What is Sewer Backflow?

The German federal flood protection guide describes two triggers: heavy rain that overloads the sewers, and river floods or high groundwater getting into the sewer network in large amounts. Blockages in the public sewer add to this. The Federal Office of Civil Protection (BBK) puts it plainly: drainage sewers are mostly not designed for flash floods.

When the water in the sewer rises, it rises in every connected house pipe too, like communicating vessels. How high is described by the backflow level. The municipality sets it, usually in its drainage bylaw. Where it hasn’t, DIN 1986-100 uses the top of the road at the connection point. In flood zones, the water in the sewer can rise to flood level, and so above the backflow level.

Every drainage point below the backflow level that is connected to the sewer without protection is at risk:

• Floor drains in the laundry room, boiler room and garage
• Toilets, showers and washbasins in the basement or a basement flat
• Washing machine and condensate connections
• Drains in window wells, basement stairwells and yards if they are connected to the sewer

What comes out is sewage from the sewer, often with faeces and germs. That makes backflow hygienically worse than clear rainwater. Wet insulation, screed and furniture often have to be removed completely afterwards.

A sign that it’s backflow and not groundwater: the water comes out of drains, smells of sewer and rises during or shortly after heavy rain.

The owner has to install backflow protection themselves. What exactly is required is set out in the municipality’s drainage bylaw. The Verbraucherzentrale sums up the consequence: if you haven’t installed protection contrary to the bylaw, you are liable for all backflow damage yourself.

Insurance doesn’t pay automatically either. According to the GDV, backflow is only covered by an additional natural hazards policy (Elementarschadenversicherung), which around 41% of residential buildings in Germany don’t have. And even then, the federal flood protection guide says: for sewer backflow, insurers only pay if a fixed protective device such as a backflow valve or lifting station has failed. Lifting stations and backflow valves also have to be checked and serviced regularly; otherwise, according to the Verbraucherzentrale, you risk your cover.

The federal flood protection guide names two effective safeguards: backflow valves and wastewater lifting stations with a backflow loop. Only drainage points below the backflow level may be connected to either.

Backflow valve

• Flaps in the pipe or the drain that close during backflow; requirements are set by DIN EN 13564-1
• For wastewater containing faeces there are separate types approved for it
• While backflow lasts, no water can drain from the basement
• According to the guide, servicing every six months is mandatory

Lifting station

• Pumps the wastewater through a pressure line with a loop above the backflow level
• The basement stays usable even during backflow
• Needs electricity; if it fails, the station stops too
• Annual servicing in a single-family home (operator rules based on DIN 1986-3)

A single central valve for the whole house is not permitted: it would also shut off pipes from above the backflow level and hinder the sewer’s ventilation. Which solution fits depends on what is connected in the basement. For a toilet or shower in a basement flat, a lifting station is usually the answer.

• Take stock: which drains in the basement are below street level, and which of them are protected? The municipality or the local sewer authority can tell you the backflow level.
• Check monthly: on a backflow valve, operate the emergency closure; on a lifting station, do a test run over two switching cycles.
• Before a storm: the BBK advises checking backflow valves before the water rises. Close emergency closures when heavy rain is forecast.
• During backflow: don’t use water in the basement, so no washing machine and no flushing. With the valve closed, it can’t drain. And don’t stay in the basement.
• Check your insurance: is backflow included in your policy, and what maintenance conditions does it set?

You need a professional for retrofitting, choosing the right type and regular servicing. A wrongly chosen or wrongly installed backflow valve only gives a false sense of security.

Drainbee can’t see into the sewer and doesn’t detect backflow in the pipe directly. It can’t monitor a plain backflow valve without a shaft.

If you have a lifting station or pump sump, Drainbee measures the level there. Especially in heavy rain, when backflow is most likely, the system has to work. If the power fails, the float sticks or the pump can’t keep up, the level rises. Drainbee reports that with a threshold alarm, and it reports the power outage too, by push or call-style alarm in the app.