Characteristics and applications of biochars derived from wastewater solids

Pyrolysis is a thermochemical decomposition process that can be used to generate pyrolysis gas (py-gas), bio-oil, and biochar as well as energy from biomass. Biomass from agricultural waste and other plant-based materials has been the predominant pyrolysis research focus. Water resource recovery facilities also produce biomass, referred to as wastewater solids, that could be a viable pyrolysis feedstock. Water resource recovery facilities are central collection and production sites for wastewater solids. While the utilization of biochar from a variety of biomass types has been extensively studied, the utilization of wastewater biochars has not been reviewed in detail. This review compares the characteristics of wastewater biochars to more conventional biochars and reviews specific applications of wastewater biochar. Wastewater biochar is a potential candidate to sorb nutrients or organic contaminants from contaminated wastewater streams. While biochar has been used as a beneficial soil amendment for agricultural applications, specific research on wastewater biochar is lacking and represents a critical knowledge gap. Based on the studies reviewed, if biochar is applied to land it will contain less organic micropollutant mass than conventional wastewater solids, and polycyclic aromatic hydrocarbons are not likely to be a concern if pyrolysis is conducted above 700 °C. Wastewater biochar is likely to serve as a better catalyst to convert bio-oil to py-gas than other conventional biochars because of the inherently higher metal (e.g., Ca and Fe) content. The use of wastewater biochar alone as a fuel is also discussed. Finally, an integrated wastewater treatment process that produces and uses wastewater biochar for a variety of food, energy, and water (FEW) applications is proposed.