Co-occurrence of endogenous denitrification and bio-induced phosphorus precipitation in aerobic granular sludge for treating low chemical oxygen demand/nitrogen wastewater.

This study investigated an aerobic granule sludge process with concurrent endogenous denitrification (ED) and bio-induced phosphorus precipitation (BIPP) in an anaerobic/aerobic sequencing batch reactor under elevated influent alkalinity and a sludge retention time exceeding 60 days. At an influent low chemical oxygen demand/nitrogen (COD/N) ratio of 2.9, the system achieved simultaneous total nitrogen and total phosphorus removal efficiencies of 85.2 % and 98.6 %, respectively. X-ray diffraction analysis suggested the presence of hydroxyapatite (HAP) in the mineral-rich cores of large granules (2.0-5.0 mm). pH-controlled batch assays showed greater Ca2+ depletion at elevated pH, while modeling predicted greater HAP supersaturation within the granules. Assuming that all depleted Ca2+ was incorporated into HAP, the estimated maximum contribution of BIPP to phosphorus immobilization was 63.2 % during the typical cycle on day 170. This value does not represent a directly measured BIPP contribution. Size-fractionated assays further showed that large granules had greater BIPP-associated phosphorus immobilization potential, whereas small granules exhibited stronger biological phosphorus removal characteristics. Cycle profiles and batch assays for ED also indicated increased nitrite-reduction potential during operation, consistent with improved nitrogen removal. Collectively, these findings highlight the potential complementary roles of ED and BIPP in nutrient removal under low COD/N conditions.