, another group of individuals who
deserve special recognition for their contributions and
technical assistance, and a group of invited experts who
reviewed the document and provided comments and, suggestions for
its improvement. Mr. Sherwood C. Reed of USACRREL was the
principal author of Part I and coordinated the activities of
the other authors and groups. Dr. James E. Smith Jr., EPA
CERI was Project Officer of this task.
Authors
Reed, S. C.; USACRREL
Wallace, A. T.; University of Idaho
Bouwer, H.; USDA
Enfield, C.; USEPA
Stein, C.; Metcalf & Eddy
Thomas, R.; USEPA
Special Contributions and Technical Assistance
Allen, D.; New Hampshire WSPCC
Aulenbach, D; Rensselaer Polytechnic Institute
Crites, R.; G. S. Nolte & Associates
Dornbush, J.; South Dakota State University
Leach, L.; USEPA
Linstedt, R. D.; Black & Veatch
Rittershaus, P.; Banner Associates
Sylvester, K.; ERM, Inc.
Review Group
Ariail, D.; USEPA
Dean, R.; USEPA
Deemer, D. D.; ERM-Southeast, Inc.
Gilbert, T.; Wisconsin DNR
Hill, G.; Wisconsin DNR
Larson, R.; Wisconsin DNR
Leach, L.; USEPA
Madancy, R.; OWR-DOI
Morgan, G.; USDA
Nickleson, M.; USAEHA
Opfer, W.; U. S. Forest Service
Pycha, C.; USEPA
Schochenmaier, L.; South Dakota DWNR
Smith, E.; USACERL
Stark, S.; Minnesota Pollution Control Agency
Troxler, R.; Georgia DNR
iii
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ACKNOWLEDGEMENTS FOR PART II
Part II was developed with the assistance of a number of
individuals. In addition to a team of authors, another group
of experts were invited to review the document and provide
comments and suggestions for its improvement. Certain of
these reviewers also provided additional contributions and/or
technical assistance. Mr. D. Donald Deemer was the
principal author of Part II and coordinated the activities of
the other authors and reviewers. Dr. James E. Smith, Jr.,
EPA, CERI was the Project Officer for the development of this
document.
Authors
Deemer, D. D.; ERM-Southeast, Inc.
Thomas, R. E.;r USEPA
Reed, S. C.; USACRREL
Witherow, J. L.; USEPA
Morgan, G.; USDA, FHA
Special Contributions and Technical Assistance
Abernathy, A. R.; Clemson University
Ariail, D.; USEPA
Clark, P. J.; Clark Engineers
Ford, J. P.; ERM, Inc.
Parmelee, D. M. ; Parmelee Incorporated
Smith, R. G.; University of California-Davis
Review Group
Aly, O. M.; Campbell Soup Company
Bledsoe, B. E.; USEPA
Crites, R. W.; G. S. Nolte & Associates
Gilde, L. C.; Campbell Soup Company
Lee, C. R.; USAEWES
Jones, A. A.,; USEPA
Opfer, W. J.; USDA, Forest Service
Overman, A. R.; University of Florida
Sutherland, J. C.; EDI Engineering & Science
Zirschky, J.; Clemson University
IV
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CONTENTS
Page
FOREWORD
ACKNOWLEDGEMENTS FOR PART I
ACKNOWLEDGEMENTS FOR PART II
CONTENTS
11
iii
iv
v
PART I. RAPID INFILTRATION
Chapter
1 INTRODUCTION
1.1 Background
1.2 Objective and Scope
1.3 References
2 PLANNING AND SITE SELECTION
2.1 General
2.2 Preliminary Screening
2.2.1 Land Area Requirements
2.2.2 Economic Factors
2.2.3 Regulatory Factors
2.2.4 Site Characteristics
2.3 Site Selection
2.4 References
3 ' SITE INVESTIGATION
3.1 General
3.2 Site Evaluation
3.3 Soils Investigation
3.3.1 Soil Texture
3.3.2 Soil Structure
3.3.3 Soil Color
3.3.4 Field Procedures
3.3.4.1 Test Borings
3.3.4.2 Test Pits
3.3.5 Laboratory Procedures
3.3.6 Evaluation of Soil Test Results
3.4 Ground Water Investigations
3.5 Infiltration and Permeability Tests
3.5.1 Infiltration Test Procedures
and Evaluation
3.5.1.1 Flooding Basin Test
3.5.1.2 Air Entry Permeameter
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1
4
5
5
5
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7
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3.5.2 Permeability Tests
3.6 References
25
26
DESIGN
4.1 General 28
4.2 Hydraulic Loading and Basin Area 28
4.2.1 Land Area Required 31
4.2.2 Wet/Dry Ratio 31
4.2.3 Application Rate 31
4.2.4 Infiltration of Standing Water 34
4.3 Subsurface Flow and Ground Water Mounding 36
4.3.1 Subsurface Flow , ,37
4.3.2 Ground Water Mounding 39
4.4 RI Basin Configuration and Application
Scheduling 42
4.5 Design Requirements for Winter Operation in
Cold Climates 44
4.6 Design of Seepage Ponds : 47
4.7 Design of Physical Elements 47
4.7.1 Dikes 47
4.7.2 Basins in Fine Textured Soils 48
4.7.3 Inlet, Distribution, and Transfer
Structures 48
4.7.4 Flow Control 49
4.8 RI System Performance 49
4.8.1 Removal of Toxic Organics 49
4.8.2 Nitrogen Management in RI Systems 50
4.8.3 Disinfection in RI Systems 52
4.9 References , ^3
CONSTRUCTION
5.1 General
5.2 Infiltration Surfaces in RI Basins
5.2.1 Cut and Fill in Coarse Soils
5.2.2 Cut and Fill in Fine Textured Soils
5.2.3 Ridge and Furrow Construction
5.3 Dike Construction
5.4 Control of Water
5.5 References
58
58
58
59
60
61
61
61
OPERATION
6.1 General
6.2 Wastewater Scheduling
6.3 Maintenance of Infiltration Surfaces
6.4 Monitoring
6.5 Winter Operations
6.6 References
62
62
63
64
64
66
VI
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Figure
1-1 Actual RI Loading Rates versus Design
Requirements
3-1 Moisture Content/Compaction versus
Permeability for a Clay Soil
3-2 Permeability versus Void Ratio
3-3 Typical Subsurface Profile
4-1 Basin Layout for Example Calculation
4-2 Subsurface Profile Section C-C for
Example Calculation
Table
1-1 Rapid Infiltration - Potential Problems
2-1 Economic Rating Factors for Site Suitability
3-1 Soil Characteristics in Field Descriptions
3-2 Textural Properties of Mineral Soils
3-3 Vertical Saturated Permeabilities
from Figure 3-2
4-1 Design Factors for Hydraulic Loading
4-2 Porosity of Selected Soils
14
19
21
43
43
3
6
11
13
20
29
35
PART II. OVERLAND FLOW
Chapter
Page
INTRODUCTION
1.1 Background
1.2 Objectives
1.3 References
CURRENT STATUS
2.1 Introduction
2.2 Site Selection
67
67
68
69
69
VI1
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2.2.1 Process Description 69
2.2.2 Soil Permeability 69
2.3 Process Design 70
2.3.1 Design Criteria 70
2.3.2 Preapplication Treatment 70
2.3.3 Storage 71
2.3.4 Distribution Systems 71
2.4 Terrace Design and Construction 71
2.5 Vegetation Selection and Establishment 71
2.6 References 72
DESIGN CRITERIA
3.1 Procedures 74
3.2 Empirical Method 74
3.2.1 Revised Criteria • •74
3.2.2 Use of Design Ranges 76
3.2.2.1 Selection of Hydraulic
Loading Rate 76
3.2.2.2 Determination of Land Area 77
3.2.2.3 Selection of Terrace Length 77
3.2.2.4 Selection of Application Period 78
3.3 Rational Design Procedure 78
3.3.1 Introduction • 78
3.3.2 Procedure 79
3.3.3 Example 79
3.4 References 83
PREAPPLICATION TREATMENT
4.1 Areas of Importance 85
4.2 Level of Preapplication Treatment 85
4.3 Solids Removal 86
4.4 Algal Interference . 86
4.5 References 86
i
STORAGE
5.1 Current Practice 88
5.2 Cold Weather Storage Requirements 89
5.3 Rainfall Considerations ,89
5.3.1 Impact on Effluent BOD Concentration 91
5.3.2 Impact on Effluent Suspended Solids
Concentration , ' 91
5.3.3 Mass Discharges , 91
5.3.4 Recommended Operating Practices 92
5.4 Storage Reservoir Design 92
5.5 References 92
DISTRIBUTION SYSTEMS
6.1 Selection 94
Vlll
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8
6.2 Surface Methods
6.2.1 Gated Pipe
6.2.2 Slotted or Perforated Pipe
6.2.3 Bubbling Orifices
6.3 Low Pressure Sprays
6.4 Sprinklers
6.5 Sizing of the Distribution System
6.6 Controls
6.6.1 Automatic Valves
6.6.2 Manual Valves
6.7 References
TERRACE DESIGN AND CONSTRUCTION
7.1 Importance of Proper Construction
7.2 Design Methods
7.3 Terrace Configurations
7.3.1 Conventional Terraces
7.3.2 Step-Up Terraces
7.3.3 Back-to-Back Terraces
7.3.4 Step-Down Terraces
7.3.5 Transitions
7.4 Drainage Channels
7.4.1 Design
7.4.2 Runoff Computation
7.4.3 Channel Types
7.4.4 Avoiding Erosion Problems
7.5 Land Grading
7.5.1 Rough Grading
7.5.2 Topsoil Handling
7.5.3 Final Grading
7.6 Supervision and Acceptance
7.7 References
VEGETATION SELECTION AND ESTABLISHMENT
8.1 Function
8.2 Selection
8.2.1 Objectives
8.2.2 Grass Types
8.2.2.1 Reed Canary Grass
8.2.2.2 Bermuda Grass
8.2.2.3 Nurse Crop
8.2.3 Combining Grass Species
8.3 Planting
8.3.1 Density
8.3.2 Scheduling
8.3.3 Soil Preparation
8.3.4 Seeding
8.3.5 Sprigging
8.4 Vegetation Development
8.4.1 Start-up Irrigation
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118
IX
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8.4.1.1 Methods
8.4.1.2 Procedures
8.4.2 Initial Management
8.5 Erosion Control
8.6 References
118
119
120
121
121
Figure
3-1
3-2
5-1
6-1
7-1
7-2
BOD_ Fraction Remaining versus Distance Down
Slope with Screened Raw Wastewater 81
BOD_ Fraction Remaining versus Distance Down
Slope with Primary Effluent 82
Recommended Storage Days for Overland Flow
Systems 90
Alternative Sprinkler Configurations for
Overland Flow Distribution 98
Types of Overland Flow Terraces 103
Types of Drainage Channels 106
Table
3-1
6-1
8-1
8-2
Suggested Overland Flow Design Ranges 75
Summary of Overland Flow Distribution Methods 95
Suitable Overland Flow Grasses 113
Seeding Density for Overland Flow Systems 116
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PART I. RAPID INFILTRATION
CHAPTER 1
INTRODUCTION
1.1 Background
Rapid infiltration (RI) is a ,very successful and cost-effective
method for wastewater management. RI systems can be designed
for wastewater treatment; or as one component in a waste re-use
plan involving recovery of the treated water and/or - storage in
an aquifer. Municipal RI systems have been in successful
operation in the United States for up to 100 years. In 1981
there were about 320 municipal RI systems in the United States,
either operating or under construction. Some 30 percent of
these systems have been implemented since 1971.
The concept depends on a relatively high rate of wastewater
infiltration into the soil followed by rapid percolation,
either vertically or laterally, away from the application
point. The best soils are relatively coarse textured, with
moderate to rapid permeabilities. In practice, wastewater
application rates have ranged from about 15 m/yr to 120 m/yr
(50-400 ft/yr) for successful RI systems. For contrast, the
typical loading on an agricultural system using wastewater
might be 1 to 2 m/yr (3-6 ft/yr) and the maximum loading
typically permitted for an on-site septic tank leachfield
system is about 29 m/yr (95 ft/yr). Figure 1-1 compares the
actual hydraulic loading at several successfully operating RI
systems to the design range recommended by the 1981 Technology
Transfer Process Design Manual [1].
The operational context for RI systems is typically earthen
basins designed for a repetitive cycle of flooding,
infiltration/percolation, and drying. The wet and dry periods
designed for these cycles are a function of wastewater and soil
characteristics, climatic conditions, and treatment goals. The
percolate from cyclic RI systems is typically suited for
indirect re-use for many purposes. If this percolate emerges
in an adjacent surface water it should be of a high quality,
equivalent to that obtained with complex and costly advanced
waste treatment (AWT) systems.
1.2 Objective and Scope
The Manual [1] includes basic criteria and information on the
design, construction and performance of municipal RI systems.
A large number of successfully operating systems have been
designed using the Manual [1] (and/or its first edition
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Range Suggested
for Design
0.2 0.5 1.5 5 15 50
Effective Hydraulic Conductivity of Soil (cm/hr)
Locations: A-Pilot Basins, Phoenix, AZ
B-Lake George, NY
C-Ft. Devens, MA
D-Boulder, CO
E-Hollister,CA
F-Corvallis, MT
G-East Glacier, MT
H-Jackson, WY
I -Eagle, ID
FIGURE 1-1
ACTUAL RI LOADING RATES VERSUS DESIGN REQUIREMENTS
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TABLE 1-1
RAPID INFILTRATION - POTENTIAL PROBLEMS
Soils
Ground Water
Layers or zones of less permeable
soils not revealed during site
investigation which impede water
movement.
Field testing conducted at dif-
ferent location or different
depth than the final system, so
design may be based on inappropri-
ate data.
Unexpected high seasonal ground
water table which Interferes with
subsurface water movement.
Inadequate capacity to move water
away from the site later-
ally or vertically in the time
allowed by design.
Final surface layer in infiltra-
tion area contains significant
clay or silt. These "fines" may
segregate during flooding, re-
settle on the surface 'and impede
future water movement.
The subsurface flow from one
basin influences the capacity
of an adjacent basin.
Design Assumptions
Construction
Less than design capacity for
water movement because back-
fill operations have reduced
soil permeability (see related
note under Construction).
Actual wastewater characteristics
(algae, suspended solids) differ-
ent than assumed.
Design based on improper use of
criteria or inadequate infil-
tration measurements.
Design ignores potential for
freezing during winter oper-
ations in cold climates.
Excess traffic and inadvertent
compaction of final basin
surfaces.
Failure to remove all of fine
soils in surface layers or zones
of unacceptable soils.
Construction activity in the
basin area when soil moisture
is too high.
Rainfall sorting of fines into
layers of low permeability during
fill operations.
Inadequate specifications for
earthwork and infiltration surface
preparation.
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published in 1977). However, there are a few recently
constructed systems, in a variety of locations in the United
States, that are not meeting all design expectations,
particularly with respect to the capability to infiltrate and
then percolate wastewater at the design rates. A preliminary
analysis of these systems indicated that the observed problems
could be grouped as shown in Table 1-1.
The basic criteria in the Manual [1] are valid and if properly
applied will result in successful performance. It is not the
intent of this supplement to repeat all of that basic
information. This supplement provides additional guidance and,
where necessary, a stronger emphasis and more specific detail
on critical aspects so that the problems listed in Table 1-1
can be avoided in future designs. In addition, this supplement
presents information developed since 1981 on performance of RI
systems with respect to nitrogen removal, organics removal, and
the need for disinfection.
1.3 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA, Center
for Environmental Research Information, Cincinnati, OH.
October 1981.
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CHAPTER 2
PLANNING AND SITE SELECTION
2.1 General
The -basic procedures and criteria for planning and site
selection are described in detail in Chapter 2 of the Manual
[1] and can also be found in other sources [2, 3, 4, 5].
2.2 Preliminary Screening
The initial procedure to identify suitable sites can be a desk
top analysis of existing information. Numerical rating
procedures are described in references [1, 3, 4] and include
consideration of soil and ground water conditions, grades,
existing land use, and flood potential. However, the
procedures in the Manual [1] do not take into account the
economic factors related to pumping from the community to the
RI site.
2.2.1 Land Area Requirements
At the planning stage it is necessary to make a preliminary
estimate of the land area that will be required for the
treatment portion of the RI system using Equation 2-1 (the
basic equation is in the Manual [13, but is modified here for
the operating period).
A = .(.1^9 HOI
(2-1)
where: A = field area for treatment, ha
Q = design daily flow, m /d
L = annual design percolation rate, m/yr
P = period of operation, wk/yr
Since RI systems typically operate on a year-round basis,
Equation 2-1 usually becomes:
_ (0.0365HQ)
~ (175
(2-2)
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Figure 2-3 in the Manual [1] relates the clean water
permeability of the most restricting layer in the soil profile
to the design wastewater percolation rate. That figure can be
used to estimate the L value for use in equations 2-1 and 2-2
above. Figure 2-3 in the Manual [1] is only valid for these
preliminary estimates and is not intended for final RI designs.
The Manual Ql]clearlystatesthisprecaution,butTts mis-use
for final designs has resulted in problems.
2.2.2 Economic Factors
Table 2-1 summarizes the economic rating factors [4] that can
be used in conjunction with the technical factors already in
Chapter 2 of the Manual [1] to evaluate the influence of
distance and elevation on site suitability. Both pumping
distance and elevation difference can have a significant
influence on cost-effectiveness of site development. In
general, a site within 10-12 km (6-7 mi) of the community may
still be competitive with other alternatives.
TABLE 2-1
ECONOMIC RATING FACTORS FOR SITE SUITABILITY [4]
Characteristic
Rating Value
Distance from wastewater source, km
0- 3
3- 8
8-16
> 16
Elevation difference, m
< 0
0-15
15-60
> 60
8
6
3
2
6
5
3
1
km x 0.6214 = miles
m x 3.281 = ft
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2.2.3 Regulatory Factors
Although the percolate from RI systems is of high quality, it
may not in some cases satisfy all factors i state ground water
quality requirements. In these situations, there are two
options:
Design for percolate recovery, via underdrains
wells, for subsequent re-use or surface discharge.
or
Demonstrate via a hydrogeological survey and analysis
that all of the percolate will emerge as base flow in
an adjacent surface water or otherwise not adversely
impact the in situ aquifer.
2.2.4 Site Characteristics
Special emphasis is necessary on site topography, and soil type
and soil uniformity as indicated by the soils and topographic
maps. Extensive cut and fill can significantly increase costs
for RI construction. Thus, sites with significant and numerous,
changes in relief over a small area are not the best choice.
As described in greater detail in Section 3.3.6, a significant
clay fraction (>10%) in the soil would generally exclude RI
construction on fill using such materials. Therefore, sites
with that type of soil and a topography that would require fill
construction are usually eliminated during the preliminary
screening process. Extremely non-uniform soils over the site
area do not absolutely preclude development for an RI system,
but they significantly increase the cost and complexity of site
investigation.
Most of the common field tests are only valid for a relatively
small area or shallow depth. Where extreme soil variability is
shown on the soils maps, it is generally best to consider other
sites. The alternative may be to construct a large-scale pilot
cell to define the hydraulic characteristics of the site. If
the tests are successful, the pilot cell can then be
incorporated into the full-scale system. The layout and
construction of the remainder of the system then requires
special care on the part of the designer and careful
coordination with those responsible for construction.
2.3 Site Selection
It is costly to conduct the extensive field investigations
required for a final design at multiple sites. It is
appropriate to make a final screening and numerical rating of
sites to verify feasibility for a single site, or to determine
which of several sites should be the focus of detailed field
investigations. Procedures discussed in Section 2.2 above can
be used for this purpose.
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2.4 References
1. U. S. Environmental Protection Agency. Technology Transfer
Process Design Manual for Land Treatment of Municipal
Wastewater. EPA 625/1-81-013. U. S. EPA, Center for
Environmental Research Information, Cincinnati, OH.
October 1981.
2. Reed, S. C. Design,. Operation and Maintenance of Land
Application Systems for Low Cost Wastewater Treatment.
In: Proceedings of Workshop on Low Cost Wastewater
Treatment. Clemson University, Clemson SC. April 1983.
3. Ryan, J. R. and R. C. Loehr. Site Selection Methodology
for the Land Treatment of Wastewater. USACRREL SR 81-28.
U. S. Army Corps of Engineers, Cold Regions Research and
Engineering Laboratory, Hanover, NH. November 1981.
4. Reed, S. C. and R. W. Crites. Handbook of Land Treatment
Systems for Industrial and Municipal Wastes. Noyes Data
Corp., Park Ridge, NJ. 1984.
5. U. S. Environmental Protection Agency. Generic Facilities
Plan for a Small Community: Stabilization Pond, Land
Treatment, or Trickling Filter. FRD 26. U. S. EPA, Office
of Water Program Operations, Washington, D. C. September
1982.
6. Large, D. W. Land Application of Wastewater and State
Water Law, Vol. I and Vol. II. U. S. Environmental
Protection Agency. EPA 600/2-77-232 and EPA 600/2/78-175.
U. S. EPA, Office of Water Program Operations, Washington,
D. C. August 1978.
8
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CHAPTER
SITE INVESTIGATION
3.1 General
A basic requirement for all engineering designs is that the
procedure will be used with valid data and information. For RI
systems, that information comes from a properly conducted field
investigation, coupled with the careful interpretation ' of the
field data by the designer. Inadequacies in these two areas
are the most common factors associated with problems in RI
systems.
Problems that should not have arisen are listed on Table 1-1.
These problems can be avoided, and it is the purpose of this
section to provide additional guidance and detail to assist in
RI site investigation. Several .requirements deserve special
emphasis:
. It is essential for the final field testing to be
conducted on the actual site and at the actual depth in
the soil profile intended for the RI system. A series
of tests may, therefore, be required as the design is
refined and the final basin configuration determined.
Extrapolation of data from nearby sites is not an
acceptable basis for design.
. Field investigation and testing can be expensive.
Cost-effectiveness and reliable results are only insured
if the investigative program is planned and conducted by
persons familiar with soils and ground water testing, as
well as having a complete understanding of the RI
concept and the design expectations regarding water
movement.
. Interpretation of field test results also requires
competence in soils, hydrogeology, and a thorough
understanding of the RI process. If suitable in-house
expertise does not exist, outside assistance is
necessary.
3.2 Site Evaluation
The first step in the site investigation involves confirmation
of the feasibility of RI for the selected location. These
procedures are in addition to the screening and selection
guidance presented in Chapter 2. Several components are
included in this more detailed phase of the evaluation:
9
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Field examination of exposed soil profiles (on and
near the site) to include road cuts, borrow pits, and
plowed fields.
Field observation of ground water indicators: Wet
spots, seepage areas, vegetation changes, ponds and
streams, and general drainage characteristics such as
standing water after rainfall.
Backhoe test pits, to a 3 m (10 ft) depth where soil
conditions permit, in the major soil types on the
site. Soil samples from critical layers (especially
the layer being considered for the future basin
surface) are collected and reserved for future
testing. Ground water seepage into the pit is
observed and the highest water level obtained
recorded. Where ground water is encountered, a
temporary water level observation tube {5 cm (2 in.)
PVC pipe with diagonal hacksaw slots} can be
installed as the pit is backfilled.
The water level in any on-site or adjacent wells is
recorded, in addition to data from any observation
tubes installed as part of step 3 above. The
elevations for all of these observation points are
obtained, as well as the elevations of adjacent
surface waters, and a preliminary water table map
prepared. , The evaluation of these data includes
consideration of potential seasonal high water
levels.
The data obtained from steps 1-4 above allow
preliminary definition of:
a. General hydrological setting
b. Soil descriptions and water table locations
c. Proposed soil layer for RI basins
d. Flow direction, depth, and discharge areas for
ground Water and the re-charge characteristics
for the site
e. Possible site modifications including fill or
excavation, underdrains, or control of natural
ground water flow
Evaluation of the data in step 5 results in one of
three conclusions for the site:
a. Site is suitable; proceed with further detailed
field testing.
b. Site may be suitable with modifications; proceed
with additional field testing and analysis.
10
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c. Site is unsuitable for RI due to factors not
revealed in preliminary screening and site
selection; no further testing or analysis is
necessary.
7. Ground water samples are taken at a later stage of
the detailed site investigation and analyzed for
chemical and biological constituents to establish the
background characteristics of the aquifer.
3.3 Soils Investigation
The soils investigation includes field and laboratory testing
and observations based on borings and test pits. Table 3-1
summarizes the factors that are included in the field
description of all soil samples obtained from test pits and
borings.
TABLE 3-1
SOIL CHARACTERISTICS IN FIELD DESCRIPTION
Characteristics
Significance
5-
6.
Estimate of percent cobbles,
gravel, sand, and fines
Plasticity of fines
Major textural class
Soil color
Wetness and consistency
Structural characteristics
stratigraphy, and geologic
origin
Influences permeability
Permeability and influence
on cut or fill construction
Permeability
Presence of minerals, indi-
cation of seasonal groundwater
Drainage characteristics
Ability to move water verti-
cally and laterally.
