Treatment of swine wastewater using multi-soil-layer based constructed wetland: Substrates assessment and efficiency improvement

Dispersed discharge of decentralized swine wastewater without effective treatment has become a widespread problem in rural areas, threatening the regional water environment. To address the above issues, performance of the multi-soil-layer based constructed wetland (MSL-CW) in treating swine wastewater during long-term operation was investigated, with the substrates type and their layout in the MSL-CW being compared. Results showed that zeolite performed best in NH+4-N removal, while iron filling had the highest removal capacities for TP, followed by cinder. Removal efficiencies of COD, NH+4-N, and TP could be substantially facilitated in the MSL-CW without reducing agents (rMSL-CW) compared to that in control CW, normal MSL-CW, and MSL-CW with the mixed substrates. The relatively excellent treatment performance of rMSL-CW indicated that changes in environmental conditions (such as ORP and aeration) induced by removing reducing agents might be the reason for the differences in treatment performances among CWs. Furthermore, the treatment efficiency of rMSL-CW could be further improved by increasing the ORP, in which applying the aeration and removing the compact surface soil were found to be conducive to COD removal and NH+4-N removal, respectively. This study suggested the promising potential for long-term operation and full-scale application of rMSL-CW for swine wastewater treatment.