Compound-specific effects of micropollutant-loaded gasification char on anaerobic digestion: implications for closed-loop wastewater treatment.

Bio-based gasification char (BGC) is being considered for use in the fourth stage of wastewater treatment aiming on organic micropollutants (OMPs) removal. However, when OMP-loaded BGC enters anaerobic digestion via sludge, the adsorbed compounds may inhibit microbial-in general and methanogenic activity in particular. This study assessed the impact of OMP-loaded BGC on the performance of anaerobic batch digestion under mesophilic conditions. Six compounds (benzotriazole, sulfamethoxazole, valsartan, metoprolol, carbamazepine and diclofenac) were tested in reactors containing OMP-loaded BGC, OMPs only and two reference conditions over 40 days. No significant differences in final cumulative methane yield were detected between the BGC and non-BGC conditions for any compound. Modified Gompertz modelling revealed compound-specific kinetic effects: sulfamethoxazole without BGC caused complete process inhibition until day 14, whereas valsartan with BGC exhibited reduced methane production. BGC was found to reduce acidification of the reactor across all compounds. The reduction in OMP concentration was found to be significantly higher in the BGC group compared to non-BGC conditions, where only SMX was fully removed, with recalcitrant compounds reaching only 14 - 36 %. This difference was particularly evident for recalcitrant compound carbamazepine, suggesting that strong adsorption might be the predominant removal pathway. These findings support the closed-loop BGC system, while also highlighting the need to consider the kinetic risks of individual compounds and the fate of adsorbed OMPs during digestion.