625477011A
Alternatives for
Small Wastewater
Treatment Systems
On-Site Disposal/
Septage Treatment and Disposal
EPA Technology Transfer Seminar Publication
This document has not been
submitted to NTIS, therefore it
should be retained.
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EPA-625/4-77-011
ALTERNATIVES FOR SMALL
WASTEWATER TREATMENT SYSTEMS
On-Site Disposal/Septage Treatment and Disposal
ENVIRONMENTAL PROTECTION AGENCY* Technology Transfer
October 1977
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the U.S.
Environmental Protection Agency Technology Transfer Program and
presented at Technology Transfer design seminars throughout the
United States.
Part I was prepared by Richard J. Otis, Sanitary Engineer, Depart-
ment of Civil and Environmental Engineering; William C. Boyle, Pro-
fessor, Department of Civil and Environmental Engineering; James C.
Converse, Associate Professor, Department of Agricultural Engineering;
and E. Jerry Tyler, Assistant Professor, Department of Soil Science,
University of Wisconsin, Madison, as a report of the Small-Scale Waste
Management Program.
Part II was prepared by Ivan A. Cooper, Senior Project Manager,
and Joseph W. Rezek, President, Rezek, Henry, Meisenheimer, & Gende,
Inc., Libertyville, 111.
NOTICE
The mention of trade names or commercial products in this publication is for
illustration purposes, and does not constitute endorsement or recommendation for
use by the U.S. Environmental Protection Agency.
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CONTENTS
Part I. On-Site Disposal of Small Wastewater Flows 1
Introduction 1
Use of Soil for Treatment and Disposal of Wastewater 7
Estimating the Infiltrative and Percolative Capacity of Soil 21
On-Site Treatment and Disposal System Alternatives 27
Alternative Selection 50
Wisconsin Small Scale Waste Management Project 51
References 54
Part II. Septage Treatment and Disposal 61
General 61
Land Disposal 66
Separate Treatment Facilities—Septage Only 75
Septage-Sewage Treatment Facilities 77
Costs 86
Summary and Conclusions 87
References 87
in
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Parti
ON-SITE DISPOSAL OF SMALL WASTEWATER FLOWS
INTRODUCTION
In 1970, approximately 19.5 million households, or nearly 30 percent of all housing units in
the United States, disposed of their wastewaters through some form of private sewerage facilities.1
This number is growing at an increasing rate, owing to population movement to rural areas where
community sewage treatment facilities are not usually available. Retired persons are moving back
to rural areas, as are young families who follow industries to the fringes of metropolitan centers.2
Most of these rural households use septic tank systems. Because of poor design, construction, or
maintenance, however, a large number of these systems fail to provide adequate sewage treatment
and disposal.
Many households, while located in rural areas, are situated in small communities or subdivi-
sions ranging in size from a few households to 100 or more. In such instances, failing septic tank
systems allow raw or poorly treated sewage to reach the ground surface, the surface body of water,
or even the ground water, creating a severe public health hazard and nuisance because of the prox-
imity of homes. Public wastewater facilities are often the only solution to this problem.
Assessing wastewater facility needs of small rural communities is difficult because of a lack of
information. The last known published status report is a survey conducted by the U.S. Department
of Agriculture in 1962.3 At that time, 92 percent of the communities with populations of less than
1,000 had no public facilities, compared with 19 percent of communities with 1,000 or more. The
results of the survey are presented in table 1-1.
Since 1962, several government programs have been initiated that attempt to abate water
pollution by providing grants-in-aid of construction for community sewerage facilities-^namely,
programs under the Federal Water Pollution Control Act (Public Law 92-500) and various State
Table 1-1 .—Number of communities with and without public sewerage facilities in 19623
Population
26-999
1,000-2,499
2,500-5,500
More than 5,500
Total
Public sewerage
facilities
Commu-
nities
with
3,803
3,079
2,027
2,926
11,835
Commu-
nities
without
42,837
1,391
349
142
44,719
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programs. Consequently, the data presented in table 1-1 need to be updated, but they do indicate
that few small communities have public wastewater facilities.
The need for improved facilities exists in many of these communities, which often were estab-
lished long before sound design and installation criteria for septic tank systems were enforced.
Some homeowners merely installed a pipe to discharge their wastewater away from the house into a
ditch or stream. More conscientious homeowners installed septic tank systems; but without good
design 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 that conveys all the wastewaters to a community treatment plant. This
central system has been preferred by government authorities, engineers, and the public for several
reasons. First, the system has been tried and proven. Much technical expertise has gone into the
theory, design, and operation of central sewerage, which has led to great confidence in it. Second,
central sewerage is usually more cost-effective than other systems because of economies of scale. It
is less costly to serve many people with one system than to serve each person individually. Third,
central sewerage allows ready application of a central (usually public) management that is respon-
sible for the proper functioning of the system.
For smaller communities and subdivisions, however, such a conventional collection and treat-
ment facility is impractical because of the financial burden it places on residents or the developer.
This is largely owing to the high cost of collecting wastewater from each home or business. Smith
and Eilers4 computed the 1968 national average of total annual costs of municipal wastewater col-
lection and treatment facilities, which showed that 65 percent of the total annual cost is for amor-
tization and operation and maintenance of the collection system. A more recent study of Sloggett
and Badgers5 of 16 small communities in Oklahoma showed a similar distribution. It is clear from
these breakdowns (see table 1-2) that the collection system is the most expensive component of any
facility.
In small communities, homes are typically scattered, which causes sewer costs to be dramati-
cally higher than those in larger communities. Sloggett and Badgers5 demonstrated that the costs
per customer rise as the number and density of customers decline. Construction costs per customer
were compared with the density and number of customers served. (See tables 1-3 and 1-4.) Both
Table 1-2.— Distribution of total annual costs for municipal wastewater collection and treatment facilities
[Percent]
Smith and Eilers^
Sloggett and Badger5
Amortization
Collection
60.3
Treatment
15.3
Collection and
treatment
72.6
Operation and
maintenance
Collection
4.7
14.2
Treatment
8.4
3.2a
Overhead
11.3
10.0
Total
100.0
100.0
"Lagoons.
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Table 1-3.—Cost of construction per customer relative to density of customers for 16 community wastewater
facilities in Oklahoma5
Item
Number of systems
Average cost per customer 1972 dollars
Average number of customers
Customers per mile of sewer
Fewer
than 30
5
1,100
96
30 to 39
5
847
119
40 to 49
1
696
310
More
than 50
5
575
256
Table I-4.— Cost of construction per customer relative to number of customers for 16 community wastewater
facilities in Oklahoma, 19725
Item
Number of systems
Average cost per customer, dollars
Customers per mile of sewer
Number of customers served
Fewer than 100
6
1,000
28.3
100 to 199
4
798
37.8
200 to 299
3
594
49.4
300 to 400
3
434
55.2
factors were shown to have a significant effect, but the density of customers was shown to have the
largest impact on per capita construction costs.
Sloggett and Badgers5 made similar comparisons using total annual costs. They found number
and density of customers to be significant. (See table 1-5.) In 1972, average annual costs per cus-
tomer ranged from $76.90 to $43.36 for communities with populations of less than 100 and of 300
to 400, respectively. In 1968,4 the national average for municipalities, large and small, was $19.80.
Because of the prohibitive costs of extending sewers, outlying sections of a community may
not be served. In 30 percent of the communities with public facilities surveyed in 1962,3 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.
In smaller communities, costs of conventional facilities can become prohibitive, exceeding
$10,000 per household for the capital portion alone and costing even more if more than secondary
treatment is required to meet water quality standards. It is not unusual for the cost of a system to
approach the total equalized value of the community.6
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 give
grants-in-aid of construction for 75 percent of the grant-eligible portions of the wastewater facility.
Such grants would offset the high per capita costs in small communities, but such communities have
difficulty obtaining them in many cases.
Noncentral Wastewater Facilities for Small Communities
A noncentral facility of several treatment and disposal systems, serving isolated individual resi-
dences or clusters of residences, may offer a less costly alternative to the conventional central facility
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in the nonurban setting. As table 1-2 indicates, approximately two-thirds of the total annual cost of
a conventional facility is for the collection system. In a community of scattered homes this propor-
tionate cost could be even higher. If the central treatment plant could be eliminated, long sewer
extensions to widely spaced homes would not be necessary. Instead, individual or jointly used sep-
tic tank systems or other treatment and disposal systems, located where the wastes are generated,
could be used. Such systems could result in substantial savings because of the following advantages.7
• Functioning septic tank-soil absorption systems (ST-SAS) can be used rather than provid-
ing new service. Often, homeowners who are not having disposal problems or who have
recently installed septic tank systems do not wish to support community action on a new
wastewater facility that will cost them more money unnecessarily. Incorporating existing
systems into the public system minimizes such opposition, as well as reduces the total
cost of the public facility.
• Isolated homes and clusters of homes can be served individually instead of by costly
sewer line extensions. This could be equally advantageous to existing communities and
newly platted subdivisions. Where growth was not expected to be great enough to
warrant sewer extensions, individual septic tank systems could be used. In cases where
substantial growth was expected, such as in newly platted subdivisions, the first few
homes built could be served by holding tanks pumped and maintained by the manage-
ment entity. When the number of homes warranted a common disposal system, it could
be built on land reserved for that purpose. This would delay construction until enough
contributors were available to pay for it.
• Less costly treatment facilities usually can be constructed, and subsurface disposal often
can be used that 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 the use of other
simple treatment methods. In addition, by limiting the area served, the maximum future
capacity can be more accurately predicted and a more optimal design can be provided.
• A more cost-effective facility may encourage smaller communities to proceed with con-
struction rather than to wait for Federal construction grants. This would speed abate-
ment 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.
• More rational planning of community growth is possible. Linear development, which is
encouraged by the construction of interceptor sewers used to collect wastes from out-
Table \-5.~Total average annual cost per customer for 16 community wastewater facilities in Oklahoma, 1972
[Dollars]
Number of customers
Fewer than 100 .... ...
100 to 199
200 to 299
300 to 400 .
Total average annual cost
0 percent
construction
grant
76.90
57.55
52.10
43.36
75 percent
construction
grant
33.06
25.39
24.09
20.72
100 percent
construction
grant
18.44
14.63
14.75
13.17
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lying clusters of homes, could be avoided. Growth could be encouraged in the more desir-
able areas by providing public service in those areas only.
• Noncentral facilities are more ecologically sound because they dispose of wastes over
wider areas. Thus, the environment is able to assimilate the waste discharge more readily,
and the need for mechanical treatment and associated energy consumption is reduced.
Management of Noncentral Disposal Systems
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 to properly manage a noncentral facility are similar to those needed with a conventional
community system.
Powers Needed by a Management Entity.7 To effectively administer on-site wastewater dis-
posal systems, an entity must be able:
• To own, operate, manage, and maintain all wastewater systems within its jurisdiction. It
must be empowered to acquire by purchase, gift, grant, lease, or rent both real and per-
sonal property. It must also have the authority to plan, design, construct, inspect, oper-
ate, and maintain all types of on-site systems whether individual septic tanks or a more
complex system serving a group of residences. The entity should have at least these "own-
ership and operation" powers within its boundaries, but it should not be limited to pro-
viding services only within its boundaries. The entity may be given jurisdictional authority
to operate, maintain, and perhaps own such systems outside its boundaries, by State stat-
ute, case law (essentially interpretations of State laws made by the courts), or terms of a
contract.
• To enter into contracts, undertake debt obligations by borrowing and issuing bonds, and
sue and be sued. These powers are more than legal niceties; without them, the entity
could not acquire the property, equipment, supplies, and services necessary to construct
and operate on-site systems.
• To 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;
therefore, nongovernmental management entities must have the authority implied or
directly granted to set and charge user fees to cover administrative costs.
• To plan and control how and when wastewater facilities will be extended to those within
its jurisdiction.
Although not necessary to provide adequate management of a noncentral facility, two addi-
tional powers are desirable:
• To enact rules and regulations on the use of on-site systems and provide for their enforce-
ment through express statutory authorization. To promote good public sanitation, the
entity should be empowered to require the abatement and replacement of malfunctioning
systems according to its plans. This power, however, may be inferred from the statutory
authorization to operate a system.
• To meet eligibility requirements for both loans and grants-in-aid of construction from Fed-
eral and State governments. Although a management entity can function without such
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loans and grants, viability of the noncentral system is strengthened when grant money is
used to offset some or most of the costs to customers. Low-income families especially
can ill-afford to finance the entire cost of their sewerage system. Experience has shown
that low-income rural families cannot pay wastewater bills in excess of $7.00 per month
or a combined water-sewage bill in excess of $14.00 per month.8 Yet charging lower rates
is difficult without public subsidy. The inequity should be especially obvious to most
nonrural residents, who typically pay considerably less.
Acceptable Management Entities. The kinds of entities that could manage a noncentral facility
vary from State to State. State constitutions, statutes, and administrative agency regulations must
be examined to determine which entities are authorized to manage on-site systems. In addition,
case laws must be checked to determine if the courts have construed the constitution, statutes, or
regulations as having given to or removed from an entity the authority to manage such a system.
Those entities that may have the necessary powers include municipalities, counties, townships, spe-
cial districts, private nonprofit corporations, rural electric cooperatives, and private profit-making
businesses.
Although they have disadvantages, the potential of noncentral facilities seems to warrant their
further investigation. Many of the possible shortcomings of this alternate facility may vanish as
some are constructed and experience is gained.
Collection and Treatment Alternatives for Noncentral Facilities. Proper facilities planning
involves a systematic comparison of all feasible alternatives for wastewater treatment and disposal
so that the most cost-effective solution, which will minimize total costs to the community and the
environment over time, can be found.
The trend toward gravity sewers with a common central treatment plant has eliminated many
worthy alternatives from consideration. If this bias can be changed and the noncentral concept
used, environmental and monetary costs of wastewater facilities in many communities could be
significantly reduced by reducing the size of, or eliminating, the collection system and by simpli-
fying the treatment facility.
The most extreme noncentral system would be one in which each home and establishment was
served by an individual septic tank system. The most cost-effective community system probably
lies between the 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 individual and joint sys-
tems.
Alternatives for dealing with wastewater treatment and disposal are numerous. To evaluate
which method is most cost-effective for a particular community would seem a monumental task. It
can be greatly simplified, however, by selecting the proper point from which to begin the design.
A wastewater facility must produce an effluent that 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 is too great for the environment
to assimilate, pollutants will accumulate. The physical characteristics of the local environment 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 large volumes of water can be easily discharged into rivers
and streams. This practice requires, however, rather high degrees of treatment before discharge to
prevent degradation of the stream. If soil is the disposal medium, lower levels of pretreatment are
required before disposal because of soil's greater assimilative capacity. The trade-off is that large
areas of land are required for absorption. When operation and maintenance costs of high levels of
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treatment for surface-water disposal are compared with costs for land disposal, however, land
disposal may be a more cost-effective alternative. A similar situation may exist for atmospheric
disposal, as in evapotranspiration (ET).
Thus, the first step in designing community wastewater facilities is to characterize the local
environment. Once it is determined what disposal media are available, systems can be designed to
fit for cost-effective comparisons. This requires a knowledge of the receiving environment's waste
assimilation capabilities. Federal and State regulatory agencies have already set effluent standards
for surface waters. The assimilative capacities of soil and ET systems are poorly understood, how-
ever, and need to be reviewed.
USE OF SOIL FOR TREATMENT AND
DISPOSAL OF WASTEWATER
Liquid Movement Into and Through Soil and Soil Materials
Proper performance of on-site wastewater disposal systems depends on the ability of the soil or
a soil material to absorb and purify the wastewater. Failure occurs if either of these functions is 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 be-
tween the grains. The size and shape of these pores is determined by the texture (particle-size distri-
bution) of the soil and the shape and packing of the individual grains. When significant amounts of
clay and organic matter are present, soil particles cement together, forming aggregates (peds).
Planar voids form between the peds. Tubular channels made by plants and animals living in the soil
and irregularly shaped discontinuous pores called vughs also are found. (See fig. I-l.)9
Soil permeability, or capability to conduct water, is not determined by soil porosity but by the
size, continuity, and tortuosity of the pores. A clayey soil is more porous than a sandy soil; yet the
sandy soil will conduct much more water because it has larger, more continuous pores. These twist-
ing pathways, with enlargements, constrictions, and discontinuities through which the water moves,
are constantly being altered. The soil structure that helps to maintain the pores is very dynamic and
may change greatly in response to changes in natural conditions, biological activity, and soil-manage-
ment practices. Repeated wetting, drying, and freezing help to form peds; plants with extensive
root systems and soil fauna activity promote soil aggregation and channeling. Mechanical compact-
ing and adding soluble salts can break down the peds, reducing the capacity of the soil to conduct
water.
Characterization of Water in Soils. Under natural drained conditions, some pores are filled with
water. The distribution of the water depends on the characteristics of the pores; the water's move-
ment is determined by its relative energy status. 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 mois-
ture potential.
The moisture potential has four components, of which the gravitational and the matric poten-
tial are the most important. The gravitational potential is the result of the attraction of water
toward the center of the earth and is equal to the weight of the water. To raise water against grav-
ity, work must be done; 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
relative to some reference level. Thus, the higher the water, the greater its gravitational potential.
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Basic structure
Basic structure
Skeleton grains
Plasma
100 p. Interpedal planar voids
Intrapedal
(a)
1 cm.
1 cm.
Figure 1-1. Schematic of (a) apedal (single-grained) and (b) pedal (aggregated) soil fragment.9
The matric potential is produced by the affinity of water for soil particle surfaces. The pores
and surfaces of soil particles hold water because of forces produced by adsorption and surface ten-
sion. Molecules within the liquid are attracted to one another, equally in all directions, by cohesive
forces. Molecules at the surface of 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 sur-
face to contract and 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 to other water molecules;
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 to a relatively low capillary
rise of 28 cm water (pressure below meniscus equals -28 cm water), whereas a pore radius of 30
microns results in a relatively high rise of 103 cm (pressure equals -103 cm water) as illustrated in
figure 1-2.10 This indicates that it takes more energy to pull water from a small pore than from
a large one. The water within the tube is at less than atmospheric pressure, as noted, because it is
pulled downward by gravity as it is pulled upward by capillary action. The water is under tension,
as the tube sucks the water into it. This negative pressure is called soil tension or soil suction and
is measured in millibars (mbars).
Adsorption forces also contribute to matric potential. Molecular forces between the surface of
the soil particles and the water cause the water to form envelopes over the particle surfaces and to be
retained in the soil. (See fig. 1-2.)
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Soil particle
Capillary
water
i
Figure 1-2. Upward movement by capillarity in glass tubes as compared with soils (after Brady10).
When soil is saturated, its pores 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 mois-
ture 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
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 soil moisture as tension increases is a function of pore-size distribution
and is characteristic for each soil type. Figure 1-3 shows the soil moisture retention curves for sand,
silt loam, sandy loam, and clay. 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 held in very fine pores. The silt loam has a greater number of
coarse pores than does the clay, so its curve lies somewhat below that of the clay. The sandy loam
has a greater number of fine pores than the sand does, so its curve lies above that of the sand.
Liquid Flow in Soils. Gravitational potential pulls water downward; matric potential attracts
water in all directions, but only if the soil is not saturated. The rate of flow increases as the poten-
tial difference of potential gradient between points increases. The ratio of the flow rate to the
potential gradient is referred to as hydraulic conductivity, K, defined by Darcy's Law:
Q = KA dH/dZ
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60-
50-
c
-------
This parameter (K) accounts for all factors affecting flow in soil, including tortuosity and size
of the pores. Thus, the measured K values for soils vary widely because of differences in pore-size
distributions and pore continuity.
The hydraulic conductivity often changes dramatically with changes in soil moisture tension.
At a tension equal to or less than zero, the soil is saturated and all pores 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 that cause flow become
capillary. As the water content decreases or tension increases, the path of the water flow becomes
more tortuous because the water travels along surfaces and through sufficiently small pores to retain
water at the prevailing water potential. The unsaturated K, therefore, usually is much lower.
To illustrate this, three soil materials with differing pore-size distributions are represented in
figure 1-4. One soil is coarse and porous (like a sand); one is fine and porous (like a clay); and one
has both large and fine pores (like a sandy loam). When there is an open infiltrative surface and a
Absent
J_
Very low
_L_L
or
Strong
Rate of application
of liquid
Degree of crusting
SAND
LOAMY SAND
SANDY LOAM
SILT LOAM
CLAY
BH Liquid
Figure 1-4. Effect of increasing crust resistance and decreasing rate of application of liquid on the rate of percolation
in soil.9
11
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sufficient water supply, all soil pores are filled, and each conducts water downward because of grav-
ity. 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 and restricts 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 capillary force; therefore, progressively smaller pores empty at in-
creasing crust resistance. This crusting leads to a dramatic reduction in K. (See fig. 1-5.)l l
If no crust is present, similar phenomena occur when the rate of water applied to the capillary
system is reduced. When there is an 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 because the smaller pores have a greater capillary attraction for water. Thus,
larger pores can fill with water only if smaller pores are unable to conduct away all the applied wa-
ter.
The reduction in K upon increasing soil moisture tension is, therefore, 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 K decreases more slowly with
increasing tension. The K curves determined in situ show such patterns. (See fig. 1-5.)
The K curves for the pedal silt loam and clay horizons demonstrate the physical effect of rela-
tively 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/d
for the silt loam), but these pores are not filled with water at low tensions and lvalues drop dramat-
ically between saturation and 20-cm tension (1.5 cm/d for the silt loam).
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 by hydrostatic pressure and capillary pull.
Phenomena contributing to the development of a clogging zone at the infiltrative surface of
soil absorption systems include: compaction, puddling, and smearing of the soil during construction;
puddling caused by the constant soaking of the soil during operation; blockage of soil pores by
solids filtered from the waste effluent; accumulation of biomass from growth of microorganisms;
deterioration of soil structure caused by exchange of ions on clay particles; precipitation of insol-
uble metal sulfides under anaerobic conditions; and excretion of slimy polysaccharide gums by some
soil bacteria.
Many systems fail, usually within a year or two, because of poor construction techniques.
Absorption of water by soils depends on preservation of a suitable soil structure, but soil structure
can be partially or completely destroyed during construction. Extensive damage does not occur in
single-grained soils (sands) but can occur in aggregated soils with high clay content. When mechani-
cal forces are applied to a moist or wet soil, the water around clay particles acts as a lubricant,
causing the soil to exhibit plasticity and soil particles to move relative to one another. Such move-
ments, 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 heavy machinery
frequently passing over the field; smearing from excavating equipment; and puddling from exposure
of the infiltrative surface for a day or more to rainfall or wind-blown silt that seals off the soil pores.
As a result, the absorption field may be clogged before it is put into service.
Compaction, smearing, and puddling occur primarily in soils containing clay. The flat clay
particles adhere to each other in dry soil, making it hard and very resistant to high compressive
forces. When wet, however, the clay plates separate when forces are applied to the soil. The water
12
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1,000 —
100
10.
