Self-Sustained Nitrite Accumulation at Low pH Greatly Enhances Volatile Solids Destruction and Nitrogen Removal in Aerobic Sludge Digestion

Abstract

Aerobic sludge digestion of waste activated sludge (WAS) is widely used as a stabilization option in small-and midsized wastewater treatment plants. However, the digestion process is often limited by low volatile solids (VS) destruction and poor pathogen removal efficiency. This study presents a novel operational strategy that achieves enhanced VS destruction and nitrogen removal by inducing sustained nitrite accumulation via a single spike of nitrite to aerobic digester operated at a natively low pH (<5.5). The strategy was demonstrated through the use of three laboratory aerobic sludge digesters, each continuously operated for over 300 days. Compared to control reactors, the strategy enhanced volatile solids destruction by 35.0-38.4%, nitrogen removal by 58.5-70.8%, and pathogen reduction by approximately 1 log. The standard oxygen uptake rate (SOUR) was reduced to 0.49 ± 0.03 mgO <inf>2</inf> /gVS/h, compared to 0.85 ± 0.01-1.68 ± 0.02 mgO <inf>2</inf> /gVS/h in the control, indicating enhanced stabilization. Free nitrous acid formed from nitrite at low pH, rather than nitrite itself, was identified to be the cause of improved digestion performance. Since the nitrite production is self-supporting, no additional ongoing costs are incurred.