COST-EFFECTIVENESS ANALYSIS
"" OF ALTERNATIVES FOR
SMALL WASTEWATER TREATMENT SYSTEMS
by
J.J. Troyan,1 and D. P. Norn's2
Prepared for the
Environmental Protection Agency
Technology Transfer
Municipal Design Seminar
on
Small Wastewater Treatment Systems
March 7 to 9,1977
Seattle, Washington
1 Project Manager, Brown and Caldwell, Eugene, Oregon
2Vice President, Brown and Caldwell, Eugene, Oregon
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COST-EFFECTIVENESS ANALYSIS OF ALTERNATIVES FOR
SMALL WASTEWATER TREATMENT SYSTEMS
In response to the increasingly acute financial problems presented to
small communities and rural residential areas in the past five years due to water
pollution control requirements, new emphasis is being placed on examining the
possible use of nonconventional sewerage systems. When conventional com-
munity systems have been proposed for these areas, the magnitude of the cost to
individual homeowners has quite often resulted only in delays in taking any kind
of action. Some research and development work has taken place over many years
on individual on-site treatment and disposal systems as well as non-convention-
al systems such as pressure sewers. However, the most recent work, initiated
because of the high cost of the conventional systems, has only begun to become
familiar to consulting engineers and public officials in the wastewater manage-
ment field.
As part of the Environmental Protection Agency's Technology Transfer
Seminar Program, the objective of this paper is to present specific information
pertinent to the cost-effectiveness analysis of sewerage systems for both small
communities and rural residential areas. Toward that end, we have included
procedures for use in determining the feasibility and desirability of using four
on-site systems and four types of community collection systems. The material
herein does not emphasize procedures for the selection of community treatment
systems for two reasons: first, the cost of collection systems is usually the
largest single part of a municipal system for small communities; and second,
detailed analysis of various small treatment systems is being provided by another
author in this same program.
In particular, this paper includes sections describing the problem condi-
tions which must be considered in selecting sewerage alternatives; the advan-
tages, disadvantages, and limitations of the on-site and community collection
alternatives being discussed in this seminar; a procedure for screening and
analyzing costs of alternatives for individual homes; and a set of case histories
taken from recent sewerage reports and facilities plans which show how and
why some of the more nonconventional systems have been analyzed.
PROBLEM CONDITIONS
The evaluation of on-site sewage disposal systems and nonconventional
community collection systems should start with three basic premises:
1. Where site conditions are suitable, the conventional septic tank-
soil adsorption system (ST-SAS) is the best type of on-site disposal
system.
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2. Where costs are reasonable, a conventional gravity sewage collec-
tion system is the best type of community system.
3. A conventional gravity collection system represents the standard of
community sanitation against which all alternatives should be
measured.
We all recognize that there are situations in which the conventional
ST-SAS would not work satisfactorily, and in which the cost of a conventional
gravity system would be exorbitant. The problem in these situations is one of
sorting out the alternatives in a rational manner and selecting quickly a short
list of alternatives which warrant detailed analysis. Fortunately, recent work
in the area of alternative collection, treatment, and disposal systems for small
flows has provided a background of information which makes possible the initial
screening of alternatives with a minimum of time and a high level of confidence.
This screening process is effective because there is a range of available alter-
natives, each of which deals with certain specific problem conditions and each
of which may be most cost-effective in a certain rather narrow set of circum-
stances. The starting point for the screening process, therefore, should be an
inventory of the problem conditions encountered in the study area. The princi-
pal problem conditions may be grouped into four categories: soils, site char-
acteristics, geology-hydrology, and climate.
Soils
Quite naturally, the nature of the surface soils has a major effect on the
function of soil absorption systems. Three factors are of particular concern:
Permeability. It is a well-recognized fact that soil absorption systems
will not work in soils that will not absorb water. Tight clays and other soils
of low permeability generally preclude the consideration of soil absorption sys-
tems. Sixty minutes per inch as measured by the percolation test is often used
as the lower limit of permeability for soil absorption systems, but the authors
believe this value to be unnecessarily restrictive especially in light of the
accuracy of the test. In a well-designed and properly constructed ST-SAS, a
percolation rate of 120 minutes per inch can be more than adequate to support
the rate of infiltration from the disposal trench into the adjacent soil. Soils
of very low permeability will also generally preclude the use of percolation-
assisted evapotranspiration systems.
Depth to Impermeable Layer. Even though surface soils may have ade-.
quate permeability, a soil absorption system will not work if a shallow imper-
meable layer prevents downward percolation of the wastewater. A shallow im-
permeable layer may lead to an accumulation of perched water which will flood
the disposal trench and cause clogging of the trench infiltrative surface. An
unsaturated soil column of about 3 ft is generally accepted as adequate for
effective draining of fine-grained soils.
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Depth to Creviced Bedrock. Experience has shown that unsaturated soil
is an excellent medium for the removal of pathogenic bacteria and viruses.
Wastewater may flow for long distances through crevices in bedrock, however,
without such purification. One goal of an adequate soil absorption system,
therefore, is to achieve an adequate distance of travel through unsaturated soil
before the wastewater enters crevices in underlying bedrock. A commonly'
accepted value for the minimum depth of unsaturated soil is two to three ft for
fine-grained soils and up to 10 ft for coarser soils.
Site Characteristics
Lot size and study area topography both influence the selection of avail-
able alternatives for wastewater disposal.
Lot Size. The average lot size, and the corrollary factor of distance be-
tween homesites, influence the feasibility of both on-site and community
sewerage systems. For example, a minimum lot size of about one acre is fre-
quently required to accommodate an adequate ST-SAS with proper allowances for
setbacks and for the house itself. It is not normally desirable to construct any
subsurface discharge system on lots smaller than 1/2 acre, and if water is
obtained from individual wells, the minimum lot size should be about one acre.
The cost of conventional community sewerage, on the other hand, in-
creases rapidly as the distance between households increases and population
decreases. For average lot sizes of two acres or more, community sewerage is
not normally the most economical solution. For lot sizes between 1/2 acre and
two acres, particularly where the ST-SAS is not a suitable solution, a careful
cost analysis may be required to select the most cost-effective method of waste-
water management.
Topography. While an adequate slope is necessary for gravity sewerage,
excessively steep or irregular topography can limit the options available. A
slope of about 25 percent is generally considered limiting for a ST-SAS, and
construction of any on-site system is difficult at that slope. Irregular, hilly
topography frequently requires numerous lift stations if a gravity collection sys-
tem is installed. This is more likely to be true if the area was developed
entirely on septic tanks, and roads were laid out without thought of later con-
struction of sewers. These conditions often shift the economics in favor of a
community pressure sewer system. Vacuum sewers are not well suited for use
in irregular, hilly topography, but may offer some cost advantage where there
is no slope, especially if soils are unstable.
Geology and Hydrology
.Depth to bedrock, soil stability, and groundwater hydrology are probably
the most important group of factors involved in the selection of the best system
for a small wastewater flow.
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Shallow Bedrock. Bedrock within five feet of the ground surface forms
an impervious layer which rules out the use of a ST-SAS. Bedrock closer than
six feet to the ground surface will probably result in excessive costs for any
gravity collection system. Closer than three ft to the surface, bedrock probably
rules out most on-site systems, except septic tank-slow sand filter-surface
discharge systems, mound systems, or, in some cases, -evapotranspiration
systems. Only pressure or vacuum collection systems are likely to provide
community sewerage at a reasonable cost in an area with shallow bedrock.
Unstable Soils. Common examples of soil instability are sandy soils
with a high water table, and fine sediments with a high water content such as
one finds in swamps and some tidal estuaries. Costs of dewatering, sheeting,
and other measures necessary to construct deep sewers in unstable soils may
cause a five-fold or greater increase in sewer construction cost. Some types
of unstable soils can cause gravity sewer lines to shift after they are installed,
changing slopes and adding high maintenance costs to high construction cost.
Seasonal High Groundwater Within Four Feet of Surface. There is abun-
dant documentation in the technical literature for the fact that high groundwater
has a detrimental effect on the function of a septic tank soil absorption system.
Inundation of the infiltrative surface due to seasonalhigh groundwater or perched
water on top of an impervious layer leads to rapid failure of the soil absorption
system which can be reversed only by an extended rest period. A commonly
used limiting value for seasonal high groundwater in fine-grained soils is three
feet below the bottom of the drainfield trench. For coarser, granular soils a
lower value may be acceptable. For a minimum-depth drainfield trench of 12
inches of gravel fill with the top of gravel at natural ground surface and covered
by a mounded soil fill about 12 inches deep for protection, the limiting minimum
depth to seasonal high groundwater is about four feet. For higher groundwater
levels, a modified, mounded soil absorption system must be used to raise the
infiltrative surface above natural ground level.
Seasonal High Groundwater Within 24 Inches of Surface. Even with the
use of a mounded system there is a limit to the acceptable groundwater height.
Water which flows downward from a mounded soil absorption system must be
able to flow laterally from under the mound into the adjacent soil mantle with-
out surfacing at the edge of the mound. To prevent surfacing at the edge of the
mound the groundwater beneath the mounded system should not rise to within
24 inches of the surface for any extended time period (equivalent of two weeks
or more). This seasonal maximum groundwater height can be assumed to be an
upper limit on any type of soil absorption system.
