ON-SITE DISPOSAL
OF
SMALL WASTEWATER FLOWS
y
Richard J. Otis,1 William C. Boyle,2
James C. Converse^ andE. Jerry Tyler4
University of Wisconsin
Madison, Wisconsin
Prepared for the
Environmental Protection Agency
Technology Transfer
1977
i
^Specialist, Department of Civil and Environmental Engineering
^Professor, Department of Civil and Environmental Engineering
"'Associate Professor, Department of Agricultural Engineering
4A ssistant Professor, Department of Soil Science
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ACKNOWLE DGEMENTS
In 1971, the State of Wisconsin provided research funds to the
University of Wisconsin to commence investigations into the on-site
disposal of wastewater. The Small Scale Waste Management Project
was established with the objectives to determine and understand the
causes of failure of septic tank-soil absorption fields, to improve
methods of site characterization, system design and system construc-
tion for on-site disposal of wastewater, to search out effective alter-
natives to the septic tank soil absorption field in problem soils, to
develop more effective management techniques to on-site wastewater
disposal systems, and to assess the implications of new wastewater
disposal technologies for land use planning.
Since that time, substantial funding has also been provided by
the Wisconsin Department of Natural Resources, the Upper Great
Lakes Regional Commission, and the United States Environmental
Protection Agency. A major portion of this report is based upon
research efforts conducted under these funding agencies. The
authors wish to acknowledge the generous support of these agencies
and the research efforts of the Small Scale Waste Management
staff--
Department of Soil Science
James L. Anderson
Fred G. Baker
Marvin T. Beatty
Johan Bouma
Richard B. Corey
Thomas C . Daniel
Joseph L. Denning
Robin F. Harris
John M. Harkin
Michael V. Jawson
Dennis R. Keeney
Fred R. Magdoff
Lawrence J. Sikora
Edward J. Tyler
William G. Walker
C. B. Tanner
Department of Civil and Environmental
Engineering - -Sanitary Engineering
Laboratory
William C. Boyle
Lester Forde
Neil J. Hutzler
Kenneth Ligman
Richard J. Otis
John T. Quigley
David K. Sauer
Robert Siegrist
Michael D. Witt
Food Research Institute
Dean O. Cliver
Bruce Donohoe
Kenneth M. Green
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Department of Bacteriology
John F. Deininger
Elizabeth McCoy
Patti J. Hantz
David H. Nero
Wayne A. Ziebell
Department of Urban and
Regional Planning
Peter W. Amato
Harrison D. Goehring
Environmental Resources Unit
David E. Stewart
Center for Resource Policy Studies
and Department of Agricultural
Economics
Richard L. Barrows
Melville L. McMillan
Marc D. Robertson
Ronald E. Shaffer
Stephen C. Smith
Department of Agricultural
Engineering
James C. Converse
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TABLE OF CONTENTS
A cknow ledgemen ts
Table of Contents
Introduction 1
Conventional Public Facilities 2
Non-Central Wastewater Facilities for Small Communities 6
Powers Needed by a Management Entity 7
Types of Acceptable Management Entities 9
Collection and Treatment Alternatives for Non-
Central Facilities 9
The Use of Soil for Treatment and Disposal of Wastewater 12
Liquid Movement Into and Through Soils 12
Soil Porosity and Permeability 12
Characterization of Water in Soils 12
Liquid Movement in Soils 1$
The Process of Pore Clogging 18
The Significance of Unsaturated Flow 20
Wastewater Treatment Capabilities of Soil Materials 22
Bacteria and Virus Removal by Soil 22
Chemical Transformations and Removals by Soil 27
Estimation of the Infiltrative and Percolative Capacity of the Soil 30
Estimation of Soil Permeability 30
The Percolation Test 30
The Crust T\est 31
Estimation of High Groundwater 35
Maintaining the Infiltrative Capacity of the Soil 38
Sizing the Absorption System 38
Estimation of Flow 38
Sidewall Versus Bottom Area Absorption 39
Distribution of Liquid Over the Infiltrative Surface 40
Gravity Distribution 40
Dosing 41
Pressure Distribution 41
Construction Practices 42
Modifying the Treated Wastewater Characteristics 43
Modifying the Wastewater Source 43
Modifying Treatment 47
Effluent Quality and Soil Clogging 49
Restoring the Infiltrative Capacity of a Clogged Absorption
Field 51
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Resting 51
Oxidizing Agents 51
Alternate Systems for Problem Soils 53
Slowly Permeable Soils 53
Shallow, Permeable Soils Over Creviced or Porous Bedrock 55
Permeable Soils With Seasonally High Groundwater 56
Mound Systems 56
Curtain or Underdrain Systems 56
Systems iNot Dependent On Soil and Site Conditions 57
Systems Discharging to Surface Waters 57
Aerobic Processes 58
Intermittent Granular Filtration 59
Disinfection Alternatives 65
Other Treatment Processes 67
Costs of Surface Water Discharge Systems 68
Evapotranspiration Systems 68
Causes of On-Site Disposal System Failure 74
References
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ON-SITE TREATMENT AND DISPOSAL OF SMALL WASTEWATER FLOWS
Small Scale Waste Management Project
University of Wisconsin-Madison/University of Wisconsin-Extension
INTRODUCTION
In 1970 approximately 19.5 million households or nearly 30 percent
of all housing units in the United States disposed of their wastewaters by
some form of private sewerage facilities (1). This number is growing
at an increasing rate, due to an emerging trend of population movement
to rural areas where community sewage treatment facilities are not
usually available. Retired persons are moving back to rural areas, as
well as young families who are following the growth of industries on the
outlying fringes of metropolitan centers (2). Most of these rural house-
holds utilize septic tank systems to dispose of their wastewater. Because
of poor design, construction or maintenance, however, a large number
of these systems are failing to provide adequate treatment and disposal
of their sewage.
Many households, while located in rural areas, are situated in
small communities or subdivisions ranging in size from a few households
to a hundred or more. In such instances, failing septic tank systems
which allow raw or poorly treated sewage to reach the ground surface,
surface body of water or even the groundwater, create a severe public
health hazard and nuisance because of the close proximity of homes.
Public wastewater facilities are often the only solution to abate the
problem.
Assessment of wastewater facility needs of small rural communities
is difficult because of the lack of information. The last known published
status report is a survey conducted by the U.S. Department of Agricul-
ture in 1962 (3). The results of the survey are presented in Table I.
At that time, 92 percent of the communities with populations less than
1000 had no public facilities as compared to 19 percent for all commu-
nities with populations above 1000 people.
Since 1962, there have been several governmental programs initi-
ated, namely the Federal Water Pollution Control Act (PL92-500) and
various state programs, which attempt to abate water pollution by
providing grants in aid of construction for community sewerage facilities.
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Consequently, the data presented in Table I needs to be updated. However,
these figures do serve to indicate that few small communities have public
wastewater facilities.
Table I. Number of communities with and without public
sewerage facilities in 1962 (3)
Size of community
United States
population
With
Without
26-999
3,803
42,837
1000-2499
3,079
1,391
2500-5500
2,027
349
Over 5500
2,926
142
Total
11,835
44,709
Certainly the need for improved facilities exists in many of these
communities. The communities often were established long before sound
design and installation criteria for septic tank systems were enforced.
Some homeowners merely installed a pipe to discharge their wastewater
into a ditch or stream away from the house. More conscientious home-
owners installed septic tank systems, but without good design criteria
and proper maintenance, many of these systems have failed. Nuisance
and public health hazards have resulted, often impeding or halting
economic development in the area.
Conventional Public Facilities
The traditional method of providing public wastewater facilities is
to construct a system of gravity collection sewers which convey all the
wastewaters to a single community treatment plant. This "central"
system is preferred by governmental authorities, engineers and the
public alike for several reasons. First, the gravity sewer system is
tried and proven. There is much technical expertise in the theory,
design and operation of central sewerage which has led to great confi-
dence in the system. Second, central sewerage is usually more cost-
effective because of economies of scale. It is less costly to serve
many people with one system rather than each one individually. Third,
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central sewerage allows ready application of central (and usually public)
management which is responsible for the proper functioning of the system.
The availability of a single entity to manage the system is quite desirable
from a regulatory authority's viewpoint because the authorities have an
entity against whom they can bring administrative or judicial action to
abate water pollution problems. Central management is also favored
by the homeowner who no longer has the responsibility for his private
system.
For smaller communities and subdivisions, however, such a conven-
tional collection and treatment facility is impractical because of the finan-
cial burden it places on the residents or developer. This is largely due
to the high cost of collecting wastewater from each home or business.
Smith and Eilers (4) computed the 1968 national average of total annual
costs of municipal wastewater collection and treatment facilities which
showed that 65 percent of the total annual cost is for amortization and
maintenance of the collection system. A more recent study by Sloggett
and Badger (5) of 16 small communities in Oklahoma showed a similar
distribution (see Table II). It is clear from this breakdown of the total
annual costs that the collection system is the most expensive component
of any facility.
Table II. Distribution of total annual costs for municipal
wastewater collection and treatment facilities
Current expenses
Amortization cost Operation & maintenance Overhead
Collection Treatment Collection Treatment Total
Smith &
Eilers
(1968X4) 60.3% 15.3% 4.7% 8.4% 11.3% 100.0%
Sloggett &
Badger (5) — 72.6% — 14.2% 3.2% 10.0% 100.0%
(lagoons)
In small communities, homes are typically scattered, which cause
the costs of sewering to rise dramatically. In their study of 16 waste-
water collection and treatment systems f Sloggett and Badger (5)
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showed that the costs per customer rise as the number and density of
customers declines. Construction costs per customer were compared
to the density and number of customers served (see Tables III and IV).
Both factors were shown to have a significant effect but the density of
customers was shown to have the largest impact on per capita construc-
tion costs.
Sloggett and Badger (6) made similar comparisons using the total
annual costs. They found both number and density of customers to be
significant (see Table V). In 1972, average annual costs per customer
ranged from $76.90 to $43.36 for communities with populations less than
100 and 300-400 respectively. The national average for all municipalities
large and small was $19.80 in 1968 (7).
Table III. Cost of construction per customer relative to
density of customers for 16 community wastewater
facilities in Oklahoma (5)
Customers per mile of sewer
Under 30 30-39 40-49 Over 50
Number of systems
5
5
1
5
Average cost/customer (1972 dollars)
$1,100
$847
$696
$575
Average number of customers
96
119
310
256
Table IV. Cost of construction per customer relative to
number of customers for 16 community wastewater
facilities in Oklahoma (5)
Number of customers served
Under 100 100-199 200-299 300-400
Number of systems
6
4
3
3
Average cost/customer (1972 dollars)
$1,000
$798
$594
$434
Customers/mile of sewer
28.3
37.8
49.4
55.2
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Because of the prohibitive costs of extending sewers, outlying
members of the community may not be served. In 30 percent of the
communities with public facilities surveyed in 1962 by the U.S. Depart-
ment of Agriculture (3) at least one-third of the residences were not
accommodated. In small communities this number would be much higher.
Thus, central sewerage often does not abate the pollution problems as
intended.
Table V. Total average annual cost per customer for
16 community wastewater facilities in Oklahoma (6)
Total average annual cost (1972 dollars)
0% construction 75% construction 100% construction
No. of customers grant grant grant
Under 100
$76.90
$33.06
$18.44
100-199
57.55
25.39
14.63
200-299
52.10
24.09
14.75
300-400
43.36
20.72
13.17
In smaller communities, where homes tend to be more scattered, the
cost of conventional facilities can become prohibitive. Costs can exceed
$10,000 per household for the capital portion alone and may be even
higher if treatment beyond secondary is required to meet water quality
standards. It is not unusual for the cost of the complete system to
approach the total equalized value of the community (8).
To help communities meet the water quality goals of the Federal
Water Pollution Control Act Amendments of 1972, the federal government
was authorized by a provision in the Act to provide grants in aid of
construction for 75% of the grant eligible portions of the wastewater
facility. The availability of these grants would help offset the high per
capita costs in small communities but, unfortunately, small communities
have difficulty in obtaining them.
The federal funds are allocated to the individual states on the basis
of need, but each state is given the power to determine how the funds are
to be spent. Only minimum requirements are set out by the Act for
states to follow in preparing a priority list of projects. For example,
the Act requires that consideration be given to the severity of thepollution
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problem, the population affected, the need for preservation of high quality
waters and national priorities. The federal regulations seem to give the
states some discretion by not requiring strict adherence to their rankings
of pollution discharges. Thus, the priority lists usually work to the disad-
vantage of small communities, in that many of them are near the bottom,
preceded by communities with larger populations and larger pollution
discharges. This emphasis denies small communities any expectation of
receiving badly needed funding for public facilities in the near future. It
is obvious from this discussion that it is impractical to expect many
small communities to construct conventional public wastewater facilities
to eliminate failing private systems.
Non-Central Wastewater Facilities for Small Communities
A "non-central" facility of several treatment and disposal systems
serving isolated individual residences or clusters of residences may
offer a less costly alternative to the conventional central facility in the
non-urban setting. As Table II indicates, approximately two-thirds of the
total annual cost of a conventional facility is due to the collection system.
In a community of scattered homes this proportionate cost could be even
higher. If the central treatment plant could be eliminated, long sewer
extensicns collecting wastes from widely spaced homes would not be
necessary. Instead, treatment and disposal could be provided where the
wastes are generated. Individual or jointly used septic tank systems or
other treatment and disposal methods could be used. Such a non-central
facility of disperse systems could result in a substantial savings because
of the following advantages (9).
1. Existing functional septic tank-soil absorption systems can be
utilized rather than providing new service. Often, homeowners who are
not having trouble or who have recently installed new septic tank systems
do not wish to support community action that will cost them more money
unnecessarily. Incorporating existing systems into the public system
minimizes this opposition, as well as reducing the total cost of the public
facility.
2. Isolated single homes and clusters of homes can be served
individually instead of extending costly sewer lines out to them. TEis
could be equally advantageous to existing communities, as well as newly
platted subdivisions. Where future growth is not expected to be great
enough to warrant sewer extensions, individual septic tank systems could
be used. In cases where substantial growth is expected, such as in
newly platted subdivisions, the first few homes built could be served by
holding tanks which would be pumped and maintained by the management
entity. When the number of homes increased to the point where a
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common disposal system is warranted, it could be built on land reserved
for that purpose. This would delay construction until the time there are
enough contributors available to pay for it.
3. Less costly treatment facilities can usually be constructed. In
addition, subsurface disposal can often be employed which requires
minimal treatment and avoids the necessity of upgrading the treatment
plant to meet changing standards for effluent discharges to surface waters.
Where subsurface disposal is not possible, the smaller flows may allow
other simple treatment methods to be used. In addition, by limiting the
area served the necessary excess capacity required for future growth is
accurately known providing a more optimal design.
4. A more cost-effective facility may encourage smaller communities
to proceed with construction rather than waiting for federal construction
grants. This would speed abatement of water pollution problems. Where
financial aids are necessary, a greater number of community facilities
could receive construction grants because of the fewer dollars required
for each project.
5. More rational planning of community growth is possible. Linear
development, which is encouraged by the construction of interceptor
sewers used to collect wastes from outlying clusters of homes could be
avoided. Growth could be encouraged in the more desirable areas by
providing public service in those areas only.
6. Non-central facilities are more ecologically sound since the
disperse systems dispose of the wastes over wider areas. Through this
practice the environment is able to assimilate the waste discharge more
readily, which reduces the need for mechanical treatment and the asso-
ciated energy consumption.
Though relatively untried, the use of individual or several jointly
used on-site treatment and disposal systems does not exclude the use of
central management. There are several methods of exerting public
(or in some cases private) central management over such facilities.
The powers needed by an entity to properly manage a non-central facility
are similar to those powers needed to manage a conventional community
system.
Powers Needed by a Management Entity Any management entity which
endeavors to effectively administer on-site wastewater disposal systems
must have the power and authority to perform vital functions (9). The
entity should be able to:
1. Own, operate, manage and maintain all wastewater systems
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within its jurisdiction. The entity must be empowered to acquire by
purchase, gift, grant, lease or rent, both real and personal property. It
must also have the authority to plan, design, construct, inspect, operate
and maintain all types of on-site systems whether the system is a typical
individual septic tank system or a more complex system serving a group
of residences. The entity should have at least these "ownership and
operation" powers within its boundaries but it should not be limited to
providing services only within its boundaries. The entity may be given
extra territorial jurisdictional authority to operate, maintain and perhaps
own such systems outside of the entities boundaries by state statute, by
case law, or as terms under a contract.
2. Enter into contracts, to undertake debt obligations either by
borrowing and/or by issuing bonds and to sue and be sued. These
powers are more than mere legal niceties because without them the entity
would not be able to acquire the property, equipment and supplies and
services necessary to construct or operate the individual or jointly used
on-site systems.
3. Raise revenue by fixing and collecting user charges and levying
special assessments and taxes. The power to tax is limited to various
public or quasi-public management entities. In lieu of taxing powers,
the non-government management entities must have the authority implied
or directly granted to set and charge user fees to cover administrative
costs.
4. Plan and control how and at what time wastewater facilities will
be extended to those within its jurisdiction.
