Treatment in a filter bed is based on the same principles as infiltration, except that the treated water is led away via an discharge pipe. When the soil is composed of densely packed clay, the ground will not be able to divert all the water and a discharge pipe must be installed in the bed. The discharge pipe usually leads to a ditch, but can also lead to a catch drain or another suitable place.
With IN-DRÄN, horizontal filter beds are often constructed in a rather large area to maximize infiltration and achieve safer sanitation. These filter beds are not very high and may be preferred, especially in cases of high groundwater or close proximity to mountains. Usually, very little water will come up to the discharge pipe then.
In some cases, a dense filter bed is the best solution, e.g. if the only possible location is near drinking water wells. To minimize the risk of contaminating drinking water wells, a an airtight sealing layer is placed in the bottom of the filter bed and all water is diverted to a safer place. When a sealing layer is used, an extra ventilation pipe must be placed in the bed under the IN-DRÄN modules to ensure good oxygen supply for the biomat since the sealing layer will not prevent oxygen from the soil from entering the bed. In areas sensitive to eutrophication (over-fertilization), a phosphorus trap can be added to the filter bed.