The highly concentrated wastewater of a rendering plant (7 g COD/l and 1.1 g TKN/l on average) is biologically pretreated in an activated sludge plant. Due to low loading of this plant, full nitrification and nitrogen removal are generally achieved. In periods of high COD loads, however, the nitrification capacity was substantially reduced. Control analyses revealed extreme phosphorus deficiency and that only nitrite, but no nitrate was formed. An extended model based on the ‘Activated sludge model No. 1’ has been used to investigate the kinetics of nitrification in the case of phosphorus deficiency, the demand for phosphorus under dynamic conditions, as well as the effect of variations in the nitrogen load at low levels of phosphate.

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