DRAFT
ALTERNATIVE CONVEYANCE SYSTEMS
SMALL DIAMETER GRAVITY SEWERS
Prepared for
United States
Environmental Protection Agency
Municipal Wastewater Branch
Waste and Hazardous Waste Treatment Research Division
Risk Reduction Engineering Laboratory
Cincinnati, Ohio
Prepared by
Richard J. Otis
Ayres Associates
Madison, Wisconsin
August 1990

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CHAPTER 1
OVERVIEW OF ALTERNATIVE CONVEYANCE SYSTEMS
INTRODUCTION
Small Diameter Gravity Sewers
Description
Small diameter gravity sewers (SDGS) are rapidly gaining popularity in
unsewered areas because of their low construction costs. Unlike
conventional sewers, primary treatment is provided at each connection and
only the settled wastewater is collected. Grit, grease and other
troublesome solids which might cause obstructions in the collector mains
are separated from the waste flow and retained in interceptor tanks
installed upstream of each connection (Figure 1.1). With the solids
removed, the collector mains need not be designed to carry solids as
conventional sewers must be. Large diameter pipes designed with straight
alignment and uniform gradients to maintain self-cleansing velocities are
not necessary. Instead, the collector mains may be smaller in diameter,
laid with variable or inflective gradients and have fewer manholes.
Construction costs are reduced because SDGS may be laid to follow the
topography more closely than conventional sewers and routed around most
obstacles within their path without installing manholes. The interceptor
tanks are an integral part of the system. They are typically located on
private property, but usually owned by the utility districts so that
regular pumping to remove the accumulated solids for safe disposal is
ensured.

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Small diameter gravity sewers (SDGS) were first constructed in Australia in
the 1960's- They were used to provide a more cost effective solution than
conventional gravity sewers to correct problems with failing septic tank
systems in densely developed urban fringe areas. The SDGS were designed to
collect the effluent from existing septic tanks. Since the tanks would
remove the suspended solids that might settle or otherwise cause
obstructions in the mains, smaller collector mains 100 mm (4 in) in
diameter, laid on a uniform gradient sufficient to maintain only a 0.45 m/s
(1.5 ft/s) flow velocity were permitted. This alternative proved to reduce
construction costs by 30 to 65%. Routine maintenance also proved to be low
in cost. As a result, by 1986 over 80 systems had been constructed with up
to 4,000 connections per scheme (Laver, 1975; South Australia Health
Commission, 1986; Tucker, 1989).
In the United States, small diameter gravity sewers were not introduced
until the mid-1970's (Otis, 1986). The first systems, located in Mt.
Andrew, Alabama and Westboro, Wisconsin, were small demonstration systems
with 13 and 90 connections respectively. The Mt. Andrew system was
constructed as a variable grade system with sections of sewer depressed
below the static hydraulic grade line (Simmons,et al.,1982). The Westboro
system was designed with uniform gradients using the more conservative
Australian guidelines (Otis, 1978). The Westboro system proved to be 30%
less costly than conventional sewers.
As knowledge of the success of these systems spread, SDGS began to gain
acceptance and by the mid-1980's, over 100 systems had been constructed (US
EPA, 1986). The designs of most of the systems constructed prior to
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1990 followed the Australian guidelines, but as experience has been gained,
engineers are finding that the guidelines can be relaxed without
sacrificing performance or increasing maintenance costs. Variable grade
systems in which the sewers are allowed to operate in a surcharged
condition are becoming more common. Minimum flow velocities are no longer
considered as a design criterion. Instead, the design is based on the
system's capacity to carry the expected peak flows without raising the
hydraulic grade line above the interceptor outlet inverts for extended
periods of time. Inflective gradients are allowed such that sections of
the mains are depressed below the static hydraulic grade line. Despite
these significant changes from the Australian guidelines, operation and
maintenance costs have not increased.
Small diameter gravity sewer systems consist of: a) house connections; b)
interceptor tanks; c) service laterals; d) collector mains; e) cleanouts,
manholes and vents; and f) lift stations.
a)	House Connections are made at the inlet to the interceptor tank.
All household wastewaters enter the system at this point.
b)	Interceptor Tanks are buried, vented, watertight tanks with
baffled inlets and outlets. They.are designed to remove both
floating and settleable solids from the waste stream through
quiescent settling over a period of 12 to 24 hours. Ample
volume is also provided for storage of the solids which must be
periodically removed through an access port. Typically, a
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single-chamber septic tank, which is vented through the house
plumbing stack vent, is used as an interceptor tank.
c) Service Laterals connect the interceptor tank with the collector
main. Typically, they are 75- 100 mm (3-4 in) in diameter, but
should be no larger than the collector main to which they are
connected, They may include a check valve or other backflow
prevention device near the connection to the main.
c)	Collector Mains are small diameter plastic pipes with typical
minimum diameters of 75 to 100 mm (3-4 in), although 30 mm (1.25
in) pipe has been used successfully. The mains are trenched into
the ground at a depth sufficient to collect the settled
wastewater from most connections by gravity. Unlike
conventional gravity sewers, small diameter gravity sewers are
not necessarily laid on an uniform gradient with straight
alignment between cleanouts or manholes. In places, the mains
may be depressed below the hydraulic grade line. Also, the
alignment may be curvilinear between manholes and cleanouts to
avoid obstacles in the path of the sewers.
d)	Cleanouts, Manholes and Vents proyide access to the collector
mains for inspection and maintenance. In most circumstances,
cleanouts are preferable to manholes because they are less
costly and can be more tightly sealed to eliminate most
infiltration and grit which commonly enter through manholes.
Vents are necessary to maintain free-flowing conditions in the
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mains. Vents in the household plumbing are sufficient except
where depressed sewer sections exist. In such cases, air
release valves or ventilated cleanouts are necessary at the high
points of the main.
e) Lift Stations are necessary where elevation differences do not
permit gravity flow. Either STEP units (See Pressure Sewer
Systems) or mainline lift stations may be used. STEP units are
small lift stations installed to pump wastewater from one or a
small cluster of connections to the collector main, while a
mainline lift station is used to service all connections in a
larger drainage basin.
Although the term "small diameter gravity sewers" has become commonly
accepted, it is not an accurate description of the system, since the mains
need not be small in diameter (the size is determined by hydraulic
considerations) nor are they "sewers" in the sense that they carry sewage
solids. The most significant feature of small diameter sewers is that
primary pretreatment is provided in interceptor tanks upstream of each
connection. With the settleable solids removed, it is not necessary to
design the collector mains to maintain minimum self-cleansing velocities.
Without the requirement for minimum velocities, the pipe gradients may be
reduced and, as a result, the depths of excavation. The need for manholes
at all junctions, changes in grade and alignment, and at regular intervals
is eliminated. The interceptor tank also attenuates the wastewater flow
rate from each connection which reduces the peak to average flow ratio
allowing reductions in the sewer diameter. Yet, except for the need to
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evacuate the accumulated solids in the interceptor tanks periodically, SDGS
operate similarly to conventional sewers.
Application
Small diameter gravity sewers have potential for wide application. They
are a viable alternative to conventional sewers in many situations, but are
particularly well-suited for low-density residential and commercial
developments such as small communities and residential fringe developments
of larger urban areas. Because of their smaller size, reduced gradients
and fewer manholes, they can have a distinct cost advantage over
conventional gravity sewers where adverse soil or rock conditions create
excavation problems or where restoration costs in developed areas can be
excessive. In new developments, construction of the sewers can be deferred
until the number of homes built warrant their installation. In the
interim, septic tank systems or holding tanks can be used. When the sewers
are constructed, the tanks can be converted for use as interceptor tanks.
