Nitrite build-up within biological wastewater treatment systems has generally been attributed to unsuitable environmental and process conditions. In this study, an enriched nitrifying culture was used to investigate the cause of nitrite build-up by controlling dissolved oxygen (DO) and total ammonia concentrations as well as pH level in a batch system. At pH 7.5, nitrite build-up was observed with a low DO level (0.5 mg/L), while no nitrite accumulation was found in the system with a high DO level (6.0 mg/L). However, nitrite peaks were found in both the high and low DO systems at pH values of 8.0 and 8.5. Conversely, at pH 7.0, no nitrite accumulated in either such system. Hence, DO level did not appear to be the dominant factor behind a nitrite build-up, and the correlation with free ammonia (FA) was erratic. On the other hand, hydroxylamine, an intermediate in nitritification by Nitrosomkonas, has been found to inhibit nitratification by Nitrobacter. Indeed, based strictly on the mechanistic pathways of nitritification, hydroxylamine appeared likely to accumulate in a nitrifying system subjected to deficient oxygen. The results of this research showed that variable levels of hydroxylamine would accumulate in a batch system under all conditions. Relative to nitrite build-up, the presence of unionized or so called free hydroxylamine (FH) fraction (as opposed to ‘total') appeared to have a consistent correlation with incomplete nitrification. In turns, FH was believed to be a major, if not principal, factor behind nitrite build-up in these batch nitrifying systems.

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