Functional identification behind gravity-separated sludge in high concentration organic coking wastewater: Microbial aggregation, apoptosis-like decay and community

Functional identification and elimination of activity-decayed sludge are helpful for improving the performance of biological treatment process. However, cell decay-associated changes in biological functions have not been explored for gravity-separated sludge. In this work, sludge flocs from the aerobic basin of a wastewater treatment plant treating high-concentration organic coking wastewater was fractionated according to settling velocity, i.e. sludge F (fast settling), sludge M (moderate settling) and sludge S (slow settling). Sludge volume index (SVI), mean floc size, dehydrogenase activity, specific oxygen uptake rate (SOUR), extracellular polymeric substances (EPS) content and aggregation interaction were investigated in the fractionated sludges. Apoptosis-like decayed cell distribution (ALDCD), a novel property of sludge, was proposed to describe sludge decay based on cell membrane variation. ALDCD of sludge F was 6.64% and 13.5% lower than sludge M and S, respectively. Microbial community and functional prediction revealed that sludge F exhibited the highest microbial potential for organic removal and sludge M had the highest potential for nitrogen metabolism while sludge S had the lowest potential for both. Our analysis suggests that the treatment efficiency might be enhanced by retaining compact sludge flocs while eliminating dispersive sludge flocs. This study also facilitates the identification and elimination of functional microbial groups from decayed sludge in wastewater treatment. [Display omitted] •Aerobic floccular sludge was gravity-separated into different fractions.•The light fraction showed the highest SVI and the lowest aggregation ability.•Apoptosis-like decayed cell distribution was proposed to evaluate sludge decay.•The apoptosis-like decay was associated with variations in microbial functions.•Slow-settling flocs have the lowest potential in COD and nitrogen removal.