Though not necessary to provide adequate management of a non-cen-
tral facility, two additional powers are desirable. These are that the
entity be able to:
a. Make rules and regulations regarding the use of on-site systems
and provide for their enforcement through express statutory authori-
zation. To promote good public sanitation, the entity should be
empowered to require the abatement of malfunctioning systems and
to require the replacement of all such systems, all according to the
plans of the entity. This power, however, may already be inferred
from the statutory authorization to operate a system.
b. Meet the eligibility requirements for both loans and grants in aid
of construction from both the federal and state governments. While
it is obvious that a management entity can function wihout being
eligible for these loans and grants, the viability of the'hon-central"
system is strenghened when grant money is used to offset some or
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most of the costs to the families served by the entity. This is
especially true considering that low -income families typically can
ill-afford to finance the entire cost of their sewerage system.
Experience has shown that low-income families cannot pay waste-
water bills in excess of $7.00 per month or a combined water-
sewage bill of $14.00 per month (10). This rate is difficult to
reach without benefit of public subsidy. The inequity should be
especially obvious to most non-rural residents who typically pay
considerably less than this amount.
Types of Acceptable Management Entities The types of entities which
could manage a non-central facility vary from state to state. The various
state constitutions, state statutes, administrative agency rules and regula-
tions must be examined on a state by state basis, to determine which types
of entities are authorized to manage on-site systems. In addition, the
case laws (essentially interpretations of state laws made by the courts)
must be checked to determine if the courts have construed the constitu-
tion, statutes or regulations to give or to remove the authority to manage
such a system from a possible entity. Those entities which may have the
necessary powers include municipalities, counties and townships, special
districts, private non-profit corporations, rural electric cooperatives and
private profit-making businesses. Each state would have to be checked
to see which are permitted.
While there are disadvantages, the potential of non-central facilities
seem to warrant further investigation. Many of the possible shortcomings
of this alternate facility may vanish as some are constructed and experi-
ence gained.
Collection and Treatment Alternatives for Non-Central Facilities Proper
facilities planning involves a systematic comparison of all feasible alter-
native methods of dealing with a wastewater treatment and disposal
problem. The purpose of this comparison is to identify the most "cost-
effective" solution which will minimize total costs to the community and
the environment over time.
The commitment by regulatory agencies and engineers to conventional
gravity sewers with a common central treatment plant, however, has
eliminated many worthy alternatives from consideration. If this bias can
be changed, the utilization of the non-central concept has the potential
of significantly reducing the environmental and monetary costs of waste-
water facilities in many communities by either reducing the size or
eliminating the collection system altogether and by simplifying the treat-
ment facility.
The most extreme non-central system would be one where each
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home and other establishment were served by an individual septic tank
system. However, the most cost-effective community system would
probably lie somewhere between the two extremes of central sewerage
and individual systems. Either because of economies of scale or because
site conditions are unfavorable for individual disposal systems, joint
systems serving several homes may be constructed. The end result may
be a mix of several individual and joint systems.
The number of alternative methods that can be considered for dealing
with a wastewater treatment and disposal problem are endless. To eval-
uate which method is most cost-effective for a particular community
would seem to be a monumental task. However, this task can be greatly
simplified by selecting the proper beginning point from which to design
the facility.
The objective which must be met by the wastewater facility is to
produce an effluent which will not accumulate harmful pollutants to
dangerous levels in the environment. The environment, of course, is
part of the treatment system, providing final purification. If the pollutant
load in the wastewater received by the environment is too great for the
environment's assimulative capacity, the pollutants will accumulate.
Therefore, to properly design a wastewater facility it is necessary to
first evaluate the physical characteristics of the local environment.
These characteristics will dictate the type and degree of treatment
required before the wastes are discharged. The receiving environment
may either be surface waters, land or the atmosphere. Usually, surface
waters are used as the receiving environment because discharging large
volumes of water into rivers and streams is an easy matter. However,
this practice requires that rather high degrees of treatment be provided
prior to discharge to prevent the degradation of the stream. On the
other hand, if soil is considered as the disposal medium, lower levels
of treatment are required before disposal, because of the soil's greater
assimulative capacity. The trade-off is that large areas of land are
required for absorption. However, when operation and maintenance costs
of high levels of treatment for surface water disposal are compared to
land costs for land disposal, land disposal may be a more cost-effective
alternative. A similar situation may exist for atmospheric disposal as
in evapotranspiration.
Thus, the point of beginning in designing community wastewater
facilities is to characterize the local environment. Once it is determined
what disposal media are available, then treatment systems can be designed
to fit for cost-effective comparisons.
This requires that the capabilities of the receiving environment for
waste assimilation be known. Federal and state regulatory agencies have
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already set effluent standards for surface waters. However, the assimi-
lative capacities of soil and evapotranspiration systems are poorly under-
stood. Therefore, it is necessary to review these areas.
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THE USE OF SOIL AND SOIL MATERIALS FOR TREATMENT
AND DISPOSAL OF WASTEWATER
Liquid Movement Into and Through Soil and Soil Materials
Proper performance of on-site wastewater disposal systems depends
upon the ability of the soil or a soil material to absorb and purify the
wastewater. Failure occurs if either of these functions are not
performed. Both are directly related to the hydraulic conductivity
characteristics of the soil, which are largely controlled by the pore
geometry of the material.
Soil Porosity and Permeability Soil is a complex arrangement of solid
particles and air and water filled pores. The size and shape of these
pores is a function of the structure or arrangement of the solid particles.
In single grained soils such as sands, the voids are simply packing pores
that exist between the individual grains. The size and shape of these
pores is a function of the texture (particle size distribution) of the soil
and the shape and packing of the individual grains. When significant
amounts of clay and organic matter are present, soil particles become
cemented together and form aggregates or peds. Planar voids will form
separating the peds. Tubular channels formed by plants and animals
living in the soil and irregularly shaped discontinuous pores called vughs
are also found in soils (see Figure 1).
Soil permeability or capability to conduct water is not determined
by the soil porosity but rather the size, continuity and tortuosity of the
pores. A clayey soil is more porous than a sandy soil, yet the sandy
soil will conduct much more water, because it has larger, more contin-
uous pores. These twisting pathways with enlargements, constrictions
and discontinuities through which the water moves are constantly being
altered as well. The soil structure which helps to maintain the pores
is very dynamic and may change greatly from time to time in response
to changes in natural conditions, biological activity and the soil-manage-
ment practices. Repeated wetting, drying and freezing help to form peds
while plants with extensive root systems and soil fauna activity promote
soil aggregation and channeling. On the other hand mechanical compac-
tion and the addition of soluble salts can cause the breakdown of the peds
reducing the capacity of the soil to conduct water.
Characterization of Water in Soils Under natural drained conditions,
some pores in soil are filled with water. The distribution of this water
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Figure 1. Schematic representation of a single-grained
(left) and an aggregated soil material (right) (11)
depends upon the characteristics of the pores while its movement is
determined by the relative energy status of the water. Water flows
downhill, but more accurately, it flows from points of higher energy
to points of lower energy. The energy status is referred to as the
moisture potential.
The moisture potential has four. components of which the gravitational
and the ma trie potential are the most important. The gravitational poten-
tial is the result of the attraction of water toward the center of the earth
by a gravitational force and is equal to the weight of the water. To raise
water against gravity, work must be done and this work is stored by the
water in the form of gravitational potential energy. The potential energy
of the water at any point is determined by the elevation of that point rela-
tive to some reference level. Thus, the higher the water, the greater its
gravitational potential.
The matric potential is produced by the affinity of water to the soil
particle surfaces. The pores and surfaces of soil particles hold water
due to forces produced by adsorption and surface tension. Individual
molecules within the liquid are attracted to other molecules equally
in all directions by cohesive forces. Molecules at the surface of
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the liquid, however, are attracted more strongly by the liquid than by air.
To balance these unequal forces, the surface molecules pull together causing
the surface to contract creating surface tension. When solids come in
contact with the surface of the liquid, the water molecules are more
strongly attracted to the solid than other water molecules and hence the
water climbs up the surface of the solid. This is referred to as capillary
rise. The upward movement ceases when the weight of the raised water
equals the force of attraction between the water and the solid. As the
ratio of solid surface area to liquid volume increases, the capillary rise
increases. Therefore, water rises higher in smaller pores.
For example, a cylindrical pore radius of 100 microns corresponds
with a relatively low capillary rise of 28 cm water (pressure below
meniscus equals -28 cm water) while a pore radius of 30 microns results
in a relatively high rise of 103 cm (pressure equals -103 cm water)as illus-
trated in Figure 2. The water within the tube is at less than atmospheric
pressure as noted because it is "pulled" downward by gravity as it is
being"pulled" upward by the forces of capillarity. The water is under
tension as the tube essentially "sucks" the water into it. This negative
pressure in soil is called soil tension or soil suction and is measured
in millibars (mbar), This implies that it takes more energy to remove
or pull water from a small pore than a large one.
In addition to the capillary phenomenon, adsorption forces also contri-
bute to the matric potential. Molecular forces between the surface of the
soil particles and the water form envelopes of water over the particle
surfaces retaining the water in the soil (Figure 2).
Figure 2. Upward movement by capillarity in glass tubes
as compared with soils (after Brady (12))
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When the soil is saturated all the pores in the soil are filled with water
and no capillary suction occurs. The soil moisture tension is zero. If the
soil drains, the largest pores empty first, because they have the least
tension to hold water. As drainage continues, progressively smaller pores
empty and the soil moisture tension increases because smaller pores have
a stronger pull to hold water. Thus, the tension represents the energy
state of the largest water filled pores. Finally, with further drainage,
only the very narrowest pores are able to exert sufficient capillary "pull"
to retain water. Hence, increasing tension or suction is associated with
drying.
The rate of decrease of moisture in soil upon increasing tension is a
function of its pore-size distribution, and is characteristic for each soil
material or type. Figure 3 shows the soil moisture retention curves for a
sand, silt loam, sandy loam and a clay soil. The sand has many relatively
large pores that drain abruptly at relatively low tensions, whereas the
clay releases only a small volume of water over a wide tension range
because most of it is strongly retained in very fine pores. The silt loam
has more coarse pores than does the clay, so its curve lies somewhat
below that of the clay. The sandy loam has more finer pores than the
sand so its curve lies above that of the sand.
60
20 40 60 80 100
SOIL MOISTURE TENSION (MBAR)
Figure 3. Soil moisture retention for four different
soil materials (11)
15
-------
Liquid Movement in Soils Water will flow from a point where it has a higher
potential to a point of lower potential. The gravitational potential acts
to move water downward while the matric potential attracts water in all
directions but only if the soil is not saturated. The rate of flow increases
as the potential difference of potential gradient between points increases.
The ratio of the flow rate to the potential gradient is referred to as the
hydraulic conductivity or K defined by Darcy's Law.
Q = KA
dZ
where: Q = flow rate
K = hydraulic conductivity
A = cross-sectional area of flow
dH
= hydraulic gradient
This parameter accounts for all the factors affecting flow within the soil
including tortuosity and size of the pores. Thus, the measured K values
for different soils vary widely due to differences in pore size distribu-
tions and pore continuity.
The hydraulic conductivity often changes dramatically with changes
in the soil moisture tension. At a tension equal to or less than zero,
the soil is saturated and all the pores in the soil are conducting liquid.
When the tension is greater than zero, air is present in some of the
pores and unsaturated conditions prevail. This condition grossly alters
the flow channel because the forces which cause flow are now associated
with capillarity. As the water content decreases or tension increases,
the path of the water flow becomes more and more tortuous since the
water travels along surfaces and through sufficiently small pores to
retain water at the prevailing water potential. Therefore, the unsaturated
hydraulic conductivity is usually much lower.
To illustrate this, three different soil materials can be considered
with pore size distributions schematically represented in Figure 4. One
"soil" is a coarse, porous material (like a sand), one is a fine porous
material (like a clay) and a third (like a sandy loam) has both large and
fine pores. With an open infiltrative surface and with a sufficient supply
of water, all the soil pores are filled and each pore will conduct water
downward due to gravity. The larger pores will conduct much more
water than the smaller ones. If a weak barrier or crust forms over the
tops of the tubes to restrict flow some of the larger tubes will drain.
Only the pores with sufficient capillary force to "pull" the water through
the crust will conduct water. The larger the pore, the smaller the
16
-------
capillary farce so that progressively smaller pores empty at increasing
crust resistance. This crusting leads to a dramatic reduction in the
hydraulic conductivity of the soil (see Figure 5).
If no crust is present, similar phenomena occur when the rate of
application of water to the capillary system is reduced. With abundant
supply, all pores are filled. If the supply is decreased, there is not
enough water to keep all pores filled during the downward movement of
the water. The larger pores empty first, since the smaller pores have
a greater capillary attraction for water. Thus, larger pores can fill
with water only if smaller pores have an insufficient capacity to conduct
away all the applied water.
HiBh
Atasant
'
I Mod»m l
llHH
JTl
Moderate
on
¦
f
01
a
Vtrv tow
_L±_
Rit« of application
of liquid
¦< Dtgrt* of "enittng'
II
SAND
LOAMY SAND
SANDY LOAM
SILT LOAM
ICnat
CLAY
Figure 4. Schematic illustration of the effect of increasing
crust resistance or decreasing rate of application
of liquid on the rate of percolation through
three "soil materials" (11)
17
-------
The reduction in K upon increasing soil moisture tension is, there-
fore, characteristic for a given soil texture and structure. Coarse soils
with predominantly large pores have relatively high saturated hydraulic
conductivities (Ksat), but K drops rapidly with increasing soil moisture
tension. Fine soils with predominantly small pores have relatively low
Ksat> but their hydraulic conductivity decreases more slowly upon
increasing tension. Hydraulic conductivity or K curves, determined
in situ show such patterns for natural soil (see Figure 5).
The K curves for the pedal silt loam and clay horizons demonstrate
the physical effect of the occurrence of relatively large cracks and root
and worm channels. The fine pores inside peds contribute little to flow.
The large pores between peds and root and worm channels give relatively
high Ksat values (25 cm/day for the silt loam), but these pores are not
filled with water at low tensions and K values drop dramatically between
saturation and 20 cm tension (1.5 cm/day for the silt loam).
The Process of Pore Clogging When liquid wastes are applied to the soil,
a clogging zone often develops at the infiltrative surface. This restricts
the rate of infiltration, preventing saturation of the underlying soil even
though liquid is ponded above. The soil is then able to conduct liquid
only if the water is able to penetrate the clogged zone under the forces
of hydrostatic pressure and capillary pull.
Several phenomena contribute to the development of a clogging zone
at the infiltrative surface of soil absorption systems. These include
(1) compaction, puddling and smearing of the soil during construction;
(2) puddling caused by the constant soaking of the soil during operation;
(3) blockage of soil pores by solids filtered from the waste effluent;
(4) accumulation of biomass from growth of microorganisms; (5) deteri-
oration of soil structure caused by exchange of ions on clay particles;
(6) precipitation of insoluble metal sulfides under anaerobic conditions;
and (7) excretion of slimy polysaccharide gums by some soil bacteria.
Studies by several investigators indicate that the physical and
biological mechanisms are the primary causes of soil clogging in an
absorption field not smeared and compacted during construction (11, 13-
27). In these instances, clogging seems to develop in three stages:
(1) slow initial clogging, (2) rapid increase of resistance leading to
permanent ponding, and (3) a final leveling off towards equilibrium.
Initial development of the clogging zone seems to be due largely to the
accumulation of suspended solids from the wastewater so that the liquid
seeps away more and more slowly between loadings. Aerobic bacteria
decompose many of the organic solids helping to keep the soil pores
open but they can function only when the infiltrative surface drains
18
-------
DRYING »•
Figure 5. Hydraulic conductivity (K) as a function of soil
moisture tension measured in situ with the crust-test
procedure" (28)
between doses to allow the entry of air. As the clogging zone begins to
form, decreasing the aerobic periods between ponding, the aerobic
bacteria eventually are unable to keep up with the influx of solids.
Permanent ponding finally results^ leading to anaerobic conditions where
oxygen is no longer present. Any dissolved oxygen in the water is
inadequate to maintain the aerobic environment necessary for the rapid
decomposition of the organic matter. Clogging then proceeds more
quickly due to the less efficient destruction of soil clogging organics
by anaerobic bacteria. Sulfides produced by reduction of sulfate by
these bacteria bind up trace elements as insoluble sulfides, causing
heavy black deposits in the clogging zone. Some anaerobic and facul-
tative organisms which grow in such an environment produce gelatinous
19
-------
materials (bacterial polysaccharides slimes or gums) which clog the soil
pores very effectively. At this point the clogging mat seems to reach an
equilibrium state where the resistance to flow changes little. Failure
of an absorption field will not occur, however, if the rate of application
does not exceed the equilibrium rate. The process can be reversed
to restore much of the original infiltrative capacity if the ponded surface
is allowed to drain and rest to permit aerobic biological decomposition
and the drying and cracking of the clogging materials.
