Adsorbent modified constructed wetlands for advanced removal of bulk organics and heavy metals from municipal wastewater effluent†

Abstract

Constructed wetlands (CWs) represent a nature-based solution (NBS) for the treatment of wastewater, employing natural processes to mitigate aquatic pollution. Nevertheless, the high specific surface area demand of CWs remains a limiting factor, particularly in urban areas. As the removal of organic and inorganic pollutants is predominantly influenced by the removal mechanisms associated with the selected substrate/adsorbents, this study aims to optimise these processes by hybridising different adsorbents such as biochar, granular activated carbon (GAC) and natural zeolite. Over a period of 18 months, five different approaches were tested, with modifications made to the various layers of the respective CW. Furthermore, the heavy metal (HM) speciation, the behaviour of pollutants and the changing environmental conditions within the wetlands were observed at different filter depths to determine the removal potential of the different adsorbents. The results indicated that the modified CW with biochar, zeolite, and GAC enabled high removal rates of bulk organics (91%) and HMs (Cd: 32%, Cr: 86%, Cu: 92%, Fe: 83%, Ni: 91%, Pb: 43%, and Zn: 96%) with significantly lower hydraulic retention time (HRT) compared to previous studies. This addresses the specific surface area as the main issue of these systems. It was also demonstrated that effective pre-treatment is crucial for maximizing the removal ability of CW systems. Thus, the use of these systems as a polishing technology for the downstream treatment of municipal wastewater is recommended.