How to select substrate for alleviating clogging in the subsurface flow constructed wetland?

Constructed wetland (CW) is a cost-effective and environmentally friendly ecological technology for contaminated water remediation, especially in dispersed communities and rural areas. Plants grow, biofilms form, and pollutants attach to the substrate, which is the main supporting structure of a subsurface flow CW (SSFCW) system. After long-term operation, the accumulation of clogs from physical, chemical, and biological processes in SSFCW substrates can easily cause clogging, thus reducing treatment efficiency reduction and service life and causing no discharge of sewage by intermittent until last indicates in the CW surface. Subsequently, stench and mosquito breeding occur, thus influencing environmental sanitation. Substrate clogging is the most serious, challenging, and inevitable problem in the long-term operation of SSFCWs. The present study reviews the effects of substrates on clogging categorized into physical, chemical, and biological clogging and analyzes the substrates that can alleviate/aggravate clogging in CWs. The recommended substrates that can relieve clogging include plastic, rubber, soil mixture, walnut shell, biochar, organic waste, alum sludge, and lightweight aggregate, while shell, steel slag, blast furnace slag, zeolite, and soil may easily generate phosphorus-clogging substances. CW substrate clogging is a mixture of three clogs with synergistic effects, and the corresponding clogging mitigation substrates mentioned above can be used to alleviate the most severe among the three types of clogs to reduce the synergy, and thus to promote stable operation and technology level of CWs. This review aims to promote the scientific selection of substrates for the stable operation and technical level of CW through targeted recommendations for substrates that relieve clogging. Future studies should focus the effects of influent water quality and substrate type on clogging, and waste as substrate to alleviate clogging, while mitigating the negative environmental impact of waste treatment.