Page 576 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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532 Chapter 14 Design of Sewer Systems
14.14.1 Common Modifications

Force mains may be aerated or the wastewater chlorinated at the pump station to prevent
odors and excessive corrosion. Pressure surge control devices are installed to reduce
pipeline pressure below a safe operating pressure during lift station start-up and shut-off.
Typically, automatically operated valves (cone or ball type) control pressure surges at the
pump discharge or pressure surge tanks. Normally, force main cleaning includes running a
manufactured “pigging” device through the line and long force mains are typically
equipped with “pig” insertion and retrieval stations. In most cases, insertion facilities are
located within the lift station and the pig removal station is at the discharge point of the
force main. Several launching and retrieval stations are usually provided in long force
mains to facilitate cleaning of the pipeline.

14.14.2 Applicability

Force mains are used to convey wastewater from a lower to higher elevation, particularly
where the elevation of the source is not sufficient for gravity flow and/or the use of gravity
conveyance will result in excessive excavation depths and high sewer pipeline construc-
tion costs.
Force mains are very reliable when they are properly designed and maintained. In gen-
eral, force main reliability and useful life are comparable to that of gravity sewer lines, but
pipeline reliability may be compromised by excessive pressure surges, corrosion, or lack
of routine maintenance.

14.14.3 Advantages and Disadvantages

Use of force mains can significantly reduce the size and depth of sewer lines and decrease
the overall costs of sewer system construction. Typically, when gravity sewers are installed
in trenches deeper than 20 ft (6.1 m), the cost of sewer line installation increases significantly
because more complex and costly excavation equipment and trench shoring techniques are
required. Usually, the diameter of pressurized force mains is one to two sizes smaller than
the diameter of gravity sewer lines conveying the same flow, allowing significant pipeline
cost reduction. Force main installation is simple because of shallower pipeline trenches
and a reduced quantity of earthwork. Installation of force mains is not dependent on site-
specific topographic conditions and is not impacted by available terrain slope, which typi-
cally limits gravity wastewater conveyance.
While construction of force mains is less expensive than gravity sewer lines for the
same flow, force main wastewater conveyance requires the construction and operation of
one or more lift stations. Wastewater pumping and use of force mains could be eliminated
or reduced by selecting alternative sewer routes, consolidating a proposed lift station with
an existing lift station, or extending a gravity sewer using directional drilling or other state-
of-the art deep excavation methods.
The dissolved oxygen content of the wastewater is often depleted in the wet-well of
the lift station, and its subsequent passage through the force main results in the discharge
of septic wastewater, which not only lacks oxygen but often contains sulfides. Frequent
cleaning and maintenance of force mains is required to remove solids and grease buildup
and minimize corrosion due to the high concentration of sulfides.
Pressure surges are abrupt increases in operating pressure in force mains that typically
occur during pump start-up and shut-off. Pressure surges may have negative effects on
force main integrity but can be reduced by proper pump station and pipeline design.