Climate
Except for the special conditions which prevail in arctic climates, clima-
tic factors have little effect on most community sewerage systems. Climate can,
however, be severely limiting for certain on-site disposal systems.
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Long, Cold Winters. Extended periods of severe cold will preclude the
use of uncovered sand filters because of ice formation on the filter surface.
A long,cold winter will generally limit annual evapotranspiration to the point
that evapotranspiration units will not function properly.
Low Net Annual Evapotranspiration. Even in warm climates excessive
rainfall may limit evapotranspiration to a value that will preclude the use of
ET systems. Quite obviously, an ET unit relying solely on evapotranspiration
will not function in an area where the net ET is zero or negative. ET units have
been designed for combined ET and percolation rates ranging from 0.16 to 1.6
inches per day.3 As a practical matter, ET units should be evaluated very
carefully in any location where the net annual ET is 24 inches or less. For a
suburban family of five with a per capita water use of 100 gallons per day, for
example, a 24-inch net annual ET would require more than 1/4 acre to construct
an adequate ET bed.
DESCRIPTION OF ALTERNATIVES
IN COST-EFFECTIVENESS ANALYSIS
Eight separate alternatives for disposal of wastewater from individual
homesites, listed in Table 1, are described briefly in this section. The alter-
natives are divided into two groups -on-site systems (Table 2) and community
collection systems (Table 3). Each alternative included in this description and
the following screening process is dependent to some extent on the problem con-
ditions described in the previous section. Community treatment systems are not
described here because their use is not dependent on the specified problem
conditions (only their cost). In addition, since collection system costs are
usually predominant in the total cost of a new sewerage system for a small
community, the basic choice in a cost-effective analysis will normally be be-
tween on-site systems and community sewers of some sort.
Description of the alternatives is presented in terms of the advantages,
disadvantages, and limitations of each.
Table 1. Alternatives in Cost-Effectiveness
Analysts
Group A. On-Site Disposal Systems
1. Septic tank - soil absorption
2. Septic tank - mound
3. Septic tank - ET/ETA
4. Septic tank - sand filter-surface discharge
Group B. Community Collection Systems
1. Conventional gravity sewers
2. Small-diameter gravity sewers
3. Pressure sewers
4. Vacuum sewers
Limitations listed for an alternative are
those characteristics which would pre-
vent the use of that alternative. Advan-
tages and disadvantages of an alter-
native are characteristics which de-
termine the relative desirability of that
alternative, but do not bear directly on
whether or not it can be used.
The descriptions given to each
alternative are drawn both from the ex-
periences of the preceding authors and
from Brown and Caldwell's findings.
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Table 2. Description of On-Site Disposal Systems
Advantages
Disadvantages
Limitations
A.I. Septic tank - soil absorption system
Simple, minimum O&M
requirements
Relatively low construction cost
Can operate in wide range of
climates
Large space requirement
Nitrate often discharged to
groundwater
Seasonal high groundwater must
be deeper than 4 ft
Impermeable soil layer or ex-
cessively permeable soils must
be deeper than 3 ft beneath
trench bottom
Chemicals not necessary
Power may not be required
Can be used in some areas ST-SAS
cannot,due to limitations of soils,
hydrology, or geology
Minimum O&M requirements
Can operate in wide range of
climates
Chemicals not necessary
Ai 2. ST - mound system
Larger space requirement than
ST-SAS
NC>3 often discharged to
groundwater
Visual impact
High construction cost
Usually requires pumping from
ST to mound
Must be 24 inches acceptable
soil above:
1. groundwater
2. restrictive soils
3. excessively permeable
soils
Must be allowed by state
regulations
Can be used in some areas where
soil disposal systems cannot
Chemicals not necessary
Power usually not required
NO3 not discharged to ground-
water from lined bed
Minimum O&M requirements
A.3. Septic tank - ET system
Very large space requirements
in most areas
Not operable in all climates
Vegetative cover should be
maintained in healthy
condition
Very high construction cost
Must have water-tight bed
lining in high-ground water
areas
Annual ET rate must exceed
annual precipitation for lined
beds
Salt accumulation in bed may
limit service life
Must be allowed by state
regulations
Not generally applicable in very
cold climates
Can be used where soil disposal
and ET systems cannot
NO3 not discharged to ground-
water
Small space requirement
Operable in wide range of
climates
A.4. Septic tank - sand filter system
Filter surface and disinfection
units require periodic main-
tenance
Disinfection necessary
Pumping necessary
State/federal discharge permit may
be required
Sampling and inspection of opera-
tion may be required
Must be permissible under state
regulations
Uncovered filters not applicable
in very cold climates
High construction and O&M costs
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Table 3. Description of Community Collection Systems
Advantages
Disadvantages
Limitations
Can be used in any climate
Pumping may not be necessary
No ST pumping required
Relatively low O&M cost if no
pumping stations
Sewage generally not septic at
treatment site
Centralized control of waste-
water treatment
B.I. Conventional gravity sewers
High construction cost
Deep manholes required
Deep excavation may be
necessary
Pump stations may proliferate
in hilly areas (high con-
struction & O&M costs)
None, if cost-effective
Can be used in any climate
Lower construction cost than con-
ventional system
Pumping may not be required
Relatively low O&M cost if no
pumping required
B.I. Small-diameter gravity sewers
Septic sewage conveyed to
treatment site
Deep excavation may be
necessary
Every new home must have a
septic tank
Large manholes not necessary,
but cleanouts must be provided
Pump stations may proliferate in
hilly areas (high construction
and O&M costs)
Must be allowed by state
regulations
Can be used in any climate
Deep manholes not necessary
Low construction cost
No infiltration/inflow
Shallow excavation
B.3. Pressure sewers
Pumping required
Relatively high O&M requirement
Septic sewage conveyed to
treatment site
Every new home must have a
septic tank and pump or
grinder-pump
Must be allowed by state
regulations
Can be used in any climate
Deep manholes not necessary
Low construction cost
Minimum infiltration/inflow
Shallow excavation
B.4. Vacuum sewers
Vacuum must be maintained
High O&M cost
Difficulty in locating mal-
functions
Must be allowed under state
regulations
Not applicable in extremely
hilly terra in
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PROCEDURE FOR EVALUATING ALTERNATIVES
The evaluation of alternatives for on-site systems and small community
systems should begin with the basic assumption of equality in system life and
performance. It makes little sense, for example, to compare a system with
unquestioned reliability to another system which may be inoperative several
weeks per year. Similarly, a system with an anticipated life of 40 years shou»ld
not be compared with a system with an anticipated life of 10 years unless allow-
ance is made for four-fold replacement of the latter. Starting with this basic
assumption of equality of service, the first step in evaluation should be a
screening process to select the most suitable alternatives. The selected alter-
natives should then be compared on the basis of life-cycle costs, and the length
of the selected life cycle should be at least equal to the projected life of the
structures that the system is designed to serve.
Screening of Alternatives
For practical application, there are two aspects to the determination of
feasibility of alternatives. The first is technical feasibility, and the second
is administrative feasibility.
Technical Feasibility. Each of the problem conditions presented and de-
scribed previously limits in some respect the application of certain sewerage
alternatives. For each problem there is a probable best response in terms of.
selecting or discarding sewerage alternatives. Note the qualifying word "pro-
bable". There are, of course, varying combinations and degrees of problem
conditions which render any absolute judgement impractical. With this quali-
fication, however, we may screen the available alternatives for technical feasi-
bility in accordance with Table 4.
Administrative Feasibility. As a practical matter, no sewerage alterna-
tive is feasible unless its construction is approved by appropriate regulatory
agencies. A mounded soil absorption system may be the best solution to a per-
ticular problem, but if present regulations forbid its use, it is not a feasible
alternative. This does not mean that we should not work for modification and
improvement of existing regulations, but each individual decision must be made
within the context of the regulations which govern at that time.
An example of this situation is the use of an on-site sand filter followed
by direct discharge to a waterway. Research indicates that sand filtration of
septic tank effluent, followed by chlorination and direct discharge to surface
waters, is an acceptable individual on-site disposal system. However, under
current regulations such a unit would usually require the issuance of a specific
NPDES permit, and the system would possibly be discarded as administratively
unworkable. The final step in screening alternatives, therefore, is the discard
of those technically feasible solutions which cannot be implemented within the
framework of existing regulations.
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Table 4. Technical Screening of Alternatives
Problem Condition
Probable Best Response
Soils
Impermeable
Shallow permeable layer over impermeable
layer or creviced bedrock
Site
Average net lot size 2 acres or more
Average net lot size less than 1/2 acre
Steep slopes
Irregular, hilly topography which.would
require deep cuts or numerous lift
stations
Geology-hydrology
Shallow bedrock
Unstable soils which result in high excavation
costs
Seasonal high groundwater within 4 ft of
surface
Seasonal high groundwater within 2 ft of sur-
face
Climate
Long, cold winters
Low net annual evaporation-transpiration
Discard ST-SASa, ST-Moundb, and ST-ETAC
Discard ST-SAS and ST-ETA
Discard conventional gravity system
Discard ST-SAS, ST-Mound, ST-ETd,and ST-ETA
Discard ST-SAS and ST-Mound
Discard conventional gravity collection system,
small-pipe gravity system, and vacuum system.
