Frozen septic tank: what to do now and how to prevent it next winter

How to tell what has frozen

What to do, in a safe order

1. Stop using water If the water isn't draining, every further bucketful may end up in the basement or the bathroom.
2. Find the spot Open the lid and look at the level. If the level in the tank is low but the drains in the house are standing full, the pipe before the tank has frozen.
3. Thaw it gently Pour warm, not boiling, water through the inspection chamber or the tank inlet, because boiling water can deform plastic pipes. If the pipe is frozen deep down and over a long section, you need a specialist with a pipe-thawing machine.
4. Don't put anything in the tank Don't use salt, antifreeze, solvents or an open flame. Chemicals kill the bacteria and pollute the soil, and a flame damages the pipes and is dangerous because of sewer gases.
5. Request pumping If the septic tank or holding tank is full, pumping frees up space. Clear the snow from the access road and the lid so that the vacuum tanker can get there.

Why a septic tank freezes in winter

• The pipe was laid too shallow or without insulation, and the frost reaches it.
• The flow is low: at a summer house in winter or during a long absence, water stands in the pipe and freezes.
• The pipe doesn't have enough fall, so water stays in it instead of draining away.
• A dripping tap or toilet: a thin trickle of water gradually turns to ice in the pipe.
• The snow has been cleared above the pipe or the field, or there's a driveway or parking space on top: compacted ground freezes deeper because the snow no longer insulates it.
• The riser and the lid are not insulated.
• On a BIO plant, the compressor is in an unheated place, or condensation has collected in the air pipe.

A holding tank freezes in winter if it is almost full, because then the top layer is close to the surface and freezes first. That's why it's worth pumping out a holding tank before the frosts or before a longer absence.

How to prevent it next winter

• Insulate the pipe and the riser under the lid: insulation boards above the pipe, an insulated lid.
• In critical sections, such as where the pipe leaves the house, it can be worth installing a heating cable.
• Don't clear the snow above the pipe and the field, and don't drive over them.
• Fix dripping taps and toilets before winter.
• Pump out the holding tank before the frosts or before you go away.
• For a summer house with a BIO plant, use the holiday mode if there is one, and check that the compressor is protected from frost.
• If the pipe freezes every winter, relay it deeper or with the correct fall.

Read about relaying a pipe on the page sewer installation, and about work in the cold season on the page septic tank installation in winter. The pipe depth and insulation are set by the building design; if there isn't one, they are assessed before the work starts.

Septic tank in winter: can it be pumped?

Yes. The vacuum tanker needs a cleared access road and a lid that is uncovered and can be opened. If the lid is deep under snow or frozen shut, free it in advance: that saves time and money. In your request, say that the tank or the pipe is frozen, and give the approximate distance from the road to the lid.

The minimum under the rules doesn't change in winter: even if the house stands empty in winter, the septic tank must be pumped as often as the municipality requires. Read more on the page how often to pump a septic tank.

BIO plant in winter

Manufacturers specify the conditions their plants work in: the AQUABLOC control unit, for example, is designed to work from −25 to +50 °C. The PURA passport also provides that the start-up of a new plant can be postponed by up to two months in cold weather. In winter it's important that the plant receives wastewater, the compressor is protected from frost and the air pipe doesn't freeze. If the plant raises an alarm, see biological plant alarm.