T3
E
U
1.0
0.1-
245 —
DRYING-
20 40, 60 80
SOIL MOISTURE TENSION, mbar
100
Figure I-5. Hydraulic conductivity (K) as a function of soil moisture tension measured in situ using the crust test.11
13
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acts as a lubricant as the clay plates move relative to one another to close channels and vughs re-
ducing the permeability of the soil to very low levels.
Not all soils are equally susceptible to this structural destruction. Tendency toward compac-
tion and puddling depends on soil type, moisture content, and applied force. Soils with high clay
content are easily puddled; sands are not affected. Clayey soils will not puddle, however, if they are
only slightly moist. Under pressure, dry clay breaks into small fragments along pedal boundaries
rather than smears, thereby keeping the large pores open.
Several studies indicate that physical and biological mechanisms are the primary causes of soil
clogging in an absorption field not smeared and compacted during construction.9'12"26 In these
instances, clogging seems to develop in three stages: slow initial clogging, rapid increase of resistance
leading to permanent ponding, and leveling off toward equilibrium. Initial development of a clog-
ging zone seems to be largely the result of accumulation of suspended solids (SS) from the waste-
water, so that the liquid seeps away more and more slowly between loadings. Aerobic bacteria
decompose many of the organic solids, thereby helping to keep soil pores open; but they can func-
tion only when the infiltrative surface drains between doses, allowing air to enter. As the clogging
zone begins to form, decreasing the aerobic periods between ponding, the aerobic bacteria eventu-
ally are unable to handle the influx of solids. Permanent ponding results, leading to anaerobic
conditions. 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
because anaerobic bacteria destroy soil-clogging organics less efficiently. 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 facultative organisms that grow in such an
environment produce gelatinous materials (bacterial polysaccharide slimes or gums) that clog soil
pores very effectively. At that point the clogging mat seems to reach an equilibrium in which the
resistance to flow changes little. An absorption field will not fail, however, if the application rate
does not exceed the equilibrium rate. The process can be reversed and much of the original infil-
trative capacity restored if the ponded surface is allowed to drain and rest, permitting aerobic bio-
logical decomposition and the drying and cracking of the clogging materials.
Infiltration not only depends on the resistance of the clogging zone but also on the capillary
properties of the underlying soil.11 For example, an identical crust with a resistance of 5 days (the
length of time for 1 cm3 to pass through 1 cm2 of barrier with a head of 1 cm) and ponded with 5
cm of liquid would induce flow rates of 8 cm/d in a sandy loam, 7 cm/d in a sand, 4 cm/d in a silt
loam, and 1.8 cm/d in a clay.27 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 conductivities.
Significance of Unsaturated Flow. Liquids flow at a much slower rate in unsaturated soil than
in saturated soil because flow only occurs in the finer pores. This slows infiltration but enhances
purification. Wastewater effluent is purified by filtration, biochemical reactions, and adsorption-
processes that are more effective in unsaturated soils because average distances between effluent
particles and the soil particles decrease as the time of contact increases. This flow phenomenon is
illustrated in figure 1-6, which shows a thin section of the C horizon of a Saybrook silt loam, a stony
sandy loam till with a Ksat of 80 cm/d. The flow velocity of water in the soil pores can be esti-
mated from its moisture retention curve (fig. 1-3). This velocity can be used to derive the time for
water to travel 1 foot (30 cm), assuming a hydraulic gradient of 1 cm/cm owing only to gravity.
Successively smaller pores empty at increasing tensions and K decreases correspondingly. (See figs.
1-4 and 1-5.) Calculated travel times increase from 3 hours at saturation to 30 hours at 30 mbar and
8 days at 80 mbar of soil moisture tension.
In structured soils it is possible for flow to be predominantly through the planar voids, bypass-
ing the interiors of the peds. High liquid application rates may result in a great deal of dispersion
14
-------
Skeleton grains
Plasma (very porous
and calcareous)
Macrovoids
Liquid
Saybrook Silt Loam (11C Stony Sandy Loam Till)
100 micron
SATURATED
AT SOmbar SUCTION
AT SOmbar SUCTION
K = 80 cm/d
One foot (30 cm) movement
in the soil in: 3 hours
(hydraulic gradient: 1 cm/cm)
K = 7 cm/d
30 hours
K = 7 mm/d
8 days
Figure I-6. Occurrence and movement of liquid in a saturated and unsaturated sandy loam till, C horizon, of Say-
brook silt loam.9
where the water passes through the planar voids without displacing the water already in the peds.
In such instances, short-circuiting of liquid through the soil occurs with associated low retention
times. Low application rates 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 move-
ment in soil columns have shown short-circuiting to be a particular problem on drained soils dosed
at relatively high rates.27
Short-circuiting in a structured soil is illustrated in figure 1-7. If the large planar voids are
drained and air-filled, liquid applied at the surface at a high rate will quickly pass through the large
pores before much of it can enter the fine pores of the peds. The retention time of most of the
liquid, therefore, is low, and only a portion of the 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. Also, 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 wastewater. The design of
absorption systems may be critical in achieving 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. When an absorption
system in a highly porous or dry structured soil is first put into service, however, and there is no
clogged zone, there is danger of inadequate purification unless design precautions are taken to
ensure that the soil remains unsaturated.
15
-------
'. '. I Added water
Clogging zone
UNCLOGGED CLOGGED
Figure 1-7. Influence of clogging zone on short-circuiting in structured soils.
Treatment Capabilities of Soil
The principal goal in liquid waste disposal for homes in unsewered areas is the purification of
the liquid before it reaches potable or recreational waters. Organic matter, chemicals, pathogenic
organisms, and viruses that are not removed before they are applied to the soil must be removed or
transformed by it. Numerous studies show that under proper conditions, soil is an extremely effi-
cient purifying medium.
Bacteria and Virus Removal. From the standpoint of public health, removal of disease orga-
nisms and viruses is the most critical function of a soil disposal field. Many field and laboratory
16
-------
studies have examined the soil's efficiency in removing pathogens and the various parameters that
affect its efficiency, such as soil type, temperature, pH, organism adsorption to soil and soil-clogging
materials, soil moisture, nutrient content, and biological antagonisms.28 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 1-8 shows removal of fecal coliforms (FC) and fecal streptococci (FS) from septic tank
effluent by two columns packed with 2 feet of Plainfield loamy sand (effective size 0.14 mm, uni-
formity coefficient 1.99).29»30 Both columns were loaded well below their Ksai rates of nearly
400 cm/d (96 gal/d/ft2), but one was loaded at twice the rate of the other. The number of bacteria
discharged from both columns reached a plateau during the first 100 days of application, then
declined. Fewer bacteria passed through the column that had the lower loading rate. Column one,
loaded at 10 cm/d (2.4 gal/d/ft2), removed approximately 92 percent of FC applied per day; col-
umn two, loaded at 5 cm/d (1.2 gal/d/ft2), removed 99.9 percent. FS and Pseudomonas aeruginosa
were also found in the effluent from the more heavily loaded column one. These organisms were
not detected in effluent from column two. During this period a clogging zone developed on the
infiltrative surface of each column, and FC counts in the effluents from both columns eventually
dropped to between 10 and 100 FC/100 ml.30
COLUMN ONE
Loaded at 10 cm/d
hrs retention time)
o
o
o
<
CD
a
o
COLUMN TWO
Loaded at 5 cm/d
(25 hrs retention time)
140
180
100
TIME, days
Figure I-8. Bacteria counts in effluents from sand columns loaded with septic tank effluent.29
17
-------
Septic tank systems installed in sands also exhibit the effects of the clogging zone in removing
indicator bacteria. Figure 1-9 shows bacterial counts from 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 to the side of the trench were similar to those found in the natural
80^9,29,30
Concurrent studies of Almena silt loam were also conducted.30 This soil has a lower capacity
to conduct liquid than the unstructured sands and most of the flow is through the larger pores
between peds. Undisturbed cores, 2-feet deep, of Almena silt loam were loaded with septic tank
effluent at a rate of 1 cm/d (0.24 gal/d/ft2). 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/d to 3 mm/d to promote slow flow through the peds rather
than through the larger cracks around them (fig. 1-7), bacterial counts decreased dramatically to
below 2/100 ml of FC, FS, and P. aeruginosa. When the loading was restored to 1 cm/d, high
counts were again observed. (See fig. 1-10.)
Virus adsorption and inactivation in soils have been of considerable interest to scientists and
engineers. When viruses enter a septic tank or other treatment process, they are likely to associate
with cells in fecal material. These masses settle, releasing some viruses, depending on turbulence
within the process. (In laboratory studies up to 89 percent of the polio virus added in fecal material
was released by vigorous shaking.31) Secondary adsorption on wastewater solids may occur in treat-
ment processes. The free and particle-adsorbed virus will then be discharged to treatment processes
or the soil absorption field.
Removal of viruses in soils results from the combined effects of sorption, inactivation, and
retention. On entering the soil, viruses 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.32 In the adsorbed position, the viruses are inactivated in a spontaneous process that is
temperature-dependent, being greater at higher temperatures.31 Virus detention in the soil is
ABSORPTION FIELD
CROSS SECTION
I-
UJ
0-
1 -
2-
3-
Bacteria, 100 ml or 100 g of soil
Trench
o
7
1
Liquid "
• ." •. "- •' V 'r '•'.-- '. 'm
••.•x... Clogged zone • .
— 1 ft -»| FS
• •• — <200
1 60,000
FC
<200
1 onn nnn
4nnn nnn
Total
coliforms
<600
5~7nn nnn
23 000 000
bacteria
X 107
0.6
3D
4 400
\_
Natural
soil
<200
<200
<200
17,000
<200
700
23,000
<600
1,800
6.7
3.7
2.8
Figure I-9. Cross section of seepage trench in sand showing bacterial counts at various points near the trench.17-19
18
-------
affected by the degree of saturation of the pores through which the virus-laden effluent flows. The
more saturated the pores, the less opportunity there is for viruses to come in contact with surfaces
to which they can adsorb.
In laboratory studies using packed sand columns, septic tank effluent was inoculated with
more than 105 plaque-forming units (PFU) per litre of polio virus type 1.31,33 ^.11 viruses were
removed in the 24-inch columns at a loading rate of 5 cm/d (1.24 gal/d/ft2) over a period of more
than one year. At a loading rate of 50 cm/d (12.4 gal/d/ft2), virus breakthrough occurred (fig.
1-11). Analysis of the sand residue after virus application indicated that adsorbed viruses in the
column were inactivated at a rate of 18 percent per day at room temperature and at 1.1 percent at
6° C to 8° C.33
In contrast, viruses were detected approximately 60 inches within columns packed with cal-
careous loamy sand and fed secondary effluent containing 3 X 104 PFU polio virus type I at a rate
of 15 cm/d (3.7 gal/d/ft2).32 Most of the viruses were adsorbed in the top 2 inches of the soil and
removal was not appreciably affected at application rates of between 15 and 55 cm/d (3.7 and 13.6
gal/d/ft2). Only deionized water desorbed the viruses, but drying for 5 days prevented desorption
even with deionized water.
Laboratory tests with ground soil from a Batavia silt loam reduced virus in septic tank efflu-
ents by 5.4 logs per cm, and with Almena silt loams to 7.9 logs per cm.33 It should be emphasized,
o
o
O
<
no
LC = Loading change from
1 cm/d to 3 mm/d
and return to 1 cm/d
180
Figure 1-10. Bacteria counts in effluent from an undisturbed core of Almena silt loam loaded with septic tank
effluent.29
19
-------
Fluid samples.
A = 5 cm (850 ml) dose
D = 50cm (8.51 ml) dose
O = input titer
4- = indicated value
o
en
0
10
20
40
50
30
DEPTH, cm
Figure 1-11. Penetration of polio virus into a 60-cm conditioned sand column at room temperature.33
60
however, that finely ground soils do not exist in the field. Channels in natural soil reduce oppor-
tunity for virus adsorption, and viruses may travel long distances when loading rates are high.
Chemical Transformation and Removal. Domestic wastewaters may contain a few chemicals
hazardous to public health or to the environment. Nitrogen and phosphorus compounds are dis-
charged in household wastewater and 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 methemo-
globinemia in infants.34 A safety limit for nitrate of 10 mg/1 as nitrogen is recommended by the
U.S. Public Health Service (USPHS).35 There are many reports of nitrate concentrations above the
10 mg/1 nitrogen limit in wells near septic tank systems.34'36"38 Accelerated eutrophication of
surface waters is also attributed to nitrate from waste discharge.37
In solution, nitrate moves freely through the soil, although there can be some denitrification
(reduction of nitrate to nitrogen gas) where organic material and an anaerobic environment occur
together. Nitrogen in septic tank effluent is about 80 percent ammonium and 20 percent organic
nitrogen, but much of it is converted biologically to nitrate as it moves through the aerated, unsatu-
rated soil immediately below the clogging zone in the seepage field.39 This is illustrated in figure
1-12, where concentrations of the various forms of nitrogen are plotted against depth below a soil
trench in a sandy soil. If anaerobic conditions prevail in the subsoil, nitrification will not occur; the
nitrogen then remains in the form of ammonium. Ammonium is readily adsorbed by soil materials
of high clay content; hence it migrates much more slowly.36-39
Phosphorus is also of environmental concern. If allowed to reach surface waters, it can acceler-
ate eutrophication because it is an essential nutrient of algae and aquatic weeds. Phosphorus enrich-
ment of ground water seldom occurs below septic tank systems, however, because it is fixed in soil
by sorption reactions or as phosphate precipitates of calcium, aluminum, or iron. Calcium is usually
found in the wastewater, and aluminum and iron are abundant in most soils.40 Phosphorus may
20
-------
0
10
20
30
§ 40
I-"
CO r-n
QC
O
no
I
Q_
LU
Q
60
70
80
90
100
110
120
130
10
20
MICROGRAMS, per gram soil
30 40 50
100
200
300
• Ammonium - N
O Nitrate- N
£ Chloride
Organic N
Figure 1-12. Concentrations of NH4-N, N03~N, organic N, and Cl in unsaturated soil below the clogged zone in
sand.29
leak into the ground water, however, where high water tables or very coarse sand and gravel occur,
or where the seepage bed has been loaded heavily for a long time. In such instances, concentrations
of phosphate above 5 mg/1 as phosphorus have been observed.37 Phosphorus can move downward
50 to 100 cm/yr through clean silica sand,41 but movement in loams, silt loams, and clays is much
slower (5 to 10 cm/yr). Thus, except in coarse soils, it takes more than 10 years for phosphorus to
move as much as 3 feet.41
Heavy metals and complex organic compounds are also effectively adsorbed by soil and re-
moved from the percolating wastewater.
ESTIMATING THE INFILTRATIVE AND PERCOLATIVE
CAPACITIES OF SOIL
Site selection criteria for on-site systems vary from folk knowledge and experience to empirical
tests, often codified into rules. The USPHS's general reference manual42 has provided guidelines
for many State, regional, and local manuals and codes of practices.
Several factors are usually considered in the selection of a site. The ability of the soil to
absorb liquid, usually estimated by the percolation test, is a common requirement. Other factors
21
-------
considered are slope, depth to ground water, nature of and depth to bedrock, likelihood of seasonal
flooding, and distance to well or surface water.42 These traditional factors have several limitations
and vary widely among codes.
Estimation of Soil Permeability
The Percolation Test. In 1926, Henry Ryon developed a test to obtain field data on failing
seepage systems.43 He dug a 1-ft2 hole to the depth of the failed system, 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 1 inch (percolation rate). To calibrate the test, Ryon inspected several failing or
near-failing systems and noted the loading of the system, soil characteristics, and percolation rate in
nearby soil. Ryon plotted curves relating permissible loading rates versus the percolation rate from
these data. It was later proposed that these curves could be used to size new soil absorption sys-
tems. Adoption of the procedure by the New York State Health Department led to its wide accep-
tance, although 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 may be predicted from its initial ability to absorb clear water.43
From Ryon's data comparing absorption rates of existing septic tank systems with the percolation
test, the measured rate is reduced by a factor ranging from 20 to 2,500 in order to size the absorp-
tion area.9 The results of the test are highly variable, however, and its use for system sizing relies
on an empirical relationship between the measured percolation rate and the actual loading rate.
Tests run in the same soil vary by as much as 50 percent;44'45 thus, the procedure is unreliable.
The Crust Test. The soil below most operating absorption systems is unsaturated because of
the clogging mat that develops at the infiltrative surface. To properly size an absorption system,
therefore, the unsaturated K characteristics of the soil must be known. Because the standard perco-
lation test does not provide these kinds of data, the crust test was developed.9-44'46'49
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 installed in
the column just below the infiltrative surface measures the soil moisture tension as water is applied
to determine the degree of saturation in the soil. (See fig. 1-13.) To maintain unsaturated condi-
tions in the soil column, a crust of gypsum and sand is placed over the soil surface. Water flowing
through the infiltrometer into the soil is restricted by the crust. This establishes a constant steady-
state flow rate, which induces a nearly uniform moisture tension in the soil beneath the crust. The
measured soil moisture tension and the equilibrium flow rate locate one point on the K curve. Addi-
tional tests run with crusts of various hydraulic resistances define the K curve as shown in figure 1-5.
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.
Although this procedure offers a direct measurement of K, it is time-consuming, requires a
skilled operator, and cannot be run economically at each site. Because K depends on the pores in
the system, however, the conductivity of a soil at various sites can be defined within statistical lim-
its.50 (See fig. 1-14.)50 Also, variability curves of soils in the same textural groups have similar K
curves.50 (See fig. 1-15 and table 1-6.)50 Therefore, by defining families of K curves for groups of
soils, the K characteristics of a particular soil or site can be predicted.
In Wisconsin, four major K types have been suggested based on the texture of the soil materi-
als.11 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 similar groupings might be made,
22
-------
Manometer
Figure 1-13. Schematic of the crust test procedure.
but they must be based on field data because differences in soil mineralogy may affect groupings.)
Typical K curves were developed from field measurements for each of these types. (See fig. 1-5.)
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.11
This information provided a design point on the curve for proper field sizing. The same procedure
could be used to select design points for other types of soil systems. (See table 1-7.)
The application rates for various soils presented in table I-711'51 represent the best estimates
available. Because of the unstructured nature of the sands and sandy loams, the rates are reasonably
accurate. Because of the nature of the flow that occurs in finer textured, structured soils, however,
there is more variability in the tensions measured under operating fields.11 In these soils the design
rates must be used with care, particularly where expandable clays are present.
Estimation of High Ground Water
To ensure adequate purification of the wastewater before it reaches ground water, a minimum
of 3 feet of unsaturated soil is necessary below the infiltrative surface. If saturated soils occur with-
in 3 feet, transmission of harmful pollutants to the ground water may result.29'33-52 Determining
if saturated conditions occur within the minimum is often difficult, however, because water-table
levels fluctuate with changing weather conditions. Typically, the water table is low during the sum-
mer and rises in the spring and fall. Ideally, the highest ground water level should be recorded, but
this is not very practical. Moreover, observations made in relatively dry years are not representa-
tive. Thus, other methods must be used to determine the high water elevation.
23
-------
1,000 &-
E
o
100 -
. \\.\ •
0.1
—» Regression line
• • — One standard deviation
about regression line
1.0
10
100
SOIL MOISTURE TENSION,
cm water
Figure 1-14. Hydraulic conductivity (K) data for Piano series (after Baker50).
24
-------
1,000
Mo
100
Group B
Piano (PI),
Batavia (Bt),
and Morley (Mo)
series
E
o
10
1.0
0.1
Group A
Ontonogon (0) and
Magnor (M) series
0.1
1.0 10
SOIL MOISTURE TENSION, mbar
100
Figure 1-15. Hydraulic conductivity (K) for Ontonogon and Magnor series and Piano, Batavia, and Morley series
(after Baker50).
25
-------
Table \-G.-Summary of morphological characteristics of soil series from figure /-7550
Soil identification
Horizon
selected
Texture
Structure
Frequency of biopores
to
Group A:
Ontonogon series:
Very fine, illitic, frigid,Typic Eutroboralf B2
Magnor series:
Fine-loamy, mixed; Aquic Glossoboralf MB
Group B:
Piano series:
Fine, silty, mixed, Mesic; Typic Arguidoll B2
Batavia series:
Fine, silty, mixed Mesic; Mollic Hapludalf B2
Morley series:
Fine, illitic, Mesic; Typic Hapludalf IIB3
Heavy loam
Heavy loam
Silty clay loam
Silt clay loam
Heavy, silty clay loam
Moderate, medium
angular blocky
Moderate, medium
subangular blocky
Moderate, medium
subangular blocky
Moderate, medium
subangular blocky
Moderate, medium
prismatic, too
coarse, blocky
structure
Few coarse, common medium
Few coarse, common medium
Few coarse, common medium
and fine
Few coarse, common medium
and fine
Few coarse, common medium
-------
Table 1-7.—Recommended maximum loading rates of septic-tank effluent for different soil types
Estimated percolation rate,
min/in
0 to 10
1 1 to 30
31 to 45
46 to 90
Soil texture
Sand
Sandy loams,
loams
Some porous
silt loams.
and silty
clay loamsb
Clays, some
compact silt
loams and
silty clay
loamsb
Loading rate3
Bouma1 1
cm/d
5
3
3
1
gal/d/ft2
1.23
0.72
0.72
0.24
Machmeier51
gal/d/ft2
1.20
0.60
0.50
0.45
Operating
conditions1 1
4 doses per day, uni-
form distribution.
trenches or beds
1 dose per day, uni-
form distribution.
trenches preferred
1 dose per day, uni-
form distribution
desirable, shallow
trenches only
Dosing and uniform
distribution de-
sirable, shallow
trenches only
aBottom area only.
^Should not be applied to soils with expandable clays.
Soil mottling sometimes indicates 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,
have a gray-brown matrix, and are described according to color, frequency, size, and prominence.53
Well-drained soils are usually brown in color because of finely divided insoluble iron and manganese
oxide particles throughout the horizon. Under reducing conditions often produced by prolonged
saturation, however, iron and manganese are mobilized until reoxidized when the soil drains. Re-
peated wetting and drying cycles quickly produce local concentrations of these oxides on pore sur-
faces, forming red mottles.54 Soil from which much of the iron and manganese has been completely
reduced, turns from brown to gray, a process referred to as gleying. The upper limit of the mottled
soil, therefore, often is a good estimate of the high ground-water level, although it may also be the
result of a periodic perched water table. The latter possibility can be confirmed by a lack of mottles
in lower horizons.55
ON-SITE TREATMENT AND DISPOSAL SYSTEM ALTERNATIVES
There are numerous strategies, including soil-dependent and nonsoil-dependent systems, to
consider for on-site treatment and disposal of wastewater. As has been discussed, however, soil can
be an excellent medium if it is managed properly. Where suitable soil exists, it should usually be
used in the system. Figure I-1656 provides a useful chart for conceptualizing the boundary condi-
tions of soils and techniques for maintaining infiltrative capacity.
