Formation and characterisation of fungal and bacterial granules under different feeding alkalinity and pH conditions

Laboratory experiments were carried out using two identical bioreactors to investigate the effect of feeding alkalinity and pH on the formation of aerobic sludge granules in wastewater treatment. Both bioreactors, of 2.4 L each in working volume and seeded with activated sludge, were operated as sequencing batch reactors (SBR) and fed at the same rate with a glucose-based synthetic wastewater. The first SBR, with a low alkalinity of 28.7 mg CaCO3/L in the influent, had a pH of about 3.0 in the reactor and achieved rapid formation of fungi-dominating granules in 1 week. The second SBR, with a high alkalinity of 301 mg CaCO3/L from the addition of 440 mg NaHCO3/L to the influent, maintained a reactor pH of around 8.1 and had a slower formation of bacteria-dominating granules, taking about 4 weeks. After granulation, both reactors performed well in organic degradation and sludge–liquid separation. However, according to examinations carried out using scanning electronic microscopy (SEM) and confocal laser scanning microscopy (CLSM), the mature fungal granules with a mean size of 7.0 mm had a loosely packed fluffy structure. Both fungi and extracellular polymeric substances (EPS) were distributed uniformly throughout the granules. The bacterial granules were smaller, with a mean size of 4.8 mm and a compact structure. EPS were distributed throughout and bacteria were mainly situated in the outer layer of the mature granules. Granulation of fungal sludge could be completed more rapidly than bacterial granulation. However, fungal granules were apparently weaker in structure and subject to more breakage and erosion than bacterial granules in aeration turbulence. The results suggest that by controlling the feeding alkalinity and reactor pH, a strategy of species selection can be developed for aerobic sludge granulation at different rates with different microbial communities and structural features.