11
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3.3.1 Soil Texture
It is obvious from the list in Table 3-1 that some prior soils
experience is essential for an effective and accurate field
description of soil samples. Table 3-2 gives an indication of
soil type and texture based on its feeling and appearance in
the field.
A moderately coarse-textured soil is the best choice for RI.
The field investigator still looks for layers or lenses in the
soil profile that could restrict water movement. Fine-textured
soils, and even sandy soils with a significant silt or clay
content (>10%), are not desirable. Soils in this category tend
to have relatively low in situ permeabilities. In addition,
the remolding of clay during construction activities for either
cuts or fills can reduce the permeability to an unacceptable
level C3]. Figure 3-1, a typical density versus moisture
content plot for a clayey soil, illustrates this loss of
permeability.
Point A on the lower curve might represent typical in situ
characteristics in the soil profile. Point A1 represents the
possible results of construction activity when higher moisture
contents might prevail. As shown by the figure, the soil
density of A1 is essentially the same as at point A, but the
permeability has been reduced by an order of magnitude by the
remolding of the clay. If the site topography requires
placement of fill for infiltration surface construction, clayey
soils are not recommended. Experience has shown that clayey
sands and clays exceeding 10% were not successful when used in
fill construction for the infiltration surfaces in RI basins
C3]. Clayey material can also be a problem for basins entirely
within cut sections and when used as fill material for dikes or
embankments. These aspects are discussed in Chapters 4 and 5.
3.3.2 Soil Structure
Soil structure refers to the aggregation of soil particles into
clusters of particles called peds. Well structured soils with
large voids between peds will transmit water more rapidly than
structureless soils of the same texture. Fine-textured soils
which are well structured, with strong peds, can be used for RI
systems. Field investigation of such sites require testing to
insure that the soil structure will not be destroyed by
application of wastewater. Structure, is best observed and
evaluated in the sidewalls of a test pit.
12
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TABLE 3-2
TEXTURAL PROPERTIES OF MINERAL SOILS [2]
Soil
Class
Feeling and Appearance
Dry Soil
Moist Soil
Sand Loose, single grains which
feel gritty. Squeezed in
the hand, the soil mass
falls apart when the pres-
sure is released.
Sandy Aggregates easily crushed;
Loam very faint velvety feeling
initially, but with continued
rubbing the gritty feeling
of sand soon dominates.
Loam 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.
Silt Aggregates are firm, but may
Loam be crushed under moderate
pressure. Clods are firm to
hard. Smooth, flour-like
feel dominates when soil is
pulverized.
Clay Very firm aggregates and
Loam hard clods that strongly
resist crushing by hand.
When pulverized, the soil
takes on a somewhat gritty
feeling due to the harsh-
ness of the very small
aggregates which persist.
Clay 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.
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 with-
out breaking. Slight tendency
to ribbon between thumb and
forefinger. Rubbed surface has
a broken or rippled appearance.
Case can bear much handling
without breaking. Pinched be-
tween 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 be-
tween thumb and forefinger
and retains its plasticity
when elongated. Rubbed surface
has a very smooth, satin feel-
ing. Sticky when wet and easily
puddled.
13
-------
Id7
E
o
.Q
O
0>
i_
0>
Q_
TJ
Q)
2
3
"S
cn
10
-8
10
I I
1900
10 \2 14
Moisture Content (%)
16
Point A: Density=2000 kg/m3, Moisture=9%, k=2 x 10'8 cm/sec
Point A': Density=2000 kg/m3, Moisture=13.5%, k=2 x 10'9 cm/sec
FIGURE 3-1
MOISTURE CONTENT/COMPACTION VERUS PERMEABILITY FOR A CLAY SOIL (3)
14
-------
3.3.3 Soil Color
The color and color patterns in a soil profile are a good
indicator of the drainage characteristics of the soil [4].
Reds, yellows, and yellow browns are representative of well
oxidized conditions; an indication of well-aerated,
non-saturated soils. More subdued shades and grays and blues
predominate if insufficient oxygen is present when saturated
conditions prevail. Imperfectly drained or seasonally
saturated soils show alternate streaks or pockets of oxidized
and reduced soil elements. This condition is termed soil
mottling. The observation of mottled soils in the test pit
walls is evidence of saturated conditions at that level at some
time in the past.
Some color photographs of soil mottling are included in
reference [2]. Experience is still required for accurate
interpretation since some mottling may have occurred in
geological time and may have no reference to the current status
of the soil profile, or mineral or organic stains may be
mistaken for mottles. However, any mottles are suspect and
should be taken as a preliminary indicator of seasonal high
ground water conditions.
3.3.4 Field Procedures
Field procedures for soils investigations include borings and
test pits. The practical depth for most test pits is about 3 m
(10 ft), so borings are required for deeper exploration and for
confirmation in the areas between test pits. Samples are
obtained from borings for soil identification and undisturbed
samplings when necessary for physical and mechanical testing.
3.3.4.1 Test Borings
Locating test borings in the areas of the site that are
actually to be used for RI basin construction is essential. At
least one boring is recommended in every major soil type on the
site. With generally uniform conditions, one boring for every 1
to 2 ha (2-5 ac) for continuous areas of up to 20 ha (50 ac)
would suffice for large scale systems. Small scale systems ( <5
ha) should consider 4-6 shallow borings spaced over the entire
site. All borings typically penetrate to below the water table
if it is within 10 to 15 m (30-50 ft) of the ground surface. A
few borings should extend all the way through the saturated
zone, if possible, to define the thickness of the aquifer for
use in subsurface flow calculations. The drilling methods_most
commonly used are hollow stem augers and wet rotary drills.
Other methods include jetting and churn drilling. Soil samples
are obtained from all borings using a 5 cm (2 in.) split-spoon
or similar device. Recommended sampling intervals are:
15
-------
Borings for monitoring wells; spoon samples at 1.5 m
(5 ft) intervals
Borings for soils investigations; spoon samples, 2
each per 1.5 m (5 ft), down to 6 m (20 ft) then one
per 1.5 m (5 ft) thereafter.
The blow counts to drive the sampling spoon are recorded for
each sample. The count for the final 30 cm (12 in.) is termed
the standard penetration test and is used to evaluate soil
density. Visual soil descriptions of each sample are made in
the field and samples preserved in plastic bags for chemical
and mineralogical analysis. If a dry drilling technique is
used, the position of the water table is observed prior to
closing the hole. Where soils are relatively uniform in depth,
it is possible to obtain spoon samples and undisturbed samples
alternately from the same boring. When the profile changes
significantly with depth, a second boring, within 3 m (10 ft)
of the first, is recommended for undisturbed samples to insure
sample continunity. One set of undisturbed samples for every
10 ha (25 ac) of RI basin area is usually sufficient.
An 8 cm (3 in.) thin wall Shelby tube is pushed at least 30 cm
(12 in.) into each major soil unit identified for undisturbed
samples. At least one tube per boring is recommended if the
soil is uniform (isotropic and continuous) and a maximum of one
tube every 1.5 m (5 ft) above the water table. The ends of the
tube are sealed and transported in an upright position to the
laboratory for analysis. It may not be possible to obtain tube
samples from gravelly sands, and loose, dry, coarse sands. In
these cases, a test pit will be necessary.
3.3.4.2 Test Pits
Test pits are strongly recommended for all RI site
investigations, since, unlike borings, they permit the direct
observation of a relatively large portion of the subsurface
profile. Two or three test pits would be considered the
minimum on even the smallest site. The various soil layers can
be visually identified and the presence of fractured
near-surface rock, hardpan, mottling, layers or lens of gravel
or clay, or other anomalies can be recorded. If the pit
extends into the ground water, it can also be used for in situ
measurement of hydraulic conductivity (see Section 3.5).
Soil texture is described for each layer using the procedures
described in Section 3.3.1. Soil structure can be examined
using a pick, or similar device, to expose the natural
cleavages and planes of weakness in the profile. Color and
mottling can also be observed in the pit wall. One end of the
backhoe pit can be excavated on a gentle incline. Following
identification of the soil layers, benches can be excavated by
16
-------
hand at appropriate levels to expose undisturbed soils for in
situ testing. These tests include in-place dry density and
field or oven-dried moisture content and, as described in
Section 3.5, may include infiltration testing. Tests are made
at the depth proposed for the RI basin surface and at least
once again, 1 m (3 ft)' deeper if the soil is uniform. Samples
from each layer are preserved in plastic bags for laboratory
analysis and a bag sample of about 40 kg (90 Ib) of soil at the
proposed infiltration surface collected for possible
re-compacted permeability testing in the laboratory.
3.3.5 Laboratory Procedures
Physical, chemical, and mineralogical tests in the laboratory
are conducted on the split-spoon and test pit samples described
in the previous section. The minimum that should be tested
consists of the soil unit that is planned for the final RI
basin infiltration surface and the soil ' unit immediately
beneath it. Composite samples can be assembled from the same
soil unit in different borings or from samples of the same soil
unit, if encountered at different depths. One set of chemical
and mineralogical data is obtained for each of the soil units
tested. That would be a minimum of two chemical and two
mineralogical tests (if needed) for each 10 ha (25 ac) of area
intended for use as RI basins.
The physical testing of these samples include gradation and
related procedures to determine the relative percentage of
sand, silt, and clay. The chemical tests may include: percent
organic matter, phosphorus, iron, manganese, potassium,
magnesium, calcium and sodium (exchangeable), base saturation,
pH, cation exchange capacity, and electrical conductivity (EC).
Mineralogical tests are only necessary when the' physical
testing indicates the presence of clay in the soil sample.
These tests are performed on the less-than-two-micron size
fraction to identify the clay minerals. The analysis is
typically done by x-ray diffraction.
Additional testing for phosphorus adsorption capacity may be
necessary in special situations. Phosphorus may be a parameter
of concern for RI in those cases where the hydrogeological
analysis shows that percolate will emerge in an adjacent
surface water which has stringent phosphorus limitations. The
Manual [1] (Section 5.4.2.3) contains procedures for estimating
phosphorus removal at RI systems. The design equation in the
Manual [1] is based on absolutely-worst-case conditions and, as
such, is unduly conservative. The rate constant is the lowest
possible value, based on a neutral pH. Most soils are either
slightly acidic or slightly alkaline, and would have a higher
rate. The equation is based on saturated flow conditions and',
therefore, predicts the shortest possible residence time for
17
-------
flow from the basin to the ground water table. The actual
residence time through this unsaturated zone will be much
longer and, therefore, P removal will be more effective than
predicted by the equation.
The design example in the Manual [1] is also too conservative
with respect to phosphorus removal during lateral flow. The
example is based on a most conservative (but impossible)
assumption that the hydraulic gradient is equal to 1 for
lateral flow, which again results in the shortest possible
residence time. For example, if a 10% gradient is assumed, the
0.2 mg/L P concentration could be achieved in a lateral
distance of 20 m (65 ft) instead of the 200 m (650 ft)
indicated by the Manual [1] (p.5-21). For the conditions
specified in that example, a travel distance of 36 m (118 ft)
would be required for the phosphorus concentration to approach
"background" levels (assumed at 0.015 mg/L for this case).
If the calculated concentration at the distance of concern
exceeds specified limits, phosphorus adsorption tests are
performed on the soils through which the percolate will flow.
Section 3.7.2 in the Manual [1] describes these tests; other
useful information can be found in references [6, 7, 8, 9]. It
is recommended that the results of the laboratory adsorption
tests be multiplied by a factor of 5 to account for the slow
precipitation of phosphorus that occurs over time in the soil
profile [10].
The undisturbed soil samples obtained with the Shelby tubes are
tested for dry unit weight, moisture content, and textural
gradation. Then, assuming a specific gravity of 2.69 (valid
for most soils) the porosity, void ratio, and degree of
saturation can be calculated using standard procedures [4, 11].
The textural gradations are plotted on the standard
semilogarithmic paper, and the effective size and uniformity
coefficients derived [4, 11]. The same calculations can also
be performed with results from field density measurements in
the test pits.
If needed, laboratory permeability tests are conducted using
the undisturbed samples. The basic procedures are described in
Soil Science Society of America Journal 46(4):866-880 and ASTM
Standards.
The results of laboratory permeability tests are plotted versus
void ratio on semilogarithmic paper as shown in Figure 3-2. A
design permeability for each hydraulic unit is selected near
the lowest end of the void ratio range, as shown on the example
Figure 3-2 and Table 3-3.
18
-------
0)
"o
0>
CO
Q)
.— O
-O =
O 3
0) O
»
«4-
0>
Q
z r
CM
'o
10
'o
CNJ
CO
cc
a
o
to
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0)
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TABLE 3-3
VERTICAL SATURATED PERMEABILITIES FROM FIGURE 3-2*
Hydraulic Permeability Void Ratio
Unit cm/sec in./h Range
Other
Physical Properties
K 1.5 x 10
_2
-3
3-0 x 10
K 1.0 x 10"
K 9-0 x 10~4
4
-4
K 2.0 x 10
K- 3.0 x 10~5
o
21 .26
4.25
1 .42
1 .27
0.283
0.043
0.46-0.62
0.47-0.66
0.48-0.67
0.49-0.68
0.49-0.69
0.52-0.70
Medium to coarse sand,
loose, D 0.2-0.3
Fine to medium sand,
loose, D 0.1-0.1, less
than 9% fines
Fine to medium sand,
loose, D 0.1-0.2,
greater than 9% fines
Fine to medium sand,
dense, D 0.1-0.2
Silty sand, D less than
0.1
Clayey sand
*These are not general factors, but are only valid for the example in
Figure 3-2
D is the 10$ soil fraction
3.3.6 Evaluation of Soil Test Results
The results of all tests are reviewed and evaluated by an
experienced engineer, geologist, or soil scientist. This is
particularly important for the chemical and mineralogical data
at sites where construction or operation may alter soil
reactions or mineral structure (chemical exchange and leaching,
cementation, swelling, etc.).
If test results indicate that a high percentage of the clay is
montmorillonite, the relatively high cation exchange capacity
may result in infiltration problems caused by swelling, even if
the total amount of clay present is small. Any combination of
either vermiculities or montmorillonites can be a problem, if
the total clay content exceeds 10%. Soils with greater than
10% clay are increasingly subject to physical changes and
20
-------
chemical leaching, and may require soil amendments for use as
RI application basins in a cut section. As previously
indicated, soils with that much clay are not recommended as
fill material for infiltration areas.
The results of the field observations and the laboratory tests
for physical characteristics are combined and plotted on a site
map and on several cross-sections through the site to provide a
basis for final design decisions regarding basin location,
water movement, etc. Figure 3-3 illustrates a typical profile.
50
J40
I 30
o
5 20
UJ
10
Soil Boring Locations
11
22
25
23
I
0
2OO
400
600
Distance (m)
800
1000x10
FIGURE 3-3
TYPICAL SUBSURFACE PROFILE
21
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3.4 Ground Water Investigations
Ground water investigations are necessary to determine the
depth to the ground water table, the direction of subsurface
flow, and, in some cases, to determine water quality parameters
for design purposes. The rate of ground water flow is
discussed in Section 3.5. Installation of at least three wells
is recommended during the site investigation. If the
preliminary study has identified the general ground water flow
direction and a tentative location for the RI basins, then one
well should be up-gradient, one in the basin area, and one
down-gradient on the ground water flow path near the planned
system boundary. These wells can also be used later for
operational water quality monitoring. If little is known about
the area, the three wells are usually spaced in a triangle over
the potential site. One well per 6-8 ha (15-20 ac) is usually
sufficient for sites up to 40 ha (100 ac). The well bottom is
usually between 3 and 10 m (10-30 ft) below the water table.
The lower 1.5 to 2 m (5-6 ft) of the well consists of machine
slotted well screen. A 5 cm (2 in.) inner diameter well screen
with 0.25 mm (0.01 in.) slots is usually satisfactory. The
bottom of the hole contains a medium to coarse sand-pack to
about 1 m (3 ft) above the well screen with a bentonite seal
above the sand pack. Figure 3-13 in reference [2] illustrates
a typical shallow monitoring well.
The elevation of the top of each well is determined and the
depth to ground water in each well periodically observed and
plotted on maps and on profiles similar to Figure 3-3. In
general, the ground water table tends to follow surface
contours and the flow trends toward adjacent surface waters.
This may assist in determining location of the monitoring
wells.
It is strongly recommended that monitoring wells be installed
prior to the normal "wet" period for the site area and observed
during this period to detect seasonal high ground water. The
use of test pits for detection of soil mottling (see Section
3.3.3 in this text) is also recommended.
3.5 Infiltration and Permeability Tests
Definition of infiltration capacity and subsurface permeability
is essential for RI system design. The infiltration rate is
defined as the rate at which water enters the soil from the
surface. When the soil profile is saturated, with negligible
ponding above the surface, the infiltration rate is comparable
to the effective saturated hydraulic conductivity of the soil
profile within the zone of influence for the tests. In both
this text and in the Manual Cl] the terms hydraulic
conductivity and permeabilty are used interchangeably.
Definition of both the vertical (K ) and horizontal (Kh)
22
-------
permeabilities in the soil profile are necessary for RI system
design.
3.5.1 Infiltration Test-Procedures and Evaluation
A number of methods have been developed to measure infiltration
rate or vertical hydraulic conductivity (K ) in the field. A
comparison of the various methods can be fecund in Table 3-2 in
the Manual [1]. It is the intent of each of these methods to
define essentially the same parameters, but their reliability
varies according to individual test conditions.
3.5.1.1 Flooding Basin Tests
A flooding basin test, with the test area at least 7m2 (75
ft ) is strongly recommended for all RI systems. Testing the
larger area provides more reliable data than the smaller-scale
cylinder infiltrometers or air entry permeameters (AEP). A much
larger volume of water is obviously required for this test.
The basic procedure is described in detail in Section 3.4.1 of
the Manual [1]. For most cases a tensiometer at 15 cm (6 in.)
and another at 30 cm (12 in.) are sufficient in lieu of the
extensive instrumentation described in the Manual [1]. The use
of picks and shovels (in lieu of special tools), and a
bentonite seal around the perimeter are also suggested. The
test basin is flooded several times to calibrate the
instrumentation and to insure saturated conditions. The actual
test, run is conducted within 24 hours of the preliminary
trials. For most soils with RI potential, this final test run
may require 3 to 8 hours.
Since it is the basic purpose of the test to define the
hydraulic conductivity of the near surface soil layers, the use
of clean water (with about the same ionic composition as the
expected wastewater) is acceptable in most cases. There are
three major exceptions when the test utilizes actual or
simulated wastewater and/or extends for a longer period.
If the wastewater expected in the actual RI system
will have a high solids content, similar liquid is
needed in the test. High solids concentrations might
come from algae carryover from ponds or from specific
industrial or commercial components in the wastewater
(whey wastes, pulp and paper, food processing, etc.).
Such solids clog the infiltration surface and clean
water tests would be a misleading basis for design.
In addition, the test also extends for a sufficient
period (perhaps several weeks) to model the actual wet
and dry cycles proposed for operation of these RI
systems. In these situations, an appropriate number
of standard flooding basin tests could be run with
clean water to define the basic characteristics of the
1.
23
-------
site. Assuming that generally uniform conditions
prevail, one longer term test can then be run to
define the influence of unique wastewater types on
infiltration. The subsurface flow characteristics are
still controlled by the clean water K values in most
situations.
2. If the system is to operate in the continuously,
flooded "seepage pond" mode, it is best for a test to
simulate that condition for a sufficiently long period
(perhaps several months) to insure that wastewater
will percolate at, expected rates. The RI design
procedures in the Manual [1] were not intended for
permanently flooded seepage pond designs. The wet and
dry periods described in the Manual [1] are intended
to restore the infiltration capacity of the surface
soils to near maximum potential every cycle through
bio-oxidation of the filtered organics. If the
infiltration surface is continuously flooded, a
different situation prevails and some other design
approach is necessary and percolate water quality
expectations modified.
3. If the borings and related site investigation reveal a
heterogeneous mixture of soils, a large scale RI cell
{0.5 to 3 ha (1 to 7 ac)} is constructed and operated
as a pilot RI unit fpr development of design criteria.
If test results are appropriate, the pilot unit can be
incorporated into the full scale system.
A minimum of one basin infiltration test on each major soil
type in the RI area is recommended. For large continuous
areas, one test for up to 10 ha (25 ac) of usable land is
typically sufficient. The test is performed on the soil layer
j.ntended for use as the basin infiltration surface in the final
operational system. Such a requirement seems self evident,but
it's disregard in the past has resulted in serious problems.
Construction of RI basins on backfilled material is to be
avoided whenever possible. When construction on fill is
absolutely necessary due to site topography, and if the soils
are within acceptable limits for clay content, a basin
infiltration test in a test fill area is recommended. The test
fill is constructed using the same equipment and procedures
that would be used for full scale construction. The test fill
should be as deep as required by the site design or 1.5 m (5
ft), whichever is less. The top of the fill area should be at
least 5 m (15 ft) wide and 5 m (15 ft) long to permit a
standard flooding basin test near the center. This approach
was developed, and used successfully to evaluate potential RI
sites in Maryland [12].
24
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3.5.1.2 Air-Entry Permeameter (AEP)
This device was developed by Dr. Herman Bouwer [13] to measure
"point" hydraulic conductivity values in the absence of a water
table. It has been used in site investigations for a number of
land treatment systems and is described in Section 3.5.2 of the
Manual [1]. The test is useful to verify site conditions in
the areas between the larger scale flooding basin tests, and
also for in situ tests in the end wall of a test pit as
described in Section 3.3.4.2 of this text. An AEP unit, used
in this manner can quickly determine the hydraulic conductivity
of all of the soil layers in the profile. The device is not
commercially available, but specifications and fabrication
details can be obtained from:
U. S. Department of Agriculture
Water Conservation Laboratory
4332 East Broadway
Phoenix, AZ 85040
3.5.2 Permeability Tests
(V
and horizontal
Definition of both vertical . .
permeability is necessary for RIV design. The vertical
component K can be inferred from the field tests described in
the previous section or measured in the laboratory with
undisturbed soil samples. Field results are the primary basis
for design in all cases. Laboratory data on undisturbed
samples are valuable for confirmation of test basin results and
for interpolation for areas between the field tests.
Laboratory values from deeper soil borings also provide data
for ground water flow and mounding analysis.
In most soils, 1C will exceed K due to soil stratification and
particle orientation. The relationship for some soils is given
in Table 3-5 in the Manual [1]. The most conservative approach
would be to assume that 1C equals K . The direct measurement
of K, in the field is suggested for ^any RI project where there
is a concern regarding ground water mounding. A number of
procedures are discussed in the Manual [1]. The auger hole
test is recommended and complete details on procedure and
calculations can be found in several references [1, 10, 13,
14]. A more recent development when the ground water is within
3 m (10 ft) of the surface using a test pit is described in
references [15, 16].
25
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3.6 References
1« U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 615/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for On-Site Wastewater
Treatment and Disposal Systems. EPA 625/1-80-012. U. S.
EPA, Center for Environmental Research Information,
Cincinnati, OH. October 1980.
3. Reed, S. C. The Use of Clayey Sands for Rapid
Infiltration Wastewater Treatment. USACRREL IR 805. U.
S. Army Corps of Engineers, Cold Regions Research and
Engineering Laboratory, Hanover, NH. December 1982.
4. Brady, J&T. C. The Nature and Properties of Soil. 8th
Edition. MacMillan Publishing Co., New York, NY. 1974.
5. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Application of
Municipal Sludge. EPA 625/1-83-016. U. S. EPA, Center
for Environmental Research Information, Cincinnati, OH.
October 1983.
6. Enfield, C. G. Evaluation of Phosphorus Models for
Prediction of Percolate Water Quality in Land Treatment.
In: Proceedings of the International Symposium on Land
Treatment of Wastewater, Vol. 1. U. S. Army Corps of
Engineers, Cold Regions Research and Engineering
Laboratory, Hanover, NH. August 1978.
7. Hill, D. E. and B. L. Sawhney. Removal of Phosphorus from
Water by Soil Under Aerobic and Anaerobic Conditions.
Journal of Environmental Quality. 10:401-405. 1981.
8. Loehr, R. C., et al. Phosphorus Considerations. In: Land
Application of Wastes, Vol 1. Van Nostrand Reinhold Co.,
New York, NY. 1979.
9. Overcash, M. R. and D. Pal. Design of Land Treatment
Systems for Industrial Wastes - Theory and Practice. Ann
Arbor Science, Ann Arbor, MI. 1979.
10. Reed, S. C. and R. W. Cries. Handbook of Land Treatment
Systems for Industrial and Municipal Wastes. Noyes Data
Corp., Park Ridge, NJ. 1984.
26
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11.
12.
13
14,
15,
16.
Terzaghi, K. and R. B. Peck. Soil Mechanics in
Engineering Practice. John Wiley & Sons, New York, NY.
1964.
Stein, C. E. Test Basins in Fill. Unpublished. Metcalf &
Eddy Inc., Silver Springs, MD. May 1983.
Bouwer, H. Groundwater Hydrology. McGraw Hill Co., New
York, NY. 1978.
U. S. Department of Interior. Drainage Manual.
Reclamation. 1978.
Bureau of
Healy, K. A. and R. Laak. Site Evaluation and Design of
Seepage Fields. Journal American Society of Civil
Engineers, Environmental Engineering Division.
