Drained water quality in sludge treatment wetlands: Effects of earthworm densities and plant species

Abstract

Earthworms can improve sludge dewatering and stabilization in sludge treatment wetlands (STWs). However, drained water quality in STWs with earthworm addition is not well known. In this study, the combination of two plant species (Phragmites australis and Typha angustifolia) and earthworm (Eisenia foetida) densities of 2.7, 5.4, 8.1, 10.8, 13.5 and 16.2 kg/m3 in six STWs were investigated to evaluate their effects on drained water quality. Meanwhile, clogging characterization of the six STWs was evaluated. The results showed that filtration rates in the three earthworm STWs were 0–0.4 cm/s higher than those in the STWs without earthworm addition. P. australis and T. angustifolia had positive effects on chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and ammonium (NH4+) removal, with removal efficiencies all over 80%. Moreover, NH4+ removal efficiency was significantly different between P. australis and T. angustifolia; the higher NH4+ removal efficiencies of 89.5–90.4% were determined in the two P. australis STWs. Meanwhile, canonical correlation analysis and principal component analysis indicated that the combination of plants and earthworms was beneficial for drained water treatment in the STWs. Furthermore, the optimum drained water treatment was determined in the P. australis STWs with an earthworm density of 10.8 kg/m3, with the mass removal efficiencies for COD, NH4+, TN and TP being 99.1%, 93.1%, 91.5% and 91.0%, respectively. Thus, it can be concluded that P. australis and earthworm addition improved drained water quality and alleviated clogging in the STWs.