Systems Dependent on Soil and Site Conditions
If a site satisfies all requirements for key factors (depth to bedrock, ground water, slope, flood
plain), then soil type and K should be determined. If the soil is moist, slowly permeable, and clayey,
27
-------
Key site factors:
Depth to bedrock
Depth to groundwater
Slope
Flood plain
[ Unsatisfactory
Alternate systems
Site alteration
Mounds
Drains
Regrading
In-house modification
Flow reduction
Waste segregation
Evapotranspiration-adsorption
Nonsoil-dependent alternatives
Surface disposal
Evapotranspiration
'cu<
Satisfactory
Soil type and hydraulic
conductivity
smical
ation
1" _
"^
i
Dosing
1
i
Chemical
oxidation
/ = infiltration rate
'CL = infiltration rate of clogged soil
ID = infiltration rate (design)
comp
= infiltration rate of compacted soil
Figure 1-16. Strategies for on-site wastewater disposal (after Bouma56).
28
-------
it is best not to construct absorption systems within the subsoil because irreparable damage may be
done to the soil by compaction, smearing, and puddling. (An alternative strategy should be used
such as constructing a mound over the natural soil that uses the soil for final disposal.)
Conventional ST-SAS. If the soil is dry or permeable, a system may be constructed to operate
satisfactorily if the proper hydraulic loading rate is selected. The loading rates recommended in
table 1-7 are based on observations from properly functioning ST-SAS. To maintain this infiltrative
rate for a reasonable span of 20 to 50 years, proper design, construction, and maintenance proce-
dures must be followed as shown. If these procedures fail, then an alternative strategy must be
investigated.
Estimation of Flow. Waste flows from homes, restaurants, motels, and so forth are intermit-
tent and subject to wide fluctuations. Variations in the number of persons contributing to the flow
and in their activities profoundly affect the daily volume of waste discharged, making accurate esti-
mates of waste-flow volumes difficult.
A study of 11 rural homes showed the average per capita flow from a single household to be
43 gal/d.57'59 The greatest flows result from laundering and bathing. (See fig. 1-17.) Social
AVERAGE FLOW, 42.6 gal/cap/d
T = Toilet
L = Laundry
B = Bath or shower
D = Dish wash
O = Other
WS = Water softener
cc
=)
O
I
CE
LU
Q-
Q.
O
QC
LU
CO
z
O
I I I I I 1
12p.m. 3a.m. 6a.m. 9a.m. 12m. 3p.m. 6p.m. 9p.m. 12p.m.
TIME OF DAY
Figure 1-17. Average daily flow pattern from 11 rural households.57
29
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events, such as family gatherings and guests staying overnight, will accentuate the peak flows shown.
In addition, the number of people living in a home may increase, through additions to the family or
sale of the house. Thus, to ensure the system will not fail when large flows occur, it is necessary to
design for the expected peak flows rather than for average per capita flow.
Household peak flows can be estimated by assuming two people occupy each bedroom. This is
realistic because the number of occupants in a home is a function of the number of bedrooms.
Figure 1-17 shows that a peak flow of about 3 gal/hr per capita, or 75 gal/d per capita. Thus, 150
gal/d per bedroom gives a reasonable estimate of the peak flow. This is the design basis recom-
mended by the USPHS,42 and has proved satisfactory.
Estimation of flow from public buildings, commercial establishments, and recreational facili-
ties is more difficult. The Small-Scale Waste Management Project (SSWMP) is determining daily and
peak flows from bowling alleys, camps, churches, schools, country clubs, self-service laundries,
marinas, motels, restaurants, service stations, shopping centers, theaters, and stadiums.
Sidewall Versus Bottom Absorption. A soil absorption system has two infiltrative surfaces: the
bottom of the trench or bed and the sidewall. When the bottom begins to clog, the waste effluent
ponds in the system and the sidewall begins to absorb liquid.11 In some soils the sidewall may be-
come the more significant infiltrative surface as clogging continues.15-43
The rate of water movement through soil is proportional to the total water potential gradient,
primarily because of gravitational and matric potentials. In an unclogged absorption system, the
potential gradient is lower for the sidewall than the bottom, because gravitational potential is zero.
As the clogged zone develops, the matric potential of the bottom may be reduced to where the sum
of the gravitational and matric potential is less than the matric potential of the sidewall. The side-
wall then becomes the dominant infiltrative surface.
Absorption systems should be designed to maximize the most significant infiltrative surface.
In the Midwest, the bottoms 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 ET rates are low, and the matric potential is lowered because of soil wet-
ness. Maximizing the infiltrative area by considering sidewalls as a reserve capacity is recommended,
but in the Midwest, the bottoms should be sized to absorb the entire estimated daily flow.
Distribution of Liquid Over the Infiltrative Surface. Local overloading of septic tank effluent
onto 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 permeable soils. Uniform appli-
cation of the waste water over the infiltrative surface is usually beneficial.
Absorption systems with uniform distribution and dosing are not necessary in all soil types to
eliminate poor purification and soil clogging. Sands and weakly structured sandy loams and loams
benefit most.60 After a system is put into service in natural sands, local overloading may cause
ground-water contamination, which goes unnoticed until clogging develops. Development of a
clogged zone may take several years. Excessive clogging caused by poor distribution tends to occur
in weakly structured soils. Uniform distribution aids in reducing the clogging because the liquid is
applied simultaneously to the entire infiltrative surface at rates no greater than the soil is able to
accept.61
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 of 2 to 4 inches per 100 feet, with the holes downward. Such a system does not pro-
vide uniform distribution. The liquid trickles out the holes nearest the inlet and at points of lowest
elevation. (See fig. 1-18.)
30
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Gravity flow;
continuous trickle of effluent
71
zn
Equilibrium
4 I I 4 4 4 4 4 4 4 I 4 4
TRADITIONAL SUBSURFACE SEEPAGE BED
Figure 1-18. Progressive clogging of the infiltrative surfaces of subsurface absorption systems.9
Clogging usually seems to start near the inlet of the absorption system and progresses down the
length of the bed.9 The large holes permit too much liquid to be discharged close to the inlet. Thus,
the soil below receives a continual trickle of water and is soon constantly ponded. Clogging develops,
forcing the liquid to infiltrate farther down the trench where the infiltrative surface is still fresh.
This sequence continues until the entire bottom is clogged (fig. 1-18). Altering the direction of the
holes or the slope of the pipe does not improve distribution significantly.62
Periodic dosing of large volumes of effluent onto the field improves distribution and allows the
soil to drain between applications. Drainage exposes the infiltrative surface to air, reducing clog-
ging.15- 43>61 Even with dosing, however, the effluent is not distributed over the entire infiltrative
surface if a 4-inch pipe is used.62
Pressure systems help provide uniform distribution. Networks of small-diameter pipes with
small holes are designed so that the entire network fills before much liquid passes out of the holes,
thereby achieving uniform distribution.60'62 These systems combine uniform distribution with
dosing, enhancing purification and reducing clogging.
31
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Proper loading of permeable soils to prevent saturated flow is vital to ensure purification of the
waste effluent. Pressure distribution systems provide this loading control. Conventional gravity dis-
tribution is ineffective.60
Pressure distribution systems also retard clogging. Because the network is designed to apply no
more liquid than an area of the bed can absorb each day, the soil remains well aerated. Absorption
fields in sand with pressure distribution have shown no evidence of clogging after 4 years of opera-
tion,60 but fields in sand with conventional distribution begin to clog after 6 months.9 The aerobic
environment maintained by pressure systems promotes the growth of microorganisms that destroy
clogging materials; it also appears to attract larger fauna, such as worms, which consume nutrients
accumulating at the infiltrative surface. The worms' burrows help break up the clogging zone.
Worm activity perhaps explains why an absorption field in a silt loam, underlaid with glacial fill and
dosed with pressure distribution at three times the USPHS42 recommended rate, has not clogged
after 3 years' operation.61 (See section on Pore Clogging.)
Construction Practices. Probably the most frequent cause of early failure of soil absorption
systems is poor construction. Rapid absorption of waste effluent by soil requires maintaining open
pores at the infiltrative surface, but often the pores are sealed during construction by compacting,
smearing, or puddling.
The following construction techniques are recommended to minimize soil clogging:63-64
• Work in clayey soils only when moisture content is low. If the soil forms a "wire"
instead of breaking apart when rolled between the hands, it is too wet.
• Do not drive excavating equipment on the bottom of the system. Trenches rather than
bed construction are preferable in clayey soils because equipment can straddle the trench,
thus reducing compaction and smearing.
• Construct in shallow systems to place the infiltrative surface in more permeable horizons
and to enhance ET. This is particularly beneficial in clayey soils because they are gener-
ally wetter for longer periods of time, especially at greater depths.
• Remove smeared or compacted surfaces. Compaction may extend as deep as 8 inches in
clays; if so, hand spade to expose a fresh infiltrative surface.
• Schedule work only when the infiltrative surface can be covered in 1 day because wind-
blown silt or raindrop impact can clog the soil.
Restoring the Infiltrative Capacity of a Clogged Absorption Field. Soil absorption systems
often fail after several years of satisfactory service because the clogging zone 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.
One effective method is resting the system.9' 13> I4.43 Resting allows the absorption field to
gradually drain, exposing the clogged infiltrative surface to air. After several months, the clogging
materials are broken down in 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 rests. Two beds, each with 50 to 75 percent of the total absorption area required,
can be constructed when the disposal system is installed. The two beds can be used alternately by
diverting the wastewater from one to the other every 6 months. If a system with one bed fails and a
new bed is constructed, provisions should be made so that the old one is not abandoned but can
easily be alternated with the new bed.
32
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The infiltrative surface also can be rejuvenated by adding oxidizing agents to the absorption
field. Oxidation serves the same function as resting, but the clogging zone is destroyed in a day or
two rather than in several months. Such a method does not necessitate taking the clogged bed out
of service; therefore, it eliminates the need for a second bed.
Laboratory and field tests indicate that chemical oxidation can restore the infiltrative surface
to nearly its original permeability.26 The preferred oxidant is hydrogen peroxide (H2O2) because
it is effective at the natural pH of absorption fields, produces no noxious byproducts, and is inex-
pensive.
H2O2 treatment is best 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 using peroxide. Five gallons of 50 percent H2O2 solution may be sufficient to reduce
the clogging developing in the bed. Because the field is still permeable, the oxidant can reach the
clogging zone more easily than it could in a sealed system. It is best to perform tank pumping and
peroxide treatment while the system is not in use; for example, during a vacation, to give the re-
agent 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 H2O2 is in progress.
Mound Systems. The conventional septic tank-soil absorption field is not a suitable system of
wastewater disposal in many areas, such as those with slowly permeable soils, excessively permeable
soils, or soils over shallow bedrock or high ground water. Mound systems are alternatives, however,
that can be used and that use the soils' ability to absorb and purify wastewater.65
Slowly Permeable Soils. Slowly permeable soils are 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 spring and fall. During these wet periods infiltrating surface
water is unable to percolate fast enough through the subsoil, and flooding occurs from lateral move-
ment of water from higher elevations through the topsoil.
To overcome these conditions, one alternative is to raise the absorption field above the natural
soil by building the seepage system in a mound of medium sand.66 This raises the seepage system
above the wet, slowly permeable subsoil and places it in a dry, permeable sand. (See fig. 1-19.) This
technique has several advantages. First, the percolating liquid enters the more permeable natural
topsoil over a large area and can spread laterally until it is 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
compacting of the wet subsoil is avoided because excavation in the natural soil is not necessary.
The design of the mound is based on the expected daily wastewater volume it will receive and
the natural soil's characteristics. The mound must be large enough to accept the daily wastewater
flow without surface seepage in the spring and fall when perched water exists in the natural soil, as
well as during the summer and winter when the water table is lower. Size and spacing of the seep-
age trenches is important to prevent 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 large enough to
conduct the effluent into the underlying soil.
A clean, medium sand is used as fill for the mound; the gravel trenches consist of 1- to iy2-inch
stones. As in any seepage trench, a clogging mat will develop at the bottom. The ultimate infiltra-
tion rate through this zone has been shown to be 5 cm/d.1 * One consideration, therefore, must be
to ensure 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
33
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Topsoil
Subsoil \ Perforated PVC pipe C|ay fi|| or topsoil
Sand fill
Topsoil
SEPTIC TANK
Plowed surface
Pump switch
PUMPING CHAMBER
V/2-to 2-inch PVC pipe
from pumping chamber
^
i.n 1-inch perforated
"' PVC pipe
Seepage trench
5/8 to 1 inch stone
PLAN VIEW
Figure 1-19. Plan view and cross section of mound system for problem soils.
liquid contributed by the upslope trench. Infiltration rates into the natural soil and basal area
required for the mound are based on the K characteristics of the least permeable soil horizon below
the proposed site.
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.
34
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Mound systems installed in these soils have been monitored since 1972 and are performing
satisfactorily.67 Proper site criteria design and construction techniques, which have been described
in detail,66 are critical for satisfactory performance. Not all sites are acceptable for the mound
design.
Shallow, Permeable Soils Over Creviced or Porous Bedrock. These soils constitute a major
group of problem soils because adequate soil is not available to purify the percolating waste before
it reaches the porous bedrock, which leads directly to the ground water. To overcome these limita-
tions, the absorption field can be raised above the natural soil by using the mound system. (See fig.
1-19.) 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 ground
water.29- 33> 68>69 Nitrates, however, will not be removed.
The mound design procedures are the same as with slowly permeable soils. The seepage system
within the fill may be of nearly any shape because the permeability of the natural soil is not a lim-
iting factor. A bed is usually more suitable than trenches. Detailed site criteria design and construc-
tion procedures70 should be followed for proper operation of the mound.
Permeable Soils With Seasonally High Ground Water. Homes should not be built in areas with
permanently high ground-water tables. In some areas, however, homes are built where the water
table is only occasionally high during the year. During high water table periods, a conventional
ST-SAS cannot function properly because of flooding of the system and improper purification. A
properly designed and constructed mound system provides sufficient unsaturated distance for puri-
fication before the effluent reaches the ground water (fig. 1-19). The mound design procedures are
the same procedures as those used with 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 prevent the perched water table from entering the base of the mound.
Detailed site criteria, design, and construction procedures are described elsewhere71 and should be
followed to ensure 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 maintain a sufficient depth of unsaturated soil for purifying the wastewater to
avoid short circuiting.72'73 Such systems are being evaluated.
Modifying the Treated Wastewater Characteristics. Although the search for improved methods
of on-site disposal has centered on the soil absorption system, more emphasis recently has been on
altering the characteristics of the effluent discharged to the soil. Improving effluent quality has
been said to enhance soil infiltration, reduce dependence on soils for final treatment, and eliminate
the need for soil in the system.
Modifying the Wastewater Source. One of the simplest ways to improve the effluent dis-
charged to the soil is to make changes at the source, either by reducing the total volume of waste
discharged or by preventing pollutants from entering the waste stream.
Flow reduction to produce lower wastewater volumes can be accomplished through water con-
servation and recycling. Reductions can be achieved by improving water-use habits or by simple
modifications in water-use appliances and plumbing fixtures. With less wastewater to treat and dis-
pose of, the life of the on-site disposal would increase.
Nearly 70 percent of the total household wastewater generated is derived from toilet, laundry,
and bath.57 Using low-flow toilets, "sudsaver" washing machines, and restricted-flow shower heads
and recycling bath and laundry wastes for toilet flushing are four commonly mentioned ways to
35
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save water. 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 17 per-
cent in rural Wisconsin homes.57 Recycling bath and laundry wastes to flush toilets could increase
the savings to 33 percent. (See table 1-8.) These savings compare well with values from other stud-
ies.74-75
Waste segregation to eliminate pollutants from the waste stream improves the quality of the
wastewater. Analysis of wastewaters generated from rural households suggests which water-use
events should be modified for the most beneficial results.58'59 (See table 1-9.)
Table \-8.-Average calculated wastewater reductions in eleven rural homes5'1
[All volumes in gallons per capita per day]
Use
Toilet
Laundry
Bath
Dishes
Water softener . .
Other . . .
Total . . .
Percent savings
Number
per
capita
per day
2 29
31
47
39
03
Average
volume
of
event
3 99
33 49
21 35
12 50
81 07
Normal
volume
9 16
10 51
10 00
4 86
2 64
5 43
42 60
With 3
gallon
flush
6 87
10 51
10 00
4 86
2 64
5 43
40 31
5.00
With
sudsaver
@
S27.86
9 16
8 64
10 00
4 86
2 64
5 43
40 73
4.00
With 15
gallon
bath
shower
9 16
10 51
7 np
4 8fi
2 64
5 43
39 65
7.00
With
all
three
used
fi 87
8 64
7 OR
4 fifi
2 fi4
K 4-3
35 49
17.00
Recycled
bath/
laundry
to toilet
Q KA
7 OR
4 Rfi
2 fi4
c 4-3
28 fi2
33.00
Table I-9.—Mean household wastewater contributions58
[mg per capita per day]
Parameter
BOD5 U . .
BOD5 F .
TOC U . .
TOC F ...
TS
TVS
TSS
TVSS ....
TOT-N
NH3-N
N03-N
TOT-P
ORTHO-P . .
Temperature °F . . . .
Flow (gal)
Number sample ....
Toilet
Fecal
flush
4,340
2,340
3,530
1,580
10,700
7,760
6,240
5,090
1,500
590
6.3
270
120
66
4.3
32 to 40
Nonfecal
flush
6,380
3,980
4,250
3,170
17,800
12,000
6,280
5,120
2,640
520
21.1
280
190
66
4.3
24 to 37
Garbage
disposal
10,900
2,570
7,320
3,910
25,800
24,000
1 5,800
13,500
630
9.6
.2
130
90
71
3.8
4 to 7
Kitchen
sink
8,340
4,580
5,000
4,110
13,800
9,730
4,110
3,840
420
32.3
1.8
420
180
80
4.8
7 to 11
Auto-
matic
dish-
washer
12,600
7,840
7,280
4,690
18,200
10,500
5,270
4,460
490
54
4.1
820
380
101
12.0
13to 15
Washing machine
Wash
10,800
6,970
7,700
5,380
37,500
14,700
7,930
4,700
580
19.4
17
1,600
410
90
15.7
24 to 27
Rinse
4,010
2,840
2,610
1,910
10,900
4,800
3,040
1,810
150
11.4
10.3
550
110
83
14.4
24 to 28
Bath/
shower
3,090
1,870
1,750
1,130
4,590
3,600
2,260
1,580
310
40
7.4
36
20
85
13.0
18 to 24
36
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Recently, the concept of segregating toilet wastes (black water) from the other household
wastewaters (gray waters) for separate treatment and disposal has drawn attention. Those planning
development in water-short areas have raised serious questions regarding the use of valuable drinking
water to transport body wastes and the comingling of black and gray waters before on-site treat-
ment and disposal. Segregating black water from other household wastewater by using a nonwater-
carrying 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 possible reductions are shown in table I-10.57"59'74
If the toilet wastes can be segregated and adequately disposed of, attention must then be
directed toward the disposal of the gray water. Gray water is relatively uncontaminated, compared
with black water. Gray water, however, contains substantial quantities of physical and chemical
pollutants as well as pathogenic indicator organisms.76 (See tables 1-11 and 1-12.)
Pollutant concentrations of gray water are similar to those of black water or combined wastes
in rural homes.59 Black water contains high concentrations of SS, nitrogen, and bacteria; gray water
also contains sufficient quantities of pollutants and pathogenic indicators to cause concern.
Little research has been done on treatment and disposal of household gray water. One method
is the ST-SAS, but simple alternatives might be more desirable in certain applications. The SSWMP
is evaluating alternative methods for gray water treatment and disposal.
Reducing flow or waste strength may increase the life of a soil absorption field, but it is not
known by what factor. For new installations, smaller absorption fields could perhaps be allowed, if
water-saving devices were used. Unless assurances are made that flow reduction or waste-segregation
facilities could not fail or be removed, however, reducing the size of the field is not recommended.
Effluent Quality and Soil Clogging. Improving 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 SS and 5-day biochemical oxygen demand (BOD5)
concentrations in the effluent.77 The relationship found was:
Adjusted area required = Area required for standard septic tank system X 3 BOD5 + TSS/250
Assuming an average effluent quality of 40 mg/1 BOD5 and 40 mg/1 SS, the calculated adjusted
absorption area is approximately two-thirds of the standard area.
In several other studies23- 50.78 on effects of effluent quality on soil infiltration, however,
only slight differences in clogging rates were found over a range of qualities tested. These differ-
ences were small in hand-packed lysimeters of sand and sandy loam loaded with septic tank effluent
(74 mg/1 BOD5 and 51 mg/1 SS) and aerobic unit effluent (81 mg/1 BOD5 and 75 mg/1 SS).78 The
Table 1-10.—Effect of toilet waste segregation on household wastewater
Parameter3
Flow
BOD5
SS
Total phosphorus
Total Kjeldahl nitrogen
Percent
reduction
22 to 31
22 to 49
36 to 67
14 to 42
68 to 99
aAlthough not shown, there would also be substantial reductions in the quantities of pathogenic organisms.
37
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Table 1-11.—Pollutant contributions by black and gray wastewater streams
76
Parameter
BOD,-
^5
SS
Nitrogen
Phosphorus
Flow
Mean,
percent
63
39
18
70
65
(
Range,
percent
51-80
23-64
1-33
58-86
53-81
3ray
Mean
(gal/cap/day)
28 5
17.2
1 9
2 8
29.4
(mg/l)
255
155
17
25
Mean,
percent
37
61
82
30
35
E
Range,
percent
20-49
36-77
67-99
14-42
19-47
lack
Mean
(gal/cap/day)
16 7
27 0
8 7
1 2
15.9
(mg/l)
280
450
145
20
Note.—Values based on results of several studies.59'74'75 Results are average values for households with typical conventional
appliances, excluding the garbage disposal.
Table 1-12.—Selected bacteriological characteristics of bath and laundry wastewaters
76
Event
Bath/shower
Laundry ...
Bacteria
Total coliforms
Fecal coliforms
Fecal streptococci
Total coliforms
Fecal coliforms
Fecal streptococci
Data
points
32
32
32
41
41
41
Mean3
number
per
100 ml
1,810
1,210
326
215
107
77
95 percent
confidence
interval
per 100 ml
530 to 6,160
330 to 4,4 10
70 to 1,510
45 to 1 020
28 to 405
19 to 305
al_og-normal data.
aerobic unit effluent produced earlier, but less intense, clogging in the sand; the reverse was true in
the sandy loam. After resting, the soils receiving aerobic unit effluent recovered more quickly. In
general, however, there was little difference in 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 efflu-
ent, aerobic unit effluent, and distilled water.23 The aerobic effluent had a significantly lower
biodegradable organic concentration than the septic tank effluent-chemical oxygen demand (COD)
concentrations of 150 mg/l and 60 mg/l, respectively—but SS concentrations were similar (40 mg/l
and 33 mg/l, respectively). More severe clogging occurred with the aerobic effluent. No clogging
occurred in the soil loaded with distilled water. It was hypothesized that finely divided SS in the
aerobically treated wastewater entered the small pores in the soil, clogging it with depth and cre-
ating a more effective barrier to flow.