However, SDGS usually are not well suited in high density developments
because of the cost of installing and maintaining the interceptor tanks.
Extent of Use in the U.S.
The use of small diameter gravity sewers has been rapidly increasing in the
U.S. They have been referred to by different names including variable
grade sewers (VGS), small bore sewers (SBS), septic tank effluent drains
(STED) and common effluent drains (CED). They are all similar in design
except that CED typically are designed to have uniform gradients with a
minimum flow velocity of 0.3 m/s (1 fps). The others do not require
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uniform gradients, but will allow inflective gradients where sections of
the sewer are depressed below the hydraulic grade line. Minimum flow
velocities may not be required.
The use of small diameter gravity sewers has been largely limited to
existing rural communities. The first SDGS system was installed in 1977
and by the early 1990's over 200 systems were operating. Increasingly,
they are been used for residential fringe developments and new subdivision
and resort developments where the topography is favorable. Frequently, the
systems built are hybrid gravity and pressure systems.
Experience with the sewers has been excellent. The sewers have proved to
be trouble-free with low maintenance requirements. As a result, confidence
with the systems has grown and the designs have become less conservative.
Mvths Versus Reality
Deterrents to the use of small diameter gravity sewers have come from both
the engineering/regulatory community and the potential users themselves.
Engineers and regulatory agencies have been reluctant to promote SDGS
because of the concern over long-term performance. The concern has been
over whether the sewers could handle the flows without backups or
obstructions occurring. This concern is fading as experience shows the
sewers to be relatively trouble-free.
Potential users have discouraged their use because of the conception that
SDGS are a "second-rate" system. Typical concerns are for odors and
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whether the system can be expanded to accommodate growth. However, where
SDGS have been installed, users have found them to perform no differently
than conventional sewers. With proper planning, expansion can be
accommodated and with proper design, odors problems are avoided.
Construction of SDGS may not be the lower in cost than conventional sewers
in all unsewered developments. The cost of installing interceptor tanks is
a significant cost. Usually existing septic tanks cannot be used as
interceptor tanks because they are not watertight and cannot be inspected
and repaired cost effectively. As in any project, all reasonable
alternatives should be evaluated before design commences.
REFERENCES
Laver, R.W. 1975. Personal communication,(August 14). South Australia
Department of Health. Norwood, South Australia.
Otis, R.J. 1986. Small diameter gravity sewers: An alternative for
unsewered communities. US Environmental Protection Agency. Water
Engineering Research Laboratory. EPA/600/s2-86/022. Cincinnati, Ohio.
Otis, R.J. 1978. An alternative public wastewater facility for a small
rural comaunity. Small Scale Waste Management Project. University of
Wisconsin. Madison, Wisconsin.
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Simmons, J.D., J.O. Newman, and C.W. Rose. 1982. Small diameter,
variable-grade gravity sewers for septic tank effluent. In: On-Site Sewage
Treatment. Proceedings of the third national symposium on individual and
small community sewage treatment. American Society of Agricultural
Engineers, ASAE publication 1-82. pp 130-138.
South Australian Health Commission. 1986. Public health inspection guide
no. 6: Common effluent drainage schemes. Adelaide, South Australia.
Tucker, L. 1989. Personal communication, (October 6). South Australia
Health Commission, Department of Local Government, Operations Branch,
Effluent Drainage Unit. Adelaide, South Australia.
U.S. Environmental Protection Agency. 1986. Innovative and alternative
technology projects: 1986 progress report. Office of Municipal Pollution
Control. Washington, D.C.
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CHAPTER 3:
SMALL DIAMETER GRAVITY SEWERS
INTRODUCTION
Small diameter gravity sewers are a system of interceptor tanks and small
diameter collection mains. The interceptor tanks, located upstream of each
connection and usually on the property served, remove grease and settleable
sol ids from the raw wastewater. The settled wastewater is discharged from
each tank via gravity or by pump (STEP unit) into the gravity collector
mains usually located in the public right-of-way. The mains transport the
tank effluents to the treatment facility.
Because the interceptor tanks remove the troublesome solids from the waste
stream, the collector mains need not be designed to carry solids. This
reduces the gradients needed and, as a result, the depths of excavation.
The need for manholes at all junctions, changes in grade and alignment, and
at regular intervals is eliminated resulting in further potential cost
savings. The sewer diameter can also be reduced because the interceptor
tank attenuates the wastewater flow to reduce the peak to average flow
ratio. Yet, except for the need to evacuate the accumulated solids from
the interceptor tanks periodically, SDGS operate similarly to conventional
sewers.
The compatibility of septic tank effluent pumping (STEP) systems with SDGS
allows an efficient low-cost hybrid collection alternative in many
unsewered developments. A hybrid design can often eliminate or minimize
the need for lift stations to reduce both capital, operation and
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maintenance costs. It is cautioned that grinder pump (GP) systems are not
compatible with SDGS because the waste solids and grease are not removed
from the waste stream before discharge to the collector main.
DESCRIPTION OF SYSTEM COMPONENTS
Typical small diameter gravity sewer systems consist of: building sewers,
interceptor tanks, service laterals, collector mains, cleanouts, manholes
and vents, and lift stations (See Figure 3.1). Other appurtenaces may be
added as necessary
Building Sewers
All wastewaters enter the small diameter gravity sewer system through the
building sewer. It conveys the raw wastewaters from the building to the
inlet of the interceptor tank. Typically it is a 100-150 mm (4-6 in)
diameter pipe laid at a prescribed slope, usually no less than 1%, made of
cast iron, vitrified clay, acrylonitrile butadiene styrene (ABS) or
polyvinyl chloride (PVC).
Interceptor Tanks
Interceptor tanks perform three important functions: 1) removal of
settleable and floatable solids from the raw wastewater, 2) storage of the
removed solids and, 3) flow attenuation. .The tanks are designed for
hydraulic retention times of 12 to 24 hours when two-thirds full of solids
to permit liquid-solid separation via sedimentation and flotation. Outlet
baffles on the tanks prevent floating solids from leaving the tank. The
tank has sufficient volume to store the solids until which time they can
removed, typically on 1 to 10 year cycles for residential connections and
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semi-annually or annually for commercial connections with food service.
Anaerobic digestion does take place within the tank which reduces the
volume of accumulated sludge and prolongs the storage time. The
interceptor tanks also provide some surge storage which can attenuate peak
flows entering the interceptor tank by more than 60% (Baumann, et al.,
1978; Otis, 1986).
Septic tanks are typically used as interceptor tanks (Figure 3.2).
Pre-cast reinforced concrete and coated steel tanks are usually available
locally in a variety of sizes. Fiberglass (fiber reinforced plastic, FRP)
and high density polyethylene tanks (HDPE) are also available regionally.
Pre-cast concrete tanks are most commonly used in SD6S systems, but
polyethylene and fiberglass tanks are gaining in popularity because they
are more watertight and lighter in weight for easier installation.
However, FRP and HDPE tanks require more care in proper bedding and
anti-flotation devices where high ground water occurs.
Service Laterals
Service laterals connect the interceptor tank to the collection main. The
laterals are usually plastic pipe no larger in diameter than the collector
main. They are not necessarily laid on a uniform grade nor with a straight
alignment (Figure 3.3).
