Effect of side-stream conditions on the release of dissolved organic matter in the side-stream enhanced biological phosphorus removal process

Abstract

The side-stream enhanced biological phosphorus (P) removal (S2EBPR) process exhibits efficient and stable P removal by anaerobic fermentation of return activated sludge (RAS) in a side-stream reactor, reducing influent carbon demand while enabling microbial selection and sludge reduction. However, the characteristics and impacts of dissolved organic matter (DOM) released under extended anaerobic conditions remain unclear. In this study, an S2EBPR system was constructed to investigate the composition of mainstream and side-stream DOM and their relationships with system performance. Results showed that humic substances (HS), proteins (PN), and polysaccharides (PS) were the primary DOM components in the side-stream reactor (14.3 ± 3.0 mg g⁻¹ VSS). An intermediate side-stream sludge retention time (SRT_SS) of ∼72 h minimized DOM by efficient utilization of labile PN and PS, while prolonged SRT_SS enhanced HS accumulation, humification, and refractory organics. Notably, a significant positive correlation was identified between effluent phosphorus and total DOM concentration, governed by a threshold effect of sludge hydrolysis. While an optimal SRT_SS efficiently supplies volatile fatty acids for polyphosphate-accumulating organisms (PAOs) with minimal DOM accumulation, excessive sludge disintegration under prolonged SRT_SS conditions releases refractory intracellular organics, which increases total DOM and inhibits PAO activity. Denitrification remained stable at SRT_SS of 72–144 h, with the effluent dissolved organic nitrogen reduced to 6–13%. Fluorescence analysis indicated that DOM was dominated by fulvic acid-like and humic acid-like substances; microbial degradation of PN-like compounds promoted DOM humification. Prolonged SRT_SS increased refractory and aromatic DOM without impairing nitrogen removal, though potential risks for downstream treatment and water quality warrant further investigation.