Synergizing Thiosulfate,zero-valent Iron And Carbon Source To Achieve Mixotrophic Advanced Denitrification For Secondary Effluent

In recent years,the eutrophication of water body is becoming more and more serious,and the increasing nitrate concentration in surface rivers and reservoirs is one of the main factors.Due to the limitation of sufficient aerobic nitrification and reflux ratio in the traditional AO process,most of the NO₃^--N in natural water comes from the secondary effluent of municipal sewage treatment plants,and there is still a risk of eutrophication when discharged into the water body.The high cost of additional organic carbon source is a common problem when heterotrophic denitrification is used in secondary effluent with low C/N ratio.In this paper,an integrated thiosulfate-and Fe~0-driven mixotrophic denitrification was constructed.Firstly,the feasibility of integrated thiosulfate-and Fe~0-driven mixotrophic denitrification was confirmed by using simulated AO effluent in the laboratory.Secondly,the effect of different C/N ratios on mixotrophic denitrification efficiency and microbial utilization of soluble microbial products（SMP）with actual secondary effluent were investigated in the sequencing batch reactor,and the effects of different S/N and C/N ratios on mixotrophic denitrification efficiency and SO₄^2-production with actual secondary effluent were explored in the continuous flow reactor.Then,the contribution of autotrophic and heterotrophic denitrification was clarified based on the material transformation study in a single batch.Finally,the mechanism of integrated thiosulfate-and Fe~0-driven mixotrophic denitrification with secondary effluent was explained through the study of p H,DO,Fe~0 reaction behavior,mixed liquid suspended solids（MLSS）and mixed liquor volatile suspended solids（MLVSS）,extracellular polymeric substances（EPS）,electron transfer system activity（ETSA）,microbial communities and denitrification functional genes.The main results are as follows:（1）Feasibility of integrated thiosulfate-and Fe~0-driven mixotrophic denitrification with simulated AO effluent in the laboratoryUnder different C/N ratios,the removal efficiency of TN and NO3^--N in the integrated thiosulfate-and Fe~0-driven mixotrophic denitrification system were higher than those in the thiosulfate-driven mixotrophic denitrification system.When the C/N ratio increased to 1.9-2.1,the maximum removal efficiency of TN and NO₃^--N with integrated thiosulfate-and Fe~0-driven mixotrophic denitrification reached 93.20%and 97.05%,respectively,and were less affected by water temperature.The contribution ratio of thiosulfate-driven autotrophic and heterotrophic denitrification in TN removal was about 7:5.The variations of p H,DO,MLSS and MLVSS showed that Fe~0 could balance p H,stabilize DO and promote microbial biomass in the integrated thiosulfate-and Fe~0-driven mixotrophic denitrification system.The analysis of microbial communities showed that the addition of Fe~0 could promote the growth and enrichment of Thiobacillus,Plasticicummulans and Terrimonas in the thiosulfate-driven mixotrophic denitrification system.（2）Denitrification performance and mechanism in sequencing batch reactor of integrated thiosulfate-and Fe~0-driven mixotrophic denitrification with actual secondary effluentUnder different C/N ratios,the removal efficiency of TN and NO₃^--N in the integrated thiosulfate-and Fe~0-driven mixotrophic denitrification system were significantly higher than those in the thiosulfate-driven mixotrophic denitrification system.When the C/N ratio was increased to2.9-3.1,the maximum removal efficiency of TN and NO3^--N in the sequencing batch reactor with integrated thiosulfate-and Fe~0-driven mixotrophic denitrification were up to 80.62%and 80.87%,respectively.Under the condition of low C/N ratio（C/N<3.1）,the simultaneous addition of thiosulfate and Fe~0 can effectively promote the utilization of humic acids in SMP by microorganisms.Under different C/N ratios,TN removal was dominated by thiosulfate-driven autotrophic denitrification,supplemented by heterotrophic denitrification.With the increase of C/N ratio,the contribution ratio of thiosulfate-driven autotrophic denitrification was relatively reduced,while the contribution ratio of heterotrophic denitrification was relatively increased.The variations of DO,p H,Fe~0 reaction behavior,MLSS,MLVSS,EPS and ETSA showed that the low DO environment and the acid-producing characteristics of thiosulfate-driven autotrophic denitrification make it difficult for the Fe~0 surface to be covered by Fe（OH）₃ or Fe O（OH）in the integrated thiosulfate-and Fe~0-driven mixotrophic denitrification system,so that Fe~0 can be continuously corroded and play a role in balancing the p H,stabilizing DO and promoting the increase of microbial biomass.The simultaneous addition of thiosulfate and Fe~0 can promoted the synthesis of polysaccharide（PS）and iron-sulfur protein,which increased the content of EPS and ETSA.The content of EPS and ETSA increased with the increase of C/N ratio,while the value of protein/polysaccharide（PN/PS）in EPS decreased.The evolution of microbial communities and functional genes showed that the addition of Fe~0 could significantly increase the relative abundance of Thiobacillus and denitrifying functional genes（nar G,nir S,nir K,cnor B,qnor B,nos Z）in the thiosulfate-driven mixotrophic denitrification system,and with the increase of C/N ratio,the relative abundance of Thiobacillus decreased and Rhodanobacter increased,and the relative abundance of denitrification functional genes also increased.（3）Denitrification performance and mechanism in continuous flow reactor of integrated thiosulfate-and Fe~0-driven mixotrophic denitrification with actual secondary effluentIncreasing the S/N and C/N ratios could effectively enhance the denitrification effect with integrated thiosulfate-and Fe~0-driven mixotrophic denitrification.The addition of Fe~0 in the thiosulfate-driven mixotrophic denitrification system can effectively reduce the accumulation of NO2^--N and improve the denitrification efficiency.When the S/N and C/N ratios were 0.8-1.0 and2.9-3.1,respectively,the maximum removal efficiency of TN and NO3^--N were 87.23%and 87.74%,respectively.The production of SO4^2-increased with the increase of S/N ratio and decreased with the increase of C/N ratio in the integrated thiosulfate-and Fe~0-driven mixotrophic denitrification system,and the addition of Fe~0 did not effectively reduce the concentration of SO4^2-in the effluent.The variations of p H,DO,EPS and ETSA showed that p H and DO both decreased with the increase of S/N ratio,while p H increased and DO decreased with the increase of C/N ratio.The addition of Fe~0 in the thiosulfate-driven mixotrophic denitrification system resulted in higher p H and lower DO.The content of EPS and ETSA increased with the increase of S/N ratio,C/N ratio and TN removal efficiency,while the PN/PS value of EPS was positively correlated with S/N ratio and negatively correlated with C/N ratio.In the thiosulfate-driven mixotrophic denitrification system,the addition of Fe~0 resulted in the decrease of EPS content due to the complexation of iron ions with carboxyl groups in EPS,the increase of ETSA due to the participation of iron in the synthesis of iron-sulfur protein,and the increase of PN/PS value made the biofilm more stable.The evolution of microbial communities and functional genes showed that the addition of Fe~0 could further increased the relative abundance of Thiobacillus,KD4-96,A4b and denitrifying functional genes（nar G,nir S,nir K,cnor B,qnor B,nos Z）in the thiosulfate-driven mixotrophic denitrification system.The growth of Thiobacillus and KD4-96 could be promoted by increasing the S/N and C/N ratios,respectively,and the relative abundance of denitrification functional genes could be effectively increased by increasing the S/N and C/N ratios.