A critical review on using biochar as constructed wetland substrate: Characteristics, feedstock, design and pollutants removal mechanisms

Constructed wetlands (CWs) are constructed systems that simulate natural wetlands and can be used to treat wastewater from several sources of pollution through physical, chemical and biological depuration processes. This work aims to critically review the updated literature on constructed wetlands (CWs) integrating biochar in the substrate. In detail, the study focuses on the characteristics of biochar that are generally integrated into this treatment ecotechnology and the processes used to prepare the materials, including conditions of thermal con-version and the kind of feedstock used (e.g., agricultural, food, and wood wastes, sewage sludge and algal marine feedstock). Based on the literature review, it is found that the feedstock must be rich in carbon and low in the mineral matter to produce good quality biochar, i.e. large pore volume and high specific surface area, thus allowing to effectively remove pollutants from wastewater. The biochar quality is affected by the conditions involved in preparing biochars (e.g., pyrolysis temperature, heating rate and carbonization time). The properties of biochar used for wastewater treatment, the effect of its implementation as CW substrate and its treatment efficiency have also been described. Several factors alter the removal efficiency of pollutants in CWs, such as substrate chemical and physical properties, hydraulic retention time, oxygenation, and redox conditions in the reed bed. In addition, the mode by which biochar is implemented in the filter and the choice of macrophyte are crucial for regulating the efficiency of the treatment system. Phragmites australis was the most used plant in the previous studies because of its large advantages. Different configurations of CWs integrating biochar into the wetland as a filling medium, were reported and compared. In vertical flow CWs (VF-CWs), which are the system mostly investigated, several studies have shown that the optimal position for the biochar substrate is the in-termediate one between two layers of inert materials, to avoid clogging of the filtration system or biochar flotation.