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Abstract

Nitrous oxide is a greenhouse gas with a global warming potential 310 times that of the contribution of CO₂. It is now recognized that the potential exists for N₂0 emission to be significant from some biological nutrient removal (BNR) processes. The control of N₂0 off-gas must therefore be considered when operating a BNR system. The main objectives of this research were to investigate the mechanisms involved in the production and reduction of nitrous oxide in biological wastewater treatment systems, and to use N₂0 off-gas as a real-time control parameter to assess ammonia oxidation in a simultaneous nitrification, denitrification and phosphorus removal (SNDPR) system. Strategies to diminish the emission of nitrous oxide from the treatment process were also studied. The data support the conclusion that a hybrid system, including suspended sludge and biofilm in the same reactor of a sequencing batch reactor (SBR) was a more effective system than a conventional suspended growth system, in terms of overall effluent quality, SND efficiency and lower emission of N₂0. In the hybrid system, nitrification occurred mostly in the suspended sludge; the biofilm played the major role in denitrification. It was also determined that N₂0 off-gas from the hybrid system was mainly a result of heterotrophic denitrification, rather than nitrification. N₂0 reduction rates were found to be higher with the existence of an external carbon source and the absence of DO. It was also observed that N₂0 reduction rates were higher for acetate, than for lactose, in the presence and absence of DO. Denitrification using stored carbon resulted in the production of more N₂0 off-gas than denitrification using an external carbon source. The largest production of N₂0 off-gas occurred when the internal carbon source was PHA during aerobic conditions. Based on the results of the investigation into the factors affecting N₂0 emissions, operating strategies for N₂0 off-gas control were suggested. These strategies were the use of lower aeration rates, continuous feeding and higher pH. The emission of N₂0 was found to have a close correlation with ammonia removal, which can be a tool for real-time assessment of ammonia oxidation. Therefore, it can be considered as a potential real-time control parameter for ammonia oxidation in a SNDPR hybrid SBR system.