Enhanced removal of biolabile oxygen-depleted dissolved organic matter by coagulation-adsorption process Improves biological stability of reclaimed water

Ensuring biological stability, signifying the maintenance of an unchanged bacterial concentration and composition during water distribution, is essential for mitigating the microbial hazards of reclaimed water. However, the interplay between chlorine-resistant bacteria (CRB) regrowth and molecular transformation of biodegradable organic matter in chlorinated reclaimed water distribution system remain unclear. This work investigated the transformation of dissolved organic matter (DOM) in reclaimed water treated by coagulation and by coagulation-adsorption over a 20-day incubation following chlorination, as well as its interactions with CRB through high-resolution mass spectrometer and high-throughput sequencing. The DOM biotransformation profile and DOM-bacteria interaction network collectively revealed the overall substrate preference and metabolic pattern of CRB (e.g., Methylobacterium-Methylorubrum, Acidovorax, and Sphingomonas), which evinced a propensity for utilizing oxygen-depleted DOM (O/C < 0.5) and producing oxygen-enriched DOM (O/C > 0.4). The incorporation of powdered activated carbon during coagulation markedly decreased the level of biodegradable DOM (4.33 mg C/L) and subsequently retarded the regrowth of CRB by one day compared to coagulation alone, attributable to the selective adsorption of DOM molecules with low O/C onto the activated carbon. This work underscores the critical role of the enhanced removal of oxygen-depleted DOM in ameliorating the biological stability of waters.