This description of soil clogging assumes that the native soil struc-
ture is left relatively intact at the infiltrative surface during construction
of the system. However, many systems fail, usually within a year or
two, because of poor construction techniques. Absorption of water by
soils depends upon preservation of a suitable soil structure, but soil
structure can be partially or completely destroyed by compaction and
smearing during construction. Extensive damage does not occur in soils
with a single-grained structure (sands) but can be very serious in
aggregated soils with high clay contents. When mechanical forces are
applied to a moist or wet soil, the water around clay particles acts as a
"lubricant" causing the soil to exhibit plasticity where individual soil
particles move relative to one another. Such movements, referred to as
compaction, puddling, or smearing, close the larger pores. Structural
damage increases as soil wetness and clay content increase. Compaction
may result from frequent passes over the field by heavy machinery,
smearing of the soil surface by excavating equipment and puddling by
exposure of the infiltrative surface for a day or more to rainfall or wind-
blown silt that seals off the soil pores. The result is that the absorption
field may be clogged before it is put into service.
Infiltration is not only dependent upon the resistance of the clogging
zone but also on the capillary properties of the underlying soil (28).
For example, an identical "crust" with a resistance of 5 days (the length
of time for 1 cm^ to pass through 1 cm^ of barrier with a head of 1 cm)
and ponded with 5 cm of liquid would induce flow rates of 8 cm/day in a
sandy loam; 7 cm/day in a sand; 4 cm/day in a silt loam; and 1.8 cm
day in a clay (28). Crusts with very high resistances would conduct
more liquid when overlying a clay than when overlying a sand. Thus,
similar clogging zones developed in different soils have different conduc-
tivities.
The Signficance of Unsaturated Flow Flow of liquid in unsaturated soil
proceeds at a much slower rate than in saturated soil because flow only
occurs in the finer pores. This slows the rate of infiltration into the
soil but enhances purification. Wastewater effluent is purified by filtra -
tion, biochemical reactions and adsorption, processes which are more
20
-------
effective in unsaturated soils because average distances between effluent
particles and the soil particles decrease while the time of contact increases.
This flow phenomenon can be illustrated by an example (11). Figure 6
shows a thin section of the C horizon of a Saybrook silt loam, which is a
stony sandy loam till with a saturated hydraulic conductivity of 80 cm/
day. The flow velocity of water in the soil pores can be estimated from
its moisture retention curve (Figure 3). This velocity can be used to
derive the time for water to travel one foot (30 cm), assuming a hydraulic
gradient of 1 cm/cm due only to gravity. Successively smaller pores
empty at increasing tensions and K decreases correspondingly (see Figures
4 and 5). Calculated travel times increase from 3 hours at saturation
to 30 hours at 30 mbars and 8 days at 80 mbars of soil moisture tension.
Skeleton grams
¦ Plasma
(very porous
and calcareous)
| Macrovoids
¦ Liquid
Figure 6. Occurrence and movement of liquid in a saturated
and unsaturated sandy loam till C horizon of Saybrook silt loam (11)
In structured soils it is possible to have flow predominantly through
the planar voids, thus bypassing the interior of the peds. High liquid
applications may result in high dispersion where the water passes through
the planar voids without displacing the water already in the peds. In such
21
SAYBROOK SILT LOAM (IIC STONY SANDY LOAM TILL)
-------
instances, short circuiting of liquid through the soil occurs with associated
low retention times. Low applications of water would displace more of the
water in the peds and have low dispersion. Differences in dispersion
related to different structures while following chloride movement in soil
columns has shown short circuiting to be a particular problem on drained
soils dosed at relatively high rates (29).
Short circuiting in a structured soil is schematically illustrated in
Figure 7. If the large planar voids are drained and air filled, a high
application rate of liquid applied at the surface will quickly pass through
the large pores before much can enter the fine pores of the peds. There-
fore the retention time of the bulk of the liquid is low and only a portion
of the entire soil volume is used to transmit the fluid.
If the application rate is low or if there is a barrier to flow, the
dispersion is low. The large pores will not fill with liquid and flow will
be through the fine pores in the peds. In this case the retention time will
be long and flow will only be through the portion of the soil most effective
in renovation.
Long liquid travel times are desirable to adequately purify the waste-
water. The design of absorption systems may be critical to achieve this
in some soils. Travel times are sufficiently long under all moisture
tensions to affect adequate purification in clay, but are too short in sand
and sandy loams when the soil is near saturation. Once a clogging zone
has developed in such permeable soils, moisture tensions reach a level
where sufficiently long travel times result. However, when an absorption
system constructed in a highly porous or dry structured soil is first put
into service without a developed clogged zone, there is danger that adequate
purification may not be achieved unless precautions are taken in design
to insure unsaturated soil conditions are maintained.
Wastewater Treatment Capabilities of Soil Materials
The principal goal in liquid waste disposal for homes in unsewered
areas is the purification of the liquid before it reaches potable or recrea-
tional waters. Organic matter, chemicals and pathogenic organisms and
viruses that are not removed prior to application to the soil must be
removed or transformed by the soil material. Numerous studies have
shown that under proper conditions, the soil is an extremely efficient
purifying medium.
Bacteria and Virus Removal by Soil From the standpoint of public health,
removal of disease organisms and viruses is the most critical function of a
soil disposal field. Many field and laboratory studies have examined the
efficiency of the soil for pathogen removal and the various parameters that
22
-------
UNCLOSGED CLOGGED
HI ADDED WATER
^ CLOGGING ZONE
Figure 7. Influence of clogging zone on short circuiting
in structured soils
affect its efficiency. Factors important in removal of pathogens by soil
include soil type, temperature, pH, organism adsorption to soil and soil
clogging materials, soil moisture and nutrient content and biological
antagonisms (30). Another key factor is the liquid flow regime in the
soil. As shown previously, unsaturated flow, induced by either a clogged
zone or application rate, enhances purification because liquid movement
is through only the smaller pores of the soil.
Figure 8 shows removal of fecal coliforms and fecal streptococci
from septic tank effluent by two columns packed with 2 feet of Plainfield
loamy sand (effective size 0.14 mm, uniformity coefficient 1.99) (31,32).
Both columns were loaded well below their saturated hydraulic conductivity
rates of nearly 400 cm/day (96 gpd/ft2) but one was loaded at twice the
rate of the other. During the first 100 days of application, the number of
bacteria discharged from both columns reached a plateau and then began
to decline. Fewer bacteria passed through the column with the lower
loading rate. Column 1, loaded at 10 cm/day (2.4 gpd/ft2), removed
approximately 92% of the fecal coliforms applied per day while column 2,
loaded at 5 cm/day (1.2 gpd/ft2), removed 99.9%. Fecal streptococci
23
-------
and Pseudomonas aeruginosa were also found in the effluent from the more
heavily loaded column 1. These organisms were not detected in effluent
from the more lightly loaded column 2. During this period a clogging zone
developed on the infiltrative surface of each column and the fecal coliform
count in the effluents from both columns eventually dropped to between
10 and 100 FC/100 ml (32).
TIME (days)
Figure 8. Bacteria counts in effluents from sand columns loaded
with septic tank effluent. Column 1 loaded at 10 cm/day
(1.5 hours retention time) and Column 2 loaded at
5 cm/day (25 hours retention time)
FC = fecal coliforms FS = fecal streptococcus (31)
Septic tank systems installed in sands also exhibit the effects of the
clogging zone in removing indicator bacteria. Figure 9 shows the bacterial
counts obtained while monitoring several points around an absorption trench
in an unsaturated medium sand soil. The kinds and numbers of bacteria
found in the liquid 1 foot (30 cm) below and 1 foot (30 cm) to the side of the
trench were similar to natural soil flora (11,31,32).
24
-------
FT
ABSORPTION FIELD
CROSS SECTION
BACTERIA /100 MLS OR PER 100 6 OF SOIL
FECAL FECAL TOTAL TOTAL
0 -
TRENCH
— 1 FT-»|
• -—
STREPTO-
COCCI
COLIFORMS
COLIFORMS
BACTERIA
«I07
1 -
o
o
<200
<200
<600
0.6
-
1
160,000
1,900,000
9,700,000
3.0
LIQUID
:i: CL066ED ZONE :
94,000
4,000,000
23,000,000
4,400
•
-
<200
17,000
23,000
6.7
3 -
• ——
<200
<200
<600
3.7
NATURAL
SOIL
<200
700
1,800
2.8
Figure 9. Cross-section of seepage trench in sand showing
bacterial counts at various points near the trench (18, 20)
Concurrent studies of Almena silt loam were also conducted (32).
This soil has a lower capacity to conduct liquid than the unstructured
sands and the majority of flow is through the larger pores between soil
peds. Undisturbed cores, 2 feet deep, of Almena silt loam were loaded
with septic tank effluent at a rate of 1 cm/day (0.24 gpd/ft^). At this
loading, effluent short-circuited through large pores and channels and
significant numbers of bacteria were found in the column effluents. When
the loading rate was reduced from 1 cm/day to 3 mm/day to promote
slow flow through the soil peds rather than through the larger cracks
around the peds (Figure 7), bacterial counts decreased dramatically to
below 2/100 ml of fecal coliforms, fecal streptococcus and P. aeruginosa.
When the loading was restored to 1 cm/day, high counts of tHese organisms
were again observed (see Figure 10).
Virus adsorption and inactivation in soils have been of considerable
interest to scientists and engineers over the years. When virus enter
the septic tank or other treatment process, they are likely associated
with cells in fecal material. These masses settle releasing some virus
depending upon turbulence within the process (up to 89% of the polio virus
added in fecal material was released by vigorous shaking in laboratory
studies (34)). Secondary adsorption on wastewater solids may occur in
25
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LC
J
Ps.a.
FS
-O O-
8
*'¦*.»¦ ,A\ •yfH100 C
\ / - SO ^
'¦»«¦«» 1 I m
-I 1 1 1 1 1 1 1 I CT
20 60 100 140 180 3
TIME (days) "
Figure 10. Bacteria counts in effluent from an undisturbed core of
Almena silt loam loaded with septic tank effluent
(LC = loading change from 1 cm/day to 3 mm/day
and return to 1 cm/day) (31)
treatment processes. The free and particle adsorbed virus will then be
discharged to subsequent treatment processes or the soil absorption field.
Removal of virus in soils occurs as the result of the combined effects
of sorption, inactivation and retention. Upon entry into the soil, virus are.
rapidly adsorbed to solid surfaces. Desorption appears to be strongly
related to the ionic strength of the applied fluid, increasing as the ionic
strength decreases (36). In the adsorbed position, the virus are inactivated
in a spontaneous process which is temperature dependent, being greater at
the higher temperatures (34). Virus detention within the soil is affected by
the degree of saturation of the pores through which the virus ladened effluent
flows. The more saturated the pores, the less opportunity there is for
virus contact with surfaces to which it can adsorb.
26
-------
In laboratory studies with packed sand columns, septic tank effluent
was inoculated with more than 10 5 plaque -forming units (PFU) per liter
of polio virus type I (34,35). All viruses were removed in the 24-inch
columns at a loading rate of 5 cm/day (1.24 gpd/ft^) over a period of
more than one year. At a loading rate of 50 cm/day (12.4 gpd/ft2),
virus breakthrough occurred (Figure 11). Analysis of the sand residue
following virus application, indicated that adsorbed virus within the column
were inactivated at a rate of 18% per day at room temperature and at
1.1% at 6 to 8° C (35).
In contrast to these results, virus were detected approximately
60 inches within columns packed with calcareous loamy sand and fed
secondary effluent containing 3 x 10^ PFU polio virus type I at a rate
of 15 cm/day (3.70 gpd/ft2) (36). Most of the virus was adsorbed in the
top 2 inches of the soil and virus removal was not appreciably affected
at application rates of between 15 and 55 cm/day (3.70 and 13.6 gpd/
ft2)„ Only deionized water desorbed the virus but drying for five days
prevented desorption even with deionized water.
Laboratory tests with ground soil from a Batavia silt loam reduced
virus in septic tank effluents by 5.4 logs per cm and Almena silt loams
material produced 7.9 logs of reduction per cm (35). It should be
emphasized, however, that soils in the field do not exist in a finely
ground state. Channels in natural soil will reduce opportunity for
virus adsorption and travel over long distances may occur when loading
rates are high.
Chemical Transformations and Removals by Soil Domestic wastewaters
may contain a few chemicals hazardous to public health or the environment.
Nitrogen and phosphorus compounds are discharged in household waste-
water which can enter ground or surface waters in sufficient quantities
to cause concern. Nitrogen, in the form of nitrate or nitrite has been
linked to cases of methemoglobinemia in infants (37). A safety limit for
nitrate of 10 mg/1 as nitrogen is recommended by the U.S. Public Health
Service (38). TTiere are many reports of nitrate concentrations above
10 mg/1 N limit in wells near septic tank systems (37,39-41). Acceler-
ated eutrophication of surface wates is also attributed to nitrate contri-
buted from waste discharge (40).
In solution, nitrate moves freely through the soil though some deni-
trification (reduction of nitrate to nitrogen gas) can occur where organic
material and an anaerobic environment occur together. Nitrogen in
septic tank effluent is about 80% ammonium and 20% organic nitrogen,
but much of it is converted biologically to nitrate as it moves through
the aerated unsaturated soil immediately below the clogging zone in
27
-------
DEPTH (cm)
Figure 11. Penetration of polio virus into a 60 cm conditioned sand
column at room temperature. Fluid samples (A5 cm (850
ml) dose, ~ 50 cm (8. 5 1) dose, • input titer,
1
-------
§
g
X
MICROGRAMS PER CRAM SOIL
-# *—T
/» °
• * ©
I*//
*£\f
.—• AMMONIUM ¦ N
o^—o NITRATF -N
—• CHLORIDE
\
I
ORGANIC - N
Figure 12. Concentrations of NH4-N, NO3-N, Organic N
and CI in unsaturated soil below the clogged zone
in sand (31)
can move downward 50-100 cm per year through "clean" silica sand (44)
but, movement in loams, silt loams and clays is much slower (5-10 cm
per year). Thus, except in coarse soils, over 10 years would be required
for the phosphorus to move as much as 3 feet (44).
Heavy metals and complex organic compounds are also effectively
adsorbed by soil and therefore removed from the percolating wastewater.
29
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ESTIMATION OF THE INFILTRATIVE AND PERCOLATIVE
CAPACITY OF THE SOIL
Criteria for site selection for on-site systems vary from folk know-
ledge and experience to various empirical methods of site testing, often
codified into rules. The U.S. Public Health Service has developed a
general reference manual (45), which has provided guidelines for many
state, regional and local manuals or codes of practice.
Several factors are usually considered in the selection of a site for
a septic tank system. The ability of the soil to absorb liquid usually
estimated by the percolation test, is a common requirement. Other
factors include slope, depth to groundwater, nature and depth to bedrock,
likelihood of seasonal flooding and distance to well or surface water (45).
These traditional factors have several limitations and vary widely between
codes.
Estimation of Soil Permeability
The Percolation Test In 1926, Henry Ryon developed a test to obtain
field data on failing seepage systems (46). He dug a hole one foot square
to the depth of the failed systems, filled it with water, allowed the water
to seep away, refilled the hole with water and recorded the time required
for the water level to drop one inch ("percolation rate"). To calibrate
the test, Ryon inspected several failing or near-failing systems and noted
the loading of the system, the soil characteristics and the percolation rate
measured in nearby soil. Ryon plotted curves relating permissible
loading rates versus the percolation rate from these data. It was later
prqposed that these curves could be used to size new soil absorption
systems. Adoption of the procedure by the New York State Health
Department led to its wide acceptance though slight changes have been
made over the years. Today it is used by nearly every state to size
on-site systems.
The percolation test is based on the assumption that the ability of a
soil to absorb sewage effluents over a prolonged period of time may be
predicted from its initial ability to absorb clear water (46). From
Ryon's data comparing absorption rates of existing septc tank systems to
the percolation test, the measured rate is reduced by a factor ranging
from 20 to 2500in order to size the absorption area (11). However, the
results of the test are highly variable and its use for system sizing relies
on an empirical relationship between the measured percolation rate and
30
-------
the actual loading rate. Tests run in the same soil vary by as much as
50% (47,48). Thus, the procedure is unreliable, requiring a more
accurate test.
The'Crust Test" The soil below most operating absorption systems is
unsaturated because of the clogging mat which develops at the infiltrative
surface. To properly size an absorption system, therefore, the unsatu-
rated hydraulic conductivity characteristics of the soil must be known.
Since the standard percolation test does not provide conductivity data of
this type the "crust test" was develqped (11,47,49-52).
The crust test is performed in situ to avoid disturbing natural pores
and to maintain continuity with the underlying soil. A carved soil column
is fitted with a ring infiltrometer (an impermeable collar with a tight
fitting lid) to control water addition to the column. A tensiometer is
installed in the column just below the infiltrative surface to determine
the degree of saturation in the soil by measuring the soil moisture
tension as water is applied (see Figure 13). To maintain unsaturated
conditions in the soil column, a "crust", made of gypsum and sand is
placed over the soil surface. When water is introduced through the
infiltrometer, flow into the soil is restricted by the crust. This esta-
blishes a constant steady-state flow rate which induces a nearly uniform
moisture tension in the soil beneath the crust. The measured soil mois-
ture tension and the equilibrium flow rate locate one point on the hydraulic
conductivity curve. Additional tests run with crusts of different hydraulic
resistances define the K-curve as shown in Figure 5. This curve can be
used for design if the range in soil tensions under the clogged zones of
mature absorption systems in similar soils is known.
