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 oxicianoxicioxic 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. (C) 2016 Elsevier B.V. All rights reserved.