Environmental Engineering Reference
In-Depth Information
5.2.2 force Main Collection system
In a typical force main collection system,
wastewater is collected to central points and
pumped under pressure to the treatment plant. The
system is normally used for conveying wastewater
long distances. The use of the force main system
allows the wastewater to flow to the treatment plant at the desired veloc-
ity without using sloped lines. It should be noted that the pump station
discharge lines in a gravity system are considered to be force mains
because the content of the lines is under pressure.
Key Point:
Extra care must be taken
when performing maintenance on
force main systems because the
content of the collection system is
under pressure.
5.2.3 vacuum Collection system
In a vacuum collection system, wastewaters are collected to cen-
tral points and then drawn toward the treatment plant under vacuum.
The system consists of a large amount of mechanical equipment and
requires a large amount of maintenance to perform properly. Generally,
the vacuum-type collection systems are not economically feasible.
5.2.4 Pumping stations
Pumping stations provide the motive force (energy) to keep the
wastewater moving at the desired velocity. They are used in both the
force main and gravity systems. They are designed in several different
configurations and may use different sources of energy to move the
wastewater (i.e., pumps, air pressure, or vacuum). One of the more com-
monly used types of pumping station designs is the wet-well/dry-well
design.
5.2.4.1 Wet-Well/Dry-Well Pumping Stations
The wet well/dry well pumping station consists of two separate
spaces or sections separated by a common wall. Wastewater is collected
in one section (wet-well section), and the pumping equipment (includ-
ing, in many cases, the motors and controllers) is located in a second
section, known as the dry well. Many different designs for this type
of system are available, but in most cases the pumps selected for this
system are of a centrifugal design. Among the major considerations in
selecting the centrifugal design are that: (1) it allows for the separation
of mechanical equipment (e.g., pumps, motors, controllers, wiring) from
the potentially corrosive atmosphere (sulfides) of the wastewater, and
(2) this type of design is usually safer for workers because they can
monitor, maintain, operate, and repair equipment without entering the
pumping station wet well.
Note:
Most pumping station wet wells are confined spaces. To ensure
safe entry into such spaces, compliance with the OSHA Confined Space
Entry Standard (29 CFR 1910.146) is required.