100(EE5):1133. October 1974.
Bouwer, H. and R. C. Rice. The Pit Bailing Method for
Hydraulic Conductivity of Isotropic or Anisotropic Soil.
Transactions American Society of Agricultural Engineers.
Vol. 26.5, 1435-1439. 1983.
27
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CHAPTER 4
DESIGN
4.1 General
The basic design procedures in the Manual [1] are still valid,
so this Chapter only provides additional detail and a stronger
emphasis on certain critical aspects. Significant advances
have been made in computer sciences in recent years and most
designers now have access to at least a microcomputer.
Information on the continuing development of computer models to
solve some of the problems encountered in RI system design can
be obtained from:
Holcomb Research Institute
International Ground Water Modeling Center
Butler University
Indianapolis, Indiana 46208
317/283-9555
Determining the design annual hydraulic loading rate is one of
the most critical aspects of RI system design. The operational
cycle _(wet/dry periods) is another important factor and is
determined independently. Combining the annual loading and the
wet/dry cycle then determines the unit wastewater application
rate. Details on these factors are presented in this chapter.
Some of ^ the discussion may seem self-evident or overly
simplistic, but a review of problem systems indicates that
these factors and their inter-relationships may not always have
been clearly understood in the past.
4.2 Hydraulic Loading Rate and Basin Area
The hydraulic loading rate is based directly upon the field and
laboratory test results for infiltration, permeability, and
hydraulic conductivity. Figure 2-3 in the Manual [1] indicates
the general relationship between these factors. That figure is
not intended for final design of RI systems.
If the site investigation reveals a specific layer of soil that
will restrict flow, the design is based on the hydraulic
conductivity of that layer regardless of its thickness. In
many cases with a near surface deposit of silts or clays, it
may be cost-effective to remove the restricting layer and
locate the infiltration surfaces in the underlying soils. If
there is not an obvious restricting layer, the "effective"
hydraulic conductivity of the profile is the mean (arithmetic,
harmonic, or geometric) of the values observed (see Section 3.5
in the Manual [1] for procedures). A small percentage of this
28
-------
"effective" hydraulic conductivity is then taken as the design
loading rate. The percentages used are based on historically
successful RI performance and allow for wastewater
characteristics, soil reactions, and the need for cyclic
operation. A further refinement was added in the Manual Cl] to
allow for a higher design percentage for the most reliable
field test procedures. If a very large pilot scale cell is
constructed and actual wastewater is used and applied for
several months, then test results could be used directly for
design with little or no modification. However, most of the
commonly used test procedures require the adjustments (safety
factors) summarized in Table 4-1.
TABLE 4-1
DESIGN FACTORS FOR HYDRAULIC LOADING
Test Procedure
Adjustment Factor for Annual Loading Rate
Basin flooding test
Air entry permeameter and
cylinder infiltrometers
Laboratory permeability
measurements
10-15$ of "effective" rate observed
2- 4$ of "effective" rate observed
4-10$ of "effective" rate observed,
or of restricting soil layer.
The hydraulic conductivity defines the amount of clean water
that can move through a unit cross-section in the soil, at unit
gradient and under saturated conditions. For example, a soil
with an "effective" vertical conductivity of 5 cm/hr (2 in/hr)
could transmit 438 m /yr (116,000 gal/yr) of water through
every 1m (11 ft ) of horizontal area:
K = 5 cm/hr
v
hydraulic gradient = 1, because of saturated vertical flow
29
-------
Clean Water Rate, L
cw
L
cw
(5 cm/h)(24 h/d)(365 d/yr)(1 m2)
(100 cm/m)
= 438 m3/yr (10,800 gal/yr ft2)
The loading can also be expressed in terms of a depth of water
on a unit area because of the dimensions involved, so:
438
cw
1 m
= 438 m/yr (1,437 ft/yr)
An annual ^ basis is used for the loading rate determination,
since it is assumed that wastewater will be applied on some
sort of regular schedule throughout the year, although the
specific wet/dry cycle has not been determined at this point in
the calculations.
Assuming for the example above, that basin flooding tests were
used to determine K on a site with deep and uniform soils, it
would be appropriate to adopt a factor of 10% from Table 4-1,
to determine the design loading rate:
Annual Wastewater Loading, L
L T = (0.10)(clean water-rate, L )
™ = (0.10)(438) cw
= 44 m/yr (144 ft/yr)
It is, of course, possible to divide this annual loading by
some other time unit and produce the apparent "average" weekly
or daily loading. In earlier texts this was done to provide a
basis for comparison with the loading rates for other land
treatment concepts. The actual unit application rate is not
derived in this manner? Confusion on this point has led to
problems. The 10%, or other safety factors, are only used to
determine the annual hydraulic loading. It is not appropriate
to adjust either Ky or K^ with these factors for shorter time
periods. Such adjustments do not define the unit rates for
infiltration or subsurface flow (see Section 4.2.4 and 4.3).
Determination of the annual loading rate is in effect a
definition of the capacity of the site to transmit wastewater
if applied at some undefined, but regular schedule throughout
the year. Most RI systems do operate on a year-round basis.
However, if some winter storage is required by the regulatory
authorities or there are seasonal constraints on operation, the
annual loading is proportionally reduced to account for the
non-operating period. System operation planned for nine months
30
-------
per year for the sample case would reduce the "annual" loading
proportionate ly :
^X44 m/yr) = 33 m/yir (108 ft/yr) .
In some cases the hydraulic loading is limited by the
capability to move water away from the application site due to
shallow confining layers or a small hydraulic gradient (see
Section 4.3.1 in this text and Section 5.7.1 in the Manual
[1]).
4.2.1 Land Area Required
The required application area for RI systems can- be determined
with equations 2-1 and 2-2. The area calculated with these
equations is the surface area that is needed for infiltration
in the final system. There have been cases where the earthwork
design has allowed encroachment for dike construction on this
area. The completed system in these cases has significantly
less than the required infiltration area.
4.2.2 Wet/Dry Ratio
A regular drying period is essential for the successful
performance of RI systems. The period required for drying is a
function of the solids and degradable organics in the
wastewater and of the climatic influences on aerobic reactions.
The ratio of loading to drying periods within a single cycle
for successful RI systems varies, but is almost always less
than 1. For primary effluent, the ratios are generally less
than 0.2 to allow for adequate drying. The ratio varies for
secondary effluents in relation to the treatment objective.
Where maximum hydraulic loading and/or nitirif ioation are the
objective, the ratio might be 0.2 or less. When nitrogen
removal is necess.ary (see Section 4.8.2) the ratio ranges from
0.5 up to 1.0 12]. Table 5-13 in the Manual [1] presents
suggested hydraulic loading/drying periods, related to
wastewater type, treatment goal, .and climatic season. In all
cases, to avoid excessive soil clogging, the hydraulic loading
period for primary, or similar, effluents does not exceed 1-2
days regardless of season or treatment goals. As discussed in
Section 4.2.4, additional time may be needed for all of the
wastewater to infiltrate.
4.2.3 Application Rate
The selected wet/dry cycle is combined with the "annual"
hydraulic loading to determine the unit application rate. The
procedure is more complex when different wet/dry cycles are
. 31
-------
selected for summer and winter operations; an example is given
below:
3
Assume: design flow = 800 m /d; soil K =5 cm/hr, use 9%
adjustment factor; so: annual hydraulic loading = 39 m/yr (128
ft/yr) treatment goals with primary effluent are to maximize
loading rates, so, from Table 5-13 in the Manual [1]:
Summer period (April - Oct, 214 d)
Winter period (Nov - March, 151 d)
Wet
(d)
2
2
Dry
(d)
7
12
Total
(d)
9
14
Summer period, each cycle = 9 d
214
Cycles per season =
= 24 cycles
Winter period, each cycle = 14 d
Cycles per season = —
= 11 cycles
Total cycles per year =35
Assuming the wastewater has similar characteristics all year,
the amount applied per cycle is the same and the summer/winter
drying periods relied on for restoring the infiltration rate.
If the wastewater has special seasonal characteristics (e.g.,
high solids for seasonal industries, etc.) it may be necessary
to allow for extra drying time (or reduced loading) during
those special periods .
Wastewater loading per cycle
= 39 (m/yr)
~35 (cycles/yr )
= 1.1 m/cycle (3.6 ft/cycle)
Application rate (R) during 2 day wet period
R =
2 d/cycle
=0.56 m/d
(1.8 ft/d)
32
-------
Since in this example the wastewater flow is constant
year-round, and no storage is provided, a greater land area
will be needed during the winter months to maintain the same
loading rate. Using a variation of equation 2-1:
Summer period,
A =
(800 nr/d) (214 d)
(1.1 m/cycle)(24 cycles)(10,000 m /ha)
Winter period,
A =
(800 m /d)(151 d)
(1.1 m/cycle)(ll cycles)(10,000 m /ha)
0.65 ha
(1.6 ac)
1.0 ha
(2.5 ac)
The winter period would control the design in this case. The
total area needed is divided into multiple basins. Table 5-14
in the Manual [1] suggests the number of basins required for
various wet/dry ratios.
For this example, use 7 basins
basin area =
= (1'°
2 ,.
(10, OOOm^ha)
= 1428 m2 (15,370 ft2)
could use 38 m x 38 m square basins (125 x 125 ft)
During the summer months, four of the basins could be operated
on an 8-day cycle, or five basins on a 10-day cycle to
approximate the 9-day wet/dry, summer cycle recommended by the
design. This would allow each one of the seven basins to be
taken out of service for maintenance for an extended period
each summer.
The unit application rate (R) derived for this example was 0.56
m/d (1.8 ft/d) . That is compared to the effective steady state
infiltration rate for soil (K ) to insure successful
performance. As a general "rule-of -thumb" the unit application
rate is usually less than 50% of the K to allow for initial
clogging by wastewater solids and organdies, if optimization of
hydraulic loading is the design goal. Even if the application
rate (R) is higher, ponding will not typically commence until
the later part of short flooding cycles. In either case, the
depth of water remaining in the basin at the end of 1-2 day
flooding periods will probably not exceed 0.3 m. The depth of
ponding increases as the flooding period is extended and/or the
concentration of solids in the wastewater increases.
33
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4.2.4 Infiltration of Standing Water
The design approach is based on the assumption that all of the
applied water will infiltrate during the very early part of the
drying cycle so that most of that period is available for
aerobic restoration of the upper soil profile. This might
require from 0.5 to 2 days, depending upon the organic content
and volume of the dose. When the restoration of aerobic
conditions takes too long, it. may be necessary to extend the
drying period, perform maintenance on the basin bottoms, or in
an extreme case, perhaps remove and replace a given depth of
the surface soils.
It may occasionally be necessary to estimate either the depth
of water remaining in the basins at some time after flooding
has stopped or the time necessary to reduce the ponded depth to
zero. _There are several alternate approaches to this problem,
depending upon whether a clogging layer has formed on the
surface or not. If surface clogging does not exist, the
equation proposed by Stefan [3] is applicable.
h = h - 2.22 (l-n)°'35(n)325 K t°-675
o v
(4-1)
where:
h = depth of water in basin after time t, cm
h = depth at end of flooding period (at t=0),cm
n = soil porosity (decimal fraction)(see Table 4-2)
KV = saturated vertical hydraulic
conductivity, cm/h
t = time, h
In the application of this equation, it is important to
remember that the properties n and K are values which apply to
the wetting front, hence they will b^e affected by the presence
of microbial growth in this (near surface) zone and are not
equal to the values which existed before wastewater
applications began. Typical initial porosity ranges for
various soil textural classes are given in Table 4-2.
34
-------
TABLE 4-2
POROSITY OF SELECTED SOILS [4]
Soil Type
Porosity (%}
Silt and Clay
Fine sand
Medium sand
Coarse sand
Grave1
Sand and gravel mix
Coarse glacial till
50-60
40-50
35-40
25-35
20-30
10-30
25-45
Based upon the studies of deVries [26], both porosity and
hydraulic conductivity can be expected to decrease to the range
of 30 to 60 percent of their initial values upon the
establishment of biological growth in the near surface soil
profile, the larger decreases corresponding to finer textured
soils. Using n and K values equal to 50% of the original
values will probably give reasonable results.
The basin is empty when h = 0. Substitution and rearrangement
of equation 4-1 allows the estimation of the time required to
drain the basin.
t ~
-
h
K~
v
(4-2)
where: t, = time to drain basin, h
d
all other terms defined above.
For example, assuming a medium sand with a KV = 8 cm/h (3
in./h)' and a porosity of 35%, estimate the drainage time if 40
cm of effluent remain in the basin at the end of the flooding
period.
35
-------
Use 50% reductions, so
K =(0.5)(8) = 4 cm/h
n = (0.5)(0.36) = 0.18
40
4
0.3
0.820'5 0.180'5
= 7.8 h
In the presence of a surface clogging layer (the usual case),
equations 4-1 and 4-2 are not applicable as they depend upon a
continuously downward moving saturated front which never occurs
under a thin surface layer of higher hydraulic resistance than
the soil beneath. An equation which roughly estimates the
decrease in water depth with time is:
In
h+d
h0+d
(4-3)
where: d = depth of clogged surface layer, cm
KVC = hydraulic conductivity of clogging layer, cm/h
all other terms are previously defined.
The parameters d and K are obviously functions of both
wastewater suspended soli&i and operating conditions. As such,
they are never known with certainty. There are several
published studies which might be used to deduce probable values
of these parameters [26, 27, 28]. Of these, the work of
deVries [26] appears to simulate closely the operating
conditions of interest. From this study it appears that K
will probably be about 0.6 to 1.0 cm/h (1.5-2.5 in./h) and V§
will probably be about 2 to ,2.5 cm (0.8-1.0 in.)., Note 'that
when surface clogging controls, the properties of the upper
soil profile do not enter into equation 4-3 in any way.
The procedures outlined above are particularly useful in
conjunction with thermal calculations for RI .systems in
northern climates where winter freezing and ice formation may
cause problems (see Section 4.5 for details).
4.3 Subsurface Flow and Ground Water Mounding
A proper design insures that the subsurface soils have the
capacity to transmit the applied wastewater down and away from
the basins at an acceptable rate to avoid failure. .Ground
36
-------
water mounding beneath the basin occurs when there is
insufficient gradient to move water away from beneath the basin
in a lateral direction. Some temporary mounding is acceptable
as long as it does not interfere with infiltration at the basin
surface and dissipates quickly enough to allow for aerobic
restoration of the near surface profile. Chapter 5 in the
Manual [1] discusses these factors in detail and presents a
graphical procedure for estimating mounding beneath an RI
basin. Supplemental information and criteria are included in
Section 4.3.3 in this text.
4.3.1 Subsurface Flow
Percolate flow in the unsaturated zone beneath an RI basin is
essentially vertical and K controls flow. If a ground water
table, impeding layer, or barrier exists at depth, a horizontal
component is introduced and flow is then controlled by some
combination of K, and K within the ground water mound or
perched mound. At the margins of the ground water mound, and
beyond, the flow is typically lateral and K, controls. Section
3.5.2 in this text, Chapter 3 in the ManuSl [1], and similar
sources [3, 4] discuss K and K_ and techniques for their
measurements.
v
The capability for lateral flow away from the application site
controls the extent of mounding that will occur beneath the
basins. The "space" available for lateral flow is the
underlying aquifer and the zone between the existing ground
water table or other barrier and whatever point is selected as
an acceptable depth beneath the ground surface. The final
basin locations and configurations should be plotted on the
soils/ground water maps and an analysis conducted to insure
that the adjacent profile has the capacity for lateral
transmissions of the applied wastewater. The first zone of
concern is the perimeter around the general basin area, since
this may directly influence mounding. The analysis must
consider the gradients and potential for flow in various
directions away from the site, in addition to the loading
schedules for various basins (see Section 4.3.2) to estimate
what proportion of the applied wastewater will flow in a
particular direction. The general analysis employs potential
flow theory and is beyond the scope of this text. The use of
this theory to construct flow lines in the presence of a
uniform flow field of constant thickness represents a
reasonable approximation to what takes place down-gradient of
the mound. The calculations, if they are required by an
approving agency, are described in the text by Bear [29], or
the computer model by Daly [44].
In many cases the RI percolate emerges as base flow in adjacent
surface water and it is necessary to predict the position of
the ground water table between the RI basins and that point of
37
-------
emergence. This will reveal if seeps or springs are likely to
develop in the intervening terrain. In addition, some state
agencies may specify a residence time for the percolate in the
soil to protect adjacent surface waters, so it may also be
necessary to calculate the travel time from the basin to the
surface water. Equation 4-4 can be used to estimate the
saturated thickness of the water table at any point
down-gradient of the basin area [5]. This value can be
converted to an elevation and plotted on maps and profiles to
identify potential problem areas.
h =
2Q.D
(4-4)
where: h = saturated thickness of the unconfined aquifer at
the point of concern, m
hQ = saturated thickness of the unconfined aquifer
beneath the rapid infiltration area, m
Q. = the lateral discharge .from the unconfined -flow
system, per unit width of the flow system, m /d m
D = the lateral distance from the RI area to the
point of concern, m
1C = effective horizontal conductivity of the soil
system, m/d (must have consistent units)
The quantity Q in equation 4-4 is determined with:
Q = (h _
i 2D. o
(4-5)
where: D.
distance to seepage face or outlet point, m
= saturated thickness of the unconfined aquifer at
point D .
other terms defined above
Figures 5-10 and 5-11 in the Manual [1] and/or other references
[6, 30, 31, 32] can also be used to estimate the saturated
thickness of the water table at a point down-gradient of the
basin area. The answers from all of these models are
meaningless unless the soil and hydraulic parameters and
boundary conditions of the site match reasonably well with
those of the model selected.
38
-------
The travel time for lateral flow is a function of the hydraulic
gradient, the distance, the
Equation 4-6 can be used for
gradient, the distance, the K, , and the porosity of the soil,
tnis purpose..
(n)(D
(4-6)
D
where: t = travel time for lateral flow from basin area to
emergence in surface waters, d
n = porosity of soils, % (as decimal; calculated from
field data, see Table 4-2 this section for ranges)
D = travel distance, m
h = saturated thickness of aquifer at RI basin area, m
h. = saturated thickness of aquifer at point of
1 emergence
K = effective horizontal conductivity of the soil
system, m/d (units must be consistent)
4.3.2 Ground Water Mounding
The material presented in the Manual [1] on mound height
analysis was that developed by Glover C33] and summarized by
Bianchi and Muckel C34]. The Manual [1] did not state nor
imply that this simplified graphical analysis would solve all
problems relating to ground water mounds. The curves were
developed for basins of square or rectangular geometry, which
lay above level, fairly thick, homogeneous, aquifers of
(practically) infinite extent. Vertical re-charge was assumed
uniform (or nearl'y so) and the height of the mound was limited
to less than one-half the thickness of the original unconfined
aquifer. Many potential RI sites will not conform well to
these conditions and a certain amount of ingenuity is required
on the part of the engineer who is analyzing such sites. In
the simplest cases the original curves may be used with
modifications. For example, a circular basin can be
approximated by a square one of equal area, allowing the use of
Figures 5-8 and 5-10 without introducing significant error
C33]. Also, as the curves were developed using unsteady state
equations, they necessarily show an ever-increasing mound
height with time. It is obvious that during prolonged resting
periods (e.g., during annual maintenance), the mound recedes to
a certain extent. The figures are still applicable, but the
principle of superposition in time must be employed. If dosing
39
-------
is stopped at time t, a uniform discharge (from the mound back
to the basin) is assumed beginning at t, thus effectively
cancelling the re-charge. The algebraic sum of the two mound
heights in time approximates the mound shape after re-charge
ends. More details on the analysis of mound decay can be found
in references [35, 36, 37].
Other conditions encountered with reasonable frequency which
may invalidate the use of the curves in the Manual [1] include
the presence of sloped water tables [37], subsurface layers of
reduced conductivity which give rise to perched mounds [38],
and the presence of points of withdrawal (e.g., a stream [39]
or a significant rise in mound height relative to the original
saturated depth [37, 39]). Although there are analytical
solutions and/or dimensionless plots from generalized solutions
which satisfy some of these more complex boundary conditions,
the designer would probably be better advised to use a computer
for these complications, especially if the mound height
analysis is thought to be critical to the success of the
project. There are a number of existing programs available to
perform these calculations, and improvements are continuously
forthcoming. Since most engineering offices have access to at
least a microcomputer, and some to larger systems, two
applicable programs are discussed here and sources given.
An interactive program based upon Glover's analysis [33] has
been written for the APPLE II+, 48K microcomputer [40]. In
addition to calculating the mound height ordinates with time
and distance (which also solves the problem discussed in
Section 4.3.2) under continuous re-charge, the program can also
solve the mound decay problem and can handle the boundary
condition of re-charge to a stream. To do this an "image
basin," discharging at the same rate as re-charge from the RI
basin, is assumed to be located on the opposite side of the
stream, equidistance from the real basin. The technique borrows
from the theory of well hydraulics using image wells and
ensures that the (mathematical) mound intersects the stream at
its water level, a necessary physical condition.
The program can also calculate the hydrograph of stream
re-charge, a computation occassionally required by regulatory
agencies where water quality limiting stream segments are
involved. The authors have designed the program to be
"user-friendly," allowing several options, creating unambiguous
displays, and allowing easy variation in parameters and
variables. A floppy disk containing the program and all
documentation can be purchased from the authors at:
Department of Civil Engineering
Colorado State University
Fort Collins, CO 80523
40
-------
If more versatility than the above program can provide is
required, or more diverse boundary conditions are encountered,
another interactive program is available which seems to be
admirably suited. It was developed by Dr. S. P. Neuman [41]
and adapted for interactive mode on a FDP-11/23 minicomputer
(256K) by Bloomsburg and Rinker [42]. The program called
UNSAT, can be purchased from:
Dr. G. L. Bloomsburg
469 Paradise Drive
Moscow, ID 83843
208/882-5726
This program can treat non-uniform flow regions having
irregular boundaries, arbitrary degrees of local anisotropy,
evapotranspiration, and percolate recovery by fully- or
partially-penetrating wells. Significant to some more
difficult analyses, certain time-dependent boundary conditions
can be handled, such as varying infiltration rates or stream
levels. The major drawback to the use of this program is the
necessity of knowing (or estimating), for each soil unit in the
flow system, the following relationship:
1. Saturated hydraulic conductivity
2. Relative conductivity (0_
-------
4.4 RI Basin Configuration and Application Scheduling
When the availability of suitable land for an RI system is
limited and preliminary mounding calculations indicate a
problem, there is no alternative to the use of subsurface
drainage to control the rise of the water table (see sections
5.7.2 to 5.7.4 in the Manual [1]). This requires underdrainage
and a discharge permit unless all the recovered percolate is
re-used. However, when land is not a limiting factor, it may
often be possible to avoid underdrainage by "optimizing" the
configuration and application scheduling of the RI basin
system; for example, by arranging the basins in a strip
configuration and staggering basin operation so that no two
adjacent basins are ever dosed sequentially. This was
discussed in Section 5.6.1 of the Manual [1]. Further
improvements are possible if the basins are spread out on the
site, although this loses the cost advantage of common dikes.
The mound height analysis, if performed correctly, usually
points to the best approach. By way of illustration, assume a
system of seven RI basins arranged as shown in Figure 4-1 in a
hydrogologic setting as shown in Figure 4-2 .
The average loading is 1,727 m3/d (60,984 ft3/d) on a bottom
area of 2.83 ha (7 acres), an average loading of 0.061 m/d (0.2
ft/d) . The horizontal hydraulic conductivity is 6.1 m/d (20
ft/d) and the specific yield is 0.20. It is planned to operate
the basins on one day of dosing and six days rest using the
sequence 1, 4, 7, 2, 6, 2, 5. Will mounding interfere with the
operation of this system? One possible method of analysis is
to assume that the intermittent loading to each basin is
equivalent to a continuous dose, averaged over the entire
system. Correcting for dike area, this goading is about 0.055
m/d (0.18 ft/d). Selecting one year (36S d) as a trial value
of t and using Figure 4-9 in the Manual [1] with L/W = 2 and W
- 132 m (432 ft.) .
i _ 0.055 _ _
R ~ V " "072 -- °'
KD
V
(6.1H15.2) _
0.2
W
132
\|(4) (465) (365)
= 0.16
h = (0.08)(0.275)(365) = 8.3 m (26 ft)
o
Referring to Figure 4-2, this is clearly an unacceptable mound
since it would have intersected the basin surfaces before 365
days had elapsed.
42
-------
I Direction of
I Groundwoter Flow
276 m (906 ft)
w
I
1
2
•A
3
4
•1
5
3
6
1
k.
