The Plant‐Covered Retention Soil Filter (RSF): The Mechanical and Biological Combined Sewer Overflow (CSO) Treatment Plant

Massive improvements of municipal wastewater treatment during the last 20 years have significantly reduced the pollution load on our bodies of water. At the same time, pollution load resulting from combined sewer overflows (CSO) has increased, which has made appropriate countermeasures necessary. Up to now, this has been achieved by introducing combined wastewater basins in accordance with the German ATV standard A 128 which deals with the design and construction of this type of installation. However, the effect of these installations is limited, according to their specific size, and especially the reduction of nutrient compounds is not sufficient, as these installations may include sedimentation as a first treatment process which does not affect the soluble substances at all. The only technology that is capable of reducing settable compounds as well as soluble compounds is a plant‐covered retention soil filter (RSF). The first retention soil filters were built in Germany about 10 years ago at a few sites to treat stormwater overflows from combined sewer systems (CSO) in order to meet advanced standards. This paper introduces the technology of RSFs for the treatment of combined water and deals with design, operation principles, some current suggestions for the construction site, and for the first time, the results of a long period of operation on the basis of long‐term investigations at RSF Fulda, which is the second oldest RSF in Germany (1994). Even so, in Germany RSFs are used for the treatment of stormwater.