Impact of Slowly Biodegradable COD and Loosely Bound Polymeric Substances Accumulation in High-Rate Activated Sludge: Implications for Bioflocculation and Organic Matter Harvesting

This study investigates the impact of loosely bound (LB-) and tightly bound (TB-) polymeric substances (PS) on bioflocculation and organic matter harvesting in High Rate Activated Sludge (HRAS) systems, operated with primary effluent wastewater to specifically investigate the bioflocculation process. A pilot-scale HRAS system was operated at a contrasted solids residence time (SRT) of 0.2 and 0.8 d to assess the composition of polymeric substances extracted from the sludge (LB- vs TB-contents, biopolymers fraction), bioflocculation capacity, settleability, and the fate of organic matter. Results demonstrate that a low SRT (0.2 d) favors the accumulation of influent slowly biodegradable COD (more than 60% based on COD mass balance) and of LB-PS with a limited biopolymer content (<30%). The high LB-PS content observed at 0.2 d SRT (259 ± 15 mgCOD/gVSS) in turn hinders bioflocculation, resulting in the formation of small and loose flocs and a higher loss of effluent suspended solids. Conversely, sludge grown at 0.8 d SRT exhibited a lower LB-EPS (116 ± 9 mgCOD/gVSS) content with a better bioflocculation, resulting in the formation of larger, more structured and fluffier flocs. A poor bioflocculation at low SRT hampered particulate and colloidal organic matter removal, ultimately limiting the harvesting of organic matter despite an increased redirection. Overall, our results provide relevant insights into the role of sludge composition (influent slowly biodegradable COD, LB-PS, biopolymers content) in the bioflocculation and resulting harvesting of organics in HRAS systems. Our results also suggest that operation of HRAS systems at a very low SRT (e.g., 0.2 d) has the potential to increase the harvesting and valorisation of the organic matter of municipal wastewater but requires a better control of bioflocculation.