Optional lateral appurtenances include check valves, "p"-traps or "running"
traps and corporation stops. Most existing SDGS systems do not include
these options, but check valves are being used more frequently to prevent
backups into low-lying connections during peak flows. "P"-traps
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have been retro-fitted on connections where odors issuing from the house
plumbing vent stack have been a problem. Corporation stops are used
primarily on "stub outs" reserved for future connections.
Collector Mains
Collector mains convey the settled wastewater to either a lift station,
manhole of a conventional gravity sewer system or directly to a treatment
plant. Plastic pipe, with solvent weld or rubber gasket joints is used
almost exclusively. Flexible, high density polyethylene pipe with heat
fused joints has also been used successfully.
Manholes and Cleanouts
Manholes and cleanouts provide access to the collector main for
maintenance. Since hydraulic flushing is sufficient to clean the mains,
the use of manholes is usually limited. Common practice is to use manholes
only at major junctions because they can be a significant source of
infiltration, inflow and sediment. Cleanouts are typically used at
upstream termini, minor main junctions, changes in pipe size or alignment,
high points, and at intervals of 120 to 300 m (400-1000 ft).
Air Release Valves and Vents
Vents are necessary to maintain free-flowing .conditions in the mains. In
SDGS systems installed with continuously negative gradients, the individual
house connections will provide adequate venting if the sewer lateral is not
trapped. In systems where inflective gradients are allowed, the high
points in the mains must be vented. Air release or a combination of air
release/vacuum valves are commonly used in combination
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with a cleanout (Figure 3.4). Individual connections located at a summit
can also serve as a vent if the service is not trapped or has a check
valve.
Lift Stations
Lift stations may be used at individual connections which are too low in
elevation to drain by gravity into the collector. They are also used on
the collector mains to lift the wastewater from one drainage basin to
another. Individual or STEP lift stations are usually simple reinforced
concrete or fiberglass wet wells following the interceptor tank with low
head, low capacity submersible pumps operated by mercury float switches
(Figure 3.5). In a few systems where the static lift is great, high head,
high capacity turbine pumps have been used successfully. This is only
possible if the wastewater affluent is screened prior to pumping to
eliminate any solids that might clog the turbines. Mainline lift stations
are similar in design to the "residential" lift stations, but because of
corrosion problems which commonly occur in the wet well, the construction
of dry wells is becoming more common to reduce corrosion problems and to
facilitate maintenance.
SYSTEM DESIGN CONSIDERATIONS
Hydraulic Design
A small diameter gravity sewer system conveys settled wastewater to the
selected outlet by utilizing the difference in elevation between its
upstream connections and its downstream terminus. It must be set deep
enough to receive flows from the majority of the service connections and
have sufficient capacity to carry the expected peak flows. Therefore,
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design decisions regarding its location, depth, size and gradient must be
carefully made to hold the hydraulic losses within the limits of available
energy. Where the differences in elevation are insufficient to permit
gravity flow from an individual connection, energy must be added to the
system by a lift pump (See Pressure Sewer Section). The number and
location of individual lift stations or STEP units generally is determined
from the comparisons of their costs of construction, operation and
maintenance with the cost of construction and maintenance of deeper and/or
larger diameter (smaller headloss) sewers. The hybridization of SDGS with
STEP is quite common.
Design Flow Estimates
The hydraulic design of sewer mains is based on the estimated flows which
the sewer must carry. Since wastewater flows vary throughout the day, the
sewer main must be designed to carry the expected prolonged peak flows,
typically the peak hour flow. Conventional sewer design assumes 380 L/d
(100 g/d) per capita times a typical peaking factor of 4 for collector
mains. This estimate includes allowances for commercial flows and
infiltration. However, experience with SDGS has shown that these design
flow estimates exceed actual flows because most SDGS serve residential
areas where daily per capita flows are less than 380L/d (100 g/d) per
capita the peak to average flow ratio is also less than 4 because the
interceptor tanks attenuate peak flows markedly.
Measured average daily wastewater flow per capita is approximately 170 L/d
(45 g/d) (Anderson and Watson, 1967; Bennett and Linstedt, 1975; Siegrist,
et al., 1976). However, in small communities and residential developments
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where little commercial or industrial activity exists, average per capita
wastewater flows in sewers may be as much as 25% less (Otis, 1978).
Household wastewater flow can vary considerably between homes but it is
usually less than 227 L/d (60 g/d) and seldom exceeds 284 L/d (75 g/d) U.S.
EPA, 1980). Typically, 190 L/d (50 g/d) is assumed for per capita
wastewater flows in residential areas. Commercial and industrial flows are
estimated individually using established criteria (U.S. EPA, 1980).
The collector mains are sized to carry the daily peak flows rather than the
average flows. In residential dwellings, the rate of wastewater discharged
from the building depends on the water use appliances and fixtures used.
Instantaneous peak flows are typically 0.3 to 0.6 L/s (5 -10 g/m) (U.S.
EPA, 1980). Maximum hourly flows of 380 L/h (100 g/h) may occur (Watson,
et al., 1967; Jones, 1974). However, the interceptor tank in SDGS systems
attenuates these peaks dramatically. Monitoring of individual interceptor
tanks showed that outlet flows seldom exceeded 0.06 L/s (1 g/m) and most
peaks ranged between 0.03 and 0.06 L/s (0.5-0.9 g/m) over 30 to 60 minutes
periods. There were long periods of zero flow (Otis, 1986). The degree of
attenuation depends on the design of the interceptor tank and /or its
outlet (see below).
In addition to wastewater flows, allowance must be made for potential clear
water infiltration/Inflow. A common source of infiltration in SDGS systems
is the building sewer and interceptor tank. In SDGS systems in which the
existing septic tanks were used as interceptor tanks, wet weather flows
have been significantly higher than dry weather flows. Leaking building
sewers, cracked tanks and poorly fitting tank covers are
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the most common sources of infiltration. Where all new tanks were
installed and the building sewers tested or replaced, the ratio of wet
weather to dry weather flows have been much lower (Otis, 1986). In all
systems, foundation drains and roof leaders may be significant sources of
inflow and SDGS projects should attempt to eliminate them during
construction.
Experience with SDGS systems has shown that the criteria used to estimate
design flows have been conservatively high. Design flows have generally
ranged from 190 to 380 L/d (50-100 g/d) per capita with peaking factors
ranging from 1 to 4. More recent designs have been based on flows per
connection of 0.006 to 0.02 L/s (0.1-0.3 g/m). These design flow estimates
have been successful because the interceptor tanks have storage available
above the normal water level to store household flows for short peak flow
periods.
Flow Velocities
Conventional sewer design is based on achieving "self-cleansing" velocities
during normal daily peak flow periods to transport any grit which may enter
the sewer, scour grease and resuspend solids that have settled in the sewer
during low flow periods. However, in SDGS systems, the primary treatment
provided in the interceptor tanks upstream of. each connection remove grit,
and most grease and settleable solids. Studies have shown that the
remaining solids which enter the collectors and any slime growths which
develop within the sewer are easily carried when flow velocities of 0.15
m/s (0.5 ft/s) are achieved (Otis, 1986). Experience with SDGS has shown
that the normal flows which occur within the systems
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are able to keep the mains free-flowing. Thus, SDGS need not be designed
to maintain minimum flow velocities during peak flows although many state
agencies require that minimum velocities of 0.3 to 0.45 m/s (1.0-1.5 ft/s)
during daily peak flow periods be maintain.