Discard ST-SAS, ST-ETA, and both conventional
and small-pipe gravity collection systems
Discard conventional and small-pipe gravity
collection systems
Discard ST-SAS
Discard ST-Mound and ST-ETA
Discard ST-ET
Discard ST-ET
aSeptic tank with conventional soil absorption system
'-'Septic tank with mounded soil absorptiqn system
GSeptic tank with evaporation-transpiration - soil absorption system
"Septic tank with evaporation-transpiration system
The engineer, in the course of the administrative screening process, must
be alert to catch both those alternatives which are technically feasible but not
permitted and those which are permitted but not technically feasible. As an
illustration of the latter case, regulations governing septic tank-soil absorption
systems in many areas permit the installation of systems which we now recognize
as substandard. A cost comparison of septic tank systems and community
sewerage systems under these circumstances will violate the basic assumption
of equality of service on which all alternative systems should be compared.
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Cost Analyses
It is not the intent of this paper to set forth specific unit costs which
should be used in evaluating alternatives for small community or on-site waste-
water systems. These costs may vary significantly from community to commun-
ity, and any professional engineer can prepare a more accurate cost comparison
from local information than we could present here. Rather, the purpose of this
paper is to point out the basic alternatives on the basis of equal life-cycle
performance.
Any on-site system should be evaluated over a period equivalent to the
life of the structure it serves. For a single-family dwelling, a reasonable life
is 50 years, and all cost comparisons which include on-site systems should be
compared over that period. To do otherwise is unrealistic. If a conventional
septic tank-SAS is known to have a short life span in an area, the fact that it
may be the apparent best solution-,based on present capital costs is not a
realistic situation, since the owner may be faced in 10 years with the cost of an
expensive sewerage project required for the protection of public health. In
extreme cases, failure to plan a sewage disposal system for the life of the
dwelling has resulted in people being forced to abandon their homes to abate
a serious health hazard. A utility function should not be allowed to govern the
useful life of the dwelling, and insofar as possible utility functions should be
selected which will be most economical over the life of the dwelling.
Both for community sewerage systems and for on-site systems the cost
analysis must include all reasonable operating and maintenance costs associated
with the systems and, where appropriate, the cost of establishing and operating
public agencies to supervise construction, operation and maintenance.
Costs for Septic Tanks. Each system which incorporates a septic tank
should include an evaluation of the following costs:
1. Initial cost of the septic tank, installed.
2. Cost of pumping the septic tank at intervals not greater than once
every four years.
3. If the tank is steel, the cost of replacement at 15-year intervals.
4. The cost of an initial construction permit (this permit will also
cover the effluent disposal system).
5. The fee for a periodic inspection at intervals not greater than two
years (this inspection will also cover the effluent disposal system).
Costs for Soil Absorption Systems. The cost of a soil absorption system,
whether subsurface or mounded, may be assumed to vary almost directly with the
amount of infiltrative surface area provided. Since the infiltrative surface area
required by regulation varies widely from state to state, and in many cases from
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county to county, some common basis is necessary for the calculation of life-
cycle costs which can be reasonably compared with the cost of other alterna-
tives. The lack of uniformity in regulations is matched by disagreement in the
scientific community regarding the basis for design of infiltrative surface area.
McGauhey and Winneberger recommended consideration of only the sidewall
area of the disposal trench. Bouma^, on the other hand, recommends considera-
tion of only the bottom area of the disposal trench. Healey and Laak recomimend
use of both the sidewall and bottom area of the disposal trench as useful in-
filtrative surface area.5 For the disposal trench configurations most commonly
required by regulations, sidewall area is about equal to bottom area, and proba-
bly either can be used. In any case, the most important factor in the design of
a soil absorption system is the provision of.sufficient infiltrative surface, and
a current conservative approach would therefore use either sidewall area or
bottom area, but not total trench area.
For any given design, the life of individual soil absorption systems will
vary depending on the site characteristics, household water use, and attention
to septic tank pumping. Some systems will fail early and some will last in-
definitely. Soil absorption systems should be evaluated on a basis which, on
the average, will provide a level of environmental sanitation equivalent to that
obtainable from conventional gravity sewerage. One way to approach this eval-
uation is to define a "median", or "control" system, which can be expected on
the average to last 15 years before failure. Using this control system as the
basis for comparison, any set of regulations may be used to compare septic
tanks in any area on a common basis using the following assumptions:
1. The life of a soil absorption system, properly installed, is directly
proportional to the amount of infiltrative surface area, so long as
soil percolative capacity is above a limiting value.
2. A gravity-dosed absorption system in fine-grained soils with 450
sq ft of infiltrative surface area per bedroom is defined as a control
system and will last for 15 years. Less infiltrative surface area
will result in a proportionately shorter life.
3. A soil absorption system, supplemented after failure with a second
identical system and an alternating valve, may thereafter be opera-
ted by alternately resting each half and will last for the life of the
dwelling.
4. Systems smaller than a control system will have to be supplemented
at shorter intervals until the total installed infiltrative surface
equals that of two control systems. Thereafter, the systems may be
rested alternately and will last for the life of the dwelling.
5. Coarse-grained soils which permit clogging in depth at the infiltra-
tive surface will accept higher application rates. '" For coarse-
grained soils it is assumed that a control system may be defined as
250 sq ft of infiltrative surface area per bedroom.
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Based on the above assumptions, the cost evaluation for soil absorption
systems should include the following items:
1. Initial construction cost of the absorption system.
2. Cost of constructing a supplementary absorption system in accor--
dance with the above assumptions on design life.
3. For systems in which the septic tank effluent is pumped to the
disposal field, include the following additional costs:
a. Initial cost of installing the pumping unit, complete with
electric power and controls .
b. Cost of replacing the pumping unit every 10 years for the life
of the dwelling.
c. Cost of power and maintenance for the pumping unit. Main-
tenance cost should include at least one service call per
year.
Costs for Evapo-Transpiration Systems. Each evapo-transpiration sys-
tem must be preceded by a septic tank, with costs as set forth above. A very
careful cost comparison should be made wherever ST-ETA units are considered
to be sure that ST-SAS units are not more cost-effective.
ET units may be expected to work in favorable climates where impermeable
soils or high groundwater prevent the use of SAS units. Lined beds are generally
required in areas with high groundwater. To date we have very little informa-
tion which may be used to develop a reliable prediction of the average life of ET
units, but for the purposes of this discussion it is assumed that problems with
salt buildup and bed linings will require complete bed replacement every 15
years. This criterion is .assumed in lieu of more definitive information.
Costs for Sand Filters. A sand filter is subject to the same limitations
on its useful life as a soil absorption system. Unattended, its infiltrative sur-
face will eventually clog, and it must be periodically renewed by scraping and
sand replenishment. The filter structure may be expected to last indefinitely so
long as it is adequately maintained. Sand filtration and direct discharge is
assumed to include effluent disinfection. This is a high-maintenance system
which the individual homeowner cannot be expected to maintain, and its use,
even where permitted by regulatory agencies, should be supervised by a quali-
fied public agency. A cost evaluation of sand filter systems should include the
following items:
1. Initial construction cost, including fencing for mild climates and
a complete cover for severe climates.
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2. Bi-monthly inspection of the disinfection equipment, and replenish-
ment of chemicals, if required.
3. Cost of power and chemicals for disinfection.
1 4. Semi-annual rejuvenation of the sand surface.
5. Replacement of the chemical feed equipment every 10 years.
6. All costs for septic tanks as set forth above.
7. If pumping to the filter is required, all costs for pumping as de-
tailed for soil absorption systems.
Community Systems. The elements of community collection and treat-
ment systems are well-known, and the costs therefore need little explanation.
It is important, however, whenever a community system is compared with in-
dividual on-site systems, that the comparison be made over a time period which
represents the life of the dwelling and which includes all costs to the individual
homeowner. This comparison should include the following costs:
1. The installed cost of the community system, recognizing that the
elements of a community collection system, except for mechanical
equipment, normally have a life expectancy of 50 years.
2. The installed cost of all on-site components of the community sys-
tem, such as house connections and septic tanks, where required.
3. For pressure systems and small-pipe gravity systems, all of the
costs associated with septic tanks as set forth previously.
Summary
We believe that the procedures for evaluation presented above will iden-
tify with a minimum effort and maximum uniformity the most suitable alternative
for handling small community sewage flows. The procedures allow for variations
in local regulations and still permit consideration of all alternatives on the basis
of equality of function over the life of the system.
