Effect of anaerobic digestion and pyrolysis on nutrient composition, heavy metals, and phosphorus recovery in manure and sewage sludge biochar

Recycling nutrients from organic residues is key to circular agriculture, but contaminant limits restrict their reuse. This study examined how anaerobic digestion (AD) and pyrolysis influence nutrient enrichment and regulatory compliance in biochar derived from six feedstocks: raw and digested manures (RM, DM) and biologically or chemically treated raw and digested sewage sludges (BTRSS, CTRSS, BTDSS, CTDSS). Samples were analyzed for pH, dry matter (DM), loss on ignition (LOI), total C and N, macro-/micronutrients, and trace elements, and evaluated against Norway’s 2025 fertilizer regulation using concentration classes and P: metal ratio thresholds. AD reduced DM, LOI, and total C but increased pH, Ca, and Mg; it enhanced P in manures but lowered it in sludges. Pyrolysis produced stable, alkaline biochar (DM > 97%) enriched in P, K, Ca, and Mg, while N and S decreased. Micronutrients (Fe, Mn, B, Mo) and heavy metals became concentrated, with Zn and Cu as key regulatory constraints, whereas Cd, Pb, Hg, and As remained below limits. Chemically treated sludges showed high Al due to coagulants. Overall, digested manure biochar at 400 °C met both P and heavy metal criteria, while most sludge-derived biochar failed eligibility, except for chemically treated raw sludge at 400 °C.