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 subsurf ace flow systems in the German speaking countries, covering the result s of research projects and also that of practical daily maintenance. T ill now some 50 - 100 horizontal flow systems with more than 50 pe in the German speaking countries are built. Approximately half of these u se sand as filter material. The results show that the purification eff iciency of these systems is in the order of that of good working conve ntional sewage plants, not only for BOD5 and GOD, but also for nitroge n and phosphorus. Very high elimination efficiences 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 1 0 m(2)/pe. Generally there seem to be few problems with winter time an d with clogging. The operation of these systems is easy and inexpensiv e. In nearly all reed beds Phragmites was predominant after a few year s. For the planting of Phragmites an easy method using halm-cuttings i s given. The long term experience with the reported systems allows us to give recommendations for design, construction and maintenance. (C) 1997 IAWQ.