Pilot plant studies on high biological nutrient removal and using settled domestic wastewaters have been carried out with the so-called Modified Renphosystem. The system consists of a bioreactor which has been divided into four zones: anaerobic, aerobic, anoxic and aerobic, respectively. In the by-pass of the bioreactor a part of the return sludge was stripped in a stripper tank. The process of P-release was accelerated by dosage of acetate. The Modified Renphosystem is characterized by distribution of the stripped sludge to the first aerobic zone and the anoxic zone. In the aerobic nitrification and P-accumulation take place. In the anoxic zone denitrification and P-accumulation run simultaneously. The experiments carried out at low F/M-ratios resulted in very low concentrations of P and N in the effluent. For achieving a high degree of nitrate removal the first part of the mainstream of the Modified Renphosystem was retrofitted to a AAO-system followed by an anoxic and aerobic zone respectively. This process configuration resulted in Pt and Nt of the unfiltered effluent of 0.4 and 2 mg/l respectively.