Sewage Ejector Pump Information
Sewage ejector pumps, or sewage grinder pumps,
are designed to pump residential or household sewage or
blackwater to a destination such as an elevated septic tank
or to a city sewer (for homes whose lower baths are at a
depth below the level of their sewer line). Even if a
building is nearly at the same level as its septic tank or
sewer line, if the geography of the site prevents sewage
from flowing fast enough on its own (two-feet per second)
then a sewage grinder or sewage ejector pump is needed.
A typical application of a sewage ejector pump
is in a home where a basement bathroom is located lower than
the height of the sewer line which leaves the home. The
sewage ejector pump lifts waste from the basement bathroom
up to the sewer line where it flows out to a septic tank or
community sewer. A sewage or septic grinder pump, (there is
more than one grinding method) reduces sewage to a finely
ground slurry of waste and water which can then be pumped or
forced to its destination. If your building's drain
system is at a level below a municipal sewer line, or if
your septic drainfield or tank and fields are uphill from
the building, you need a sewage grinder pump and a
forced-main sewer system.
Pumping
stations in sewage collection systems, also called lift stations, are
normally designed to handle raw sewage that is fed from underground
gravity pipelines (pipes that are laid at an angle so that a liquid can
flow in one direction under gravity). Sewage is fed into and stored in
an underground pit, commonly known as a wet well. The well is equipped
with electrical instrumentation to detect the level of sewage present.
When the sewage level rises to a predetermined point, a pump will be
started to lift the sewage upward through a pressurized pipe system
called a sewer force main or rising main from where the sewage is
discharged into a gravity manhole. From here the cycle starts all over
again until the sewage reaches its point of destination – usually a
treatment plant. By this method, pumping stations are used to move waste
to higher elevations. In the case of high sewage flows into the well
(for example during peak flow periods and wet weather) additional pumps
will be used. If this is insufficient, or in the case of failure of the
pumping station, a backup in the sewer system can occur, leading to a
sanitary sewer overflow – the discharge of raw sewage into the
environment.
Sewage pumping stations are typically designed so that one pump or one
set of pumps will handle normal peak flow conditions. Redundancy is
built into the system so that in the event that any one pump is out of
service, the remaining pump or pumps will handle the designed flow. The
storage volume of the wet well between the 'pump on' and 'pump off'
settings is designed to minimize pump starts and stops, but is not so
long a retention time as to allow the sewage in the wet well to go
septic.
Sewage pumps are almost always end-suction centrifugal pumps with open
impellers and are specially designed with a large open passage so as to
avoid clogging with debris or winding stringy debris onto the impeller.
A four pole or six pole AC induction motor normally drives the pump.
Rather than provide large open passages, some pumps, typically smaller
sewage pumps, also macerate any solids within the sewage breaking them
down into smaller parts which can more easily pass through the impeller.
The interior of a sewage pump station is a very dangerous place.
Poisonous gases such as methane and hydrogen sulfide can accumulate in
the wet well; an ill-equipped person entering the well would be overcome
by fumes very quickly. Any entry into the wet well requires the correct
confined space entry method for a hazardous environment. To minimize the
need for entry, the facility is normally designed to allow pumps and
other equipment to be removed from outside the wet well.
Traditional sewage pumping stations incorporate both a wet well and a
'dry well'. Often these are the same structure separated by an internal
divide. In this configuration pumps are installed below ground level on
the base of the dry well so that their inlets are below water level on
pump start, priming the pump and also maximizing the available NPSH.
Although nominally isolated from the sewage in the wet well, dry wells
are underground, confined spaces and require appropriate precautions for
entry. Further, any failure or leakage of the pumps or pipework can
discharge sewage directly into the dry well with complete flooding not
an uncommon occurrence. As a result, the electric motors are normally
mounted above the overflow, top water level of the wet well, usually
above ground level, and drive the sewage pumps through an extended
vertical shaft. To protect the above ground motors from weather, small
pump houses are normally built, which also incorporate the electrical
switchgear and control electronics. These are the visible parts of a
traditional sewage pumping station although they are typically smaller
than the underground wet and dry wells.
More modern sewage pumping stations do not require a dry well or pump
house and usually comprise only a wet well. In this configuration,
submersible sewage pumps with closely coupled electric motor are mounted
within the wet well itself, submerged within the sewage. Submersible
sewage pumps are mounted on two vertical guide rails and seal onto a
permanently fixed 'duckfoot', which forms both a mount and also a
vertical bend for the discharge pipe. For maintenance or replacement,
submersible sewage pumps are raised by a chain off of the duckfoot and
up the two guide rails to the maintenance (normally ground) level.
Reinstalling the pumps simply reverses this process with the pump being
remounted on the guide rails and lowered onto the duckfoot where the
weight of the pump reseals it. As the motors are sealed and weather is
not a concern, no above ground structures are required, excepting a
small kiosk to contain the electrical switchgear and control systems.
Due to the much reduced health and safety concerns, and smaller
footprint and visibility, submersible pump sewage pump stations have
almost completely superseded traditional sewage pump stations. Further,
a refit of a traditional pump stations usually involves converting it
into a modern pumping station by installing submersibles in the wet
well, demolishing the pump house and retiring the dry well by either
stripping it, or knocking down the internal partition and merging it
with the wet well.
