Use of photo-driven algal-bacterial aerobic granular sludge system to close carbon cycle in biological nutrients removal and sludge anaerobic digestion units in wastewater treatment plants

This study established a closed photo-driven algal-bacterial aerobic granular sludge (AGS) system coupling with an automatic biogas capture unit and with a working volume of 800 mL operated at a gas circulation rate of 0.75 L/min and liquid pH controlled at 8.0. This closed system was used to investigate the feasibility of algal-bacterial AGS to capture and fix carbon (C) from nutrients removal and anaerobic digestion (AD) processes. Results showed a high total C emission of 0.93 kg-CO₂/kg-chemical oxygen demand (COD) under the external O₂ supply condition. However, under the photo-driven mode with photosynthesis as the sole O₂ supplier, the coexisting microalgae fixed all inorganic C from the nutrients removal process with a negative C emission of −0.25 kg-CO₂/kg-COD and −0.33 kg-CO₂/kg-COD under illumination at 24 klux and 36 klux, respectively. During this process more HCl was required to neutralize the increased pH under a stronger light intensity, which can be replaced by biogas (as an acid reagent) automatically introduced into the closed system as needed. Meanwhile, a CO₂ removal rate of 40 mg-C/(g-mixed liquor volatile suspended solids (MLVSS)•d) and a biogas upgrading rate of 184 mL-biogas/(g-MLVSS•d) were achieved under 24 klux. It was estimated that the CO₂ removal capacity can cover all CO₂ produced from AD of the grown biomass in this system. In addition, photosynthetic O₂ could simultaneously facilitate aerobic phosphorus uptake and ammonia oxidation at a low dissolved oxygen of 0.7–1.4 mg/L. Results from this work suggest that a properly designed and operated algal-bacterial AGS is promising for closing C cycle in wastewater treatment plants. © 2023 Elsevier B.V.