A rainwater filter for rough cleaning

The rainwater filter of a cistern is generally referred to as the first effective coarse filter. Above all, it serves to ward off the intrusion of small animals, leaves and plant parts. Downstream filter units ensure a finer postfiltering. The placement of the filters varies and can be done both in the cistern and on the supply lines.

Filter types and placements

Usually the rainwater filters are located in the downpipes, which lead, for example, from the roof into the cistern or they are installed as a cistern filter directly in the tank. In underground cisterns, they can be mounted above the tank as a soil filter.

Normally, the filter unit of a rainwater filter consists of a grid, perforated plate or sieve. For concrete cisterns so-called filter plates made of cellular concrete can be installed in the water inlet path. The simple mechanical principle of operation makes it easy to build a rainwater filter yourself.

Filter construction and mechanical action principle

The structure of a rainwater filter can be mounted as a simple flow filter that filters rainwater for the cistern directly on the pass. Typical representatives of this type of construction are filter types that are installed in downpipes.

Another type has a cavity next to the water flow through which the rainwater is flushed. These types of construction are also referred to as pot filter. In the "pot" different filter elements can be mounted:

  • Filter gravel in a gravel filter
  • Sand filter with very fine gravel
  • filter cartridge
  • Substrate as a catalyst
  • filter basket
  • filter net
  • filter screen

Additional effect by water movement

The features of the rainwater filter are different depending on the type and manufacturer. Attention should be paid to a permanently possible access, for example by a flap or a lid for the revision and possibly emptying, emptying or replacement of the filter elements.

Depending on the mode of action, rainwater filters can use vertical, horizontal or combined water flow directions. Here, the following properties and additional effect properties are used:

  • Horizontal turbulence
  • Vertical turbulence
  • Water quantity regulation by buffer storage (retention filter)
  • Boundary of a funnel
  • Standing metal element in cone shape

Additional filters and water purification

If the rainwater is to be used as a useful water, for example, for the toilet flush or the washing machine, an additional filter in the house waterworks or in the cistern is inevitable. Even today, methods of trickle filtering are more often used than with a gravel filter. The effect is enhanced by a soaked water inlet, which prevents turbulence of settled dirt particles.

Due to the floating removal on the surface of the cistern water, the water which has become cleanest by the downpouring of the dirt particles is removed. At the intake manifold or the water sampling hose normally another suction filter is mounted.

Frost resistance and load capacity

An important criterion for selecting one or more rainwater filters is the area that "delivers" the rainwater. Most manufacturers specify the maximum roof areas for which the respective filter is suitable.

While frost resistance is not a problem with underground installation, this aspect must be considered when installing in downpipes. In some models, the downpipe filter automatically "shuts off" in winter and transfers the rainwater and condensation into the sewage system.

When using a rainwater filter installed in the ground or underground, pay attention to how strong it is. Almost all manufacturers offer two classes that are either walkable or passable. Even when self-construction must be paid to the subsequent traffic load.

Tips & Tricks

As an ideal combination, you can work with a flow filter and a whirl filter. Together with a calm water inlet, these two filter units are enough to make rainwater suitable for use with utility water.

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