Autotrophic Denitrification-based Processes For Nitrate Removalfromwater With Low Organic Carbon

Nitrate?NO₃^-?is one of the most common contaminants in aquatic ecosystem.With the development of society and acceleration of urbanization,NO₃^-pollution has become a serious environmental problem in surface water and groundwater systems.In order to make the biological denitrification method more effective for NO₃^-removal in water with low organic carbon,and based on the studies of autotrophic denitrification technology,this study developed biofilm-electrode autotrophic/ heterotrophic denitrification and particulate pyrite autotrophic denitrification methods,respectively,to treat NO₃^-contaminantin the low organic carbon water of groundwater and nitrified domestic wastewater,which provided the new ways for biological denitrification of the low organic carbon water.The developed biofilm-electrode autotrophic/heterotrophic denitrification reactor?BER-AHD?was run at fixed and varying C/N to determine the optimum required current density.Central Composite Design?CCD?and Response Surface Methodology?RSM?were used to determine the optimum C/N and current densities of 1.13 and 239.6 mA/m2,respectively.The NO₃^-removal efficiency was 100% at the optimum conditions.Through the maximum possible number?MPN?method,the content of autotrophic and heterotrophic denitrifying bacteria was determined to be 2.0×103 and 2.0×105/mL,respectively.Using the analysis of dissolved CO2 and COD,autotrophic and heterotrophic denitrifying bacteria were determined to be synergistic,that is,the produced CO2 by heterotrophic denitrification could be used as the inorganic carbon source in the process of autotrophic denitrification,and the utilization of CO2 further promoted the heterotrophic denitrification.Compared to the traditional electrode biofilm technology,the developed method not only improved the removal rate and reduced secondary pollution,but also effectively improved the utilization of hydrogen and reduced energy consumption.Due to the anode corrosion and NH4+-N accumulation in the effluent of BER-AHD,the process is suitable forNO₃^-treatment in low organic carbon groundwater,but not for NH4+residual nitrified domestic wastewater.Hence,about the nitrified domestic wastewater treatment,the particulate pyrite autotrophic denitrification?PPAD?was determined to be an effective method in the microorganism acclimation study.The denitrification rate was 0.86 mg/?L·h?,which was lower than that of 1.19 mg/?L·h?in elemental sulfur oxidizing denitrification?SOD?.The up-flow bed packed reactor was developed to compare the PPAD and SOD performances.When empty bed contact time?EBCT?was 2.9 h,the average NO₃^--N removal efficiencies were 39.7% and 99.9%,respectively.In this study,although the denitrification rate of PPAD column was lower,the alkalinity consumption and by-product production were much lower than that of SOD.Furthermore,the performance of PPAD was optimized through batch and column experiments to treat nitrified domestic wastewater.The results showed that the denitrification rate constant kin the microcosms was the highest?0.473 d-1?at the pyrite dosage of 125 g/L,biomass concentration of 1,250 mg VSS/L and pyrite particle size of 0.815-1.015 mm.The up-flow bed packed reactor?column experiment?was used to investigate the PPAD for nitrified domestic wastewater treatment.The results showed that total inorganic nitrogen?TIN?removal efficiency was 89.7% in the column with pyrite particles,quartz sands and oyster shells?P+S+OS?and only 70.1% in the column with pyrite particles and quartz sands?P+S?when the EBCT was 5.8 h,showing that the addition of oyster shells was beneficial to improve PPAD performance.Through the analysis of by-products,the mixotrophic denitrification was determined to occur in the P+S+OS column,which was the main reason for the increase inTIN removal efficiency.In summary,this study has developed BER-AHD and PPAD technologies that were suitable for the NO₃^-treatment inthe waterwith low organic carbon and give the new idea of mixotrophic methods based on the autotrophic denitrification to treat NO₃^-contaminant in different polluted water.