Investigation of nitrite build-up within an enriched nitrification process

Lei Yang, Purdue University

Abstract

Nitrite build-up is usually observed in biological wastewater treatment systems under unsuitable environmental conditions and process operations, including ammonia shock loading, high pH, low temperature, low D.O. level, and short SRT. Discharging nitrite into receiving waters can cause several problems, such as nitrogenous oxygen demand (NOD) and nitrite toxicity to aquatic organisms. However, nitrite might also be considered as the preferred intermediate in a nitrification-denitrification sequence, in order to streamline the involved reactions. In this research, an enriched nitrifying culture was used to investigate the factors causing nitrite build-up in batch systems. Three environmental parameters were controlled in the systems: pH, D.O. level, and the initially applied total ammonia concentration. Ten different conditions, composed by variation of these three parameters, were studied in this research. At pH 7.5, nitrite build-up was observed at a free ammonia (FA) level as low as 0.92 mg N/L in a batch nitrifying system with a low D.O. level; however, no nitrite build-up was found even at a FA level up to 7.2 mg N/L in the same system but with a high D.O. level. The FA parameter, therefore,did not appear to be the sole factor causing nitrite build-up. For both low and high D.O. levels, nitrite build-up was found with pH 8.0 and 8.5 while no nitrite accumulated at the pH level of 7.0. Therefore, D.O. level was also not the sole factor that caused nitrite build-up. Hydroxylamine, which was found to accumulate in both low and high D.O. studies during this research, appeared to exhibit an inhibitory effect on nitritification. Hence, nitrite build-up in these systems was also believed to be due to hydroxylamine accumulation. Since the degree of nitrite build-up was proportional to the pH, free hydroxylamine (FH), not total hydroxylamine, appeared to be the cause of nitrite build-up. Total nitrogen sag curves were observed in this research. This so-called "sag" behavior is visually similar to that associated with oxygen depletion in a stream or river (i.e., D.O. sag). In this case, though, the sag curves depicted a temporary loss of total nitrogen concentration versus time, but their cause remains unresolved.

Degree

Ph.D.

Advisors

Alleman, Purdue University.

Subject Area

Civil engineering|Environmental science|Microbiology|Sanitation

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