Microalgal-bacterial granular sludge under constant dark and weak light conditions: Morphology, performance and microbial community

As a sustainable wastewater treatment process, it is unclear whether the microalgal-bacterial granular sludge (MBGS) can adapt to the continuous light-deficiency environment, which hinders the further application of this technology under natural outdoor conditions when the sunlight is insufficient. Herein, this study investigated the performance of MBGS in treating simple and complex organic wastewater under respective constant dark and weak light (20 mu mol m- 2 s- 1) conditions. It was found that MBGS could survive under constant dark and weak light conditions for approximately 7 and 15 days, respectively. During the initial 7 days' operation, the average removal efficiencies of organics, nitrogen and phosphorus for simple/complex wastewater were 39.0 %/38.1 %, 22.4 %/19.1 % and 58.6 %/29.9 % under dark conditions, which were 55.8 %/80.1 %, 77.8 %/86.7 % and 76.4 %/90.9 % under weak light conditions, respectively. It was observed that weak light promoted the granule size increase and extracellular polymeric substances production of MBGS. Microbial analysis showed that the dark condition was beneficial to Proteobacteria growth of MBGS while weak light was more conducive to the increase of Cyanobacteria. Firmicutes became the main taxa of MBGS under complex carbon source conditions, potentially contributing to the decomposition of complex organics under light-insufficient conditions. This study will advance our knowledge on MBGS under light-limited conditions.