There are a number of possibilities for decentralised or on-site infiltration systems, which are described in the following section. It should be noted that often combinations of the different systems are used.

Infiltration beds

Infiltration beds represent the simplest form of local infiltration. The rainwater is let run onto a vegetation-covered surface where it infiltrates. In areas with high groundwater table or fine-grained soils, infiltration can be very slow and may result in standing water.

Open ditches

The use of open ditches and swales adjacent to streets, roads, highways, and small parking lots is a special case of surface infiltration. The infiltration capacity depends on factors such as the porosity of the soil and the suspended solids load in the stormwater, and it may not be possible to infiltrate all the runoff into the ground using only swales and ditches, when the catchment area is too large compared to the available infiltrating surface. Good vegetation growth in open ditches is essential, since the vegetation re-aerates the soil surfaces when they become clogged with fine sediments carried by the runoff. In addition to swales and ditches percolation trenches can be installed.

Figure 7: Swale

Figure 8: Infiltration ditch (Source: Universität Trier)

Percolation basins

A percolation basin is constructed by excavating a pit, filling it with gravel or crushed stone, and then backfilling over the top of the rock. The rock media provide the porosity for temporary storage of water so that it can then slowly percolate into the ground. Basins should not be submerged by groundwater, as this will hinder their proper function. In order to reduce the risk of clogging of the rocks by suspended particles, it is advantageous to filter the stormwater before it enters the rockfilled basin by passing either through a granular filter bed or a fine mesh geotextile material. The filter media needs to be removed and replaced from time to time.

Pipe trench

A pipe trench is a special case of a percolation basin where the stormwater is stored not only in the rock media, but in the pipe itself. Clogging can also occur in the pipe trench, so that filtering of stormwater should be part of each pipe trench installation.