In most catchments trade waste forms a significant fraction of the total mass load which waste water treatment plants (WWTPs) are required to process. In large cities it is often assumed that WWTPs will not be greatly impacted by any individual trade waste discharges because of the significant dilution afforded by the comparatively large volume of influent overall. The absence of any evidence to the contrary (e.g. biological process failures) is often used to support the case for dilution based risk mitigation. In this work we report on an investigation into intermittent inhibition of a biological nitrification process at a major metropolitan WWTP in Australasia. Analysis of process records excluded operational issues as a cause of lower than expected air demand during such inhibition low air demand (low oxygen uptake rate) and peak in ammonia indicates inhibition. Reanalysis of continuous influent composition records from instrumentation at the plant inlet works allowed incidences of inhibition to be clearly linked to very short lived arrival of a specific industrial trade waste. Targeted automatic grab sampling at times the instrumentation indicated the particular trade waste was present allowed detailed analytical chemistry analysis of concentrated samples and resulted in identification of chloro- and nitro- substituted phenols as the cause of the biological inhibition.