Maximum velocities should not exceed 4-5 m/s (13-16 f/s). At flow
velocities above this limit, air can be entrained in the wastewater that
may gather in air pockets to reduce the hydraulic capacity of the
collector. Drop cleanouts or manholes should be employed where the pipe
gradient results in excessive velocities.
Hydraulic Equations
Hydraulic equations used for design of the sewer mains are the same as
those used in conventional gravity sewers. However, unlike conventional
gravity sewers, sections of SDGS systems are allowed to be depressed below
the hydraulic grade line such that flows may alternate between open channel
and pressure flow. Therefore, separate analyses must be made for each
section of sewer in which the type of flow does not change.
Both Manning's and Hazen-Williams pipe flow formulas are used. Roughness
coefficients used range from 0.009 to 0.015 for Manning's "n" and 100 to
150 for Hazen-Wil1iams "C". Typical values used are 0.013 and 140
respectively (Otis, 1986). Nomographs and hydraulic elements graphs may be
found elsewhere (WPCF, 1982).
Design depths of flow allowed in the sewer mains have been either half-full
or full. Most older systems designed with uniform gradients
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have used half-full conditions to dictate changes in pipe size. However,
systems with variable gradients allow the collector main to be surcharged
at capacity. In these systems, pipe size changes are dictated by the
relative elevation of the hydraulic grade line to any service connection
elevation.
Design procedures follow conventional sewer design except in sections where
pressure flow occurs. In these sections, the elevation of the hydraulic
grade during daily peak flow conditions must be determined to check that it
is lower than any interceptor tank outlet invert. If not, free-flowing
conditions will not be maintained at every connection. Where the hydraulic
grade line is above a tank invert, the depth of the sewer can be increased
to lower the hydraulic grate line, or the diameter of the main can be
increased to reduce the frictional headloss or a STEP unit can be installed
at the affected connection to lift the wastewater into the collector. If
short term surcharging above any interceptor tank outlet inverts is
expected, check values on the individual service lateral may suffice to
prevent backflow.
Collector Mains
Layout
The layout of SD6S is a dendriform or branched system similar to that of
conventional sewers except that the mains are usually not laid down the
street center line so that expensive pavement restoration is avoided. In
most cases, SDGS are located along side of the pavement in the street
right-of-way. If there are numerous services on both sides of the street,
collectors may be provided on both sides to eliminate pavement crossings.
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Another alternative is to locate the collectors down the back property
lines to serve a whole block with one collector. The backlot alternative
may be the most accessible to homeowners since most septic tank systems are
located in the backyard. Therefore, homeowners are not required to reroute
the building sewer to the front of the lot, but access for interceptor
maintenance may be limited. Since new interceptor tanks are usually
installed, SDGS are installed most often in the front of the lots. If
necessary, the building drains are reversed to direct the flow to the
front.
A1ignment and Grade
The horizontal alignment of SDGS need not be straight. Obvious obstacles
such as various utilities, large trees, rock outcrops, etc. should be
avoided with careful planning, but unforeseen obstacles can often be routed
around by bending the pipe. The radius of the bend should not exceed that
recommended by the manufacturer.
The gradient of SDGS must provide an overall fall sufficient to carry the
estimated daily hourly peak flows, but the vertical alignment need not be
uniform. Inflective gradients, where sections of the main are depressed
below the static hydraulic grade line, are permissible if the invert
elevation is controlled where the flow in the pipe changes from pressure to
open channel flow. The elevation of these summits must be established such
that the hydraulic grade line does not rise above any upstream interceptor
tank outlet invert during peak flow conditions. Adequate venting must also
be provided at the summit. Between these critical summits, the profile of
the sewer should be reasonably uniform so unvented
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air pockets do not form which could create unanticipated headlosses in the
conduit and excessive upstream surcharging.
Pipe Diameter
The pipe diameter is determined through hydraulic analysis. It varies
according the to the number of connections and the available slope. The
minimum diameter used is typically 100 mm (4 in), but 50 mm (2 in) pipe has
been used successfully in recent projects (Meza, 1989). Where the smaller
pipe is used, the interceptor outlets have flow control devices to control
peak flows to a maximum rate and check valves at each service connection to
prevent flooding of services during peak flow periods.
Depth
The depth of burial for the collector mains is determined by the elevation
of the interceptor tank outlet invert elevations, frost depth or
anticipated trench loadings. Either condition may control. In most cases,
it is not attempted to set the depth such that all connections can drain by
gravity. Where gravity drainage is not possible STEP lift stations are
used at the affected connections. An optimum depth is selected to minimize
the costs between excavation and the installation of STEP units. However,
the depth must not be less than that sufficient to prevent damage from
anticipated loadings. Where the pipe is not buried below pavement or
subject to traffic loadings, the minimum depth is typically 0.75 m (30 in).
Pipe manufacturer should be consulted to determine the minimum depth
recommended. In cold climate areas, the frost depth may determine the
minimum depth of burial unless insulated pipe is used.
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Pipe Materials
Polyvinyl chloride (PVC) plastic pipe is the most commonly used pipe
material in SDGS systems. Standard dimension ratio (SDR) 35 is used in
most applications, but SDR 26 may be specified for road crossings or where
water lines are within 3 m (10 ft). For deep burial, SDR 21 may be
necessary. Where the use of STEP units is anticipated, only SDR 26 or 21
should be used for the collector mains because of the compatibility of pipe
fittings. Typically, elastomeric (rubber ring) joints are used, however,
for pipe smaller than 75 mm (3 in), only solvent weld joints may be
available.
Flexible, high density polyethylene (HOPE) has been used infrequently, but
successfully. Pipe joining is by heat fusion.
Service Laterals
Typical service laterals are 100 mm (4 in) PVC pipe, although laterals as
small as 30 mm (1-1/4 in) have been used. The service lateral should be no
larger than the diameter of the collector main to which it is connected.
The connection is typically made with a wye or tee fitting. Where STEP
units are used, wye fittings are preferred.
Occasionally, check valves are used upstream of the connection to the main
to prevent flooding of the service connection during peak flows. If used,
it is important that the valve be located at the collector main connection.
Air binding of the service lateral can occur if the valve is located near
the interceptor tank outlet (Bowne, 1989).
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Interceptor Tanks
Location
The interceptor tanks should be located where they are easily accessible
for periodic removal of accumulated solids. Typically, they are located
near the house between the house and the collector main adjacent to or in
place of the existing septic tank. If the collector main is located on the
opposite side of the building, reversal of the building drainage may be
desirable, but not necessary. Access for maintenance is the critical
factor in location. In some projects, the tank has been located in the
public right-of-way to eliminate the need for the utility district to enter
private property to pump the tank (Figure 3.6).
Design
Prefabricated, single-compartment septic tanks are typically used for
interceptor tanks in SDGS systems. Most projects standardize the use of
3785 L (1000 gal) tanks for all residential connections. For commercial
establishments, local septic tank codes are commonly used to determine the
necessary volume. For a given volume, several tank designs may be
available locally. Shallow tanks, or tanks with the greater water surface
area for a given voluae are preferred designs because of the greater flow
attenuation that they proyide.
