-Septic systems: If the final soil treatment unit is a mound, is preceded by a pretreatment unit, or the liquid flowing out of the septic tank cannot flow to the trench by gravity, a pump station is needed. A pump station is a separate small concrete tank or second compartment of a septic tank containing a pump designed to move the liquid. A pump may also be necessary to move liquid from the septic tank to a pretreatment unit. The pump operates on a float-controlled switch. When the storage area in the tank is full it will pump the contents to the soil treatment unit. Timers can also be used in place of floats to spread effluent application evenly throughout the day. The pump has an emergency alarm indicator circuit to warn the homeowner when the water level is too high. If this happens, the problem needs immediate attention! Be sure to know where this alarm is, what it means, and what to do when it is activated. The manhole cover on lift stations should be at the surface for easy year round access.
(info taken from www.extension.umn.edu/distribution/naturalresources/components/6583-03.html )

-Sewer systems: Pumping stations in sewage collection systems 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 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 detention time as to allow the sewage in the wet well to go septic.

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.
(info taken from http://en.wikipedia.org/wiki/Pumping_station)