In vermifiltration system, the earthworms promote organic matter degradation of sewage sludge (e.g. the increase in VSS reduction) mostly through two strategies, which are earthworm feeding and earthworm microorganism interaction [2]. Earthworm feeding improves the VF treatment performance through the digestion
action of earthworm, while earthworm–microorganism interaction promotes the organic matter degradation through improving the microbial activity and composition. In the present study, stable isotopic abundance was firstly used to estimate the amount of earthworm cast, and then to determine the contribution of earthworm feeding to the enhanced VSS reduction.