CASE HISTORIES
The preceding sections of this paper have described problem conditions
which should be considered in analyzing wastewater systems for small commun-
ities, alternative systems which might be used as tools to overcome particular
problems, and a recommended procedure for determining which system or systems
would be best suited for any one of a number of conditions. In the following
section, we present a set of five case histories drawn from recent facilities
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plans in order to show why and how relatively innovative and nonconventional
wastewater systems have been developed, analyzed, and in some cases select-
ed as the most cost-effective solution to a sewerage problem in a small com-
munity. While not all alternatives described earlier in this paper are represent-
ed in these case histories, the cases do provide a good spectrum of responses
to small-community sewerage problems. It should be noted that these case
histories do not represent examples of the proposed cost-analysis procedure
described in the previous section, but are included both as sources of informa-
tion and knowledge, and as subjects of discussion in the Design Seminar.
CASE HISTORY NO. 1
GLIDE-IDLEYLD PARK, OREGON
The Glide-Idleyld Park area is an unincorporated community in Douglas
County, Oregon with a present population of about 2,500. The area is without
public sewers at the present time, and all wastewater is treated in septic tanks
and disposed of through drainfields. Due to unsuitable soils and high ground-
water conditions, however, numerous drainfield failures have occurred. Because
of this problem the Douglas County Department of Public Works undertook a
study of wastewater management for the area in 1975. The material for this case
history is taken from that study.
Physical Characteristics
The Glide-Idleyld Park area of Douglas County is located in a scenic
setting along the North Umpqua River on a highway leading to Crater Lake
National Park. The area is noted for fine trout fishing and scenic beauty. There
has also been high interest in recent years in home-building in the area. Other
information found in the sewerage study regarding physical characteristics of
the area is presented below.
Climate. The region has a temperate climate with moderately warm sum-
mers and wet, but mild winters.
Soils and Geology. Clayey soils are found throughout the study area,
while outcroppings of rock occur near the surface in some areas.
Groundwater. High groundwater is prevalent throughout the study region.
While abundant evidence of surface water pollution was available and presented
in the report, the extent of groundwater pollution due to failing drainfields was
unknown.
Topography. The study area contains a wide variety of slopes, elevation,
and physical features, all appealing to the many visitors to the area. The
terrain varies from gently rolling fields to steeply sloping hillsides with solid
rock evident in many places.
14
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Site Characteristics. The total study area contains approximately 13,000
acres, is 11 miles in length, and averages 1.75 miles in width. At the present
time most of central Glide is divided into lots ranging from 20,000 sq ft to 1 or
2 acres. Many of the lots do not lend themselves to subdivision. Present low-
density residential areas have a preponderance of 1-acre to 7-acre parcels, few
of which could be subdivided. . Densities used for future planning were 1.5
homes per acre for the more urban portions of the study area, and two acres per
home for outlying areas.
Alternatives and Screening Process
While no screening process is explicitly presented in the sewerage study,
the following facts are presented, and, in total, do provide the basis for de-
tailed analysis of the alternatives examined in the cost-effectiveness analysis.
1. Sixty percent of existing ST-SAS units are failing in the study area.
2. Analysis of soils in the area showed that only a very small portion
was suitable for installation of drainfields due to the abundance of
clayey to rocky soils. In some areas shallow surface soils over-
lie basalt bedrock.
3. Three previous studies of the area had recommended community
sewerage, all by conventional gravity sewers.
4. Public response in surveys and at public meetings indicated that
nearly 80 percent of the residents favored installation of sanitary
sewers.
5. No action had been taken to implement any of the initial three
sewerage plans, each time because of cost.
6. In 1974 the County Engineer's Office had conducted a study of
pressure sewers. The report on that study contains the following
paragraph in the opening general discussion: "Though there are
similar systems, such as vacuum sewers, or wastewater quantity
reduction by reuse of 'grey water1, it is believed pressure sewers
hold the most promise in difficult terrain and where a number of
homes are of concern. "^
As a result of the above factors, the cost-effectiveness analysis in the
sewerage study examined two methods of wastewater collection - pressure
sewers and conventional gravity sewers. In addition, two types of treatment
were considered, both disposing of final effluent by discharge to the North
Umpqua River during the winter and by land application during the low-flow sum-
mer months.
15
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Cost-Effectiveness Analysis
An analysis of the cost-effectiveness of alternatives for community
sewerage in the Glide-Idleyld Park area included both the collection system and
the treatment and disposal system. Emphasis in the study was given to the
collection system, since it had been the most expensive item in previous
studies.
Collection System. Both construction costs and operation and mainten-
ance costs are presented in the Douglas County report in terms of present worth.
Unit construction costs for the conventional collection system alternative
were obtained from county, state, and federal data, plus information from two
consulting engineering firms. Operation and maintenance costs were obtained
from local sewerage agencies in terms of both dollars per mile of line and dol-
lars per person served. A detailed analysis of the required lengths and sizes
of the conventional system components was conducted, including "Bstimates of
necessary segments of force main and amounts of rock excavation. The re-
sulting costs of the conventional system are shown in Table 5.
Unit construction costs for the pressure sewer collection system alterna-
tive were more difficult to obtain. The county gathered necessary costs from
national suppliers of materials and local contractors in order to get the best
possible data on labor and equipment needs for installing pipelines, pumps,
septic tanks, and electrical instrumentation. The service connection at each
home was laid out to include pumping of septic tank effluent from a small wet
well to the local pressure collection sewer. In the design, 612 existing homes
and small businesses were to be served by 568 pumps at an overall average cost
of $1,155 per connection, excluding treatment and collection system costs.
Operation and maintenance costs were obtained from agencies throughout the
United States presently using pressure sewer systems, from suppliers of effluent
pumps in Oregon, and from the local water supply agency. In terms of 1975
Table 5. Cost of Conventional Collection System
Present worth, dollarsc
Component
Gravity main
Pressure mains
Rock excavation
Service connections
Mainline pump stations
Totals
Construction
2,523,400
424,400
169,000
515,200
772,000
4,404,000
O&M
104,900
38,700
0
0
118,000
261,600
Total
2,628,300
463,100
169,000
515,200
890,000
4,665,600
Rounded to nearest $100 and calculated using 5.875 percent interest over a 20-year design period.
16
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prices each pumping unit was estimated to require $50 per year in maintenance,
while the pipelines were estimated to cost $100 per mile per year. The result-
ing costs for the pressure sewer collection systems are shown in Table 6.
A comparison of Tables 5 and 6 shows that the total present worth of the
pressure sewer system is almost exactly one-half the cost of the conventional
collection system, a difference of $2.3 million. As expected, O&M costs for
the pressure sewer system are higher than for the conventional system - by
more than 60 percent, or $166,000, but the initial construction cost of the pres-
sure system is only 44 percent of the initial cost of the conventional system -
a savings of almost $2.5 million.
Treatment Plant. Since treatment and reuse of effluent is'not feasible in
the Glide area, and insufficient land is available to rely on land application as
a year-round disposal method, treatment and river discharge with summer land
application was chosen as the only practical alternative. Two methods of treat-
ment were considered - extended aeration with effluent polishing by microstrain-
ing, and aerated lagoons followed by intermittent sand filtration. As noted in
the report: "The extended aeration option is common in Oregon and found to be
generally acceptable. Lagoons, however, have not been generally accepted due
to poor effluent quality." Since no intermittent sand filters were in operation in
Oregon at the time the study was being conducted, it was not known if the State
Department of Environmental Quality would accept such a recommendation.
Therefore, for the purposes of the study, extended aeration and micro strain ing
was used for determination of user charges for a complete project. At the same
time, the county stated its intention to continue evaluation of the cheaper la-
goon-sand filtration alternative. Comparative costs for the two treatment sys-
tems are shown in Table 7.
User Charges
An assessment of user charges for the recommended project - collection
by pressure sewers and treatment by extended aeration, microstraining, and
Table 6. Cost of Pressure Sewer System
Present worth, dollarsc
Component
Pressure mains
Service connections
Rock excavation
Mainline pump stations
Totals
Construction
, 900,900
853,700
126,000
76,000
1,956,600
O&M
28,900
374,300b
0
24,700
427,900
Total
929,800
1,228,000
126,000
100,700
2,384,500
aRounded to nearest $100 and calculated using 5.875 percent interest over a 20-year design period.
Includes both initial and future connections.
17
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Table 7. Comparison of Glide Area Treatment Costs
Process3
Extended aeration + microstraining
Stabilization ponds + intermittent
sand filtration
Construction
cost
$350,000
$150,000
Annual O&M
cost
$28,000
$ 5,000
Each facility sized for a 0.3 mgd design capacity.
summer land application - was also made. Included were components for both
initial costs and O&M costs for a single-family residence as follows:
Initial Annual
cost cost
Initial assessment for system
cost and lot hookup $1,500
Annual charge for initial assess-
ment @ 6 percent interest
over 20 years $129
Annual operation and maintenance
charge 114
Total annual charge to initial single-
family users $243
An annual charge for septic tank pumping is included in the annual O&M charge,
based on the local rate of $35 for pumping a single tank. Assessments for
future hookups were estimated to be $1,800, or $300 more than for initial users.
CASE HISTORY NO . 2 . BELLEVUE, IDAHO
Bellevue, Idaho is a community of approximately 550 persons located in
south-central Idaho. The case history described below presents a cost-effec-
tiveness analysis of the alternatives of (1) continuing the use of individual sub-
surface disposal systems or (2) sewering the community with conventional gra-
vity sewers. The description and analysis are taken from a facilities plan pre-
pared for Elaine County, Idaho by Brown and Caldwell.
