Formation, extracellular polymeric substances and microbial community of aerobic granules enhanced by microbial flocculant compared with poly-aluminum chloride

Abstract

This study is the first reported research in which microbial flocculant (MBF) was applied to accelerate the formation and strengthen the stability of aerobic granules in sequencing batch reactors. To better evaluate the effects of MBF, poly-aluminum chloride (PAC) was used as a reference. The main metrics were the characteristics of granule formation, extracellular polymeric substances (EPS) and characteristics of the microbial community. MBF resulted in granules that matured 13 days earlier than in the control reactor (without flocculant), but 17 days later than PAC-fed granules. Scanning electron microscope images showed granules from both MBF and control reactors were covered with a mesh structure; in contrast, the surface of PAC-fed granules was smoother. All three flocculant treatments achieved high contaminant removal rates. EPS analysis demonstrated that protein and β-polysaccharide were uniformly distributed in the MBF-fed granules, while a bilayer consisting of a protein shell and β-polysaccharide core existed in PAC-fed granules. Both MBF and PAC strengthen the resistance of granules to hydrolase. At the genus level and compared to that of the control reactor, the microbial community in the MBF-fed and PAC-fed reactors was dominated by relatively heavy bacteria that improved sludge settling performance. Furthermore, the MBF reactor had the lowest abundance of EPS producers, while the PAC reactor had the highest abundance of these microorganisms.