Inlet and outlet baffles are provided in conventional septic tanks to
retain solids within the tank. These baffles are adequate for SDGS
applications. The inlet baffle must be open at the top to allow venting of
the interceptor tank through the building plumbing stack vent. On the
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outlet, various "gas deflection" baffles or outlet screens may be used to
capture low density or neutral buoyancy solids that might otherwise pass
through the tank (Figure 3.7). These devices are not necessary, however,
since these solids have not caused problems in SDGS systems.
Flow control devices may also be used on interceptor outlets to limit peak
flow rates to a predetermined maximum. Surge chambers were added to
interceptor tanks in early projects (Simmons, et al., 1982). The surge
chamber contained a standpipe with a small orifice drilled near the bottom
(Figure 3.8). During peak flow periods, the chamber provides storage for
the wastewater while the orifice controls the rate of flow from the tank.
These chambers are no longer used because the orifices plug readily so the
chambers are not effective in flow attenuation (Ref: Markle, R., 1989).
They also require about 0.3-0.4 (1.0-1.5 ft) of headloss which may require
deeper burial of the collectors and, as a result, higher construction
costs. Where flow control is desirable, it has been incorporated into
outlet screening devices made from plastic such as polypropylene in such a
manner that the typical freeboard provided in the tank is sufficient for
the necessary storage volume (Figure 3.9).
Water tightness is a critical criterion in selection of an interceptor
tank. For that reason, existing septic tanks are seldom converted to
interceptor tanks. Earlier systems attempted to use the existing septic
tank at each home to reduce construction costs. It was found that septic
tanks are difficult to inspect and repair properly. SDGS systems with
significant numbers of old tanks all have high ratios of wet weather to dry
weather flows (Otis, 1986). Common practice now is to replace all
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tanks. This practice has the added advantage of requiring the property
owner to replace the building sewer to ensure greater watertightness. Some
projects incorporate the replacement of the building sewer to allow
inspection of the building plumbing to eliminate roof leaders, foundation
drains and other unwanted connections that contribute clear water inflow.
Access to the tank for periodic inspections and solids removal is required.
A sufficiently large opening over the tank inlet or outlet to allow
inspection and effective sludge removal should be provided. However,
because of the septic wastes, personnel must not enter the tank. The
opening should be a minimum of 45 cm (18 in) square or in diameter. A
watertight riser terminating 15 cm (6 in) above grade with a bolted or
locking air tight cover is preferred to a buried access.
Material
Prefabricated septic tanks are typically used for interceptor tanks. They
are available in reinforced concrete, coated steel, fiberglass and high
density polyethylene. Unfortunately, the quality of manufacture varies
from locality to locality. Therefore, it is necessary to carefully inspect
and test random tanks for structural soundness for the intended application
and for watertightness. Coated steel tanks are not recommended because the
coating is easily damaged.leading to severe corrosion and short tank life.
All tank joints must be designed to be watertight. The joints include tank
covers, manhole risers and covers and inlet and outlet connections.
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Rubber gasket joints for inlet and outlet connections are preferred to
provide some flexibility in case of tank settlement.
Manholes and Cleanouts
In most SDGS systems, cleanouts have replaced manholes except at major
junctions at mains. Since hydraulic flushing is all that is necessary to
maintain the mains in a free-flowing condition, cleanouts provide
sufficient access to the mains. Cleanouts are less costly to install than
manholes and are not a source of infiltration, inflow or grit. Since the
SDGS system is not designed to carry grit, elimination of manholes is
strongly recommended. They also eliminate what was a common source of
odors in SDGS systems.
Cleanouts are typically located at upstream termini of mains, junctions of
mains, changes in main diameter and at intervals of 120 to 300 m (400 -
1000 ft) (Figures 3.10). Cleanouts may also be used in place of drop
manholes (Figure 3.11). The cleanouts are typically extended to ground
surface within valve boxes.
Manholes, if used, are located only at major junctions. The interiors
should be coated with epoxy or other chemical resistant coating to prevent
corrosion of the concrete.. The covers used .are typically gas-tight covers
to limit the egress of odors and inflow of clear water.
Where depressed sections occur, the sewer must be well vented. Cleanouts
may be combined with air relief valves at high points in the mains (Figure
3.4) or an open vent cleanout installed (Figure 3.12).
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Valves
Air release, combination air release/vacuum and check valves may be used in
SDGS systems. Air release and combination air release/vacuum valves are
used for air venting at summits in mains that have inflective gradients in
lieu of other methods of venting. These valves must be designed for sewage
applications with working mechanisms made of type 316 stainless steel or of
a plastic proven to be suitable (Bowne, 1989). The valves are installed
within meter or valve boxes set flush to grade and covered with a water
tight lid (Figure 3.4). If odors are detected from the valve boxes, the
boxes may be vented into a small buried gravel trench beside the boxes.
Check valves are sometimes used on the service connections at the point of
connection to the main to prevent backflow during surcharged conditions.
They have been used primarily in systems with 50 mm (2 in) mains. Many
types of check valves are manufactured, but those with large, unobstructed
passageways resilient seats have performed best. Wye pattern swing check
valves are preferred over tee pattern valves when installed horizontally
(Bowne, 1989). Although the systems with 100 mm (4 in) mains have operated
well without check valves, they can provide an inexpensive factor of
safety. An alternative method used to prevent pumping backups in some
projects has been to provide an interceptor tank overflow pipe to the drain
field of the abandoned septic tank system. In Australia, a "bounder trap"
is included at every connection which provides an overflow to the groomed
surface if backups occur (Figure 3.13)
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Odors and Corrosion
Odors are a commonly reported problem with SD6S systems. The settled
wastewater collected by SDGS is septic and therefore contains dissolved
hydrogen sulfide and other malodorous gases. These gases tend to be
released to the atmosphere in quantity where turbulent conditions occur
such as in lift stations, drop cleanouts or hydraulic jumps which occur at
rapid and large changes of grade or direction in the collector main. The
odors escape primarily from the house plumbing stack vents, manholes or wet
well covers of lift stations.
The odors have been easily controlled by either minimizing turbulence or
sealing air outlets. Drop inlets have been effective in eliminating most
odors at lift stations (Figure 3.14). Gas tight lift station covers should
be installed if odors are persistent and odor control provided for the
fresh air vent. An effective odor control measure is to terminate the vent
in a buried gravel trench (Figure 3.15). Carbon filters have been used
successfully, but require regular maintenance. Manholes should be replaced
with cleanouts, but if used, gas tight covers can be used. Odors from
house plumbing vents can be controlled most easily by sealing the vent on
top of the interceptor tank outlet baffle or by installing water traps in
the service lateral.
The atmosphere created by the released gases is very corrosive. Corrosion
is a common problem in lift stations. Corrosion resistant materials must
be used (See Pressure Sewer Systems). More recent SDGS systems have used
wet well/dry well design for lift stations to reduce the exposure of
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components to the corrosive atmosphere.
OPERATION AND MAINTENANCE CONSIDERATIONS
Administration
Utility or special purpose districts are commonly formed to administer,
operate and maintain SDGS systems located outside municipal boundaries.
These districts vary in structure and powers from state to state, but they
typically have most of the powers of municipal government except for
methods of generating revenues.
