CO 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 CO can be applied for microalgal-bacterial granular sludge (MBGS) process. This article applied CO for improving MBGS process. It was found that the physical structure of MBGS with no CO addition appeared to have a trend to be loose and disintegrated, with a sludge volume index at 5 min (SVI) of over 150 mL/g and an average pore size of 35 nm in 60 d operation. However, CO could maintain the compact and integrated structure of MBGS with a SVI lower than 50 mL/g and an average pore size ranging from 10 to 13 nm. CO could enhance the production of extracellular polysaccharides and aromatic protein, thus favoring the granular stability of MBGS. CO 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, CO 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 CO addition, the ammonia removal related enzymes of glutamate dehydrogenase and glutamine synthetase could be negatively and positively impacted by CO, respectively. Mass balance analysis of carbon indicated that CO 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 CO as additional carbon source. Hence, CO 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.