Sanitary sewer
systems are considered to be wastewater collection and treatment systems.
Sewers and wastewater treatment systems are considered to be the most common
form of pollution control in the country. Exact specifications for design vary
by state and local jurisdiction. According to an online course in gravity sewer
design based on rules from the State of Florida and Dade County:
“Gravity sanitary sewer systems are a collection of
underground pipes or drains used primarily to convey wastewater from a
community to an authorized point of discharge such as a lift station, pump
station or a wastewater treatment facility.”
“A gravity sanitary sewer system transports wastewater
mainly by gravity along a downward-sloping pipe and it should be designed based
on the pipe size and slope to maintain adequate flow towards the discharge
point without surcharging manholes or pressurizing the system.”
Sewer System Design and Construction
Typical
designs are for gravity sewer sewers but lift pumps, also known as lift stations,
may be employed where gravity drainage is unsuitable due to soil, topography, or
bedrock issues. In addition, there may be pump stations that convey the wastewater
to the treatment plant.
Accurate
estimation of daily wastewater flow rates is very important to sewer system design, and future additions to the system must also be considered. Minimum, average, and peak
daily flow rates are estimated. Peak daily flow rates are used to design for adequate
capacity. Estimating design capacity is typically done by house size or number
of bedrooms for household septic systems. I assume it is done similarly for sewer
lines.
The totality of the drainage area is known as
the tributary area or basin. If there is an addition to an existing sewer
system, then as-built drawings and sewer maps must be consulted. Since drainage
around the buried components of the system may affect those components,
additional information about the water table and soil permeability is
desirable.
The design
period must be considered. Sewer mains and laterals less than 18” can be
designed to last 100 years. Other mains, trunks, interceptors, and outfalls
should be built to last 50-100 years. EPA recommends 50+ years.
It is useful
to derive a ‘design average flow’ rate for the wastewater, in gallons per day (gpd).
This may be the same as the average annual daily flow rate for a system that is
used evenly, or it may be changed to accommodate peak usage where there is increased
seasonal usage. These wastewater flow calculations are important to deriving
the ultimate system size. Population, population density, per capita flows,
residential flows, commercial flows, and industrial flows must all be
considered. Two other important considerations are groundwater infiltration (GWI)
and stormwater inflow. For subsurface wastewater conveyance systems what leaks
in as well as what leaks out must be considered. These extraneous flows, GWI
and inflow, and are commonly known as infiltration/Inflow (I/I) flows. They
must be estimated and added into the flow rate calculations. I/I flows can vary
considerably by system.
The hydraulic
design of the sewer pipe is evaluated based on the size and type of the pipe,
design period, design depth and peak flows. Wastewater flow is of two types: open
channel flow or pressure flow.
“When flow fills the conduit and the Hydraulic Grade
Line (HGL) rises above the sewer crown, the flow is classified as pressure
flow. When the conduit is partially full and the HGL is below the sewer crown
and a freewater surface develops in the sewer, the flow is classified as an
open channel flow.”
Open channel flow is the basis for a gravity sewer system. Hydraulic
slope (S) in sewers refers to the slope of the channel bottom and is the basis
for estimating flow. The types of open channel flow in a gravity sewer system
are steady flow, unsteady flow, uniform flow, non-uniform flow, steady uniform
flow, unsteady uniform flow, steady non-uniform flow and unsteady non-uniform
flow. Steady and unsteady flow refers to the depth of flow being constant flow or
not over time. Uniform and non-uniform flow means that the depth of flow is
constant or not over a location. In steady-uniform flow the depth of flow is
constant with respect to time and location and the hydraulic grade line (HGL)
is parallel to the sewer invert slope. Steady-uniform flow is the default
analysis. “Unsteady uniform flow occurs when the HGL remains parallel to the
sewer invert and fluctuates up and down as the rate of flow fluctuates with
time.” This type of flow is uncommon. Steady non-uniform flow is considered
to be the situation when multiple laterals are entering main lines but is
usually estimated as steady uniform flow.
“Unsteady non-uniform flow develops during the onset and
termination of peak flows. However, design of sewers based on this flow regime
is seldom required, as it involves extensive calculations for flow routing,
wave and water surface profiles.”
The Manning
Equation is used to determine open channel flow characteristics. It was first
presented by Philippe Gauckler in 1867 and re-developed by Robert Manning in
1890.
It is important
to maintain minimum slope and velocity so that solids are not deposited in the
lines. This can help prevent the need for future maintenance due to solids and
sulfides. Adequate velocity helps to keep the pipes clean of solids and greasy
scum and to prevent problems with corrosion and odors caused by sulfides.
“Slope is the key criterion in designing sewer collection
systems to avoid odor and corrosion problems due to sulfides in wastewater.
Sewer systems designed with long runs at minimum slopes are prone to sulfide
generation due to long residence times, poor oxygen transfer, and deposition of
solids.”
“Sewer system design is based on achieving self-cleaning
velocities during normal daily peak flow periods to transport any grit which
may enter the sewer system, scour grease and re-suspended solids that have
settle in sewer during low flow.”
