Mechanism of toxicity formation and spatial distribution in activated sludge treating synthetic effluent containing bisphenol A (BPA)

•The mechanism of organic sludge toxicity formation in wastewater treatment was studied.•The organic sludge toxicity was directly caused by BPA biodegradation rather than adsorption.•The organic toxicity centralized in the inner section of sludge flocs.•The organic sludge toxicity fluctuated positively with original BPA concentration.•The sludge toxicity significantly affected bacteria community.

Organic sludge toxicity is a critical issue for sludge land applications. To investigate sludge organic toxicity formation and spatial distribution properties, sludge was acclimated using synthetic effluent containing various bisphenol A (BPA) concentrations in sequential aerobic activated sludge reactors (SBRs). Acute sludge toxicity, which was characterized by the inhibition rate of the luminescent bacterium Photobacterium phosphoreum, was the focus of this study. The original influent COD of the SBR was approximately 300 mg/L; the sludge retention time (SRT) and hydraulic retention time (HRT) were controlled at 20 d and 12 h, respectively. The results indicated that a positive correlation existed between sludge toxicity and the influent BPA concentration. Furthermore, the toxicity was centralized in the intracellular and inner sections of extracellular polymeric substance (EPS) in sludge flocs. BPA concentrations were controlled at 2.5 mg/L, 5.0 mg/L, 7.5 mg/L and 20 mg/L; the sludge toxicities, measured by comparing the control SBR with the blank, were 2.99%, 4.05%, 6.99% and 10.64%, respectively, at the end of 28-day operation. In the 12-h SBR treatment process, aqueous and sludge phase BPA was completely removed within 2 h. However, the maximum sludge toxicity appeared in the 8th hour. The DGGE analysis suggested that the existence of BPA in wastewater increased bacterial diversity in activated sludge. However, the bacterial similarity between sludge in SBRs decreased with increasing BPA concentration. Lastly, the bacterial similarity between blank sludge and control sludge of 20 mg/L BPA was 0.47.