Though this procedure offers a direct measurement of K, it is time
consuming and requires a skilled operator. It is not a test which can
be run economically at each site. However, since the hydraulic conduc-
tivity of a soil is dependent upon the pores in the system, the conductivity
of a soil at various sites in the soil map unit can be defined within statis-
tical limits(see Figure 14) (53). Also, it has been found that variability
curves of different soils in the same textural groups have similar conduc-
tivity curves (see Figure 15 and Table VI) (53). Therefore, by defining
families of K-curves for groups of soils the hydraulic conductivity char-
acteristics of a particular soil or site can be predicted.
In Wisconsin, four major hydraulic conductivity types have been
suggested based on the texture of the soil materials (28). These textural
groupings include the sands; sandy loams and loams; silt loams and some
silty clay loams; and the clays and some silty loams. (In other regions
31
-------
Table VI. Summary of morphological characteristics
Soil series from Figure 14 (53)
Soil
identification
Horizon
selected
Texture
Structure
Frequency of
biopores
Group A
Ontonogon series B2
Very fine, illitic,
frigid
Typic Eutroboralf
Magnor series IIB
Fine-loamy, mixed
Aquic Glossoboralf
heavy loam moderate, medium few coarse,
angular blocky common
medium
heavy loam moderate, medium few coarse,
subangular blocky common
medium
Group A
Piano series B2
Fine-silty, mixed,
Mesic
Typic Arguidoll
Batavia series B2
Fine-silty, mixed
Mesic
Mollic Hapludalf
Morley series IIB3
Fine, illitic,
Mesic
Typic Hapludalf
silty clay
loam
silty clay
loam
heavy, silty
clay loam
moderate, medium
subangular blocky
moderate, medium
subangular blocky
moderate, medium
prismatic, too
coarse, blocky
structure
few coarse,
common
medium
and fine
few coarse,
common
medium
and fine
few coarse,
common
medium
32
-------
similar groupings might be made but they must be based on field data
since differences in soil mineralogy may affect these groupings.) Typical
hydraulic conductivity curves were developed from field measurements
for each of these conductivity types (see Figure 5).
.To make these curves useful in designing soil absorption fields for
septic tank systems, soil moisture tensions were measured under the
clogging zones of several operating absorption fields (28). This informa-
tion provided a design point on the curve for proper field sizing. This
same procedure could be used to select design points for other types of
soil systems (see Table VII).
The application rates for various soils presented in Table VII repre-
sent the best estimates available to date. Because of the unstructured
nature of the sands and sandy loams the rates are reasonably accurate.
However, because of the nature of the flow which occurs in finer textured,
structured soils there is more variability in the tensions measured under
operating fields (28). In these soils the design rates must be used with
care particularly where expandable clays are present.
Figure 13. Schematic diagram of the crust test procedure.
33
-------
Table VII. Recommended maximum loading rates of septic tank
effluent for different soil types
Estimated Loading rate + r
percolation cm/day (gpd/ftz)
rate (min/inch) Soil texture Bouma(28) Machmeier(54)
Operating
conditions (28)
0-10
sand
5 (1.23) (1.20)
10-30 sandy loams, 3 (0.72) (0.60)
loams
30-45 * some porous 3 (0.72) (0.50)
silt loams,
and silty
clay loams
45-90 *clays, some 1 (0.24) (0.45)
compact silt
loams and
silty clay
loams
4 doses/day
uniform distribution
trenches or beds
1 dose/day
uniform distribution
trenches preferred
1 dose/day
uniform distribution
desirable
shallow trenches
only
dosing and uniform
distribution
desirable
shallow trenches
only
"^Bottom area only.
* Should not be applied to soils with expandable clays.
34
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SOIL MOISTURE TENSION
(cm woter)
Figure 14. Hydraulic conductivity data for Piano series.
Regression line is solid line, and dashed lines
indicate one standard deviation about
regression line (after Baker (53))
Estimation of High Groundwater
To insure adequate purification of the wastewater before it reaches
groundwater, three feet of unsaturated soil is necessary below the infil-
trative surface. If saturated soils ever occur within the three feet
minimum, transmission of harmful pollutants to the groundwater may
result (31-36). To determine if saturated conditions do occur within
the minimum is often difficult, however, because water table levels
fluctuate in response to changing weather conditions. Typically, the
water table is low during the summei; while in the spring and fall, it
rises. Ideally, the highest groundwater level should be observed
when it occurs, but this is not very practical. Moreover, observations
35
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0.1 1.0 10 100
SOIL MOISTURE TENSION (mBAR)
Figure 15. Hydraulic conductivity groups;
Group A: Ontonogon and Magnor series
Group B: Piano, Batavia and Morley series
(After Baker (53))
made in relatively dry years do not represent those that occur in normal
years. Thus, other methods must be used to determine the high water
elevation.
Soil mottling is sometimes an indicator of the presence of seasonally
high water levels. Mottles are spots of contrasting colors found in soils
subject to periodic saturation. The spots are usually bright yellow-orange
-red surrounded by gray-brown matrix and described according to their
color, frequency, size and prominence (55). Well-drained soils are
usually brown in color due to the presence of finely divided insoluble iron
and manganese oxide particles distributed throughout the horizon.
However, under reducing conditions often produced by saturation over
36
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prolonged periods, the iron and manganese is mobilized until reoxidized
when the soil drains. Repetitive wetting and drying cycles quickly
produce local concentrations of these oxides on pore surfaces forming
red mottles (56). Soil fiom which much of the iron and manganese has
been completely reduced loses its brown color becoming grey by a
process referred to as gleying. Therefore, the upper limit of the
mottled soil is often a good estimate of the high groundwater level
though it may also be due to a periodic perched water table. The
latter instance may be confirmed by the lack of mottles or gleying
in lower horizons (57).
-------
MAINTAINING THE INFILTRATIVE CAPACITY OF THE SOIL
Loading rates recommended in Table VII are based on observations
from properly functioning septic tank-soil absorption systems. If a
system is to operate satisfactorily for a reasonable length of time at
these loading rates, then the infiltrative capacity of the soil must be
maintained. This requires that proper design, construction and main-
tenance procedures be followed.
Sizing of the Soil Absorption System
Estimation of Flow Waste flows from single homes, restaurants, motels,
etc., are intermittent and subject to wide fluctuations. Variation in the
number of persons contributing to the flow and their activities have
profound effects on the daily volume of waste discharged. Accurate
estimates of waste flow volumes are therefore difficult.
A study of eleven rural homes showed the average per capita flow
from a single household to be 43 gpd (58-60). The greatest flow contribu-
tions come from the laundry and bathing events (see Figure 17). Social
events, such as family gatherings and overnight guests, will accentuate
the peak flows shown. In addition, the number of people occupying the
home may increase through additions to the family or sale of the house.
Thus, the use of the average per capita flow for design purposes is
unwise. Rather, it is necessary to design for the expected peak flows
to ensure the system will not fail when large flows occur.
Peak flows can be empirically estimated for households by assuming
two people occupy each bedroom. This is realistic since the number of
occupants in a home is a function of the number of bedrooms. Figure 16
shows that a peak flow of about 3 gal/hr/capita can be expected which
corresponds to approximately 75 gallons per person for the entire day.
Thus, 150 gpd/bedroom gives a reasonable estimate of the peak flow.
This is the design basis recommended by the Public Health Service (45)
which has proved satisfactory in practice.
Estimation of flow from public buildings, commercial establishments
and recreational facilities is more difficult. The Small Scale Waste Manage-
ment Project is presently determining various daily and peak flows from
bowling alleys, camps, churches, schools, country clubs, laundromats,
marinas, motels, restaurants, service stations, shopping centers,
theaters and stadia.
38
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T - TOILET
L - LAUNDRY
B - BATH OR SHOWER
D - DISH WASH
4 —i 0 - OTHER
WS- WATER SOFTENER
AVERAGE FLOW - 42.6 GPCD
TIME OF DAY
Figure 16. Average daily flow pattern from eleven rural
households (58)
Sidewall Versus Bottom Area Absorption A soil adsorption system has
two infiltrative surfaces; the horizontal bottom of the trench or bed and
the vertical sidewall. When the bottom area begins to clog, the waste
effluent ponds in the system and the sidewall begins to absorb liquid
(28). In some soils the sidewall may become the more significant infil-
trative surface as clogging continues (16, 46).
The rate of water movement through soil is proportional to the total
water potential gradient primarily due to the gravitational and matric
potentials. In an unclogged absorption system, the potential gradient is
lower for the sidewall area than the bottom area because the gravitational
potential is zero. As the clogged zone develops, the matric potential of
the bottom area may be reduced to where the sum of the gravitational
and matric potential is less than the matric potential of the sidewall area.
39
-------
The sidewall then becomes the dominating infiltrative surface.
Absorption systems should be designed to maximize the most signi-
ficant infiltrative surface. For the Midwest, the bottom areas are more
important because of changes in moisture tensions occurring in the soil
during wet seasons. The horizontal gradients can be reduced to levels
lower than the vertical gradients because of relatively low natural
drainage rates. This is particularly true in early spring and late fall
when evapotranspiration rates are low. The matric potential is lowered
because of soil wetness. Maximizing the infiltrative area through consid-
eration of sidewalls as a reserve capacity is recommended but in the
Midwest, the bottom area should be sized to absorb the entire estimated
daily flow.
Distribution of Liquid Over the Infiltrative Surface
Localized overloading of septic tank effluent on the soil often occurs
because of poor distribution. This may result in poor purification of the
effluent in highly permeable soils and accelerate clogging in slowly perme-
able soils. Uniform application of the wastewater over the infiltrative
surface is usually beneficial.
Absorption systems with uniform distribution and dosing are not
necessary in all types of soil to eliminate poor purification and soil
clogging. Sands and weakly structured sandy loams and loams would
benefit most (61). After a new system is put into service in natural
sands, local overloading may cause unnoticed groundwater contamination
until clogging develops. Development of a clogged zone may take several
years. Conversely, excessive clogging due to poor distribution tends to
occur in weakly structured soils. Uniform distribution aids in reducing
the clogging by applying the liquid simultaneously to the entire infiltrative
surface at rates no greater than the soil is able to accept (62).
Gravity Distribution Liquid' flow by gravity is the most common method
of distributing waste effluent over the infiltrative surface of the soil
absorption field. Perforated 4-inch diameter pipe is laid level or at a
uniform slope to 2 to 4 inches per 100 feet with the holes downward.
Such a system does not provide uniform distribution. The liquid trickles
out the holes nearest the point of inlet and at points of lowest elevation
(see Figure 16).
Clogging usually seems to start near the inlet of the absorption
system and progresses down the length of the bed (11).
40
-------
The large holes permit too much liquid to be discharged close to the inlet.
Thus, the soil below these points receives a nearly continuous trickle of
water and is soon constantly ponded. Clogging develops forcing the
liquid to infiltrate further down the trench where the infiltrative surface
is still fresh. This sequence continues until the entire bottom is clogged
(Figure 17). Altering the orientation of the holes or changing the slope
of the pipe does not improve distribution significantly (63).
TRADITIONAL SUBSURFACE SEEPAGE BED
Gravity flow; continuous trickle of effluent.
— — — 1
~
'"~T' '" " --
I ~
-—T
I ~
~ \ I ~ ~ ~ ^
—___ i
l *
i i
i ~ ~ I i j i ~ ^ ^
V
Equilibrium
Figure 17. Progressive clogging of the infiltrative surfaces
of subsurface absorption systems (11)
Dosing Periodic dosing of large volumes of effluent onto the field
improve distribution and provides an opportunity for the soil to drain
between applications. Drainage exposes the infiltrative surface to air,
reducing clogging (16,46,62). However, even with dosing, the effluent
is not distributed over the entire infiltrative surface if the 4-inch pipe
is used (63).
Pressure Distribution Pressure systems help provide uniform
41
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distribution. Networks of small diameter pipe with small holes are
designed such that the entire pipe network fills before much liquid passes
out the holes, thereby achieving uniform distribution (61-63). These
systems combine uniform distribution with dosing enhancing purification
and reducing clogging.
Proper loading of permeable soils to prevent saturated flow is vital
to insure purification of the waste effluent. Pressure distribution systems
provide this loading control. Conventional gravity distribution is ineffec-
tive (61).
Pressure distribution systems also retard clogging. Since the network
is designed to apply no more liquid than an area of the absorption bed can
absorb each day, the soil remains well aerated. Absorption fields in sand
with pressure distribution have shown no evidence of clogging after four
years of operation (61) while fields in sand with conventional distribution
begin to clog after six months (11). The aerobic environment maintained
by pressure systems promotes the growth of microorganisms which destroy
clogging materials and appears to attract larger fauna, such as worms,
to consume nutrients accumulating at the infiltrative surface. The worm's
burrows help break up the clogging zone. Worm activity perhaps explains
why an absorption field in a silt loam underlain with glacial fill dosed with
pressure distribution at three times the USPHS (45) recommended rate
has not clogged after three years of operation (62).
Construction Practices
Probably the most frequent cause of early failure of soil absorption
systems is poor construction techniques. Rapid absorption of waste
effluent by soil requires maintaining open pores at the infiltrative surface,
but often the pores are sealed during construction by compaction, smearing
or puddling of the soil by excavating equipment.
Compaction, smearing and p.uddling occur primarily in soils containing
clay. The flat clay particles adhere to each other in dry soil making it
hard and very stable to high compressive forces. However, when wet,
the clay plates separate when forces are applied to the soil. The water
acts as a lubricant as the clay plates move relative to one another to
close channels and vughs reducing the permeability of the soil to very
low levels.
Not all soils are equally susceptible to this structural destruction.
Tendency toward compaction and puddling depends upon the soil type, the
moisture content and the applied force. Soils with high clay contents
are easily puddled while sands are not affected. However, soils with
clay will not puddle if they are only slightly moist. Instead, under pres-
sure, dry clay breaks into small fragments along pedal boundaries rather
42
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than smearing, thereby keeping the large pores open.
Careful construction techniques, following the recommendations
below, will minimize this cause of soil clogging (64,65).
1. Work should be done in clayey soils only when the moisture
content is low. If the soil forms a "wire" instead of breaking apart
when attempting to roll it between the hands, then it is too wet.
2. Excavating equipment should not be driven on the bottom of the
system. Trenches rather than bed construction is preferable in clayey
soils because equipment can straddle the trench, thus reducing compaction
and smearing.
3. Shallow systems should be constructed to place the infiltrative
surface in more permeable horizons and to enhance evapotranspiration.
This is particularly beneficial in clayey soils because they are generally
wetter for longer periods of time, especially at greater depths.
4. Any smeared or compacted surfaces should be removed. Compac
tion may extend as deep as 8 inches in clays. This requires hand spading
to expose a fresh infiltrative surface.
5. Work should be scheduled only when the infiltrative surface can
be covered in one day because wind blown silt or raindrop impact can
clog the soil.
Modifying. the Treated Wastewater Characteristics
While the search for improved methods of on-site disposal has
centered largely around the soil absorption system, recently more
emphasis has been put on altering the characteristics of the effluent
discharged to the soil. Improving the quality of effluents has been
purported to enhance soil infiltration, reduce the dependence on soils
for final treatment or eliminate the need for soil altogether.
Modifying the Wastewater Source One of the simplest ways to improve
the effluent discharged to the soil is to provide changes at the source,
either by reducing the total volume of waste discharged or by preventing
entry of pollutants into the waste stream.
Flow reduction to produce lower wastewater volumes can be accom-
plished through water conservation and recycle. Reductions can be
achieved through improved water use habits or by simple modifications
in water-use appliances or plumbing fixtures. With less wastewater to
43
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Event
No/cap/day
Average
size of
event
Normal
use
With
3 gal/
flush
With
sudsaver @
27.86
With 15
gal bath
shower
With all
three
used
Recycle
bath/laun.
to toilet
Toilet
2.29
3.99
9.16
6.87
9.16
9.16
6.87
0
Laundry
.31
33.49
10.51
10.51
8.64
10. 51
8.64
8.64
Bath
.47
21.35
10.00
10.00
10.00
7.05
7.05
7.05
Dishes
.39
12.50
4.86
4.86
4.86
4.86
4.86
4.86
W. Soft.
.03
81.07
2.64
2.64
2.64
2.64
2.64
2.64
Other
--
--
5.43
5.43
5.43
5.43
5.43
5.43
Total
- —
_ —
42.60
40.41
40.73
39.65
35.49
28.62
(% savings)
(5%)
(4%)
(7%)
(17%)
(33%)
Table VIII. Average calculated wastewater reductions in eleven rural homes
(All volumes in GPCD) (58)
-------
Event
Fecal
Nonfecal
Kitchen
Automatic
Clothes
C lothes
toilet
toilet
Garbage
sink
dish
washer-
washer-
Bath/shower
Parameter
flush
flush
disposal
usage
washer
wash
rinse
BODc U
4340
6380
10900
8340
12600
10800
4010
3090
BOD* F
2340
3980
2570
4580
7840
6970
2840
1870
TOC U
3530
4250
7320
5000
7280
7700
2610
1750
TOC F
1580
3170
3910
4110
4690
5380
1910
1130
TS
10700
17800
25800
13800
18200
37500
10900
4590
TVS
7760
12000
24000
9730
10500
14700
4800
3600
TSS
6240
6280
15800
4110
5270
7930
3040
2260
TVSS
5090
5120
13500
3840
4460
4700
1810
1580
TOT-N
1500
2640
630
420
490
580
150
310
NH3-N
590
520
9.6
32.3
54
19.4
11.4
40
NO3-N
6.3
21.1
.2
1.8
4.1
17
10.3
7.4
TOT-P
270
280
130
420
820
1600
550
36
ORTHO-P
120
190
90
180
380
410
110
20
Temp. °F
66
66
71
80
101
90
83
85
Flow (gal)
4.3
4.3
3.8
4.8
12.0
15.7
14.4
13.0
No./samples 32-40
24-37
4-7
7-11
13-15
24-27
24-28
18-24
Table IX. Mean wastewater contributions from household events, mg/capita/day (59)
-------
treat and dispose of, the life of the on-site disposal would increase.
