The short answer
A septic tank high water alarm goes off when the liquid in a tank or pump chamber rises above its normal level. It usually means a pump has stopped, a float switch has stuck, the outlet is blocked or the drainage field isn't taking water. Cut back on water use, check the power, and get an engineer to find the cause before the system overflows.
Most simple gravity septic tanks do not have an alarm at all. They are fitted where something can stop the flow, such as a pump.
Where alarms are fitted
Pump chambers. When effluent has to be lifted, for example to a drainage field uphill of the tank, a pump chamber sits after the tank. A float switch runs the pump and a second, higher float triggers the alarm if the level keeps rising.
Sewage treatment plants. Many plants have a control panel with alarms for a high liquid level and for a failed air blower.
Cesspools. A level alarm warns that the tank is nearly full and needs emptying. Without one, the first warning can be a backed-up toilet.
The alarm is usually a box on an outside wall or in a garage, with a light and a buzzer. Some send a text message.
What sets it off
- A failed pump, from a burnt-out motor, a jammed impeller or a tripped circuit.
- A stuck float switch, often fouled by fat, wipes or a tangled cable. The pump may be fine but never told to run.
- A blockage in the outlet pipe or rising main.
- A drainage field that has stopped soaking away, so water backs up even though the pump is working.
- Too much water at once, from a busy household, a running toilet or rainwater getting in.
- A power cut. Pumps and blowers stop, and levels start to rise.
What to do when the alarm sounds
Start by reducing water use. A pump chamber normally has some spare capacity above the alarm level, but not much, so hold off on washing, baths and showers.
Check whether the power is on and whether a breaker or RCD has tripped at the consumer unit or the control panel. If it has tripped once and resets cleanly, the alarm may clear as the pump catches up. If it trips again, leave it off. Repeated resetting can damage a pump with a fault.
Leave the chamber closed. Opening a sewage chamber exposes you to gases, and nothing inside can be fixed safely by hand. Silence the buzzer if the panel allows, but don't ignore the light.
Then call an engineer, telling them what you have checked. Many alarms give a few hours to a day of warning before an overflow, depending on water use.
False alarms
A float can stick in the up position or a cable can be damaged, so the alarm sounds with a normal level. That still needs looking at, because a faulty alarm will not warn you of a real problem.
Keeping the alarm useful
Test it occasionally if the panel has a test button, and have the floats checked at each service. If the alarm box has a battery backup, replace the battery as the maker recommends so it works during a power cut. Keep wipes, fat and sanitary products out of the drains, because they are what usually jams floats and pumps.
Our engineers repair pump chambers, floats and alarms on septic tanks and treatment plants. See septic tank repairs, treatment plant repairs and our pumping stations page. If the alarm has gone off because the system is overflowing, our guide to what to do when a septic tank overflows has the next steps.