Recommended minimum
slopes from the recommended standards for wastewater flow (RSWF) are given in
the table below.
Minimum velocities for all sewer lines are at 2 ft/sec when “flowing full.” Manning’s formula is applied using an “n” value of 0.013. Daily or hourly peak flows can be estimated. Peak hourly flow rates can be important since the wastewater flows can vary significantly by hour.
System layout
includes the design of the main and trunklines and where to install lift or pumping
stations, if applicable. Surface topography is generally followed where possible
for slope maintenance. Sewer lines typically run through the middle of a street
so that they can more easily collect wastewater from both sides of the street. Other
location issues include location relative to water lines and other utilities, easements,
and rights-of-way. Other considerations include the location of manholes, terminal
cleanouts, service connections, inverted siphons, junction chambers, and other
structures or devices.
Manholes
provide access for observation and maintenance. They are recommended to be at
least 48 inches in diameter by RSWF but the minimum is 24 inches.
“As per RSWF, manholes shall be installed: at the end of
each line; at all changes in grade, size, or alignment; at all intersections;
and at distances not greater than 400 feet for sewers 15 inches or less, and
500 feet for sewers 18 inches to 30 inches, except that distances up to 600
feet may be approved in cases where adequate modern cleaning equipment for such
spacing is provided; As per MD WASD design requirements, manhole to manhole
runs shall be kept in a range no more than 400 feet without permission. Greater
spacing may be permitted in larger sewers.”
The flow channel of a manhole should be constructed to conformable
to that of the flow channel within the sewer main with a similar slope. Manholes
are made of precast or poured concrete. Gasketed flexible watertight connections
are recommended, or any watertight connection arrangement that allows differential
settlement of the pipe and manhole wall to occur. The covers also must be
watertight in order to prevent inflow. If a sewer inflow is 2 ft or higher
above the manhole channel, then a drop manhole is required. A drop manhole
design is shown below. Manholes are typically inspected for damage and
watertightness before the system is activated.
Inverted siphons,
or depressed sewers, are designed to stand full even if there is no flow. The
siphon is designed to “carry the flow under an obstruction such as stream or
depressed highway and to regain as much elevation as possible after the
obstruction has been passed.” They are often constructed with multiple
barrels. Adequate self-cleansing velocities and maintenance flexibility under widely
variable flow conditions are important considerations. The design should permit
cleaning and maintenance.
Sewer connections,
or service laterals, are the pipes that connect individual homes and buildings
to a main sewer line. They are typically 4-, 5-, or 6-inch sewer-grade pipes with
a minimum slope of 2%. Design schematics for shallow and deep laterals are shown below.
Equalization storage tanks may be installed
where peak flow rates may exceed design capacity. The tank contents collect wastewater
during peak flow and feed it back into the sewer pipe system during times of
lower flow. Sewer lines must be installed deep enough to prevent freezing and
to accommodate wastewater from basements. Pipes are typically different grades
of plastic such as PVC but may also be iron-based, cement, concrete, or vitrified
clay. Plastic pipes are more flexible.
Corrosion due
to hydrogen sulfide (H2S), other sulfides, and sulfuric acid can be a problem for
sewer pipes. Metal pipes are more susceptible so may need corrosion-resistance
measures.
Stream
crossings should be minimized and where used should be perpendicular to stream
flow and have no gradient. They must be at least 1 foot below the base of the stream
where it cuts rock, 3 feet below the base of the stream where there is soil, and
more than 3 feet if it is a wide stream.
Sewer pipes
must be at least 6-12 inches away from water main lines and preferably more. Any
connections between pipes must be as far away from the water main as possible. Structural
support may be required where sewer and water lines cross. The water or sewer
lines may be required to either be encased in a watertight sleeve or the sewer
pipe must be constructed to the standards of water pipe and be pressure tested
to 150 psi.
Construction may
have to contend with high water tables which can make pipes buoyant.
Watertightness is a factor for any pipe joints and service connections. There
are also requirements for making the trenches, bedding the pipes, and backfilling
around them. The ultimate goal of these measures is to prevent infiltration and
root penetration.
“As per RSWF, deflection tests shall be performed on all
flexible pipes. The test shall be conducted after the final backfill has been
in place at least 30 days to permit stabilization of the soil-pipe system.”
“No pipe shall
exceed a deflection of 5 percent. If deflection exceeds 5 percent, the pipe
shall be excavated. Replacement or correction shall be accomplished in accordance
with requirements in the approved specifications.”
“As per MD-WASD standards, the deflection, or deformation
of the pipe due to external loading, shall not exceed approximately 7.5
percent. Deflection shall be determined by passing an approved go/no go mandrel
through the gravity sewer main. Deflection will be based on the average inside
diameter as presented in ANSI/AWWA C900.”
A lamping test uses
light:
“For lamping test, each section of installed sewer lines
shall show, from each end, a full circle of light.”
