Rate control-based strategy to enhance biological nitrogen removal during anoxic/oxic sludge digestion

Abstract

Anoxic/oxic (A/O) sludge digestion is a waste-activated sludge (WAS) treatment practice that simultaneously reduces volatile suspended solids reduction and removes nutrients from digested sludge. This study aims to optimize the performance of A/O sludge digestion by achieving the optimum alkalinity and nitrogen balance through rate control. Rate control involves oxic/anoxic cycling based on the activity of WAS (i.e. ammonification, nitrification, oxygen utilization, and denitrification rates) and controlling the processes of amonification, nitrification, oxygen ultilization, and denitrification in WAS. This strategy is different from pH, oxidation–reduction potential, dissolved oxygen controls, or fixed time control methods. Laboratory-scale batch aerobic digestion and A/O digestion tests were performed to verify the feasibility of the approach. The optimum operational cycles for the aeration and mixing were determined as 3.5 and 2.5 h, respectively (6 h full cycle). The sludge reduction in A/O digestion was greater than that in the aerobic digestion (51.83 vs. 48.13%) at a sludge retention time of 23 d. The total nitrogen reduction in A/O digestion was significantly higher than that in the aerobic digestion (65.20 vs. 12.27%). The performance of the proposed rate-control strategy for A/O digestion is superior to those of other experimental control schemes. In addition, this technique produces a sludge dewatering liquor with low nitrogen and phosphorus concentrations.