By lifting wastewater to a higher elevation, pump stations are installed either in the gravity system or at the headworks of the wastewater treatment plant. Areas not suitable for gravity systems may use a pressure sewer collection system. This discussion reviews the construction differences and features of different pump station types.
A wastewater collection system is a network of pipes, containment basins, pumps, valves and manholes that are used to collect and move the liquid to a treatment facility.
Ideally, a system is designed so that the wastewater flows by gravity. However, soil conditions and topography are sometimes factors that may impede gravity flow. Rocky soil or a high water table may be an impediment to the installation of a gravity sewer. Long pipe running through flat surface areas or hilly contours will prohibit gravity flow.
[[{"type":"media","view_mode":"media_large","fid":"305","attributes":{"alt":"The components used in the submersible pump system are designed for dependable service in the harsh environment in which they operate","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]Where gravity flow is not practical, a lift station may be required. A lift station, also called a pump station, is a below grade basin that collects wastewater and pumps it up to a higher elevation. Pump stations are found in gravity sewers, pressure sewers and at the headworks of a wastewater treatment facility.
Submersibles are a popular type of pump used in these stations. Submersible pumping systems are compact and often less costly to install than other system types. Being a below grade installation, these systems do not obstruct sight lines and are virtually silent during operation.
The components used in the submersible pump system are designed for dependable service in the harsh environment in which they operate. The pumps are made of rugged iron castings, often protected with powdered coated epoxy. The pumps are available with abrasive resistant mechanical seals, long life bearings and a special cord entry system designed to prevent the liquid from penetrating into the motor. Since these pumps are submerged in a liquid temperature which is usually below 100-deg F, the motor will have a favorable heat dissipation rate. This enables the motor to remain cool, even during continuous operation.
The most common basin materials are concrete and fiberglass, with the smaller systems usually being fiberglass. Most systems are provided with rail systems, which simplify the installation and removal of the pumps from the basin when servicing is required.
Pump Station Types
- Distinguished by the variation in their construction rather than by their equipment:
- Indoor / basement installations
- Outdoor residential installations
- Vacuum pumping systems
- Self-priming pump systems
- Vertical suspended pump systems
- Dry pit pump stations
- Cast in place submersible stations
- Most have a motor, pump, electrical distribution & control system, valves, suction & discharge piping, and wet well.
The discharge piping is usually PVC or stainless steel (SS). Brackets and lifting cables or chains are typically constructed from SS. Check valves, shut-off valves and air relief valves will be installed in most systems. The basin will have a cover that can be made from a variety of materials: fiberglass, steel and aluminum, with a rated aluminum cover with access hatch being the most common on larger systems.
Pump operation is controlled by liquid level sensors that are connected to a control panel. The most common liquid level sensor is a float switch, while transducers and other pressure sensing devices are often used. The control panel will contain the motor contactors, overloads, pump protection features and alarm functions. The control panel is located in a ventilated area outside the basin, either mounted on a stand near the cover or on a nearby wall surface.
There are two types of pumps used in submersible pump stations. Solids handling pumps are used in larger systems that have a high flow rate requirement. Grinder pumps are found in smaller systems, used in pressure sewers, and are low flow-high head pumps.
Pump stations are a major investment and are expensive to operate and maintain. As technology improves, operation and maintenance will require special skills. These factors, along with increasingly stringent regulatory requirements, underscore the need for a properly designed pump station.
Pumps & Systems, March 2007