Nearly 70% of the total wastewater generated in the homes is derived
from the toilet, laundry and bath (58). The most substantial water savings
can therefore be made in these areas. Low flow toilets, "sudsaver" washing
machines, restricted flow shower heads, and recycling of bath and laundry
wastes for toilet flushing are four commonly mentioned water saving devices.
By reducing the toilet flushing volume to 3 gallons, clothes washing to 28
gallons by using a sudsaver, and baths and showers to 15 gallons, average
water use could be reduced to 17% in rural Wisconsin homes (58). Recycling
bath and laundry wastes to flush toilets could increase the savings to 33%
(see Table VIII). These savings compare well with values from other studies
(66,70).
Waste segregation to elminate pollutants from the waste stream improves
the quality of the wastewater. Analysis of wastewaters generated from various
water-use events in rural households suggest which events should be modified
for the most beneficial results (59,60) (see Table IX).
Recently, the concept of segregating toilet wastes (black water) from the
other household wastewaters (grey waters) for separate treatment and disposal
has drawn attention. Serious questions have been raised by those planning
development in water-short areas regarding the use of valuable drinking water
to transport body wastes and the practice of co-mingling the black and grey
waters prior to on-site treatment and disposal. Segregation of black water
from other household wastewater by use of a non-water carriage toilet could
conserve water resources and reduce the volume and pollutant load discharged
to on-site disposal systems. Daily savings would vary considerably at a given
home and between homes. Ranges of recent possible reductions are shown in
Table X (58-60,66).
If the toilet wastes can be segregated and adequately disposed of, atten-
tion must then be directed toward the disposal of the grey water. Grey water
has been considered to be relatively uncontaminated, compared to black water.
However, grey water contains substantial quantities of physical and chemical
pollutants as well as pathogenic indicator organisms (see Tables XI and XII).
Pollutant concentrations of grey water are similar to those of black
water or combined wastes in rural homes (60). Black water contains high con-
centrations of suspended solids, nitrogen and bacteria but grey water also
contains sufficient quantities of pollutants and pathogenic indicators to cause
concern.
To date, little research has been done on treatment and disposal of house-
hold grey water. One method is the septic tank-soil absorption system. However,
simple alternatives might be more desirable in certain applications. The Small
Scale Waste Management Project is evaluating alternative methods for grey
water treatment and disposal.
46
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The reductions in flow or waste strength may increase the life of
a soil absorption field, but if so, by what factor it is not known. For
new installations, it has been suggested that the smaller absorption fields
could perhaps be allowed if water saving devices are used. However,
unless assurances are made that the flow reduction of waste segregation
facilities could not be removed or fail, reduced field sizing is not recom-
mended.
Table X. Effect of toilet waste segregation
on household wastewater
Parameter*
% Reduction
Flow
22-31%
bod5
22-49
Suspended solids
36-67
Total Phosphorus, P
14-42
Total Kjeldahl
Nitrogen, N
68-99
*Although not shown, there would also be substantial
reductions in the quantities of pathogenic organisms.
Modifying Treatment Another method to improve effluent quality is to
provide better treatment than the septic tank offers. Of the numerous
alternatives available, small extended aeration units and intermittent
sand filters seem to be the most promising.
Aerobic treatment is often suggested because the process can
produce a higher quality effluent than the septic tank. Many different
process designs exist, but extended aeration is most commonly used
for small waste flows. Two compartment tanks are typically used,
one for aeration and the other settling. Some designs provide a small
septic tank ahead of the aeration tank.
The performance of several aerobic units of different designs has
been compared to septic tanks operating under both laboratory and field
conditions (71-73). Two years of evaluation showed the aerobic units can
47
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Table XI. Pollutant contributions bv black and grey
wastewater streams*(l05)
Grey Black
Mean
Range
Mean
Mean
Mean
Range
Mean Mean
Pollutant
%
%
g/c/d
mg/l
%
%
g/c/d mg/l
B°d5
63
51 -80
28.5
255
37
20-49
16.7 280
Suspended solids
39
23-64
17.2
155
61
36-77
27.0 450
Nitrogen, N
18
1-33
1.9
17
82
67-99
8.7 145
Phosphorus, P
70
58-86
2.8
25
30
14-42
1.2 20
Flow
65
53-81
29.4 gal/c/d
35
19-47
15.9 gal/c/d
¦^The values shown are based on the results of several studies (60,66,70).
The results are average values for households with typical conventional
appliances, excluding the garbage disposal.
Table XII. Selected bacteriological characteristics of
bath and laundry wastewaters (105)
Data Mean1 95%C.I.
Event Bacteria Points #/100 ml #/100 ml
Total coliforms
32
1810
530-6160
Bath/shower
Fecal coliforms
32
1210
330-4410
Fecal Strept.
32
326
70-1510
Total coliforms
41
215
45-1020
Laundry
Fecal coliforms
41
107
28-405
Fecal Strept.
41
77
19-305
1
Log normalized data.
48
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achieve a higher degree of treatment than septic tanks. Removals of
biodegradable organic material from the waste by aerobic units were
significantly higher than those achieved by septic tanks, but suspended
solids concentrations in all effluents were nearly identical (see Figures
19 and 20 and Table XV). However, the septic tanks were more stable.
Periodic upsets resulted in substantial variability in aerobic unit effluent
quality.
Effluent Quality and Soil Clogging Improving the effluent quality before
discharge to the soil may inhibit clogging. In studies with packed columns
of sands, sandy loams and loams, clogging was found to be a function of
the sum of the suspended solids and BOD5 concentrations in the effluent
(74). The relationship found was:
Adjusted area _ Area required for
required standard septic X 3 / BOD5 + TSS
tank system J —
Assuming an average effluent quality of 40 mg/1 BODg and 40 mg/1 of
suspended solids, the calculated adjusted absorption area is approximately
two-thirds of the "standard" area.
However, in several other studies (24, 53, 75), • investigating effects
of effluent quality on soil infiltration, only slight differences in clogging
rates were found over a range of qualities tested. Differences in rates
of clogging were small in hand-packed lysimeters of sand and sandy
loam loaded with septic tank effluent (74 mg/1 BODg and 51 mg/1 suspended
solids) and aerobic unit effluent (81 mg/1 BODg and 75 mg/1 suspended
solids) (75). The aerobic unit effluent produced earlier but less intense
clogging in the sand, but the reverse was true in the sandy loam. Upon
resting, the soils receiving aerobic unit effluent recovered more quickly.
In general, however, there was little difference between the soil clogging
characteristics of the two effluents.
In studies with undisturbed cores of Almena silt loam (percolation
rates of 70 min/in in the topsoil and 100 min/in in the subsoil) columns
were continuously ponded with septic tank effluent, aerobic unit effluent
and distilled water (24). The aerobic effluent had a significantly lower
biodegradable organic concentration than the septic tank effluent (chemical
oxygen demand concentrations of 150 mg/1 and 60 mg/1 respectively),
but the suspended solids concentrations were similar (40 mg/1 and 33 mg/1
respectively). More severe clogging occurred with the aerobic effluent.
No clogging occurred in the soil loaded with distilled water (see Figure 18).
It was hypothesized that finely divided suspended solids in the aerobically
49
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treated wastewater were to enter the small pores in the soil which clogged
the soil with depth creating a more effective barrier to flow.
Figure 18. Measured moisture pressures in six columns of Almena
silt loam, ponded with distilled water (nos. 1 and 3),
septic tank effluent (nos. 4 and 6) and aerobic unit
effluent (nos. 2 and 5) (24)
Subsequent studies designed to test the solids clogging hypothesis
indicated that the initial saturated hydraulic conductivity is more signifi-
cant than effluent quality (104). Undisturbed cores of Almena silt loam
were paired according to their initial saturated hydraulic conductivity,
one pair representing a high and low initial K a[. Three sets of four
replicates each were dosed with 1 cm/day of septic tank effluent (48 mg/1
BOD5, 27 mg/1 TSS), aerobic unit effluent (27 mg/1 BOD5, 61 mg/1 TSS),
and tap water. The length of time to when each column remained ponded
between daily doses was recorded. The aerobic columns showed mean
ponding times of 21.3 weeks, the septic tank set 20.6 weeks, and the tap
Soil Moitture Pretture (Cm Woter)
o Mechomcolty Treated
Effbent
50
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water 18.3 weeks. When initial K values were compared between all
three sets the ponding times for the nigh Ksat columns was 28 weeks while
the ponding times for the low Ksat columns was 14.8 weeks.
These studies to date seem to indicate that, in unstructured soils,
such as sands and sandy loams, applied effluent quality may affect the
degree of clogging. A similar effect has not been found in finer textured
soils.
Restoring the Infiltrative Capacity of a Clogged Absorption Field
Soil absorption systems often fail after several years of satisfactory
service because the clogging zone eventually develops to a point where
insufficient amounts of effluent pass through it. Methods are being sought
to rejuvenate old fields so that failed systems need not be replaced.
Resting One effective method is resting the system (11,14,15,46). Resting
allows the absorption field to gradually drain exposing the clogged infiltra-
tive surface to air. After several months, the clogging materials are
broken up to physical and biochemical processes, restoring the infiltrative
capacity of the system. A second bed must be available to allow continued
use of the disposal system while the failed bed is resting. Two beds can
be constructed when the disposal system is first installed at the outset,
each with 50 to 75 percent of the total absorption area required. The
two beds can then be used alternately by diverting the wastewater from
one to another every six months. If a system with only one bed has
failed and a new one is contructed, provisions should be made such
that the old one is not abandoned, but can easily be alternated with the
new bed.
Oxidizing Agents The infiltrative surface also can be rejuvenated by the
addition of oxidizing agents to the absorption field. The oxidizing agents
perform the same function as resting but the clogging zone is destroyed
v ithin a day or two rather than several months. Such a method does not
necessitate taking the clogged bed out of service which eliminates the need
for a second bed.
Laboratory and field tests indicate that chemical oxidation can restore
the infiltrative surface to near its original permeability (27). The oxidant
preferred is hydrogen peroxide (H202) because it is effective at the natural
pH of absorption fields, produces no noxious byproducts and is inexpensive.
Hydrogen peroxide treatment would best be used in a preventive
maintenance program because smaller quantities are cheaper and safer
to handle. Routine septic tank pumping could be coupled with absorption
field maintenance with peroxide. Five gallons of 50% H2O2 solution may
51
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be sufficient to reduce the clogging developing in the bed. Since the field
is still permeable, the oxidant can reach the clogging zone easier than in
a sealed system. Tank pumping and peroxide treatment would best be
performed while the system is not in use, for example, during a vacation,
to give the reagent time to work without being diluted with peak effluent,
and to allow aerobic conditions to become well established in the bed.
Evaluation of preventive maintenance with hydrogen peroxide is in progress.
52
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ALTERNATIVE SYSTEMS FOR PROBLEMS SOILS
There are many areas where the conventional septic tank-soil absorp-
tion field is not a suitable system of wastewater disposal. Sites with very
slowly permeable soils, excessively permeable soils, or soils over shallow
bedrock or high groundwater, for example, are simply not suited for the
conventional system. However, alternate systems can be used which still
utilize the capabilities of soil to absorb and purify wastewater (76).
Slowly Permeable Soils
Slowly permeable soils constitute a major group of problem soils.
Soils with percolation rates faster than 120 min/in often have seasonal
perched water tables within 2 feet of the ground surface, especially during
the spring and fall. Infiltrating surface water during these wet periods is
unable to percolate through the subsoil fast enough and flooding occurs
from lateral movement of water through the topsoil from higher elevations.
Such conditions are not suitable for conventional soil absorption systems.
To overcome these conditions, one alternative is to raise the absorp-
tion field above the natural soil by building the seepage system in a mound
of medium sand (77). This raises the seepage system above the wet slowly
permeable subsoil and places it in a dry permeable sand (see Figure 19).
There are several advantages to this. First, the percolating liquid enters
the more permeable natural topsoil over a large area and can safely move
out laterally until absorbed by the less permeable subsoil. Second, the
clogging zone that eventually develops at the bottom of the gravel trench
within the mound will not clog the sandy fill to the degree it would in the
natural soil. Finally, smearing and compaction of the wet subsoil is
avoided, since excavation in the natural soil is not necessary.
The design of the mound is based upon the expected daily wastewater
volume it will receive and the natural soil characteristics. It must be
sized such that it can accept the daily wastewater flow without surface
seepage when perched water exists in the natural soil in the spring and
fall, as well as when the water table is lower during the summer and
winter. Size and spacing of the seepage trenches is important to avoid
liquid from rising into the fill below the trenches when the water table
is high. In addition, the total effective basal area of the mound must be
sufficiently large to conduct the effluent into the underlying soil.
A clean, medium sand is used as the fill material in construction of
53
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PLAN VIEW
Figure 19. A plan view and cross-section of a mound system
for problem soils
54
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the mound and the gravel trenches constructed within consist of 1 - 1-1/2
inch stone. As in any seepage trench, a clogging mat will develop at its
bottom. The ultimate infiltration rate through this zone has been shown
to be 5 cm/day (28). Therefore,one consideration must be to insure that
sufficient trench bottom area is available for the design flow.
If more than one trench is included, another consideration is the
spacing between trenches. The area between trenches must be long enough
for the underlying natural soil to absorb all the liquid contributed by the
upslope trench. Infiltratibn rates into the natural soil is based on the
hydraulic conductivity characteristics of the least permeable soil horizon
below the proposed site. The basal area required for the mound is based
on this as well.
To distribute the wastewater to each of the trenches a pressure
distribution network is used. This provides uniform application which is
necessary to prevent local overloading and eventual surface seepage.
Mound systems installed in these soils have been monitored since
1972 and are performing satisfactorily (78). However, application of
proper siting criteria design and construction techniques which have been
described in detail (77) are critical for satisfactory performance. Not
all sites are acceptable for the mound design.
Shallow, Permeable Soils Over Creviced or Porous Bedrock
Shallow, permeable soils over creviced or porous bedrock constitute
a major group of problem soils because inadequate soil is available to
purify the percolating waste before it reaches the porous bedrock which
leads directly to the groundwater. To overcome these limitations, the
absorption field can be raised above the natural soil by using the mound
system (see Figure 19). This increases the amount of soil available for
percolation and with uniform application of effluent, purification will be
adequate by the time the percolating effluent reaches groundwater (31,35,79,
80). However, nitrates will not be removed.
The design of the mound follows the same procedures as described
for the mound in slowly permeable soils. However, the seepage system
within the fill may have nearly any shape desired, since the permeability
of the natural soil is not a limiting factor. A bed is usually more suit-
able than trenches. Detailed site criteria design and construction proce-
dures are described elsewhere (81) and should be followed to assume
proper operation of the mound.
55
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Permeable Soils With Seasonally High Groundwater
Mound Systems Homes should not be built in areas with a permanently
high groundwater table. However, in some areas, homes are built where
the water table is high only occasionally during the year. During high
water table periods, a conventional septic tank-soil absorption system
cannot function properly due to flooding of the system and improper puri-
fication. A properly designed and constructed mound system provides
sufficient unsaturated distance for purification before the effluent reaches
the groundwater (Figure 19). The design of the mound follows the same
procedures as described for the mound in slowly permeable soils but the
seepage system within the mound is usually a rectangular bed. Normally
the permeability of the natural soil is not a limiting factor but the mound
must be designed to prevent the intrusion of the perched water table to
the base of the mound. Detailed site criteria, design, and construction
procedures are described elsewhere (82) and should be followed to assume
proper performance of the system.
Curtain or Underdrain Systems Conventional subsurface trenches can be
constructed where periodic high water tables are a problem if the natural
soil is drained. Agricultural drain tile is used to lower the water table
and to discharge the water to the ground surface. Careful placement of
the drain is necessary to insure a sufficient depth of unsaturated soil is
maintained for purifying the wastewater to avoid short circuiting (83,84).
Systems of this type presently are being evaluated.
There are disadvantages to these alternate systems. Construction
of mound systems depends upon a source of suitable fill material and
relatively large lots. Mounds cost $2500 to $3500 or more to construct
depending on hauling costs. Underdrain systems may be cheaper but
systems not dependent on soil for disposal sometimes may be more
desirable.
