Distinct granulation pathways of aerobic granular sludge under poly aluminum chloride enhancement

Aerobic granular sludge (AGS), a novel strategy for nutrient removal which exhibits compact structure, good settleability, and resilience against high organic load, has been considered as a highly potential wastewater treatment technology. However, the long start-up period for granulation prevented its widespread development. In this study, the distinct pathways of PAC-enhanced AGS granulation were systematically investigated. Four identical sequencing batch reactors (SBR) with different PAC dosages (with 0, 50, 100, 400 mg/L effective Al³⁺ respectively) were applied. It was observed that the presence of PAC accelerated granules formation, promoted mechanical strength as well as denitrification rate of granules, and thus notably enhanced removal efficacies of COD, NH₄⁺-N, NO₂^- and NO₃^-. According to the dissolved oxygen (DO) distribution inside the sludge and the denitrification rate (SDNR) measurements, distinguishing structures of granules under different PAC addition were discovered. Comparatively, AGS under low PAC addition (i.e., 50 mg/L) resulted in the largest granule size, the biggest anaerobic zone and the highest denitrification rate. Presumably, for the system with the low PAC addition (50 mg/L), appropriate aluminum ions (Al³⁺) neutralized part of the negative charge on the microorganism surface, thereby promoting cells aggregation. In contrast, a high dosage of PAC (400 mg/L) induced excessive Al³⁺ absorbed on the cell surface after neutralization, which increased the repulsive force between microorganisms, leading to more cavities and channels existed inside the granules. Therefore, granules under low PAC dosage (i.e., 50 mg/L) presented large anaerobic zone and high denitrification rate, thus favored the best internal structure and nutrients removal efficiencies.