18
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Physical Characteristics
Physical characteristics described in the report include climate, soils
and geology, groundwater hydrology and quality, topography, and site char-
acteristics.
Climate. At an elevation of 5,200 ft, Bellevue has a long winter and a
short, rather cool summer. The mean annual snowfall is approximately 100
inches, most of which falls between November and March. Mean daily mini-
mum temperatures are below freezing generally from September through May.
Soils and Geology. As part of an ancient lake-bed, the ground beneath
Bellevue is comprised of alternating layers of clay, sand, silt, and gravel.
The principal soil association in the vicinity of the community is characterized
as a well-drained, shallow, gravelly loam. It has been described by the Soil
Conservation Service as poor for subsurface disposal due to its coarse texture,
which creates a high possibility of groundwater contamination.
Groundwater. Depth to groundwater in and around the community is
generally greater than 25 ft. Groundwater quality is excellent both above and
below Bellevue and is presently suitable for both domestic and agricultural
uses. In particular, samples taken below Bellevue, with respect to the direc-
tion of groundwater flow, show that no perceptible change has occurred in
groundwater quality over the past 20 years.
Topography. The ground surface is quite flat both within the present
community and in areas immediately adjacent to the present city limits. The
maximum slope in this area is approximately 5 percent.
Site Characteristics. While the present city limits enclose an area of
about 500 acres, sizes of existing lots were not available to the study. The
cost-effectiveness analysis, however, considered a range of costs for various
lot sizes.
Alternatives and the Screening Process
Two basic alternatives were considered for Bellevue: (1) continuing the
use of septic tank-soil absorption systems, and (2) community sewers followed
by some form of treatment and surface discharge. On-site alternatives other
than the ST-SAS were not considered for the following reasons:
1. Present ST-SAS systems have experienced a low failure rate.
2. No groundwater degradation is noticeable in wells downstream of
the community in samples taken 20 years apart.
3. Population growth projections do not indicate that groundwater de-
gradation will be a future problem.
19
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4. In general, groundwater depths and soil permeabilities are suitable
fonthe use of the ST-SAS.
Cost-Effectiveness Analysis
An analysis of the cost-effectiveness of the use of the ST-SAS for the
community of Bellevue is presented below in two parts: (1) an analysis of total
annual costs of a new ST-SAS versus the annual cost of collection sewers for
various let sizes, and (2) the total annual cost of a complete sewerage system
comprised of collection and trunk sewers, plus three types of community treat-
ment .
Homeowner Costs. The cost of septic tank-drainfield systems is rela-
tively constant regardless of lot size, assuming there is sufficient acreage to
handle the required system to begin with and the topography and soil types are
suitable. Community sewerage cost to the homeowners, on the other hand,
varies directly with lot size. As the size of lot is reduced and houses are con-
structed more densely, the overall cost of sewers to the homeowner is reduced,
since the cost for essentially the same length of sewer line is split between
more homeowners. It is therefore possible to compare the cost of septic tank-
drainfield systems with the varying cost of community sewerage to determine the
lot size at which it becomes more economical to use a community waste treat-
ment and disposal system. This comparison, on the basis of cost alone, is not
the only factor which enters into a decision regarding minimum lot size for septic
tank systems, however. Although cost does represent a limiting condition for
planning purposes, other factors include availability of public water supply and
public acceptance of septic tanks. Where all other factors are substantially
equal, public sewers should not be planned for residential areas where their
overall cost would be greater than that of septic tank systems.
The cost of septic tank systems in Blaine County is presented below in
Table 8. Installation costs were obtained from contacts with installers in the
study area. The overall cost estimate is based on construction of a three-
bedroom home using Idaho regulations and assuming a percolation rate of 10 min/
inch. The resulting design criteria is 165 sq ft per bedroom. The annual costs
were computed for a 20-year period at 8 percent interest. Operation and main-
tenance costs included both the periodic cost of pumping the tanks, and the
administrative cost associated with a proposed maintenance district. The opera-
tion and maintenance costs also include an allowance for a maintenance district.
Obviously, if implementation of such a district is either delayed or ignored, the
cost to the homeowner will be less.
Costs of community sewerage consist of three basic items: (1) construc-
tion and maintenance of collection sewers, (2) construction and maintenance
of trunk sewers, and (3) construction, operation, and maintenance of some type
of community waste treatment and disposal system. An estimate of the cost of
collection sewers alone was made based upon construction costs and assessment
20
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Table 8. Estimated Construction and Maintenance Cost for Individual Disposal Systems
Item
Septic tank system costs
Initial installation
Septic tank, 1000 gallon
Dra infield, 250 lineal feet
Permit0
Total, initial installation cost
Replacement drainfield
Operation and maintenance costs
Administrative cost of
Septic tank maintenance district
Septic tank pumping ($80 every 4 year)c
Total,annual septic tank costs
Construction
cost, dollars3
370
1250
20
1640
1250
-
-
Annual
cost, dollars'3
-
•
-
150
38
30
20
238
aAll costs are based on an EPA index of 350.
b Based on 8 percent interest and a 20-year planning period. The replacement drainfield is assumed to be
required after 10 years and the economic life of the entire septic tank system is'assumed to be 40 years.
c Based on information from the County Health District.
formulas in two nearby communities where community sewerage systems had
been constructed within the past five years. The results of that estimate are
shown in Fig. 1.
It is apparent from a review of Table 8 and Fig. 1 that at a one-acre gross
lot size the annual cost of a septic tank-soil absorption system equals the
annual cost for local collection sewers alone. In developing areas, between
25 and 40 percent of gross lot size is normally used for streets, schools, public
facilities, and small commercial zones. When the costs of trunk sewers, treat-
ment and disposal are included, the result is that an even smaller lot size is •
more economically served by septic tank systems, although the cost to the
homeowner for these latter components may not be more than 10 to 25 percent of
the actual cost due to state and federal grants. The break-even gross lot size
may be as small as 0.25 acre if expensive forms of treatment and disposal are
necessary. In these cases the governing factors on lot sizes for ST-SAS use
will be setback distances and the possible requirement for a replacement drain-
field. An estimate of homeowner costs for conveyance and treatment, excluding
the influence of grant monies, ranged between $60 and $150 per dwelling, de-
pending on the type of treatment. The lower value was for land application by
infiltration-percolation while the higher value was for tertiary treatment and
river discharge.
Community Costs. Costs determined for Bellevue in the study included
initial construction cost, average annual operation and maintenance cost,
21
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. o
CONSTRUCTION COST
0
0.5 1.0 1.5 2.0 2.5
AVERAGE GROSS LOT SIZE, ACRES
I CITY OF KETCHUM SEWERAGE
a. BASED ON ASSESSMENT FORMULA USED
PROJECT. UPDATED TO EPA INDEX 350.
b. BASED ON 8% INTEREST AND 20 YEAR PLANNING PERIOD.
Figure 1 Cost of Sewerage
annualized construction cost, and total annual cost. The annualized construc-
tion cost was calculated using a design period of 20 years and an interest rate
of 6.125 percent. Table 9 shows cost comparisons between the continued use
of the ST-SAS, and construction of collection sewers plus various types of
treatment and disposal. Overall, use of the ST-SAS is 30 percent less expen-
sive on the basis of total annual cost, than the least expensive form of com-
munity sewerage - even with the conservative assumption of implementation of
a maintenance'district.
Table 9. Community Sewerage Costs
Waste Management Option
ST-SAS
Collection sewers, gravity
Collection sewers + activated
sludge (AS) treatment
Collection sewers + AS +
removal of N and P
Collection sewers + conveyance +
lagoon treatment + disposal by
infiltration- percolation
Cost, million dollars
Constr.
cost
1.40a
1.33
2.72
3.43
1.91
Annual
constr.
cost
0.115
0.094
0.203
0.270
0.144
Avg.
O&M
cost
0.044
0.004
0.052
0.087
0.065
Total
annual
cost
0.159
0.098
0.255
0.357
0.209
alncludes new ST-SAS plus drainfield replacement after 10 years for new home.s . For existing dwelling
units, only drainfield replacement cost is included.
22
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Given the previously determined facts that there were no over-riding
detrimental social or environmental impacts, continued use of the ST-SAS was
recommended for Bellevue.
CASE HISTORY NO . 3
WESTBORO, WISCONSIN
Westboro is a community of 200 persons located in Taylor County in north-
central Wisconsin. It presently has no municipal wastewater collection or
treatment system, and all buildings in the community are served by septic tank
systems, 80 percent of which were found to be discharging wastes to the ground
surface in 1971. While a wastewater plan was developed in 1967, prohibitive
costs and lack of available federal funding have prevented implementation of a
community sewerage system. In the interest of abating their sewage disposal
problems, the Westboro residents agreed to cooperate with the Small Scale
Waste Management Project, (SSWMP) of the University of Wisconsin in an effort
to develop an alternative plan which might result in a more cost-effective facility.
