Achieving nitrogen removal via nitrite in a pilot-scale continuous pre-denitrification plant.

Nitrogen removal via nitrite (the nitrite pathway) is beneficial for carbon-limited biological wastewater treatment plants. However, partial nitrification to nitrite has proven difficult in continuous processes treating domestic wastewater. The nitrite pathway is achieved in this study in a pilot-scale continuous pre-denitrification plant (V=300 L) treating domestic wastewater by controlling the dissolved oxygen (DO) concentration at 0.4-0.7 mg/L. It is demonstrated that the nitrite pathway could be repeatedly and reliably achieved, with over 95% of the oxidized nitrogen compounds at the end of the aerobic zone being nitrite. The nitrite pathway improved the total nitrogen (TN) removal by about 20% in comparison to the nitrate pathway, and also reduced aeration costs by 24%. FISH analysis showed that the nitrite oxidizing bacteria (NOB) population gradually reduced at low DO levels, and reached negligible levels when stable nitrite pathway was established. It is hypothesized that NOB was washed out due to its relatively lower affinity with oxygen. A lag phase was observed in the establishment of the nitrite pathway. Several sludge ages were required for the onset of the nitrite pathway after the application of low DO levels. However, nitrite accumulation increased rapidly after that. A similar lag phase was observed for the upset of the nitrite pathway when a DO concentration of 2-3 mg/L was applied. The nitrite pathway negatively impacted on the sludge settleability. A strong correlation between the sludge volume index and the degree of nitrite accumulation was observed.