In this work, we used a statistical approach to determine the best biochar production conditions to use with a diverse range of biosolid (BS) samples collected from across Scotland. BS biochar was then characterised using different instrumental characterisation techniques; and tested (in terms of removal efficiency/adsorption capacity) for P removal from wastewater. This process was optimised through a series of batch and dynamic tests using synthetic P solutions and final wastewater effluent. The toxicity of BS biochar was considered using seed germination tests. P solubility and speciation was explored using different chemical extractants to draw relationships between extractable/bioavailable P and the physicochemical properties of the biochar.

Pilot scale trials were performed to assess how the BS biochar would operate under environmentally relevant conditions and at a larger scale. A five-week trial was carried out at the Scottish Water Horizons Development Centre in Scotland in September-October 2023. High P removal/recovery potential was achieved even at low P concentrations, bringing the residual effluent P level below 0.5 mg Ptotal/L (the new EU limit for recipients). Quality analysis indicated ~8% P2O5 was present on the final biochar and only trace levels (well below legislative limits) of organic and inorganic contaminants were observed (i.e., Cd, PFAS/PFOS, PAHs).