Abstract Summary
Stricted legislation and a heightened emphasis on environmental stewardship are transforming conventional wastewater treatment plants (WWTPs) into greener and more efficient biorefineries, producing valuable bioproducts that are increasingly prevalent in society. Notably, the recent wastewater treatment Directive (UE) 2024/3019 introduces new objectives of energy neutrality and significant nutrient reduction in wastewater discharges. To achieve these goals, stricter limit values for nitrogen and phosphorus have been implemented. Furthermore, all WWTPs serving populations equivalent to more than 150,000 inhabitants will be required to implement nutrient removal processes. These legislative changes will inevitably increase operating costs for WWTPs, driven by the need for enhanced energy consumption to remove nitrogen and increased dosing of chemical reagents to precipitate phosphorus. This is necessary to comply with the stringent discharge parameters. The aim of this study is to produce high-quality treated water and biofertilizers from domestic wastewater through two distinct scenarios. The first scenario showcases the world's largest 100% solar anaerobic photobiofactory, which leverages purple phototrophic bacteria (PPB) (CBE JU project DEEP PURPLE). The technology's development has progressed from Technology Readiness Level (TRL) 5 during pilot-scale trials at the Estiviel WWTP in Toledo, Spain, to TRL 7 with the construction of a demonstration plant at the Linares WWTP. Employing a photobioreactor system with a treatment capacity of 350 m³/d (1500 population equivalents), the effluent consistently meets or exceeds European legislative standards (TP < 2 mg/L, TSS < 15 mg/L, BOD5 < 23 mg/L, and COD < 40 mg/L). Moreover, the process's flexibility allows for treated water to be adapted for agricultural applications, minimizing nitrogen and phosphorus uptake and serving as a valuable nutrient-rich irrigation resource. Additionally, phototrophic biomass has proven to be a low-cost feedstock for formulating biofertilizers with a C:N:P ratio of 100:15:13 and an N:P:K ratio of 100:98:25. The second scenario focuses on producing a valuable product, struvite (Aquavite®), recovered from nutrient-rich wastewater streams in WWTPs (BBI B-FERST project). The recovery efficiency of phosphorus(III) oxide (P2O5) in the Aquavite® production system exceeded 80%, while ammonium (NH4+) removal efficiency approached 40%. With a treated flow rate of up to 100 m³/d, the system produces an estimated 15 tons per year of P2O5-enriched struvite (28%), which can be used as a raw material for fertilizer formulation. Both the BBI DEEP PURPLE and BBI B-FERST projects provide full-scale solutions for nutrient recovery from urban wastewater, promoting the concept of a circular economy."