7
r
>
^
140 m
(460ft)
FIGURE 4-1
BASIN LAYOUT FOR EXAMPLE CALCULATION
A B
^ /J> A ^
7.6m
(25ft)
•> ^
S7 GWT
15.2m
(50ft)
Impervious
v^
FIGURE 4-2
SUBSURFACE PROFILE SECTION C-C FOR EXAMPLE CALCULATION
43
-------
However, the previous analysis does not reflect the actual
operation. Each basin is loaded at 0.43 m/d (1.4 ft/d) for one
day, then allowed to drain and rest for six days. At worst,
only basins with touching corners (not sides) are dosed on
consecutive days, thus, minimizing mound interference. An
analysis based upon a single basin shows the maximum mound
height is 1.60 m (5.3 ft) at the end of one day, but then
recedes to less than 0.09 m (0.30 ft) by the end of the seventh
day (using the principle of superposition in time discussed in
Section 4.3.2). Using the principles of superposition in time
and space together, the time history of mound heights'at a few
critical points within the space together, the time history of
mound heights at a few critical points within the system; for
example, points A or B can be calculated. Such a plot shows
that mounding is not a serious problem with this basin
arrangement and application scheduling. If the analysis
indicated that mounding would likely be a problem, the design
could be changed slightly to alleviate it. For example, the
dikes could be made a little wider and/or the basins could be
changed from square to rectangular (long axis perpendicular to
ground water flow direction), or arranged in a strip.
4.5 Design Requirements for Winter Operation in Cold
Climates
Rapid infiltration systems can operate successfully on a
year-round basis in cold climates. Systems in Massachusetts,
New York, Michigan, Wisconsin, South Dakota, Montana, and Idaho
have all operated through the winter without significant
difficulty. Proper thermal protection for the pipes, pump
stations, valves and related plumbing is essential for
wintertime operation. This is covered in other references [7].
The unique concern for RI is prevention of a permanent,
impermeable ice barrier at, or in, the near surface soils in
the basin. Two possible problems can develop:
. The standing water in the basin freezes into a layer of
ice that, because of entrapped vegetation, cannot float
to the surface. If the next increments of applied
wastewater are not warm enough to melt through the ice
layer, the basin can be in the failure mode for the
rest of the winter.
If at the end of the infiltration period, the .soils
drain too slowly, the water remaining in the soil pores
may freeze, rendering the surface impermeable. If this
frozen, saturated soil layer is not thawed, the basin
is in the failure mode.
The successfully operating systems in northern climates tend to
have relatively coarse, rapidly drained soils and operate with
dilute raw sewage or relatively warm wastewater from
44
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conventional primary or secondary treatment systems. Suggested
operating procedures to deal with some of these problems are
discussed in Chapter 6. Design features which have been
successful include:
1. Ridge and furrow configuration on the basin bottom
combined with a floating ice sheet. The ice gives
thermal protection, and rests on the ridge tops in the
final infiltration stage.
2. Inducing snow drifting with snow fences in the basins
and then flooding beneath the snow cover. The snow
retards freezing, but the water equivalent of the
melted snow is a negligible contribution to the
hydraulic load in the basin.
3. By-pass some of the final cells in a long detention
facultative lagoon to retain some of the heat
originally present in the wastewater.
4. Design one or more of the RI cells for winter operation
in the seepage pond mode with continuous flooding and a
floating ice cover. These cells can be drained, dried,
and restored each spring. Any special nitrogen limits
or other water quality limits must be considered, since
removal efficiencies are reduced at low temperatures.
5. Based on thermal and hydraulic calculations, adjust the
wet/dry ratio during the critical periods so the near
surface soil never irreversibly freezes (see discussion
below and reference [8] for details).
Wastewater storage for winter conditions is not included in any
of the alternatives listed above. Storage may be necessary to
equalize flow variations, and storage may be provided for
emergencies, but winter storage is not an absolutely necessary
component in RI system design for the contiguous United States.
A cost-effective design attempts to understand and adjust for
the low temperature conditions rather than increasing costs by
including long-term storage.
Storage may be needed during the winter in cold climate areas,
however, if high nitrogen removal is a system requirement.
Ice starts to form as soon as the surface layer of water
reaches 0°C (32°F) and the latent heat is withdrawn. The ice
thickness that will develop can be predicted with equation 4-7,
which is valid for ponds and for the standing water in a
wet/dry RI operation.
45
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I = C N(AT) (t)
(4-7)
where: I = ice thickness, cm
C = coefficient, depending on environmental conditions
= 1.7 for snow cover on basin or snow on top of ice
= 3.0 for no snow.
(see reference 7 for other conditions)
AT =
(0 C - TA)
A
t
= average air temperature over period t, C
= time, days (this is the time, in 'wet/dry RI
operations after the water at the surface reaches
ObC (32°F)
The factor (AT)(t) in equation 4-7 is called, the freezing index
and is an environmental characteristic ' for a particular
location. It can be calculated from weather records or .typical
values can also found in reference [7] and similar sources.
The design calculations are based on the coldest winter of
record in 20 years, or even longer if data are available.
Equations 4-2 or 4-3 and 4-7 can be solved for winter operating
systems to determine how long it takes for infiltration to be
complete and if any ice. will form. Any ice formed has to be
melted by the next increment of wastewater., . The energy input
required is given by equation 4-8.
E =
m
100
(4-8)
where:
m
= energy required to melt the ice, cal
6 ,3
= latent heat of ice, 80 X 10 cal/m
I = ice thickness, cm
r
A = surface area of ice, m"
The energy available in the next increment of wastewater is
dependent on the incoming liquid temperature. Only a fraction
of the heat released in cooling the incoming water to 0 C
(32°F) is available for ice melting, since there will also be
significant heat losses to the atmosphere. The situation where
both the soil and the water in the soil pores freeze is a more
46
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complex, two-phase problem and reference
consulted.
[8] should be
A -design based on a floating ice layer is recommended for
critical situations. Even under wet/dry operations when the
ice is resting on the basin bottom, it provides'insulation and
retards freezing of the soil. Absence of the ice layer would
require a more frequent flooding cycle to prevent soil
freezing. Stefan [8] shows that the presence of a floating ice
layer protects the soil from freezing for a period of over 27
days as compared to 2.5 days for freezing to commence in an
unprotected soil under the same conditions. It is. essential
that the ice layer re-float at every flooding cycle.
4.6 Design of Seepage Ponds
The Manual [1] presents criteria for RI design that are based
on the assumption that the system will be operated on a wet/dry
cycle so that the near surface soils can be aerobically
restored. Those criteria were not intended for basins or
infiltration ponds expected to be permanently flooded. It
should be understood that the percolate from seepage ponds will
not be comparable in quality to a properly managed RI system
designed for a regular wet/dry cycle. Seepage ponds in
suitable soils 'can infiltrate large volumes of water where
water quality is not an issue. Specific criteria for their
design is not included in this text.
4.7 Design of Physical Elements
Most of the physical elements for RI basins are similar to
wastewater .lagoons. These include dikes, access ramps, inlet
and transfer structures, and flow control devices. Chapter 4
in reference Cll] provides details on some of these features,
as well as Section 5.9 in the Manual [1].
4.7.1 Dikes.
The dikes of RI basins intended for wet/dry cyclic operation do
not need to be more than 1 m (3 ft) high and could be even less
in many cases. Extra freeboard is not recommended for routine
wastewater containment. If a basin does not infiltrate at the
expected rate, restoration of the surface is necessary and
extra freeboard in that basin does not solve the problem.
Dikes that are higher than necessary can actually contribute to
operating problems through the extra runoff and potential for
erosion of soil fines. Some of the basins in a system,
however, can be designed for emergency storage and these dikes
can be higher. In these cases the dike includes sufficient
freeboard and incorporates wave protection • at the water line
zone. All basins designed for flooding for more than a few
weeks duration might include an outlet or other positive
47
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drainage features 'so the basin can be drained quickly when
maintenance is necessary. This is because the time period
required for natural drainage through the basin bottom may be
unacceptably long.
The dikes are compacted to prevent seepage through them. The
top of the dike often is a road for vehicle access and, if
possible, should be pitched to the outside so that all drainage
and runoff goes away from the basin. Erosion control for the
inner dike surface is necessary, both during construction and
during system operation. The washout of soil fines from this
source has sealed the basin surfaces in a number of systems.
Grass, or similar vegetation, may eventually stabilize the
slope, but the use of a silt fence or similar porous barrier at
the toe of the slope is suggested during construction and may
still be necessary during operation. It is also necessary to
provide a ramp or other easy access for maintenance equipment
to enter the basins.
4.7.2 Basins in Fine-Textured Soils
If the soils in the basin bottom contain a significant fraction
of fines (silts and/or clay >10%), then stabilization with
grass may be necessary C12]. Flooding the bare surface of such
soils may suspend some of the fine soil fraction in the water.
The repetitive sorting and re-deposition of those fines on the
surface can eventually reduce the infiltration rate
significantly. A surface layer of gravel or similar material
may hold the fines in place, but the interface may not dry
properly and a clogging, biological layer may develop. Other
mixed-in amendments (sand, gravel, lime, woodchips) have not
been effective [12H- A grass cover is the only effective
treatment for this condition known to date. These RI systems
tend to be near the low end of the range of hydraulic loading
and the grass cover may actually improve the infiltration
capacity of the basin surface.
4.7.3 Inlet, Distribution and Transfer Structure
Small basins with low velocity, low volume wastewater flow may
only require a simple splash block at the discharge pipe.
Large basins may require a more complex arrrangement to dampen
the entering flow to prevent erosion of the basin and/or the
adjacent dike. In all cases, uniform wastewater application
over the entire basin surface is necessary. This may
automatically occur with a high application rate on finer
textured soils, but may require some structural assistance on
coarse soils to insure uniform distribution. The distribution
system might range from a network of pipes and troughs to
sprinklers in the extreme case.
48
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In many cases, several basins or sets of basins may be flooded
at the same time so interbasin transfer structures may be
necessary. In general, these are similar to those used for
lagoons [11]. Figure 5-4 in the Manual [1] illustrates a
transfer structure with removable rings to control the depth of
water in a basin. A device of this type gives the operator
greater control and permits optimization of basin use.
4.7.4 Flow Control .
Depending on the size of the system, the design can provide for
a control system ranging from simple manual valves to a fully
automated operation involving time switches, float switches,
tensiometers, and automatic valves. In either case, it is
necessary for regular visits to all of the basins by the
operator to observe conditions and make necessary adjustments.
It is essential that the design provide sufficient flexibility
so that adjustments in loading and application rate for a
particular basin can be made by the operator. The adjustable
transfer structure discussed above is one example. It is also
absolutely essential that particular attention be paid to the
preparation of the O & M Manual for the system?It is critical
for theoperatortounderstand clearly theRI concept, what
controls and adjustments are available, and what the
consequences of these adjustments might be.
4.8 RI System Performance •
Basic information on the performance of RI systems can be found
in Chapters 1, 2, and 5 of the Manual [1] and other sources [2,
4, 14, 20, 24, 25]. The percolate after a few meters of travel
in the soil is of such a quality that recovery and re-use for
unrestricted agricultural irrigation is acceptable. The system
can be managed to satisfy nitrogen and other drinking water
limitations in the percolate, but the direct on-site recovery
and re-use as potable water is not recommended without
extensive further treatment.
There has been additional research and study on the removal of
toxic organics and nitrogen in RI systems since publication of
the Manual [1] and the need for disinfection is still a concern
for many. Further details on these three topics are given
below.
4.8.1 Removal of Toxic Organics
The discussion on organics in the Manual Cl] was limited to
removal efficiencies for pesticides and BOD. The confidence in
BOD removal efficiency is based on a reasonably large
historical data base. A few researchers have since monitored
specific organic chemicals in rapid infiltration systems [13,
49
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14, 15, 16, 17] and have demonstrated significant reductions in
many toxic organic chemicals. Analysis of organic chemicals is
expensive and the analytical techniques are relatively new,
resulting in a limited data base for specific compounds
identified in municipal wastewaters. Therefore,
"rule-of-thumb" removal efficiencies are not available and are
not expected to be available in the foreseeable future. For
this reason, the engineer needs to forecast the removal
efficiency based on an understanding of the processes in a RI
system.
A model has been developed [16] which separates a RI system
into three computation compartments involving the standing
water, the surficial soils, and the deeper soil layers. The
model was developed in the laboratory, but not tested on a full
scale RI system.
An interactive computer aided design code has been written in
Fortran 77 along with a user's guide. The code can run on most
64K byte microcomputers having Fortran 77 compilers. Copies of
the information are available in Simulation Waste Access to
Groundwater: A User's Guide [19] which can be obtained from
the International Groundwater Modeling Center, Butler
University, Indianapolis, Indiana, 46208.
4.8.2 Nitrogen Management in Rapid Infiltration Systems
The discussion in Sections 5.2.2 and 5.4.3.1 in the Manual [1]
describes the nitrification/denitrification mechanisms in
rapid-infiltration systems for nitrogen removal. The
importance of temperature, pH, and the necessity of organic
carbon (as an energy source for the denitrifying bacteria) is
discussed. Equation 5-1 estimates the amount of total nitrogen
that may be removed based on the total organic carbon (TOC) in
the applied wastewater. Nitrogen removal versus infiltration
rate is shown in Figure 5-2 and hydraulic loading rates versus
percent nitrogen removal is provided in Table 5-2 (all in the
Manual [1]). Research on RI systems has combined new and
former design and operaion techniques for improved nitrogen
control. If high nitrates are not objectionable, the design
develops the maximum hydraulic loading potential. However, if
the objective is to maximize total nitrogen removal, maximizing
hydraulic loading is sacrificed to design for optimum
denitrification.
In systems designed for maximum hydraulic loading, high nitrate
concentrations will be experienced in the percolate near the
beginning of each loading period. These short pulses of high
nitrate-nitrogen may peak up to an order of magnitude greater
than the average. The pulse is a result of nitrification of
the ammonium stored in the soil profile during the previous
loading period. This high nitrate water is released as a slug
50
-------
when the capillary water is displaced during the next loading
period. Four to five days drying has been shown to be an
adequate time to provide ample soil re-oxygenation for
nitrification of this applied ammonium. After the high nitrate
pulse, the nitrate-nitrogen concentration drops to a much lower
value (see Section 5.2.2 of the Manual [1]). The average total
nitrogen concentration, including the high peaks, will likely
range between 9 and 20 mg/L.
When the objective of design is to maximize total nitrogen
removal, a more detailed procedure is used. Loading schedules
directly affect ammonium adsorption during wetting, while
drying schedules affect oxygen mass transfer and diffusion into
soil. To achieve complete nitrification of the ammonium
nitrogen, the amount of ammonium nitrogen applied during
loading is balanced against the amount of oxygen entering the
soil profile during drying. Otherwise, ammonium nitrogen
accumulates in the soil until its adsorption capacity for
ammonium nitrogen is saturated, and then ammonium nitrogen, as
well as nitrate-nitrogen, appears in the reclaimed water.
Loading periods must be long enough to maximize ammonium
adsorption and develop an anoxic environment to allow
denitirification. The organic carbon source provided must be
sufficient to support adequate denitrification. The small
amount of available carbon in secondary effluent can be a
limiting factor. Drying periods must be long enough to allow
near ultimate soil re-oxygenation. This period is determined
by the infiltration rate and the moisture release capability of
the soil at the site. The design procedure is summarized in
these six steps:
1. Determine the mass of ammonium that can be stored in
the soil profile per unit area for the unsaturated
depth using the published equations [21] to calculate
the ammonium adsorption ratio and exchangeable
ammonium, percentage.
2. Calculate the length of the loading period required
for maximum ammonium adsorption, using the
infiltration measurement described in Section 5.4 of
the Manual [1] and the known ammonium concentration of
the applied effluent.
3. Estimate the mass flow of oxygen and the mass of
diffused oxygen that can be accumulated in the soil
profile for a specific drying period, using the
published technique [23].
4. Balance the ammonium adsorption with the available
oxygen to establish the length of loading and drying
periods for optimum nitirification of the system.
51
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5.
6.
Practical lengths of loading and drying periods might
also be considered to fit the operation and
maintenance schedule of the municipal system
operators; i.e., five to nine days loading and two to
five days drying, respectively.
Balance nitrate-nitrogen produced against the mass of
organic carbon entering the soil.
Optimize denitrification by reducing the infiltration
rate in the flooded basin.
Using these six steps, the designer can expect total nitrogen
removal in excess of 60% and the discharge of total nitrogen to
be below 10 mg/L, for typical municipal wastewaters during warm
weather.
The infiltration rate can be fine-tuned to enhance
denitrification by operating at reduced depth of ponding or by
adding suspended solids. However, this will result in
additional sacrifice of loading and will increase the land
areas required. Selecting a site which naturally has low
infiltration rates or compacting the soil surface can assure
high total nitrogen removal rates, but either increases the
land area needed. Reducing the infiltration rate, increases
contact time between the soil micro-organisms, and
nitrate-nitrogen, and more efficient denitrification can be
accomplished. One of the most important factors for producing
denitrification is to have near-saturated conditions in the
soil media [20H.
Thus/ optimum total nitrogen removal in land treatment systems
can be achieved by adjusting the loading period to insure
complete nitrification of the ammonium nitrogen in the sewage
and adjusting the infiltration rate to provide the needed level
of denitrification. The use of primary effluent allows maximum
total nitrogen removal at higher infiltration rates due to its
higher organic carbon content. Nitrification reaction rates in
the soil will be significantly reduced in the winter in cold
climate areas. Winter storage, similar in duration to that
described in Part II of this supplement, is necessary for those
RI systems designed for maximum nitrogen removal.
4.8.3 Disinfection in RI Systems
The need for disinfection and the effectiveness of virus and
pathogen removal in RI systems is discussed in the Manual [1]
(Chapter 5) and in more recent texts [2, 24, 25]. Each one of
these references discusses numerous projects, studies, and
evaluations. In general, they show that RI system percolate
can be of very high quality with respect to viral and bacterial
content. There have been some studies where some viruses were
52
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detected in the percolate when high virus concentrations were
present in the wastewater, rainfall, during drying periods was
heavy, and the wastewater was applied at high rates to very
coarse soils. There has never been any evidence of any
water-related disease problem related to the operation of any
land treatment system in the United States.
Chlorination of wastewater can reduce the concentration of
bacteria and virus significantly. Chlorination of wastewater
can also significantly increase the number and concentration of
refractory toxic organic compounds which can then reduce the
effectiveness of RI treatment for their removal. Since the RI
system itself will remove bacteria and virus effectively, there
is no need for wastewater Chlorination prior to application.
In the general case,the ground water sources for down-gradient
water supplies will be protected. On-site recovery of water
for drinking purposes is not recommended without appropriate
treatment. The worst case for down-gradient impacts might be
high wastewater loading rates or seepage ponds on coarse soils
with a shallow water table and nearby recovery for drinking
water. These conditions are typically identified during the
site investigation and design. In some cases, with such ground
water use very close to the RI system, it may be prudent to
provide standby disinfection for the recovered water.
Disinfection at that point will be more effective and provide a
greater degree of overall health protection than Chlorination
of wastewater prior to application on the RI basin.
4.9 References
1. U. S. Environmental Protection Agency. Technology Transfer
Process Design Manual for Land Treatment of Municipal
Wastewater. EPA 625/1-81-013. U. S. EPA, Center for
Environmental Research Information, Cincinnati, OH.
October 1981.
2. Reed, S. C. and R. W. Crites. Handbook of Land Treatment
Systems for Industrial and Municipal Wastes. Noyes Data
Corp., Park Ridge, NJ. 1984.
3. U. S. Department of Interior. Drainage Manual, Bureau of
Reclamation. USGPO Stock No. 024-003-00117-1. 1978.
4. Bouwer, H. Groundwater Hydrology. McGraw Hill Co., New
York, NY. 1978.
5. Sylvester, K. Land Treatment by Rapid Infiltration,
Unpublished. ERM, Inc., West Chester, PA, April 1984.
53
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6. Motz, L. H. Hydrogeology in Land Application of
Wastewater. In: Proceedings of Water Forum "81, Vol 1.
American Society of Civil Engineers. San Francisco, CA.
August 1981.
7. U. S. Environmental Protection Agency. Cold Climate
Utilities Delivery Design Manual. EPA 600/8-79-027. U. S.
EPA, Center for Environmental Research Information,
Cincinnati, OH. September 1979.
8. Stefan, H. G. Heat Loss from Rapid Infiltration Basin in
Winter. Journal American Society of Civil Engineers,
Environmental Engineering Division. Vol. 108, EE1, p.141.
February 1982.
9. Eckenfelder, W. W. Industrial Water Pollution Control,
McGraw Hill Co., New York, NY. 1966.
10. Healy, K. A. and R. Laak. Site Evaluation and Design of
Seepage Fields. Journal American Society of Civil
Engineers, Environmental Engineering Division.
100(EE5):1133. October 1974.
11. U. S. Environmental Protection Agency. Design Manual for
Municipal Wastewater Stabilization Ponds. EPA
625/1-83-015. U. S. EPI, Center for Environmental
Research Information, Cincinnati, Ohio. October 1983.
12. Reed, S. C. The Use of Clayey Sands for Rapid
Infiltration Wastewater Treatment. USACRREL IR 805. U.
S. Army Corps of Engineers, Cold Regions Research and
Engineering Laboratory, Hanover, NH. December 1982.
13. Bouwer, E. J., P. L. McCarty, and J. C. Lance. Trace
Organic Behavior in Soil Columns During Rapid Infiltraion
of Secondary Wastewater. Water Resources Research.
15:151-159. 1981.
14. Bouwer, H. , R. C. Rice, J. C. Lance, and R. C. Gilbert.
Rapid-Infiltration System for Wastewater Renovation and
Beneficial Re-use. EPA 600/2-82-080. PB 82-256941. U.
S. Environmental Protection Agency. 1981.
15. Bouwer, H. and R. C. Rice. Renovation of Wastewater at
the 23rd Avenue Rapid Infiltration Project. Journal Water
Pollution Control Federation. 56:76-83. 1984.
16. Enfield, C. G., D. M. Walters, J. T. Wilson, and M.
Piwoni. Behavior of Organic Pollutants during Rapid
Infiltration of Wastewater into Soil: Part II
Mathematical Description of Transport and Transformation.
Journal of Environmental Quality. (In Press).
54
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17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
Hutchins, S. R., M. B. Tomson, and C. H. Ward. Trace
Organic Contamination of Groundwater from a Rapid
Infiltration Site: A Laboratory-Field Coordinated Study.
Environmental Toxicology and Chemistry. Vol. 2, No. 2, pp
195-216. 1983.
Tomson, M. B., J. Dauchy, S. Hutchins, C. Curran, C. J.
Cook, and C. H. Ward. Groundwater Contamination by Trace
Level Organics from a Rapid Infiltration Site. Water
Resources Research. 15:1109-1116. 1981.
Walters, D. M. and C. G. Enfield. Simulated Waste Access
to Groundwater: A User's Guide. Butler University,
Indianapolis, IN.
Bennett, E. R. and L. E. Leach. Field Studies of Rapid
Infiltration Treatment of Primary Effluent. American
Society of Civil Engineers Specialty Conference on
Environmental Engineering. Boulder, Colorado. July 1983.
Lance, J. C. Land Disposal of Sewage Effluents and
Residue. Groundwater Pollution Microbiology. pp. 197-224
Gabriel Britton and Charles P. Gerba, eds. John Wiley &
Sons,:Inc., New York, NY. 1984.
Leach, L. E. and C. G. Enfield. Nitrogen Control in
Domestic Wastewater Rapid Infiltration Systems. Journal
Water Pollution Control Federation. Vol. 55, No. 9, pp.
1150-1157. September 1983.
Lance, J. C., F. D. Whisler, and R. Bouwer. Oxygen
Utilization in Soils Flooded with Sewage Water. Journal
of Environmental Quality. Vol. 2. November 1973.
Middlebrooks, E. J., ed. Water
Publishers, Ann Arbor, MI. 1982.
Re-Use. Ann Arbor
Page, A. L., T. J. Gleason, J. E. Smith, Jr., I. K.
Iskandar, and L. E. Sommers. Proceedings of the 1983
Workshop on Utilization of Municipal Wastewater and Sludge
on Land. University of California, Riverside, CA. 1983.
deVries, J. Soil Filtration of Wastewater- Effluent and
the Mechanism of Pore Clogging. Journal Water Pollution
Control Federation. 44:565-573. 1972.
Ripley, D. P. and Z. A. Saleem. Clogging in Simulated
Glacial Aquifers due to Artificial Recharge. Water
Resources Research. 9:1047-1057. 1973.
55
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28. Behnke, J. J. Clogging in Surface Operations for
Artificial Ground Water Recharge. Water Resources
Research. 5:870-876. 1969.
29. Bear, J. Hydraulics of Groundwater. McGraw Hill Book
Co., New York, NY. pp. 282-299. 1979.
30. Singh, R. Prediction of Mound Geometry Under Recharge
Basins. Water Resources Research. 12:775-780. 1976.
31. Marino, M.A. Artificial Ground Water Recharge, 1.
Circular Recharging Area. Journal of Hydrology.
25-201-208. 1975.
32.
Khan, M. Y. and D. Kirkham. Shapes of Steady State
Perched Groundwater Mounds. Water Resources Research.
12:429-436, 1976.
33. Glover, R. E. Mathematical Derivations as Pertaining to
Ground Water Recharge. USDA, Agricultural Research
Service, (mimeo.) 81 pp. 1961.
34.
37
38
Bianchi, W. C.: and C. Muckel. Ground Water Recharge
Hydrology. USDA, Agricultural Research Service. ARS
41-161. December 1970.
