Continuous-flow experiments have been carried out to achieve effluents with low COD and a minimum ammonium-nitrogen concentration. The system was aerated by air injection. The air passing through bottom-resting perforated tubes by a plastic mesh. Mixtures of milk and water, pH 11 units and containing 3500 mg COD/1 (similar to dairy sewage) were fed into the reactor. The liquor temperature was near 30 degrees C. A mixed culture of isolates from dairy effluents, adapted at 30 degrees C for 19 days and mixed with sludge from a domestic wastewater plant, was used as an inoculum. The inlet flows were 8, 10, and 12.6 l/d. Experiments were continued to the pseudostationary state. The pH generally became stabilized around 8.5. With increasing flow, the average COD values and the final ammonium-nitrogen concentrations decreased until a certain flow value; at which they were almost stable. The same behaviour was seen with the concentrations of nitrate and nitrite-nitrogen, nitrification and denitrification phenomena taking place simultaneously. At the end of the experiments, the effluent was characterized by low COD and nutrients. The biomass content was 5.9 g N, 5.5 g P2O5, and 0.9 g K2O for each 100 g biomass. This could be used as a source of nutrients after mineralization.