Leak Detection
Water and air
pressure may be utilized to determine leakage rate.
“As per RSWF, the leakage exfiltration or infiltration
shall not exceed 200 gallons per inch of pipe diameter per mile per day for any
section of the system. An exfiltration or infiltration test shall be performed
with a minimum positive head of 2 feet.”
A sewer system is
recommended to be pumped out before an infiltration test is performed. Pumping
into or out of calibrated drums is the recommended method for infiltration and
exfiltration testing.
The water level for testing
should provide a minimum head of 2 feet in a lateral connected to the test
portion, or, in the event there are no laterals in the test or a minimum
difference in elevation of 5 feet between the crown of the highest portion of
the drain or sewer and the test level.
“Where infiltration or exfiltration exceeds the allowable
limits specified herein, the defective pipe, joints, or other faulty
construction shall be located and repaired.”
Air pressure
tests may also determine leakage rates. Procedures vary by pipe material.
“As per RSWF, the air test shall, as a minimum, conform
to the test procedure described in ASTM C 828 for clay pipe, ASTM C 924 for
concrete pipe, ASTM F 1417 for plastic pipe, and for other materials test
procedures approved by the regulatory agency.”
Smoke Testing for Leak Detection
Smoke testing may
be done to find leaks and even to locate lost manholes. A smoke test uses
non-toxic smoke and high-capacity blowers to blow smoke through the sanitary
sewer pipes. Pumping smoke down through the sewer line can be used to locate
leaks. The smoke can enter people’s houses and reveal system leaks within the
house. If there are no leaks the smoke will exit via the house plumbing vents.
Finding plumbing leaks is secondary to the main goal which is to find leaks in
the public part of the sewer system. Typically, a section of the sewer system
is plugged off and workers walk around the neighborhood looking for smoke. It
is best to prepare for a smoke test.
“Prior to testing date, pour ½ gallon of water in all
basement floor drains or seldom used sink/shower drains and be sure to flush
any seldom used toilets. This ensures
that drain traps are filled with water and smoke will not enter the house
through any normal, working sewer-line connections. This “traps” water in the
pipes, and is called “filling your traps.”
Below is shown a smoke test planned for this week in a town
in West Virginia.
Sewer Lateral/Service Connection Inspection
I have a little
bit of experience inspecting sewer lateral connections in small-town sanitary sewer
additions. We would go out to each house connection before the contractor
covered up the new connection. These are usually 4-inch lines going into an 8-inch
or larger main sewer line, or sewer main. We would record the drop from the house
to the main sewer line. We would make sure the proper sewer line was used. We
would note and GPS any nearby manholes we found. We would note where the sewer
line went into the house, where it met the main sewer line, and the location of
the cleanout line to the surface, usually close to the house. We would do a water
test with a bucket of water to confirm water was flowing down the lateral. We
would do an air pressure test which involves plugging off both ends of the line
and observing whether the pressure was sustained without dropping. On the near
end, a device called an inflatable pipe plug balloon plug, pipe pressure test plug,
plumbing test ball, and sewer test plug with a pressure gauge was attached, and
the pressure was increased to about 3.5 psi for about 2.5 minutes. If there was
no drop in pressure, the test would be good. This is an easy way to do leak
detection on a short section of sewer lateral. Other states, localities, and
situations may require different pressure drop limits. We would also sketch a
drawing to indicate the house, the lateral, the main line, and any nearby manholes,
if applicable. Typically, it was a short and non-complicated process.
We also permitted the laterals. In addition to
the new laterals, there would often be existing household sewage systems, or septic
systems, that would now be required to be abandoned. This involves getting an
abandonment permit, having the septic tank pumped out, crushing it inward,
filling the space with gravel, and covering it over with topsoil.
References:
Smoke
Testing Sewers in Your Community. Information Flyer. Thrasher Group and Town of
Elizabeth. November 2024.
Gravity
Sanitary Sewer Design and Construction. Second Edition. Prepared by a Joint
Task Force of the Environmental and Water Resources Institute and the Pipeline
Division Committee on Pipeline Planning of the American Society of Civil
Engineers and the Collection Systems Subcommittee of the Technical Practice
Committee of the Water Environment Federation. Edited by Paul Bizier.
ASCE Manuals and Reports
on Engineering Practice No. 60 WEF Manual of Practice No. FD-5. 2007. Front Matter
Gravity
Sewer Design. PDH online Course C606 (8 PDH). Instructor: Jorge Acevedo, PE. 2020. Gravity Sewer Design
Manning
Formula for Determining Open Channel Flow. OpenChannelFlow.com. Manning
Formula for Determining Open Channel Flows | Open Channel Flow
Municipal
Wastewater. U.S. EPA. Municipal
Wastewater | US EPA
Primer
for Municipal Wastewater Treatment Systems. U.S. EPA. September 2015.
Primer
for Municipal Wastewater Treatment Systems
Smoke
Testing: What Homeowners Need to Know. Superior Smoke. Smoke
Testing: What Homeowners Need to Know
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