Insights on mechanisms of excess sludge minimization in an oxic-settling-anaerobic process under different operating conditions and plant configurations

• Published in: Chemosphere (Oxford) 2023-01, Vol.312, p.137090-137090, Article 137090
• Main Authors: Corsino, Santo Fabio, Carabillò, Michele, Cosenza, Alida, De Marines, Federica, Di Trapani, Daniele, Traina, Francesco, Torregrossa, Michele, Viviani, Gaspare
• Format: Article
• Language: English
• Subjects: Activated sludge; Anaerobic side-stream reactor; Biological nutrients removal; Excess sludge minimization; Oxic-settling-anaerobic (OSA) process; Wastewater treatment
• ISSN: 0045-6535; 1879-1298
• DOI: 10.1016/j.chemosphere.2022.137090

In the present research, insights about the mechanisms of excess sludge minimization occurring in an oxic-settling-anaerobic (OSA) were provided. The investigation involved two systems operating in parallel. In particular, a conventional activated sludge (CAS) system as control and a system implementing the OSA process both having a pre-denitrification scheme were considered. Five periods (P1–P5) were studied, during which several operating conditions and configurations were tested. Specifically, the hydraulic retention time (HRT) in the anaerobic reactor of the OSA system (P1 8 h, P2–P3 12 h, P4 8 h, P5 12 h) and the return sludge from the anaerobic to the anoxic (scheme A) (P1–P2) or aerobic (scheme B) mainstream reactors (P3–P5) were investigated. The results highlighted that the excess sludge production in the OSA was lower in all the configurations (12–41%). In more detail, the observed yield (Yobs) was reduced from 0.50-0.89 gTSS gCOD−1 (control) to 0.22 -0.34 gTSS gCOD−1 in the OSA process. The highest excess sludge reduction (40%) was achieved when the OSA was operated according to scheme B and HRT of 12 h in the anaerobic reactor (P3). Generally, scheme A enabled the establishment of cell lysis and extracellular polymeric substances (EPS) destructuration, leading to a worsening of process performances when high anaerobic HRT (>8 h) was imposed. In contrast, scheme B enabled the establishment of maintenance metabolism in addition to the uncoupling metabolism, while cell lysis and EPS destruction were minimized. This allowed obtaining higher sludge reduction yield without compromising the effluent quality. [Display omitted] •An OSA process with a novel plant layout enabled excess sludge reduction up to 40%.•Cell lysis and EPS destruction were enhanced by operating under prolonged anaerobiosis.•Cell lysis and EPS hydrolysis impaired effluent quality and sludge settling features.•Maintenance and uncoupling metabolism enabled higher sludge reduction (26–40%).•Nitrifiers were affected by prolonged starvation under not aerobic conditions.