35. Hantush, M. S. Growth and Decay of Ground Water Mounds in
Response to Uniform Percolation. Water Resources
Research. 3:227-234. 1967.
36. Haskell, E. E., Jr. and W. C. Bianchi. Development and
Dissipation of Ground Water Mounds beneath Square Recharge
Basins. Journal American Water Works Association.
47:349-353. 1965.
Bauman, P. Technical Development in Ground Water
Recharge. Advances in Hydroscience. 2:209-279. V. T.
Chow, (Ed.). 1965.
Khan, M. Y. and D. Kirkham. Shapes of Steady State
Perched Groundwater Mounds. Water Resources Research.
12:429-436. 1976.
39. Brock, R. P. Dupuit-Forchheimer and Potential Theories
for Recharge from Basins. Water Resources Research.
12:909-911. 1976.
40.
Molden, D. J., D. K. Sunada, and J. W. Warner.
Microcomuter Model of Artificial Recharge. In:
Proceedings of First National Conference on Microcomputers
in Civil Engineering. pp. l-5a. W. E. Carroll (Ed.).
Orlando, FL. November 1983,
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41. Neuman, S. P., R. A. Feddes, and E. Bresler. Finite
Element Analysis of Two-Dimensional Flow in Soil
Considering Water Uptake by Plants: I-Theory. Soil
Science Society of American Proceedomgs. 39:224-230.
March-April 1975.
42. Bloomsburg, G. L. and R. E. Rinker. Ground Water Modeling
with an Interactive Computer. Ground Water. 21:208-211.
March-April 1983.
43. Brooks, R. H. and A. T. Corey. Properties of Porous Media
Affecting Fluid Flow. In: Proceedings American Society
of Civil Engineers, Journal Irrigation and Drainage
Division. 92:1R2. 1966.
44. Daly, C. J. A Procedure for Calculating Ground Water Flow
Lines, USACRREL SR 84-9. U. S. Army Corps of Engineers,
Cold Regions Research and Engineering Laboratory, Hanover,
NH. April 1984.
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CHAPTER 5
CONSTRUCTION
5.1 General
The most critical elements during construction of RI systems
are the infiltration surfaces, earthwork for dike construction,
and control of surface and subsurface flow when necessary.
Special attention to all these factors is required to insure a
successful system.
5.2 Infiltration Surfaces in RI Basins
An appropriate design locates the basins in the "best" soil
materials on the site. In some existing systems this principle
was sacrificed to obtain gravity flow to and among the basins.
Gravity flow is desirable, but is not the priority requirement
for site development. The design, if at all possible, also
avoids basin construction on backfilled materials. If fill is
absolutely necessary, then a test fill is used as described in
Chapter 3 and criteria developed from that experience for
construction of the full scale system.
The construction of RI systems utilizes the standard equipment
and machinery used for conventional earthwork. However, most
construction contractors have little or no experience with RI
basin construction. It may be necessary to overcome the
traditional attitudes that "successful" earthwork requires
maximum compaction, maximum soil density and structural
stability of the soil. At a very early stage in the project it
is necessary to insure that all construction personnel
understand the RI concept and the need to avoid any action that
will unnecessarily reduce the hydraulic conductivity of the
basins. In addition, the contract specifications have to be
very explicit regarding the procedures and limitations on
earthwork. Any soil layers or zones to be removed and wasted
are carefully delineated on plans and profiles, and then
rigorously controlled during the actual excavation and disposal
process.
5.2.1 Cut and Fill in Coarse Soils
The field density test is the normal quality control procedure
for placement of fill and related earthwork for most
construction. There have been examples where field density was
the only control on placement of fill for RI basins. The
specifications might require that the density of the fill in
the basin area should not exceed some percentage (75-80%) of
the optimum density. In one project with such specifications,
the basins failed.
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The field density test is not by itself sufficient to insure
adequate quality control during RI construction. As shown in
Figure 3-1(this text),if the field moisture happened to be on
the "wet" side of optimum for a particular soil it would be
possible to achieve a specified density limit and at the same
time reduce the permeability significantly. Placement of all
fill in the infiltration area of a basin~ or excavation
activities within 0.3 m (1 ft) of the final basin surface is
only recommended when the soil moisture content is on the "dry"
side of optimum. Since the construction contractor has no
control over natural rainfall events, the contract is written
to be sufficiently flexible to allow for this downtime. The
grading plan includes measures to prevent surface runoff from
entering the basin area and even sprinkling for dust control is
avoided in the same area when equipment is operating at or near
the final grade.
The final infiltration surface in the basin needs to be
uniformly graded to allow distribution of wastewater and
utilization of the entire soil profile for treatment. Small,
apparently insignificant depressions need to be avoided, since
these will be the last to drain, resulting in a local
accumulation of solids leading to infiltration failure in those
areas. Basins with a slight grade have been successful, and
when positive drainage features are used, they are located at
the lowest spot in the basin. The normal construction efforts
to achieve the required uniform grades {+5 cm (2 in.) is
typical} can, with many soils, result in some"" compact ion of the
surface layer. The following procedures are recommended:
Fill Areas
1.
2.
3.
Cut Areas
1. ,
2.
Bring fill to specified elevation.
Fine grade to specified tolerance.
Both 1 a,nd 2 are only done when soil is on "dry" side
of optimum moisture.
Rip the surface to a depth of at least 0.6 m (2 ft)
and cross rip in a perpendicular direction.
Break up clods and other consolidated material thrown
up by the ripper. Disk-harrow with light tractor or
other vehicle with low ground pressure tires is
usually suitable.
Cut to specified elevation.
Fine grade to specified tolerance.
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3. Steps 1 and 2 when soil moisture is on the "dry" side
of optimum, for the last 0.3 m (1 ft) of cut.
4. Rip the surface to a depth of 0.6 m (2 ft).
5. Disk and harrow surface using low ground pressure
vehicles.
5.2.2 Cut and Fill in Fine-Texture Soils
The same basic precautions given in the previous section also
apply to fine-textured soils. The requirements for low
moisture content during earthwork activities are even more
important in this case. It is assumed that the design has
avoided basin fills (see Chapter 4) when clay and/or, silt
content exceeds about 10%, so this section is only concerned
with cut sections and fills of acceptable materials.
As described in Chapter 3 and 4, the presence of significant
silt and clay in the surface layer of RI basins may eventually
result in infiltration problems due to the sorting and
deposition on the surface of these fine fractions. The only
surface treatment that has been shown to be effective is the
establishment of a grass cover on such soils [IJ. A
combination of Coastal Bermuda (11 kg/ha) (10 Ib/ac) and Dallis
grass (22 kg/ha) (20 Ib/ac) was recommended for one project in
the Southern United .States. Local agronomic experts can be
consulted for recommendations. Seeding is only done at an
appropriate season for the locality, and temporary sprinklers
may be needed to establish grass in dry areas. Wastewater
flooding does not then start until the grass is well
established.
5.2.3 Ridge and Furrow Construction
Ridge and furrow construction can offer advantages for both
grass covered and bare soil basins. A ridge and furrow
configuration provides some increase in the infiltration area
available at the surface. It also allows for more rapid
aeration of the soils, since the ridge tops are dry when the
last increments of wastewater are infiltrating in the furrow.
A ridge and furrow basin bottom can extend the time between
maintenance operations, since solids will tend to accumulate in
the furrow bottoms, leaving the ridges clean. A final
situation where ridges and furrows are suggested are those
locations in extreme cold climates where the design is based on
maintaining a floating ice cover in the winter. The ice layer
rests on the ridge tops and infiltration continues in the
furrows.
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Special equipment may be available for ridge and furrow
construction if the site is in an area where surface irrigation
is normally practiced. A cross-section With furrows at least
30 cm (12 in.) deep was recommended for one RI project in the
Southern United States. Whatever configuration is constructed,
it is necessary that it be easily reproduced by the system
operator, since reconstruction of the ridges is required
periodically.
5.3 Dike Construction
Basic construction procedures and controls used for embankments
are suitable for RI dikes. Erosion control for dike soils is
necessary during construction and the use of silt fences or
other barriers is recommended.
5.4. Control of Water
The design and the grading plan includes consideration of
surface runoff requirements during construction, and these
elements (i.e., ditches, temporary berms, etc.) are typically
installed at a very early stage of construction. Such
temporary elements are then removed when construction is
complete, if not incorporated in the final site drainage plan.,
The,surface runoff from site roads and outer dike slopes is no
different than normal precipitation runoff, so special measures
for water quality control are not required.
The design may include trenches or underdrains to intercept and
re-direct native ground water in the vicinity of the RI basins.
These features typically require a surface outlet, but are not
designed to intercept wastewater percolate. Therefore, a
discharge permit is not necessary. These drains should also be
installed at a very early .stage of construction, as should
monitoring wells where the final location will not interfere
with construction activities. The routine observation of these
wells and drainage features during construction may indicate if
additional work of this nature is necessary. In one situation,
even though some ground water diversion was installed, it was
observed to be insufficient to prevent basin interference by a
seasonally high ground water table.
5.5 References
1. Reed, S. C. The Use of Clayey Sands for Rapid
Infiltration Wastewater Treatment, USACRREL IR 805, U. S.
Army Corps of Engineers, Cold Regions Research and
Engineering Laboratory, Hanover, NH. December 1982.
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CHAPTER 6
OPERATION
6.1 General
In some cases, the design of existing RI systems did not
provide sufficient flexibility for operational adjustments and
basin optimization. This is often compounded by an inadequate
O & M Manual so that the operator may not know how to make
adjustments or understand their significance. There have been
cases where all basins were loaded at the same time in what was
supposed to be a cyclic operation, or where each basin in turn,
was loaded until it was "full" before flow was switched to the
next. It is absolutely essential that the operator be given
sufficient information and resources so that design
expectations can be realized.ThemajorconcernsforRT
systems are wastewater scheduling, maintenance of basin
bottoms, and winter operations [1, 2]. The O & M procedures
for pumps and other equipment, dikes, and similar features are
not unique to RI systems and can be found elsewhere.
6.2 Wastewater Scheduling
In some cases, the design may specify different loadings for
various basins in the system if their soils are distinctly
different. In the typical case, however, the design is based
on a uniform application for all basins as derived from the
field and laboratory test results. It is not unusual for some
of the basins to have a higher capacity than the design rate,
or some less. The operator observes and records the volume of
water applied to each basin and the time required for
infiltration to be complete for every cycle. A staff gauge or
a similar marker in each basin is also recommended. The
operator can then observe the actual rate of infiltration in
the final stages after flooding has been stopped. The operator
also notes the location of wet spots and small areas of ponded
water during the drying period so these can receive special
attention at the next scheduled maintenance.
A routine evaluation of these data permits optimization of the
basins by adjusting the application schedules and loadings on a
regular basis. It is not often possible to alter the pump
capacity to increase or decrease flow rate, so the adjustment
requires a change in the flooding period for a particular
basin. These data also give warning of when clogging is
reaching an unacceptable level. Definition of a generally
applicable rule is not possible, but a preliminary
"rule-of-thumb" suggests that all standing water at the end of
the flooding period should infiltrate within the first 1/3 of
the drying period. If the final infiltration takes more than
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one-half of the drying period, then maintenance is necessary.
In all but the coarsest soils, it is recommended that each
bas"\n .in tlrie sYstem be allowed to dry and then the bottom
scarified or the crust removed once a year during the warmest
and driest season of the year, regardless of its infiltration
capacity at the time.
In many systems the actual wastewater flow in the first few
years of operation may be significantly less than the ultimate
design capacity, and ' an appropriate operation program is
needed. The best approach is to rotate operation among basins
or sets of basins, using only the number needed for the current
flow. These are loaded at the full design rates. This will
also provide an early confirmation that all of the basins have
the capability to function at design rates. Operational
flexibility to make these types of. adjustments requires
multiple cell RI systems. A minimum of three cells is
suggested for even the smallest RI system. The operator may
also have to' make adjustments, or install additions to any
distribution network in the basin, if it appears that
wastewater infiltration is not uniform.
6.3 Maintenance of Infiltration Surfaces
Regular maintenance of the infiltration surfaces in the RI
basin is required and a useful O & M manual contains a
tentative schedule and procedure. Once the system is
operational, the evaluation of the routine observations
discussed above will allow optimization of this maintenance
schedule.
Equipment for routine maintenance typically
tractor, or other towing vehicles, with low
tires and disk/harrow, or similar devices,
surface soil layer. The number and sizes of
varies with the size of the RI system. Small
contract annually, for these services unless
other municipal uses for the equipment. Since
seldom required, it is not necessary to have
dedicated for RI maintenance.
consists of a
ground pressure
to scarify the
this equipment
RI systems may
there are some
deep ripping is
such equipment
These maintenance activities are only conducted when the basin
is dry and the moisture content in the surface soil layer is on
the dry side of optimum, as described in Chapter 5. The first
step is to remove any thick deposits of organic material that
may have accumulated in the low spots and re-grade to eliminate
the low spots. Any deposits of soil eroded from the dikes are
also removed. The final step is scarification of the entire
surface.
Maintenance of grass covered surfaces is similar, but instead
of disking, the surface can be aerated when necessary with a
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spiked drum or similar device. The grass is cut and removed at
least once a year. Experience may show, however, that more
frequent mowing is necessary at a particular location.
Emergency maintenance should also be restricted to dry soils.
If positive drainage elements do not exist in a basin, it is
necessary to excavate a small sump with a backhoe or dragline,
pump out any standing water, and allow the surface to dry prior
to access with heavy equipment.
Deep ripping loosens a consolidated soil. However, if
deposition of organics is the cause of reduced permeability,
ripping cannot provide long-term benefits, since the ripping
only re-distributes the material. When acceptable permeability
cannot be restored, then removal and replacement of the
affected soil layer is recommended. Fort Devens,
Massachusetts, restored their basins in this manner after 20
years of operation [3]. Approxmately 1 m (3 ft) of the upper
soil was removed and replaced with similar material from an
adjacent borrow pit.
6.4 Monitoring.
The water quality monitoring requirements are established by
the regulatory authorities. It is assumed that the design
provided appropriate sampling points and the O & M Manual
defined appropriate techniques. The water levels in all of the
observation wells incorporated in the final system are
routinely observed and recorded. The O & M Manual_ (or a
similar document) explains what the operator should do with the
data and how to interpret it.
A regular tour of the site and the general vicinity is
recommended to insure the integrity of the dikes and to look
for seeps or springs in unexpected locations. These may not
occur for months or even years after system start-up, depending
on soil conditions and topography. Small seeps can often be
corrected with additional fill material. In some cases,
cut-off trenches and/or subsurface drainage may be necessary.
The appearance of a seep in an unexpected location is evidence
that lateral flow is occurring in a direction not predicted by
the design. Additional monitoring wells may, therefore, be
required on such a flow path.
6.5 Winter Operations
The presence of weeds and other random vegetation is not
typically a problem for many RI systems in either winter or
summer. Ft. Devens, Massachusetts operates on a year-round
schedule, but there is no special effort to manage or remove
the vegetation. However, the Ft. Devens system receives
relatively warm primary effluent and the soils are relatively
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coarse and infertile. Any ice that forms in the winter is
melted by ^the next application and the basins drain rapidly.
Any location in a cold region that does not have similar
conditions needs to pay special attention to the vegetation,
and either cut it close to the surface or burn it off in late
fall. Basins in more fertile soils may require vegetation
management during the warm season as well.
Those systems designed for a floating ice cover will require
special operation early in each winter to develop the ice
sheet, and then will have to rely on flow meters or other
remote sensing devices for operation since the basin surface
cannot then be seen beneath the ice layer.
Formation of the ice layer requires continuous flooding, after
the onset of low temperature winter conditions, to maintain
some standing water in the basin. The initial ice layer should
be 8 to 10 cm (3-4 in.) thick for ridge and furrow basins, to
bridge the gap between ridges. Equation 4-7 can be rearranged
and solved for the time required to produce a specified ice
thickness under particular temperature conditions, as shown in
the example below.
- -(I)'
AT]
(see equations 4-7 for definition of terms)
Assuming 10 cm (4 in.) of ice is required, and that the ambient
air temperaure remains at about -10°C (14°F) the time required
to form the ice, when no snow cover is present (C = 3) would
be:
t =
10
To) = 1-1 day
The predicted time of 1.1 day assumes the water is already at
0 C (32 F). Since it is likely that warmer wastewater is used,
there will be additional time required for cooling. Assuming
for this example that the total time required would be about
two days, it would be necessary to maintain a depth of standing
water over the entire basin for that period. Since water is
also infiltrating at a relatively high rate during the same
period, maintaining several centimeters of standing water might
be difficult if the system has rapidly permeable soils and a
low application rate.
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6.6 References
1. U. S. Army Corps of Engineers. Land Treatment Systems
Operation and Maintenance, EM 1110-2-504. Washington, D.
C. November 1983.
2. Reed, S. C. Design, Operation and Maintenance of Land
Application Systems for Low Cost Wastewater Treatment.
In: Proceedings of Workshop on Low Cost Wastewater
Treatment. Clemson University, Clemson, SC. April 1983.
3. Satterwhite, M. B., B. J. Condike, and G. L. Stewart.
Treatment of Primary Sewage Effluents by Rapid
Infiltration. USACRREL TR 87-48. U. S. Army Corps of
Engineers, Cold Regions Research and Engineering
Laboratory, Hanover, NH. 1976.
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PART II. OVERLAND FLOW
CHAPTER 1
INTRODUCTION
1.1 Background
When the second edition of the Technology Transfer Process
Design Manual for Land Treatment of Municipal Wastewater [1]
was published in October 1981, the overland flow (OF) process
was just beginning to gain recognition as an effective
wastewater treatment alternative. The experience gained and
successes reported from these early OF projects have led to a
substantial increase in the use of the OF process for smaller
communities. As of March 1984, there were approximately 48
municipal OF systems in operation or in some stage of design or
construction.
Overland flow treatment of municipal wastewater has finally
moved from the research/demonstration stage to routine
implementation. The guidance provided in the 1981 Manual [1]
has been instrumental in giving the design engineers and
owner/operators a sound basis upon which to design and operate
OF systems. However, as with any new and/or unfamiliar
process, the guidance can be improved as additional information
is gained from construction and operational experience.
1.2 Objectives
The overall objective of this chapter is to update and refine
the information contained in the Manual [1] with the more
recent experiences gained during the design, construction, and
operation of an increasing number of municipal OF systems.
This supplement is. not intended to replace the Manual [1] which
continues to provide a firm base of understanding for the
technology. Therefore, the Manual [1] will be referenced
extensively in this document with existing sections repeated
herein only when necessary to clarify a point.
The information provided in Part II is based principally on two
sources of new information: (1) field investigations of six OF
systems which were placed into operation in 1983 and 1984,
ranging in size from 265 to 11,356 m /d (0.07 to 3.0 MGD) and
three systems which were under construction in early 1984; and
(2) recently available data from research/demonstration
projects.
Although it is premature to assess adequately the effectiveness
of the municipal OF facilities, it appears that most, if not
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all, of them will be capable of meeting their discharge
criteria. The field investigations also uncovered a number of
design, construction, and operational factors which could be
modified to improve their performance and reliability. These
factors are organized by subject matter in this text. _ As long
term operating experience is gained from municipal OF
facilities, it will be possible to develop further refinements
and to optimize engineering criteria.
1.3 References
1. u. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
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CHAPTER 2
CURRENT STATUS
2.1 Introduction
There has been considerable experience gained regarding site
selection, design, construction, and operation of municipal OF
systems since publication of the Manual [1]. This is organized
by subject matter into individual chapters within this text and
briefly summarized in this chapter.
2.2 Site Selection
Experience has indicated two areas where the Manual Hi] needs
clarification. These are the use of the terms "slope" and
"terrace, " and the use of soil permeability data in selecting
sites.
2.2.1 Process Description
The OF process consists of applying wastewater along the upper
portions of sloping, grass covered fields and allowing it to
flow over the vegetated surface to runoff collection ditches.
Throughout this chapter, the uniformly sloped areas which
receive wastewater for treatment will be called "terraces."
The runoff collection ditches into which the treated effluent
flows will be called "drainage channels." In the Manual [1],
the term "slope" was used to describe a terrace, but the latter
term is more appropriate and is more compatible with
conventional agricultural engineering terminology.
The OF process was developed to overcome the limitations --to
land treatment which are created by soil types of low
permeability. OF differs from the other two principal land
treatment processes, slow rate and rapid infiltration, in that
it is not dependent on infiltration and the treated effluent is
discharged as a point source. As continued operating
experience is gained from municipal OF systems, it is expected
that the consideration and use of the process will increase.
Furthermore, as already experienced in some states where OF is
in use, both designers and regulatory officials are taking a
less conservative attitude towards the process design criteria.
2.2.2 Soil Permeability
Although the OF process was developed to overcome the
restrictions of low permeability soils, it can also be used on
sites which do not contain heavy clay and silt soils [2, 3].
Table 2-9 of the Manual [1] suggests that the most suitable
69
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soils for OF are those with permeabilities of less than 0.5
cm/h (0.2 in./h). However, both designers and regulatory
personnel have used that value as an inflexible limit. This
can result in sites being eliminated from consideration for OF
when they are, in fact, suitable.
OF can be designed successfully on sites where the surface soil
permeability is greater than 0.5 cm/h. Consideration for
potential ground water contamination (particularly nitrates) is
necessary if significant percolation is likely to occur (see
Section 4.5.2 of the Manual [1]). Although an artificial
barrier can be constructed on sites where no natural barriers
exist [2, 3], it is usually not cost-effective to do so.
2.3 Process Design
2.3.1 Design Criteria
The recently constructed municipal OF systems have utilized the
design guidance provided in the Manual [1] in a conservative
fashion. Although these facilities have not yet generated much
performance data, observation of a .number of them indicates
they will be capable of performing according to their discharge
criteria. Since the publication of the Manual [1], a number of
independent research and demonstration projects have al.so been
conducted to study the OF process. Most of these projects
either have been completed or are in their final stages of work
after up to ten years of evaluations. In general, these
projects have verified and/or expanded the data base for
treatment and the experiences from many of them were considered
in the development of this document. It is worthy to note that
some of the more recent studies contain initial results
regarding topics not studied prior to the publication of the
Manual [1], such as the removal of toxic organics [4, 5, 6].
Overall, these new sources of information support changes
regarding preapplication treatment and storage.
2.3.2 Preapplication Treatment
The OF experience to date, as discussed in Sections 3 and 4,
leads to the following conclusions:
OF systems are capable of performing satisfactorily
with only minimal levels (i.e., screening,
comminution) of preapplication treatment [1, 7, 8,
9, 10].
For certain treatment objectives (e.g., nitrogen
removal), OF systems using minimal preapplication
treatment perform better than those using higher
levels of preapplication treatment [1, 7, 11, 12,
13].
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Algal solids are difficult to remove in OF systems
[7, 10, 14, 15, 16] and preapplication treatment
processes which encourage algal growth frequently
result in greater discharge of suspended solids.
Inadequate screening, resulting in clogging of
distribution systems, is a common problem found in
municipal OF systems.
2.3.3 Storage
OF systems often require less storage than slow rate land
treatment systems for the same climatic conditions (see Chapter
5). In most cases, there is no need for storage during
rainfall events. Because of the potential for algae
production, storage cells should be designed as off-line,
rather than on-line, components of the treatment system.
2.3.4 Distribution Systems
Most municipal OF systems designed to date use surface or low
pressure distribution methods to minimize energy costs and
aerosol generation. Although these methods can be used
effectively, it is difficult to balance the hydraulics to
achieve uniform flow to all terrace areas. Consequently, it is
also difficult to achieve and maintain uniform sheet flow down
the terraces. These factors do not lessen the advantages of
using surface and low pressure systems for municipal
wastewaters, provided they are considered during design and
construction as discussed in Chapter 6.
2.4 Terrace Design and Construction
Observation of recently completed municipal OF facilities has
shown that terrace construction has not always achieved the
design goal of uniform sheet flow. In efforts to conserve
costs, or because -of a lack of understanding of the importance
of grading, many OF terraces have not been designed and
constructed in a manner and to the tolerances which will
provide uniform sheet flow. Detailed recommendations for
improving terrace design and construction practices are
provided in Chapter 7.
2.5 Vegetation Selection and Establishment
The OF process requires water-tolerant turf grass. Thus, the
options of vegetation selection are not nearly as flexible as
for slow rate land treatment. The more intensive operating
schedule of an OF system diminishes the significance and value
of harvesting the grass crop. It is best to leave the cut
grass on the terrace for the first year or two to build up a
mulch layer, thereby accelerating system acclimation and
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providing better overall performance. Moreover, since the
primary purpose of the OF system is wastewater treatment .and
not crop production, the cash value of the grass should not be
given more importance than it deserves. An over-emphasis on
cash value can lead to operating procedures which are
counter-productive to good wastewater treatment performance.
Recommended procedures for vegetation selection, establishment,
and management are discussed in Chapter 8.
2.6 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. Reed S. C. and R. W. Crites. Handbook of Land Treatment
Systems for Industrial and Municipal Wastes. Noyes Data
Corporation, Park Ridge, NJ. 1984.
