Microalgal ammonium assimilation and carbon mitigation in a dual-stage process integrating anaerobic treatment and a membrane photobioreactor.

Achieving carbon-neutral municipal wastewater treatment remains a critical challenge amid global climate change. Microalgae possess advantages in carbon capture and nutrient recovery, and their integration into municipal wastewater treatment systems is emerging as an active research area. This study developed an integrated process coupling anaerobic treatment and a membrane photobioreactor (MPBR) with CO2 supplementation for synthetic municipal wastewater treatment to achieve nutrient recovery, and carbon emission reduction. After anaerobic treatment, 96.0 ± 1.8% of the total organic carbon (TOC) was removed, resulting in an effluent TOC to total nitrogen (TN) ratio was approximately 0.14. Varied CO2 supplementation strategies were evaluated during MPBR operation, finding that moderate CO2 supplementation (10%) enhanced microalgal ammonium assimilation and achieved TN removal efficiency of 75.4 ± 6.9%. Nitrogen removal predominantly occurred through assimilation during light periods, while in the dark period, assimilation accounted for 42.6%. These were supported by molecular and enzymatic evidence, including increased activity of Rubisco and glutamine synthesis (GS) enzymes, along with upregulated expression of the glnA gene. Furthermore, 51% and 66% of influent carbon and nitrogen, respectively, were recovered into biomass, indicating effective carbon fixation and nitrogen recovery. Overall carbon emission assessment revealed a net carbon emission of 0.07 g CO2 equivalent/L wastewater, suggesting its carbon neutrality potential. This performance was attributed to energy recovery from biogas, efficient microalgal carbon fixation and additional energy recovery from lipid-rich biomass. Overall, the integrated anaerobic and microalgal-bacterial process is a promising strategy for simultaneous nitrogen recovery, and carbon sequestration for municipal wastewater treatment.