Insights into phosphate removal and recovery from wastewater using biosolids biochar: Pyrolysis optimisation, mechanistic and column studies

Biosolids, or sewage sludge (SS), is a byproduct from wastewater treatment plants (WWTPs) and drinking water production. SS can contain contaminants, which often generates secondary pollution post-disposal; therefore, pyrolysis is a promising approach for industrial-scale SS management (when compared to incineration, landfill, and direct agricultural use). Here, we determined optimal SS pyrolysis conditions using a Response Surface Methodology approach taking into consideration SS type, pyrolysis temperature and time. An optimised SS biochar (SSB-O) was then characterised using SEM-EDX, TGA/DSC, BET, FTIR, XRD and XPS. The biochar was then applied to phosphate adsorption from wastewater. Adsorption/desorption for phosphate (PO43−-P) was evaluated through a series of batch and dynamic experiments using artificial and real WWTP effluent. Results showed that high PO43−-P removal was achieved with a qmax,exp of 12.7 mg/g in equilibrium studies, and the SSB-O was able to bring residual effluent PO43−-P below 0.5 mg Ptotal/L or 90 % of Ptotal have been removed (new EU Urban Wastewater Treatment Directive 2024/3019). In column experiments, breakthrough time (for effluent P concentration < 0.1 mg/L) was 11,520 min (192 h) for an EBCT of 30 min, which corresponded to 668 Bed Volumes. Instrumental characterisation indicated that adsorption mechanisms were dominated by inner-sphere complexation onto metal ligands (mostly with aluminium). The work showed that SSB-O produced from a WWTP where an aluminium rich drinking water sludge was recirculated through the plant resulted in the best biochar, which could then serve as a water treatment material and as a medium to recover PO43−-P from WWTP effluents.