Co-treatment of reject water from sludge dewatering and supernatant from sludge lime stabilization process for nutrient removal: A cost-effective approach

[Display omitted] •Sludge lime stabilization supernatant (SLSS) was reused to treat reject water.•High content of calcium and alkalinity in SLSS favors P removal from reject water.•SLSS supplemented alkalinity and carbon for biological N removal from reject water.•SLSS raised sludge flocs, and mitigated fouling of MBR for reject water treatment.•Adding SLSS increased microbial abundance and biodiversity, and enriched nitrifiers. A cost-effective method for reject water treatment by reusing supernatant from sludge lime stabilization (SLS) process was investigated to achieve simultaneous COD, nitrogen and phosphorus removal. SLS supernatant was characterized as an alkaline wastewater containing high concentrations of calcium ion, alkalinity, biodegradable COD and ammonium nitrogen. Dosing SLS supernatant into a membrane bioreactor for reject water treatment enhanced COD and phosphorus reduction by 105.1% and 184.6%, increased pH of mixed liquor, formed large sludge flocs and mitigated membrane fouling. Illumina-MiSeq sequencing showed that SLS supernatant addition also increased microbial abundance and biodiversity, enriched nitrifying biomass, and converted predominant ammonia oxidizing genera from Nitrosospira to Nitrosomonadaceae_uncultured and Nitrosomonas. The combined treatment of reject water using a proposed step-feeding oxic/anoxic/oxic process can utilize biodegradable COD in SLS supernatant for denitrification, fully compensate alkalinity for nitrification, and save costs of phosphorus precipitants, external carbon and alkalinity. The results suggest that co-treatment of reject water and SLS for nutrient removal was technically and economically effective.