Production of potato starch produces wastewaters with high concentrations of COD, nitrogen and phosphorus. The best known and most economical solution for the nearly full elimination of these substances is the two-stage anaerobic-aerobic treatment. The anaerobic pretreatment must only go so far as to maintain enough COD for biological nitrogen and phosphorus elimination in the aerobic stage. To optimize this process, several tests have been carried out on nitrification/denitrification by means of the intermediate product nitrite. The carbon consumption amounts to only 60% in comparison with denitrification via nitrate. The essential parameter for regulating the process is the concentration of free ammonia in the reactor. Concentrations of 1 to 5mg NH3/l inhibit the nitratation but not the nitritation. A separation of both partial steps is possible. The content of ammonia was controlled by means of continuous NH4 and pH measuring.

The process is suitable for many wastewaters with low COD/TKN ratios and high nitrogen concentrations. If there are undesired nitrite peaks in an aerobic treatment plant caused by high pH values and temperatures, specific nitrogen elimination by means of nitrite is a reliable treatment system.

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