Two treatment plants - in the drainage area of Lake Neusiedl in Burgenland (Wulkatal, Eisbachtal)-with nitrification-denitrification and simultaneous precipitation of P were operated with an anoxic -anaerobic aeration tank to obtain biological P-removal. The results indicated that a complete denitrification is essential in the first tank to obtain release and biological uptake of phosphorus. Low loading in the Wulkatal-Plant resulted in incomplete denitrification and a biological P-removal of only 60 %. The dosage of a reduced amount of precipitant indicated a combined action of chemical and biological P-removal. In the Eisbachtal Treatment Plant with the same mode of operation - two aeration tanks of which the first was kept anoxic-anaerobic - a biological P-removal of more than 90 % was obtained under optimal process conditions. At low temperatures the removal efficiency was reduced. This could, however, be compensated by a reduced amount of precipitant. An average addition of about 1 g Fe/gP which is far below the theoretical amount for precipitation resulted in effluent concentrations below 1 mg P/1 over a period of two and half years.
At the high loaded Main Treatment Plant of Vienna (F/M ratio about 1 ky/kg.d) which has plug flow conditions in the aeration tanks and ferrous sulfate addition for the control of bulking, a significant biological P-removal has been observed. According to measurements at the 400.000 m3/d plant about 50 % of the P is removed by additional biological uptake.