3. Ketchum, L. H. et al. Overland Flow Treatment of Poultry
Processing Wastewater in Cold Climates. EPA
600/52-81-093. U. S. Environmental Protection Agency,
Robert S. Kerr Environmental Research Laboratory, Ada,
OK. July 1981.
4. Jenkins, T. F., et al. Assessment of the Treatability of
Toxic Organics by Overland Flow. CRREL Report '83-3. U.
S. Army Corps of Engineers Cold Regions Research and
Engineering Laboratory, Hanover, NH. 1983.
5. Zearth, M., et al. 'Removal of Toxic Organics by Overland
Flow. Department of Civil Engineering, University of
California Davis - Davis, CA. 1984.
6. Enfield, C. G., et'al. Mathematical Description of
Volatile Toxic Organics on Overland Flow. U. S.
Environmental Protection Agency, Robert S. Kerr
Environmental Research Laboratory> Ada, OK. '1984.
7. Smith, R. G. and E. D. Schroeder. Demonstration of the
Overland Flow Process for the Treatment of Municipal
Wastewater - Phase II Field Studies. Department of Civil
Engineering, University of California, Davis - Davis,
California. 1982.
8. Thomas, R. E., K. Jackson, and L. Penrod. Feasibility of
Overland Flow for Treatment of Raw Domestic Wastewater.
EPA 660/2-74-087. U. S. Environmental Agency, Robert S.
Kerr Environmental Research Laboratory, Ada, OK. 1974.
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9- Bledsoe, B. E. Development Research for Overland Flow
Technology. EPA 600/9-31-022. In: Proceedings of
National Seminar on Overland Flow Technology for Municipal
Wastewater. U. S. Environmental Protection Agency, Robert
S. Kerr Environmental Research Laboratory, Ada, OK.
1980.
10. Abernathy, A. R. Overland Flow Treatment of Municipal
Sewage at Easley, SC. EPA 600/2-83-015. 1983.
11. Martel, C. J., T. F. Jenkins, and A. J. Palazzo.
Wastewater Treatment in Cold Regions by Overland Flow.
CRREL Report 80-7. U. S. Army Corps of Engineers, Cold
Regions Research and Engineering Laboratory, Hanover, NH.
1980. ;
12. Thomas, R. E. Preapplication Treatment for Overland Flow.
In: Proceedings of International Symposium of State of
Knowledge in Land Treatment of Wastewater. Volume I, pp.
305-312. U. S. Army Corps of Engineers, Cold Regions
Research and Engineering Laboratory, Hanover, NH. 1978.
13. Smith, R. G., et al. Performance of Overland Flow
Wastewater Treatment Systems Summary Report. Department
of Civil Engineering, University of California, Davis -
Davis, California. 1982
14. Witherow, J. L. and B. E. Bledsoe. Algae Removal by the
Overland Flow Process. U. S. Environmental Protection
Agency, Robert S. Kerr Environmental Research Laboratory,
Ada, OK. 1982.
15. Hall, D. H., et al. Municipal Wastewater Treatment by the
Overland Flow Method of Land Application.
EPA-600/2-79-178. 1979.
16. Peters, R. E., C. R. Lee, and D. J. Bates. Field
Investigations of Overland Flow Treatment of Municipal
Lagoon Effluent. Technical Report EL-81-9. U. S. Army
Engineer Waterways Experiment Station, Vicksburg, MS.
September 1981.
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CHAPTER 3
DESIGN CRITERIA
3.1 Procedures
The Manual [1] describes two procedures for the design of an OF
system. The most commonly used method is an empirical approach
which was developed from successfully operating OF systems.
The alternative presented in the Manual [1] is the preliminary
development of a rational design procedure [2, 3, 4]. Advances
in both of these methods are presented below.
3.2 Empirical Method
3.2.1 Revised Criteria
Most, if not all, of the recently constructed OF facilities
were designed using the empirical method. Hydraulic loading
rates, application rates, terrace lengths, and other variables
were selected to be within .the range of design criteria listed
on Table 6-5 of the Manual [1]. Recent studies [5, 6, 7] have
shown excellent performance with hydraulic loading1 rates
greater than the upper limit of the ranges shown on Table 6-5
of the Manual [1], thereby implying that the ranges given in
Table 6-5 [1] are conservative. However, these high rate
application projects have been research/demonstration projects
with near ideal construction and operational control. The use
of high loading rates has not yet been observed on full-scale
systems which are operated and controlled exclusively by a
municipality. Most of the municipal systems were designed and
are currently operating at the low end of the range given in
Table 6-5 of the Manual tl]• Therefore, major modifications to
Table 6-5 [1] cannot ,be supported by the results from current
operating systems. However, on the basis of current knowledge,
most municipal OF systems can operate successfully at loading
rates nearer to the median of the design ranges^Conservative
designs near the low end of the range may negate the advantage
of using the OF process. Such conservatism can result in a
non-discharging slow rate land treatment system which is
unnecessarily costly due to the extensive preparation of
terraces. It may also result in an intermittent overland flow
system Which produces a poorer quality effluent because the
terraces often become too dry for good performance.
Table 3-1 is a revised version of Table 6-5 of the Manual [1].
Table 3-1 shows a range of values for each type of
preapplication treatment based on successfully operating
systems. The relationship between application rate and
hydraulic loading rate is discussed in Section 6.4.2 of the
Manual [1]. These ranges allow the designer to select specific
74
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TABLE 3-1
SUGGESTED OVERLAND FLOW DESIGN RANGES
Preapplication
Treatment
Application
Rate
m /h m
Hydraulic
Loading Rate
cm/d
Screening/Primary 0.07 - 0.12'
2.0 - 7.0
Aerated Cell 0.08 - 0.14
(1 day detention)
2.0 - 8.5
Wastewater Treat-
ment pond°
0.09 - 0.15
2.5 - 9.0
Secondary
0.11 - 0.17
3.0 -10.0
a. m /h m x 80.5 = gal/h ft
b. cm/d x 0.394 = in./d
c. Does not include removal of algae
d. Recommended only for upgrading existing secondary
treatment.
75
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design values on the basis of several inter-related factors
such as effluent discharge requirements, climate, and the type
of distribution system used. Rates near the upper end of the
range may be used during warm months, while values near the low
end are suggested if soil temperatures drop below approximately
10°C or if maximum removal efficiency is desired [!]• Slope
lengths and application periods are discussed in Sections
3.2.2.3 and 3.2.2.4, respectively.
3.2.2 Use of Design Ranges
3.2.2.1 Selection of Hydraulic Loading Rate
The suggested design ranges shown on Table 3-1 are divided into
four categories on the basis of the level of preapplication
treatment provided. Within each of the four categories, the
specific design criteria are shown as ranges, rather than as a
single number. The range is provided to account for the
variable conditions which may be encountered at different OF
sites throughout the country. The principal variables are the
climatic environment of the project and the effluent discharge
limitations imposed on the system.
The first step in using Table 3-1 is to establish the level of
preapplication treatment which will be provided. Next, the
hydraulic loading rate can be selected on the basis of climate
and effluent limitations. For use with Table 3-1, climatic
conditions can be divided into three categories as follows:
Cold climates - Those which require the same number of
storage days for OF as for slow rate systems. This is
explained in more detail in Chapter 5. The
geographical areas defined by this category are those
at and above the 80 day storage line shown on Figure
5-1 of this document.
Moderate climates - Those locations between the 40 day and
80 day storage lines shown on Figure 5-1.
Warm climates - Those where the OF system will not be
affected by short duration freezing temperatures. The
warm climate zones are defined as those at and below
the 40 day storage line shown on Figure 5-1.
Also for use with Table 3-1, the effluent discharge criteria
can be divided into three categories as follows:
Least stringent - Defined as EPA's secondary treatment
quality for trickling filters and wastewater treatment
ponds.
76
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Moderately stringent - Intermediate levels of BOD and
suspended solids (between 10/10 and secondary limits
defined above), but no nutrient limitations.
Most stringent - High ,levels of BOD and suspended solids
removal (i.e., 10/10 or better) and/or the inclusion of
nutrient limitations.
When the project location is in a warm climate zone (at or
below the 40 day storage line on Figure 5-1) and the effluent
discharge limitations fall into the least stringent category
(as defined above), the highest application rate (and hydraulic
loading rate) in a particular preapplication treatment category
can be selected. Conversely, when the project is in a cold
climate (at or above the 80 .day storage line on Figure 5-1) and
the effluent discharge limitations fall into the . most
restrictive category (as defined above), the lowest rates in a
particular preapplication treatment category can be considered.
If both variables (climate and discharge limitations) are in
the moderate categories, rates near the median of a specific
range are suggested. Using the same logic, all other
combinations of the two principal variables (climate and
discharge limitations) will fall somewhere within the range of
application rates and hydraulic loading rates in Table 3-1.
3.2.2.2 Determination of Land Area
The land area is determined by combining the facility design
flow, the hydraulic loading from Table 3-1, and the number of
operating days for the system. The equations in Section 6.4.8
in the Manual [1] can be used for this purpose. The operating
days for a particular facility are based on the storage factors
in Chapter 5 of this .text and the designer's best judgement
regarding any other non-operating periods. It should be noted
that once the hydraulic loading rate is selected and the land
area calculated, the designer still may have the option of
selecting a surface, low pressure, or high pressure system,
provided the application rates are maintained within the ranges
presented in Table 3-1.
3.2.2.3 Selection of Terrace Length
The length of individual OF, terraces is controlled by the
choice of distribution system and the effluent discharge
limitations. Surface and low pressure distribution systems
require terrace lengths of 20 to 30 meters, while high pressure
sprinkler distribution systems require a terrace length of 10
to 20 meters greater than the diameter of the sprinkler
pattern. The three categories of discharge criteria listed in
Section 3.2.2.1 also influence selection of terrace length.
When the discharge limitations fall into the least stringent
category, the shortest length can be considered, while the most
77
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stringent category will require the longest length
lowest application rate.
and the
3.2.2.4 Selection of Application Period
Table 6-5 of the Manual [1] suggests that application periods
can be from 8 to 12 hours per day, and that the application
frequency can be from 5 to 7 days per week. Although these
remain as good guidelines for satisfactory treatment
performance, they are not rigid boundary conditions. Of
particular importance is the fact that the application periods
(at the rates shown in Table 3-1) can be increased up to 24 h/d
for periods of two to, four weeks during warm weather with no
adverse impact on treated effluent BOD and suspended solids
concentrations. Continuous application (using 0.03 m /h-m) at
24 h/d, 7 d/wk was successful for over a year using primary
effluent at a pilot project in New York State [8]. At3another
project, with a higher application rate (0.18 m /h • m) ,
continuous operation for 24 h/d for extended periods of time
(i.e., greater than two weeks), even during warm weather, was
found to result in decreased performance, particularly with
respect to nitrogen removal C2] .
The capacity for accepting longer application periods for a few
weeks allows portions of a system to be shut down completely
for mowing and/or maintenance while the remainder of the system
continues to treat the design volume of wastewater at hydraulic
loading rates higher than the design average. It also allows
stored wastewater to be added to the daily design flow and be
satisfactorily treated without adding to the total land
requirements.
Based on previous experiences, it follows that the application
periods can be flexible, but they usually fall within a range
of 6 to 12 hours, regardless of the climate and discharge
categories used for selecting the design hydraulic loading
rate. Selecting a specific application period between 6 and 12
h/d for individual terraces has not been found to be a critical
part of the design procedure. However, there are certain
advantages (explained in Section 6.5) to operating a system 24
hours a day, while still maintaining a 6-12 h/d application
period for the individual terraces.
3.3 Rational Design Procedure
3.3.1 Introduction
A rational design procedure is presented in Section 6.11.2 of
the Manual [1] as a model which describes BOD removal as a
function of terrace length and application rate, where the
application rate has the units m /h.m of slope width. This
78
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procedure was developed by Smith and Schroeder at the
University of California-Davis [2] as an alternative to the
empirical design approach.
3.3.2 Procedure
The rational design equation is presented in the following form
Cl, 2]:
c -c A -kz/qn
z = A exp ' ^
(3-1)
where:
C
= concentration of BOD,, in applied wastewater, mg/L
C = concentration of BOD_ at a distance (z) down the
terrace, mg/L
c = minimum achievable effluent concentration, mg/L
(determined to be 5 mg/L BOD [2])
A = empirically determined coefficient dependent on the
value of q
z = distance down terrace, m
k = empirically determined rate constant
q = application rate, m /h.m terrace width, the valid
range for model shown on Figures 3-1 and 3-2
n = empirically determined coefficient, exponent
The preceding equation has been developed further [4] as a
family of curves of (C -c/C ) vs z for different values of q.
These curves, as shown1 on figures 3-1 and 3-2 for screened
wastewater and primary effluent, respectively, can be used for
selecting design application rates to achieve needed BOD
reductions.
3.3.3 Example
The following example was developed by Smith and Schroeder [2].
It is presented here only to illustrate the design procedure
just described.
79
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Assume the following information is known:
1. Applied wastewater = screened raw municipal
sewage
2. Plow (Q) = 3,000 m3/day
3. Influent BOD5 (CQ) = 150 mg/L
4. Required effluent BOD_ (C) = 20 mg/L
The necessary design calculations are:
1. Compute the required removal ratio C-5/C
C-5
20-5
150
= 0.10
Select application rate (q) in valid range of
model
Select q = 0.37 m /h
m
Determine required value of terrace length (z),
referred to as distance down-slope in Figure 3-1.
z = 41 .5m
Select application period (P) .
P = 12 h/d
Compute q for area calculation, applying a safety
factor of 1.5 [2].
q
0.37 nr/h.m
173
= 0.25 m /h-m
80
-------
0.02
0.01
FAMILY OF LINES
REPRESENT DIFFERENT
APPLICATION RATES, (m3/h-m)q.
10 20 30 40
DISTANCE DOWN SLOPE, m
FIGURE 3-1
BOD5 FRACTION REMAINING VS. DISTANCE DOWN SLOPE
WITH SCREENED RAW WASTEWATER (4)
81
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FAMILY OF LINES
REPRESENT DIFFERENT
APPLICATION RATES, (m3/h-m)q
0.01
20 30
. DISTANCE DOWN SLOPE, m
FIGURE 3-2
BOD5 FRACTION REMAINING VS. DISTANCE DOWN SLOPE
WITH PRIMARY EFFLUENT (4)
82
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6. Compute required total area.
application frequency.
Area =
Assume 7 d/wk
Area =
(q)(P)
- (3,000 m3/d)(41.5 m)
(0.25 m /h.m)(12 hr/d)
Area = 41,500 m = 4.2 ha (10.4 ac)
The rational design procedure presented above has been tested
on several OF research systems [2, 3, 9, 10]. Field
investigations at Ada, OK £9] verified the design equation with
respect to terrace length and expanded it to determine terrace
length for desired concentrations of suspended solids and
ammonia. Those field investigations [9] ran from July through
October when maximum and minimum average monthly air
temperatures were 35°C (95°F) and 9 C (48 F), respectively.
Use of the equation for other than warm weather conditions was
not tested. Field investigations at Easley, SC [10] partially
validated the BOD removals defined by Equation 3-1.
As more experience is gained, this model, or some similar first
order relationship, may become the basis for all designs. One
of the major present benefits is the support that this
procedure provides for the safety and conservatism of the
criteria in Table 3-1, and the related empirical method
described in this Chapter. It shows that the empirical design
procedure (Table 3-1) has an adquate safety factor and that
additional safety factors are not needed when using the
empirical method.
3.4 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. Smith, R. G. and E. D. Schroeder. Demonstration of the
Overland Flow Process for the Treatment of Municipal
Wastewater - Phase II Field Studies. Department of Civil
Engineering, University of California, Davis - Davis,
California. 1982.
83
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3. Martel, C. J., et al. Development of a Rational Design
Procedure for Overland Flow Systems. CRREL Report 82-2.
U. S. Army Corps of Engineers, Cold Regions Research and
Engineering Laboratory, Hanover, NH. 1982.
4. Smith, R. G. Development of a Rational Basis for Design
and Operation of the Overland Flow Process. EPA
600/9-31-022. In: Proceedings of National Seminar on
Overland Flow Technology for Municipal Wastewater, Dallas,
TX. 1981.
5. Ketchum, L. H. et al. Overland Flow Treatment of Poultry
Processing Wastewater in Cold Climates. EPA
600/52-81-093. U. S. Environmental Protection Agency,
Robert S. Kerr Environmental Research Laboratory, Ada, OK.
July 1981.
6. Wightman, D., et al. High-Rate Overland Flow. Water
Resources Research, Volume 17, No. 11, pp. 1679-1690.
Pergamon Press Ltd. 1983.
7. George, D. B., D. Wightman, and J. Witherow. High-Rate
Overland Flow Treatment of Municipal Wastewater. LCC
Institute of Water Research, Lubbock, TX. 1983.
8. Clark, P. J.
Project Report,
Marsh Pond/Overland Flow Pilot Plant
Clark Engineers, Rochester, NY. 1983.
9. Witherow, J. L. and B. E. Bledsoe. Design Model for the
Overland Flow Process. U. S. Environmental Protection
Agency, Robert S. Kerr Environmental Research Laboratory,
Ada, OK. 1984.
10. Abernathy, A. R., J. Zirschky, and M. B. Borup. Overland
Flow Wastewater Treatment at Easley, SC. Clemson
University, Clemson, SC. 1984.
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CHAPTER 4
PREAPPLICATION TREATMENT
4.1 Areas of Importance
Experience with OF technology continues to show several areas
of preapplication treatment to be especially important to the
cost-effectiveness and performance of the process. These are:
The level of preapplication treatment
Effective solids removal to prevent clogging of the
distribution system
The effect of algae on suspended solids removal
4.2 Level of Preapplication Treatment
Section 6.3 of the Manual [1] recommends that preapplication
treatment consisting of screening and comminution is adequate
for OF systems. Where the treatment site is not well isolated,
aeration is also recommended to control odors during storage
and/or application. Data from research studies [2, 3, 4, 5,3
continue to confirm the ability of OF systems to produce an
effluent superior to secondary effluent when applying screened
raw municipal wastewater. Thus, it is generally not
cost-effective to provide high levels of treatment prior to OF.
Ynfact,IncaseswherenitrogenremovalIsa major
consideration, high levels of preapplication treatment can
actually interfere with this objective. Greater nitrogen
removal can be achieved when the applied wastewater has a
higher carbon to nitrogen ratio [6]. Thus, better nitrogen
removal can be expected when applying screened raw wastewater
than when applying either primary or secondary effluent.
Presently, many regulatory authorities require higher levels of
preapplication treatment than screening or comminuting. A
single-cell aeration pond with a detention time of
approximately one day is an approach which is gaining
acceptance. These short detention time aerated cells are in
use in several recently constructed OF systems. Limited
performance data are available from these systems. However,
this method offers the following potential advantages:
A level of preapplication treatment which often
satisfies those state guidelines which require higher
levels of preapplication treatment than screening or
comminution.
85
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4.3
Positive odor control.
Minimal sludge management (as compared to
conventional primary and. secondary treatment
processes).
Relatively simple and inexpensive construction and
operation.
Reduced potential for .algal growth.
Solids Removal
A common problem with some newly constructed municipal OF
systems has been solids clogging of the distribution system.
The result is poor wastewater distribution onto the terraces
and increased maintenance time for opening clogged orifices in
the distribution system. Although this problem can never be
completely eliminated, it can be substantially reduced by
providing an adequate screening system as part of
preapplication treatment.
4.4 Algal Interference
The Manual ClD indicates that algal solids have been difficult
to remove from some stabilization pond effluents . Further
experience C2, 5, 7, 8, 9] has confirmed that algal solids are
not reliably removed by OF systems . Preapplication treatment
that generates algae (i.e., stabilization ponds) increases the
effluent suspended solids concentration from OF systems
compared to preapplication treatment which does not encourage
algal growth. Appropriate selection of preapplication
treatment processes, coupled with appropriate design of storage
facilities, is an important consideration when designing to
minimize algal solids in system discharges.
4.5 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. Smith, R. G. and E. D. Schroeder. Demonstration of the
Overland Flow Process for the Treatment of Municipal
Wastewater - Phase II Field Studies. Department of Civil
Engineering, University of California, Davis - Davis,
California. 1982.
86
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3. Thomas, R. E., K. Jackson, and L. Penrod. Feasibility of
Overland Flow for Treatment of Raw Domestic Wastewater.
EPA 660/2-74-087. U. S. Environmental Protection Agency,
Robert S. Kerr Environmental Research Laboratory, U. S.
EPA, Ada, OK. 1974.
4. Bledsoe, B. E. Development Research for Overland Flow
Technology. EPA 600/9-31-022. In: Proceedings of
National Seminar on Overland Flow Technology for Municipal
Wastewater. U.S. Environmental Protection Agency, Robert
S. Kerr Environmental Research Laboratory, Ada, OK. 1980.
5. Abernathy, A. R. Overland Flow Treatment of Municipal
Sewage at Easley, SC. EPA 600/2-83-015. 1983.
6. Smith, R. G., et al. Performance of Overland Flow
Wastewater Treatment Systems Summary Report. Department
of Civil Engineering, University of California, Davis -
Davis, California. 1982
7. Witherow, J. L. and B. E. Bledsoe. Algae Removal by the
Overland Flow Process. U. S. Environmental Protection
Agency, Robert S. Kerr Environmental Research Laboratory,
Ada, OK. 1982.
8. Hall, D. H., et al. Municipal Wastewater Treatment by the
Overland Flow Method of Land Application. EPA
600/2-79-178. 1979.
9. Peters, R. E., C. R. Lee, and D. J. Bates. Field
Investigations of Overland Flow Treatment of Municipal
Lagoon Effluent. Technical Report EL-81-9. U. S. Army
Engineer Waterways Experiment Station, Vicksburg, MS.
September 1.981.
87
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CHAPTER 5
STORAGE
5.1 Current Practice
Section 6.5.1 of the Manual [1] states that the EPA computer
programs C2] EPA-1 and EPA-3 (storage programs based on
freezing temperatures, rainfall, and snow accumulation
constraints) may be used to estimate, conservatively, the
winter storage requirements for OF systems. It further states
that in areas of the country below the 40-day storage contour
shown on Figure 2-5 of the Manual [1], OF systems can generally
operate all year without storage. One example is a large
industrial OF system in Texas [3J which has operated
successfully for 20 years without any storage.
Using EPA-1 and EPA-3 produces a conservative design of storage
facilities for municipal OF systems. This conservatism is
fostered by several factors, including:
EPA-1 and EPA-3 were developed primarily for slow
rate land treatment systems.
Lack of operating experience with OF operation in
various geographical areas.
Concern by state regulatory authorities over cold and
wet weather performance.
The use of storage facilities for other objectives
(e.g., preapplication treatment, reduction of the
length of the operating days, weekend shutdown, etc.)
in addition to non-operation only during inclement
weather.
A small amount of storage does provide considerable operational
flexibility to a system. Even where climatic conditions are
mild enough to avoid cold weather storage, two to five days of
storage capacity will allow the convenience of weekday
operation, if desired. It will also provide added flexibility
for terrace maintenance and mowing. This flexibility is
especially important for small to moderate size systems.
However, a large percentage of recent designs are combining
storage as an inseparable part of the preapplication treatment
process (e.g., the storage volume is added to the volume of a
stabilization pond). Such designs have limited operational
flexibility to allow control of algal production or to remove
the algae prior to application to the OF system.
88
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5.2 Cold Weather Storage Requirements
Experience has been mixed regarding the ability of OF systems
to operate during cold weather conditions. Studies in New
Hampshire [4], Wyoming [5], and Indiana [6] have shown that OF,
operated in'the typical cyclic pattern, could not always meet
secondary effluent requirements. On the other hand, an OF
system treating primary effluent in New York State [7] has
operated successfully through two winters without storage and
met secondary effluent requirements by operating continuously
for 24 hours a day, 7 days per week. That system operated
under very severe climatic conditions, yet consistently met or
exceeded secondary treatment quality.
General guidance for storage at overland flow systems is
provided in Figure 5-1 which is a revised storage contour map
similar to Figure 2-5 found in the Manual [1]. Figure 5-1 is
only a general guide, based on the following conditions:
Storage equivalent to that required for slow rate
land treatment systems is shown for systems in
moderate and cold climates (i.e., at and above the
40-day contour line on Figure 5-1).
Storage for cold weather operation is not shown for
systems in warm climates (i.e., those below the
40-day contour line on Figure 5-1).
A minimum of two to five days storage is recommended
for systems in warm climates to provide operational
flexibility and convenience.
The designer and operator must recognize that, if storage is
provided according to Figure 5-1, the storage facility must be
reserved for those days which actually require storage. In
moderate climate regions, there are frequently a significant
number of favorable application days intermixed with
unfavorable days during the winter. During these favorable
days, it may not be possible to apply the full design loading
rate, but it may very well be possible to apply at a reduced
loading rate without violating the effluent discharge
limitations. This procedure can be used to reduce the storage
requirements for systems located between the 40- and 80-day
contour lines. On the other hand, in the cold climate zones
(those above the 80-day storage line), it may not be possible
to use intermittent operation during the winter without
violating the effluent discharge limitations.
