A review of the sustainability of anaerobic reactors combined with constructed wetlands for decentralized wastewater treatment

Anaerobic Reactors (ARs) and Constructed Wetlands (CWs) have gained popularity as efficient and low-cost technologies for decentralized wastewater treatment. However, when used individually, each presents some disadvantages. For instance, low, or even negative, nutrient removal efficiencies have been reported in ARs, and operational failures (e.g. clogging) are frequent in CWs. Combined AR + CW systems have been proposed as a potential solution to overcome the limitations of individual units, where the AR effluent is fed into the CW to achieve better water quality and to avoid operational failures. In this review, the removal performance of combined AR + CW systems were compared with individual ARs, individual CWs, and conventional units (AAO, MBR and stabilization ponds). In addition, the pollutant removal processes (for organic matter, suspended solids and nutrients) and some aspects of combined systems' environmental, economic and social sustainability are deeply discussed herein. This literature analysis provided quantitative evidence of more efficient pollutant removal in combined systems over individual units and conventional processes. Also, the combined system's advantages are highlighted, including simple and low-cost construction, low operational and maintenance requirements, minimum energy consumption, reduced land occupation and low GHG emissions. In summary, this review supports the conclusion that AR + CW combined systems are an effective, robust, flexible and highly sustainable technology for decentralized wastewater treatment, especially for developing countries with limited resources and infrastructure.