56
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SYSTEMS NOT DEPENDENT ON SOIL AND SITE CONDITIONS
At some sites, the soils may be totally inadequate as a treatment
and disposal medium as the lot may be too small to accommodate a proper
absorption system. In such instances, on-site wastewater disposal systems
not dependent upon soil disposal but which discharge the treated wastewater
to surface waters or the atmosphere are necessary.
Systems Discharging to Surface Waters
There are numerous alternative treatment processes currently avail-
able to treat small flow wastewaters. To make a proper decision on these
alternatives, the homeowner must examine both the in-house wastewater
modification processes as discussed earlier, as well as treatment options
which might best meet local water quality objectives. A general outline
of these alternatives is presented in Table XIII.
Table XIII
In-house
Water conservation-flow control, reuse
Waste segregation - non-water carriage toilets,
low flow toilets
Biological processes
Aerobic-suspended growth, fixed media, emergent
vegetation
Anaerobic-septic tanks, fixed media systems
Physical-Chemical Processes
Filtration
Ion exchange
Adsorption
Chemical flocculation/coagulation
Disinfection-halogens, ultraviolet, ozone
Land application
Soil absorption
Irrigation
Lagoons (absorption)
E vapotr an sp ira tion
57
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Systems designed to discharge treated wastewaters to surface waters
must be capable of producing a high quality effluent. The U.S. Environ-
mental Protection Agency has currently set concentration maximums of
30 mg/1 BOD5 and 30 mg/1 suspended solids for municipal treatment plants
which discharge to water courses. Lower maximums might be set for
scattered individual systems, discharging to small intermittent streams.
It might be expected that bacteria counts in effluents from individual
systems could not exceed the maximums of total and fecal coliforms of
1000/100 mis and 200/100 mis respectively, recommended for recreational
waters (85). In addition, limitations on the nutrients, nitrogen and phos-
phorus, will likely be required for discharges to lakes or impoundments.
Aerobic Processes Although a number of the options listed in Table XIII
have been field evaluated, many others are untried in the context of small
flows application. Currently, aerobic treatment processes have received
the greatest attention as an alternative to the septic tank. Over 75 years
of experience with this biological process in larger scale applications
makes it a logical candidate for small flow on-site treatment.
Some of the first controlled aerobic processes for household waste-
water treatment were small trickling filters. Nichols (86) described
aerated pebble filters following septic tanks and Frank and Rhymus (87), in
one of the first large research projects on household wastewater disposal,
detailed the construction and operation of lath trickling filters. Relatively
little experience has been currently reported with fixed media biological
filters, but the potential of synthetic media filters in either the stationary
or rotary mode is most promising. The most recent operational exper-
ience by SSWMP with a rotating biological contactor has not been good
owing primarily to shaft breakage in both laboratory and field units (88).
On the other hand,an anaerobic -submerged media system, initially designed
for ships, has proven to be very stable and relatively maintenance free (88).
The first notable research on the adaptation of the activated sludge
process (extended aeration) to household use was conducted at Purdue Univer-
sity in the early 1950's (89). A very simple prototype receiving toilet wastes
only produced an effluent with average BOD5 and suspended solids concentra-
tions of 28 mg/1 and 42 mg/1 respectively. Similar studies were conducted
at Ohio State University employing a proprietary extended aeration package
plant. Average BOD5 and suspended solids concentrations of 24 mg/1 and
43 mg/1 respectively were reported during a 23-month period with rela-
tively trouble-free service (90).
In 1970, the National Sanitation Foundation (NSF) issued its "Standard
40", pertaining to individual aerobic treatment units (91). This standard
outlined criteria for the evaluation of the units and presented a procedure
58
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by which they would be tested and certified. Several states now require
NSF certification as a prerequisite for approval of aerobic units.
In addition to controlled studies 01 aerobic units, there are several
reports as to how they function under actual conditions (72,92,93,94).
Although some information on effluent quality exists, probably the most
valuable information is that on operation and maintenance.
Bennett and Linstedt reported on results from several homeowner
operated field units in Colorado (92). Their results indicated a mean
value for BOD5 and suspended solids of 150 mg/1. They cited lack of
proper maintenance and an adverse effect of surge flows for this poor
performance. Voell and Vance (93) provided data on a large number of
field operated aerobic units in a New York county. Average values for
BOD5 and suspended solids were about 90 mg/1. The lack of proper main-
tenance was again cited for this performance. Six different aerobic
treatment units were installed and monitored over an eight-month period
by Glasser (94). Average values for BOD5 and suspended solids were
reported to be 48 and 85 mg/1 respectively. Glasser recommended
maintenance and supervision no less than four times per year.
The performance of several aerobic units of different designs have
been compared to septic tanks operating under both laboratory and field
conditions (72,95). Two years of evaluation showed the aerobic units can
achieve a higher degree of treatment than septic tanks. Removals of
biodegradable organic material from the waste by aerobic units were
significantly higher than those achieved by septic tanks, but suspended
solids concentrations in all effluents were nearly identical (see Figures
20 and 21 and Table XIV). However, the septic tanks were more stable.
Periodic upsets resulted in substantial variability in aerobic unit effluent
quality.
Periodic carryover of solids were the major reasons for effluent
quality deterioration from aerobic units. Bulking sludge (sludge that will
rot settle), toxic chemical additions from the home and instability due to
excessive buildups of sludge seemed to be most common causes of carry -
over. Several design modifications have been suggested to help prevent
some of the operational problems but regular servicing is necessary to
insure proper functioning (72). Inspections should be made at least once
every two months and excess solids pumped every eight to twelve months
(72,95).
Intermittent Granular Filtration Field experience to date with on-site
aerobic treatment processes has indicated that additional polishing of
effluents will be necessary prior to surface discharge in order to meet
current EPA effluent guidelines.
59
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Treat-
ment BODg cod (Unfiltered) COD (Filtered) TSS1 Fecal Coliforms1 Fecal Strep1
(Sys- (mg/1) (mg/1) (rag/1) (mq/1) (no./ml) (no./ml)
terns) r .
Mean
95% Conf.
Int.
Mean
951 Conf.
Int.
Mean
95% Conf.
Int.
Mean
95% Conf.
Int.
Mean
95% Conf.
Int.
Mean
95% Conf.
Int.
Coef.
of Var,
Range
Coef.
of Var.
Range
Coef.
sf Var.
Range
Coef.
Df Var.
Range
Coef.
of Var.
Range
Coef.
of Var.
Range
Aerobic
units
o. of samples
• loq-normal distribution
Table XIV. Comparison of septic tank and aerobic unit effluent characteristics (71,72)
-------
Filtration appears to be one of the most promising alternatives
currently available to provide this polishing step. Whether the filtration
is provided by granular beds or by mechanical filter systems employed
as a part of the biological process or as a separate process, depends
upon economics, effectiveness and maintenance requirements.
Granular filtration appears to be particularly well suited to on-site
sytem design. At least two basic flow configurations have been success-
fully tested in the field; the recirculating sand filter and the intermittent
sand filter.
The recirculating sand filter system consists of a septic tank, a
recirculation tank, and an open sand filter (96). Wastewater is dosed
on to the filter by a submersible pump located in the recirculation tank.
The sump pump is actuated by a time clock and is sized to pump between
5 to 10 gallons per minute for single households. A recirculation ratio
of 4:1 (recycle to forward flow) is recommended. The recirculation tank,
normally the same size as the septic tank, receives flow from the septic
tank and the recirculated portion of the filter effluent. Baffles provide
proper mixing of the septic tank and filter effluents prior to recycle.
Filter effluent recycle flow is controlled by a rubber float valve located
in the filter effluent return line. When the recirculation tank is filled,
filter effluent is discharged from the system.
The filter bed consists of 3 feet of coarse filter sand with a desired
effective size of 0.6 to 1.5 mm and a uniformity coefficient of less than
2. 5. Approximately 12 inches of graded gravel support the sand and
surround the underdrain system. The filter is designed to operate at a
flow rate of 3 gallons per day per square foot based on raw septic tank
flow. It is estimated that approximately 1 inch of sand should be removed
once per year in order to avoid serious ponding conditims. After 12 inches
of sand have been removed, new sand would be added.
Results for a household system indicate that effluent BOD5 values
average less than 5 mg/1 and TSS values less than 6 mg/1 (96).
In the intermittent granular filter, pretreated wastewater is applied
over a 2 to 3 foot deep bed of sand and the filtrate collected by under-
drains. The sand remains aerobic and serves as a biological filter,
removing not only suspended solids, but also dissolved organics. A
summary of filter performance based upon a review of the literature is
given in Figure 22 (97).
Filters receiving septic tank or aerobic unit effluent have been
tested under field and laboratory conditions. A typical filter sysem is
61
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300
o>
E
V)
o
-J
o
CO
o
111
Q
z
lii
CL
(/>
(/)
100
30
10
1 1 .1
EFFLUENT SUS
i i 1 1
iPENOEO SOLIDS
.>*¦ /
/ /
ill l
\ y
i
i
-AEROBIC UNITS-
/J*
^ /
^SEPTIC TANKS I
, , ¦ ^
f A Ber
/ x voe
. • Gla
/ — Oti
i i i i
inett ft Linstedt (92)
ill and Vance (93)
sser (94)
s et al (72)
iii i
PERCENTAGE
Figure 20. Comparison of septic tank and aerobic unit effluent
concentrations of suspended solids
depicted in Figure 23. Of major concern in sizing intermittent granular
filters are the trade-offs between effluent quality and maintenance require-
ments as depicted in Figure 22.
Effluent quality of sand filtered septic tank and aerobic unit effluents
appear in Tables XV and XVI for field systems operated for over two years
(98,99). It may be noted that relatively little difference is shown between
aerobic unit-sand filter effluent and septic tank-sand filter effluent for
comparable loading conditions although the aerobic unit system employed
a finer sand (0.19 mm as compared with 0.45 mm). It is apparent that
effluents from this filter could meet current EPA standards for BOD and
TSS, but would require further pretreatment for coliforms or phosphorus.
Excellent ammonia conversion is also produced by both systems.
62
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Table XV. Septic tank-sand filter effluent quality data
Septic tank Sand filter Chlorinated
effluent effluent effluent
BOD (mg/1)
123
9
3
TSS (mg/1)
48
6
6
Total Nitrogen-N (mg/1)
23.9
24.5
19.9
Ammonia-N (mg/1)
19.2
1.0
1.6
Nitrate-N (mg/1)
0.3
20.0
18.9
Total Phosphorus-P (mg/1)
10.2
9.0
8.4
Orthophsophate (mg/1)
8.7
7.0
7.9
Fecal coliforms (#/100 ml)
5.9xl05
6.5xl03
2
Total coliforms (#/100 ml)
9.0xl05
1.3xl03
3
Note: Loading rate average: 5 gal/day/sq ft (0.2 m/day)„
Effective size--0.45 mm; uniformity coefficient--3.0.
Filter runs are dependent upon grain size, hydraulic loading, influent
organic strength, and maintenance techniques. There is apparently a
substantial difference in clogging mechanisms in septic tank effluent loaded
f.lters and aerobic unit loaded filters (88,98,99). Recommended filter
operation schedules for a septic tank-sand filter system are presented in
Table XVII. It is recommended that two filters be employed in an alter-
nating mode, each designed for a hydraulic loading rate of 5 gal/day/ft^.
When one filter becomes ponded, it is taken out of service, raked to a
depth of 2 to 4 inches, and rested prior to reapplication of wastewater.
After a second loading period, the top 4 inches of sand from that filter
should be replaced with clean sand.
Aerobic unit-sand filter systems do not normally require a second
filter (99). An application rate of 5 gal/day/ft^ is suggested with a six-
month maintenance interval. Removal of the solids mat, along with 1 inch
of sand, and replacement with 1 inch of clean sand is the only required
63
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Table XVI. Aerobic unit-sand filter effluent quality data
Aerobic unit Sand filter Chlorinated
effluent effluent effluent
BODc (mg/1) 26
TSS (mg/1) 48
Total Nitrogen -N (mg/1) 39.
Ammonia-N (mg/1) 0.
Nitrate-N (mg/1) 33.
Tota 1 Phosphor us -P (mg/I) 31.
Orthophosphate-P (mg/1) 28.
Fecal coliforms (#/100 ml) 1.9x
Total coliforms (#/100 ml) 1.5x
2-4
4
9-11
7
1
37.5
38.3
4
0.3
0.4
8
36.8
37.6
8
23.1
24.0
1
22.6
23.4
104
1.3 x 103
8
105
1.3 x 104
35
Note: Loading rate average: 3.8 gal/day/sq ft (0.15 m/day).
Effective size--0.19 mm.
Uniformity coefficient--3.31.
Table XVII. Septic tank-sand filter operation schedule
Sand
Effective size
Uniformity
Loading and resting period
(mm)
coefficient
(months)
0.2
3-4
1
0.4
3
3
0.6
1.4
5
64
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Figure 21. Comparison of septic tank and aerobic effluent
concentrations of biodegradable organic material
maintenance step. Reapplication of wastewater is possible immediately
after maintenance is performed. Experience has shown that periodic
biological and hydraulic upsets of the biological process can be assimi-
lated by the sand filters, however, extended periods of upset will lead
to shorter filter runs.
Disinfection Alternatives Where disinfection of final effluent is required,
several alternative systems have proven to be effective. The use of dry
feed chlorinators will normally produce effluents which will meet current
EPA standards (Tables XV and XVI). Unfortunately, a major problem
associated with the use of dry feed chlorinators is the lack of control of
dose to the wastewater. Periodic high chlorine concentrations were
determined over an extended field testing program (97). Methods to
65
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EFFECTIVE SIZE OF SAND (mm)
Figure 22. Trends of percent BOD reduction and required
maintenance of literature sand filters treating
septic tank wastewater (97) (BOD (ave.) of
septic tank wastewater -- 94 mg/1)
more effectively control hypochlorite feed are available, however (88).
In light of the toxicity of chlorine, consideration must also be given to
dechlorination of effluent prior to final surface water discharge.
Initial studies with ultraviolet irradiation of sand filtered household
effluents have proven to be most promising. Four months of operating
data with a commercially available UV unit are presented in Table XVIII
(88). Long - term tests are continuing with these units in several field
installations. One major drawback to UV irradiation is the high initial
capital investment. As greater demand for this type of system increases,
costs will likely decrease, however.
66
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WW \
INSULATED COVER
aVENT 1
-nStflfl
• . '
, * . . *
1 .
1 • . . • ! • ' ' '
J - SAND ':
• . « 1
• « 1
IpeVgravel
v COARSE •
; o STONE0 .
DISTRIBUTION
PIPE
Jl
i i-
' I l ' •
* l l ••
. - i I
. I I' • •
¦ I i: '¦
i •
O Q
„ o
o ° o
CONCRETE SLAB
"Z
• 11
.1 I
o o
6
TTY\T\
COLLECTION PIPE
Figure 23. Profile of intermittent sand filter
Other alternative methods of disinfection include iodine, bromine
salts, formaldehyde and ozone. Experience with iodine disinfection using
an iodine saturator has been excellent, but iodine costs are high (88).
Feeding of bromine salts appears to be too complex for small flows appli-
cation and ozone treatment also involves relatively complex equipment.
Little experience has yet been gained with formaldehyde feed equipment.
Other Treatment Processes A number of other unit processes are avail-
able for small flow application, but field experience is very limited.
Chemical feed equipment is available but maintenance is relatively high
(95). The use of ion exchange and carbon adsorption techniques is within
the realm of practicality but maintenance and operation requirements
are high as are costs. Currently, treatment "packages" employing a
number of these unit operations are being field tested at the University
of Wisconsin on both combined and grey water effluents from households.
67
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Table XVIII. Coliform analysis of sand filter and UV water
purifier unit effluent
Aerobic unit - UV Septic tank UV
sand filter effluent effluent sand filter effluent effluent
Fecal coliform
(#/100ml) 11-13 <1 (2.6-4.4) x 103 <1
Total coliform
(#/100 ml) 64 - 75 <1 (3.6-5.1) x lO*3 <1
Costs of Surface Water Discharge Systems Costs of the two systems
described above are largely dependent upon the volume of wastewater to
be treated, the availability of quality filter sand and the amount of main-
tenance required by the system. A cost analysis involving the application
of septic tank effluent and aerobic unit effluent onto sand filters has been
performed (Table XIX) (99). Assumptions in the analysis include a three
bedroom home, a family size of five, wastewater production of 50 gal/
cap/day (0.19 m3/cap/day) and the availability of a sand with effective
size =0.4 mm and uniformity coefficient of = 3. 5. It is noted that
sampling costs are not included in the cost analysis. Since discharge
is to surface waters, state regulatory agencies may require some type
of monitoring program.
The cost ranges presented in Table XIX suggest that the two alterna-
tives examined have similar, albeit high costs, when compared with septic
tank-soil absorption fields. These costs would likely be reduced some-
what if water conservation was practiced. It must be recognized, however,
that isolated systems can only be evaluated on a case by case basis and
conclusions on cost effectiveness cannot be drawn by examining national
average.
