Shorter nitrification is reached through Nitrobacter bacteria activity inhibition. Nitrobacter bacteria responsible for nitrite nitrogen oxidation are sensitive to free ammonium. The presence of free ammonium depended on a high ammonia nitrogen concentration in wastewater and its pH. It was the pH of the wastewater that was the decisive parameter in Nitrobacter bacteria activity inhibition in stable temperatures and at stable ammonia nitrogen concentrations. Reaction control in the aeration phase of the treatment process guaranteed the concentration of free ammonium within the 1 to 6 mg NH3/l range and at the same time concentration of free nitrous acid did not exceed 0.04 mg HNO2/l. It allowed nitrification to be significantly shortened and to receive build-up of nitrite nitrogen up to 300 mg NO2-N/l. Nitrification rate was about 0.06 g N/g MLSS·d.

Measured oxidation rates of particular groups of nitrifying bacteria as oxygen uptake rates by Nitrosomonas and Nitrobacter bacteria respectively made control of shorter nitrification easier and gave insight into the activity of both nitrifier groups.

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