Effect of side-stream conditions on the release of dissolved organic matter in 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 VSS). Intermediate side-stream sludge retention time (SRTSS) of ~72 h minimized DOM by efficient utilization of labile PN and PS, while prolonged SRTSS enhanced HS accumulation, humification, and refractory organics. Denitrification remained stable at SRTSS 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 SRTSS increased refractory and aromatic DOM without impairing nitrogen removal, though potential risks for downstream treatment and water quality warrant further investigation.