Evapotranspiration Systems
Evapotranspiration (ET) may provide a means of wastewater disposal
in some localities where site conditions preclude soil absorption. Evapora-
tion of moisture either from the soil surface or by transpiration by plants
is the mechanism of ultimate disposal. Thus, in areas where the annual
evaporation rate equals or exceeds the rate of annual moisture addition
68
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Table XIX. Initial capital costs and annual operation and
maintenance costs
Unit Cost, in dollars
Septic tank (1000 gal)
Equipment and installation cost 350-450
Maintenance cost 10/yr
Aerobic treatment unit
Equipment and installation cost 1300-2000
Maintenance cost 35/yr
Operation cost, 4 kwhr/day @ 4^/kwhr 60/yr
Wet well pumping chamber
Equipment and installation cost 250-350
Operation cost,^ 1/4 kwhr/day @ 40/kwhr 4/yr
Sand filter
Equipment and installation cost 10 - 15 /sq ft
Maintenance cost 1 /sq ft/yr
Chlorination and settling chamber
Equipment and installation cost 700-1000
Operation cost (chemical) 40/yr
Ultraviolet irradiation unit
Equipment and installation cost 1100-1500
Operation costj^ 1-1/2 kwhr/day @ 4^/kwhr 20/yr
Maintenance cost, cleaning and lamp replacement *
Does not include pump replacement.
~Undetermined.
from rainfall or wastewater application. ET systems can provide a
simple means of liquid disposal without danger of surface or ground-
water contamination. ET systems can also be designed to supplement
soil absorption in slowly permeable soils.
If evaporation is to occur continuously, three conditions must be
69
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met (100). First, there must be a continuous supply of heat to meet the
latent heat requirement (approximately 590 cal/gm of water evaporated
at 15°C). Second, the vapor pressure in the atmosphere over the
evaporative surface must remain lower than the vapor pressure at that
surface to create a vapor pressure gradient , between the surface and the
atmosphere. This gradient is necessary to remove the vapor either by
diffusion, convection or both. These two conditions of energy supply
and vapor removal are influenced by meteorological factors such as air
temperature, humidity, wind velocity radiation and vegetative cover.
Energy can also be added from heat in the water itself or from biolo-
gical activity.
The third condition is that there be a continuous supply of water
to the evaporative surface. This depends on the matric potential of the
soil and its hydraulic conductivity. The soil material must be fine
textured enough to draw the water up from the saturated zone to the
surface by capillary action but not so fine as to restrict the rate of
flow to the evaporative surface.
When evaporation exceeds water input, therefore, evapotranspira-
tion can be used to dispose of wastewater. Evapotranspiration systems
have been designed to evaporate the entire wastewater volume or to
supplement absorption in slowly permeable soils.
A typical ET bed system consists of a 1-1/2 to 3 foot depth of
selected sand over an impermeable plastic liner (101). A perforated
plastic piping system with rock cover is often used to distribute septic
tank effluent in the bed. The bed layout may be in a square pattern on
relatively flat land or in a series of trenches for sloping topography.
A sketch of the cross-section of a typical bed is shown in Figure 24.
The surface area of the bed must be large enough such that sufficient
evapotranspiration occurs to prevent the water level in the bed from
rising to the surface. This requires that the annual evaporatiai rate
must be significantly higher than the annual rainfall.
In order to provide for a maximum evaporation rate, the water in
the bed must be raised to the soil surface as rapidly as it is evaporated.
This is accomplished by using a uniform sand in the approximate size
range of D50 = 0.10 mm. A sand of this size is capable of raising
water a distance of about three feet by capillary action. In this way the
surface of the bed is kept moist even though the standing water level
within the bed may fluctuate.
Evapotranspiration also is influenced by the vegetation on the
disposal field. Trees and bushes with a large silhouette catch more
70
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4 INCH
PERFORATED
PIPE
— ¦ ~t T1 — '
v^T- 'JV777-:7*\ T:-: .T- .T •. t.m
'xuz/uvxiouyc^cr
/ IMPERMEABLE
PLASTIC LINER
WASHED SAND
Figure 24. Typical evapotranspiration bed (101)
advected heat, similar to a clothesline. On the other hand, when vege-
tation is dormant, ET is much reduced. Snow cover reflects solar
radiation which reduces ET. In addition, temperatures below freezing
require more heat to change frozen water to vapor.
Thus, care must be used in selecting a site suitable for evapotrans-
piration. A procedure has been outlined to estimate the maximum ET
that can be expected from disposal fields for areas in the United States
east of the Mississippi River (102).
A study aimed at evaluating the design parameters for non-discharging
ET systems is being conducted by the Department of Civil, Environmental
and Architectural Engineering at the University of Colorado at Boulder
(101). The study involves the use of twenty-eight outdoor lysimeter units,
two feet in diameter, and twenty-eight inches deep. Several full-scale
ET systems in use at private homes are also being monitored for
purposes of data correlation.
The design of an ET bed is based on the annual weather cycle for
the location involved. Evaporation rates are highest during the summer
months, but the study shows that winter evaporation rates are extremely
71
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important to the application of the system. Summer evaporation has been
found to be approximately forty percent of the pan value, while winter
values are about seventy percent. The average* design evaporation value
can be established from the annual pattern as shown in Figure 25. This
rate can be matched with the total expected inflow based on household
wastewater generation rate and rainfall.
A mass diagram approach is used to establish the storage require-
ments of the bed (101). Vegetative cover can increase the ET rate
during the summer growing season but if this increased rate is to be
utilized, additional storage within the bed must be provided for the
winter season. Lawn grass has been found to increase evaporation
rates slightly during June, July and August, but winter evaporation rates
are reduced with respect to bare soil.
t- *
H
% H
w UI
Ui 2
* 5
3° p
20
10
0
- FULL -
"jAN FEB MAR APR MAY JUNE JULY AUG SEPT
if)
U
5
ft
*§
5®
¦a u-
o o
_i
z -
< >-
o °
© (j
«E
ae
O
a.
<
>
ui
0 20
0 IS
0.10
0.05
0.00
PAN EVAPORATION-
' LYSIMETCR EVAPORATION^*
• TOTAL LOADING (SEWAGE ~ RAINFALL
SEWAGE LOAOING (0 04 GPD/ft*)
_l 1 1 L_
J I I 1 1
JAN FEB. MAR. APR MAY JUNE JULY AUG SEPT
MONTH-
Figure 25. Evapotranspiration bed water balance characteristics
Alfalfa can produce an evaporation rate of 0.6 gpd/ft2 at the peak
of the growing season. Design year found sewage ET rates have been
72
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found to be in the range of 0.04 gallons per day per square foot of bed
in the Boulder, Colorado, area (101). This results in a bed area of
approximately 5000 square feet for an individual home.
Evapotranspiration can theoretically remove significant volumes of
effluent from subsurface disposal systems in late spring, summer, and
early fall, particularly if high-silhouette, good transpiring bushes and
trees are present. However, the practical application of non-discharging
ET bed systems is limited to areas of the country where pan evaporation
exceeds rainfall by at least twenty-four inches per year and where winter
monthly evaporation is in excess of monthly precipitation by a value of
two inches for each and every month. Also, extreme freezing conditions
on deep snow cover should not exist where the systems are used. The
decrease of ET in winter at middle- and high-latitudes greatly limits
ET for winter disposal; under freezing conditions ET would be totally
inadequate. Thus, in high latitude, cool-winter locations evapotrans-
piration cannot be relied upon (102).
Locations for possible application of evapotranspiration disposal
systems exist in semi-arid regions of the U.S., including parts of the
Southwestern states of Texas, Oklahoma, Colorado, New Mexico, Utah,
Arizona, California and Nevada. Even in these areas, household water
conservaticn should always be considered as part of the system (101).
The cost of an ET bed system is relatively high. In-place costs
including excavation, suitable sand fill, liner and piping are in the range
of 70 to 90£ per square foot of bed surface (101). When the costs of
the septic and piping are included, the total system costs range from
$3000 to $5000 per house (or about $1.00/sq ft). Studies are presently
underway at the University of Colorado to develop a mechanical waste-
water evaporation system that will have a greater range of application
throughout the country (101). The increased range of use results from
the concept of minimizing the precipitation catchment surface as it
relates to the evaporation surface area. If the ratio of evaporation
surface area to precipitation catchment area (E/PC) is high enough,
precipitation becomes a minor factor in evaluating the utility of these
systems. Thus, in locations where evaporation potential is high,
but rainfall precludes the use of soil based ET systems,mechanical
devices might be applied. The ultimate value of this approach will be
dependent on the cost of commercially manufactured units.
The type of unit being evaluated in the prototype studies at the Univer-
sity of Colorado is a multiple, concentric disk system rotating on a common
shaft. Preliminary studies are underway to determine optimum rotation
speed, disk size and submergence, and disk material and surface char-
acteristics. This work is due to be completed in the fall of 1977 (101).
73
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CAUSES OF ON-SITE DISPOSAL SYSTEM FAILURE
Failure of on-site disposal systems do occur, while the symptoms
are usually easy to recognize, the causes are not. A septic tank
system passes through several stages of development before it is put
into service. The site has been evaluated, a design made, approval
granted by the regulatory agency and the system constructed. Following
construction, the system becomes the responsibility of the owner who
must operate and maintain it. At each stage, errors can be inadver-
tently made which shorten the life of the system. These can be avoided.
In an investigation of eight systems in silty soils, six major
problems were identified which may occur from the time of the initial
site evaluation to construction (103). Where one or several of these
problems occurred, the result was failure within three years. The
six problems were as follows.
1. Poor site evaluation by the installer.
2. Failure of the regulatory agency to reject applications with
poor siting or design.
3. Design specifications not followed during construction.
4. Poor construction procedures followed by the installer.
5. Mistakes overlooked during the site inspection by the regu-
latory agency.
6. System overloading due to increased wastewater volume
following installation.
In those systems investigated which were designed and installed
through close cooperation with the regulatory agency, soil scientist
and installer, early failure was avoided.
In addition to being properly designed and constructed, regular
maintenance must be performed if the on-site system is to function
satisfactorily. Settled solids accumulate (in the septic tank and must
be periodically removed or they are washed out into the soil resulting
in clogging of the absorption field and subsequent failure. It is recom-
mended that the septic tank be pumped at least every three years. If
74
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a garbage grinder is used, which is not recommended, the tank should
be pumped more often.
Reliable methods of site evaluation, system design, installation and
management, together with an effective educational and regulatory program,
best prevent premature failure of on-site systems.
75
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REFERENCES
1. U. S. 1970 Census, unpublished data collected by Rezek, Henry,
Meisenheimer and Gender, Inc. Libertyville, Illinois (1975).
2. Beale, C. L. and G. V. Fuguitt, "Population Trends of Non-
Metropolitan Cities and Villages in Subregions of the United States",
CDE Working Paper 75-30, Center for Demography and Ecology,
University of Wisconsin, Madiscn, Wisconsin (September, 1975).
3. U. S. Department of Agriculture, "Status of Water and Sewage
Facilities in Communities without Public Systems", Agricultural
Economics Report No. 143, Washington, D.C. (October, 1968).
4. Simth, R. and R. G. Eilers, "Costto the Consumer for Collection
and Treatment of Wastewater", Water Pollution Control Research Series,
17090-07/70, U. S. Environmental Protection Agency, Washington, D.C.
(July, 1970).
5. Sloggett, G. R. and D. D. Badger, "Economics of Constructing
and Operating Sewer Systems in Small Oklahoma Communities",
Bulletin B-718, Agricultural Experiment Station, Oklahoma State Univer-
sity (April, 1975).
6. Sloggett, G. R. and D. D. Badger, "Economics of Constructing
and Operating Sewer Systems in Small Oklahoma Communities",
Bulletin B-718, Agricultural Experiment Station, Oklahoma State
University (April, 1975).
7. Smith, R. andR. G. Eilers, "Cost to the Consumer for Collection
and Treatment of Wastewater", Water Pollution Control Research Series,
17090-07/70, U. S. Environmental Protection Agency, Washington, D.C.
(July, 1970).
8. Northwestern Wisconsin Regional Planning and Development
Commission, "Model Facilities Plan for Three Unsewered Communities
in Northwestern Wisconsin", Proposal for study submitted to the
Wisconsin Department of Natural Resources. Spooner, Wisconsin (1974).
9. Otis, R. J. and D. E. Stewart, "Alternative Wastewater Facilities
for Small Unsewered Communities in Rural America", Annual Report
to the Upper Great Lakes Commission, Small Scale Waste Management
Project, Room 1, Agriculture Hall, University of Wisconsin, Madison,
Wisconsin (July, 1976).
76
-------
10. Commission on Rural Water, Water and Wastewater Problems in
Rural America, Washington, D. C. (1973).
11. Bouma, J., W. A. Ziebell, W. G. Walker, P. Olcott, E. McCoy
and F. D. Hole, "Soil Absorption Septic Tank Effluent--A Field Study
of Some Major Soils of Wisconsin", University Extension and Geological
and Natural History Survey, Bulletin 20, University of Wisconsin-Madison
(1972).
12. Brady, N. C. The Nature and Properties of Soils, MacMillan
Publishing Co., Inc., New York (1974).
13. McGauhey, P. E. andR. B. Krone, "Soil Mantle as a Wastewater
Treatment System", SERL Report No. 67-11, Sanitary Engineering
Research Laboratory, University of California-Berkeley (December, 1967).
14. Bendixen, T. W., M. Berk, J. P. Sheehy and S. R. Weibel, Studies
on Household Sewage Disposal Systems, Part II., USPHS Environment
Health Service, Cincinnati, Ohio (1950).
15. Weibel, S. R., T. W. Bendixen and J. B. Coulter, Studies on
Household Sewage Disposal Systems, Part III, USPHS, Robert A. Taft,
Sanitary Engineering Center, Cincinnati, Ohio (1954).
16. McGauhey, P. A. and J. T. Winneberger, "Studies of the Failure
of Septic Tank Percolation Systems", JWPCF, 36, 5 (May, 1964) pp.
593-606.
17. Mitchell, R. and Z. Nero, "Effect of Bacterial Polysaccharide
Accumulation on Infiltration of Water Through Sand", Applied Micro-
biology, 12, (1964), p. 219.
18. Jones, ]. H. and G. S. Taylor, "Septic Tank Effluent Percolation
Through Sands Under Laboratory Conditions", Soil Science, 99, (1965),
pp. 301-309.
19. Thomas, R. E., W. A. Schwartz and T. W. Bendixen, "Soil
Chemical Changes and Infiltration Rate Reduction Under Sewage
Spreading", Soil Science Society of America Proceedings, 30, (1966),
pp. 641-646.
20. Thomas, R. E., W. A. Schwartz and T. W. Bendixen, "Pore Gas
Composition Under Sewage Spreadings ", Soil Science Society of
America • Proceedings, 32, (1968), p. 419T
21. Laak, R., "Influence of Domestic Wastewater Pretreatment on Soil
Clogging", JWPCF, 42, 8 Part I, (August, 1970), p.. 1495.
77
-------
22. de Vires, J., "Soil Filtration of Wastewater Effluent and the
Mechanisms of Pore Clogging", JWPCF, 44, 4 (April, 1972), p. 565.
23. Rice, R. C. "Soil Clogging During Infiltration of Secondary
Effluent", JWPCF, 46, 4 (April, 1974), pp. 708-716.
24. Daniel, T. C. and J. Bouma, "Column Studies of Soil Clogging
in a Slowly Permeable Soil as a Function of Effluent Quality", Journal
of Environmental Quality, 3, 4 (1974), pp. 321-326.
25. Kropf, F. W., K. A. Healy and R. Laak, "Soil Clogging in Subsur-
face Absorption Systems for Liquid Domestic Wastes", presented at
Seventh Conference of the International Association of Water Pollution
Research, Paris, France (September, 1974).
26. Magdoff, F. R. and J. Bouma, "The Development of Soil Clogging
in Sands Leached with Septic Effluent", Home Sewage Disposal,
Proceedings of the National Home Sewage Disposal Symposium, ASAE
Publication Proc-175 (December, 1974).
27. Harkin, J., "Causes and Remedy of Failures of Septic Tank Seepage
Systems", presented at Second National Conference on Individual On-Site
Wastewater Systems, National Sanitation Foundation, Ann Arbor,
Michigan (November 5-7, 1975).
28. Bouma, J., "Unsaturated Flow During Soil Treatment of Septic
Tank Effluent", Journal of Environmental Engineering, ASCE, 101,
EE6 (December, 1975), pp. 967-983.
29. Anderson, J. L. and J. Bouma, "Water Movement Through Pedal
Soils: I. Saturated Flow. II. Unsaturated Flow", Soil Science Society of
America. Journal (in press).
30. Gerba, C. P., C. Wallis, and J. L. Melnick, "Fate of Wastewater
Bacteria and Viruses in Soil", Journal of the Irrigation and Drainage
Division, ASCE, 101, IR3, (Sept., 1975), pp. 157-174.
31. Ziebell, W. A., "Removal of Fecal Bacteria from Wastewater of
Individual Homes During Treatment by Conventional and Experimental
Methods", M.S. Thesis, Department of Civil and Environmental
Engineering, University of Wisconsin-Madison (1975).
