Effect of Shear stress on the cultivation and characteristics of aerobic granules

Activated sludge was inoculated into four column sequencing batch reactors (SBRs) with the glucose as carbon source. The shear stresses of 0.189, 0.267, 0.327 and0.377 N/m(2) were carefully and scientifically calculated in four SBRs, respectively. The effect of shear stress on the aerobic granulation was studied. The characteristics of the aerobic granules under different shear stress were also investigated. Results showed that aerobic granulation was influenced significantly by the shear stress. With increasing of the shear stress from 0.189N/m(2) to 0.327N/m(2), the granules became more compact, its diameter increased and the water capacity decreased, correspondingly. While shear stress rose to 0.377 N/m(2), the granules changed looser, its diameter decreased, and the water capacity increased, respectively. Aerobic granules appeared cloudy yellow color and elliptic balls with smooth surface. No significant difference of the morphology and structure of granules could be identified under different shear stresses. Aerobic granules had various microbial communities with the very clear outline. A plenty of protozoans and metazoans were enmeshed on the surface of sludge, such as epistylises, vorticellas, rotifers, parameciums and other swimming ciliates, while rods, coccus and figgus were the chief bacteria. Depending on its resistance to shear force, rods and rotifers would be the dominant cultures under the high shear stress of 0.377 N/m(2). Extracelullar polymeric substances with good adsorption would be the key constituent for the stabilization of aerobic granules.