Performance Of Nitrogen And Antibiotics Removal From The Effluent Of Wastewater Treatment Plants By Biomass-iron Mixture Denitrification Biological Filter

The regeneration and utilization of the effluent of wastewater treatment plants（WWTPs）have been paid more and more attention.However,the high nitrogen content and emerging contaminants（ECs）represented by antibiotics are the bottlenecks that limit the recycling of secondary effluent.On the basis of our previous research,a filter material based on mixotrophic denitrification mode（heterotrophic-iron-driven autotrophic denitrification）was developed with the biomass-iron mixture（BIM）as raw material,and the biological denitrification filter（BIM-DNBF）filled with BIM filter material was constructed.The denitrification performance and characteristics of BIM-DNBF without the addition of organic carbon sources were investigated.Meanwhile,the removal of trace-level antibiotics by BIM-DNBF and the influence of antibiotics on denitrification were explored.The present work will hopefully assist in providing the theory and guidance for the polishing and recycling of the effluent of WWTPs.The main results of this research are as follow:（1）Two kinds of BIM filter materials(BIM_bs and BIM_rs)were prepared by two cheap carbon source materials with iron-based materials.Through the evaluation of the denitrification performance of two BIM filter materials,it was found that the denitrification efficiency of two BIM filter materials was similar(the total nitrogen removal efficiency of BIM_bs and BIM_rs were 83.00%～84.84%and 85.37%～89.67%).By comparing the denitrification rate of two filter materials,it was found that the denitrification rate of the BIM_bs filter material was higher.The total nitrogen removal efficiency of BIM_bs filter material reached 84.08%±1.62%at 300 min,while that of BIM_rs filter material was 57.25%±2.35%,so BIM_bs was selected as the BIM filter material used in subsequent research.By analyzing the characteristics of surface structure of BIM_bs filter material,it was found that the filter material has a large specific surface area,rich pore structure,and numerous oxygen-containing functional groups（-OH,C=O,O-C=O,etc.）,which can promote the formation and proliferation of biofilms.The elemental analysis of BIM_bsfilter material showed that the carbon in BIM_bs filter material accounted for 18.648%and it dropped to 13.350%after 7 months（the decrease is less than 30%）,so BIM_bsfilter material has a longer service life.In addition,although the amount of organic matter dissolved from BIM_bs filter material was relatively high during the first 5 days of the dissolution experiment and then gradually decreased to 0.79～1.35 mg·g^-1·d^-1,higher organic carbon can promote the proliferation of biofilms.In practical applications,the influence of organic matter on the quality of the initial effluent can be reduced by refluxing the effluent.（2）BIM-DNBF was constructed with BIM_bs filter material,and the start-up of BIM-DNBF needs about 9 days（total nitrogen removal rate is higher than 70%）.The research on the denitrification performance of BIM-DNBF under different hydraulic load and influent nitrate concentration showed that when the hydraulic load was 7.5m³·m^-3·d^-1 and inlet nitrate concentration was 15 mg/L,BIM-DNBF has the best denitrification performance（the total nitrogen removal efficiency of BIM-DNBF is81.47%～89.41%）,and the concentrations of NH₄⁺-N and COD in the effluent of BIM-DNBF were 0.29～0.48 mg/L and 27.13～29.23 mg/L,respectively.The results of high-throughput sequencing showed that the dominant bacterial in BIM-DNBF included heterotrophic denitrification bacteria（mainly Pseudomonas、Thauera and Erysipelothrix）and iron-driven autotrophic denitrification bacteria（mainly Brevundimonas and Acidovorax）.Nitrogen metabolism pathways in BIM-DNBF based on the PICRUSt function prediction indicated that the relative abundance of denitrifying functional genes（such as nar GHI,nap AB,nos Z）was high.Enzymes（Nar and Nap）corresponding to nar GHI and nap AB functional genes play a key role in iron-driven autotrophic denitrification.Although bacteria cannot gain energy from the coupling of ferrous oxidation and nitrate reduction catalyzed by Nap,the organic carbon provided by the BIM filter material not only provides electrons for heterotrophic denitrification,but also acts as a"co-substrate"to supplement the energy required for the growth of iron-driven autotrophic denitrification bacteria,thereby promoting the coupling of heterotrophic and iron-driven autotrophic denitrification in BIM-DNBF.（3）Research on the removal efficiency of BIM-DNBF on antibiotics and nitrogen under influence of antibiotics with different concentrations showed that the reduction effect of BIM-DNBF on OFC（Ofloxacin）was better than that on SMZ（Sulfamethazine）.The highest removal efficiency of BIM-DNBF on SMZ and OFC was 40.49%～41.61%（inlet SMZ concentration is 50μg/L）and 89.41%～89.98%（inlet OFC concentration is 30μg/L）,respectively.In addition,it was found that antibiotics with a concentration higher than 50μg/L can inhibit the denitrification,but the inhibitory intensity of 30～150μg/L antibiotics on the denitrification was small（the decrease in total nitrogen removal efficiency was less than 20%）.Based on the analysis of high-throughput sequencing,the decline of richness and diversity of microorganism communities under the influence of antibiotics was found.The main phyla and genus involved in the degradation of antibiotics may be Bacteroidot and unclassified＿f＿＿Comamonadaceae（belonging to the Comamonas family）,respectively.Under the influence of antibiotics,the relative abundance of the main heterotrophic denitrification bacteria（Pseudomonas and Erysipelothrix）and iron-driven autotrophic denitrification bacteria（Brevundimonas and Acidovorax）decreased significantly,which adversely affected the mixotrophic denitrification and led to the decrease of the efficiency of BIM-DNBF（<20%）.The analysis of the nitrogen metabolism pathway showed that antibiotics reduced the relative abundance of the nos Z genes and the input of OFC also inhibited the coupling of ferrous oxide nitrate reduction catalyzed by Nap in periplasmic space,thereby affecting the efficiency of the coupling of heterotrophic and iron-driven autotrophic denitrification in BIM-DNBF.