Phosphorus (P) is essential for life and vital for agricultural fertilizers, with growing demand driving interest in recovering P from secondary resources like municipal sewage sludge (MSS). MSS contains nutrients such as P, potassium, calcium, sulfur, and magnesium, making it a potential agricultural resource. However, contaminants, such as heavy metals, organic pollutants, and pathogens, along with variability in MSS composition limit their direct use. Pyrolysis offers a sustainable solution, converting MSS into nutrient-rich biochar, while stabilizing organic matter and reducing contaminants. Factors such as temperature, feedstock composition, and reactor design play crucial roles in optimizing phosphorus recovery and heavy metal removal. This thesis investigates the optimization of the pyrolysis process for phosphorus recovery and heavy metal removal from MSS through biochar production. In Papers I and II, three types of MSS – digested and undigested samples – were pyrolyzed at 500-900 °C using a fixed bed. Chemical composition analysis of the biochars was combined with thermodynamic equilibrium calculations (TECs) to model phosphorus speciation. The results showed that over 90% of phosphorus was retained in the biochar, with temperature having a minimal effect on phosphorus release. Furthermore, copper (Cu), zinc (Zn), and cadmium (Cd) concentrations in the biochar were reduced to meet Swedish agricultural standards. Paper III explored the behavior of trace elements during pyrolysis in a rotary pyrolyzer and identified temperature-dependent patterns in their speciation. The optimal pyrolysis temperature for pollutant removal and carbon preservation was found to be between 600-700 °C. Paper IV expanded these findings by engineering biochar to enhance nutrient recovery and minimize environmental risks. Co-pyrolysis of MSS with wheat straw and bakery waste improved the nutrient profile of biochar while immobalizing heavy metal concentrations compared with MSS biochar alone. This engineering approach facilitated the formation of plant-available phosphorus compounds such as KPM_gO₄ and enhanced the suitability of biochar for agricultural applications.