Subsequent studies designed to test the solids clogging hypothesis found initial Ksai to be
more significant than effluent quality.79 Undisturbed cores of Almena silt loam were paired
according to their initial -Ksat; one pair represented a high and one a low initial Ksat. Three sets of
four replicates each were dosed with 1 cm/d of septic tank effluent (48 mg/l BOD5, 27 mg/l total
38
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suspended solids (TSS)), aerobic unit effluent (27 mg/1 BOD5, 61 mg/1 TSS), and tapwater. The
length of time until 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-
water 18.3 weeks. When the three sets' initial Ksai values were compared, mean ponding time for
the high Ksat columns was 28 weeks and for the low Ksai columns was 14.8 weeks.
These studies indicate that in unstructured soils, such as sands and sandy loams, applied efflu-
ent quality may affect the degree of clogging. A similar effect has not been found in finer textured
soils.
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 waste-
water disposal systems not dependent on soil disposal, but which discharge the treated wastewater
to surface waters or the atmosphere, are necessary.
Systems designed to discharge treated wastewaters to surface waters must produce a high-
quality effluent. The EPA has set concentration maxim urns of 30 mg/1 BOD5 and 30 mg/1 SS for
municipal treatment plants that discharge to water courses. Lower maximums might be set for scat-
tered individual systems that discharge to small, intermittent streams. It might be expected that
bacteria counts in effluents from individual systems could not exceed total and fecal coliform maxi-
mums of 1,000/100 ml and 200/100 ml, respectively, recommended for recreational waters.80
Limitations on the nutrients nitrogen and phosphorus are likely to be required for discharges to
lakes or impoundments.
Aerobic Processes. Although a number of the options outlined have been field evaluated,
many others have not been tried with small flows. Aerobic treatment processes have received the
greatest attention as an alternative to the septic tank. More than 75 years' experience with this
biological process in larger scale applications makes it a logical candidate for small-flow, on-site
treatment.
Small trickling filters were among the first controlled aerobic processes for household waste-
water treatment. Nichols81 described the use of aerated pebble filters following septic tanks, and
Frank and Rhymus,82 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
reported with fixed-media biological filters, but the potential of rotary and stationary synthetic
filters is most promising. The SSWMP's most recent experience with a rotating biological contactor
has not been good, owing primarily to shaft breakage in laboratory and field units.83 An anaerobic,
submerged-media system, initially designed for ships, has proved very stable and relatively mainte-
nance-free.83
The first notable research on adapting the activated sludge process (extended aeration) to
household use was conducted at Purdue University in the early 1950's.84 A very simple prototype,
receiving toilet wastes only, produced an effluent with average BOD5 and SS concentrations of 28
mg/1 and 42 mg/1, respectively. Similar studies were conducted at Ohio State University using a
proprietary extended aeration package plant. Average BOD5 and SS concentrations of 24 mg/1 and
43 mg/1, respectively, and relative trouble-free service were reported during a 23-month period.85
In 1970, the National Sanitation Foundation (NSF) issued its "Standard No. 40" on individual
aerobic treatment units.86 It outlines criteria for evaluating such units and presents a procedure for
testing and certification. Several States require NSF certification for aerobic units.
39
-------
In addition to controlled studies of aerobic units, there are several reports on how they func-
tion under actual conditions.87'90 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 Colo-
rado.88 Their findings indicated a mean value for BOD5 and SS of 150 mg/1. They cited lack of
proper maintenance and an adverse effect of surge flows for this poor performance. Voell and
Vance89 provided data on a large number of field-operated aerobic units in a New York county.
Average values for BOD5 and SS were about 90 mg/1. The lack of proper maintenance was again
cited. Six aerobic treatment units were installed and monitored over an 8-month period by Glasser.90
Average values for BOD5 and SS were reported to be 48 mg/1 and 85 mg/1, respectively. Glasser
recommended maintenance and supervision at least four times per year.
The performance of several aerobic units of different designs has been compared with septic
tanks operating under both laboratory and field conditions.87'91 Two years' evaluation showed
aerobic-unit removals of biodegradable organic material from the waste were significantly higher
than those achieved by septic tanks; SS concentrations in all effluents were nearly identical. (See
figs. 1-20 and 1-21; table 1-13.)92 The septic tanks were more stable, however. Periodic upsets
resulted in substantial variability in aerobic-unit effluent quality.
Periodic carryover of solids was the major reason for effluent quality deterioration from aero-
bic units. Bulking sludge (sludge that will not settle), toxic chemical additions from the home, and
instability because of excessive buildups of sludge, seemed to be the most common causes of carry-
over. Several design modifications have been suggested to help prevent some operational problems,
300
100
ra
- Otis et al.87
A Bennett and Linstedt88
X Voell and Vance89
• Glasser90
20 30 40 50 60 70 80 90 95
PERCENTAGE
Figure I-20. Comparison of septic tank and aerobic unit effluent concentrations of suspended solids (SS).
98
40
-------
D)
in
Q
O
CO
600
100
10
i Septic tanks
I
I
I
I
I
1—r
- Otisetal.87
A Bennett & Linstedt88
X Voell and Vance89
o Glasser90
I
I
I
I
10 15 20
30
70
80
90
95
40 50 60
PERCENTAGE
Figure 1-21. Comparison of septic tank and aerobic effluent concentrations of biodegradable organic material.
98
Table 1-13.—Comparison of septic tank and aerobic unit effluents
BOD5, mg/l:
Mean
Coefficient of variant .
95-percent confidence interval
Range
COD (unfiltered),mg/l:
Mean
Coefficient of variant
95-percent confidence interval
Ranae
Treatmem
Aerobic units
47 (63)a
0.79
38 to 57
0 to 208
136 (69)a
0.45
121-150
26 to 349
t system
Septic tank
158(94)a
0.50
142-174
20-480
360 (97)a
0.36
335 to 386
66 to 780
41
-------
Table 1-13.--Comparison of septic tank and aerobic unit effluents—Continued
Characteristic
Treatment system
Aerobic units
Septic tank
COD (filtered), mg/l:
Mean 75 (68)a
Coefficient of variant 0.52
95-percent confidence interval 65 to 84
Range 7 to 210
TSSb,mg/l:
Mean 53 (69)a
Coefficient of variant : 0.19
95-percent confidence interval 45 to 64
Range 4 to 252
FCb:
Mean 107 (67)a
Coefficient of variant 0.32
95-percent confidence interval 74 to 153
Range 1 to 2,200
FSb:
Mean 32.9 (70)a
Coefficient 0.44
95-percent confidence interval 22.7 to 47.6
Range 1 to 1,930
Total nitrogen, mg-IM/l:
Mean 37.6 (38)a
Coefficient of variant 0.36
95-percent confidence interval 32.2 to 42
Range 15.8 to 77.6
Ammonium-N, mg-N/l:
Mean .02b (44)a
Coefficient of variant 1.10
95-percent confident interval .01 to .08
Range 0 to .08
Nitrite-nitrate-N, mg-N/l:
Mean 30.1 (46)a
Coefficient of variant 0.52
95-percent confidence interval 25.5 to 34.8
Range .3 to 71.6
Total phosphorus, mg-P/l:
Mean 35.2 (36)a
Coefficient of variant .69
95-percent confidence interval 27 to 43.5
Range 6.8 to 140
Orthophosphate, mg-P/l'
Mean 28.9 (32)a
Coefficient of variant .42
95-percent confidence interval 24.5 to 33.3
Range 6.8 to 51.2
285 (93)a
0.36
264 to 306
47 to 531
54 (93)a
0.17
47 to 62
11 to 695
4,210 (94)a
0.22
2,879 to 6,158
5 to 180,000
38.2 (97)a
0.87
20.1 to 72.4
Oto 11,200
55.3 (53)a
0.42
48.9 to 61.6
9.7 to 124.9
38.7 (63)a
0.45
34.3 to 43
.1 to 90.7
0.56b (67)a
2.63
.39 to .82
Oto 74.5
14.6 (54)a
.80
11.4 to 17.7
3.8 to 90
11.5(43)a
.38
10.2 to 12.8
20.4
aNumber of samples.
b Log-normal distribution.
42
-------
but regular servicing is necessary to ensure proper functioning.87 Inspections should be made at
least once every 2 months and excess solids pumped every 8 to 12 months.87-91
Intermittent Granular Filtration. Field experience with on-site aerobic treatment processes
indicates that additional polishing of effluents will be necessary before surface discharge to meet
current EPA effluent guidelines. Filtration appears to be one of the most promising alternatives
available for polishing. Whether the filtration is provided by granular beds or by mechanical filter
systems used as a part of the biological process or as a separate process depends on economics,
effectiveness, and maintenance requirements.
Granular filtration seems particularly well suited to on-site system design. At least three basic
flow configurations have been successfully tested in the field: the buried sand filter, the recircu-
lating sand filter, and the intermittent sand filter.
The buried sand filter is constructed below the soil surface. A bed is excavated and underdrain
collectors are installed and covered with approximately 12 inches of gravel. Normally, 24 inches of
sand (usually greater than 0.4 mm effective size with a uniformity coefficient of less than 4.0) are
placed over the gravel, followed by influent drain tile in 10 to 12 inches of gravel. The bed is cov-
ered with at least 6 inches of topsoil. Allowable wastewater loadings vary from State to State but
range between 0.75 and 1.5 gal/d/ft2, depending on appliance loading and on pretreatment. These
systems usually perform excellently unless overloaded, but the bed's inaccessibility dictates a more
conservative sizing than might be otherwise used for open systems.
The recirculating sand filter system consists of a septic tank, a recirculation tank, and an open
sand filter.93 Wastewater is dosed onto the filter by a submersible pump in the recirculation tank.
The sump pump is activated by a timer and is sized to pump 5 to 10 gallons per minute for single
households. A recirculation ratio of 4:1 (recycle to forward flow) is recommended. The recircula-
tion tank, usually the same size as the septic tank, receives flow from the septic tank and the recir-
culated portion of the filter effluent. Baffles provide proper mixing of the septic tank and filter
effluents before recycling. Filter-effluent recycled flow is controlled by a floating rubber valve
located in the filter effluent return line. When the recirculation tank is filled, filter effluent is dis-
charged from the system.
The filter bed consists of 3 feet of coarse 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 gal/
d/ft2, based on raw septic tank flow. It is estimated that approximately 1 inch of sand should be
removed once a year to avoid serious ponding conditions. After 12 inches of sand have been re-
moved, new sand should be added. Results of a household system study indicate that effluent BOD5
values average less than 5 mg/1 and TSS values less than 6 mg/1.93
In the intermittent sand filter, pretreated wastewater is applied over a bed of sand 2- to 3-feet
deep and the filtrate collected by underdrains. The sand remains aerobic and serves as a biological
filter, removing SS and dissolved organics. A summary of filter performance based on a review of
the literature is given in figure 1-22.94
Filters receiving septic tank or aerobic unit effluent have been tested under field and labora-
tory conditions. A typical filter system is depicted in figure 1-23. Of major concern in sizing inter-
mittent granular filters are the trade-offs between effluent quality and maintenance requirements
(see fig. 1-22).
The quality of sand-filtered septic tank and aerobic unit effluents appears in tables 1-14 and
1-15 for field systems operated for over 2 years.95-96 Relatively little difference is shown between
aerobic-unit, sand-filtered effluent and septic-tank, sand-filtered effluent for comparable loading
43
-------
6.0
5.0 -
CO
+-*
"D
01
4.0
o
? 3.0
Q
O
_l
O
o:
Q
2.0
1.0
Maintenance
< 1 month
Bed depth > 30 inches _
Maintenance
< 3 months
Maintenance ~2* 6 months
Maintenance | /
< 4 months I /
I /
_L
Maintenance *3* 18 months
_L
JL
_L
0.10 0.20 0.30 0.40 0.50 0.60
EFFECTIVE SIZE OF SAND, mm
0.70
0.80
Figure I-22. Trends in percent of BOD reduction and required maintenance of intermittent sand filters treating sep-
tic tank wastewater (average BOD, 94 mg/l).94
Insulated cover
S
.\ \ \
\ \\\
Vent
Splash
O plate
Sand
Pea gravel
0 Coarse
o
O
stone
Concrete slab
Collection pipe
Figure 1-23. Profile of intermittent sand filter.
44
-------
Table \-'\4.-Septic tank-sand filter effluent quality data
Parameter
BOD mg/l
TSS mg/l
Total nitrogen (N) mg/l
Ammonia-nitrogen mg/l
Nitrate-nitrogen mg/l
Total phosphorus (P) mg/l . . .
Orthophosphate-phosphorus mg/l . . .
Fecal coliforms (number per 100 ml)
Total coliforms (number per 100 ml)
Septic
tank
effluent
123
48
23.9
19.2
.3
10.2
8.7
5 9 X 105
9.0 X 105
Sand
filter
effluent
9
6
24.5
1.0
20.0
9.0
7.0
6.5 X 103
1.3X 103
Chlori-
nated
effluent
3
6
19.9
1.6
18.9
8.4
7.9
2
3
Note.—Loading rate average, 5 gal/d/ft2 (0.2 m/d; effective size, 0.45 mm; uniformity coefficient, 3.0.
Table 1-15.—Aerobic unit-sand filter effluent quality data
Parameter
BOD5 mg/l
TSS mg/l
Total nitrogen (N) mg/l
Ammonia-nitrogen mg/l . ... ....
Nitrate-nitrogen, mg/l
Total phosphorus (P) mg/l
Orthophosphate-phosphorus mg/l
Fecal coliforms Iog1 r\$l\
Fecal streptococci log10WI
Aerobic
unit
effluent
31.0
41.0
37.8
1.4
32.3
29.5
25.0
5.3
4.4
Sand
filter
effluent
3.5
9.4
34.8
0.3
33.8
20.3
18.9
4.0
3.2
Chlori-
nated
effluent
4.0
7.0
38.3
0.4
37.6
24.0
23.4
0.9
1.5
Note.—Loading rate average, 3.8 gal/day/ft2 (0.15 m/day); effective size, 0.19 mm; uniformity coefficient, 3.31.
conditions, although the aerobic system used a finer sand (0.19 mm, compared with 0.45 mm).
Such filtered effluents could meet current EPA standards for BOD and TSS but would require fur-
ther pretreatment for coliforms or phosphorus. Excellent ammonia conversion is produced by both
systems.
Filter runs depend on grain size, hydraulic loading, influent organic strength, and maintenance
techniques. There is apparently a substantial difference in clogging mechanisms in septic-tank,
effluent-loaded filters and aerobic-unit, effluent-loaded filters.83-95'96 Recommended filter opera-
tion schedules for a septic tank-sand filter system are presented in table 1-16. It is recommended
that there be two alternating filters, each designed for a hydraulic loading rate of 5 gal/d/ft2. When
one filter ponds, it can be taken out of service, raked to a depth of 2 to 4 inches, and rested before
wastewater is reapplied. After a second loading period, the top 4 inches of sand from the filter
should be replaced with clean sand.
Aerobic-unit, sand-filter systems do not normally require a second filter.96 Suggested applica-
tion rate is 5 gal/d/ft2. Removing the solids mat and 1 inch of sand and adding 1 inch of clean sand
are the only required maintenance steps. These should be done every 6 months. Wastewater can be
reapplied immediately after maintenance. Experience shows that periodic biological and hydraulic
45
-------
Table 1-16.—Septic tank-sand filter operation schedule
Effective size, mm
0.2
0.4
0.6
Uniformity
coefficient
3-4
3
1 4
Loading
and
resting
period,
months
1
3
5
upsets of the biological process can be handled by the sand filters; however, extended periods of
upset will lead to shorter filter runs.
Disinfection Alternatives. If the final effluent requires disinfecting, several alternative systems
have proved effective. The use of dry feed chlorinators will normally produce effluents that meet
EPA standards (tables 1-14 and 1-15). Unfortunately, a major problem associated with the use of
dry feed chlorinators is controlling the dose to the wastewater. High chlorine concentrations were
found periodically during extended field testing.94 There are ways to effectively control hypo-
chlorite feed, however.83 In light of the toxicity of chlorine, consideration must also be given to
dechlorination of effluent before final surface water discharge.
Initial studies with ultraviolet (UV) irradiation of sand-filtered household effluents have been
most promising. Four months of operating data with a commercial UV unit are presented in table
1-17.83 Long-term field tests are continuing with these units. One major drawback to UV irradia-
tion is high initial capital investment. As greater demand for this type of system increases, however,
costs are likely to decrease.
Iodine as a disinfectant for on-lot disposal may prove very practical. Because iodine is only
slightly soluble in water, a saturator holding iodine crystals can serve as the solution feed device.
The appropriate aliquot of feed solution added to the treated wastewater can be controlled by a
pump, electronic signal, or pressure valve. Iodine is an excellent bactericide, virucide, and cysticide.
Also, it does not combine with ammonia or react readily with other trace organics in wastewaters.
Iodine is expensive, but proper feed control should result in cost-effective disinfection for on-lot
treatment.83
Other disinfectants include bromine salts, formaldehyde, and ozone. The feeding of bromine
salts appears to be too complex for small-flows application; ozone treatment also involves relatively
complex equipment. Little experience has been gained with formaldehyde feed equipment.
Other Treatment Processes. A number of other unit processes are available for small-flow
application, but field experience is very limited. Chemical feed equipment is available, but mainte-
Table \-M.-Coliform analysis of effluents from sand filter and ultraviolet (UV) units
[N per 100 ml]
Coliform
Fecal
Total
Aerobic unit
Sand filter
1 1 to 1 3
64 to 75
Ultraviolet
< 1
<1
Septic tank
Sand filter
(2.6-4.4) X 103
(3.6-5.1) X 103
Ultraviolet
<1
<1
46
-------
nance is relatively high.91 The use of ion exchange and carbon adsorption techniques is feasible,
but maintenance and operation requirements are high, as are costs. Treatment packages using a
number of these unit operations are being field tested at the University of Wisconsin on both com-
bined and gray water effluents from households.
Evapotranspiration Systems. Evapotranspiration may provide a means of wastewater disposal
in some localities where site conditions preclude soil absorption. Evaporation 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 added moisture from
rainfall or wastewater application, ET systems can provide a simple means of liquid disposal without
danger of surface or ground-water contamination. The ET systems can also be designed to supple-
ment soil absorption in slowly permeable soils.
If evaporation is to be continuous, three conditions must be met.97 First, there must be a
continuous supply of heat to meet the latent heat requirement (approximately 590 cal/g 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 be-
tween the surface and the atmosphere. This gradient is necessary to remove the vapor by diffusion.
convection, or both. Meteorological factors, such as air temperature, humidity, wind velocity, radi
ation, and vegetative cover, influence both energy supply and vapor removal. Energy can also be
added from heat in the water itself or from biological activity. Third, there must be a continuous
supply of water to the evaporative surface. This depends on the soil's matric potential and K. The
soil material must be fine textured enough to draw up the water 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.
A typical ET bed system consists of a IVfc- to 3-foot depth of selected sand over an imperme-
able plastic liner.98 A perforated plastic piping system with rock cover is often used to distribute
septic tank effluent in the bed. The bed may be square-shaped on relatively flat land, or a series of
trenches on slopes. A cross section of a typical bed is depicted in figure 1-24. The surface area of
the bed must be large enough for sufficient ET to occur to prevent the water level in the bed from
rising to the surface. This means that annual evaporation rate must be significantly higher than
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 uniform sand in the size
J_
T
Impermeable
plastic liner
Figure 1-24. Typical evapotranspiration bed.98
47
-------
range of D50 (0.10 mm). This size sand is capable of raising water about 3 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 vegetation on the disposal field. Trees and bushes
with large silhouettes received more advective heat. When vegetation is dormant, ET is greatly
reduced. Snow cover reflects solar radiation, which reduces ET. In addition, when temperatures
are below freezing more heat is required to change frozen water to vapor. Thus, care must be taken
in selecting a site. A procedure has been outlined to estimate the maximum ET that can be ex-
pected from disposal fields east of the Mississippi River."
The Department of Civil, Environmental and Architectural Engineering at the University of
Colorado at Boulder98 is conducting a study of design parameters for nondischarging ET systems.
The study involves 28 outdoor lysimeter units, each 2 feet in diameter and 28 inches deep. Several
full-scale ET systems in use at private homes are also being monitored so that data can be corre-
lated.
Design of an ET bed is based on the local annual weather cycle. Evaporation rates are highest
during the summer, but the Colorado study shows that winter evaporation rates are extremely
important to the functioning of the system. Summer evaporation has been found to be approxi-
mately 40 percent of the pan value; winter values are about 70 percent. The average design evapo-
ration value can be established from the annual pattern, as shown in figure 1-25. This rate can be
matched with the total expected inflow based on household wastewater generation rate and on rain-
fall. A mass diagram is used to establish the storage requirements of the bed.98
Vegetative cover can increase the ET rate during the summer growing season; but if this in-
creased rate is to be used, the bed must provide additional storage in winter. Lawn grass has been
found to increase evaporation rates slightly during June, July, and August; in winter, when the
ground is bare, evaporation rates are reduced.
Alfalfa can produce an evaporation rate of 0.6 gal/d/ft2 at peak growing season. Design year-
round sewage ET rates have been found in the range of 0.04 gal/d/ft2 of bed in the Boulder, Colo.,
area.98 This results in a bed area of approximately 5,000 square feet for one 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 tran-
spiring bushes and trees are present. In practice, nondischarging ET bed systems are limited to areas
of the country where pan evaporation exceeds annual rainfall by at least 24 inches and where winter
evaporation exceeds precipitation by a value of 2 inches every month. The decrease of ET in winter
at middle and high latitudes greatly limits its use; under freezing conditions ET would be inade-
quate.99
Evapotranspiration disposal systems could work in semiarid regions, such as those in Texas,
Oklahoma, Colorado, New Mexico, Utah, Arizona, California, and Nevada. Even in these areas,
household water conservation should always be considered as part of the system.98
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
-------
LU
Q c
DC
LU
<
•5- LU
UJ I
LJJ tfl
tr >
30
20
10
Capillary
moisture
only
Capillary
moisture
only
_L
Jan. Feb.
Mar.
Apr. May June
July
Aug. Sept.
0.20
to
LU
tr
o
z
0 TJ
< 0)
O -°
-1 •&
it
< ^
cc
O
Q.
