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

Abstract

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.