Abstract

Effective nitrification of ammonia at high nitrogen loading rates, up to 7.5 kg NNH+4 m−3 d−1 was obtained in a nitrifying activated sludge unit working at a hydraulic retention time of 1.3 h and 20°C. Bicarbonate, used as a carbon source for autotrophic microorganisms, allowed maintainance of a system very stable against different shocks due to point deficiencies of oxygen or other accidents. Ammonia conversion to nitrate was normally between 97 and 99.9%. Temporary accumulations of nitrite were quickly transformed into nitrate. The relative low specific conversion rate of the biomass in the reactor (0.5–0.7 g-NNH+4 g-VSS−1 d−1) was compensated by the capacity of the unit to retain high concentrations of biomass (up to 15 g-VSS l−1) due to the excellent characteristics of the biomass (Sludge Volume Index of 12 ml g-VSS−1; Zone Settling Velocity of 9 m h−1). Present results show that activated sludge units can be operated efficiently as a high-rate nitrifying technology. This well-known, robust and easy to operate technology is a clear alternative to the airlift or fluidised-bed systems, which can be applied for the nitrification of high load wastewater at full scale.