Any conventional septic system designed so the drain field is at a higher elevation than the tank needs a pump to transfer effluent from the tank to the drain field. A float switch inside the tank tells the pump when to come on, and a separate float switch operates an alarm that comes on when the water level exceeds a preset minimum. Aerobic septic systems also need alarms, even if the ground doesn't slope.
The Need for a Septic Alarm
In a gravity-fed septic system, sewage flows from the house to a holding tank, solids settle to the bottom, and the overflow exits through the outlet to the drain field. When a pump is needed to transfer the waste to a higher elevation -- which is common -- it gets triggered by a float similar to the one in a toilet tank. A malfunction of either the pump or the float can create serious problems, such as a tank overflow or a sewage backup in the house. If the system has an alarm, the problem can be corrected early and the worst consequences avoided.
The septic alarm is operated by a separate float switch, which should be wired to a separate electrical circuit so that it will still function if the breaker controlling the pump trips. When the water level reaches the preset cut-in point for the float switch, it may activate a buzzer, a light or some other alert mechanism, which may be located on the tank, the side of the house, in the basement or in a utility space. Some communities require alarms to be connected to a municipal grid so the authorities can monitor malfunctioning septic systems.
What the Alarm Means
When a septic alarm goes off, it means the water level in the tank is too high, and that usually means the pump is malfunctioning. The reason could simply be that the breaker controlling it has tripped, and all you need to do is reset it. It could also mean that the pump is clogged and needs to be serviced, or that a persistent leak is causing it to work too hard. A worst case scenario is that the drain field is clogged and the pump can't clear the tank. This might happen during a period of heavy rain, and it means the drain field is saturated; you should switch off the pump and refrain from using the plumbing until things dry out.
Aerobic Septic Systems
An aerobic septic system works by pumping compressed air through sewage to stimulate decomposition, and this type of system also needs an alarm. It's controlled by a switch that is sensitive to air pressure, and when it goes off, it usually means the compressor pump is malfunctioning. This type of system usually includes a separate holding tank for treated water and a pump to direct the water to pop-up sprinklers that distribute it over the drain field. This tank usually has a separate alarm connected to a float switch, and when it goes off, it means the pump isn't working.