Determination of Phosphorus and Metal Leaching Potential in Sewage Sludge Biochar

Abstract Sewage sludge is produced at large volumes at sewage treatment plants, while at the same time it contains significant amounts of microbial, organic and inorganic contaminants. Conversion of sludge into biochar may make nutrients available to soil while it converts dangerous biochar contents, such as metals, into insoluble forms, thus making it safe for land application. This study looked into biochar production from three different sludge samples, at three pyrolytic temperatures, 300 °C, 500 °C and 700 °C. The original biomass was characterized in terms of pH, ash, volatile matter and fixed carbon content, along with the determination of initial metal Cr, Ni, Fe, Cu, Zn and Pb concentrations. Biochar samples were also characterized in terms of pH and S BET analyses. Phosphorus and metal leaching tests were conducted on both sewage sludge and biochar samples, in order to assess the safety and efficiency of applying these materials into water bodies. The results indicated that sludge pyrolysis could be an effective treatment technique for a difficult-to-manage waste, by producing valuable and safe to apply products (biochar). Specifically, biochar leached up to 15 times less heavy metals than the unpyrolized sludge samples. Phosphorus leaching from biochar samples is more stable compared to unpyrolized sludge samples, which could be of outmost importance for agronomic applications, as a fertilizer/soil conditioner.