The sewer utility should be responsible for maintenance of the entire
system. This includes all interceptor tanks and any appurtenances such as
STEP units located on private property. Typically, the utility district
assumes responsibility for all SDGS components downstream from the
interceptor tank inlet. In some projects, the responsibility for
maintenance of the components located on private property have been left to
individual property owner. This practice is favored by some districts to
avoid the need to enter private property. However, since the interceptor
tank is critical to the proper performance of the SDGS system,
responsibility for maintenance should be retained by the district. It is
strongly recommended that the district assume ownership of the interceptor
tank and the components downstream of the tank to ensure access and timely
appropriate maintenance.
To obtain access to the SDGS components located on private property,
perpetual general easements are typically secured from the owner. The
easements can take several forms, but general easements or easements by
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exhibit are recommended over metes and bounds easements because of the time
and expense of writing metes and bounds. In most cases, the easements are
obtained without compensation to the owner. Where it is necessary to cross
private property with the collector mains, metes and bounds easements are
usually used.
An example of a general easement appears below:
KNOW ALL MEN BY T1CSE PRESENTS:
That, in consideration of One Dollar and other good and valuable consideration paid to
the undersigned respectively, hereinafter referred to as GRANTORS by the util Itv
district, hereinafter referred to as GRANTEE, the receipt whereof is hereby
acknowledged, the GRANTORS each, for their respective heirs, distributees, personal
representatives, successors and assigns, do hereby grant, bargain, sell, transfer,
convey, release, quit claiM and reaise unto the GRANTEE, its successors and assigns, a
PERPETUAL EASEtCNT to erect, construct, install, lay, use, operate, Maintain, inspect,
alter, clean, reaove and replace sever pipes, puaps, interceptor tanks and all
appurtenances necessary and incident to the purposes of the easeaent. and, in connection
with the saae. tojjorarily to place Machinery and Materials which May be necessary to
effect the purposes of the easeaent upon lands of the respective GRANTORS situate in the
ftf county and state TOGETHER WITH the right of Ingress and egress over adjacent
lands of GRANTORS, their respective heirs, distributees, personal representatives,
successors and assigns, as the saae May be required in order to effect the purposes of
the easeaent. The location of the easeaent on the lands of each GRANTOR is respectively
shown on Sheet No. 	 of Contract No. 	 for the contract drawings of the local entity.
dated	.
The GRANTEE expressly agrees that any and all disturbance to the surface of the lands of
the GRANTOR will be prta^tly repaired and to the extent possible restored to their
pre-existing condition, whether such disturbance takes place during the initial
installation or at any tlae thereafter as May be occasioned by subsequent repairs or
¦aintenance to the said sanr line and Interceptor tank with the niniwnt area.
Executed at the local entity on the respective dates as follows:
Date Signature	Street Address Tax Acct.
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Operation and Maintenance Manual
An operation and maintenance manual is essential to every project.
Although most maintenance tasks are relatively simple and usually do not
involve mechanical equipment, the manual does provide a valuable reference
for location of components and services and typical drawings detailing the
design and construction of each component. In addition, the manual should
contain a comprehensive maintenance log to document all maintenance
performed and any performance problems and the corrective actions taken.
A good manual should contain, at a minimum, the following:
1.	Description of the system
A description of the botu system and each of its components
should be provided. The component descriptions should
include the function of each, their relation to adjacent
components and typical performance characteristics. Specific
design data, shop drawings, as-built plans and profiles of
the collector mains and detailed plan drawings of each
service connection are essential.
2.	Description of the system operation
Normal operation, emergency operation situations and
procedures and failsafe features should be described.
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3.	System testing, inspection, and monitoring
The purpose and methods of all recommended testing,
inspections and monitoring should be described. Sample
recording forms should also be included.
4.	Preventive maintenance procedures and schedules
A clear description of all preventive maintenance procedures
is needed with specific schedules for their performance.
5.	Troubleshooting
A description of common operating problems, how they may be
diagnosed and procedures to correct them is helpful.
6.	Safety
Safety practices and precautions should be described to alert
personnel to the potential hazards and methods to avoid or
mitigate them. The dangers of working with septic wastes
which generate dangerous hydrogen sulfide and methane gases
must be emphasized.
7.	Recordkeeping Logs and Forms
Sample recordkeeping forms and logs should be provided.
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8.	Equipment Shop Drawings and Manuals
Shop drawings and installation and maintenance manuals of all
major equipment should be included. Manufacturers and their
suppliers should be listed with contact names, addresses and
telephone numbers.
9.	Utilities List
A list of all utilities in the project area, location maps
and contact names, addresses and phone numbers should be
provided.
10. System Drawings
Complete as-built drawings of the system are necessary.
Detailed drawings of the service connections showing the
precise location of all components with maintenance logs for
each should be included.
Staff and Equipaent Requirements
Operation and maintenance requirements of SDGS systems are generally simple
in nature requiring no special qualifications for maintenance staff other
than familiarity with the system operation. The operator's
responsibilities will be limited largely to service calls, new service
connection inspections and administrative duties. In most systems,
interceptor tank pumping is usually performed by an outside contractor
under the direction of the utility district.
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ent is also limited. A truck mounted centrifugal suction
to /ide most emergency operation equipment needs,
hould be purchased to reach between cleanouts. Other
-ovided by outside contractors as needed.
ing for SDGS maintenance personnel is not necessary,
lis, however, are desirable. If a significant number of
ns include STEP units, an understanding of pumps and
5 is also helpful. (For a small number of such units, it
utility district to retain local plumbing and electrical
available for any necessary repairs.) The staff should
ngers of exposure to sewer gases and to avoid entry into
less nroperly protected. Since a significant portion of
ited private property, it is important that the staff
ation skills and a willingness to work with people.
ory
ems have few mechanical parts, the need to maintain a
ory is limited. However, if individual STEP units are
ystem requiring that spare pumps and controls must be
'gency repairs. A minimum of two spare pumps and the
/itches and controls should be maintained. Pipe and pipe
kept on hand to repair any pipeline breaks that may
:eptor lids and riser rings should also be kept.
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As-BuiIt Drawings
As -built drawings of the entire SDGS system including all on-lot
facilities is essential to maintenance of the system. Curvilinear
alignments and few manholes or cleanouts make locating the collector main
routes difficult unless accurate drawings tying the location of the line to
permanent structures are developed. As-built drawings of each individual
service connection should also be made. These drawings are necessary when
repairs are needed or when the components must be located to avoid damage
due to other construction activities.
Maintenance
Normal
Normal maintenance is generally limited to call-outs by users. The
call-outs are usually due to plumbing backups or to odors. In nearly every
case reported, the plumbing backups were due to obstructions in the
building sewer. Although the building sewer is the property owner's
responsibility, most utilities have assisted the owner in clearing the
obstruction. Odor complaints are common. As with the plumbing backups,
faulty venting in the building plumbing is usually the cause. If improved
venting fails to eliminate the odor complaints, the interceptor inlet vent
can be sealed or running traps placed in the service lateral to prevent the
sewer main from venting through the service connection.
Preventive
Preventive maintenance includes inspection and pumping of the interceptor
tanks, inspection and cleaning of the collection mains and inspection and
servicing of any STEP units or lift stations.
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Interceptor Tanks: The interceptor tanks must be evacuated of solids
periodically to prevent solids from entering the collector mains.
Prescribed pumping frequencies are typically 3 to 5 years, but operating
experience indicates that a longer time between pumpings, of 7 to 10 years,
is adequate. Restaurants and other high use facilities, such as taverns,
require more frequent pumping. Common practice is to pump tanks serving
these facilities every 6 to 12 months. Tank inspection is usually
performed immediately after the tank has been evacuated to check for
cracks, leaks, baffle integrity and general condition of the tank. If
effluent screens are used on the tank outlet, they must be pulled and
cleaned by flushing with water. Annual flushing of the screens is
recommended if they are to be effective.
