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 sloped 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 gasses, 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 maximising 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 the 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 pumping stations do not require a dry well or pump house and usually, consist only of a wet well. In this configuration, submersible sewage pumps with a closely coupled electric motor are mounted within the wet well itself, submerged within the sewage. Submersible pumps are mounted on two vertical guide rails and seal onto a permanently fixed “duck foot”, which forms both a mount and also a vertical bend for the discharge pipe. For maintenance or replacement, submersible pumps are raised by a chain off of the duck foot 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 duck foot where the weight of the pump reseals it. As the motors are sealed andthe 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 pumping stations have almost completely superseded traditional sewage pumping stations. Further, a refit of a traditional pumping station 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.