Copyrolysis of Municipal Sewage Sludge with Agricultural Residues: A Theoretical and Experimental Study for Tailored Biochar Production

Municipal sewage sludge (MSS) has been identified as a promising feedstock for producing biochar with potential applications as a soil conditioner and animal feed. However, the high heavy metal content and limited availability of nutrients, such as phosphorus (P), pose significant challenges. This study aimed to improve the quality of MSS-derived biochar through copyrolysis with wheat straw (rich in K and Si) and bakery waste husks (rich in K) at temperatures of 500, 650, and 900 °C. Thermodynamic equilibrium calculations (TEC) were performed using FactSage and HSC Chemistry to predict the stability of P-bearing compounds and the fate of heavy metals in the biochars. The morphology and physicochemical properties of the biochars were examined by using SEM and Brunauer–Emmett–Teller (BET) analyses. The results indicate that increasing the proportions of wheat straw and bakery waste husks, along with higher pyrolysis temperatures, reduced the biochar yield. TEC demonstrated that these blends enhanced the formation of plant-available phosphates compared with pure MSS biochar. This improvement was primarily because of the formation of K/Mg-bearing phosphates in different amorphous and crystalline phases, such as K2P2O7, CaK2P2O7, KPMgO4, and KZnPO4, instead of Fe/Al-based phosphates. Additionally, copyrolysis reduced the concentrations of heavy metals such as cadmium (Cd), lead (Pb), and zinc (Zn) in the biochars compared to MSS pyrolysis alone. However, it had no significant effect on the copper (Cu), chromium (Cr), and nickel (Ni) levels. In conclusion, copyrolysis with wheat straw and bakery waste husks not only improved the nutrient profile and physicochemical properties of MSS-derived biochar but also mitigated heavy metal contamination. Additionally, this method reduced the presence of heavy metals, making it a more suitable alternative to biochar produced through monopyrolysis for use in agricultural applications.