Most utilities do not perform the pumping themselves. Private pumpers are
usually hired through annual contracts to pump a designated number of tanks
each year and to be on call for emergency pumping. The septage removed is
usually land spread or discharged into a regional treatment plant. During
the pumping operations, utility district personnel should be present to
record the depth of sludge and thickness of any scum in each tank so that
the schedule can be altered according to need.
Collector Mains: Periodic inspection and cleaning of the collector mains
is usually recoonended maintenance functions. Hydraulic flushing is most
often recommended for cleaning. Pressure hoses to push pigs through the
mains has also been suggested as a cleaning method, but is not recommended
if the collector mains are SDR 35 pipe. Reported performance of systems
has been good and, therefore, inspection and flushing has not been deemed
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necessary by most utilities and has seldom been performed. In systems
where the mains have been inspected, no noticeable solids accumulations
have been noted. The experience with SDGS in Australia is similar. Many
large systems there have been operating over 30 years without main
cleaning. However, regular flushing is still recommended for long flat
sections in which daily peak flow velocities are less than 0.15 m/s (0.5
ft/s).
Lift Stations: Mainline lift stations should be inspected on a daily or
weekly basis. Pump operation, alarms and switch function should be checked
and running times of the pumps recorded. The discharge rate of each pump
should be calibrated annually.
Emergency Operation
Mainline or service lateral obstructions and lift station failures require
that emergency actions be taken to limit the time the systems is out of
service to prevent environmental or property damage that might occur. It
requires that the utilities have defined emergency operation procedures.
Obstructions: If an obstruction occurs, the utility must be able to
respond quickly such that backups do not occur at upstream service
connections. Usually the obstruction will be.caused by construction debris
which cannot be renoved by simple flushing. It may require that the main
be excavated to remove the obstruction. While the obstruction is cleared,
the utility must be prepared to pump from the cleanout, manhole or
interceptor tank immediately above the obstruction to a cleanout or manhole
below. A contractor's centrifugal suction pump or truck mounted pump works
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well for this.
Fortunately, obstructions have been rare. All reported obstructions have
occurred soon after construction or after a service connection has made.
Construction debris has been the cause. Obstructions from other causes
have not been reported.
Lift Stations: Lift stations may fail due to loss of power or a mechanical
failure. Standby emergency generators can be provided for power during
prolonged outages, but the generators can be costly and require regular
maintenance. Because of the costs, many small communities have provided
added storage at the lift station (Figure 3.16) and/or truck mounted pumps
that can pump from the wet well to a downstream hose connection on the
forcemain (Figure 3.17). This latter method also works well for mechanical
failures.
Record Keeping
Good record keeping of all operation and maintenance duties performed is
essential for preventive maintenance and trouble shooting when problems
occur. A daily log should be kept and maintenance reports on all equipment
filed. Flows at the mainline lift stations should be estimated daily by
recording the pump running times. This is h.elpful in evaluating whether
infiltration or inflow problems are developing. A record of each service
call and corrective action taken should be filed by service connection
identification number. This record should include tank inspection and
pumping reports. These records are particularly useful if reviewed prior
to a service call out.
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Troubleshooting
Odors
Odors are the most frequently reported problem with SDGS systems. Odors
typically occur at lift stations and from house plumbing stack vents,
particularly at homes located at higher elevations or ends of lines. Odors
are most pronounced where turbulence occurs. The turbulence releases the
obnoxious gases dissolved in the wastewater.
Odors at lift stations have been successfully eliminated by installing drop
inlets that extend below the pump shut off level. This eliminates most of
the turbulence. Other successful corrective measures include soil odor
filters (Figure 3.15), air tight wet well covers and vents that extend 3 to
5 m (10-15 ft) above grade.
Odors at individual connections originate from the collection main. If a
sanitary tee or similar baffle device is used at the interceptor tank inlet
or outlet, the top of the tee can be sealed or capped to prevent the gases
escaping into the building sewer. P-traps or running traps on the service
lateral have also been used (Figure 3.13). In some cases, extension of the
main further upslope to where it can be terminated in a vented subsurface
gravel trench has been employed successfully. The trench filters the odors
before venting the gas to the atmosphere.
Corrosion
Corrosion is a problem that is largely limited to lift stations and
manholes. Nonferrous hardware must be used in lift station wet wells.
Concrete manholes and wet wells must be coated with corrosion resistant
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materials. Alternatively, corrosion problems can be reduced in lift
stations by using wet well/dry well construction with a well vented wet
well.
infiltration/Inflow
Clear water infiltration/inflow was a common problem with earlier SDGS
systems that used a high percentage of existing septic tanks for
interceptor tanks. Leaking tanks or building sewers were the primary entry
points of clear water. Systems that have installed all new interceptor
tanks and pressure test building sewers and tanks have few
infiltration/inflow problems.
SYSTEM COSTS
Construction
SDGS systems have resulted in reported savings of 0 to 50% in comparison to
conventional gravity sewers. The unavoidable costly component of SDGS is
the installation of the interceptor tanks which in some instances, have
caused the construction costs of SDGS to exceed the estimated costs of
conventional sewer construction. However, the pretreatment provided by
SDGS eliminates the need for primary treatment which may reduce the cost of
the treatment facility.
Construction costs are airectea directly by site factors and system design.
Site factors include topography, depth to bedrock, depth to ground water,
soil type and other factors that can affect the cost of pipeline
installation. Some of these factors can be mitigated through thoughtful
design. The design itself can also reduce construction costs independent
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of site factors.
In a study of 10 systems, construction costs of individual components were
ranked from most costly to least costly as follows (Otis, 1986):
1.	Col lector mains
2.	Interceptor tanks (including service lateral)
3.	Mainline lift stations
4.	Pavement restoration
5.	Crossings (road, stream, utility)
6.	STEP 1ift stations
7.	Manholes
8.	Site restoration
9.	Force main
10.	Cleanouts
This ranking suggests in which areas efforts should be made in system
design and construction methods to reduce the total costs.
Costs of installing the collector mains and the inteceptor tanks typically
accounts for over 50% of the total costs of construction. Pipe
installation costs are affected most by the depth of excavation. Where
frost does not control the depth at which the sewers must be installed,
shallow placenent can reduce the total costs .significantly. Consideration
should be given to eliminating gravity drainage for basement drains.
Greater use of individual STEP units can also reduce the required depth of
the collectors. Several projects have shown that hybrid systems using
pressure connections into gravity collectors can be cost effective in areas
of undulating topography. Reducing the depth may also eliminate the
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need for some mainline lift stations. Shallow placement will allow the use
of continuous trenching equipment as well.
The cost of installation of the interceptor tanks and service laterals
includes the cost of evacuation and abandonment of the existing septic
tank. Installation costs should be reduced by combining more than one
connection on one tank. However, this is seldom done.
Often times the cost of the service connection is affected by the attitude
of the contractor towards working on private property. Many contractors
dislike working on private property because of the insistence of the
property owner for complete restoration of their property. Several methods
have been used to mitigate this problem to control the cost. Video taping
of each site prior to construction helps to resolve complaints concerning
appropriate restoration. Letting a separate contract for the service
connections to allow a smaller contractor who is typically more accustomed
to working with property owners to perform the work has been effective.