0.15
0.10
0.05
0.00
Pan evaporation
Total loading (sewage and rainfall)*
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Figure I-25. Evapotranspiration bed water-balance characteristics.
rainfall precludes the use of soil-based ET systems, mechanical devices might be applied. The ulti-
mate value of this approach depends on the cost of commercially manufactured units.
The kind of unit being evaluated in the prototype studies at the University of Colorado is a
multiple, concentric disc system rotating on a common shaft. Preliminary studies are under way to
determine optimum rotation speed and disc size, submergence, material, and surface characteristics.
49
-------
ALTERNATIVE SELECTION
The choices available to the individual for wastewater disposal are numerous, yet only a few
will prove economically and environmentally acceptable. The selection process should involve eval-
uation of technical, economic, and administrative feasibility. All three are extremely important to
the successful execution of a project.
Table 1-18 lists some options available. It is first necessary to evaluate design constraints, such
as soil types, site topography, geological characteristics, climate, and water-quality objectives. Once
Table \-18.-On-sitesystem alternatives
Process
A. In-house:
1. Water conservation:
a. Flow control
b. Reuse
2. Waste segregation:
a. Non-water-carriage toilet:
(1) Chemical
(2) Biological
(3) Recycle
(4) Incinerating
Very low flow toilet . . .
b. Gray-water treatment . .
3. Household product selection
4. Appliance selection
B. Wastewater carriage:
1. Gravity sewer
2. Small-diameter gravity sewer
3. Pressure sewer
4. Vacuum sewer
C. Anaerobic:
1. Septic tanks
2. Fixed media:
a. Sand/granular
b. Synthetic media
D. Aerobic:
1. Suspended growth:
a. Activated sludge:
(1) Batch
(2) Continuous
b. Lagoons
2. Fixed media:
a. Sand-intermittent filter . . .
b. Sand expanded
c. Coarse media:
(1) Trickling filter
(2) Rotating disks
(3) Tray/media contractors
3. Emergent vegetation
Objective
Reduce water
Reduce pathogens, flow, BOD, N, P, solids
Reduce P, solids, BOD, pathogens
Control waste products, P, toxic metals, chlorinated compounds
Reduce flow, waste constituents
Carry wastewater
Reduce solids, grease, pathogens?
Reduce solids, BOD, pathogens, N
Reduce solids, BOD, pathogens?
Reduce solids, BOD, nitrification, pathogens
Reduce solids, BOD, pathogens
Reduce solids, BOD, pathogens, nitrification
Reduce solids, BOD, pathogens, nitrification
Reduce N, P, BOD, pathogens
50
-------
Table l-18.-O/?-s/fe system alternatives—Continued
Process
Objective
E. Physical-chemical:
1. Adsorption
2. Ion exchange
3. Chemical precipitation:
a. Suspended
b. Fixed
4. Disinfection:
a. Halogens
b. Ultraviolet unit . . .
c. Ozone
5. Holding tank
F. Land application:
1. Soil absorption
2. Mounds
3. Irrigation
4. Lagoons (adsorption)
5. Evapotranspiration . .
Polish organics
NH3,M+
Reduce P, solids, BOD, pathogens
Reduce pathogens
Storage
Dispose solids, BOD, pathogens, P, nitrification
these have been delineated, an orderly selection of options can be evaluated through cost analysis.
Finally, an appropriate institutional framework must be developed to ensure appropriate construc-
tion, operation, and maintenance of the system. Sample flowsheets are depicted in table 1-19.
Although many of these systems have been proved technically feasible, extensive field testing to
determine process reliability and effectiveness of institutional controls is still needed.
Costs of Alternative On-Site Treatment and Disposal Systems
The costs of alternative on-site treatment and disposal systems are not well documented, owing
to an insufficient data base. Unit costs, based primarily on Wisconsin studies, are presented in table
1-20. These costs are estimates; systems should be evaluated on a site-by-site basis.
WISCONSIN SMALL SCALE WASTE MANAGEMENT PROJECT
In 1971, the State of Wisconsin provided research funds to the University of Wisconsin to com-
mence investigations into the on-site disposal of wastewater. The SSWMP 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 construction for on-site dis-
posal of wastewater; to search out effective alternatives 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 USP plan-
ning.
Since then, substantial funding also has been provided by the Wisconsin Department of Natural
Resources, the Upper Great Lakes Regional Commission, and the United States Environmental Pro-
tection Agency. A major portion of Part I of this publication is based on research efforts conducted
51
-------
Table 1-19.—Sample flowsheets
Environmental design constraints
Good soil:
Single home
High bedrock:
Single home
Poor soil topography:
Surface water with BOD 10 mg/l, SS 10 mg/l, FC < 2007
100:
Single home
Lake with BOD 10, SS 10, M< 1, P < 1, FC <
100/100:
Single home
Surface water with BOD 10 mg/l, SS 10 mg/l, FC < 200/
100:
Small community
Process3
C1-F1
C1-F2
or
A1a-C1-D2aE4b
A1a-D1-D2a 4a
or
A1a-C1-D2aE4b
A1-C1-D2a-C2a-E3a 4b
or
A2b-C1-E2-D2a-E3bE4a
A1a-C1-B2-F1
or
A2b-(E-5)-C1-B3-D1-D2aE4b
Cost
Low
High
High
High
High
High
High
Moderate
Moderate6
Maintenance
Low
Low
High
High
High
High
High
Moderate13
Moderate13
Reliability
Excellent if properly
constructed and maintained
Excellent if properly
constructed and maintained
Data unavailable
Data unavailable
Data unavailable
Data unavailable
Data unavailable
Depends on institutional
control
Depends on institutional
control
en
tsD
aSee also table 1-18.
bRelative to centralized system.
-------
Table 1-20.— Capital costs and operation and maintenance costs
Unit
Cost, dollars
Septic tank (1,000 gal):
Installed cost 350 to 450
Operation cost per year 10
Aerobic:
Installed cost 1,500 to 2,500
Maintenance per year 65 to 110
Power per year, 2.4 to 7.4 kwh/d (@ 4d/kwh) 35 to 60
Sand filter-
Installed cost 15 to 20/ft2
Maintenance per year 1/ft2
Pump chamber:
Installed cost:
Chamber 200 to 250
V2-hp sump pump and controls 300 to 350
Operation cost3 per year, 0.1 kwh/d @ 4d per kwh 21
Chlorine and settling chamber:
Installed cost 1,000 to 1,200
Operation cost3 per year (chemical) 26
Ultraviolet irradiation:
Installed cost 1,100 to 1,500
Power, 1 Vi kwh/d @ M per kwh 20
Maintenance per year 40 to 60
Soil absorption system:
Installed cost 1 to 1.25/ft2
Maintenance per year —
Mound:
Installed cost 2,500 to 4,000
Maintenance per year 18 to 25
Power per year 2
Evapotranspiration:
Installed cost 75
-------
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
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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54
-------
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55
-------
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38W. G. Walker, J. Bouma, D. R. Keeney, and P. G. Olcott, "Nitrogen Transformation During
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56
-------
40J. Beek and F. A. M. de Haan, "Phosphate Removal by Soil in Relation to Waste Disposal,"
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44 J. Bouma, "Evaluation of the Field Percolation Test and an Alternative Procedure to Test
Soil Potential for Disposal of Septic Tank Effluent," Soil Sci. Soc. Am. Proc., 35, 6, 871-875, Nov.-
Dec., 1971.
45K. A. Healy and R. Laak, "Factors Affecting the Percolation Test,"./. Water Pollut. Cont.
Fed., 45, 7,1508-1516, July 1973.
46J. Bouma, D. I. Hillel, F. D. Hole, and C. R. Amerman, "Field Measurement of Unsaturated
Hydraulic Conductivity by Infiltration Through Artificial Crusts," Soil Sci. Soc. Am. Proc., 35, 2,
362-364, Mar.-Apr. 1971.
47J. Bouma and J. L. Denning, "Field Measurement of Unsaturated Hydraulic Conductivity by
Infiltration Through Gypsum Crusts," Soil Sci. Soc. Am. Proc., 36, 5, 846-847, Sept.-Oct. 1972.
48 J. Bouma, F. G. Baker, and P. L. M. Veneman, "Measurement of Water Movement in Soil
Pedons Above the Water Table," Geological and Natural History Survey Information Circular No.
27, University of Wisconsin, Madison, Wis., 1974.
49 F. G. Baker and J. Bouma, "Measurement of Soil Hydraulic Conductivity and Site Selection
for Liquid Waste Disposal," paper presented at the Second National Conference on Individual On-
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51 R. E. Machmeier, "Design Criteria for Soil Treatment Systems," paper presented at the Amer-
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52W. A. Ziebell, D. H. Nero, J. F. Deininger, and E. McCoy, "Use of Bacteria in Assessing Waste
Treatment and Soil Disposal Systems," Home Sewage Disposal, Proc. Am. Soc. Ag. Eng., 175, Dec.
1974.
53So«7 Survey Manual, U.S. Department of Agriculture Handbook, No. 18, Washington, D.C.,
1951.
54M. J. Vespraskas and J. Bouma, "Model Experiments on Mottle Formation Simulating Field
Conditions," Geoderma, 15, 217-230, 1976.
55 M. J. Vespraskas, F. G. Baker, and J. Bouma, "Soil Mottling and Drainage in a Mollic Hapludalf
as Related to Suitability for Septic Tank Construction," Soil Sci. Soc. Am. Proc., 38, 3, 497-501,
May-June 1974.
57
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56 J. Bouma, "Subsurface Application of Sewage Effluents." In Land Use Planning, edited by M.
T. Beatty and G. W. Peterson, Ch. 30, Section IX, American Society of Agronomy, 1977.
57M. D. Witt, "Water Use in Rural Homes," Small Scale Waste Management Project, University
of Wisconsin, Madison, Wis., 1974.
58M. D. Witt, R. Siegrist, and W. C. Boyle, "Rural Household Waste Characterization," Ho me
Sewage Disposal, Proc. Am. Soc. Ag. Eng., 175, Dec. 1974.
59 R. Siegrist, M. D. Witt and W. C. Boyle, "The Characteristics of Rural Household Wastewater,"
J. Environ. Eng. Div., Am. Soc. Civ. Eng., 102, EE 3, 533-548, June 1976.
60J. C. Converse, J. L. Anderson, W. A. Ziebell, and J. Bouma, "Pressure Distribution to Im-
prove Soil Absorption Systems," Home Sewage Disposal, Proc. Am. Soc. Ag. Eng., 175, Dec. 1974.
61 J. Bouma, J. C. Converse, J. Carlson, and F. G. Baker, "Soil Absorption of Septic Tank Efflu-
ent in Moderately Permeable Fine Silty Soils," Transactions, Am. Soc. Ag. Eng., 18, 1094-1100,
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62J. C. Converse, "Distribution of Domestic Waste Effluent in Soil Absorption Beds," Trans-
actions, Am. Soc. Ag. Eng., 17, 2, 299-304,1974.
63 R. J. Otis and J. Bouma, "Notes on Soil Absorption Field Construction for Septic Tank Sys-
tems," Small Scale Waste Management Project, University of Wisconsin, Madison, Wis., June 1973.
64 J. Bouma, "Using Soil for Disposal and Treatment of Septic Tank Effluent Following the
Current Health Code," Small Scale Waste Management Project, University of Wisconsin, Madison,
Wis., 1974.
65J. Bouma, "Innovative On-Site Soil Disposal and Treatment Systems for Septic Tank Efflu-
ent," Home Sewage Disposal, Proc. Am. Soc. Ag. Eng., 175, Dec. 1974.
66J. C. Converse, 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, University of Wisconsin, Madison, Wis., Apr. 1975.
67J. Bouma, 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," J. Environ. Quality, 4, 3, 382-388, July-Sept. 1975.
68J. Bouma, 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 Management for Waste Management, Ottawa, Canada, Oct. 1973.
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Type Disposal Systems for Septic Tank Effluent. II. Nutrient Transformations and Bacterial Popu-
lations," J. Environ. Quality, 3, 3, 228-234, July-Sept. 1974.
70 J. C. Converse, R. J. Otis, and J. Bouma, "Design and Construction Procedures for Fill Sys-
tems in Permeable Soils With Shallow Creviced or Porous Bedrock," Small Scale Waste Management
Project, University of Wisconsin, Madison, Wis., Apr. 1975.
71 J. C. Converse, R. J. Otis, and J. Bouma, "Design and Construction Procedures for Fill Sys-
tems in Permeable Soils With High Water Tables," Small Scale Waste Management Project, Univer-
sity of Wisconsin, Madison, Wis., Apr. 1975.
58
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72J. L. Anderson and J. Bouma, "Sizes of Subsurface Trenches and Associated Drains Deter-
mined by the Dupuit-Forchheimer Approximation," Small Scale Waste Management Project, Uni-
versity of Wisconsin, Madison, Wis., Apr. 1975.
73E. L. Decoster, "The Hydrodynamics of an Artificial Groundwater Mound Developed as Part
of a Subsurface Waste Disposal System," master's thesis, Department of Civil and Environmental
Engineering, University of Wisconsin, Madison, Wis., 1976.
74S. Cohen and H. Wallman, "Demonstration of Waste Flow Reduction from Households,"
Environmental Protection Technology Series, EPA-670/2-74-071, U.S. Environmental Protection
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ment," Completion Report Series No. 66, Environmental Resource Center, Colorado State Univer-
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to Facilitate On-Site Surface Disposal," Small Scale Waste Management Project, University of Wis-
consin, Madison, Wis., Nov. 1976.
77
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Engineering Experiment Station, Ames, Iowa, Aug. 1916.
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The Treatment of Sewage from Single Homes and Small Communities," Public Health Bulletin,
No. 101, U.S. Public Health Service, Washington, D.C., 1920.
83Small Scale Waste Management Project, Final Report to the U.S. Environmental Protection
Agency, University of Wisconsin, Madison, Wis. (in preparation).
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85Ohio State University, "A 23-Month Study of Individual Household Aerobic Sewage Treat-
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59
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88E. R. Bennett and K. O. Linstedt, "Individual Home Wastewater Characterization and Treat-
ment," Completion Report Series No. 66, Colorado State University, Fort Collins, Colo., 1975.
89 A. T. Voell and R. A. Vance, "Home Aerobic Wastewater Treatment Systems—Experience in
a Rural County," Ohio Home Sewage Disposal Conference, Ohio State University, Columbus, Ohio,
Jan. 1974.
90M. B. Glasser, "Garrett County Home Aeration Wastewater Treatment Project," Bureau of
Sanitary Engineering, Maryland State Department of Health and Mental Hygiene, Baltimore, Md.,
1974.
91N. J. Hutzler, "Evaluation of On-Site Wastewater Treatment Processes Receiving Controlled
Simulated Wastewater," master's thesis, independent study report, Department of Civil and Envi-
ronmental Engineering, University of Wisconsin, Madison, Wis., 1974.
92R. J. Otis, W. C. Boyle, and D. K. Sauer, "The Performance of Household Wastewater Treat-
ment Units Under Field Conditions," Home Sewage Disposal, Proc. Am. Soc. Ag. Eng., 175, Dec.
1974.
93M. Hines, "The Recirculating Sand Filter; a New Answer for an Old Problem," Proceedings of
the Illinois Private Sewage Disposal Symposium, Champaign, 111., Sept. 29-Oct. 1, 1975.
94 D. K. Sauer, "Intermittent Sand Filtration of Septic Tank and Aerobic Unit Effluents Under
Field Conditions," master's thesis, Department of Civil and Environmental Engineering, University
of Wisconsin, Madison, Wis., 1975.
95D. K. Sauer, W. C. Boyle, and R. J. Otis, "Intermittent Sand Filtration of Household Waste-
water Under Field Conditions," J. Environ. Eng. Div., Am. Soc. Cir. Eng., 102, EE 4, 789-803, Aug.
1976.
96D. K. Sauer, "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, Mich., Nov. 16-18, 1976.
97D. Hillel, Soil and Water Physical Principals and Processes, New York: Academic Press, 1971.
98Personal communication, E. R. Bennett, Department of Civil and Environmental Engineering,
University of Colorado, Boulder, Colo., 1977.
99 C. B. Tanner and J. Bouma, "Influence of Climate on Subsurface Disposal of Sewage Efflu-
ent," Proceedings of the Second National Conference on Individual On-Site Wastewater Systems,
National Sanitation Foundation, Ann Arbor, Mich., Nov. 5-7,1975.
60
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Part II
SEPTAGE TREATMENT AND DISPOSAL
GENERAL
The first priority of the EPA's program to abate water pollution has been to provide adequate
wastewater treatment for sewered communities. According to the 1970 census,1 however, 16.6
million housing units, or more than 24.5 percent of the housing units in the United States, relied on
septic systems for wastewater disposal.
Users
The geographical distribution of septic systems, as seen in figure II-l and table II-l, shows that
States with more than 35 percent use are located in New England, the Southeast, and the Pacific
Northwest. Most North-Central, Northeastern, and Southeastern States have only a slightly lower
use of these on-site disposal facilities. The Southwestern States' use of septic tanks is between 10
Percent of
households using
septic tanks
E
-------
Table 11-1 .—Sewage disposal characteristics for the United States3'"1
Alabama . .
Alaska .
Arizona . . .
Arkansas
California
Colorado
Connecticut
Delaware
Washington, D.C
Florida
Georgia . .
Hawaii
Idaho
Illinois . .
Indiana
Iowa .
Kansas
Kentucky .
Louisiana
Maine .
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Montana .
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico . .
New York
North Carolina
North Dakota
Ohio . .
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Utah
Vermont
Housing units on
public sewers
Number
566,307
55,511
446,304
355,684
6,084,632
612,659
608,603
130,259
277,068
1,509,682
848,516
161,438
137,891
3,072,266
1,060,942
662,320
594,758
536,388
778,247
169,975
953,470
1,339,304
1,947,137
864,984
338,581
1,173,688
154,581
385,860
147,743
132,475
1,890,977
230,737
4,824,525
733,848
128,967
2,565,317
686,240
448,967
2,798,522
197,947
363,611
140,258
671,248
2,989,684
258,649
72,264
Percent
of total
50.80
62.69
77.11
52.85
87.22
82.47
62.82
74.44
99.52
60.61
57.85
74.78
57.87
83.20
61.97
69.35
75.52
50.57
67.90
50.11
77.23
72.83
68.43
70.92
48.56
70.47
64.21
75.44
86.07
53.26
82.03
71.60
98.34
45.32
64.32
74.41
73.17
61.04
72.13
64.41
45.18
63.30
51.76
78.49
82.93
48.23
Housing units with
septic tanks
Number
385,345
18,629
114,433
220,287
853,013
113,290
354,585
39,860
454
938,352
474,455
50,558
93,146
554,603
589,794
257,889
163,918
312,856
287,481
140,409
243,728
490,365
847,433
307,441
209,115
359,278
74,198
105,320
21,988
109,015
404,241
65,781
1,289,253
687,572
53,074
779,510
203,174
275,944
985,014
107,544
334,210
62,366
457,008
654,283
49,249
68,265
Percent
of total
34.56
21.03
19.77
32.73
12.23
15.25
36.60
22.78
0.16
37.67
32.35
23.42
39.09
15.02
34.45
27.00
20.82
29.50
25.08
41.39
19.74
26.67
29.78
25.21
30.00
21.57
30.82
20.59
12.81
43.83
17.53
20.42
20.93
42.46
26.47
"22.61
21.66
37.52
25.39
34.99
41.53
28.14
35.24
17.18
15.79
45.56
Housing units with
other
Number
163,139
14,423
18,013
96,999
38,324
16,689
5,633
4,870
871
42,743
143,654
3,844
7,266
65,080
61,061
34,829
28,808
211,328
80,245
28,817
37,271
9,120
50,509
47,070
149,514
132,617
11,974
20,266
1,951
7,231
10,123
25,722
44,883
197,859
18,457
102,566
48,413
10,559
96,502
1,843
106,996
18,970
168,672
164,950
3,976
9,315
Percent
of total
14.63
16.29
3.11
14.41
0.55
2.25
0.58
2.78
0.31
1.72
9.79
1.78
3.05
1.76
3.57
3.65
3.66
19.93
7.01
8.50
3.02
0.50
1.78
3.86
21.44
7.96
4.97
3.96
1.13
2.91
0.44
7.98
0.73
12.22
9.21
2.98
5.16
1.44
2.48
0.60
13.29
8.56
13.0C
4.32
1.2£
6.21
62
-------
Table 11-1 .—Sewage disposal characteristics for the United States3-^ —Continued
State
Virginia ....
Washington
West Virginia
Wisconsin
Wyoming ...
Total
Housing units on
public sewers
Number
906,030
786,551
304,151
994,926
86,983
48,187,675
Percent
of total
71.02
65.28
51.31
70.26
75.94
71.18
Housing units with
septic tanks
Number
408,213
403,909
187,028
371,567
23,349
6,601,792
Percent
of total
27.49
33.52
31.55
26.24
20.38
24.52
Housing units with
other
Number
170,580
14,464
101,600
49,549
4,217
2,904,375
Percent
of total
11.49
1.20
17.14
3.50
3.68
4.30
aTotal housing units = 67,693,842.
and 20 percent. On a local level, many counties in New Jersey, New York, California, and other
States have over 50,000 housing units that use on-site waste disposal systems, but statewide use
appears less significant. Areas with over 100,000 housing units using on-site waste disposal systems
include suburban New York, Los Angeles, and Miami.2
The use of a septic system requires periodic maintenance that includes pumping out the
accumulated scum and sludge, which is called septage. Kolega3 has reported a septage buildup of
between 65 and 70 gallons per capita per year in properly functioning septic systems.
Various recommendations exist for time periods between pumping out a septic tank, most
between 2 and 5 years. After a hauler pumps out a homeowner's septage, it must be disposed of in
a safe, cost-effective, and convenient manner. Table II-2 shows the estimated statewide septage
generation per year, based on pumping the average 1,000-gallon septic tank every 4 years.
Septage Characteristics
Septage is a highly variable anaerobic slurry having large quantities of grit and grease; a highly
offensive odor; the ability to foam; poor settling and dewatering abilities; high solids and organic
content; and, quite often, an accumulation of heavy metals. Tables II-3, II-4, and II-5 present the
results of previous research work compiled by EPA's Cincinnati research group,4 as well as extreme
values reported in the literature.
Graner6 reports septage in Nassau and Suffolk (N.Y.) counties with characteristics similar to
medium to strong wastewater; in Maine Goodenow9 found samples with total solids (TS) and sus-
pended solids (SS) over 130,000 mg/1 and 93,000 mg/1, respectively. In Alaska, Tilsworth7 ob-
tained septage samples with 5-day biochemical oxygen demand (BOD5) over 78,000 mg/1 and
chemical oxygen demand (COD) over 700,000 mg/1. The EPA mean concentrations are good indi-
cators of septage concentrations when compared with other researchers' data.
