Population Dynamics, Biokinetics and Gaseous Nitrogen Production from Partial Nitrification Reactors Operated Under Oxygen Limited Conditions

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Biological nitrogen removal based on partial nitrification and denitrification via nitrite is a cost-effective but operationally complex alternate to conventional nitrification and denitrification via nitrate. We achieved stable long term (t=232d) partial nitrification characterized by near complete ammonia removal (82±18%, n=97) and nitrite accumulation (80±18%) with only pH (7.5 ± 0.1) and solids retention time (3.0d) control. During reactor operation, specific ammonia oxidation activity measured as specific oxygen uptake rate (sOUR) was 4-30 times that for nitrite oxidation, which in turn was close to that for endogenous metabolism. Based on a 16S rDNA clone library, a majority of the reactor populations were closely related to Nitrosomonas europaea. No clones related to nitrite oxidizing bacteria were detected. In concurrence, based on quantitative polymerase chain reaction (qPCR), ammonia oxidizing bacteria (AOB) were the predominant microbial population fraction in the reactor, varying in the range 0.67 ± 0.49 over the period of reactor operation. Using qPCR based direct measures of AOB concentrations in conjunction with sOUR measures, the maximum specific growth rate (μmax) of AOB in the partial nitrification reactor was 0.52 ± 0.45 d-1. Notably, both variations in AOB concentrations as well as their specific activities contributed to observed variations in overall partial nitrification dynamics.  Significant NO production as much as 1.4 ppm was observed. Reactor nitrite concentrations were the strongest positive trigger for biological NO generation. In contrast, reactor operating DO concentrations were negatively correlated with NO generation. Abiotic processes did not contribute significantly to NO production.

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