Backflow protection: preventing flooded basements

When the sewer system is overloaded: What is backflow?

It doesn't always have to involve a large river overflowing its banks in the flood of the century and putting the whole city centre under water– the risk of basements being flooded occurs far more frequently. This is because our sewer networks are designed only for medium rainfall events, both for economic reasons and to ensure that they are self-cleaning. In the event of heavy rain or a blocked sewer pipe, the system can become overloaded: The sewer system is then no longer able to completely drain off the approaching water and a "backflow" occurs. That means the water will find its way back and leak wherever it can: at best from manhole covers embedded in the street and at worst from the drains, sinks and toilet in your basement, laundry or hobby room.

And apart from the damage caused to the building and household goods in such a case, pumping out the water and draining the basement takes a lot of time, money and effort. In addition, insurance companies may refuse indemnification if a standard-compliant backflow protection device is not fitted, or does not comply with relevant laws, rules and regulations.

Below the flood level it can be dangerous

In principle, there is only a risk of flooding below the so-called flood level - marked as a red line in the picture below. If the sewer system overflows, the water is forced back into the waste water pipes. Above the flood level, the water disperses onto the street or gardens.

What can backflow protection look like - both active and passive?

In order to keep your feet dry in the basement even when the sewer system is full, backflow protection is essential. A whole range of standards and laws exist for selecting the right solution. Basically, there are two distinct types:

• Passive backflow protection with the aid of anti-flooding devices
• Active backflow protection with the aid of lifting units

Passive backflow protection: watertight drain thanks to anti-flooding device

If the water from the sewer system wants to go back into the house: can't you just close the waste water pipe? Yes, you can - but only subject to very strict conditions and rules. First of all, anti-flooding devices may only be used if:

• The discharge points are located above the sewage system, i.e. there is a natural fall down to the sewer
• The rooms are intended for subordinate use only, i.e. no significant damage to health or property can occur in the event of flooding
• The number of users is limited and the users have access to a toilet above the flood level, and
• Use of the affected discharge point is not required in the event of a backflow.

In addition, anti-flooding devices must

• Be constructed in such a way that they do not experience wear under the influence of domestic waste water
• Close automatically in the event of backflow and ensure unobstructed water drainage after the backflow has ended, and
• Be regularly maintained.

In short, anti-flooding devices are, at first glance, a compact and cost-effective solution, but they have some disadvantages compared to lifting units. For example, the sanitary appliances cannot be used during a backflow situation and, if the devices malfunction, there is a risk of water entering the basement.

Active backflow protection: Only lifting units make things really safe

The fact that backflow occurs when the sewer system is overloaded is unavoidable. Possible unpleasant consequences, however, can be prevented. A backflow loop with a connected waste water lifting unit offers safe protection here – even in the event of a power failure, the basement will not get wet and you have a small safety volume in the tank that you can use. The schematic diagrams below clearly show how it works: in the event of backflow, the water cannot rise any higher than the red flood level; it spills out of the gully and floods onto the street. The loop is located above the flood level, so no water can enter the building from the outside this way. One small disadvantage: In everyday use, the waste water from the building cannot flow directly into the sewer system, but must first be transported through the backflow loop with the help of a waste water lifting unit. An advantage of the lifting unit, however, is that the installations connected to it can even be used during backflow conditions.

What type of lifting unit is needed for what?

Common waste water lifting units feature a tank in which the waste water is initially collected. When the water level in this tank reaches a certain level, the pump is started up and lifts the water out via the backflow loop. When the level in the tank has dropped far enough, the pump stops automatically. "DIN EN 12050: Waste water lifting plants for buildings and sites" defines 3 types of waste water lifting units, each of which is subject to specific regulations:

• Type 1: Lifting units for waste water containing faeces from flushing lavatories and urinal systems
• Type 2: Lifting units for slightly contaminated waste water or rainwater without faeces
• Type 3: Lifting units for waste water containing faeces and subject to limited use

In order to guarantee the function of a waste water lifting unit in the long term, DIN EN 12056-4 requires that the unit be inspected regularly by a suitably qualified person:

• Quarterly in commercial buildings
• Half-yearly in multiple dwelling buildings
• Annually in single-family houses

Where do which standards relating to backflow protection apply?

When it comes to reliable backflow protection – not only with the help of a lifting unit, but also with an anti-flooding device – a whole series of standards must be observed. These are standards for defined areas of application (inside and outside buildings) and rules that revolve very specifically around the products to be used. Here is an overview:

Fundamentals of drainage system construction:

• DIN EN 12056 Gravity drainage systems inside buildings
• DIN EN 752 Drain and sewer systems outside buildings
• DIN 1986-100 Drainage systems for buildings and premises – Part 100: Specifications in relation to DIN EN 752 and DIN EN 12056 (German only)

Product-related standards:

• DIN EN 12050 Waste water lifting plants for buildings and sites
  - Part 1: Lifting plants for wastewater containing faecal matter
  - Part 2: Lifting plants for faecal-free wastewater
  - Part 3: Lifting plants for limited applications
  - Part 4: Non-return valves for faecal-free wastewater and wastewater containing faecal matter

When designing and implementing a drainage system, these standards should be strictly adhered to.

Summary

Basically, there are two different ways of protecting basements from flooding when the public sewer system is overloaded.

Firstly, by means of anti-flooding devices: Here, the flow into the waste water pipe is interrupted by, for example, backflow flaps[RP1] . It should be noted here that during backflow conditions, i.e. when the pipe is closed, the connected sanitary objects cannot be used. And if an anti-flooding device is out of order or defective, there is no protection against backflow.

And secondly, by waster water lifting units: Here, the waste water is transferred with the help of a pump into the sewer via a backflow loop above the flood level. Only this loop provides absolute protection against backflow, even if the lifting unit is occasionally out of service or defective.