Using carbon dioxide-added microalgal-bacterial granular sludge for carbon-neutral municipal wastewater treatment under outdoor conditions: Performance, granule characteristics and environmental sustainability

Microalgal-bacterial granular sludge (MBGS) process has a gorgeous prospect for municipal wastewater treatment, but the research on the treatment of complex organic wastewater by MBGS process with CO 2 addition under outdoor conditions is not enough. Therefore, this paper evaluated the feasibility of CO 2 -added MBGS process for complex organic wastewater disposal under natural day-night cycles. The results showed that the addition of CO 2 overall improved the removal efficiency of pollutants. Typically, the removal efficiency of total phosphorus increased averagely from 88.5 % to 95.0 % in 12-h day cycle and from 26.2 % to 45.3 % in 12-h night cycle. The addition of CO 2 increased the size of MBGS from 1.0 mm to 16.5 mm within 30 days due to extracellular polymeric substances secretion and the dominant filamentous microalgae on granules. The decrease of catalase activity and malondialdehyde content indicated that CO 2 reduced oxidative damage and maintained the normal growth of MBGS. Further estimates of the collected gas showed that CO 2 -added MBGS process could reduce global CO 2 emissions by one hundred million tons per year. This study is expected to contribute to the goal of carbon neutrality in the area of wastewater treatment by MBGS process. • CO 2 -added microalgal-bacterial granular sludge (MBGS) was evaluated outdoors. • CO 2 addition promoted the phosphorus removal for both day and night cycles. • CO 2 favored the MBGS growth with filamentous microalgae on the granule surface. • CO 2 -added MBGS process contributed to the global carbon neutrality.