Treatment performance and mechanism of source-separated blackwater using an earthworm-based reactor for rural applications

Although earthworms are widely used in domestic sewage treatment, their pathways for pollutant transformation, evolution of microbial community structure, and functional metabolic mechanisms during blackwater treatment remain unclear. This study constructed an earthworm-based reactor (ER) for treating blackwater. Results indicated that compared with the control reactor (CR) without earthworms, the ER achieved higher removal rates for COD, NH₄⁺-N, total kjeldahl nitrogen, and TN by 5.96%, 7.92%, 11.93%, and 5.37%, respectively. The introduction of Eisenia fetida promoted the conversion of suspended COD to dissolved COD, and enhanced the ammonification of organic nitrogen and the nitrification of NH₄⁺-N. Microbial community analysis revealed significant enrichment of genera such as Ferruginibacter, Lysobacter, the JG30-KF-CM45 group, members of family Isosphaeraceae and Chitinophagaceae in ER. These genera exhibited positive correlations with COD and nitrogen removal, suggesting their potential roles in carbon and nitrogen transformation. The enrichment of autotrophic and heterotrophic nitrifying bacteria in the ER was associated with enhanced nitrogen transformation, and FAPROTAX predictions supported increased denitrification potential. Collectively, these correlative data suggest that Eisenia fetida may enhance carbohydrate and nitrogen metabolism. However, this system did not significantly improve phosphorus removal; therefore, the effluent requires appropriate treatment based on its intended application. This study provides a theoretical basis for earthworm-based reactors in treating rural blackwater.