Degradation sequence, multi-phase distribution, and destabilization mechanisms in anaerobic treatment of real coal chemical wastewater.

Coal chemical wastewater (CCW) is a toxic refractory industrial effluent with a complex composition, for which anaerobic digestion offers a promising treatment approach. This study established an upflow anaerobic sludge bed reactor to treat CCW, focusing on the degradation sequence of organic pollutants, their distribution characteristics, and the system destabilization mechanisms. During stable operation, the average chemical oxygen demand removal efficiency reached 81.8%, with most biochemical reactions occurring within 0-8 h. Gas chromatography showed that the removal rates of benzene derivatives, phenolic compounds, polycyclic aromatic hydrocarbons (PAHs), long-chain alkanes, and nitrogen-containing heterocyclic compounds (NCHs) all exceeded 80%. Benzene derivatives and simple phenolics showed low residual levels, whereas PAHs and NCHs underwent intracellular accumulation. Under high-strength CCW stress, extracellular polymeric substances (EPS) increased to 116.2 mg/g-MLSS (proteins) and 9.5 mg/g-MLSS (polysaccharides), while redox-active components decreased. The intracellular accumulation of PAHs and long-chain alkanes reached 5.6 mg/L and 27.5 mg/L, respectively. The secondary structure of proteins indicated that the α-helix/(β-sheet + random coil) ratio in tightly-bound EPS decreased to 36.24%, accompanied by reduced intermolecular hydrogen bonding (37.7%) in EPS, indicating a looser structure. Microbial community composition and metabolic gene expression analyses revealed that system instability was caused by a combination of microbial richness and diversity decline, and downregulation of genes related to basal metabolism and organic matter degradation. The system exhibited a metabolic trade-off strategy that prioritized organic degradation over fundamental metabolic functions. This study provides new insights into the degradation mechanisms and stability regulation of anaerobic systems treating CCW.