Experiments were carried out to establish whether nitrous oxide (N2O) could be used as a non-invasive early warning indicator for nitrification failure. Eight experiments were undertaken; duplicate shocks DO depletion, influent ammonia increases, allylthiourea (ATU) shocks and sodium azide (NaN3) shocks were conducted on a pilot-scale activated sludge plant which consisted of a 315 L completely mixed aeration tank and 100 L clarifier. The process performed well during pre-shock stable operation; ammonia removals were up to 97.8% and N2O emissions were of low variability (<0.5 ppm). However, toxic shock loads produced an N2O response of a rise in off-gas concentrations ranging from 16.5 to 186.3 ppm, followed by a lag-time ranging from 3 to 5 h ((0.43–0.71) × HRT) of increased NH3-N and/or NO2− in the effluent ranging from 3.4 to 41.2 mg L−1. It is this lag-time that provides the early warning for process failure, thus mitigating action can be taken to avoid nitrogen contamination of receivin
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