Bioremediation and nutrients removal from wastewater by algal-bacterial granular sludge under two Cr(VI) exposure patterns

Abstract

Algal–bacterial granular sludge (ABGS) is promising for simultaneous heavy metal bioremediation and nutrients removal. However, its long-term stability and adaptive behavior under chronic low-level heavy metal stress and different exposure patterns remain poorly understood. In this study, the performance and stability of ABGS were systematically compared during 95-day intermittent and continuous exposure to 2 mg/L Cr(VI)-containing wastewater. Both exposure patterns resulted in decreased granular sludge settleability and chlorophyll a content, while granular structural integrity was well maintained, with integrity coefficient reaching a final value of 1.4 %. Nutrient removal was initially inhibited but rapidly recovered to approximately 80 % total phosphorus and 70 % total nitrogen by the end of test period. Notably, intermittent Cr(VI) exposure was more conducive to the recovery of microbial activity than continuous exposure, indicating a stronger adaptive resilience of ABGS under fluctuating stress conditions. The two exposure patterns showed only minor differences in Cr(VI) removal, and both exhibited enhanced extracellular polymeric substances secretion and Ca/Fe accumulation within granules, facilitating Cr immobilization. These findings highlight the long-term resilience and adaptive responses of ABGS under chronic low-level Cr(VI) stress in addition to the critical role of exposure patterns in shaping system stability and microbial activity. This study provides new insights into optimization of biological treatment strategies for long-term treatment of heavy metal-containing wastewater in practice.