Capture of wastewater ammonium by microalgal-bacterial granular sludge: A green-to-green engineering solution for carbon neutrality

With rapidly evolving global climate change, municipal wastewater treatment has been identified as a nonneglected carbon source contributing to about 1–2% of total carbon emissions worldwide. Given such a situation, the development of carbon–neutral municipal wastewater treatment process has become highly desirable and pressing. However, almost all wastewater ammonium is biologically converted to nitrogen gas in the present biological treatment processes of municipal wastewater, making ammonium recovery impossible. It is shown in this article that the current biological nitrogen removal (BNR) processes indeed are a significant carbon source, implying that nitrogen neutrality largely determines carbon neutrality of wastewater treatment. To tackle this challenge, ammonium recovery instead of BNR should be incorporated into future municipal wastewater treatment processes. Although several advanced bio/electrochemical approaches have recently been developed for ammonium recovery from municipal wastewater, their engineering and economic feasibility have not yet been determined. Instead, the nature-inspired green approach for recovery of wastewater ammonium should deserve more attention and effort. It is shown that an integrated anaerobic fixed-film membrane bioreactor-microalgae bacterial granular sludge process coupled with heat exchange can create a significant carbon sink, likely leading to carbon-negative municipal wastewater treatment. Consequently, it is reasonable to consider that the way forward is to explore a green-to-green engineering approach through harnessing the synergies among wastewater treatment, energy-heat-nutrients recovery, and carbon dioxide capture.