32. McCoy, E. and W. A. Ziebell, "The Effects of Effluents on
Groundwater: Bacteriological Aspects", Individual On-Site Wastewater
Systems, Proceedings of the Second National Conference, National
Sanitation Foundation Ann Arbor, Michigan (November, 1975).
78
-------
33. Ziebell, W. A., D. H. Nero, J. F. Deininger and E. McCoy, "Use
of Bacteria in Assessing Waste Treatment and Soil Disposal Systems",
Home Sewage Disposal, Proceedings from the National Home Sewage
Disposal Symposium, ASAE Pub. Proc-175 (December, 1974).
34. Green, K. M., "Sand Filtration for Virus Purification of Septic
Tank Effluent", Ph.D. Thesis, Department of Bacteriology, University
of Wisconsin-Madison (1976).
35. Green, K. M. and D. 0. Oliver, "Removal of Virus from Septic
Tank Effluent", Home Sewage Disposal, Proceedings from the National
Home Sewage Disposal Symposium, ASAE Pub. Proc-175 (December,
1974).
36. Lance, J. C., C. P. Gerba and J. L. Melnick, "Virus Movement
in Soil Columns with Secondary Sewage Effluent", Applied and Environ-
mental Microbiology, 32, 4, (October, 1976), pp. 520-526.
37. Groundwater Contamination, Proceedings of 1961 Symposium, Tech.
Report W61 -5, Robert A. Taft Sanitary Engineering Center (April, 1961).
38. Gruener, N. and H. I. Shuval, "Health Aspects of Nitrates in
Drinking Water", Developments in Water Quality Research, H. I. Shuval
ed., Proceedings of the Jerusalem International Conference on Water
Quality and Pollution Research (June, 1969).
39. Preul, H. C., "Travel of Nitrogen Compounds in Soils", Ph.D.
Thesis, Department of Civil Engineering, University of Minnesota,
Minneapolis (1964).
40. Dudley, J„ G. and D. A. Stephenson, "Nutrient Enrichment of
Ground Water from Septic Tank Disposal Systems", Inland Lake Renewal
and Shoreland Management Demonstration Project Report, Environmental
Resources Unit, University of Wisconsin-Extension, Madison, Wisconsin
(November, 1973).
41. Walker, W. G., J. Bouma, D. R. Keeney and P. G. Olcott,
"Nitrogen Transformation During Subsurface Disposal of Septic Tank
Effluent in Sands: Part IL Ground Water Quality", Journal of Environ-
mental Quality, 2, 4 (October-December, 1973), pp. 521-525.
42. Walker, W. G., J. Bouma, D. R. Keeney and F. R. Magdoff,
"Nitrogen Transformations During Subsurface Disposal of Septic Tank
Effluent in Sands: Part I. Soil Transformation", Journal of Environ-
mental Quality, 2, 4 (October-December, 1973), pp. 475-480.
79
-------
43. Beek, J. and F. A. M. de Haan, "Phosphate Removal by Soil in
Relation to Waste Disposal", Proceedings of the International Conference
on Land for Waste Management, Ottawa, Canada (October, 1973).
44. Sikora, L. J. and R. B. Corey, "Fate of Nitrogen and Phosphorus
in Soils Under Septic Tank Waste Disposal Fields", Transactions, ASAE,
19, 5 (1976), pp. 866-875.
45. U. S. Public Health Service, Manual of Septic Tank Practice,
U. S. Government Printing Office, Washington, D. C. Pub, No. 526
(1967).
46. McGauhey, P. H. and J. H. Winneberger, "Summary Report on
Causes and Prevention of Failure of Septic Tank Percolation Systems",
SERL Report No. 63-5 Sanitary Engineering Research Laboratory,
University of California, Berkeley (May, 1963).
47. Bouma, J., "Evaluation of the Field Percolation Test and an Alter-
native Procedure to Test Soil Potential for Disposal of Septic Tank
Effluent", Soil Science Society of America Proceedings, 35, 6 (November
-December, 1971), pp. 871-875.
48. Healy, K. A. and R. Laak, "Factors Affecting the Percolation Test",
Journal Water Pollution Control Federation, 45, 7 (July, 1973), pp. 1508-
49. Bouma, J., D. I. Hillel, F. D. Hole, and C. R. Amerman, "Field
Measurement of Unsaturated Hydraulic Conductivity by Infiltration
Through Artificial Crusts", Soil Science Society of America Proceedings,
35, 2 (March-April, 1971), pp. 362-364.
50. Bouma, J. and J. L. Denning, "Field Measurement of Unsaturated
Hydraulic Conductivity by Infiltration Through Gypsum Crusts", Soil
Science Society of America Proceedings, 36, 5 (September-October,
1972), pp. 846-847.
51. Bouma, J., F. G. Baker and P. L. M. Veneman, "Measurement
of Water Movement in Soil Pedons Above the Water Table ", University
of Wisconsin-Extension, Geological and Natural History Survey Infor-
mation Circular No. 27, Madison, Wisconsin (1974).
52. Baker, F. G. and J. Bouma, "Measurement of Soil Hydraulic
Conductivity and Site Selection for Liquid Waste Disposal", presented
at Second National Conference on Individual On-Site Wastewater Systems,
NSF, Ann Arbor, Michigan (November 5-7, 1975).
80
-------
53. Baker, F. G., "Variability of Hydraulic Conductivity Characteristics
in Selected Structured and Nonstructured Soils", Water Resources Research
(in press).
54. Machmeier, R. E., "Design Criteria for Soil Treatment Systems"
presented at the Winter Meeting, ASAE, Chicago, Illinois (December
15-18, 1975).
55. Soil Survey Manual, U.S. Department of Agriculture Handbook
No. 18, Washington, D. C. (1951).
56. Vespraskas, M. J. and J. Bouma, "Model Experiments on Mottle
Formation Simulating Field Conditions", Geoderma, 15, (1976), pp. 217-
230.
57. Vespraskas, M. J., F. G. Baker and J. Bouma, "Soil Mottling
and Drainage in a Mollic Hapludalf as Related to Suitability for Septic
Tank Construction", Soil Science Society of America Proceedings, 38,
3 (May-June, 1974), pp. 497-501.
58. Witt, M.D., "Water Use in Rural Homes", Small Scale Waste
Management Project, Room 1, Agriculture Hall, University of Wisconsin,
Madison (1974).
59. Witt, M.D., R. Siegrist and W. C. Boyle, "Rural Household
Waste Characterization", Home Sewage Disposal, Proceedings of the
National Home Sewage Disposal Symposium, ASAE pub Proc-175
(December, 1974).
60. Siegrist, R., M. Witt and W. C. Boyle, "The Characteristics
of Rural Household Wastewater", Journal of the Environmental Engineering
Division, ASCE, 102, EE 3, (June, 1976), pp. 533-548.
61. Converse, J. C., J. L. Anderson, W. A. Ziebell and J. Bouma,
' Pressure Distribution to Imrpove Soil Absorption Systems", Home
Sewage Disposal, Proceedings of the National Home Sewage Disposal
Symposium ASAE pub Proc-175 (December, 1974).
62. Bouma, J., J. C. Converse, J. Carlson and F. G. Baker, "Soil
Absorption of Septic Tank Effluent in Moderately Permeable Fine Silty
Soils", Transactions of ASAE, 18, (1975), pp. 1094-1100.
63. Converse, J. C., "Distribution of Domestic Waste Effluent in Soil
Absorption Beds", Transactions ASAE, 17, 2 (1974), pp. 299-304.
81
-------
64. Otis, R. J. and J. Bouma, "Notes on Soil Absorption Field Construc-
tion for Septic Tank Systems", Small Scale Waste Management Project,
Room 1, Agriculture Hall, University of Wisconsin, Madison (June, 1973).
65. Bouma, J., "Using Soil for Disposal and Treatment of Septic Tank
Effluent Following the Current Health Code", Small Scale Waste Manage-
ment Project, Room 1, Agriculture Hall, University of Wisconsin, Madison
(1974).
66. Cohen, S. and H. Wallman, "Demonstration of Waste Flow Reduction
from Households", Environmental Protection Technology Series, EPA-
670/2-74-071, U. S. Environmental Protection Agency, Cincinnati, Ohio
(September, 1974).
67. Olsson, Eskil, etal., "Household Wastewater", The National Swedish
Institute for Building Research, Box 26 163 - 102 52 Stockholm 27, Sewden,
1968.
68. Ligman, K., N. Hutzler, and W. C. Boyle, "Household Wastewater
Characterization", Journal of the Environmental Engineering Division,
ASCE Proc. Paper 10372, 100, EE1, (February, 1974), pp. zvl-zla.
69. Laak, R., "Relative Pollution Strengths of Undiluted Waste Materials
Discharged in Households and the Dilution Waters Used for Each", Manual
of Grey Water Treatment Practice--Part II, Monongram Industries,Ioc, (1975).
70. Bennett, E. R. And E. K. Linstedt, "Individual Home Wastewater
Characterization and Treatment", Completion Report Series No. 66,
Environmental Resource Center, Colorado State University, Fort Collins,
Colorado Ouly, 1975).
71. Otis, R. J., W. C. Boyle and D. K. Sauer, "The Performance of
Household Wastewater Treatment Units Under Field Conditions", Home
Sewage Disposal, Proceedings of the National Home Sewage Disposal
Symposium, ASAE Proc-175 (December, 1974).
72. Otis, R. J. and W. C. Boyle. "Performance of Single Household
Treatment Units", Journal of Environmental Engineering Division ASCE,
Proc. paper 11895,"T02, EE 1, (February, 1976), pp. 175-189.
73. Hutzler, N. J., R. J. Otis and W. C. Boyle, "Field and Laboratory
Studies of Household Wastewater Treatment Alternatives", Proceedings
Ohio Home Sewage Disposal Conference, Ohio State University, Columbus,
Ohio (1974).
82
-------
74. Laak, R., "Pollutant Loads from Plumbing Fixtures and Pretreat-
ment to Control Soil Clogging", On-Site Waste Management Volume IV,
Hancor, Inc. (June, 1974).
75. Winneberger, J. H., L. Francis, S. A. Klein and P. H. McGauhey,
"Biological Aspects of Failure of Septic Tank Percolation Systems--Final
Report", Sanitary Engineering Research Laboratory, University of
California, Berkeley (August, 1960).
76. Bouma, J., "Inovative On-Site Soil Disposal and Treatment Systems
for Septic Tank Effluent", Home Sewage Disposal Proceedings of the
National Home Sewage Disposal Symposium ASAE Proc-175 (December,
1974).
77. Converse, J. C., R. J. Otis, J. Bouma, W. G. Walker, J. L.
Anderson and D. E. Stewart, "Design and Construction Procedures for
Mounds in Slowly Permeable Soils With Seasonally High Water Tables",
Small Scale Waste Management Project, Room 1, Agriculture Hall,
University of Wisconsin, Madison (April, 1975).
78. Bouma, J., J. C. Converse, R. J. Otis, W. G. Walker and W. A.
Ziebell, "A Mound System for On-Site Disposal of Septic Tank Effluent
in Slowly Permeable Soils with Seasonally Perched Water Tables",
Journal of Environmental Quality, 4, 3 (July-September, 1975), pp. 382-
3W.
79. Bouma, J., J. C. Converse and F. R. Magdoff, "A Mound System
for Disposal of Septic Tank Effluent in Shallow Soils over Creviced
Bedrock", Proceedings of the International Conference on Land Manage-
ment for Waste Management'1, uttawa, uanaaa (UctoDer, ly/6).
80. Magdoff, F. R., D. R. Keeney, J. Bouma and W. A. Ziebell,
"Columns Representing Mound-Type Disposal Systems for Septic Tank
Effluent. II. Nutrient Transformations and Bacterial Populations",
Journal of Environmental Quality, 3, 3 (July-September, 1974), pp.
228-234. ~
81. Converse, J. C., R. J. Otis and J. Bouma. "Design and Construc-
tion Procedures for Fill Systems in Permeable Soils With Shallow
Creviced or Porous Bedrock", Small Scale Waste Management Project,
Room 1, Agriculture Hall, University of Wisconsin, Madison (April,
1975).
82. Converse, J. C., R. J. Otis and J. Bouma, "Design and Construc-
tion Procedures for Fill Systems in Permeable Soils With High Water
Tables", Small Scale Waste Management Project, Room 1, Agriculture
Hall, University of Wisconsin, Madison (April, 1975).
83
-------
83. Anderson, J. L. and J. Bouma, "Sizes of Subsurface Trenches and
Associated Drains Determined by the Dupuit-Forchheimer Approximation",
Small Scale Waste Management Project, Room 1, Agriculture Hall,
University of Wisconsin, Madison (April, 1975).
84. Decoster, E, L.# "The Hydrodynamics of an Artificial Ground-
water Mound Developed as Part of a Subsurface Waste Disposal System",
M.S. Thesis, Department of Civil and Environmental Engineering,
University of Wisconsin-Madison (1976).
85. Water Quality Criteria, Federal Water Pollution Control Adminis-
tration, U.S. Department of the Interior, Washington, D.C. (1968).
86. Nichols, C. S., "Sewage Disposal for Village and Rural Homes",
Bulletin No. 41, Iowa State Engineering Experiment Station, Ames,
Iowa (August, 1916).
87. Frank, L. andC. P. Rhymus, "Studies of Methods for the Treat-
ment and Disposal of Sewage--The Treatment of Sewage from Single
Homes and Small Communities", Public Health Bulletin, No. 101,
USPHS, Washington, D.C., 1920.
88. Small Scale Waste Management Project--Final Report to U.S.EPA,
University of Wisconsin-Madison (in preparation).
89. Perry, R. R., L. E. Rigby and D. E. Bloodgood, "Summary of
Studies on Aerobic Sewage Treatment for Individual Homes", Unpublished
Report, Purdue University, Lafayette, Indiana (1954).
90. Ohio State University, "A 23-Month Study of Individual Household
Aerobic Sewage Treatment Systems", Special Report 220, Engineering
Experiment Station, Columbus, Ohio (July, 1961).
91. National Sanitation Foundation, "Standard No. 40: Individual Aerobic
Wastewater Treatment Plants", Ann Arbor, Michigan (November, 1970).
92. Bennett, E. R. and K. O. Linstedt, "Individual Home Wastewater
Characterization and Treatment", Completion Report Series No. 66,
Colorado State University, Fort Collins, Colorado (1975).
93. Voell, A. T. and R. A. Vance, "Home Aerobic Wastewater Treat-
ment Systems--Experience in a Rural County", Ohio Home Sewage
Disposal Conference, Ohio State University, Columbus, Ohio (January,
1974).
84
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94. Glasser, M. B., "Garrett County Home Aeration Wastewater Treat-
ment Project", Bureau of Sanitary Engineering, Maryland State Depart-
ment of Health and Mental Hygiene, Baltimore, Md. (1974).
95. Hutzler, N. J., "Evaluation of On-Site Wastewater Treatment
Processes Receiving Controlled Simulated Wastewater", M.S.Independent
Study Report, Department of Civil and Environmental Engineering,
University of Wisconsin, Madisai, Wisconsin (1974).
96. Hines, M., "The Recirculating Sand Filter; A New Answer for an
Old Problem", Proceedings Illinois Private Sewage Disposal Symposium,
Champaign, Illinois (September 29-October 1, 1975).
97. Sauer, D. K., "Intermittent Sand Filtration of Septic Tank and
Aerobic Unit Effluents Under Field Conditions", M.S. Thesis, Department
of Civil and Environmental Engineering, University of Wisconsin, Madison
(1975).
98. Sauer, D. K., W. C. Boyle and R. J. Otis, "Intermittent Sand
Filtration of Household Wastewater Under Field Conditions", Journal of
the Environmental Engineering Division, ASCE, 102, EE 4, Proc. paper
12295, (August, 1976), pp. 789-803. —
99. Sauer, D. K., "Treatment Systems for Surface Discharge of On-
Site Wastewater", Proceedings of the Third National Conference on
Individual On-Site Wastewater Systems, National Sanitation Foundation,
Ann Arbor, Michigan (November 16-18, 1976).
100. Hillel, D., Soil and Water Physical Principals and Processes,
Academic Press, New York (1971).
101. Bennett, E. R., personal communication, Department of Civil and
Environmental Engineering, University of Colorado, Boulder (1977).
102. Tanner, C. B. and J. Bouma, "Influence of Climate on Subsurface
Disposal of Sewage Effluent", Proceedings of the Second National
Conference on Individual Onsite Wastewater Systems, National Sanitation
Foundation, Ann Arbor, Michigan (November 5-7, 1975).
103. Carlson, J. and J. Bouma, "On-Site Investigation of Some Small
Scale Waste Disposal Systems in Dane County", Small Scale Waste
Management Project, Room 1, Agriculture Hall, University of Wisconsin,
Madison (1974).
104. Baker, F. G., 'Reduced Flow in Dosed Soil Columns", Journal
Environmental Engineering Division, ASCE (in press).
85
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105. Siegrist, R., "Segregation and Separate Treatment of Black and
Grey Household Wastewaters to Facilitate On-Site Surface Disposal",
Small Scale Waste Management Project, Room 1, Agriculture Hall,
University of Wisconsin-Madison, Wisconsin (November, 1976).
86
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