Research On Removal Performance Of Chromate By Biofilm Under Solid Phase Denitrification Condition

In recent years, the co-existence of different oxidized contaminants such as Cr⁶⁺ and NO₃^--N in natural water or wastewater is a common and growing problem. Although some physicochemical treatment methods, such as electrodialysis, reverse osmosis, membrane filtration and ion exchange are effective for the simultaneous removal of Cr⁶⁺ and NO₃^--N, the high-cost and the generated concentrated wastes that require subsequent disposal are still problems that need to be solved. Traditional bio-reduction could greatly improve the removal rates of Cr⁶⁺ and NO₃^--N, while there were some other problems, such as the deterioration of effluent water quality caused by the difficult control of soluble carbon source addition and the difficult immobilization of generated Cr3+ in the presence of cellular secretion and small molecular organics. To solve the above problems, study on the simultaneous removal of Cr⁶⁺ and NO₃^--N and immobilization of the generated Cr3+ using biodegradable meal box ?BMB? as carbon source and biofilm carriers was performed.In this study, the domestication of soil which was collected from northeast farmland of China and biofilm formation of BMB were performed firstly. After two months of acclimation, the denitrification sludge was mature and biofilm was formed. Effects of different factors, such as inoculation, DO, pH, NO₃^--N concentrations, NOX and additive amounts of BMB, on the denitrification performance of BMB were subsequently investigated. Research further showed that the denitrification rate of the system could reach the maximum value of 1.72 mg/?L·h? in the presence of the blend of fresh BMB and active sludge as inoculation,20 min N2 aeration, pH 7.0,100 mg/L NO₃^--N and 5 g BMB. In addition, the releasing performance of organic matters from BMB and recycling and reusing performance of BMB confirmed the feasibility of BMB serving as solid carbon source and biofilm carriers. The mixture of BMB attached with biofilm and active sludge was added to the reactor to study the effects of hydraulic loading rates and NO₃^--N loading rates under continuous condition on denitrification performance of the system. Research showed that the best denitrification performance and low concentrations of effluent organic matters were achieved with 0.55 m3?m2·d? of hydraulic loading rate and 0.14 kg/?m3·d? of NO₃^--N loading rate. Furthermore, scanning electron microscopy and fourier transform infrared spectroscopy also confirmed the feasibility of BMB serving as solid carbon source and biofilm carriers.To further study the simultaneous removal performance of Cr⁶⁺ and NO₃^--N, the effects of chromium loading rates under continuous condition on denitrification performance and the effects of NO₃^--N loading rates on Cr⁶⁺ removal performance were subsequently conducted. Results indicated that when Cr⁶⁺ concentration was lower than 20 mg/L, nitrogen removal rate could reach 100%, whereas further increase of Cr⁶⁺ concentration could heavily inhibit the denitrification performance; in addition, under denitrification condition, the high effective reduction of Cr⁶⁺ and efficient immobilization of Cr3+ were achieved when Cr⁶⁺ concentration was less than 20 mg/L, while the majority of generated Cr3+ remained in wastewater with higher influent Cr⁶⁺ concentrations. Then the mutual interaction of Cr⁶⁺ and NO₃^--N was investigated. Results indicated that NO₃^--N concentrations had little effects on Cr⁶⁺ removal, while obvious inhibition on NO₃^--N removal rates was observed with the increase of Cr⁶⁺ concentration. Cr distribution between solid and liquid phase demonstrated that reduction and immobilization of Cr⁶⁺ were mainly occurred in the biofilm,while intercellular compartment in the biofilm played an important role in the complexation with extracellular polymeric substances. Scanning electron microscopy-energy dispersive spectrometer and transmission electron microscopy-energy dispersive spectrometer further confirmed the Cr distribution in apparent. Besides, microbial community analysis showed that Cr⁶⁺ concentrations increased with decreasing the variety and abundance of the major of microbial community.