A novel method to determine maximal nitrification rates by sewage sludge at a non-inhibitory nitrite concentration applied to determine maximal rates as a function of the nitrogen load

A new method was developed to determine the maximal nitrification capacity of sewage sludge. Samples of sludge were taken from two domestic waste water treatment plants and were chemolithotrophically grown in recycling reactors. The experimental duration was typically in the order of hours. The maximal nitrification rates were estimated from the concentrations of nitrogen compounds in effluents. These rates could be easily determined by first order kinetics, since substrates were supplied in excess and growth was generally negligible. However, accumulation of nitrite affected the ammonia oxidation rates. At 3–5 mM nitrite these rates were halved as compared to 0–1 mM. Since the accumulated nitrite is continuously diluted without loss of sludge, recycling reactors are especially suitable to study nitrification. Subsequently, this method was applied to investigate the development of the maximal nitrification capacity as a function of the nitrogen load. For this purpose, sludge was subcultured with synthetic waste water in recycling reactors. Since heterotrophic ammonium assimilation was negligible, the final capacities were proportional to the nitrogen load. However, these capacities were lower than expected. Since the numbers of protozoa had increased, predation might have reduced the nitrifying population