The geometric mean heavy-metal content of residential septage from Lebanon, Ohio, was com-
pared with geometric means found in raw and digested sludge from 33 U.S. sewage treatment plants
and with metal content in Danish and Swedish sludge (see table II-5). On an mg/kg dry-weight
basis, domestic septage contains one-half to two orders of magnitude less heavy metal than does
municipal sludge.4
63
-------
Table \\-2.-Estimatedhouseholdseptagegeneration by State3'1
[Millions]
State
Alabama
Alaska
Arizona ....
Arkansas
California
Colorado
Connecticut
Delaware
Washington, D.C
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky . . .
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
m3/yr
0.36
0.02
0 11
0 21
0.81
0 11
0.34
0 00
0.00
0.89
0.45
0 05
0.09
0 52
0.56
0.24
0.16
0.30
0 27
0 13
0 23
0 46
0.80
0 29
020
0 34
gal/yr
96.3
4.7
28 6
55 1
213.3
28 3
88.6
1 0
0.11
234.6
118.6
12 6
23.3
138 7
147.4
64.5
41.0
78.2
71 9
35 1
609
122 6
211.9
76.9
52.3
89 8
State
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyoming
Total .
m3/yr
0.07
0 10
002
0 10
0.38
006
1.22
0 65
0.05
0 74
0.19
0 26
0.93
0 10
0.32
0.06
0.43
0.62
0.05
0.06
0 39
0 38
0.18
0.35
0.02
15.67
gal/yr
18 5
26.3
5 5
27 3
101.1
164
322.3
171 9
13.3
1949
50.8
690
246.3
26 9
83.6
15.6
114.3
163.6
12.3
17.1
102.1
101 0
46.8
92.9
5.8
4 141.91
aBased on pumping a 1,000-gallon septic tank every 4 years.
Bacteriology
Septage contains predominately gram-negative, nonlactose fermenters. Many of these micro-
organisms, such as Pseudomonas, are considered aerobic and have been found in septic tanks.
Numerous obligate anaerobes are present; but only spore-forming types, including Clostridium
lituseburence and Clostridium perfringens, have been recovered. Calabro12 was unsuccessful at
isolating non-spore-forming obligate anaerobes, such as bacteriodes; because they are exceedingly
oxygen-sensitive, the pumping operation may expose anaerobes to incident oxygen, thereby killing
them. Figure II-2 compares specific types of microorganisms from 12 septage and septic tank sew-
age samples, with 95-percent confidence limits. The standard plate count per millilitre was deter-
mined after 48 hours incubation under aerobic and anaerobic conditions at 24° C ± 1°. When the
septic tank is pumped, sludge, intermediate wastewater, and upper layer of scum are mixed, yielding
aerobes and anaerobes.
The presence of aerobes in a septic tank can be explained by either the dissolved oxygen of the
incoming sewage providing sufficient oxygen to allow limited aerobic growth, or by chemostatic
displacement of effluent by the influent furnishing a relatively constant number of aerobic micro-
organisms. It is fortunate that Pseudomonas and similar aerobic bacteria are found in the septic
tank, as they add limited lipid and detergent degradation capabilities.
64
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Table 11-3.— Septage characteristics3'*
Parameter
Total solids
Total VS
Total SS
VSS .
BOD5
COD
TOC
TKN
NH3-(N)
NOn
^2
NOo
J
Total P .
PO,
4
Alkalinity
Grease
pH (units)
LAS
EPA mean
concentra-
tion
40,000.0
26,000.0
15,000.0
518 1000
5,000.0
45 000.0
1 5 000 0
600.0
1500
50.7
53.2
150 0
564.0
51 0200
9561 0
6 to 9.0
1500
Minimum
reported
6 1,1 32.0
54,500.0
7310.0
53 660.0
6440.0
7 500.0
43160
4 66.0
46.0
8<0.1
8<0.1
5200
5 10.0
7 522 0
4604 0
6 1.5
4 110.0
Maximum
reported
9 130,475.0
971,402.0
993 378.0
1051 500.0
778,600.0
7 703 000.0
896 000 0
5 1,900.0
8 380.0
81.3
1111.0
4760 0
5170.0
74 1900
423 368 0
612.6
4200.0
Varia-
bility6
115
16
301
14
179
469
73
29
63
13
110
38
17
8
39
8
2
aAII values in mg/l, except where noted.
Values represent ratio of maximum to minimum values.
Table 11-4.—Septage metal concentrations3'*
Metal
Al
As
Cd
Cr
Cu
Fe
Hg
Mn
Ni
Pb
Se
Zn
EPA mean
concentra-
tion
50 0
0 1
0 5
1 0
8 5
200 0
0 1
50
1 0
2 0
0 1
50 0
Minimum
reported
420
4003
4005
4030
80 30
430
40 0002
4050
40 20
41 50
40 02
833 00
Maximum
reported
4 200 0
405
410 8
83 0
4340
4750 0
440
4 32 0
828 0
431 0
403
4 153 0
Varia-
bility6
100
17
21R
10
1 13
2RO
20 000
RA
140
01
1C
K
aAII values in mg/l.
Values represent maximum to minimum values.
Calabro12 estimated the gross relative stability of septage, septic tank sewage, and domestic
wastewater using methylene blue as a redox indicator of biological activity. Septage samples
changed color in 5 hours, septic tank sewage in 6 to 21 hours, and raw domestic sewage in 17 to 21
hours.
65
-------
Table 11-5.— Heavy metal content of septage and municipal sludge3'
Metal
Cd
Cr
Cu
Hg
Mn
Ni
Zn
Lebanon,
Ohio
5.5
21 0
28.1
0.24
106.0
28.5
1,280.0
Sal otto
43.0
1 050 0
1,270.0
6.5
475.0
530.0
2,900.0
Other
U.S.
69.0
840.0
960.0
28.0
400.0
240.0
2,600.0
Denmark
10.0
110.0
340.0
7.8
350.0
37.0
2,600.0
Sweden
9.3
170.0
670.0
5.8
400.0
65.0
1,900.0
aAli values in mg/kg dry solids.
A Septage
B Septic-tank sewage
P 8-
1 ?-
O
01 6-
5J 5-
1 4-
1 3-
£
w 2-
o
o"
g 1 -
i — i
j — i 95 percent confidence limits
r^
- — .
1
—
r—\
<
i
...
—
- —
r-T
„.
~
—
...
r-n
i
...
r—i
<
...
ABAB AB AB AB AB
AEROBIC ANAEROBIC SYNTHETIC E.coli LACTOSE NONLACTOSE
FERMENTERS FERMENTERS
Figure 11-2. Comparison of specific types of micro-organisms, with 95 percent confidence limits (after Calabro).
LAND DISPOSAL
Septage disposal on land can include surface spreading and sub-sod injection (SSI), spray irriga-
tion, trench and fill, sanitary landfills, and lagooning. All land disposal alternatives require analyses
of soil characteristics, seasonal groundwater levels, neighboring land use, groundwater and surface
water protection and monitoring, climate, and site protectors such as signs and fences.
Land spreading requires a knowledge of land slopes, which are often limited to 8 percent, and
of runoff conditions. Other requirements may include storage facilities for times when land applica-
tion is inadvisable, crop management techniques, odor control procedures, and loading criteria.
Loading criteria generally are determined by agricultural considerations that result in the limiting of
organic and heavy metals.
66
-------
Loading Factors
Nitrogen. In most agricultural areas, available existing nitrogen (N) is far below levels needed
for optimum crop yield. As a result, artificial sources of nitrogen, such as commercial fertilizer, are
generally added. Nitrogen is available as a plant nutrient in the form of the ammonium ion, which
is retained on negatively charged soil particles.13 Septage is rich in available ammonia, with about
25 percent of the total 5 to 8 lb/1,000 gal of nitrogen occurring in this form. Soil bacteria will
transform NH3-N to NO3-N, but much of this nitrogen may not be available for plant use if
hydraulic loadings cause the highly soluble NO3-N to be leached below plant roots. Nitrogen also
may be lost if poor drainage conditions exist. This causes rapid denitrification, converting nitrate to
nitrogen gas.
Nitrogen must be applied at rates less than or equal to plant nitrogen uptake requirements.
Otherwise, excess nitrates could form and contaminate groundwater or surface water through leach-
ing or runoff. Nitrate concentrations above 10 mg/1 in drinking water may cause health problems,
particularly infant methemoglobinemia (nitrate cyanosis). Nitrate pollution in surface waters also
will lead to accelerated eutrophication of lakes and streams.
Maine has reported in "Maine Guidelines for Septic Tank Sludge Disposal on the Land"14 that
a loading criteria of 62,500 gallons per acre per year on well-drained soils and 37,500 gallons per
acre per year on moderately well-drained soils will not result in pollution caused by excess nitro-
gen. These loadings result in an application of 500 pounds per acre per year in well-drained soils
and 300 pounds per acre per year in moderately well-drained soils. Maine officials report that wells
monitored at sites that follow these criteria show no signs of pollution.
Phosphorus and Potassium. Both phosphorous and potassium are basic requirements for plant
growth. Land application of septage usually results in a phosphorous surplus and a potassium defi-
ciency. Both elements, however, tend to become fixed in the soil and are not likely to leach out.
For this reason, nitrogen requirements usually govern the organic considerations in application
rates.
Heavy Metals. The phytotoxic metals—zinc (Zn), nickel (Ni), and copper (Cu) and cadmium
(Cd) are foliage-limiting factors in the amount of sludge that may be applied to the land. How these
metals are retained in the soil is a complex and poorly understood process, but workable estimates
of limits based on soil cation exchange capacity (CEC) have been proposed by researchers in Wis-
consin.13
The CEC can be estimated by a displacement procedure that yields an exchange capacity in
milliequivalents (meq) per 100 grams of soil. A lifetime application has been proposed13 that
would limit the amounts of phytotoxic metal applied in terms of Zn equivalent. Further research
on lifetime metal-loading limits is underway, as some heavy metals seem to become tied up in the
soil over time. This is the result of a reversion effect linked with a solid state diffusion of metal into
crystalline soil structures. Attenuation of the effects of phytotoxic metals in sludges when they are
overapplied to the land may be attributed to this mechanism.
The Wisconsin metal-loading criterion limits the Zn equivalents to 10 percent of the CEC,
based on Cu being twice as toxic as Zn and Ni four times as toxic as Zn. Other researchers have
proposed relative toxication ratios other than 1:2:4.
The calculation of permitted lifetime loading of metal from septage is expressed as:13
ML =
67
-------
where:
ML = maximum loading to soil, tons of sludge per acre
CEC = cation exchange capacity of soil, meq/100 g
Zn = zinc content of sludge, mg/1
Cu = copper content of sludge, mg/1
Ni = nickel content of sludge, mg/1
Cadmium presents a special problem because of its mobility and its potential for accumulating
in the edible portions of plants. Effects are cumulative and insidious. For example, excessive ali-
mentary cadmium intake manifests itself in humans as Itai-Itai (Ouch-Ouch) disease.15 This hazard
is endemic to the Jintsu River region in Japan. The affected area is situated below the Kamioka
mine. High concentrations of cadmium, lead, and zinc have been traced to the mine's effluent,
which drains into the Jintsu River. The area has shown high concentrations of cadmium in rice,
fish, and river water. Drainage from the mine varied from 0.005 mg/1 to 0.6 mg/1 Cd at a pH of 7 to
8. Farther downstream, the river water contained almost no cadmium, yet suspended material had
concentrated the cadmium, showing a concentration of 363 pm to 382 pm. Solids had carried over
into the rice paddies where rice roots concentrated the cadmium.16 Rice roots were analyzed and
found to contain up to 1,300 pm Cd.17 People complaining of renal dysfunctions were diagnosed
as having Itai-Itai disease. Other symptoms include advanced skeletal deformations and weakened
bones that fracture easily. Friberg16 hypothesized that bone structure weakness was caused by
cadmium having replaced calcium in bone material.
One recommendation for Cd limits is based on Wisconsin findings that 2 pounds or more per
acre showed a significant increase in metal concentration in plants over control plants. The pro-
posed limits are 2 pounds per acre per year and a lifetime loading of 20 pounds per acre.13
The proposed limits of phytotoxic metals and cadmium are reported to be low enough to pro-
tect reasonably well-chosen disposal sites. Based on Lebanon, Ohio, septage and Salotto findings, as
shown in table II-5, approximately eight times as much septage as municipal sludge could be applied
to the land, using cadmium as the limiting factor. Using the phytotoxic metals limit, approximately
five times as much septage could be applied.4
Metal-Loading Calculation.13 A sample metal-loading calculation for septage application rates
based on a combination of phytotoxic metals and cadmium, again assuming average Lebanon,
Ohio, septage and a soil CEC of 10 meq/lOOg, follows.
Septage Concentration:
Zn = 50 mg/1
Cu = 8.5 mg/1
Ni = 1.0 mg/1
Cd = 0.5 mg/1
Total metal equivalent loading:
65 X CEC = 650 Ib per acre
Septage metal equivalent per ton:
50+ 2(8-5) +4(1.0) = Si? = °-142 Ib metal equivalent per ton septage
oUu ouu
68
-------
Lifetime loading permitted:
650
0.142 = 4,577.5 tons per acre
Yearly loading limit due to Cd:
- prr ~ » c = 2,000.0 tons septage per acre for 2 Ib Cd
pm Ld 0.5
Lifetime Cd loading permitted :
20 Ib per acre X ' = 20,000.0 tons per acre
Cadmium loading, therefore, is limiting on a yearly basis (2,000.0 tons per acre per year), and
phytotoxic metal equivalents are limiting on the lifetime of the site (4577.5 tons per acre per year).
The yearly loading based on cadmium of 2,000 tons per acre per year translates to 0.469 mil-
lion gallons per acre per year, or 4.8 times the application rate based on the limiting nitrogen load-
ing in this example of 500 pounds per acre per year. A well-drained site receiving this septage
would have a phytotoxic metal-loading lifetime of 11 years at the nitrogen application rate.
Pathogens. The natural digestion process in a septic tank does not always result in a pathogen-
free material, as related by Calabro,1 2 who found salmonella and other potentially dangerous orga-
nisms in septage. For this reason, care must always be taken in handling it.
Evidence that pathogens are reduced when septage is exposed to atmospheric conditions is
based on work by the Metropolitan Sanitary District of Greater Chicago and others. Table II-6
shows that after 7 days only 1 percent of the original coliforms survived. Table II-7 shows basically
the same reduction for sludge cake applied to the land. Table II-8 shows the number of days of
storage required in a laboratory study14 to reduce several viruses and bacteria to 99.9 percent of
the original values at various temperatures.
The soil reportedly has removed pathogens by various mechanisms, predominately filtration,
soil inactivation, and die-off. Pathogen travel is usually restricted to the order of feet from point of
application unless runoff or channeling occur, potentially polluting surface and ground water.
Table 1 1 -&.— Fecal coliform counts of stored digester supernatant exposed to atmospheric conditions^ 8
Days
0
2
7
14
21
35
Fecal coliform
counts
(per 100 ml)
a800 000
b20 000
8 000
6 000
<2 000
<20
Percent
survival
100 00
2 50
1 00
0 75
<0 25
<0 01
aFecal coliform count just before lagooning.
bFecal coliform count after lagooning.
69
-------
Table 11-7.—Disappearance of fecal coliforms in sludge cake covering a soil surface
19
Days after sludge application
Fecal coliforms/gm
sludge cake, dry
weight
1
2
3
5
7
12 ... . .
3,680,000
655 000
590,000
45,000
30,000
700
Table 11-8.—Laboratory study on number of days' storage required for 99.9 percent reduction of virus and bacteria in
sludge2-®
urgamsm
Poliovirus 1
Echo virus 7
Echo virus 1 2
Coxsackie virus A9
Aerobacter aerogenes
Escherichia coli .
Streptococcus faecalis
4°C
110
130
60
12
56
48
48
20° C
23
41
32
21
20
26
28° C
17
28
20
6
10
12
14
Number of days
The Wisconsin guidelines13 for sludge disposal on agricultural land do not recommend raw
sludge spread without treatment. Partially digested septage may be applied if some preventive meas-
ures are followed, such as lagooning before land disposal or immediate liming of septage.
Disposal Methods
Surface Application. This method of septage disposal is perhaps the most frequently used in
the United States today. Future studies should give consideration to stabilization and additional
pathogen reduction before surface application of septage to land because no discussion of health
hazards in this respect is available. With surface application techniques, some nitrogen loss occurs
through ammonia volatilization; the highest losses occur with spray irrigation.
Land Spreading. The hauler truck that pumps out the septic tank is frequently the vehicle
that applies septage to the land. Consideration should be given to intermediate holding facilities
before land application. Storage is necessary during or just before precipitation to prevent runoff of
contaminated water. In colder climates, land application should be limited to unfrozen surfaces to
prevent runoff during thaws. Pathogen die-off during storage, as mentioned before, also indicates
the necessity of storage.
With a storage facility, disposal can be performed by the hauler truck or by a tank wagon,
usually pulled by a farm tractor. The choice is one of economics. A larger operation may choose to
have its trucks on the road with septage spreading performed by a separate crew, thus freeing the
70
-------
more expensive tank truck to perform cleanout functions. A smaller septage hauler may prefer to
use one vehicle to perform both tasks, thus leveling the workload by spreading septage during slack
hauling periods. In some instances, soil conditions may require the use of flotation-type tires that
are not suitable for long-distance highway use. This would dictate the use of separate collection and
spreading vehicles.
Ridge-and-Furrow. This method has been used to dispose of sludges on relatively level land,
usually that limited to 1.5-percent slopes. No instances of this method were found during this
study. Although this method can be used to distribute septage to row crops during their growth,
care should be taken to ensure these crops are not for human consumption.
Spray Irrigation. Spray irrigation of septage necessitates storing it in a lagoon before disposal.
Portable pipes and nozzle guns are used rather than fixed or solid ones. Because the septage must
be pumped at 80 to 100 psi through 3/4-inch to 2-inch nozzle openings, a screening device at the
lagoon's pump suction is mandatory. Spray irrigation also offers the greatest potential for offensive
odors; thus a knowledge of wind patterns and a well-located site are important during design stages.
Subsurface Application. Soil incorporation techniques offer better odor and pest control than
surface spreading techniques and reduce the likelihood of inadvertent pathogen contamination to
humans. Disadvantages include full incorporation of all nitrogen because ammonia volatilization is
eliminated; this action reduces any nitrogen-loading safety factor from ammonia loss in surface
spreading. Costs are greater than for surface spreading because a storage lagoon or tank and sub-
surface injection equipment are necessary. A resting period of 1 to 2 weeks is required before
equipment can be driven over the waste-incorporated land.13 Three methods have been used to
inject septage into the land.
Plow furrow cover (PFC). A typical setup consists of a moldboard, a furrow wheel, and a
colter. Septage is placed in a narrow furrow and immediately plowed over.
Sub-sod injection. This technique uses a device that injects a wide band or several narrow bands
of septage into a cavity 6 to 8 inches below the surface. Some equipment forces the injection swath
closed.
Terreator. This is a patented device that drills a 9.5-cm hole with an oscillating chisel point. A
tube places the septage as deep as 50 cm below the surface at a rate of 24.8 litres per linear metre (2
gallons per linear foot).21 Kolega21 found that subsurface application of 300 pounds of nitrogen
per year in a well-drained soil did not produce any noticeable ground water quality variation with
PFC, SSI, or terreator methods.
Burial. Methods include disposal in trenches, sanitary landfills, leaching lagoons, or settling
lagoons with infiltration-percolation beds. Foul odors are endemic to these operations until a final
soil cover is placed over the open surfaces of trenches or landfills. Lagoon management practices,
such as proper inlet design, site location, and liming, minimize these problems.
Site selection is important, not only to control odor but also to minimize potential ground
water and surface water pollution problems. Many States require wells' sampling and ground water
monitoring as operational checks.
Trenches. Disposing septage in trenches is similar to disposing it in lagoons, except that
trenches are usually a smaller scale alternative. Septage is placed sequentially in one of many
trenches in small lifts, 6 to 8 inches, to minimize drying time.22 When a trench is filled with sep-
tage, 2 feet of soil should be placed as a final covering, and a new trench opened. New York recom-
mends trenches be a maximum of 7 feet deep. Sufficient room must be left between trenches for
movement of heavy equipment. The trench-and-fill technique is often used at sanitary landfills.
71
-------
Sanitary Landfill. When a sanitary landfill accepts septage, leachate production and treatment
must be investigated. For moisture absorption, New Jersey recommends a starting value of 10 gal-
lons of septage to each cubic yard of solid wastes. Septage should be prevented from entering land-
fills in areas with more than 35 in/yr rainfall if leaching prevention and control facilities or an
isolated hydrogeological rock stratum are not present.
A 6-inch earth cover should be applied daily to each area that was dosed with septage. A
2-foot final cover should be placed within a week after the placement of the final lift.23 Many
designers suggest a maximum cell height of 8 feet.24 Using the New Jersey criterion and an 8-foot
cell height, 1,000 gallons of septage could be distributed on 340 ft2.
Leaching Lagoons. Connecticut23 has been advocating leaching lagoon systems of earthen
anaerobic-aerobic sludge digestion cells. Septage is discharged into a manhole at the edge of a
lagoon and exits about one-third the distance into the cell near the bottom. The lagoon bottom is
not sealed, and at least one-third of the lagoon is above ground level to facilitate liquid removal by
hydrologic gradient and envirotranspiration. The minimum depth of the lagoon is 3 to 5 feet.
Sludge is periodically removed, and effluent from this anaerobic lagoon flows through a controlled
outlet to an aerobic leaching lagoon. Adding lime is suggested to maintain pH at 6.8 to 7.2. When
introduced with the septage into the manhole, lime settles at the end of the anaerobic leaching
lagoon influent pipe and exerts little or no effect on lagoon pH.2 3 Parallel sets of these series
lagoons are recommended. The capacity of each cell is equal to 0.1 of yearly volume, based on 50
to 70 gallons per capita per year of contributing population.
Massachusetts23 requires a minimum of a 6-foot-deep anaerobic lagoon and six percolation
beds, each having 1 ft2/gal/d of design capacity. The lagoon design requirements call for a sizing of
1 gallon per capita per day, with a minimum of 20 days' retention at average flow. The recom-
mended discharge below the liquid level can stir up sediment and release foul odors. Acton, Mass.,
now allows haulers to discharge over riprap into the lagoon, which, they report, lessens odor prob-
lems.
Disposal Lagoons. These are usually a maximum of 6 feet deep, allow no effluent or under-
drain system, and require small (6- to 12-inch) application rates and sequential loading of lagoons
for optimum drying. Series or parallel series lagoons with 2 years' capacity each and a 2-foot maxi-
mum depth are called for in New York State guidelines.11 After drying, solids can be bucketed out
and disposed in a sanitary landfill and the lagoon used for further applications, or 2 feet of soil may
be placed over the solids as a final cover. Many States report odors having been controlled by plac-
ing the lagoon inlet pipe below the liquid level and having water available for haulers to immediately
wash spills into the lagoon inlet pipe.
