Experimental Study On Treatment Of Wastewater Treatment Plants Effluent By External Inorganic Electron Donor Enhanced Denitrification Biofilter

The advanced denitrification of wastewater treatment plants（WWTPs）effluent plays an important role in the effectiveness of water environmental treatment.At present,biological denitrification is still the main way of advanced denitrification in the WWTPs effluent.However,the content of bioavailable organic matter in the WWTPs effluent is low,resulting in insufficient electron in the process of biological denitrification and low denitrification rate.In this paper,enhanced denitrification biofilter（DNBFs）was used to treat the WWTPs effluent with a low C/N ratio,which improved the advanced denitrification efficiency,and was of great significance to improve the environmental quality of receiving water and avoid water eutrophication.The influences of the types and combinations of external electron donors on pollutant removal were explored,the enhanced denitrification mechanism was analyzed based on the research of the dynamics,reaction rate,key enzyme activities,electronic transport system activity（ETSA）,microbial community structure and other aspects,which provided a theoretical basis for advanced denitrification of the WWTPs effluent with a low C/N ratio.The main conclusions are as follows:（1）Ceramsites with average particle sizes of 2-4 mm,4-12 mm and 12-20 mm were selected as the medium of DNBFs respectively,the influences of ceramsite particle size and hydraulic retention time（HRT）（1.5 h,3 h and 6 h）on pollutant removal were studied.The results showed that with the increase of average particle size,the average removal rates of total nitrogen decreased from 76.9%to 46.4%and 41.6%,respectively.With the increase of HRT from 1.5 h to 3 h,the denitrification effect of the DNBFs was the best,and the extension of HRT from 3 h to 6 h could effectively alleviate the influence of the decrease of water temperature on the denitrification rate.The DNBFs medium with smaller particle size improved the activity of nitrifying,denitrifying enzymes and ETSA.The dominant denitrifying bacteria in DNBFs were Acinetobacter and Flavobacterium,and the relative abundance of denitrifying bacteria was higher on DNBFs with smaller particle size.（2）Sulfur and pyrite with different mass ratios was selected as the inorganic electron donor for the DNBFs.The influences of mass ratio of sulfur to pyrite（1:0,1:1 and 1:2）and HRT（1.5 h,1 h and 0.5 h）on pollutant removal in the DNBFs were investigated.The results showed that with the decrease in the mass ratio of pyrite,the average removal rates of total nitrogen decreased from 73.4%to 58.7%and 53.0%,respectively,and the production of sulfate increased from 13.61 mg/L to 15.56 and 17.19 mg/L,respectively.With the decrease of HRT from 1.5 h to 1 h,the oxidation and utilization capacity to sulfur and pyrite increased as the microorganisms matured.In addition,with the increase of water temperature,denitrifying bacteria had a higher activity and nitrogen removal efficiency was improved.When the mass ratio of sulfur to pyrite was 1:2 and the HRT was 1 h,the denitrification effect was the best.The average concentrations of COD_Cr,NH₄⁺-N and TN in the effluent met the Class V standard of the Environmental Quality Standards for Surface Water（GB3838-2002）.The mass ratios with higher pyrite improved the activity of nitrifying,denitrifying enzymes and ETSA.The dominant heterotrophic denitrifying bacteria in this experiment were Acinetobacter and Thauera,and the dominant autotrophic denitrifying bacteria were Thermomonas and Thiobacillus.（3）The iron-carbon medium composed of iron scrap and activated carbon was placed in the DNBFs.The influences of iron carbon volume ratio（3:1,2:1 and 1:1）and HRT（1.5 h,1 h and 0.5 h）on pollutant removal were investigated.The results showed that with the increase of the iron carbon volume ratio,the average removal rate of total nitrogen decreased from 77.9%to 61.1%and 60.5%,respectively.Iron-carbon microelectrolysis enhanced denitrification was the result of the combined action of electrode reaction,redox reaction,flocculation precipitation reaction and physical adsorption reaction,which could maintain a high denitrification efficiency for the DNBFs at a low temperature.Therefore,when HRT was 1.5 h（at the beginning of the operation stage）,the denitrification efficiency was the highest with a low water temperature.When the iron carbon volume ratio was 1:1 and the HRT was 1.5 h,the denitrification efficiency was the best.The average concentrations of COD_Cr,NH₄⁺-N,TN and TP in the effluent met the Class V standard of the Environmental Quality Standards for Surface Water（GB3838-2002）.The activity of nitrifying,denitrifying enzymes and ETSA was improved in the DNBFs with a lower iron carbon volume ratio.The dominant heterotrophic denitrifying bacteria in this experiment were Acinetobacter and Thauera,and the dominant autotrophic denitrifying bacteria were Thermomonas,Hydrogenophaga and Thiobacillus.