Effect of Step Aeration on a Municipal Sewage Aerobic Granular Sludge System

In order to achieve simultaneous nitrogen and phosphorus removal in low-C/N urban sewage, a sequencing batch reactor (SBR) was operated in anaerobic/aerobic (A/O) mode. Keeping the total aeration volume at 900 L, the aeration strategy was adjusted. The uniform aeration of 2.81 L.(h.L)(-1) was changed to "high/low aeration" with high strength 4.22 L.(h.L)(-1)before low strength 1.88 L.(h.L)(-1), and "low/high aeration" with low strength 1.88 L.(h.L)(-1) before high strength 4.22 L.(h.L)(-1). The experiment investigated the nitrogen and phosphorus removal performance and sludge characteristics of the system under different aeration strategies. The results showed that the nitrogen and phosphorus removal performances of the system under high/low aeration were the best. The effluent concentrations of NH4(+)-N, NO2(-)-N, NO3(-)-N, and TP were 0, 0.15, 8.12, and 0.04 mg.L(-1), respectively. The removal rates of total nitrogen (TN) and total phosphorus (TP) were as high as 78.33% and 99.19%, respectively. Simultaneous nitrification endogenous denitrification (SNED) was clear, with the SNED ratio at 77.08%. Compared with uniform aeration, the system nitrification rate and denitrification rate increased, and the denitrification rate reached 14.33 mg.(g.h)(-1), which was the maximum value during the whole operation; the solidity, sedimentation performance, and stability of granular sludge were improved, and the sludge volume index (SVI) was 23.49 mL.g(-1). After adjusting the aeration strategy to low/high aeration, the nitrogen and phosphorus removal performance of the system deteriorated, and the removal rates of TN and TP were reduced to 51.26% and 58.32%, respectively. However, the system had the best nitrification performance with ammonia oxidation rate and nitrate production rate at 14.92 mg.(g.h)(-1)and 7.50 mg.(g.h)(-1), respectively, which were their maximum values during the whole operation. Simultaneously, the filamentous bacteria in the granular sludge multiplied, the granular structure became loose, the sedimentation and stability all worsened, and the SVI rose to 40.76 mL.g(-1).