5.3 Rainfall Considerations
There continues to be a common misconception that OF systems
cannot be operated during rainfall events. However, since OF
89
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2 TO 5 DAYS STORAGE
FOR OPERATIONAL FLEXIBILITY.
o
SCALE
500 I OOP
__d
KILOMETERS
FIGURE 5-1
RECOMMENDED STORAGE DAYS
FOR OVERLAND FLOW SYSTEMS
90
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systems are specifically designed for surface runoff,
arbitrarily prohibiting wastewater application during rainfall
will serve no purpose unless the discharge violates the
conditions of the discharge permit and impairs water quality in
the receiving stream.
5.3.1 Impact on Effluent BOD Concentration
Experience has shown that rainfall of any intensity has little
effect on effluent BOD concentrations. Studies with screened
raw and primary effluent municipal wastewaters showed effluent
BOD concentrations during rainfall events to be only slightly
eleyated above normal operating values, but-well below the 30
mg/L secondary treatment standard [8, 9]. Similar results have
been found at several industrial OF systems treating food
processing [10], textile [11], and pulp and paper [12]
wastewaters. Most of the industrial experiences showed that
the effluent BOD concentration increased slightly during
moderate rainfall intensities and/or short duration storms, but
actually decreased below normal operating values during high
intensity and/or long duration rainfall events. This decrease
can most likely be attributed to dilution.
5.3.2 Impact on Effluent Suspended Solids Concentration
The impact of rainfall on the suspended solids concentration of
the treated effluent can be more significant than the influence
on BOD. Increased concentrations of suspended solids have been
found to consist principally of non-volatile solids resulting
from varying degrees of soil erosion [8, 13]. Elevated levels
of suspended solids are typically found in the rainfall runoff
from newly constructed systems where the vegetation has not had
sufficient time to provide thorough coverage of the terraces,
where runoff channels have not been stabilized, or where the
design/construction practices did not follow sound soil
conservation and agricultural principles. Mature OF systems
with stable soils and established vegetation show little or no
increase in suspended solids concentrations. In one project
where the impact of rainfall was studied [14], the suspended
solids concentration in the discharge resulting solely from
rainfall of approximately 1.5 cm was similar to the following
day's discharge resulting solely from applied wastewater. In
some cases, erosion protection (e.g., rip-rap, concrete lining,
and other soil stabilization procedures) of drainage channels
may be necessary to further reduce suspended solids discharges.
5.3.3 Mass Discharges
Even though the effluent BOD and suspended solids
concentrations during rainfall are similar to dry weather
conditions, the mass discharge of these constituents obviously
increases proportionally to both the intensity and duration of
91
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the rainfall event. For systems with discharge limitations
which are specified in mass units, heavy* rainfall events can
cause a violation of the mass discharge limitations during the
actual event C14, 15]. However, it is unlikely that the 30-day
average limits in the discharge permit will be violated.
5.3.4 Recommended Operating Practices
While rainfall increases the mass of most pollutants discharged
from an OF system, it also results in increased stream flow and
minimal impact on receiving water quality. Therefore, the
operating permits for OF systems need not prohibit application
of wastewater during rainfall events. Recognizing this, some
state regulatory agencies and EPA regions have written permits
based on flow which increase the mass discharge limits during
rainfall and/or replace them with a concentration limit. The
U. S. Army Engineer Waterways Experiment Station has
recommended a detailed procedure for developing permits which
take the above factors into consideration [9].
5.4 Storage Reservoir Design
To minimize the impact of algae on treatment performance, the
storage reservoir should be designed as an off-line component
of the system and used only as the need dictates. The storage
reservoir should be emptied as soon as possible by blending the
stored wastewater containing algae with pre-treated wastewater
prior to its application to the OF system.
5.5 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. Whiting, D. M. Use of Climatic Data in Estimating Storage
Days for Soil Treatment Systems. EPA-600/1-76-250. U. S.
Environmental Protection Agency, Office of Research and
Development. 1976.
3. Thornthwaite, C. W. An Evaluation of Cannery Waste
Disposal by Overland Flow Spray Irrigation. Publications
in Climatology 22. 1969.
4. Martel, C. J., T. F. Jenkins, and A. J. Palazzo.
Wastewater Treatment in Cold Regions by Overland Flow.
CRREL Report 80-7. U. S. Army Corps of Engineers Cold
Regions Research and Engineering Laboratory, Hanover, NH.
1980.
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5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Borelli, J., et al. Overland Flow Treatment of Domestic
Wastewater in Northern Climates. University of Wyoming,
Laramie, WY. 1984.
Ketchum, L. H. et al. Overland Flow Treatment of Poultry
Processing Wastewater in Cold Climates. EPA
600/52-81-093. U. S. Environmental Protection Agency,
Robert S. Kerr Environmental Research Laboratory, Ada, OK.
July 1981.
Clark, P. J. Marsh Pond/Overland Flow Pilot Plant
Project Report. Clark Engineers, Rochester, NY. 1983.
Figueiredo, R. F., R. G. Smith, and E. D. Schroeder.
Rainfall and Overland Flow Performance. Journal American
Society of Civil Engineers, Environmental Engineering
Division, Vol. 110, No. 3, p. 678. June 1984.
Peters, R. E., C. R. Lee, and D. J. Bates. Field
Investigations of Overland Flow Treatment of Municipal
Lagoon Effluent. Technical Report EL-81-9. U. S. Army
Engineer Waterways Experiment Station, Vicksburg, MS.
September 1981.
Gilde, L. C. and O. M. Aly. Personal Communications.
Campbell Soup Company, Camden, NJ. 1984.
Deemer, D. D. Overland Flow Treatment of Textile Mill
Wastewater. In: Proceedings of the 1984 Triangle
Conference on Environmental Technology. Duke University,
Durham, NC. 1984.
Deemer, D. D. Personal Notes.
Marietta, GA. 1984.
ERM-Southeast, Inc.,
Witherow, J. L. Overland Flow in Warm Climates. In:
Proceedings of the 44th Annual Louisiana Conference on
Water Supply, Sewage, and Industrial Waste. 1981.
Witherow, J. L. et al. Meat Packing Wastewater Treatment
by Spray Runoff Irrigation. EPA 600/2-76-224. In:
Proceedings of the Sixth National Symposium on Food
Processing Wastes. 1976.
Witherow, J. L.
Systems - II. In:
Waste Conference.
1975.
Small Meat-Packers Waste Treatment
Proceedings of the 30th Industrial
Purdue University, Lafayette, IN.
93
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CHAPTER 6
DISTRIBUTION SYSTEMS
6.1 Selection
OP achieves treatment primarily through contact between the
applied wastewater and the soil medium. Other factors being
equal, the distribution method which achieves the best sheet
flow pattern will produce the best quality of effluent. As
stated in Section 6.6 of the Manual [1], wastewater
distribution on OF systems can be achieved by surface methods,
low pressure sprays, and moderate to high pressure impact
sprinklers. Observation of recently constructed systems has
added considerable knowledge about the importance of selection
and design. A summary of the advantages and limitations of
various types of distribution systems is provided in Table 6-1.
6.2 Surface Methods
Surface distribution methods are favored by many regulatory
authorities and engineers because they offer potentially lower
operating costs and minimal aerosol generation. Because of the
low aerosol potential, state regulations usually require less
buffer zone area for surface systems, especially compared to
high pressure sprinkler methods
Achieving and maintaining
uniform flow onto and across the OF terraces can be
limitation of surface distribution systems
They require
careful installation to achieve the leveling required for
uniform flow through each orifice, and periodic maintenance
thereafter. Hydraulically balancing and maintaining
distribution becomes increasingly difficult as the elevation
differences within the system increase,
Uniform sheet flow
over the terraces can;generally be accomplished in the presence
of a thick cover crop. Spreading can also be improved by
applying the wastewater onto a gravel layer and/or splash
blocks. The gravel layer is often underlain by a plastic
membrane. Sufficient grade must be provided at the point of
application to prevent backflow under the distribution pipes
and resulting non-uniform distribution.
6.2.1 Gated Pipe
Gated pipe is probably the best choice of the surface
application methods. However, because the gated pipe is
located above ground, it has a potential for freezing and
settling in addition to the limitations previously mentioned.
Pipe-runs exceeding 100 m (300 ft), make it difficult to attain
uniformity of discharge between gates without careful
individual gate adjustment. Reasonably good results can be
attained for longer runs by feeding from the high side of the
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TABLE 6-1
SUMMARY OF OVERLAND FLOW DISTRIBUTION METHODS
Methods
Advantages
Limitations
Surface Methods (6.2)*
General (6.2)
Low energy costs
Minimize aerosols
and wind drift
Small buffer zones
Difficult to achieve
uniform distribution
Moderate erosion
potential
Gated Pipe (6.2.1)
Same as General
plus
Easy to clean
Easiest of surface
methods to balance
hydraulically
Same as General plus
Potential for
freezing and settling
Slotted or perforated
pipe (6.2.2)
Same as General
Same as Gated Pipe plus
Small openings clog
Most difficult to
balance hydraulically
Bubbling Orifices (6.2.3)
Same as General plus
Not subject to
freezing/settling
Only the orifice
must be leveled
Same as General plus
Difficult to clean
when clogged
Low Pressure Sprays (6.3)
Better distribution
than surface methods
Less aerosols than
sprinkler systems
Low energy costs
Nozzles subject
to clogging
More aerosols and wind
drift than surface
methods
Sprinklers (6.4)
Most uniform
distribution
High energy costs
Aerosol and wind drift
potential
Large buffer zones
a. Section where discussed
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terrace and balancing the pressure gain from the elevation drop
across the terrace against the pressure loss generated by the
friction of the wastewater moving through the pipe. The
advantages of gated pipe are that the movable gates allow
relatively easy flushing of debris which tend to build up in
the openings. The use of a surge flow wastewater feed method
has been found to minimize the amount of gate plugging [2].
With careful adjustment, the gates can be used_ to attain
reasonable uniformity of discharge along the pipe in spite of
minor local height variations.
6.2.2 Slotted or Perforated Pipe
The grade of slotted or perforated pipe must be carefully
established and regularly maintained since there is no other
adjustment possible. Small slots and/or perforations are not
as easily cleaned as the gated pipe openings, and it is often
necessary to cut large openings in the top of the pipe for
purposes of maintenance. Recommendations to use slotted or
perforated pipe are limited to small systems having relatively
short pipe-runs which are easy to adjust.
6.2.3 Bubbling Orifices
Bubbling orifices are essentially small riser pipes (often
1-inch diameter) connected to underground distribution
laterals. Typically, they discharge onto some type of concrete
dispersion pad. Bubbling orifices have the same limitations as
other surface distribution systems except for pipe settlement
and freeze damage, since only the orifice, not the pipeline,
must be carefully leveled.
6.3 Low Pressure Sprays
2
Low pressure sprays {i.e., fixed nozzles <1.38 kg/cm (20 psi)}
are essentially a variation or extension of the bubbling
orifice. They are available in many types and variations. The
principal limitation of low pressure sprays is plugging from
particles too large to pass the orifice and partial plugging
from the buildup of smaller particles. Fine screening or
double comminution before the distribution system or straining
within the system can be used to prevent plugging by large
particles. Flush outlets on the ends of the distribution
laterals are helpful in clearing the lines of any large
particles. It is usually necessary to open these outlets for
only a few minutes, with the discharge applied to the terrace.
The pulsation caused by the flushing also tends to clear the
fixed spray nozzles of accumulated small debris that has built
up and partially plugged them.
The principal advantages of low pressure spray systems over
surface methods are better distribution of the wastewater onto
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the terrace and less sensitivity to local variations in
elevation. While they produce less mist than high pressure
sprinklers, they produce more mist than surface methods. Low
pressure spray systems can usually provide adequate
distribution of municipal wastewater on OF terraces. Splash
blocks and/or gravel layers are often used to prevent erosion.
6.4 Sprinklers
2
Medium {1.38 to 3.45 kg/cm (20 to 50 psi)} to high {>3.45
kg/cm (50 psi)} pressure impact and gear driven agricultural
type sprinklers have been used extensively to apply industrial
wastewaters to OF systems, but only to a limited extent on
municipal OF systems. The potential limitations of those
sprinklers are non-uniform distribution during windy
conditions, the risk of aerosol generation and the higher
energy requirements associated with pumping. State regulations
usually require a greater buffer zone area for sprinkler
systems than for surface and low pressure distribution methods.
Gear driven sprinklers produce less mist than impact-
sprinklers, but generally have a higher initial cost. They are
also more subject to clogging if the wastewater contains
stringy-type solids.
Sprinklers provide the most uniform distribution of wastewater
onto the terraces, thereby making it easier to achieve uniform
sheet flow with less maintenance. Minor variations in height
have little effect on discharge and major variations in height
between terraces can be easily overcome in design. Medium to
high pressure sprinklers have less tendency to collect debris
and become plugged, but will be plugged by particles too large
to pass the nozzle. Screening should be used to reduce the
particle size within the distribution system to one which will
readily pass through the nozzles. As with low pressure sprays,
flush outlets on the ends of the laterals are quite effective
in clearing the pipe of any large particles.
Sprinkler distribution systems have not been shown to provide
higher levels of performance than surface and low pressure
spray methods when treating municipal wastewaters. However, on
several industrial OF systems treating higher concentrations of
BOD and suspended solids than typically found in municipal
wastewater, high pressure sprinklers were found to be the only
satisfactory distribution method [3].
Vertical distribution risers connected to the lateral pipeline
through a flexible coupling allow easy cleanout of any riser
stoppages and protect the buried lateral pipe from breakage due
to vibration and impact. The piping configurations in Figure
6-1 show both the effective and non-effective methods for
placing lateral lines and sprinklers on the terrace. The use
of part-circle sprinklers as shown in Figures 6-1(b) and 6-1(f)
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SPRAY
RADIUS
RUNOFF COLLECTION
CHANNEL
SPRINKLER
(HALF CIRCLE).
(a)
SPRAY DIAMETER
SPRINKLER
I (FULL CIRCLE)
RUNOFF COLLECTION
CHANNEL
, SPRAY DIAMETER „
C SPRINKLER
(FULL CIRCLE)
(c)
SPRAY DIAMETER
SPRINKLER
(FULL CIRCLE)
SPRAY
!_ RADIUS
RUNOFF COLLECTION
CHANNEL
ROAD
(e)
SPRINKLER
(HALF CIRCLE)
RECOMMENDED
WOT RECOMMENDED
FIGURE 6-1
ALTERNATIVE SPRINKLER CONFIGURATIONS
FOR OVERLAND FLOW DISTRIBUTION
98
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is not recommended because they will have the tendency to allow
some wastewater to drift back, untreated, into the collection
channel (b) or road (f) from the adjacent terrace. Also,
part-circle sprinklers sometimes have a tendency to rotate out
of their specific part-circle pattern. A very slight wind will
cause the configuration shown in Figure 6-1(d) to spray most of
the water on one terrace, thereby causing an overload of that
terrace and little or no wetting of the other one.
6.5 Sizing of the Distribution System
A majority of the full-scale municipal overland flow systems
have been sized to apply the full daily design flow to the
system over a six to twelve hour period, five days a week.
This mode of operation minimizes the labor required to operate
the system, but it may increase both the capital and energy
costs for the system. For example, if the 24-hour wastewater
volume is applied to the system during an 8-hour period each
day, the piping distribution system must be sized for three
times the design flow, and greater yet if the system is only
operated five days a week. Additionally, pumping capacity must
also be increased by the same order of magnitude. Minimum
capital and energy costs are usually achieved by designing for
seven-day, 24-hour operation of the pumping and .distribution
system with no terrace receiving wastewater for more than
twelve hours at any one time, followed by at least a twelve
hour rest (see Section 3.2.2.4). The designer and the
municipality should consider these factors during the design
and planning stage and select the operating mode which is best
for their particular situation.
There has been a tendency to design OF systems with large zones
controlled by remotely operated valves. This increases the
size of the distribution system compared to the use of smaller
zones with additional, but smaller, remotely operated valves.
Large zones can also limit operational flexibility.
6.6 Controls
Both manual and automatic controls have been used successfully
for operating OF systems. The advantages of manual controls
are their simplicity and the fact that the operator will have
frequent contact with and opportunity for observation of the
system. These advantages must be balanced against the need for
more operator time and full dependency on the operator to
control the application periods. Such demands on an operator's
time frequently result in the overwatering of some terraces and
the underwatering of others. Totally manual controls are not
recommended for pumped distribution systems. As a minimum, a
low level cutoff switch should be provided for pump protection.
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The primary advantages of automation are better utilization of
operator time, and in some cases, more accurate control of the
application periods. Automation can range from a pump
protection cutoff switch to a programmable control system
capable of making decisions based on feedback from the
operating system. The best system is a compromise which allows
the operator to program any portion of the system to operate at
any time for any pre-selected length of time. This flexibility
is very helpful in providing the desirable short application
and rest periods during start-up (see Section 8.4), while still
maintaining the capability of providing longer application and
rest periods during routine operation (see Section 3.2.2.4).
The controls should also provide minimum safety detection
features to protect against damage. In addition, the operator
should have the capability of manually bypassing the control
system in case of failure and for maintenance purposes. The
advantages of automatic controls increase with larger systems.
6.6.1 Automatic Valves
Both pneumatic and hydraulic remote controls have proven to be
quite dependable for field installations. Pneumatically and
hydraulically operated diaphragm valves (requiring pressure to
close) have given good service for many years on numerous land
treatment systems under all climatic conditions with few
operational problems. Electric controls located in the field
are generally not suitable because they can be affected by
lightening and are a safety hazard. Piloted valves have been
unsatisfactory in all cases where the wastewater was used as
the operating medium because of clogging, even when strainers
are used. The use of a clean, pressurized, external fluid as
an operating medium generally provides satisfactory service.
Ball, butterfly, and plug valves with external cylinder
operators, closed by air pressure and opened by spring tension,
provide satisfactory on/off service.
6.6.2 Manual Valves
Ball, plug, and gate valves all provide satisfactory manual
on/off service. Butterfly valves are also capable of providing
satisfactory service for primary effluent, but are not
recommended for use with raw wastewater since solids can
interfer with the valve operation. Globe and angle valves
generally permit close regulation of flow, and are therefore
satisfactory for throttling purposes. Other types of valves
should be used for throttling only if specifically recommended
for that purpose by the manufacturer.
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6.7 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981. . .:
2. Borelli, J., et al. Overland Flow Treatment of Domestic
Wastewater in Northern Climates. University of Wyoming,
Laramie, WY. 1984. ,
3. Gilde, L. C., and O. M. Aly. Personal Communications.
Campbell Soup Company, Camden, NJ. 1984.
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CHAPTER 7
TERRACE DESIGN AND CONSTRUCTION
7.1 Importance of Proper Construction
The primary purpose of land grading, also called land forming
and land leveling, is> to re-shape the soil surface to assure
the uniform movement of water across the OF terraces.
Attainment and maintenance of smooth sheet flow down each
terrace is necessary to achieve optimum OF process performance.
Observation of current practices indicates that grading and
finishing require more attention in design and during
construction. The purpose of this chapter is to provide more
detail than that found in the Manual [1].
7.2 Design Methods
The design of OF terraces and drainage channels utilizes many
conventional agricultural engineering principles. There are
four basic methods which are used for land grading design [2,
3]: (1) the plane method, (2) the profile method, (3) the
plan-inspection method, and (4) the contour adjustment method.
In all methods, the designer must provide a 10 to 40% excess of
cut in relation to fill to compensate for the shrink-swell
potential of the soil. With some soils, the percentage can be
even higher. Experience with a specific type of soil is the
only way to determine the actual percentage of excess cut
required. The local ;USDA Soil Conservation Service personnel
can often provide advice in this area.
It is beyond the scope of this text to provide a detailed
description of the various land grading design methods.
However, since these techniques are not commonly used in
munxcipal wastewater treatment engineering, the consultant is
encouraged to seek assistance, if necessary, from an
agricultural engineer or other qualified professional for this
portion of the design.
7.3 Terrace Configurations
There are four basic types of terrace configurations used in OF
design. These are: (1) conventional, (2) step-up, (3)
back-to-back, and (4) step-down. The four configurations are
shown on Figure 7-1 and described in more detail in the
following sections. The choice of which configuration to use
should be based on the existing site conditions and the
economics of construction. More than one type of terrace can
be used on the same site.
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TERRACE.
(2-8%) \
ORIGINAL GROUND
SURFACE (2-8%)
CONVENTIONAL
TERRACE
TERRACE
FRONT SLOPE
ORIGINAL GROUND
SURFACE (< 2%)
STEP-UP
TERRACE
. ORIGINAL GROUND
SURFACE (<1%)
.TERRACE
(2-8%)
BACK-TO-BACK
TERRACE
TERRACE
BACK SLOPE
ORIGINAL GROUND
. SURFACE (> 8%)
TERRACE
STEP-DOWN
TERRACE
FIGURE 7-1
TYPES OF OVERLAND FLOW TERRACES
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7.3.1 Conventional Terraces
Conventional terraces are used where the existing field grade
generally meets the criteria (2 - 8%) for overland flow
presented in the Manual [1]. Localized cutting and filling_is
accomplished as necessary to fully meet grade criteria.
Individual terraces are then formed by drainage channel
construction.
7.3.2 Step-Up Terraces
Step-up terraces are used where the existing field grade is
less than that desired for OF (i.e., <2%) and it is necessary
to increase existing field grades. The front slope of the
adjacent lower terrace provides one side of a v-channel, while
the terrace itself provides the other side. If additional
channel depth is required, it may be attained by additional
excavation, by construction of a ridge on the upper edge of the
lower terrace, or a combination of these two methods.
7.3.3 Back-to-Back Terraces
Back-to-back or "humpback" terraces are also used where the
existing field grades are less than that desired for OF (i.e.,
<2%). They are most economical when the existing field grades
are very flat (<0.3%!). When the existing field grades are
between 0.3% and 2.0%, step-up terraces are usually more
economical to construct than back-to-back terraces. However,
on sites where land availability is extremely limited,
back-to-back terraces may be justified when the existing field
grades are up to 1.0% because, although they will not be as
economical to construct, they will provide higher land
utilization (i.e., greater terrace area) than step-up terraces.
Back-to-back terraces do not require construction of individual
terrace drainage channels because the entire area of the
terraces where the lower portions intersect acts as a broad
triangular channel. Where the lower portions intersect at
appreciably different heights, the,lower portion of the higher
terrace should be cut back on a 4:1 or flatter slope to prevent
erosion and facilitate mowing.
7.3.4 Step-Down Terraces
Step-down terraces are used to reduce field grades on sites
where the existing grades are greater than that desired for OF
(i.e., >8%). The drainage channel is constructed at the lower
edge of the adjacent higher terrace before the step-down to the
lower terrace is started. The upper terrace backslope, or
step-down, is typically constructed on a 4:1 slope.
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7.3.5 Transitions
The overall grading plan must include transitions from
individual terraces to adjacent, unused areas, as well as to
other terraces and runoff channels. Safety in construction and
future maintenance operations (e.g., mowing) must be considered
in designing these transitions. Using ordinary caution,
equipment is generally considered safe to operate on 4:1 or
flatter slopes [4].
7.4 Drainage Channels
7.4.1 Design
Drainage channels and discharge structures should be designed
to handle the discharge from the entire area which they will
drain, not just the area of the OF terraces.' Drainage channels
should have enough discharge capacity to handle the peak rate
of runoff from a 25-year/24-hour frequency storm, plus 0.1 to
0.2 m (4-8 in.) freeboard. Channels are ordinarily designed
using the Manning formula. For unlined channels, an n value of
0.06 is normally used to design for capacity and an n value of
0.03 to compute for velocity. Channel velocities do not
ordinarily exceed 1.5 m/s (5 ft/s), although this limit is
influenced by factors such as soil type and vegetation in the
channel. Design of drainage channels and discharge structures
involves several steps, including runoff computation, selection
of channel type, erosion control, and consideration of specific
grading techniques.
7.4.2 Runoff Computation
Various satisfactory methods are available for estimation of
design runoff. The Rational method [3] is one of the most
commonly used and various charts, nomographs, tables, and
computer programs are available to expedite design runoff
computations. A C value of 0.50 to 0.55 is satisfactory for
most overland flow systems. Cook's method [3] is somewhat
simpler and gives similar results to the Rational method when
the above C value is used. Channels must be designed
specifically for each site in order to effect economy of
construction.
7.4.3 Channel Types
Drainage channels are of three basic types: 1) the ridge type,
2) the more common ridge and trough type] and 3) the trough or
ridgeless type. These are shown on Figure 7-2. The ridge
front slope provides one side of the V-channel and the terrace
provides the other side for ridge type channels. The ridge and
trough type channel is constructed by excavating a trough and
using the excavated soil to form the ridge. The trough or
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RIDGE CHANNEL
ORIGINAL ,
GROUND SURFACE
RIDGE
FRONT SLOPE
RIDGE
BACK SLOPE
RIDGE AND TROUGH CHANNEL
ORIGINAL
GROUND SURFACE
TROUGH CHANNEL
FIGURE 7-2
TYPES OF TERRACE DRAINAGE CHANNELS
106
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ridgeless type channel is usually constructed by excavating the
channel and disposing of the excavated soil in another
location. For erosion protection and ease of mowing (if
grass-lined channels are used), it is generally best to keep
the ridge front and back slopes at a 4:1 slope and never
steeper than a 3:1 slope.