The material for this case history is taken from the report of that investigation.10
Physical Characteristics
The Westboro Sanitary District encompasses, segments of Silver Creek as
it flows southward to the east of the town center and then turns westward along
the southern edge of the community. The decline of the lumber industry has
reduced the population from 900 at the turn of the century to the present 200
residents and left the community with a cheese factory, a small machine tool
company, and a sawmill to provide local employment. Detailed physical infor-
mation as presented in the SSWMP report is summarized below.
Climate. While no specific climatic information is given in the project
report, Westboro is located in the area of Wisconsin which typically has rela-
tively short, mild summers and long, cold winters.
Soils and Geology. The soils in and around Westboro are primarily deep,
well- to somewhat poorly-drained loams and silt loams over sandy glacial till.
In general, the presently populated areas of the community are located on soils
judged unsuitable for septic tank systems. Suitable soils can be found within
the city limits, including thick deposits of well-graded sand in one location.
Mucky peat soils predominate in the southern half of the community.
Groundwater. No direct information is presented in the report on ground-
water depths. The existence of the mucky, peat soils in the southern half of
the community, however, indicates near-surface levels in that area, while
groundwater levels are apparently much lower in the northern half of town. Well
monitoring since March 1975 has shown seven of 30 wells sampled to be bac-
teriologically unsafe, one of which has also had nitrate concentrations consis-
tently above the drinking water standard of 10 mg N/l.
23
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Site Characteristics. No steep slopes exist within the community. The
central area of Westboro, known as the Front Street area and including the
business district, is divided into small lots approximately 150 ft by 50 ft in
size.
Alternatives and Screening Process
As a result of the sanitary survey taken in 1971, the Wisconsin Depart-
ment of Natural Resources had declared the sewage disposal situation to be a
nuisance and a menace to health and comfort, and had issued an order to
Westboro to construct a community collection and treatment system or to stop
all private homes from discharging sewage into Silver Creek. Under this order,
both the initial 1967 study and the latest study were completed. The following
factors were brought out in the SSWMP report relative to the screening process
to develop alternatives.
1. Both lot sizes and soils prevent the replacement of most of the fail-
ing septic tank systems, thereby eliminating that alternative for
most of the community.
2. The community was divided geographically into five separate areas.
3. Two of the five areas were considered too sparsely developed for a
. collection system and soils were judged suitable for either a con*-
ventional ST-SAS or a ST-Mound system. Individual systems were
therefore recommended as the cost-effective solution in these two
• areas.
4. In two of the remaining three areas physical conditions were also
judged suitable for individual systems of some sort, but it was
decided that a common collection system offered the greatest
advantage because of the density of homes.
5. A collection-system was also considered the best alternative for the
fifth area, the central area, which includes the business district
and homes with lot sizes too small to permit the construction of
replacement drainfields or other on-site treatment and disposal
alternatives.
Several alternatives were developed for cost-effectiveness analysis of
their ability to serve the three areas considered worth sewering. Two areas
(the Front Street central area and Joseph's Addition) were combined in each of
the alternatives because of the limited number of disposal sites available in
each one separately. Alternatives for these two areas included using both
pressure and small-diameter gravity sewers to convey septic tank effluent for
disposal in a sand bank east of town along Silver Creek.
24
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Four alternatives were considered for the remaining area^(Grossman's
Addition):
1. Pressure collection of septic tank effluent and disposal with the
Front Street and Joseph's Addition areas.
2. Small-diameter gravity sewer collection of septic tank effluent with
the following disposal alternatives:
a. Pumping to the Front Street and Joseph's Addition gravity
system.
b. Soil absorption
c. Sand filtration-chlcrlnation and discharge to Silver Creek.
Cost-Effectiveness Analysis
The analysis of cost-effectiveness of both the previous conventional
gravity sewer alternatives and the new alternatives included consideration of
total cost using present worth, consideration of environmental impact, and
judgement regarding system reliability. Cost analyses were made using a
20-year system life and seven percent interest. Service connection costs were
not included in any alternative.
Conventional Alternatives. The alternatives analyzed in 1967, involving
use of conventional gravity sewers, a community treatment plant, and surface
water discharge, were updated to recent costs. The cost of installation of new
individual systems in the two sparsely populated areas was added to each of
these alternatives to permit comparison with the six new alternatives examined
by SSWMP. The present worth of the two conventional systems is shown in
Table 10. These two systems were 17 to 20 percent more costly than the least
Table 10. Cost Comparison of Conventional Alternatives
Present worth, $1,000C
Alternative Description
Alternative #1
Conventional gravity sewers +
extended aeration package
plant + discharge to Silver
Creek
Alternative #2
Conventional gravity sewers +
raw sewage stabilization pond
+ discharge to Silver Creek
Collection
136.3
136.3
Treatment
170..1
185.5
Individual
systems
12.0
12.0
Total
cost
318.4
333.8
Includes both capital costs and O&M costs.
25
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expensive new alternative analyzed below and neither conventional alternative
was recommended for implementation.
New Alternatives. Present worth of the six new alternatives was deter-
mined and listed as shown in Table 11. Of the new alternatives, Alternative
No. 5 is the least expensive, costing $266,416, or approximately $3,861 per
household. This cost per household is significantly less than the cost of con-
ventional alternatives 1 and 2 ($4,614 and $4,838, respectively).
Table 11. Cost Comparison of New Alternatives
Present worth, $l,000a
Alternative
Description
Alternative #1
Part A: S.D.c gravity
sewers to drainfield.
Part B: S.D. gravity
sewers to drainfield.
Alternative #2
Part A: S.D. gravity
sewers to drainfield.
Part B: Press, sewers
to drainfield.
Alternative #3
Part A: S.D.. gravity
sewers to sand filter.
Part B: S.D. gravity
sewers to drainfield.
Alternative #4
PartA: S.D. gravity
sewers to sand filter.
Part B: Press, sewers
to drainfield.
Alternative #5 . •
All S.D. gravity
sewers to single
drainfield
Alternative #6
All pressure sewers
to single drainfield
Grossman's
addition
(PartA)
124.5
124.5
148.0
-
148.0
d
d
Front St.
& Joseph's
addition
(Part B)
145.2
185.3
145.2
185.3
d
d
Joint
system
269.7
309.8
293.2
333.3
254.4
294.2
Individual
systems
12.0
12.0
12.0
12.0
12.0
12.0
Total
cost
281.7
321.8
305.2
345.3
266.4
306.2
alncludes both capital cost and O&M cost.
bSum of costs for Part A and Part B.
cSmall - diameter.
"Individual cost not given in report.
26
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The environmental impact of new Alternative No. 5 was expected to be
minimal. Only nitrogen in the form of nitrate was expected to leach through
the soil to the groundwater basin in significant amounts. Some of the nitrogen
was also expected to reach Silver Creek due to the short distance between the
soil absorption field and the creek.
Finally, in selecting new Alternative No. 5 the report states: "The
reliability of this type of facility has not been established, but its selection is
warranted because it is designed from extensive experience with smaller sys-
tems and its cost and environmental impact are a significant improvement over
the conventional central facilities."
Since completion of the project report decisions have also been made to
serve both the sparsely developed Queenstown and Appaloosa Lane areas of
Westboro with small-diameter gravity sewers. In the Queenstown area, site
investigation for possible design of septic tank-mound systems showed that
there was insufficient area on the occupied lots to construct the intended system,
and adjacent land could not be purchased. In the meantime, the Westboro
Sanitary District decided that it would also be beneficial to extend small-dia-
meter gravity sewers to, and slightly beyond, the Appaloosa Lane area. At the
present time, therefore, it appears that the entire community will be served by a
sewerage system comprised of individual septic tanks, small-diameter gravity
sewers, and a community soil absorption system.
CASE HISTORY NO. 4
FOUNTAIN RUN, KENTUCKY
The city of Fountain Run is located in Monroe County, in south-central
Kentucky. The planning area for the 201 Study is the area served by the
Fountain Run Water District, including both the city of Fountain Run and a por-
tion of Monroe County. The study area is without a public sewerage system at
the present time, and all wastewater is disposed through either septic tank-soil
absorption systems or privies. Of the existing ST-SAS units, approximately 20
percent are located on soils with permeabilities less than 0.5 in per hour (120
min. per in). In addition, an estimated 30 percent of the systems are pro-
ducing surfacing effluent during the winter months at least. All material for
this case history is taken from the facilities plan report for the area. H
Physical Characteristics
The study planning area covers about 2,240 acres in the western portion
of Monroe County. The population of both the county and the city of Fountain
Run declined significantly during the 1960's, leaving approximately 320 people
in the city and some 440 in the water district. Detailed physical characteristics
are presented below.
27
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Climate. The climate in the Fountain Run planning area is temperate.
Freezing temperatures occur less than 85 days annually, and there are approxi-
mately 50 days with maximum temperatures above 90 F. The average annual
snowfall and total precipitation depths are 10 inches and 50 inches, respec-
tively. Estimated annual evaporation is 40 inches.