Placement of the interceptor tanks in the public right-of-way eliminates
the need to enter private property altogether. This latter approach is
seldom used because of space restrictions and the additional cost to the
property owner for longer building sewer connections.
Operation and Maintenance
The most significant operation and maintenance costs of projects reviewed
are labor, interceptor tank pumping and system depreciation. An operator
must be on call at all times, but the time required for preventive
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maintenance is small. Most projects do not employ full time staff, finding
that 5 to 10 hours per week is sufficient for preventive maintenance.
Interceptor tank pumping is usually performed by outside contractors. Most
projects are pumping each tank every 2 t 3 years which has been found to be
more frequent than necessary. Pumping of residential tanks every 7 to 10
years appears to be sufficient. Commercial establishments, particularly
those with food service may require pumping every 6 to 12 months.
Other operating and maintenance costs include administration, utilities,
insurance and occasional repairs. These costs account for 20 to 30 percent
of the total operation and maintenance costs.
User Charges/Assessaents
User charges typically include administration, operation and maintenance,
depreciation and debt retirement costs. In most projects, flat rates for
residential connections are charged because water meters are not provided.
Surcharges are usually placed on commercial connections based on assumed
water use.
Flat rates are also frequently used for assessments. These may take the
form of hookup charges. A two tiered system is common. The first tier is
for connections made at the time of system construction. The second is for
future connections. Existing users at the time of construction are
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usually provided the interceptor tank and service lateral while future
users must pay for the tank and lateral in addition to the hookup fee.
SYSTEM MANAGEMENT CONSIDERATIONS
User Responsibilities
Typically, the user is responsible for only the building sewer from the
building to the interceptor tank. If a STEP unit is included as part of
the service connection, the owner is also responsible for providing power
to the control panel. Beyond these limited responsibilities, the owner
must also see that access to all components of the system located on the
property is maintained.
Sewer Utility Responsibilities
The utility is usually responsible for the installation, operation and
maintenance of the entire system commencing at the inlet to the interceptor
tank. Outside contractors may be employed to perform some tasks such as
installing service connections or pumping of the interceptor tanks.
DESIGN EXAMPLE
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REFERENCES
Anderson, J.S. and K.S. Watson. 1967. Patterns of household usage. J. Amer.
Water Works Assoc. 59:1228-1237.
Baumann, E.R., E.E. Jones, W.M. Jakubowski and M.C. Nottingham. 1978.
Septic tanks. In: "Home Sewage Treatment", Proceedings of the second
national home sewage treatment symposium. American Society of Agricultural
Engineers. St. Joseph, Michigan.
Bennett, E.R. and E.K. Linstedt. 1975. Individual home wastewater
characterization and treatment. Completion report series no. 66.
Environmental Resources Center. Colorado State University. Fort Collins,
Colorado.
Bowne, W.C. 1989. Consulting Engineer. Personal communication. Eugene,
Oregon.
Jones, E.E.,Jr. 1974. Domestic water use in individual homes and hydraulic
loading of and discharge from septic tanks. In: Proceedings of the
National Home Sewage Disposal Symposium. Amer. Soc. Agricul. Engr. ASAE
publication 1-75. St. Joseph, Michigan.
Laver, R.W. 1975. Personal communication ( August 14). South Australia
Department of Health. Norwood, South Australia.
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Merkle, R. 1089. Muskingham County Department of Public Works. Personal
communication. Zanesville, Ohio.
Meza, D.B. 1989. Personal communication. State Water Resources Control
Board. Divison of Loans and Grants. Sacramento, California.
Orenco Systems, Inc. 1990. Roseburg, Oregon.
Otis, R.J. 1986. Small diameter gravity sewers: An alternative for
unsewered communities. US Environmental Protection Agency. Water
Engineering Research Laboratory. EPA/600/s2-86/022. Cincinnati, Ohio.
Otis, R.J. 1978. An alternative public wastewater facility for a small
rural community. Small Scale Waste Management Project. University of
Wisconsin. Madison, Wisconsin.
Siegrist, R.L., M. Witt, and W.C. Boyle. 1976. Characteristics of rural
household wastewater. J. Env. Engr. Div., Amer. Soc. Civil Engr.
102:553-548.
Simmons, J.D., J.O. Newman, and C.W. Rose. 1982. Small diameter,
variable-grade gravity sewers for septic tank effluent. In: On-Site Sewage
Treatment. Proceedings of the third national symposium on individual and
small community sewage treatment. American Society of Agricultural
Engineers, ASAE publication 1-82. pp 130-138.
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South Australian Health Commission. 1986. Public health inspection guide
no. 6: Common effluent drainage schemes. South Australia.
Tucker, L. 1989. Personal communication, (October 6). South Australia
Health Commission, Department of Local Government, Operations Branch,
Effluent Drainage Unit. Adelaide, South Australia.
U.S. Environmental Protection Agency. 1986. Innovative and alternative
technology projects: 1986 progress report. Office of Municipal Pollution
Control. Washington, D.C.
U.S. Environmental Protection Agency. 1980. Onsite wastewater treatment and
disposal systems design manual. EPA-625/1-80-012. Research and Development,
MERL. Cincinnati, Ohio.
Water Pollution Control Federation. 1982. Gravity Sanitary Sewer Design and
Construction. WPCF Manual of Practice No. FD-5. New York, New York.
Watson, K.S., R.P. Farrell and J.S. Anderson. 1967. The contribution from
the individual home to the sewer system. J. Water Pollution Control
Federation. 39:2039-2054.
Weatherby & Associates, Inc. 1989. Jackson, California.
Zanesville
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ALTERNATIVE CONVEYANCE SYSTEMS
SHALL DIAMETER GRAVITY SEWERS
CHAPTER 1: OVERVIEW OF ALTERNATIVE CONVEYANCE SYSTEMS
Figure 1.1: Schematic of a Small Diameter Gravity Sewer System
(Otis, 1986)
CHAPTER 3: SMALL DIAMETER GRAVITY SEWERS
Figure 3.1: Components of a Small Diameter Gravity Sewer System
Figure 3.2: Typical Pre-Cast Concrete Interceptor Tank
Figure 3.3: Service Lateral Installation Using a Trenching Machine
(Otis, 1986)
Figure 3.4: Typical Combination Cleanout and Air Release Valve
Detail (Weatherby & Associates, Inc. 1984)
Figure 3.5: Typical STEP Lift Station Detail
Figure 3.6: Alternative Locations for Interceptor Tanks
Figure 3.7: Typical Interceptor Tank Inlet and Outlet Baffles
Figure 3.8: Typical Surge Chamber Detail (Simmons, et al., 1982)
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EPA Library Region 4
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97
Figure 3.9:	Interceptor Outlet Flow Control Device (Orenco, 1989)
Figure 3.10:	Typical Cleanout Detail (Otis, 1986)
Figure 3.11:	Drop Cleanout Detail (SABESP, 1990)
Figure 3.12:	Ventilated Cleanout Detail (Otis, 1986)
Figure 3.13: Australian Boundary Trap Detail (South Australia
Health Commission, 1989)
Figure 3.14: Mainline Lift Station with Drop Inlets (Otis, 1986)
Figure 3.15: Soil Odor Filter Detail
Figure 3.16: Mainline Lift Station with Emergency Storage (Otis,
1986)
Figure 3.17: Emergency Pumping Manhole (Otis, 1986)
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