In industrial wastewater, especially from food industry, the concentrations of the organic compounds are usually high, whereas the contents of nitrogen and phosphorus are often low. For the aerobic treatment, the addition of nutrients to the industrial wastewater can be required. For ecological and economic reasons, this nutrient addition must be kept to a minimum. Unintentional nitrification and denitrification lead to an additional demand of nitrogen and should therefore be avoided at such plants. Observations from two treatment plants (50 000 m3/d, 40 t COD/d) proved that the nitrogen dosage can be controlled by monitoring the ammonia concentration. If the control procedure also considers the N/COD ratio in the raw wastewater (including the N dosage) and the organic sludge load of the last couple of days, very low effluent concentrations (NH4–N in the range of 0.3–0.5 mg/l) can be achieved and the nitrogen dosage is low. If there are periods with nitrogen in excess, too, a minimum nitrification capacity has to be maintained by means of nitrogen addition in periods of deficiency. A control procedure for phosphorus addition is to keep a fixed P/COD-ratio in the raw wastewater (including P dosage). The PO4–P concentration is monitored in order to limit the maximum phosphorus dosage. Following this procedure, considerable savings of phosphorus have been achieved, keeping very low effluent concentrations (average Total-P<0.3 mg/l).

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