Simultaneous nitrogen and phosphorus removal from domestic wastewater in an aerobic granulation system operated at different anaerobic-aerobic durations

The present work investigated the effects of anaerobic-aerobic durations on simultaneous nitrogen and phosphorus removal for the treatment of domestic wastewater in a sequential batch reactor (SBR). Three different anaerobic-aerobic durations, namely M1 (60 min anaerobic and 105 min aerobic), M2 (90 min anaerobic and 75 min aerobic) and M3 (15 min anaerobic and 150 min aerobic) were examined. In this study, M1 with an anaerobic : aerobic ratio of 0.6 : 1 appeared to be the most optimal anaerobic-aerobic duration to ensure the highest simultaneous nitrification and denitrification (SND) and enhanced biological phosphorus removal (EBPR) efficiency of 53.7 +/- 4.6% and 83.4 +/- 2.3%, respectively. Unsatisfactory SND was observed throughout the study. The poor SND efficiency in this study was mainly caused by ineffective nitrification. The nitrification was adversely affected by the high average chemical oxygen demand/nitrogen (COD/N) ratio of 14 in the influent, and the disproportion in the ratio of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in the aerobic granules. The shortening of aerobic duration in M2 caused the lowest SND and EBPR efficiency of 26.1 +/- 0.5% and 34.8 +/- 3.7%, respectively. The nitrification and the uptake of orthophosphate by polyphosphate-accumulating organisms (PAOs) during the aerobic reaction were hampered due to the presence of high free nitrous acid (FNA) concentration. Meanwhile, prolonging the aerobic duration in M3 reduced the denitrification efficiency, possibly due to the formation of smaller anaerobic/anoxic zones within the aerobic granules and insufficient carbon source availability, thus contributing to an SND efficiency of 64.2 +/- 3.7%. Finally, this paper demonstrates the importance of maintaining a proper anaerobic : aerobic ratio in governing the formation of aerobic granules with excellent properties, nutrient-removal abilities and healthy microbial dynamics.