Pump stations in wastewater collection systems are typically designed to handle raw wastewater that is fed into underground gravity pipelines (pipes that are inclined so that a liquid can flow in one direction under gravity). Wastewater is introduced and stored in a well, commonly known as a wet well. The well is equipped with electrical instrumentation to detect the level of wastewater 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 if the sewage is transported a significant distance. The pumping station may be called a pumping station if the pump only empties into a nearby gravity manhole. [3]From here, the cycle begins again until the wastewater reaches its destination, usually a treatment plant. With this method, pump stations are used to move waste to higher elevations. In the case of high wastewater flows into the well (for example, during periods of peak flow and wet weather) additional pumps will be used. If this is insufficient, or in the case of failure of the pumping station, a blockage in the sewage system may occur, leading to an overflow of the sanitary sewer, that is, the discharge of untreated wastewater into the environment.
Wastewater pumping stations are typically designed so that one pump or set of pumps handles normal peak flow conditions. Redundancy is built into the system so that in the event that one pump is out of service, the remaining pump or pumps will handle the designed flow. The wet well storage volume between the “pump on” and “pump off” configurations is designed to minimize pump starts and stops, but is not a retention time long enough to allow the wastewater in the wet well to become septic.
Sewage pumps are almost always end-suction centrifugal pumps with open impellers and are specially designed with a large open passage to prevent clogging with debris or winding fibrous debris into the impeller. A four- or six-pole AC induction motor typically drives the pump. Instead of providing large open passageways, some pumps, typically smaller wastewater pumps, also macerate the solids within the wastewater by breaking them down into smaller parts that can more easily pass through the impeller.
The inside of a wastewater pumping station is a very dangerous place. Poisonous gases, such as methane and hydrogen sulfide, can accumulate in the wet well; An ill-equipped person who enters the well is affected by the fumes very quickly. Any wet well entry requires the correct confined space entry method for a hazardous environment. To minimize the need for entry, the installation is typically designed to allow pumps and other equipment to be removed from the outside of the wet well.
Traditional wastewater pumping stations incorporate both a wet well and a "dry well." Often these are the same structure separated by an internal division. In this configuration, the pumps are installed below ground level at the base of the dry well so that their inlets are below the water level at pump startup, priming the pump and also maximizing the available NPSH. Although nominally isolated from wastewater in the wet well, dry wells are underground, confined spaces and require appropriate precautions for entry. Additionally, any failure or leak from pumps or pipes can discharge wastewater directly into the dry well with complete flooding not an uncommon occurrence. As a result, electric motors are typically mounted above the overflow, the upper water level of the wet well, usually above ground level, and drive wastewater pumps via an extended vertical shaft. To protect the above-ground motors from the elements, small pump houses are usually built, which also incorporate the electrical switchgear and control electronics. These are the visible parts of a traditional wastewater pumping station, although they are typically smaller than wet and dry underground wells.
Most modern pumping stations do not require a dry well or pump house and generally consist of a wet well only. In this configuration, submersible wastewater pumps with a closely coupled electric motor are mounted within the wet well itself, submerged within the wastewater. Submersible pumps are mounted on two vertical guide rails and sealed in a permanently fixed "duck foot", which forms both a support and a vertical elbow for the discharge pipe. For maintenance or replacement, submersible pumps are lifted by a chain from the duck foot and up the two guide rails to the maintenance level (usually the ground). 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 and the weather is not a concern.
Due to greatly reduced health and safety concerns, and reduced footprint and visibility, submersible pump sewage pump stations have almost completely replaced traditional sewage pump stations. Additionally, a retrofit of a traditional pumping station typically involves converting it to a modern pumping station by installing submersibles in the wet well, demolishing the pump house, and removing the dry well, either by stripping it or tearing down the internal partition and merging it with the wet well.