Mechanism of activated carbon enhanced activated sludge (ACEAS) in treating recalcitrant chemical wastewater

Recalcitrant chemical accident wastewater, especially nitrobenzene-containing wastewater, has posed a significant treatment challenge due to complex hazardous compounds as well as elevated toxicity. Activated carbon combined activated sludge processes can effectively remove hazardous organic pollutants from chemical wastewater. However, the interaction mechanism between activated carbon and activated sludge remains unclear. This study proposed the activated carbon enhanced activated sludge (ACEAS) process for treating nitrobenzene-containing wastewater. Combined with material characterization and metagenomic analysis, the removal efficiency of nitrobenzene was evaluated, and the interaction mechanisms between activated carbon and activated sludge was further investigated. The key findings include: The effluent nitrobenzene concentration in conventional activated sludge (AS) process was 6.5 and 9.2 times higher than in the original ACEAS (OS) and regenerated ACEAS (RS) processes, respectively. Without activated carbon replenishment, chemical oxygen demand (COD) removal efficiency in OS and RS processes increased by 10.19%-15.86% and 11.41%-14.60%, respectively, compared to AS process during long-term operation (6-24 h). Due to the formation of biofilms on the surface of activated carbon, and the content of C-O/C=O and C-N/C=N on OS increased by 13.2% and 17.3%, respectively, compared to original activated carbon (OC). Eventually, four enhanced mechanisms of activated carbon were proposed, each contributing to distinct degradation stages in the ACEAS system. In prophase, activated carbon might reduce toxicity and improve microbial degradation capacity by adsorption. During metaphase, biofilms on activated carbon surface further diminished adsorption/desorption effect. In the telophase, microbial carrier's fixation affected strengthens, reshaping microbial community structure, functional gene expression, and metabolic pathway selection, thereby enhancing activated sludge degradation efficiency.