In Germany there are both highly industrialized regions with large towns as well as extended rural areas with small communities. Most of these communities need regular sewage disposal systems. Sewage treatment plants have to be adapted to special situations in rural villages. Ponds fulfil the requirements in an excellent way. Many hundred of ponds are in operation and three kinds of pond systems are in use:
facultative ponds (more than 1000 plants) for serving less than about 1000 i+ie*;
artificially aerated ponds (more than 300 plants) for treating sewage of about 1000 to 10000 i+ie especially when food industries are connected or as an individual solution for those industries;
ponds combined with trickling filters or rotating bio-filters (more than 100 plants) as a solution for treating preferably domestic sewage from about 1000 to 4000 i+ie.
Lay out, design figures, recommendations for construction and operation, effluent results anc costs are presented on the basis of experiences with numerous plants. Actual effluent results are considerably below German Effluent Quality Standards. Facultative ponds with 10 m2/ie specific surface area produce the following effluent quality: COD ≤ 90 mg/L, BODr ≤ 25 mg/L, NH4≤N = 15 mg/L, PO4-P ≤ 6 mg/L. Heavy algae blooms cause an increase of effluent concentrations: 100 µg Chlorophyll-α represent about 9 mg COD and 3 mg BOD5 on the average.