Any of the channel types may be used with any terrace
configuration except the back-to-back terrace. The
characteristics of the site determine the relative desirability
of the channel types. When working with flat cross slopes, it
may be of value to combine the three types of channels in the
same terrace, phasing from the ridge type to the ridge and
trough type to the trough type. This increases the channel's
gradient, thereby improving drainage.
7.4.4 Avoiding Erosion Problems
Observation of operating systems indicates that many runoff
channels suffer from excessive erosion before an adequate
vegetative cover is established. One cause of this erosion is
excessively steep channel sides where water, especially from
the terraces, drains into the channel. Such grades should
never be steeper than 4:1.
A second cause of erosion is water from a tributary drainage
channel entering a larger drainage channel at a higher
elevation than the invert of the larger channel. In such
cases, the depth of the tributary channel should be lowered to
the invert of the larger channel through a transition section.
The transition section should be long enough so that the
velocity does not exceed 1.5 m/s (5 ft/s) within it, and the
side slopes should be cut back at 4:1. If such a transition is
impractical, a concrete flume or some other type of drop
structure should be used to convey the water safely into the
lower channel.
Another cause of erosion is excessive velocity before the
vegetation becomes established. All drainage channels having
flow velocities greater than 0.9 m/s (3.0 ft/s) when calculated
using a Manning n of 0.25 should be protected with some type of
cover material such as staked down jute or nylon matting with
wood fiber. Concrete lined channels, rip-rap, and straw or hay
mulch using an injected asphaltic or other binder may also be
suitable. Finally, pipes may be used to convey the drainage
water from the OF terraces to the discharge or some point
within the system where it can be safely released. Pipes are
particularly suitable when step-down terraces must be used.
Piped-drainage also provides highly efficient land utilization.
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7.5 Land Grading
Although the construction of OF terraces and drainage channels
involves the use of some standard construction equipment and
practices, it is unique in that it also includes certain
equipment (i.e., a land plane) and construction practices which
are not commonly used by many of the contractors who bid on OF
projects. The following is a discussion of certain aspects of
the land grading operations which are specific to OF systems.
7.5.1 Rough Grading
Settlement of the soil after rough grading is a major cause of
failure to achieve uniform terrace surfaces. Where the
construction schedule will allow, the soil should be left to
settle for three months to a year following the rough grading
operation. The terraces should then be checked for excessive
settlement and corrections made, if necessary. Recognizing
that this length of interruption to the construction schedule
usually is not practical, the following procedure has been
found to provide satisfactory results.
During terrace construction, the earth should be placed in 15
cm (6 inch) layers (or lifts) and compacted, as a minimum, to
the density of the adjacent undisturbed soils. Excavation,
fill, and compaction should be accomplished with a dozer, pan,
scraper, or other appropriate pieces of earth moving equipment.
Clods, if any, should be crushed before they are buried.
Because it is necessary to avoid excessive compaction,
equipment operators should be instructed to avoid a
follow-the-leader route, but rather distribute the wheel load
over as much of the tract as possible without increasing the
earth moving distance. A lift should be built all the way
across a fill area before the next lift begins, except when an
existing depression occurs within the fill area. Existing
depressions should be brought level with adjacent areas using
15 cm (6 in.) lifts before beginning the normal fill procedure.
7.5.2 Topsoil Handling
Observation of construction practices on OF sites shows that
topsoil is frequently stripped, stockpiled, and then replaced
as the last step in the grading operation. While this may be
necessary in some cases, it can be an unnecessary and costly
activity in other cases. Utilization of the subsoil may
require only the application of the proper fertilizer, which is
generally a much less expensive operation than topsoil
preservation. Fertilization has proven to be an effective and
economical method of utilizing exposed subsoil in several
industrial overland flow systems. Soil samples, to the depth
of the expected excavation, can be obtained and sent to a soils
laboratory for analysis. The laboratory can recommend whether
108
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the subsoil can support plant growth and the type and amount of
fertilizer and/or other soil amendments required.
If the subsoil cannot support plant growth, even with the
addition of fertilizers, the topsoil is stockpiled, the cuts
over-excavated, and the topsoil replaced. The fill areas must
also be stripped, the fills partially made with the materials
available, and the topsoil replaced. This is an expensive
procedure, and it increases the difficulty of the final grading
operations because the roots and other matter contained in the
topsoil often interfere with achieving a smooth surface finish.
When topsoil preservation is essential, the topsoil from one
terrace should be stockpiled on an area requiring little cut or
fill. The cuts and fills in this terrace are then completed
and the topsoil from the adjacent terrace stripped and used for
dressing the surface of the first terrace. Then, progressing
across the field, the topsoil should be moved from the adjacent
terrace as the terrace forming is completed until the last
terrace is dressed with the stockpile from the first terrace.
This procedure minimizes the time and expense of topsoil
removal and replacement.
7.5.3 Final Grading
A land plane, also called a land leveler or long frame
bottomless scraper, is the ideal equipment for final grading.
It is only effective on loose soil. The soil is usually
loosened by disking with a heavy disk. If large clods are
turned up, the area is re-worked with a lighter disk or
sheepsfoot roller to break up the clods. The land plane is
then passed over each terrace as a finishing operation.
Integrity of the terrace must be maintained during the planing
operation. The first two passes of the land plane are usually
made at opposite 45° angles to the long axis of the terrace
with the last pass being parallel to it. The purpose of this
cross planing is to prevent the development of a long
sinusoidal wave approximately twice the length of the land
plane. If any portion of the terrace is scraped bare of loose
soil during the planing operation, that area should be
re-scarified and the terrace completely re-planed. Surface
deviations not exceeding 1.5 cm (0.05 ft) from the planned
slopes can be achieved with three passes of the land plane.
There are various types and sizes of land planes and the
selection of one for a specific site is important. A land
plane capable of doing excellent work on a field 1.6 by 1.6 km
(1 mi by 1 mi) probably cannot be turned around on a terrace 30
m (100 ft) long by 60 m (200 ft) wide. A laser controlled land
plane, the ultimate in finish grading equipment, can be used if
the terrace was designed as a plane. Planing need be
accomplished only in the direction of the longest axis of the
terrace. The many turns required in cross planing can be
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eliminated if a laser controlled land plane is,used.
Although other equipment can be used for final grading, it is
usually with greater difficulty and/or poorer results than with
the use of a land plane. Laser equipped dozers are efficient
for finishing some terraces. Motor graders can also be used
for final grading. The use of a landscape box is suggested to
smooth out the tracks and other irregularities left by a motor
grader. These alternatives are recommended only if a land'
plane is not available.
7.6 Supervision and Acceptance
Although it is beyond the scope of this chapter to provide
detailed instructions for supervising the terrace construction
operations, the following is a check list of some items which
are critical to the satisfactory completion of the terraces:
1.
2.
3.
4.
5.
The specified lift depth (max. 15 cm) is not
exceeded.
The specified compaction (to pre-construction
density) is maintained in each lift.
The specified grades are attained on the edges of the
terraces.
The specified channel depth and width are maintained.
The channel transitions are constructed as shown on
the plans.
Terrace areas which are scraped bare of loose soil
during the land planing operation, are re-scarified
and the terrace completely re-planed.
The pipeline backfill
compacted.
is properly placed and
8.
Stumps, concrete, and other materials are not buried
within the overland flow site.
7.7 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
110
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2. Land Leveling Irrigation. Chapter 12. SCS National
Engineering Handbook. Section 15. U.S. Department of
Agriculture, Soil Conservation Service. March 1959.
3. Frevert, R. K., et al. Soil and Water Conservation
Engineering. John Wiley and Sons, Inc., New York, NY.
1966.
4. American Society of Agricultural Engineers Standard.
Design, Layout, Construction, and Maintenance of Terrace
Systems. A.S.A.E. S268.2, Revised and reclassified as a
Standard. A.S.A.E., Saint Joseph, MI. December 1978.
Ill
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CHAPTER 8
VEGETATION SELECTION AND ESTABLISHMENT
8 . 1 Function
A well established vegetative cover is essential for the
efficient performance of OF systems. The purpose of this
chapter is to provide greater detail on vegetation selection
and establishment than found in Sections 6.8.3 and 6.10.2.2 of
the Manual
8.2 Selection
8.2.1 Objectives
The Manual Cl3 lists the desirable characteristics for an OF
cover crop. However, a high percentage of new OF systems appear
to be placing great emphasis on cash crop production to the
detriment of slope protection and wastewater treatment. This
is especially important in the early stages of cover crop
establishment and in the start-up of wastewater treatment.
Some grasses, most notably some of the improved Bermuda grass
varieties, are established only by sprigging or sodding.
Sodding is usually too expensive, while sprigging at standard
agricultural rates does not provide adequate initial coverage
for either slope protection or the start-up of wastewater
treatment. The problem is further compounded when the early
cuttings are removed as hay rather than being allowed to remain
on the terraces as a protective mulch.
8.2.2 Grass Types
Grasses suitable for use in OF systems are described in Table
8-1. Additional information on these grasses can be found in
various references on forage grasses C2, 3]. The primary
purpose of the vegetation in an overland flow system is to
facilitate the treatment of wastewater. The market value of
the crop is only of secondary importance. If a grass will not
grow under a particular set of OF conditions, no matter what
its other desirable characteristics, it is of no benefit. The
most common grasses used on OF systems have been Reed canary
grass and various Bermuda grass varieties.
8.2.2.1 Reed Canary Grass
While Reed canary grass is sometimes slow in establishment, it
forms a very tough, dense sod, spreading by seed, rooting from
the joints of stems in soil contact, and vigorous thick
rhizomes which push out from the crown. It has one of the
longest growing seasons of all the cool season grasses and
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113
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continues growing throughout hot summers if adequate moisture
is available. It is one of the most drought-resistant of the
cool season grasses and may be expected to become the dominant
grass on many OF sites.
The market value of Reed canary grass is often described as low
by many references which show it to have relatively low protein
values. Under normal growing conditions, it is raised on wet
land where it is usually past its prime before it can be
harvested as a hay crop. However, some research [4] has shown
that Reed canary grass grown on an OF system can exceed the
commonly reported values for alfalfa in both protein and
nutrient content.
8.2.2.2 Bermuda Grass
Common Bermuda grass becomes readily established from seed
during warm weather and may establish an initial dominance
during the warm season. Unless used as a summer nurse crop
(see Section 8.2.2.3), however, Bermuda should be planted only
as far north as it grows naturally. This is true for any of
the warm season grasses. If planted in the fall or early
spring in a mixture of cool season grasses, only unhulled seed
should be planted, since unhulled seed (as opposed to hulled
seed) is more likely to survive during conditions which are not
conducive to germination. The various Bermuda grasses will
generally become dormant after the first hard frost, but will
continue to provide the thick turf needed for satisfactory
treatment performance. If an improved Bermuda grass variety
which requires sprigging (e.g., Coastal Bermuda) is selected,
overseeding with a nurse crop is required to establish a
satisfactory cover until the sprigged grass is well
established.
8.2.2.3 Nurse Crop
When the primary grass selected is a slow starting species or
one with a low initial density, a number of other varieties may
be used as a nurse crop to provide the initial density needed,
even though some of these nurse crops may last no longer than
the first growing season. Various warm season varieties of
grass could be selected for this purpose, including common
Bermuda, which will provide rapid cover when planted during hot
weather and will eventually be crowded out by the improved
variety Coastal. If the time of year is suitable; annual rye
grass or another quick growing cool season grass will serve the
same purpose.
8.2.3 Combining Grass Species
Planting a mixture of several grasses generally achieves the
most satisfactory results by providing a high initial density
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and the capability to apply wastewater as quickly as possible.
The selection of grasses to include in a mixture is dependent
on the climate, the time of planting, and the availability of
irrigation water during start-up. In areas where warm season
grasses such as Bermuda will grow, a mixture of warm season and
cool season grasses is recommended. In cooler climates, a
mixture of all cool season grasses is best.
Recommendations from local grass specialists can be helpful in
the selection process. Such'specialists might be found through
the local USDA Soil Conservation Service, county agents, or
other local agricultural specialists. However, when using
agricultural advisors, it is very important for them to
understand that the primary objective of the project is
wastewater treatment and the grasses selected must adapt to the
imposed wastewater application conditions. If. this is not
done, inappropriate recommendations may result.
8.3 Planting
8.3.1 Density
The amount of seed to plant depends on the type of grass
selected, expected germination, water availability, and time
available for cover crop development. Table 8-2 provides
general guidelines for seed densities which can be used when
constructing a new OF system. The low density rates are used
for temporary stabilization and erosion control or if water for
cover crop development is in short supply. These rates are
equivalent to commonly used seeding rates for pastures. The
moderate density rates represent a typical range of seeding
when planting at the optimum time for development by natural
rainfall. The high density rates will provide the fastest
development of a dense cover crop, but these rates require an
adequate supply of irrigation water whenever sufficient
moisture for germination and growth is not provided by natural
rainfall.
It is necessary to convert the density values in Table 8-2 to a
seeding rate (kg/ha or Ib/ac) by dividing by the number of
seeds per kilogram (or per pound) for a specific grass. These
latter values can be provided by suppliers or found in standard
references [2,3]. If a mixture of several grasses is used, the
seeding rate for each grass must be adjusted proportionally.
8.3.2 Scheduling
Planting time is affected by location, climate, variety of*
grass,. availability of irrigation water, capability of
sprinkler irrigation, construction schedule, expected rainfall,
and other factors. In general, cool season grasses should be
planted from spring through early summer or early fall through
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late fall. The availability of water and the capability of
sprinkler irrigation adds considerable flexibility to these
planting times. For example, with proper irrigation, cool
season grasses can even be started in the middle of summer.
However, this requires considerable attention to detail and is
not a recommended practice. Warm season grasses generally
should be planted from late spring through early fall, although
this can vary according to area and climate.
TABLE 8-2
SEEDING DENSITY FOR OVERLAND FLOW SYSTEMS
Level
Density,,
(seeds/m )
Low Density
1,450 - 2,690
Moderate Density
4,300 - 8,600
High Density
15,500 - 20,450
f)
seeds/m
0.093
seeds/ft'
Local agricultural advisors can give advice regarding the
optimum time as well as the limits of time for grass planting.
The timing of the planting schedule is complicated when it is
desired to plant both cool and Warm season grasses. However,
there are time overlaps in both spring and fall for most
grasses. Also, some varieties of grass seed, if planted out of
season and protected by a growing cover, simply will lie
dormant until climatic conditions are suitable for their
germination and growth. A second option is to plant the two
seed types separately at the optimum planting time for each.
The second planting (commonly called overseeding) will require
a seeder which will not damage the initial stand or mar the
soil surface.
In order to have the greatest potential for establishing a good
vegetative cover, it is important to plan the construction
schedule so that completion of the final grading of the
terraces coincides with the optimum time for planting the
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selected grasses. If this cannot be accomplished, it is
necessary to plant a nurse crop which will germinate and grow
at the time construction is completed and then to pverseed with
the primary grasses at their optimum planting time. If the
construction is completed at a time when even a nurse crop will
not germinate, the terraces and drainage channels will require
protection (straw mulch or other suitable covering) from
erosion. The construction and planting schedule is most
critical in cold climates where there is generally less time
available for establishing vegetation.
8.3.3 Soil Preparation
At the completion of finish grading, fertilizer and soil
amendments (e.g., lime) are applied according to soil test
recommendations and immediately incorporated to a depth of 10
cm (4 in.) or more. Any standard equipment can be used for
applying this material provided it gives uniform spreading and
does not damage the slopes. A smoothing harrow is usually
pulled behind the disk harrow to eliminate ridges and provide
additional cultivation in forming a seed bed. Additional
passes of the same or other equipment may be necessary to
prepare an adequate seed bed. Any procedure is satisfactory
provided it leaves the soil surface in a smooth condition and
does not create ridges or leave vehicle tracks.
8.3.4 Seeding
Brillon seeders have been found to be very effective planters
for OF systems. This equipment distributes the seed and covers
it with a small amount of soil (commonly called cultipacking)
in the same operation. Although, other planters can be used
satisfactorily, the best results are generally achieved when
the seed is cultipacked into the soil. Only light tractors
should be used for seeding or other operations such as
cultipacking and there should be no wheel tracks left on the
terraces. The seeding and cultipacking operations should be
carried out parallel to the steepest terrace slopes, even
though this is clearly contrary to the conventional wisdom for
erosion
control. If contour planting is followed, the
depressionsareso shallow that the slightest runoff breaks
through the minute depressions or weak spots, creating a rill
or wash. Up and downslope planting prevents the concentration
of water so that only sheet erosion or that from raindrop
splash takes place. Actual soil movement may be greater, but
the smooth uniform surface is not as likely to be destroyed.
Areas other than terraces may be seeded according to the best
operating characteristics of the planting equipment.
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8.3.5 Sprigging
Broadcast planting is the perferred method of sprigging an OF
terrace. The soil is prepared in the same manner as for
seeding. Then 3.5 to ^4.4 m /ha (40-50 bu/ac) of sprigs are
broadcast. Sprigs are pressed into the soil using a weighted
disk harrow with the disks set straight. The soil is firmed by
cultipacking up and down the slope. One cm ( 0.5 in.) of
irrigation water should be applied by a sprinkler system
immediately. Broadcasting freshly cut stems is an alternative
method of propogating Coastal Bermuda grass. The quantity
should be increased to 7.8 to 8.7 in /ha (90-100 bu/ac) but the
same procedure as for sprigging is used. Immediate irrigation
is even more critical when using stems .than sprigs and no
delays should be allowed between cutting the stems and
planting.
Other planting methods are satisfactory, provided they leave
the soil surface in a smooth condition and, do not leave ridges
or vehicle tracks. If less than 2.4 m /ha (27 bu/ac) are
planted, the terraces require overseeding with a nurse crop to
provide cover until a permanent sod forms. A nurse crop should
also be overseeded if the planting islate jn the season when a
sod may not have time to form before frost.
Certain precautions are necessary in planting sprigs. Only
live, freshly dug sprigs should be planted. If they cannot be
planted immediately following digging, they should be kept
moist and cool. They must be wet down and turned often to
prevent heating. Sprigs must be well covered during hauling to
prevent drying by sun and wind. Allowing sprigs to dry out
before planting is probably the most common cause of failures
to obtain stands. The tips of sprigs should be above ground as
sprigs buried more than two inches deep may not grow. The area
from which sprigs are dug should be free of weeds, soil borne
diseases, and insect pests.
8.4 Vegetation Development
When seeding and/or sprigging is used, a period of growth and
acclimation is necessary between planting the vegetation and
the application of wastewater at full design loading rates.
Certain management practices can be used to accelerate the
development of the vegetation and to minimize the acclimation
period.
8.4.1 Start-Up Irrigation
8.4.1.1 Methods
Grass growth on seeded systems can usually be started with
natural precipitation by selecting the appropriate planting
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time. However, irrigation is strongly recommended when
sprigging an OF system. In either case, the use of irrigation
will almost always provide more rapid grass development than
relying on natural precipitation. Since- irrigation can add
considerable cost to the development of the system, it is
usually used only when the construction and start-up schedule
does not provide sufficient time for grass development by
rainfall.
Either fresh water or wastewater can be used to promote the
growth of vegetation, with the irrigation rates selected to
provide sufficient moisture for grass growth, but not enough to
create runoff from the system. On seeded systems using medium
to high pressure sprinkler distribution systems, the sprinkler
system can be used for the application of irrigation water.
Surface and low pressure distribution systems cannot be used
for such purpose because they do not provide uniform coverage
of irrigation water. A temporary system of portable irrigation
pipe with medium to high pressure sprinklers is required for
these systems. Temporary irrigation pipe and sprinklers are
required for sprigged terraces, even if the wastewater
distribution system consists of high pressure sprinklers. The
temporary system is required to wet the lower portion of the
terrace beyond the reach of the sprinkler patterns.
8.4.1.2 Procedures
The following irrigation procedures have been used successfully
in providing rapid vegetative growth on new OF systems. They
have been found to be applicable in many areas of the country
and with many types of grasses C5].
The first watering is initiated as soon as possible after
planting the grass, but is not started unless there is an
adequate water (or wastewater) supply to continue irrigation as
needed. The first watering of each terrace lasts for 15
minutes. Subsequent waterings of 15 minutes each are repeated
hourly up to the point of runoff within the sprinkler pattern.
Irrigation is stopped as soon as a surface film of water
appears within the sprinkler pattern of a terrace. Fifteen
minute waterings, repeated hourly, are re-started as soon as
the soil surface appears dry. The sprinklers should be
operated at pressures recommended by the manufacturer.
Whenever rainfall occurs to the extent that it serves the same
purpose as the irrigation, the irrigation can be discontinued
until the soil surface within the sprinkler pattern appears
dry.
When the grass within the sprinkler pattern reaches an average
height of 2.5 cm (1 in.), the irrigation schedule can be
changed to one watering per day for one week. The following
week, the irrigation schedule can be reduced to one application
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of water every other day and then twice a week for the
following week, each time applying water long enough to achieve
total wetting of the soil, but allowing no runoff. Upon
completion of the above schedule, water should continue to be
applied to achieve a steady growth pattern. Application
frequency will vary with daily temperatures and rainfall, but
watering to the point of runoff should not be needed more than
twice per week.
When the terraces are irrigated only by the wastewater
distribution system (with medium to high pressure sprinklers),
the same procedure is used until the grass within the sprinkler
pattern reaches an average height of approximately 2.54 cm (1
in.). At this point, the number of watering cycles can be
successively increased to force water downslope toward the
bottom of the terrace. Extreme caution is necessary at this
point, as runoff must never be allowed to the extent of causing
erosionTTherefore,Itisimportanttosuperviseall
applications of irrigation water. As the grass development
proceeds to the extent that the entire terrace can be wetted,
the irrigation schedule can be changed to one watering per day
for one week. The following week, the irrigation schedule can
be reduced to every other day and then to twice a week for the
following week, each time applying water long enough to achieve
total wetting of the terrace, but stopping irrigation just
before runoff into the terrace channel occurs.
8.4.2 Initial Management
The initial level of treatment at new OF systems may not meet
the effluent discharge limitations, particularly if the
hydraulic loading rate is at the full design rate. The system
begins to become acclimated during the first few months of
wastewater application, and treated effluent quality continues
to improve during this period.
One practice which has been found to improve the efficiency of
an OP system during the start-up and acclimation period is to
allow at least the first three cuttings of grass to remain on
the soil surface. This mulch layer helps to prevent erosion,
and increases the organic and nutrient content of the soil,
thereby providing improved conditions for biological activity.
If wastewater must be treated at the earliest possible time,
the terraces can be sodded or seeded at the highest density
shown on Table 8-2 and irrigated according to the guidance
provided in Section 8.4.1 in order to accelerate development.
The use of wastewater for irrigation to hasten the development
of the vegetative cover not only shortens the OF system
acclimation period and produces a higher initial quality of
effluent, but also reduces alternate treatment and/or storage
requirements.
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Short application periods with short rest periods, such as 15
to 30 minutes of wastewater application with 45 to 90 minutes
rest, will also produce a better initial quality of effluent
and may further shorten the acclimation period. This technique
also reduces or prevents damage from erosion, since it reduces
flow velocities while still allowing the soil microorganisms to
become acclimated to the waste. These short application and
rest cycles can be gradually increased over several weeks to
the regularly planned application periods.
8.5 Erosion Control
The first several months after seeding is the time that the
system is most susceptible to erosion damage. Frequent
inspections are necessary during this period to note and repair
erosion damage. The inspection is especially important after a
heavy rainfall. Small channels can be corrected easily with
hand labor by filling or by making small coffer dams of soil
and/or mulch in the channels, and re-seeding or sodding. More
extensive damage may necessitate repair with equipment (i.e., a
land plane or other device), but the use of equipment on the
terraces after seeding should be avoided except when absolutely
necessary.
8.6 References
1. U. S. Environmental Protection Agency. Technology
Transfer Process Design Manual for Land Treatment of
Municipal Wastewater. EPA 625/1-81-013. U. S. EPA,
Center for Environmental Research Information, Cincinnati,
OH. October 1981.
2. McBickar, M. H. and J. S. McBickar. Approved Practices in
Pasture Management. Second Edition. Interstate Printers
and Publishers, Inc., Danville, IL. 1963.
3. Heath, M. E., D. S. Metcalfe, and R. F. Barnes,
Forages/The Science of Grassland Agriculture. Third
Edition. The Iowa State University Press, Ames, Iowa.
1975
4. Thornthwaite, C. W. An Evaluation of Cannery Waste
Disposal by Overland Flow Spray Irrigation. Publications
in Climatology 22. 1969.
5. Ford, J. P. Grass Development on Overland Flow Terraces.
Unpublished. ERM-Southeast, Inc. Marietta, GA. 1984
«U.S. GOVERNMENT PRINTING OFFICE:! 993 .750-002/60129
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