Sewer backflow during heavy rain: non-return valve, lifting pump, what to do?

In a heavy storm, water does not always come in through the door or the cellar window. Often it rises out of the floor drain, the toilet or the shower in the cellar. That is sewer backflow. Here is how it happens and how to protect yourself.

During exceptional rainfall, Brussels Environment explains, run-off is so heavy that the sewers become saturated. Because your installation is connected to the public sewer, the water levels even out, as in communicating vessels. Anything lower down, a drain, toilet or washing machine in the cellar, becomes a way out.

The Flemish Environment Agency (VMM) points out that the risk is greater when the house is connected to a low-lying section of the sewer or when the connection is deep, for instance below the cellar floor. It also notes that, for economic reasons, a sewer cannot be designed for the most extreme downpour.

A non-return valve lets water out of the house and closes when it tries to come back in. The flood guide of the Walloon Public Service (SPW) places it in a watertight inspection chamber outside, or indoors where the sewer enters the house. Fitting should generally be left to a specialist, may require the agreement of the sewer network operator, and you need to check that the pipe can withstand the pressure.

The VMM asks that all the house’s drains be mapped first: there must be no pressure-sensitive connection between the valve and the public sewer. The valve then needs to be checked regularly and cleaned if necessary, because debris caught in it stops it from closing. The SPW considers regular maintenance by a professional essential. A valve buried too deep or hidden under tiles is a valve nobody checks.

Not to be confused with the valve on the discharge pipe of a lifting pump, which we covered in our article on pump maintenance: that one stops pumped water from flowing back into the tank.

A closed valve keeps the sewer out, but it also keeps your own wastewater in. The VMM stresses this: in heavy rain, the public sewer can be so full that the water from the house can no longer drain away. If the storm lasts, it builds up behind the valve.

Brussels Environment therefore advises combining the valve with a tank or a pump. For cellars it goes further: in its view a non-return valve is not always reliable, and it is better to seal off the sewer openings in the cellar and use a pump to lift the water up to the ground-floor wastewater pipes.

That is the principle of a lifting station: water from the cellar collects in a tank and is pumped to a higher point. The SPW requires the discharge to be above the level at risk of flooding, and the power supply to be kept out of the water. Our article on lifting stations explains how to size them.

In Brussels, Vivaqua, which manages the sewers, offers anti-flooding guidance: an adviser examines the installation and suggests rerouting pipes, pumps or valves. On its territory, the City of Brussels grants, under conditions, a subsidy for this diagnosis and for the work. In Wallonia, the SPW recommends non-return valves on wastewater drains. In Flanders, the VMM publishes practical fact sheets on valves and pump pits. Before any work, ask your municipality about the rules for connecting to the sewer.

Identify the low points where the sewer can come back up: floor drains, toilet, shower and appliance connections in the cellar.

Check whether you have a non-return valve, where it is and whether you can reach it.

Have it checked and cleaned regularly, following the manufacturer’s instructions.

Test your lifting pump before the storm season.

Move appliances and valuables in the cellar up off the floor.

Keep the contact details of your insurer, your municipality and a repair service reachable at night close at hand.

Do not put yourself in danger: sewer water is contaminated, so wear boots and gloves. If necessary, switch off the electricity, and only switch it back on after a professional has checked it. As long as the sewer is under pressure, use as little water as possible in the house: it has nowhere to go. As the SPW advises, note the highest water level and take photos. Once the water has gone, air and dry the rooms and check that wastewater drains normally again.

In Belgium, home fire insurance covers natural disasters, including flooding and sewer backflow, Assuralia points out. Wikifin (FSMA) notes one exception: an insurer may refuse to cover sewer overflow or backflow for a home built in a risk zone after the zone was officially designated. Read your policy conditions and report the claim as soon as possible, with photos and a list of damaged items.

We install, inspect and repair pumps and lifting stations throughout Belgium, and we fit and maintain non-return valves on the sewer connection, in an inspection chamber or in the cellar. Before proposing a valve, a pump or both, we look at how water leaves your house and where it can come back in. And if the sewer backs up in the middle of a storm, our emergency service answers 24 hours a day, 7 days a week on +32 4 325 10 02.