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Partial nitrification to nitrite (nitritation) can be achieved in a continuous process without sludge retention by wash out of nitrite oxidising bacteria (NOB) while retaining ammonia oxidising bacteria (AOB), at elevated temperatures (the SHARON process) and, as demonstrated in this paper, also at low dissolved oxygen (DO) concentrations. Enriched AOB was attained at a low DO concentration (0.4 mg l(-1)) and a dilution rate of 0.42 day(-1) in a continuous process. A higher oxygen affinity of AOB compared to NOB seemed critical to achieving this. This was verified by determining the oxygen half saturation constant, Ko, with similar oxygen mass transfer resistances for enriched AOB and NOB as 0.033+/-0.003 mg l(-1) and 0.43+/-0.08 mg l(-1), respectively. However, the extent of nitritation attained was found to be highly sensitive to process upsets.

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Nitrifier growth parameters-the maximum growth rate (l Amax ) and yield (Y A )-were estimated by measuring the rate of carbon dioxide uptake and additional rates of oxygen uptake and ammonia (or nitrite) use. Batch tests in a combined titrimetric and offgas analyzer with enriched Nitrobacter and Nitrosomonas cultures and an activated sludge sample were performed. The measured l Amax values for the Nitrobacter and Nitrosomonas cultures were 0.67 6 0.03 day 21 and 0.54 6 0.09 day 21 , while the Y A values were 0.072 6 0.01 g volatile suspended solids (VSS) Á g nitrogen (N) 21 and 0.14 6 0.02 gVSS Á gN 21 , respectively. For the activated sludge sample, l Amax was observed to increase with pH (l Amax 5 0.72 Á 3.3 (pH-7.1) ) over the range 6.8 to 7.1. All l Amax and Y A values determined by this method were similar to those previously reported. Compared with other l Amax and Y A estimation methods, this method allows for unique l Amax and Y A estimations for given conditions from a single experiment. Water Environ. Res., 79, 2437Res., 79, (2007.