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Experiments were conducted in order to establish whether N2O could be used to predict nitrification failure (through non-invasive means). Previous research had shown a strong correlation between N2O gas and NH3 in the effluent, giving rise to the possibility N2O can be used as an indicator for failure in the nitrification process. Two pilot-scale activated sludge plants were used, each with two lanes. The smaller consisted of a 601 aeration tank and a 20 1 clarifier; the larger pilot plant had an aeration tank of 315 1 and a clarifier of 1001. The small pilot plant experiments showed that N2O gas was given off almost immediately from O-2 deprivation/NH3 shock loads, but did not follow the expected trend of the time lag of NH3 in the effluent. This led to further investigation in the hydrodynamics and mixing characteristics of aeration basins, where a second larger pilot plant was used. Further experiments were conducted of high NH3 loadings and T O-2 deprivation, showed that work of was reproducible. However, it was also shown that with partial nitrification failure, a different N2O response of a continual rise was observed.