A novel photosynthesis-based strategy for improved sewage treatment

Photosynthesis provides a cost-effective and straightforward approach to wastewater treatment (WWT) with the potential for nutrient recovery via biomass accumulation. To overcome biomass recovery challenges in traditional systems, this study proposes a photosynthesis post-treatment following microbial fuel cells (MFCs) treatment, using biomass immobilized on polypropylene fiber carriers. This approach reduces centrifugation volume, improves biomass recovery, and enhances contaminant removal. An MFCs-photosynthesis treatment system, comprising two identical 50-L photosynthesis reactors (F1 and F2), was installed in a WWT plant and operated for over 300 days in two phases: phase I with undisturbed biomass precipitation and phase II with the precipitate removed. Up to 99 % NH₄⁺ removal was achieved in both reactors, primarily through assimilation by photosynthetic microorganisms during phase I at hydraulic retention times (HRTs) exceeding 300 h and via nitrification in phase II, when the HRT was reduced by half. Organic matter (OM) removal efficiencies averaged 70 % for chemical oxygen demand and 92 % for biochemical oxygen demand across the two reactors at HRTs exceeding 150 h. A nanofiltration process was implemented to remove residual phosphate remaining after the MFC and photosynthesis treatments. Using a commercial NF90 membrane, the system achieved approximately 99 % PO₄³⁻ removal at a constant permeate flux of 1.3 mL/min. Microbial analysis of the biomass in the photosynthetic reactors identified Chlamydomonas and Cyanobacterium as dominant genera, key in NH₄⁺ and OM removal and nutrient-to-biomass conversion. Overall, this approach represents a sustainable and efficient strategy for WWT and resource recovery.