| Enhanced biological phosphorus removal (EBPR) system has long been considered an environmentally friendly process that offers effectively treating phosphorus in wastewater. Many advantages of granules were reported including good settling ability and ability to withstand shock-loadings. However, wastewater treatment plants (WWTPs) in China receive both industrial and municipal influents in China due to the progress of the urbanization. Heavy metal ions are common contaminants of industrial wastewaters. Therefore, the aim of this study were:to investigate the long-term effect of low concentrations of Cr(Ⅵ) and Cr(Ⅲ) loading on granule-based EBPR system performance; to discuss the long-term toxic mechanisms of Cr(Ⅵ) and Cr(Ⅲ) on EBPR system by comparing the variation of granular, the microbial composition, the polyhydroxyalkanoates (PHAs) and extracellular polymers (EPS) in the presence or absence of Cr(Ⅵ) and Cr(Ⅲ). And we also investigate the inhibition effect of Cr(Ⅵ) and Cr(Ⅲ) on the growth kinetics of PAOs in EBPR systems.P removal efficiency was significantly inhibited when Cr(VI) was higher than0.5mg·L-1, though it was not inhibited at≤0.4mg·L-1Cr(Ⅵ). Aerobic phosphorus uptake rate (PUR) was more sensitive than anaerobic phosphorus removal rate (PRR) response to Cr(Ⅵ) loading. Total COD removal efficiency was not affected under Cr(Ⅵ) exposure. The ratio of anaerobic COD removal to total COD removal changed. At stage IV, V and VI, COD removal efficiency in anaerobic declined substantially (around to20%,60%and80%compared with that of control). MLVSS/MLSS ratio significantly increased and sludge bulking was observed. The granules could not maintain the elliptical morphology after the long-term Cr(Ⅵ) loading exposure and filamentous bacteria became dominant on the granule surface. Fluorescence in situ hybridization (FISH) analysis indicate that The proportion of PAOs markedly decreased with the operation of Cr(Ⅵ) treatment, while the fraction of GAOs and other bacteria significantly increased. This revealed that GAOs and other bacteria could be better resistance to Cr(Ⅵ) toxicity than PAOs. The effect of Cr(Ⅵ) on both PHAs synthesis and consumption investigation indicated that PHA degradation in aerobic was more sensitive to Cr(Ⅵ) than PHA synthesis in anaerobic. The increased Cr(Ⅵ) concentration caused an EPS gradually enhancement. PS was excessively produced by the microorganisms. Polysaccharide/Protein (PS/PN) was significantly negatively correlated to the average particle diameter (r=-0.937, p<0.01), indicating that high PS/PN ratio was adverse to the granule stability, indicating low PS/PN was advantage to granule stability. The characteristics of the granule-based EBPR system had been markedly inhibited and could not revive under long-term Cr(Ⅵ) shock loading. It was mainly due to the failure of PAOs recovery which was the functional bacteria in EBPR system.P removal efficiency was significantly inhibited when Cr(Ⅲ) was higher than1.0mg·L-1. Aerobic PUR and anaerobic PRR were significantly decreased response to Cr(Ⅲ) loading. Total COD removal efficiency and the ratio of anaerobic COD removal to total COD removal were changed slightly under Cr(Ⅲ) exposure. The granules with an irregular shape was observation, indicating that the granules could not maintain the elliptical morphology. FISH analysis indicated that the bacteria proportion in the system was changed. The proportion of PAOs markedly decreased, while the fraction of GAOs and other bacteria significantly increased. PHA degradation in aerobic and PHA synthesis in anaerobic were both declined when Cr(Ⅲ) was1.0mg·L-1. EPS gradually decreased which mainly caused by the reduction of PN. However, PS production was slightly increased. The characteristics of the granule-based EBPR system had been markedly inhibited and could not revive under long-term Cr(Ⅲ) shock loading.In EBPR systems, when Cr(Ⅵ) concentrations were0.25,0.5,1.0,2.5,5.0mg·L-1, the specific growth rate of PAOs was0.5,0.3769,0.1560,0.0533,-0.3075d-1, respectively. And the inhibition constant (Ki) was0.4258mg·L-1. When Cr(Ⅲ) concentrations were0.5、1.0、2.5、5.0、10.0mg·L-1, the specific growth rate of PAOs was0.5728、0.5158、0.2621、0.1512、-0.2213d-1respectively. The inhibition constant (Ki) was1.4578mg·L-1. |
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