Soils and Geology. Bedrock in the study area is limestone, interbedded
with chert and dolomite. The predominant soil association is described as in-
cluding deep, well-drained clayey and loamy soils. According to information
provided by the Soil Conservation Service, a substantial portion of the area with-
in the city limits is free of limitations on subsurface disposal systems. As noted
previously, however, about 20 percent of existing systems are located on soils
with permeabilities less than 0.5 in. per hour.
Groundwater. Little information was available to the facilities plan inves-
tigators. One well located south of the planning area is reported to have a
depth of 39 feet. No information is presented in the report regarding ground-
water quality. No public water supplies use groundwater sources, however,
since they are inadequate from a hydraulic standpoint to sustain withdrawal rates
necessary for domestic consumption.
Site Characteristics. The planning area lies in the upper reaches of two
small watersheds and contains both gently rolling hills and moderate slopes at
elevations from 700 to 850 feet. Lot sizes are typically larger than 0.75 acre.
The smallest lot in Fountain Run is approximately 12,000 sq. ft. , or slightly
larger than one-quarter acre.
Alternatives and the Screening Process
A separate, explicit screening process is not presented in the facilities
plan for Fountain Run. The following facts, drawn from the analysis of alterna-
tives, form the basis for analysis of a^limited set of wastewater management
alternatives.
1. Optimum operation of existing individual disposal systems was
considered, but it was judged impractical to try to upgrade them to
the level of current septic tank system technology.
2. Implementation of any regional solution was also rejected quickly,
since the closest town, in a neighboring county, is 12 miles away,
and an estimate of the capital cost of an interceptor system to de-
liver Fountain Run's sewage to the neighboring city exceeded one •
million dollars and was nine times more expensive than any local
alternative.
3. A previous engineering report on sewerage for Fountain Run had
been prepared using conventional gravity sewers with oxidation
pond treatment. The data from that study was available to update
as one alternative.
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4. A substantial portion of the soils within and adjacent to the city
limits is suitable for disposal by soil absorption.
Because of the above factors, four alternatives were considered, as de-
scribed below.
A. Conventional gravity sewer collection system with one of two types
of treatment: (1) a package, complete-mix, activated sludge unit
followed by soil infiltration-percolation, (2) an oxidation lagoon
with disposal by infiltration-percolation.
B. Community-wide collection by an effluent sewer system consisting
of individual septic tanks with siphons or pumps, and small- .
diameter plastic pipelines carrying partially treated sewage by both
pressure and gravity; treatment in an oxidation pond and disposal
by infiltration-percolation.
C. Collection of septic tank effluent for clusters of houses in small-
diameter pressure and gravity sewers; disposal in soil absorption
systems in suitable soils. Individual septic tank systems used
where appropriate.
D. Completely individual on-site disposal throughout the community,
with 20 percent of the systems forced to rely on non-standard
systems due to soil conditions.
Cost-Effectiveness Analysis
An analysis of the cost-effectiveness of alternatives for sewerage in the
Fountain Run Water District included detailed cost estimates of collection and
treatment systems, plus an environmental assessment. All cost analyses were
put in terms of present worth for comparison, using a 20-year design period and
an interest rate of 6.125 percent. The salvage value of all capital expenditures
was estimated and subtracted from total present worth (capital + O&M costs) to
determine net present worth.
Alternative A. The collection system for this alternative included over
20,000 ft of 8-inch line, 1,800 ft of 6-inch line, a pumping station, and 600 ft
of force main. The total construction cost for the collection system was esti-
mated to be $339,600, with an estimated annual expenditure of $9,000 for opera-
tion and maintenance. Net present worth of the collection system after subtrac-
ting salvage value, but including O&M costs, was determined to be $390,100.
Two methods of treatment were examined, and were selected on the basis of the
consultant's past experience with treatment systems for small communities.
Net present worth of the 2-acre oxidation pond including O&M costs was
$81,600, while that of the package activated sludge unit was $89,500. Three
types of disposal systems were analyzed for this alternative: (1) an intermittent
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sand filter system and effluent discharge, (2) spray irrigation, and (3) infiltra-
tion-percolation. Including O&M costs, net present worths of the three disposal
systems were $61,100, $74,900, and $53,900, respectively. Infiltration-perco-
lation was therefore chosen as the disposal method used in both Alternative A-l
and Alternative A-2. Present worth values for both alternatives are shown in
Table 12.
Alternative B. The collection system for this alternative included about
20,000 ft of 2-inch to 4-inch plastic pipe without manholes. Some would be
pressure lines, but most were designed to accept gravity flow. Also included in
the collection system were some 3,800 ft of 8-inch gravity line, septic tanks and
siphons or pumps, plus five larger main-line pumps and one pumping station.
Operating costs included in the net present worth of the collection system were
pump operation and maintenance, septic tank pumping on a 5-year cycle, and
flushing of small-diameter lines as needed. Net present worth of the Alternative
B collection system alone, including O&M costs, was estimated to be $246,900.
Community treatment and disposal for this alternative was by the same oxidation
pond and infiltration-percolation basin combination used in Alternative A-2. The
component costs of this alternative are shown in Table 12.
Alternative C. Design criteria for the soil absorption systems used in
Alternative C included an assumed average household wastewater flow of 200
gallons per day and an application rate of 0.33 gallons per sq ft per day (approxi-
mately 400 sq ft per bedroom) to the trench sidewalls in each of two half-systems
(600 sq ft per half-system) to be used in alternate years. Public management of
all on-site and community wastewater facilities was considered critical to this
alternative. ' Homes were grouped in several patterns before selecting the final
one, to achieve an optimum mix of low cost, simple operation, disposal to the
most suitable soils, and ability to accommodate future growth. The resulting de-
sign included 22 individual on-site systems and 22 systems with two or more
households or businesses using a common disposal field. The 22 community sys-
tems called for construction of 950 ft of 8-inch gravity sewer, 10,400 ft of 4-inch
Table 12. Cost Comparisons for Fountain Run Alternatives
Present worth, $1,000
Alternative
A-l
A-2
B
C
D
Construction cost3
Collection
system
287.9
287.9
176.9
Treatment
and disposal
47.9
86.1
86.1
228. 2b
206. 4b
Operation
and
maintenance
197.8
151.6
119.4
74.5
61.9
Total
cost
533.6
525.6
382.4
302.7
268.3
Includes initial and 10-year expansion costs, less salvage value.
DTotal of collection, treatment, and disposal-costs.
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gravity sewer for septic tank effluent, and 1200 ft of 2-inch and 3-inch lines,
also for septic tank effluent. O&M costs were assumed to include pump opera-
tion and maintenance, line flushing and repair, servicing septic tanks on a re-
gular schedule, inspection of disposal field condition, repair and mowing of
disposal fields, and periodic alternation of flow in the fields. These costs
were estimated to total $6,110 per year. Components of the total present worth
of $302,700 for this alternative are shown in Table 12.
Alternative D. Because on-site disposal was being used with some degree
of success in the area, the consultant considered community management of in-
dividual on-site systems as an alternative. The cost analysis used construction
costs of $1,200 for standard ST-SAS units and 50 percent more, or'$1,800 per
unit, for construction of non-standard systems for the existing 20 percent of
systems located in soils described as having severe limitations for subsurface
disposal. Total present worth of this alternative was determined to be about
$268,300 as shown in Table 12.
Evaluation of Alternatives. The on-site disposal alternative (D) not only
had the lowest total present worth of all alternatives evaluated, but was also
given the highest rating with respect to environmental impact. The report also
states, however, that the difference in estimated cost between Alternatives C
and D was probably less than the level of precision used in estimating Alterna-
tive D. In addition, there was no significant difference in the environmental
rating of any of the five alternatives. From the standpoint of implementation,
Alternative D was not recommended because of uncertainty of the actual costs of
systems required in areas with poor soils. Public opinion, including that of the
Water District Commissioners, favored Alternative C. Considering all factors,
Alternative C, community subsurface disposal, was selected as the recommended
plan, and a user charge of $7.10 per month was estimated necessary to support
the wastewater services provided by the selected plan.
CASE HISTORY NO. 5
EASTRYEGATE, VERMONT
The community of East Ryegate, Vermont is one of three villages which
make up the town of Ryegate in the southeastern corner of Caledonia County, in
the Connecticut River Valley. At the time the facilities plan for East Ryegate
was being conducted a small combined collection sewer system served the esti-
mated 140 persons in the community and discharged raw sewage into both the
Connecticut River and a small drainage channel. The NPDES permit compliance
schedule for Fire District No. 2 of East Ryegate required that a subsurface dis-
posal system be in operation by June 1, 1976. The report which serves as the
basis of this case history is a revision of an August, 1972 report.^
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Physical Characteristics
Soils, groundwater, and site characteristics of the study area are de-
scribed below. Climate was not described in the facilities plan, since it did
not bear on any of the alternatives evaluated.
Soils. Information taken from soil borings made during the investigation
indicated that the soils beneath the village are comprised of sandy loam,
coarse sand, and silty gray clay. The clay layer appears to exist beneath the
entire study area and varies from 2 ft below the surface in the western corner of
the village to 12 ft in the village center. Percolation rates in most soil borings
were less than 10 minutes per inch; many were on the order of 1 minute per inch.
