Effect of Zn(II) on the characteristics of aerobic granules

Aerobic granules were cultivated successively in a column sequencing batch reactor (SBR) with glucose as the carbon source. The effect of zinc on the characteristics of aerobic granules was studied. Results show that the physical properties, morphology, and amount of extracellular polymeric substances (EPSs) of aerobic granules were influenced by the concentration of zinc(II) in the SBR. At a Zn2+ concentration of 0.5-10.0 mg/L, the aerobic granules had stable values of sludge volume index (SVI), specific gravity (SG), and mixed liquid suspended solid (MLSS) concentration in the reactor, and the sludge comprised aggregated granules. While the Zn2+ concentration increased to 20.0 mg/L, the compact granules became diffuse granules. The SG of granules decreased to 1.0089 and the SVI increased to 44.0 mL/g. This indicates that the granules had less settleability. Therefore, a part of loose sludge was washed out, and the MLSS in the reactor reduced to 2730 mg/L. The microorganisms of aerobic granules excreted more EPSs as the Zn2+ concentration increased from 0.5 to 20.0 mg/L. The quantity ratio of extracellular polysaccharide/extracellular protein (EPSC/EPN) was in the range 6.36-6.91 at a Zn2+ concentration of 0.5-10.0 mg/L. As the Zn2+ concentration increased to 50.0 mg/L, the granules became loosely bound flocs. Furthermore, amount of sludge was washed out, the MLSS of the SBR and SO of the sludge decreased, and the SVI increased rapidly. The EPSC and EPN contents decreased dramatically. Meanwhile, the quantity ratio of EPSC/EPC increased to 9.24 and 11.18, respectively, at Zn2+ concentrations of 20.0 and 50.0 mg/L.