Pilot-scale phosphate recovery with biosolids biochar: Performance, phosphorus bioavailability and biochar quality assessment

Phosphorus (P) recovery from wastewater using biochar made from biosolids represents a promising solution to improve P circularity not only in the water sector but more broadly. This study provides a ‘proof the concept’ for a pilot scale P recovery system based on P adsorption onto biosolids biochar (SSB-O). The generated P-loaded biochar is then assessed in terms of P bioavailability and for its quality (given its potential to be applied to soil). The pilot trial explored the SSB-O's capacity to remove phosphate (PO43−-P) from final wastewater effluent at environmentally relevant levels, whilst meeting stringent discharge limits for P as per the revised EU Urban Wastewater Treatment Directive (<0.7 mg P/L). The pilot unit was packed with ~17 kg of SSB-O and tested with an empty bed contact time (EBCT) of 15 mins, equivalent to a 150 L/h flowrate through the reactor. PO43−-P removal efficiency reached a maximum of 92%, while the process maintained <0.7 mg P/L discharge during the operational period of 383 h (for a treated wastewater volume of 57.6 m3; equivalent to 859 Bed Volumes (BV)). The spent SSB-O contained ∼8% P2O5 and was rich in macro elements (K, Ca, etc.). Trace concentrations of heavy metals (including Pb, Cd, Ni, Cr, As, Zn), PAHs, and PFAS/PFOS were also detected, but these were well below threshold limits set by the current European Biochar Certificate (EBC) and World Biochar Certificate (WBC) biochar quality guidelines. Only Cu exceeded the limits defined by the World Biochar Certificate. P bioavailability and heavy metal mobility were assessed using four different chemical extractants. Although only a limited fraction of P was readily available, further studies are required to substantiate the biochar's agronomic value and confirm its long-term phosphorus release characteristics. Minimal risk of heavy metal leaching was also observed. While further development is required, this study demonstrates the potential of biosolid-derived biochar as an adsorbent for phosphate removal from wastewater, as well as the possibility of reusing the P-enriched biochar across various primary and secondary sectors.