| Activated sludge process is often used in the biological treatment of the tannery wastewater.Soluble microbial product?SMP?and extracellular polymers substances?EPS?are the main microbial metabolites produced by activated sludge microorganisms to degrade organic matter in wastewater.There are a lot of difficult-to-degrade organic matters and heavy metal chromium in the tannery wastewater,which will cause changes in the components and contents of EPS and SMP when it entered the system of activated sludge.The heavy metal chromium could be adsorbed and cooperated with microbial metabolites,it will stabilize the form of chromium and pose a potential threat to the advanced treatment of wastewater.Therefore,municipal aerobic activated sludge was used as the simulation material to explore the adsorption efficiency of activated sludge EPS on heavy metal chromium,and to investigate the influencing factors such as adsorption time and occurrence form of heavy metals.At the same time,according to tightness degree,EPS was divided into loosely-bound EPS?LB-EPS?,tightly-bound EPS?TB-EPS?and soluble bound EPS?S-EPS?to explore the changes of EPS components and the distribution of chromium ions under?exposure to?different chromium concentrations.Finally,this study explored characteristics of heavy metal chromium in the EPS stratified components and microbial metabolites of activated sludge and effluent in the biological treatment process of tannery wastewater.The main research results are as follows:?1?As the initial concentration of chromium ions increase,the adsorption amount of chromium ion by aerobic activated sludge EPS increased,and the adsorption effect was obvious below 15 min.Heavy metal chromium has significant effect on the content of polysaccharide in EPS.The distribution of chromium in the EPS bonding layer is higher than that in mucous layer.Fluorescence spectrum showed that Cr???and the humic acid,fulvic acid and protein substances interact in the bonding layer of EPS.?2?During the biological treatment of tannery wastewater,the contents of EPS components in each structural layer alternately changes between proteins and polysaccharides,and the content of EPS components in the bonding layer was 2.248?16.66 mg·g-1.There was a strong similarity between SMP and S-EPS in activated sludge.The contents of polysaccharide and protein of EPS decreased in the primary treatment,which ranging from2.248 to 3.438 mg·g-1.The content of EPS protein was significantly increased in the secondary treatment,the content of polysaccharides in SMP increased by 94%,TB-EPS increased by 93%.The results showed that there was a process of extracellular polymer substances from the inside to the outside in biological treatment.?3?According to Pearson correlation analysis,the chromium distribution was positively correlated with PS/PN in EPS?P<0.05?and positively correlated with PS?P<0.01?.There was a significant positive correlation between DOC and PN?P<0.01?.This indicated that the contents of different components in EPS had different binding capacities to Cr,and confirmed that PS/PN played a key role in the complexation of chromium.?4?Combined with Zeta potential,it can be seen that the negative charge of SMP in the effluent water is significantly reduced,and the Zeta potential of each structural layer of EPS is significantly increased.The results of spectral detection showed that the fluorescence peak of TB-EPS changed little,which may be due to the small influence of external water quality fluctuations.Aromatic proteins and tryptophan/tyrosine substances in S-EPS and LB-EPS were continuously degraded with biological treatment,and the accumulation of furic acid and humic acid showed that biological treatment could effectively degrade organic macro-molecular substances in tannery wastewater,while humic acid and low molecular substances were not easily biodegraded and accumulated. |
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