Effects of loading rates and plant species on sludge characteristics in earthworm assistant sludge treatment wetlands

Abstract

Sludge treatment wetlands (STWs) are widely used to treat surplus sludge in recent years. However, the effects of plant species and loading rates on sludge characteristics in earthworm assistant STWs remain unclear. In the current study, six STWs planted with two plant species (Phragmites australis, Typha angustifolia) were investigated under four loading rates (60, 80, 90 and 120 kg DS/m²/yr) regarding the influence on sludge characteristics. Furthermore, earthworms were added in three STWs to evaluate their role on sludge stabilization during resting period. Results showed that the best sludge dewatering (dry solids (DS) of 45.0%) and stabilization (volatile solids to total solids (VS/TS) of 40.5%) were determined in the P. australis STWs at the loading rate of 80 kg DS/m²/yr. Furthermore, VS/TS and Escherichia coli contents in earthworm STWs were 5.5-11.2% and 12-39% lower than that in the control without earthworm addition. Meanwhile, earthworm also decreased the nutrient contents in STWs. However, earthworms had insignificant effects on heavy metal contents in STWs. Nevertheless, the bioavailability of Cd and Cr in STWs were decreased by earthworm addition, with an acid-soluble fraction of Cd and Cr reduced by 11.2-18% and 2.5-7.5%, respectively. In conclusion, sludge characteristics can be improved by earthworm addition in P. australis STWs.

Keywords: Earthworms; Loading rates; Plant species; Sludge treatment; Sludge treatment wetlands.