Microalgal-bacterial granular sludge process outperformed aerobic granular sludge process in municipal wastewater treatment with less carbon dioxide emissions.

The aerobic granular sludge (AGS) process and microalgal-bacterial granular sludge (MBGS) process were comparably applied for municipal wastewater treatment in sequencing batch reactors with a height to diameter ratio of eight. For morphological appearances, the yellow aerobic granules were strip-shaped (4.0 mm × 0.62 mm) while the green microalgal-bacterial granules were elliptical-shaped (2.0 mm × 0.75 mm). The dominated rod-shaped bacteria (e.g., Acidobacteria and Bacteroidetes) and the slender configuration might be associated with the strip shape of aerobic granules under weak acid conditions. The nutrients removal performances by MBGS process were generally slightly better than AGS process. In addition, nutrients removal mechanisms were identified to elucidate how organics, ammonia, and phosphorus were removed by AGS process and MBGS process, respectively. Mass balance calculation estimated that MBGS process appeared to achieve much less CO emission (5.8%) compared with AGS process (44.4%). Overall, it proved that MBGS process, with the merits of potentially low energy cost, limited CO emission, and excellent performance, showed more prospects in municipal wastewater treatment than AGS process.