Comparison of Land Disposal Practices
The land disposal methods discussed are compared in table II-9. The comparison assumes a
moderately well-drained soil, nitrogen-loading requirements, and northern climatic conditions
(requiring use of holding tanks or lagoons during inclement weather). In surface-spreading tech-
niques, approximately one-quarter to one-half of the ammonia nitrogen may be lost, raising the
amount of nitrogen that can be added.
Cost comparisons are not included because only very limited information is available and costs
of existing systems vary widely depending on whether land must be bought and amortized over the
life of the project, be rented, or already is municipally owned; the amount of regrading, clearing,
and grubbing, if necessary; and access requirements.
72
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Table 11-9.— Land disposal characteristics
Land disposal method
Acres required
@10,000 gal/d,
250 d/yr
Characteristics
Advantages
Disadvantages
Surface: Application
Spray irrigation.
370, plus storage and buffer
zone
Ridge and furrow
400, plus storage
oo
Hauler truck spreading..
400, plus storage
Farm tractor with tank
wagon spreading
Subsurface application:
Tank truck with plow-
furrow-cover (RFC)..
400, plus storage
420
Large orifices for nozzle; irri-
gation lines to be drained
after irrigation season
Land preparation
Larger volume trucks require
flotable tires; 500-to 2000-
gallon trucks ok; 800- to
3000-gallon capacity
Requires additional equip-
ment
Single plow mounted on
truck; not usable on wet
or frozen ground
Can be used on steep or
rough land
Lower power requirements
than spray irrigation; can
be used in furrows, on
crops not grown for hu-
man consumption
Same truck can be used for
transport and disposal
Frees hauler truck during
high usage periods
Minimal odor; storage lagoon
optional for pathogen con-
trol
High power requirements;
odor problems; possible
pathogen dispersal
Storage lagoon needed for
pathogen destruction and
when ground is wet or
frozen
Limited to 1.5 percent slopes;
storage lagoon; some odor
Some odor immediately after
spreading; storage lagoon;
limited to 8 percent slopes
Some odor immediately after
dispersal; storage lagoon;
limited to 8 percent slopes
Limited to 8 percent slopes;
longer time needed for dis-
posal operation than for
surface disposal
-------
Table 11-9.—Land disposal characteristics—Continued
Land disposal method
Acres required
@ 10,000 gal/d,
250 d/yr
Characteristics
Advantages
Disadvantages
Farm tractor with RFC.,
420
Sub-sod injection (SSI).
420
Burial:
Trench.
15
Lagoon .
30
Sanitary landfill.
195, working surface
Septage discharge into furrow
behind single plow; sep-
tage spread in narrow
swath and immediately
plowed; not usable on wet
or frozen ground
Septage placed in opening
created by tillage tool; not
usable in wet, frozen, or
hard ground
New trenches opened when
old ones filled; long-term
land commitment after
operations end
Sludge bucketed out to land-
fill from bottom of lagoon;
settled water usually flows
to percolation/infiltration
beds
Septage mixed with garbage
at controlled rates; possi-
ble leachate and collection
requirements
Minimal odor; storage lagoon
optional for pathogen con-
trol
Injector can be mounted on
rear of some trucks; mini-
mal odor; storage lagoon
optional for pathogen con-
trol
Simplest operation; no slope
limits; no climatological
limits
No slope limits; no climato-
logical limits
No topographic limits, simple
operation
Limited to 8 percent slopes;
more time needed for ap-
plication than in surface
disposal
Limit land to 8 percent; more
time needed for applica-
tion than in surface dis-
posal; keeps vehicles off
area for 1 to 2 weeks after
injection
Odor problems; high ground-
water restrictions; vector
problem
Odor problems; high ground-
water restrictions; vector
problem
Odor problems; rodent and
vector problems; limited
to areas with less than 35
inches yearly rainfall or
have leachate collection or
be isolated from ground-
water
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SEPARATE TREATMENT FACILITIES-SEPTAGE ONLY
Alternatives for treating septage at a separate treatment facility include aerated lagoons,
anaerobic-aerobic processing, composting, the BIF Purifax Process, and chemical treatment.
Aerated Lagoon
Aerated lagoons can be used to treat septage if the aerators have the required oxygen transfer
capacity and create sufficient turbulence to prevent solids deposition. Howley17 reported severe
foaming problems, but he did obtain a volatile suspended solids (VSS) reduction of 23.8 percent
and a COD reduction of 73.9 percent, using hydraulic retention times of 1 to 30 days in bench scale
units. Howley found 1.8 pounds of oxygen was required to destroy 1 pound of VSS at loadings
between 0.03 and 1.3 pounds VSS/f3/day.17 He reported that 18,500 gallons per day per million
gallons of aerated lagoon design capacity, operating at 50 percent design sewage flow, should not
cause overloading.
Brookhaven, Long Island, N.Y., using lagoon treatment of septage, experienced reductions of
62.5 percent in BOD, 51 percent in TS, and 49 percent in SS from influent strengths averaging
5,600 mg/1, 3,700 mg/1, and 2,700 mg/1, respectively. Without equalization facilities, this process
was prone to biological upsets. Grit and scum chambers and three large settling lagoons now buffer
flow to the 50,000 gal/d septage system. The effluent from a final settling lagoon is chlorinated and
discharged to sand recharge beds. Accumulated sludge is removed to a nearby landfill.
Anaerobic-Aerobic Process
The anaerobic-aerobic process uses an anaerobic lagoon or digester, then an aerated lagoon. A
pilot anaerobic-aerobic treatment process, with sand beds for filtering final effluent, reported 99
percent BOD, COD, and SS removal, and 90 percent removal of total nitrogen and total phosphor-
ous.25 Anaerobic digesters, which are useful in reducing high concentrations of volatile solids (VS)
and BOD, are addressed later in this section.
Composting
Composting is an alternate septage disposal technique offering good bactericidal action26"28
and a 25-percent reduction in organic carbon.
In aerobic composting, septage is mixed with dry organic matter for moisture control and for
easier air penetration so that aerobic conditions can be maintained. Aerobic composting is generally
recognized as superior to anaerobic composting because it provides better odor control, higher
temperatures for pathogen control, and requires shorter periods for stabilization.
Process Stages. There are three stages in composting. In the initial stage temperatures go from
cryophilic (5° C to 10° C) to mesophilic (10° C to 40° C) regions. Active composting can begin
within days and operates in the thermophilic (40° C to 80° C) region, which tends to be self-
limiting because of competing mechanisms. When there is an abundance of substrates, bacterial
populations increase, thereby raising temperatures. Temperatures above 60° C inhibit microbial
growth, lowering population but also lowering temperatures to the point where they are optimum
for renewed growth. The third stage is substrate limiting. This curing stage operates under two
successive temperature regions-mesophilic (40° C to 10° C) and cryophilic (10° C to 5° C).
Design. Composting sites should have ample room for movement of heavy equipment and
should have a receiving tank to equalize septage and collect leachate and surface water. Primary
75
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screening for removal of larger unwanted material is advised. After it is mixed with dry organic
matter, compost is shaped into windrows, cubes, or hemispheres. Moisture level is controlled by
either controlling dry, organic material/septage ratios or by aeration. Pile aeration can be achieved
by natural draft, mechanical mixing, forced (bottom) aeration, or turning over the compost.
Lebo System. The Lebo System is in operation in South Tacoma and Bremerton, Wash., and is
being constructed for Lewis County and Kent, Wash. A patented preaeration process is used before
septage is sprayed on piles of sawdust, wood shavings, or other dry organic material. A 1- to 2-inch
application is covered with additional sawdust, and front-end loaders form the mixture into piles to
minimize heat loss. Natural draft aeration, possible because the mixture is bulky, eliminates the
need for turning or forced aeration. The 50- to 60-percent moisture content material is said to
attain a pile temperature of 65° C.29 The pile is cured in 3 months.
Beltsville System. The Beltsville System, devised by the United States Department of Agricul-
ture, is operating on dewatered sludge in Washington, D.C.; Bangor, Maine; Durham, N.H.; Orange
County, Calif.; and Johnson City, Tenn. Camden, N.J., will use the Beltsville forced aeration sys-
tem on 8-percent sewage sludge, with licorice root as the bulking. The Beltsville System usually
mixes sludge with wood chips in long windrows and has piping facilities to alternately blow and
suck air through 0.66-cm (0.25-in) holes in 15-cm (6-in) pipe, covered by 30 cm (1 foot) of wood
chips or screened compost, to maintain aerobic bacterial action.26'27 Some turning of the
windrows is suggested. After several weeks, the compost can be screened and the wood chips re-
cycled for further composting.
End Use. Some composting facilities attempt to market their end product. This method has
rarely been successful because of a lack of public acceptance and other factors. Using end products
from municipal facilities as soil conditioners in parks and on golf courses has been acceptable.
In a study conducted by Western Washington Research and Extension Center,2 9 Lebo com-
post applied to sweet corn initially produced no significant change in yield; however, there was an
increased yield in subsequent years when compost and fertilizer were added to commercial ferti-
lizer.
Purifax
The BIF Purifax Process oxidizes screened, degritted, and equalized septage with dosages of
chlorine, from 700 mg/1 to 3,000 mg/1, under moderate pressure. Chlorine replaces oxygen in
organic molecules, rendering this material unavailable to bacteria as a food source, thereby stabiliz-
ing and deodorizing the septage. The Purifaxed septage changes color from black or deep brown to
straw. The process initially releases CO2, which separates liquids and solids quickly by causing the
solids to float.
Purifax treatment results in a highly acidic slurry, pH 1.7 to 3.8. If mechanical dewatering or
lagoon separation of the liquids or solids is contemplated, chemicals should be added for pH control
of the resultant liquid fraction.
Locations using the Purifax Process in treating septage and sludge in lagoons for liquid-solids
separation have had periodic solids separation and odor problems. Sand drying beds appear to be
the most efficient method of liquid-solids separation of Purifaxed septage. Adequate ventilation of
covered sand drying beds is mandatory to prevent operators from inhaling any NC13 released.22
Chemical Treatment
Raw septage is chemically treated with lime and ferric chloride at an Islip, Long Island, N.Y.,
facility. After the septage is screened, degritted, and equalized, about 190 pounds of lime per ton
76
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dry solids and 50 gallons per ton dry solids of a standard strength ferric chloride solution are flash-
mixed with the septage. The solids-liquid separation step occurs in a clariflocculator. An observed
significant-solids carryover problem indicates the separation unit may have been undersize. The
liquid fraction is chlorinated and discharged to ground water recharge beds, and the underflow
solids from the clariflocculator are vacuum filtered. Long-term relative stability of the lime-ferric,
chloride-septage mixture is unknown.
Tilsworth7 found good liquid-solids separation only after adding huge amounts of chemicals.
Separation occurred with 10,000 mg/1 lime, 10,000 mg/1 ferric sulfate, 4,000 mg/1 lime and ferric
sulfate mixture, or a 3-percent concentration of a cationic polymer.
Feige et al.8 added similar quantities of lime (±180 pounds per ton dry solids) to obtain
acceptable septage dewatering on sand drying beds. The long-term fate of limed, dewatered septage
in landfills and on land needs to be addressed.
SEPTAGE-SEWAGE TREATMENT FACILITIES
Because of their number and location, sewage treatment plants are one of the most frequent
acceptors of septage and must be included in any comprehensive study of alternate treatment
schemes. Septage can be disposed of in a treatment facility by adding it to the liquid stream or the
sludge stream. In either case, a properly designed septage handling facility, including screening,
degritting, and equalization, is recommended.
Septage frequently is considered a high-strength wastewater and is dumped into an upstream
sewer or placed directly into various unit processes in a treatment plant (fig. II-3). At several facili-
I
S
o
Grit
tank
Septage
addition point
Aerobic digester.
Anaerobic digester.
Purifax Process,
Zimpro Process,
Lime conditioning,
or other
/ \
Primary , Aeration ... Thrifipr U-»
tank ' tank/trickling filter I
V /
• — • !Z3 —
1
CL2
», ., ... 1
Solids* •* ^ ^
conditioning Stream
!».. J
rf r
Sand drying Mechanical "~~"
beds dewatering
* f *
S
t
Solids to
land or sea
Figure 11-3. Septage addition points in wastewater treatment plants.
77
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ties, septage is considered a sludge because it is the product of an anaerobic settling/digestion tank,
and it has approximately the same TS concentration as raw municipal sludge. The septage applica-
tion points, if treated as a sludge, may include sludge stabilization, sand bed drying, or a mechanical
dewatering process. The decision of where to apply the septage should be determined after a statis-
tically significant sampling and analysis of a locale's septage, including:
Solids loading \
Oxygen demand
Toxic substances
Foaming potential
Nutrient loading (N and P), where required
These factors, combined with a plant's layout, design capacity, present loading, and the follow-
ing criteria, provide the design professional with sufficient information for a reasonable septage
treatment scheme for a wastewater treatment facility.
When septage is added to an upstream sewer or discharged at a treatment plant, there should
be a suitable hauler truck discharge facility. It should include a hard-surfaced ramp that leads to an
inlet port and is able to accept a quick-disconnect coupling directly attached to the truck's outlet.
This significantly reduces odor problems. Washdown water should also be provided for the hauler
so that spills can be cleaned up. Recording the time and volume and the name of the hauler is vital
for operation and billing purposes. The Columbia Avenue Plant in Portland, Oreg., and Seattle
Metro's Renton facility use a plastic charge plate or magnetically coded card and card reader to
obtain such information.
Pretreatment
Treatment plants handling septage have experienced better operation when septage is pre-
treated. Pretreatment generally includes screening, using bar screens with 3/4- to 1-inch openings;
grit removal; and pre-aeration or prechlorination if it is an aerobic process. Grit removal by cyclone
classifiers has been done successfully in Babylon and is included in the new Bay Shore plant, both
on Long Island, N.Y. Usually, separation of inorganic matter larger than 150 mesh is sufficient.
Equalization/storage tanks with 2 days' average septage flow and mixing capability should also be
provided. To further attenuate odors, enclosing the storage tanks and ozonization in tank vent lines
should be considered. Pumping equipment should be used to apply a continuous dose of septage
into the desired unit. Operators report slug or intermittent doses of septage are difficult to treat
and may seriously upset biological treatment systems.
Primary Treatment
Feige's8 report for the EPA indicated that neither natural settling nor adding lime or polyelec-
trolytes resulted in consistent liquid-solids septage separation. Tilsworth7 characterized raw septage
as relatively nonsettleable, as determined by a settleable-solids volume test, from 0 to 90 percent
with 24.7 percent as the average volume.
Tawa30 found that poor settling characteristics generally could be expected from septage and
that it could be divided into three types. Type 1, from septic tanks pumped before necessary, set-
tled well and was found in 25 percent of the samples. Type 2, from normally operating systems,
showed intermediate settling characteristics and was found in 50 percent of the samples. Type 3,
which exhibited poor settling, was found in 25 percent of the samples and was from tanks overdue
for pumping. All samples were between 1 and 6 years old.
Unless chemicals are added to it, septage settles very poorly. In a study on treatment of
Alaskan septage, Tilsworth7 found that only 50 percent of the samples settled by more than 10
78
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percent after 30 minutes, as shown in figure II-4. About one-third of the samples are not repre-
sented in the figure because they settled less than 1 percent during the 30-minute test period.
Elutriation, in terms of the settling of septage in a septage-sewage mixture, is reported to yield
better results. Carroll31 reports that up to 75 percent of septage SS can be expected to settle in a
sewage treatment plant's primary sedimentation basins. An EPA study found 55 to 65 percent SS
removals in a primary clarifier, but only 15 to 25 percent BOD removals.32
Activated Sludge
Septage may be added to the activated sludge process if additional aeration capacity is avail-
able, the plant is organically and hydraulically loaded below design capacity, the septage metals
content can be diluted sufficiently, and foaming potential is low or controllable. Very limited
quantities of septage may be added without changing the sludge-wasting rates.
At the Weaverville Wastewater Treatment Plant in Trinity County, Calif., 400 gal/d slug dumps
were handled without significant upset at a 0.5 million gallons per day plant flowing at 40-percent
capacity.
In a report to the U.S. Forest Service,31 CH2M/Hill recommended various levels of septage
addition for several kinds of activated sludge plants. This information, modified by authors' field
investigations,22 is presented in figures II-5 and II-6.
CO
Q
_l
O
co
LLJ
CO
0.01 0.1
1 2 5 10 20 40 60 80 90 95 98 99 99.9
PROBABILITY OF OCCURRENCE
Figure II-4. Probability of solids reduction—liquid interface height after 30 minutes settling of Alaskan septage
samples.
79
-------
88
80
72
64
56
48
40
32
24
16
8
1234567 8 9 10 11 12 13 14 15
I
WASTEWATER TREATMENT PLANT CAPACITY, mgd
Figure II-5. Septage additions to activated-sludge wastewater treatment plants (no equalization facilities).31
o
o
o
Q
Q
LU
t
LU
CO
With primary (
treatment
16
The use of slug dumping of septage may depend on limiting the increase in mixed-liquid sus-
pended solids (MLSS) to 10 percent per day to maintain a relatively stable sludge, as shown in fig-
ure II-5. Higher loadings and wasting rates than the resident aquatic biomass is acclimated to may
result in a poor-settling sludge.33 Severe temporary changes in loading beyond the 10- to
15-percent MLSS increase may cause a total loss of the system's biomass.31
Package treatment plants should not accept septage for slug dumping if their design capacity is
less than 100,000 gal/d.31 In a study for the U.S. Forest Service, CH2M/Hill determined that pack-
age treatment plants can treat septage at approximately 0.1 percent of the plant design capacity,
whereas modified activated sludge can treat septage at twice the rate of a package plant. Conven-
tional activated sludge plants can treat septage at about four times the rate of package plants.31
In plants with holding and metering facilities, septage may be bled into the waste-flow stream
at considerably greater rates than would be allowable if only slug-dumping procedures were avail-
able.
An EPA study32 fed septage at a controlled rate of 2 to 13 percent of the total influent flow
to one of two activated sludge units. With a control unit food-to-microorganism (F/M) ratio of 0.4
and a septage-sewage unit F/M of 0.8, effluent BOD and SS characteristics were similar. Effluent
COD of the unit receiving septage increased when septage was loaded at 10 to 13 percent of plant
80
-------
o
CO
HI
Q
o
LU
to
LU
or
Q_
o
LU
CO
Activated sludge without
primary treatment
Activated sludge
with primary
treatment
8 12 16 20 24 28
SEPTAGE ADDED, 1,000gal/d PER mgd PLANT CAPACITY
Figure 11-6. Septage addition to wastewater treatment plants (with equalization facilities).
flow. When a lower F/M ratio of 0.5 to 0.6 was used in the septage unit, it had a superior perform-
ance because nocardia, a procaryotic filamentous actinomycete often associated with bulking, was
controlled.
Figure II-6 is based on research reported in the literature and field investigations.22 Again, it
demonstrates that package plants with design capacities under 100,000 gal/d should not accept sep-
tage. Depending on the present plant flow compared with the design plant flow, a biological treat-
ment reserve can be estimated that will allow for a certain level of septage to be adequately treated.
Under identical loading conditions, the ratio of septage addition to various kinds of treatment
plants would be:
Treatment plants
Relative
volumes
of
septage
addition
Package plants
Activated sludge (no primary treatment)
Activated sludge (conventional)
Aerated lagoons
1.00
2.08
4.83
6.00
81
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Figure II-6 represents continuous septage addition to a facility for a fully acclimated biomass.
It is recommended that an initial septage feed to an unacclimated system should be substantially
less than shown, that is, on the order of 10 percent of the graph values. Further gradual increases in
daily septage loading should be made over a 2- to 3-week period up to the maximum amount shown
in the figure. Oxygen capacity must be checked continuously and gradual changes made in sludge
age.
Figure 11-7 shows the additional oxygen requirements when septage is added in activated
sludge treatment plants. Treatment facilities should be analyzed to determine if oxygen require-
ments or mixing requirements are controlling factors.
Because septage has higher oxygen demands than raw sewage on a unit BOD5 basis, an addi-
tional oxygen supply for activated sludge plants that accept septage having primary treatment
would be 40 pounds of O2 per 1,000 gallons of septage added. For plants without primary treat-
ment, an additional 80 pounds of O2 per 1,000 gallons of septage added should be provided. Pack-
age treatment plants have an oxygen requirement similar to plants without primary treatment.
Feng34 has shown higher sludge ages (10 days versus 4 days) result in higher percentage BOD
removal and less sludge production than do lower sludge ages (fig. II-8). Wasting must be adjusted
gradually with increased loads to obtain a sludge age that produces the optimum balance between
o
o
Q
LU
CC
5
o
LU
O
X
o
<
z
g
H
Q
Q
Without primary
treatment
M
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SEPTAGE ADDED, 1,000 gal/d
Figure II-7. Additional oxygen required for septage additions in activated-sludge treatment plants.31
82
-------
100
Weak septage BODR = 1,250 mg/l
Strong septage BODR = 11,000 mg/l
5 10 15 20 25 30 35 40 45 50 55 60 65 70
PERCENT SEPTAGE
Figure II-8. BOD5 removal from septage-sewage mixtures in batch activated sludge process.34
aeration tank efficiency and good settling characteristics. A high sludge age generally produces a
light sludge with poor settling ability but good substrate removal characteristics. The reverse is
often true for a very young sludge.
At one New York plant, septage is bled into the liquid stream at a rate inversely proportional
to that of the sewage flow.23 The procedure takes advantage of a larger excess aeration capacity
during lower loading times. Orange County, Fla.,35 added septage proportionate to sewage flow
rates. Both plants have experienced some operational problems.
Some odor and foaming problems have been reported in aeration systems; however, the odor
usually dissipated within 6 to 24 hours,7-10 and foaming was not apparent in all cases. Commercial
defoamers, such as decyl alcohol, and aeration-tank spray water have been used to reduce foaming.
Attached Growth Systems
Systems that use attached growth aerobic treatment processes, such as trickling filters and
rotating biological contactors, are usually more resistant to upsets from changes in organic or
hydraulic loadings and are suitable for septage treatment.23'36-37
In trickling filters, additional recirculation has been shown to adequately dilute septage con-
centrations and diminish chances of plugging the media. At Huntington, Long Island, N.Y., 30,000
83
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gal/d septage is treated at a 1.9 million gallons per day facility.23 BOD5 reductions of 85 to 90
percent have been observed concurrent with SS reductions of 85 percent.
Rotating biological contactors use a long detention time and a continually rotating biological
medium that is reportedly resistant to upsets. At Ridge, Long Island, N.Y., a BOD reduction of 90
percent, a COD reduction of 67 percent, and a total suspended solids reduction of 70 percent were
reported. Flow equalization of a low-strength septage and a surface loading of 2 gal/d/ft2 produced
these results.
