EI 11C201
CEMP-E
1 March 1997
b. Large stations. Pumping stations serving large areas of the installation, and especially
stations where the entire wastewater flow or major portions thereof must be pumped to the
treatment facility, will be designed so far as practicable to operate on a continuous basis. The
rate of pumpage must change in increments as the inflow to the station varies. This mode of
operation will normally require two or more wastewater pumps of the constant or variable speed
type, operating in single or multiple pump combinations, as required to match the incoming
flowrates.
3-4. DEPRESSED SEWERS.
a. Velocity and flow analysis. Since a depressed sewer, or inverted siphon, is installed below
the hydraulic grade line, the pipe will always be full of wastewater under pressure, even though
there may be little or no flow. Thus, the design requires special care to secure velocities that will
prevent clogging due to sedimentation of solids. The velocity should be as high as practicable,
with a minimum requirement set at 1 meter per second (3.0 feet per second). Hydraulic
calculations may be based on the Manning formula or Hazen-Williams analysis. A minimum
Manning roughness coefficient of 0.015 is recommended due to possible accumulations of
grease and other materials on pipe walls. The pipe will be as small as the available head
permits except that pipe smaller than 150-millimeter (6-inch) is not permitted. Inasmuch as the
sewer must be of sufficient size to discharge the extreme peak flows, better velocities for the
normal range of flows can often be obtained by using several small pipes instead of one large
pipe. This requires an entrance box equipped with a diversion gate for the periodic alternation of
pipes in service and with an overflow weir so arranged that, when the flow exceeds the capacity
of one pipe, the excess can overflow to the other pipes. However, conditions might be such that
two or three pipes in lieu of one would not be advantageous or necessary. Each case will be
analyzed individually.
b. Cleaning and inspection. Depressed sewers should be flushed frequently and inspected to
make sure that obstructions are removed. Therefore, manhole structures or cleanout chambers
will be required at each end of the sewer to allow access for rodding and pumping.
c. Pipe materials. Since a depressed sewer must withstand internal pressures greater than
atmospheric, pipe materials required for use will be as indicated for force mains.
3-5. HYDROGEN SULFIDE IN SEWERS. Two of the most important problems occurring in
wastewater collection systems are (l) the corrosion of sewers and appurtenances, and (2) the
propagation and emission of odorous and toxic gases. Both of these problems can be attributed
Environmental Protection Agency (EPA) publication, Process Design Manual for Sulfide Control
in Sanitary Sewerage Systems, for a complete discussion of this topic. Sewers will be designed
hydraulically in accordance with EPA guidelines established therein to prevent excessive
generation of H2S. In general, small diameter sewers designed to maintain velocities greater
than 0.6 meters per second (2.0 feet per second), and sufficient air-to- wastewater contact,
normally experience no significant buildup of H 2S. Larger sized sewers may be susceptible to
H2S formation, but rates of generation can be reduced through proper design, with
concentrations limited to less than 1.0 milligram per liter.
a. Corrosion control. Where it is determined that the potential exists for damaging H 2S
concentrations that will cause microbiological induced corrosion (MIC) or acid corrosion, such as
new sewer connections to older systems with a history of H 2S problems and deteriorating
sewers, pipe materials must be selected to resist MIC and attack from sulfuric acid. Chapter 8
3-8
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