Groundwater. Groundwater is found below the impervious clay layer
throughout the study area. Test pits dug south of-the community in October,
1973 showed groundwater levels varying from a minimum depth, of 4.5 ft to over
10 ft. No information is presented in the report regarding groundwater quality.
It can be assumed that present quality is adequate for domestic uses, since the
community water supply well is located in the village center and supplies un-
treated water for domestic use.
Site Characteristics. Terrain throughout the village of East Ryegate is
quite flat, with ground elevations ranging from about 470 ft to 490 ft above sea
level. While no specific lot sizes are described in the report, it does state
that the 40 dwellings in the study area are located in a dense settlement and
that many of the lots do not have enough open area to accommodate an 1800-
sq ft drainfield.
Alternatives and Screening Process
Four basic alternatives were set forth for study in the investigation, one
with five variations. In order, the alternatives were as follows:
I. No action
II. Treatment and Water Reuse
III. Municipal Extended Aeration
IV. Septic Tank and Subsurface Disposal
a. Individual Private Systems
b. Joint Private Systems
c. Individual-Municipal System
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d. Municipal System - Gravity Flow
e. Municipal System - Force Main
The screening process for these alternatives involved analyzing each
alternative from the standpoint of the following questions:
1. Does the alternative comply with state regulations and federal
guidelines?
2. Does the alternative offer the best treatment process for the costs
involved ?
3. Is the alternative capable of meeting the implementation schedule
for the District as set forth by the NPDES permit?
Alternatives I and II were both eliminated in the initial screening phase
since I does not comply with either state or federal requirements and II does
not comply with laws of the State of Vermont.
Cost-Effectiveness Analysis
Following elimination of Alternatives I and II, the remaining alternatives
were examined in detail as described below. Present worth analyses were
based on the use of a 20-year design life and 7 percent interest.
Alternative III. Proposed initially in a report by another consulting
engineer in 1970, this alternative would use an addition to the existing collec-
tion system to convey the community's sewage to a package extended aeration
plant. While this alternative would end the pollution of surface streams and
proposes a feasible treatment method, it involves high operation and mainten-
ance costs and requires more power than any other alternative. In addition, the
costs for this alternative include extensive use of the existing collection system,
which the State Department of Water Resources subsequently determined to be
inadequate through an I/I analysis. The Department's cost-effective solution
to this problem was construction of a new collection system for sanitary sewage
only, and retention of the old system as a storm sewer. Actual construction
costs of the collection system for this alternative are higher, therefore, than
the $96,000 included in the total capital cost in Table 13.
Alternative IVa. Installation of private septic tank systems for each in-
dividual residence in East Ryegate assumes that every residence can be served
by such a system. As a practical matter, the report notes several obstacles to
that assumption: (1) the soils fronting on one of the four streets in the village
are underlain by a clay layer less than 2 ft from the ground surface: (2) only a
few dwellings have as much as 1800 sq ft of open area in which to construct an
ST-SAS; (3) if systems could be placed on all lots, the leachate from those in
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the central partrof the village would flow toward the public water-supply well.
The costs shown in Table 13 assume that each residence can install a properly
designed ST-SAS at a cost of $1,100.
Alternative IVb. As an attempt to improve the possibility of individual
disposal, Alternative IVb was developed. It proposes the combination of waste-
water from several dwellings in small collection systems to lots which have
suitable soils and size to accommodate the combined flow in accordance with
state regulations. Disadvantages attributed to this alternative include the facts
that: (1) there may not be sufficient suitable lots to serve all dwellings, and
(2) problems of ownership and user's rights may occur when neighbors start dis-
charging wastes to one another's property. The difference in capital cost in
Table 13 between this alternative and Alternative IVa is attributed to engineering,
land, and administrative costs.
Alternative IVc. The concept behind this alternative was to provide those
dwellings which cannot be served by Alternatives IVa or IVb with a community-
wide system. Those residences which could use subsurface disposal would not
be permitted to connect to the municipal system under this alternative. The
community system would be designed to collect wastewater flows into an inter- .
ceptor for conveyance by either gravity or force main to a municipally owned and
operated septic tank-soil absorption system. Time required to inspect lots and
notify owners whether they are to construct an individual system or connect to
the community system might exceed the time allowed in the NPDES permit com-
pliance schedule. In addition, the cost of the community system could be pro-
hibitive to the small number of dwellings connecting to it initially. Of the total
Table 13. Cost Comparison of Sewerage Alternatives for East Ryegate, Vermont
Alternative and
component
III.
IVa.
IVb.
IVc.
IVd.
IVe.
Municipal extended
aeration
Individual subsurface
systems
Joint private systems
Individual - municipal
system
Municipal system -
gravity flow
Municipal system -
force main
Total capital
cost, dollars
253,200
33,000
38,500
129,800
312,000
271,700
Avg . annual
O&M cost,
dollars
6,100
900
900
2,000
1,600
2,000
Present worth,
dollars9
302,900
42,500
48,000
145,300
310,400
274,300
3Present worth of salvage value deducted for all alternatives using community facilities.
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capital cost shown in Table 13, about $83,000 is for collection and disposal,
$12,500 for treatment, and $34,000 for engineering, land, and administrative
costs.
Alternative IVd. Under this alternative a low-lying area north of the
village would serve as a subsurface disposal field for the entire community.
While use of the proposed disposal site would permit installation of a conven-
tional gravity collection system, the disposal area itself would have to be filled
with approximately 17,000 cu yd of imported soil. State approval would have to
be obtained for this alternative because of the proposed use of the fill system,
which is not presently permitted for general use in Vermont. Capital costs in
Table 13 are composed of the following components: Collection and disposal
system - $236,000; treatment system (septic tanks) - $26,400; engineering,
land, and administrative costs - $49,600.
Alternative IVe. In this alternative, the collection system would con-
sist of conventional gravity sewers conveying all wastewater to a pumping sta-
tion in the northeast corner of the village, and a force main from the pumping
station to a municipal ST-SAS located south of the village. Soils at the pro-
posed soil absorption system site appear suitable. As noted in the facility plan,
the cost of this system is beyond the means of the District without federal aid,
but is nominal with federal aid. Of the total capital cost of $271,700 for this
alternative, $195,000 is for the collection and disposal systems, $26,400 for
the treatment system, and $49,600 for engineering, land, and administrative
costs.
Evaluation of Alternatives . Results of the evaluation of the six alterna-
tives were as follows:
1. Alternatives I and II were not recommended for the reasons given
previously.
2. Alternative III was not recommended because of its high operation
costs and power requirements, and because, as presented, it uses
a portion of the existing inadequate collection system.
3. Alternatives IVa and IVb were not recommended basically because
available soils data indicates that subsurface disposal may be
infeasible throughout much of the residential area.
4. Alternative IVc was not recommended because of the high annual
cost which would have to be paid by the few users of the community
portion of the system.
5. Alternative IVd was not recommended due to poor soil and high
groundwater at the proposed disposal site, and because the exten-
sive modifications necessary to make the site suitable may not be
acceptable to the state.
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6. Alternative IVe is therefore recommended as the most cost-effective
method of abating existing stream pollution.
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REFERENCES
Romero, J.C. "The Movement of Bacteria and Viruses through Porous
Media". Ground Water, Vol. 8, No. 2. March 1970.
Cotteral, J.A. andNorris, D.P. "Septic Tank Systems" . Jour. SEP,
ASCE, Vol. 95, No. SA-4. August 1969.
McGauhey, P. H.; Krone, R.B.; and Winneberger, J.H. "Soil Mantle as
a Wastewater Treatment System". SERL Report No. 66-7, University of
California, Berkeley. September 1966 .
Bouma, J. "Unsaturated Flow During Soil Treatment of Septic Tank Efflu-
ent". Tour. EED, ASCE. December 1975.
5. Healy, K.H. andLaak, R. "Problems with Effluent Seepage" . Water and
Sewage Works. October 1974.
6. McGauhey, P.H. and Winneberger, J.H. "Final Report on a Study of
Methods of Preventing Failure of Septic Tank Percolation Systems". SERL
Report No. 65-17. University of California, Berkeley. October 1965.
7. Department of Public Works, Douglas County, Oregon. "Glide-Idleyld
Park Sewerage Study, Douglas County, Oregon". December 1975.
8. Douglas County Engineer's Office. - "Pressure Sewer Systems" . Roseburg,
Oregon. May 1974.
9. Brown and Caldwell. "Wastewater Facilities Plan Project Report, Blaine
County". A report prepared for Blaine County, Idaho. November 1976.
10. Otis, R.J. and Stewart, D.E. "Alternative Wastewater Facilities for
Small Unsewered Communities in Rural America" . A report of the Small
Scale Waste Management Project, University of Wisconsin, Madison.
July 1976.
11. Parrott, Ely, and Hurt Consulting Engineers, Inc. "Sewerage Facilities
Plan, Fountain Run, Kentucky" . A report prepared for the Fountain Run
Water District. July 1976.
12. Dufresne-Henry Engineering Corporation. "Facilities Planning Report on
Wastewater Collection and Treatment". A report prepared for Fire Dis-
trict No. 2, Ryegate, Vermont. March 1975.
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