Horizontal subsurface flow systems in the german speaking countries: summary of long-term scientific and practical experiences; recommendations

There are now more than 15 years of experience with horizontal subsurface flow systems in the German speaking countries, covering the results of research projects and also that of practical daily maintenance. Till now some 50 - 100 horizontal flow systems with more than 50 pe in the German speaking countries are built. Approximately half of these use sand as filter material. The results show that the purification efficiency of these systems is in the order of that of good working conventional sewage plants, not only for BOD₅ and COD, but also for nitrogen and phosphorus. Very high elimination efficiencies far better than 90% for BOD, COD and P are easily possible; similarly for elimination of N, when loading rates are below 20 mm/d, or the area is larger than 10 m²/pe. Generally there seem to be few problems with winter time and with clogging. The operation of these systems is easy and inexpensive. In nearly all reed beds Phragmites was predominant after a few years. For the planting of Phragmites an easy method using halm-cuttings is given. The long term experience with the reported systems allows us to give recommendations for design, construction and maintenance.