Identification of key toxic substances in pyrolysis wastewater and proposal of improvement strategies for pretreatment

A comprehensive assessment of the biotoxicity changes of coal pyrolysis wastewater (CPW) during the treatment process is particularly important to alleviate the predicament of unstable biochemical treatment effects. However, in-depth research on reducing the biotoxicity of CPW is still limited. This study conducted an analysis of the composition of CPW, studied the corresponding relationship between pollutants and biotoxicity changes, clarified the key toxic substances (KTSs). And the effects of four traditional extraction systems and wet air oxidation (WAO) on reducing biotoxicity, chemical oxygen demand, total phenols and specific KTSs were evaluated and optimization strategies were proposed. The results showed that phenols contributed the most to biotoxicity. The biotoxicity reduction rates of each extraction system were as follows: butyl acetate (96.6 %) > methyl isobutyl ketone (95.1 %) > isopropyl ether (92.4 %) > trioctylamine (79.5 %). When using trioctylamine extraction, the phenol removal efficiency was the highest. However, due to the strong selectivity, it cannot remove nonphenolic pollutants in a broad-spectrum manner, which was not conducive to reducing overall biotoxicity. The removal rate of nitrogenous heterocyclic compounds (NHCs) was not ideal during extraction and biochemical treatment, and there was still a significant presence in the biochemical effluent. But after WAO treatment, NHCs can be completely removed, and the ratio of ammonia nitrogen to total nitrogen can be increased from less than 25 % to approximately 85 %. After WAO, the B/C ratio of the effluent increased from 0.24 to above 0.34 and the biotoxicity was significantly reduced. Although the biotoxicity was effectively reduced in the enhanced treatment process, the effluent still performed potential biotoxicity, which need to be further explored in order to reduce environmental risk.