Nutrient removal in small wastewater treatment plants is often beneficial especially if the effluent must be discharged into sensitive or guarded waters or into groundwater. The presented studies optimised an inexpensive method of subsequent enhanced wastewater treatment. The developed reactor is similar to a concentrated subsoil passage. The fixed bed reactor is divided in two sections to achieve aerobic and anoxic conditions for nitrification/denitrification processes. To enhance phosphorus removal, ferrous particles are addedto the aerobic zone. Two series of column tests were carried out and a technical pilot plant was built to verify the efficiency of the process. The results show that this method can be implemented successfully. The aerobic processes, nitrification and phosphorus removal by absorption, were not problematic. Elimination rates higher than 90 % were measured. The capacity of phosphorus removal is principally limited. After consumption of the ferric ions the reactor filling must be renewed. Denitrification strongly depends on the availability of a degradable organic substrate. To balance the substrate load and the nitrate flow a simple device was created. Weekly monitoring of the BOD and nitrate effluent concentration seems to be sufficient to find an appropriate adjustment. A technical-scale pilot plant was built and the first test drives have been started.

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