CO₂ improves the microalgal-bacterial granular sludge towards carbon-negative wastewater treatment

As a promising wastewater treatment technology, little is known about whether the greenhouse gas CO₂ can be applied for microalgal-bacterial granular sludge (MBGS) process. This article applied CO₂ for improving MBGS process. It was found that the physical structure of MBGS with no CO₂ addition appeared to have a trend to be loose and disintegrated, with a sludge volume index at 5 min (SVI₅) of over 150 mL/g and an average pore size of 35 nm in 60 d operation. However, CO₂ could maintain the compact and integrated structure of MBGS with a SVI₅ lower than 50 mL/g and an average pore size ranging from 10 to 13 nm. CO₂ could enhance the production of extracellular polysaccharides and aromatic protein, thus favoring the granular stability of MBGS. CO₂ could change the aqueous environment, e.g. lowering the pH values, which resulted in different microbial communities as well as metabolic potentials of MBGS. As for the reactor performance, CO₂ could significantly improve the removals of organics and phosphorus, evidenced by the enhancement of genes encoding acetate-CoA ligase and ATPase, respectively. Although the mass ratio of algae to bacteria was elevated by CO₂ addition, the ammonia removal related enzymes of glutamate dehydrogenase and glutamine synthetase could be negatively and positively impacted by CO₂, respectively. Mass balance analysis of carbon indicated that CO₂ could provide additional carbon source as well as enhance the buffering capacity for the MBGS system. Further estimations suggested that the MBGS process could achieve a carbon-negative objective for municipal wastewater treatment by supplying CO₂ as additional carbon source. Hence, CO₂ supply for MBGS process in municipal wastewater treatment can be deemed as a two-birds-one-stone strategy, i.e. maintaining the granular stability and eliminating the carbon emission. This article can advance our basic knowledge on MBGS process towards environment-sustainable wastewater treatment.