Earthworm effects on biosolids characteristics in sludge treatment wetlands

As an ecological sludge treatment technology, sludge treatment wetlands (STWs) have the advantages of low investment and low energy consumption, have attracted more and more attention around the world. However, the role of earthworm on biosolids characteristics in STWs is not well known. In the present study, six STWs with two plant species (Phragmites australis, Typha angustifolia) and earthworm addition were investigated to evaluate their effects on sludge characteristics. Furthermore, feasibility for sludge land application after STWs was assessed. The results showed that the best sludge dewatering (total solids 38.3% during the feeding period) and stabilization condition (volatile solids to total solids 21.1%, dehydrogenase activity 0.65 g (TPF-tetrazole red formazan)/g (sludge) after two months of rest period) were determined in the earthworm assistant STWs planted with P. australis. The rapidly available phosphorus and available potassium contents in sludge were increased by the earthworm addition. The lowest Escherichia coli (600 MPN/g) and volatile fatty acid content (188.7 g/kg) were detected in the earthworm assistant STWs planted with P. australis. Scanning electron microscope analysis showed that sludge structure in the bottom was more loosen than in the surface, and the most loosen structure was observed in the earthworm assistant STWs planted with P. australis. The highest germination index (95%) and highest biomass (100 g/m2) of the tested crop (Brassica chinensis) were observed in the system with sludge (from STWs planted with P. australis): soil = 1:3 and 1:1, respectively. Overall, the addition of earthworm in STWs exhibited a positive effect on sludge characteristics, which enhanced the quality of treated sludge for land application.