Aerobic Digestion
An alternative to considering septage as a concentrated wastewater is to assume it is the prod-
uct of an unheated digester and, therefore, a sludge. Many researchers have reported good results in
aerobic digestion of septage or septage-sewage mixtures. Jewell1 ° reports odors diminished, but
time needed to produce an odor-free sludge varied up to 7 days.
Tilsworth7 reported a high degree of septage biodegradability at a 10-day aeration time, result-
ing in a BOD reduction of 80 percent and a VSS reduction of 41 percent. Chuang,25 treating
anaerobically digested septage with an aerobic digester, reported 36-percent VS removal at 40 days'
aeration under a loading of 0.0016 pound VS per cubic foot per day. After 22 to 63 days' aeration,
Howley17 found a 43-percent VSS reduction and a 75-percent COD reduction.
Orange County, Fla., adds septage to aerobic digesters at the rate of 5 percent of total sludge
flow and obtains good reductions at a loading of 0.15 pound VS per cubic foot per day. Bend,
Oreg., obtained good removal by adding 13 percent septage to 87 percent sludge at a loading of
0.02 pound VS per cubic foot per day, with 15 to 18 days' aeration time.38
Tilsworth7 observed gas transfer characteristics for septage and found that a, the ratio of gas
transfer efficiency to tapwater, and j3, the ratio of O2 saturation concentration to tapwater,
approached unity after 1 to 2 days' aeration. Before 1 day, a and |3 were in the range of 0.4 to 0.6.
Jewell39 found both dewatering and settleability improved with aeration, but that aeration
time required to effect significant improvement varied.
Before adding septage to the aerobic digestion process, aeration capacity, toxic metal or chemi-
cal accumulations, and increased solids to be disposed of should be investigated. Investigators con-
sistently have reported initial repulsive odors and foaming problems.7'17'25'35'39
When considering septage addition to aerobic digesters, recommendations should include
screening, degritting, flow equalization, and analyses of excess digestion capacity and peripheral
effects on other processes such as solids handling. An initial septage addition should be limited to
approximately 5 percent of the existing sludge flow. Further septage additions should be gradual.
Studies in high-temperature auto-oxidation of septage are planned40 and may prove promising
as a low-cost, efficient, solids-destruction technique.
Anaerobic Digestion
Septage in Tallahassee, Fla., is treated in an unheated (20° C to 30° C) anaerobic digester.
With an influent septage concentration of 17,700 mg/1 TS, a VS reduction of 56 percent was
reported after an 82-day retention time at a loading of 0.01 pound VSS per cubic foot per day.
Large quantities of grit in the septage required draining and cleaning of the open digester after only
3 years' operation. Leseman and Swanson41 analyzed volatile acid distribution concentrations in
84
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the digester contents. The volatile acid-to-alkalinity ratio varied from 0.34 to 0.83. The 8-month
volatile acid concentration averaged 703 mg/1 and ranged from 408 mg/1 to 1,117 mg/1 at a con-
sistent pH of 6.0. The progression of volatile acid concentrations in the digester, from two to five
carbon acids, showed acetic as 276 mg/1, propionic as 294 mg/1, isobutyric as 14 mg/1, butyric as 49
mg/1, isovaleric as 28 mg/1, and valeric as 42 mg/1. The digester had an open cover, so gas produc-
tion could not be monitored. Supernatant from this digester is pumped to the sewage sludge
anaerobic digester.
Jewell10 reported a 45-percent reduction in VSS from a bench-scale digester loaded at 0.05
pound VSS per cubic foot per day, with a 15-day hydraulic retention time. Gas production varied
from 4.2 to 7.6 cubic feet per pound COD up to a loading of 0.08 pound VSS per cubic foot per
day, at which point gas production fell off dramatically, possibly indicating poisoning of the system
by a toxic chemical concentration from an unknown source.
Chuang25 reported a 92-percent VS removal from a heated anaerobic digester loaded at 0.08
pound VSS per cubic foot per day with a 15-day hydraulic retention time. Incoming solids ranged
from 0.3 percent to 8 percent, and TS reduction was more than 93 percent. BOD reductions aver-
aged 75 percent, from 6,100 mg/1 influent to 1,500 mg/1 effluent.
Howley17 recommends a maximum septage addition of 2,130 gal/d to each 14,500 gallons
sewage sludge added per day per million gallons of digester capacity, with a detention time of 30
dayf and a loading of 0.08 pound VSS per cubic foot per day. Good operation of anaerobic diges-
ters requires that toxic materials be limited.
Septage should be screened, degritted, and equalized before it is added to single-stage diges-
ters. Digesters should be cleaned on a regular schedule, such as every 2 to 3 years, or as required.
Monitoring digester performance includes long-term evaluation of volatile acid/alkalinity ratios
and gas production. Mixing is vital to preventing a sour digester from developing point-source fail-
ure from a septage load containing high volatile acid concentrations.
In systems with multiple tanks, all the preceding suggestions should be followed. Spreading
the septage among a number of digesters reduces septage concentrations. Recycling material from
the bottom of a secondary digester or from another well-buffered primary digester at a rate of up to
50 percent of the raw feed per day has been found helpful. Temperature and mixing should also be
adjusted for maximum performance.42
Mechanical Dewatering
Islip, Long Island, N.Y., uses a vacuum filter to dewater 100,000 gal/d of chemically condi-
tioned septage. A design basis of 6 pounds per hour per square foot of surface area was used and
appears to be satisfactory.43 Adding lime at a rate of about 190 Ib/ton of dry solids and 50 gal/ton
of dry solids standard concentration ferric chloride solution are added before vacuum filtering.
In a study at Clarkson College, Crowe44 had successful results with vacuum filtration of mix-
tures of raw septage and digested sludge with up to 20 percent raw septage by volume. Chemical
preconditioning with lime, ferric chloride, and polymers was required at doses typical of domestic
sludge. He observed dewatering characteristics similar to those of mixtures without septage. The
filtrate contained only 5 to 10 percent of the raw septage COD.
Sand Drying Beds
Sand drying has been used to dewater septage with varying success. Anaerobically digested
septage is reported to require two to three times the drying period of digested sludge.41 After
85
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treatment in aerated lagoons and batch aerobic digesters, dewatering simulation studies yielded a
septage capillary suction time (CST) on the order of 200 seconds versus about 70 seconds for sew-
age treatment plant sludges. A lower CST can be correlated to a faster dewatering time. The CST's
of raw septage were found to range from 120 to 825 seconds; the mean was 450 seconds.45 Adding
lime to septage before sand bed dewatering has vastly improved dewatering characteristics. Feige8
found that adding 180 pounds of lime per ton dry solids, or 30 pounds per 1,000 gallons of septage
based on 40,000 mg/1 TS, raised the pH to 11.5 and dried to 25 percent solids in 6 days and 38
percent solids in 19 days. An application depth of greater than 8 inches is not recommended be-
cause it slowed the drying process. The filtrate analysis showed that most heavy metals were tied
up in the solids, fecal coliforms were killed effectively, fecal streptococci were more resistant than
fecal coliforms, and odors were significantly reduced. Filtrate quality was generally good, but
further treatment before discharge was recommended.23
Perrin found other chemicals worked well in modifying the ability of septage to dewater.
From a mean initial CST of 450 seconds, septage showed a dewatering ability of 50 seconds after
adding an average of either 1,360 mg/1 ferric chloride, 1,260 mg/1 alum, 1,360 mg/1 Purifloc C-31,
or 2,480 mg/1 Purifloc C-41.45 Perrin also studied the effects of freezing on dewatered samples of
septage after treatment in aerated lagoons or batch aerobic digesters. Freezing lowered the CST
from 225 seconds to 42 seconds, an 80-percent decrease in dewatering time.
If septage is to be placed on sand drying beds, treatment to a consistent CST range of 50 to 70
seconds is recommended. Further treatment of underdrainage would be required in most cases.
COSTS
Of all the alternatives investigated, land disposal was reported to have the lowest operation and
maintenance costs, from $1.50 to $5.00 per 1,000 gallons, exclusive of the cost of the land.
Lagoon treatment is reported to cost between $5.00 and $10.00 per 1,000 gallons. The cost of
septage treatment in sewage treatment plants varies widely, but typically runs about $15.00 per
1,000 gallons. Composting by the Lebo process is reported to cost approximately the same as dis-
posal in wastewater treatment plants. Physical chemical treatments, such as the Purifax Process and
chemical stabilization, range from average costs similar to those found in disposal at treatment
plants to double or triple that figure.
A nationwide survey of 42 wastewater treatment plants2 2 determined that only about one-half
charged for septage disposal based on treatment costs (fig. II-9). Some charge prohibitive rates to
avoid septage; others place a minimal charge on septage to ensure against illegal dumping at an
unauthorized site. At those plants surveyed, the average charge for septage was $15.18 per 1,000
gallons. An additional 20 to 30 plants contacted, however, either placed no charge on septage dis-
posal or levied only a yearly fee, most often in the range of $50 to $300 per truck.
Many variables affect treatment costs, including local funding requirements; eligibility for
State or Federal funds; necessity for industrial cost recovery formats; local taxes assessed in lieu of,
or to offset, treatment plant expenses; level of pollutant removal capacity; climate; present loading
versus design plant capacity; and cost of land. It is easy to understand, therefore, the broad range
of charges for treatment plant septage disposal.
An estimate was performed to determine a reasonable charge a homeowner could expect to
pay for having a 1,000-gallon septic tank^ cleaned, assuming no additional work was needed. It was
based on a 15-mile haul to the disposal point, 2 hours travel time per load, vehicle depreciation and
insurance of $4,000 per year, and estimated union wages. Depending on the level of profit and a
disposal cost not exceeding $15, a reasonable charge appears to be $40 to $60.
86
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01
>
cr
D
to
CO
U-
O
LLJ
O
cc
LU
CL
5 10 15 20 25 30 35+
AVERAGE CHARGE, $15.18 PER 1,000 GALLONS
Figure II-9. Septage disposal charges at 42 wastewater treatment plants, in dollars per 1,000 gallons of septage.
Fees charged to homeowners ranged from a low of $20 to $25 per 1,000 gallons in parts of
Long Island, N.Y., to around $100 per 1,000 gallons in areas of New Jersey, Connecticut, and
Oregon. Rural areas in New England had slightly lower charges, $25 to $40 per 1,000 gallons; in
the rest of the country, charges were mostly in the range of $40 to $60 per 1,000 gallons. These
charges depend on the distance from the septic tank to the disposal point (especially if more than
15 miles) and the disposal fee.
SUMMARY AND CONCLUSIONS
Various alternatives for septage disposal have been presented. Good design practices and con-
scientious operation are necessary to preclude septage from polluting the environment.
The method chosen should depend on an evaluation of local needs by the design professional,
cost/effectiveness of solutions, and environmental weighing of impact factors.
For example, although land disposal appears most cost/effective, local constraints concerning
land use, odors, or poor soil may preclude this option. Similarly, a more expensive option, such as
composting, may prove viable if it meets such local requirements as those on land restrictions or
odor prevention and conversion of excess wood waste into a marketable product.
REFERENCES
11970 U.S. Census.
2"On-Site Domestic Waste Disposal," report prepared for the U.S. Environmental Protection
Agency, by Miller, Inc., 1975.
87
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3 J. J. Kolega and A. W. Dewey, "Septage Disposal Practice," paper presented at the National
Home Sewage Disposal Symposium in Chicago, 111., Dec. 9-10, 1974.
4 James F. Kreissl, Memo on Septage Analysis, U.S. Environmental Protection Agency, Cincin-
nati, Ohio, Feb. 2, 1976.
5Maine Municipal Association, "Septage Treatment/Management Proposal," presented to the
Environmental Protection Agency, May 1976.
6W. F. Graner, Scavenger Waste Disposal Problems, report presented to the Suffolk County
(New York) Department of Health, 1969.
7T. Tilsworth, "The Characteristics and Ultimate Disposal of Waste Septic Tank Sludge,"
Report No. IWR-56, Institute of Water Resources, University of Alaska at Fairbanks, Nov. 1974.
8W. A. Feige et al., "An Alternative Septage Treatment Method: Lime Stabilization/Sand-Bed
Dewatering," U.S. Environmental Protection Agency Technology Series, Sept. 1975.
9R. Goodenow, "Study of Processing Septic Tank Pumpings at Brunswick Treatment Plant,"
J. Maine Wastewater Control Assoc., 1,2,1972.
10W. J. Jewell, J. B. Howley, and D. R. Perrin, "Treatability of Septic Tank Sludge," Water
Pollution Control in Low Density Areas, University Press of New England, 1975.
11 "Draft Guidelines for the Design and Operation of Septic and Sewage Treatment Plant
Sludge Disposal Facilities," Department of Environmental Conservation, State of New York,
undated.
12J. F. Calabro, "Microbiology of Septage," doctoral dissertation, University of Connecticut,
Storrs, Conn., 1971.
13"Guidelines for the Application of.Wastewater Sludge to Agricultural Land," Technical Bul-
letin No. 88, Department of Natural Resources, Madison, Wis., 1975.
14 "Maine Guidelines for Septic Tank Sludge Disposal on the Land," Miscellaneous Report
155, University of Maine at Orono and Maine Soil and Water Conservation Commission, Apr. 1974.
15E. J. Chou and Y. Okamoto, "Removal of Cadmium Ion from Aqueous Solution," J. Water
Pollut. Cont. Fed., 48, 12, Dec. 1976.
16L. Friberg, M. Piscator, and G. Norberg, Cadmium in the Environment, CRC Press, Cleve-
land, Ohio, 1971.
17J. B. Howley, "Biological Treatment of Septic Tank Sludge," master's thesis, Department of
Civil Engineering, University of Vermont, 1973.
18U.S. EPA Notice of intent to issue a policy statement of acceptable methods for the utiliza-
tion or disposal of sludge from publicly owned wastewater treatment plants, Metropolitan Sanitary
District of Greater Chicago, 1974.
19C. Lue-Hing, B. T. Lyman, and J. R. Peterson, Report No. 74-21, Metropolitan Sanitary
District of Greater Chicago, 1974.
20 G. Berg, "Virus Transmission by the Water Vehicle," Health Library Sci. 2, 2, 90,1966.
88
-------
21 J. J. Kolega et al., "Land Disposal of Septage (Septic Tank Pumpings)," Pollution: Engineer-
ing and Scientific Solutions, edited by Euval S. Barrekette. New York: Plenum Publishing Com-
pany, 1972.
221. A. Cooper and J. W. Rezek, "Septage Disposal in Wastewater Treatment Facilities," paper
presented at the Third Annual On-Site Waste Disposal Conference, Ann Arbor, Mich., Nov. 17,
1976.
23"Guidelines for Septage Handling and Disposal," New England Interstate Water Pollution
Control Commission, Boston, Mass., 1976.
24S. Weiss, Sanitary Landfill Technology, Noyes Data Corporation, Park Ridge, N.J., 1974.
25F. S. Chuang, "Treatment of Septic Tank Wastes by an Anaerobic Aerobic Process," Deeds
and Data Supplement, J. Water Pollut. Cont. Fed. Highlights, 13, 7, 3, July 1976.
26F. Epstein, G. B. Willson, W. D. Burge, D. C. Mullen, and N. K. Enkiri, "A Forced Aeration
System for Composting Wastewater Sludge," J. Water Pollut. Cont. Fed., 48, 688, 1976.
27E. Epstein and G. B. Willson, "Composting Raw Sludge," Municipal Sludge Management,
Proceedings of the National Conference on Municipal Sludge Treatment, Pittsburgh, Pa., 123,1974.
28G. B. Willson and J. M. Walker, "Composting Sewage Sludge: How?" J. of Waste Recycling
14, 5, 1973.
2 D. W. James, "Composting for Municipal Sludge Disposal," paper presented at the 43rd
Annual Pacific Northwest Pollution Control Convention, Seattle, Wash., Oct. 1976.
30Anthony Tawa, Research Assistant, University of Massachusetts, Amherst, Mass., Personal
communication.
31R. G. Carroll, "Planning Guidelines for Sanitary Wash Facilities," Report to the U.S. Depart-
ment of Agriculture, Forest Service, California Region, CH2M/Hill, Jan. 1972.
32S. M. Bennett, J. A. Heidman, and J. K. Kreissl, "Feasibility of Treating Septic Tank Waste
by Activated Sludge," U.S. Environmental Protection Agency Report, in press.
33T. H. Feng, Professor of Civil Engineering, University of Massachusetts, Amherst, Mass.,
Personal communication.
34T. H. Feng and H. L. Li, "Combined Treatment of Septage with Municipal Wastewater by
Complete Mixing Activated Sludge Process," Report No. Env. E. 50-75-4, Division of Water Pollu-
tion Control, Massachusetts Water Resources Commission, May 1975.
35G. C. Cushnie, Jr., "Septic Tank and Chemical Pumpings Evaluation," master's thesis,
Department of Civil Engineering, Florida Technical University, 1975.
36"Town of Wayland, Mass., Report on Disposal of Septic Tank Pumpings and Refuse,"
Weston and Sampson Engineers, Nov. 1969.
37Design Criteria of Ridge, N.Y., Development Plant, Richard Fanning and Associates,
undated.
38"The Feasibility of Accepting Privy Vault Wastes at the Bend Waste Treatment Plant,"
report prepared for the City of Bend, Oreg., by C. & G. Engineers, Salem, Oreg., June 1973.
89
-------
39W. J. Jewell, J. B. Rowley, and D. R. PeBrin, "Design Guidelines for Septic Tank Sludge
Treatment and Disposal,"Prog. Water Technol, 7, 2,1975.
40W. T. Jewell, "Waste Organic Recycling Services—Septic Tank Sludge Treatment and Utiliza-
tion," proposal to the U.S. Environmental Protection Agency, region 1, June 1974.
41 Personal Communication, Wm. Leseman and Jerry Swanson, Water Pollution Control
Department, City of Tallahassee, Fla.
42C. Zickefoose and R. B. J. Hayes, Anaerobic Sludge Digestion, EPA 430/9-76-001, Munici-
pal Operations Branch, U.S. Environmental Protection Agency, Feb. 1976.
43W. F. Cosulich, "Stop Dumping Cesspool Wastes," Am. City, 87, 2, 78-79, Feb. 1968.
44T. L. Crowe, "Dewatering Septage by Vacuum Filtration," master's thesis presented to
Clarkson College of Technology, Potsdam, N.Y., Sept. 1974.
45D. R. Perrin, "Physical and Chemical Treatment of Septic Tank Sludge," master's thesis,
Department of Civil Engineering, University of Vermont, Burlington, Vt., 1974.
90
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METRIC CONVERSION TABLES
Recommended Units
Description
Ltngth
Area
Volume
Mass
Force
Moment or
torque
Flow (volumetric}
Unit
meter
kilometer
millimeter
micrometer or
micron
square meter
square kilometer
square millimeter
hectare
cubic meter
litre
kilogram
gram
milligram
tonne
newton
newton meter
cubic meter
per second
liter per second
Symbol
m
km
mm
ium or iW
m2
km2
mm'
ha
m3
1
g
mg
t
N
N m
m3/s
I/.
Comments
Basil SI unit
The hectare (10,000
m2} ui recognised
multiple unit and will
remain m interna-
tional use
Basic SI unit
1 tonne = 1,000 kg
The newton is that
force that produces
art acceleration of
1 m/s2 m a mats
of 1 kg
The meter is mea
lured perpendicular
to the line of action
of the force N
Not a joule
Customary
Equivalents*
39 37m = 3281 ft =
1094yd
06214 mi
0 03937 m
3937X 105m= 1 X 104 A
10 76 sq ft = 1 196sqyd
03861 jq mi = 247 t acres
0 001550 sqm
2471 acres
35 31 cut! 1 308cuyd
1 057 Q1 = 02842 gal -
08107 X 10^* acre ft
2 205 Ib
0 03527 02- 1543gr
001 543 gr
09842 ton (long) =
1 102 ton (short)
0 2248 Ib
= 7 233 poundals
07375lbft
23 73 pound* ft
2,1l9cfm
ISSSgpm
Description
Velocity
linear
angular
Viscosity
Pressure or
stress
Temperature
Work, energy,
quantity of heat
Power
Application of Units
Description
Precipitation,
run-off.
evaporation
Flow
Discharges or
abstractions.
yields
Usage of water
Unit
millimeter
cubic meter
per second
liter per second
cubic meter
per day
cubic meter
per year
liter per person
per day
Symbol
mm
m3/s
t/s
m3/d
m3/year
I/person/
day
Comments
For meteorological
purposes, it may be
convenient to meat-
sure precipitation in
terms of mass/unit
area (kg/m2)
1 mm of ram =
1 kg/m2
1 l/s=864m3/d
Customary
Equivalents*
35 31 cfs
IS 85gpm
01835gpm
264 2 g»l/year
0 2642 gcpd
Description
Density
Concentration
BOO loading
Hydraulic load
per unit area,
e g , filtration
rates
Aif supply
Optical units
Recommended Units
Unit
meter per
second
millimeter
per second
kilometers
per second
radians per
second
pascal second
centipoise
new! on per
square meter
or pascal
ki to new ton per
square meter
or kilo pascal
bar
Celsius (centigrade)
Kelvin (abs }
joule
k((o joule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/s
Pas
Z
N/m?
or
Pa
kN/m2
or
kPa
bar
°C
°K
J
kJ
W
kW
J/s
Comments
1 joule - 1 N m
where meters are
measured along
the line of action
of force N
1 watt * 1 J,'i
Customary
Equivalents*
3.281 fps
0 00326 Hpi
2,237 mph
9 S49 rpm
06722poundal(s)/sqft
1450X 10 7 Reyn(^t)
0 0001450 Ib/sqm
0 14507 Ib/iq in
14 SO Ib/iq m
("F-321/1 8
°C + 273 2
2 778 X 10 7
ttwhr =
3 725 X 10 7
hp-hi =07376
fMb = 9478 X
2778X10'4kwhr
44 25 ft Ibs/mm
1.341 hp
3412Btu/hr
Application of Units
Unit
kilogram per
cubic meter
milligram per
filer (water)
kilogram per
cubic meter
per day
cubic meter
per square meter
per day
cubic meter or
liter of free air
per second
lumen per
square meter
Symbol
kg/m3
mg/l
kg/m3/d
m3/m2/d
m3/s
l/s
lumen/m2
Comnwntt
The density of water
under standard
conditions is 1,000
kg/m3 or 1,000 g/l
or 1 g/ml
If this is converted
to a velocity, it
should be expressed
m mm/s (tmm/s =
864m3/m2/day)
Customary
Equivalents*
0 06242 Ib/cu ft
1 ppm
0 06242 Ib/cu ft/day
3281 cu ft/sq ft/day
0 09234 ft candle/sq ft
•Miles are U S statute, qt and gal are U S liquid, and oj and Ib are avoirdupois
* U S. GOVERNMENT PRINTING OfTOfc 1977- 7 57 - HO /6604
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