Hartenstein observed that 1 g worm could convert approximately 4 g of activated sludge in 5 days in their lab-scale (10 cm-diameter petri-dishes) experiments on vermicomposting of activated sludge [40]. According to the data, the earthworm biomass of VF was 20.5–62.5 g L−1 in the present study, and thus the amount of sewage sludge needed might be 640–1970 g SS (wet weight) d−1. However, the organic loading of the VF was
1.12 kg VSS (dry weight) m−3 d−1, and thus the influent sludge amount in the VF was 249 g SS (wet weight) d−1 in consideration of the water content and the VSS/SS ratio of sewage sludge. The data showed that the organic food from the influent sludge seemed not enough to meet all the earthworm growth in the VF.
The reduction of earthworm individual numbers as the operation time (Fig. S1 of the SD) complemented and confirmed the conclusion. Further, quantitatively analysis of earthworm effects showed that earthworm feeding played a less important role in enhanced VSS reduction than earthworm–microorganism interaction in the vermifiltration system. Thus, the above results implied that the strategy to improve VF treatment performance
should focus on earthworm–microorganism interaction, but not earthworm feeding. Thus, another possible approach was to control the earthworm biomass in the vermifiltration system through the harvest of the earthworms. The earthworm biomass decreased, the competition among earthworm individuals and the demand of organic food reduced, and then the activity of earthworm individuals and the interaction of earthworm microorganism might be improved and strengthened.