Performance Of Simultaneous Nitrate And Sulfide Removal Process Based On High Elemental Sulfur Yield

It will pose a serious threat on ecological safety and human health by disorderly discharging wastewaters which contain high-concentration nitrate and sulfide.Therefore,looking for green and efficient treatment technologies for such wastewater has become the focus of environmental protection in recent years.The simultaneous nitrate and sulfide removal process can effectively remove sulfide and nitrate from wastewater,and obtain elemental sulfur,which could realize waste treatment and resource recovery.However,it needs to be studied that how to realize high yield of elemental sulfur,how to quantify the amount of elemental sulfur accurately and what the characteristics of elemental sulfur is in the novel process.Therefore,this study compared the performance of simultaneous nitrate and sulfide removal process with abiotic assay,in order to determine the proportion of elemental sulfur produced by biological reactions;The influence of substrate concentration and substrate ratio were investigated on the facets of substrate removal performance,product conversion performance and elemental sulfur yield.High-throughput sequencing technology was used to analyze the microbial community and key functional microbial populations in the system.Moreover,the characteristics of elemental sulfur in the effluent and sludge of the process was investigated.The morphology,distribution and chemical characteristics of elemental sulfur in the system were analyzed,which can provide a reference for the separation and recovery of elemental sulfur in the simultaneous nitrate and sulfide process.The main conclusions are as follows:1)Comparing the performances of simultaneous nitrate and sulfide removal process with abiotic assay,the role of microorganisms in the production of elemental sulfur was explored.Within the tested concentration range（60～300 mg S/L）and fixed hydraulic retention time（24h）,the abiotic process did not have the function of nitrate removal,and the sulfide removal percentage was only 21.53～25.83%.The main product of sulfide oxidation was thiosulfate（10.13～12.55%）,the second was elemental sulfur（2.59～7.49%）,and the third was sulfate（1.08～3.77%）.In the simultaneous nitrate and sulfide removal process,the removal percentages of nitrate and sulfide were 74.93～95.43%and 90.67～96.88%,respectively.The main nitrogen-containing product of the process was nitrogen（82.00～105.67%）.The main product of sulfide oxidation was sulfate（45.18～75.36%）,followed by elemental sulfur（19.21～48.32%）and thiosulfate（0.0～16.29%）.The contribution of biological reactions to nitrate reduction,sulfide oxidation and elemental sulfur production were 74.93～95.42%,64.12～75.35%and 14.48～29.99%,respectively.2)The influence of substrate concentration on the performance of simultaneous nitrate and sulfide removal process was revealed,as well as the characteristics of microbial community.Within the tested concentration range（120～720 mg S/L）and fixed hydraulic retention time（20h）,the optimum concentration of sulfide and nitrate was 480 mg S/L and 84 mg N/L,respectively,considering the substrate removal and products conversion;while the removal percentages of sulfide and nitrate reached 95.93%and 95.49%,respectively.When the influent sulfide concentration was in the range of 360 and 600 mg S/L,the main products were elemental sulfur and nitrogen,and their yields reached 78.5%and 115.3%,respectively.According to Pearson correlation analysis and principal component analysis,it was beneficial for the conversion of the final product from sulfate to elemental sulfur by increasing the influent substrate concentration（p<0.05）,furthermore,the amount of effluent elemental sulfur was positively correlated with the substrate concentration,which accounted for 76.64～88.32%of the total elemental sulfur production.High-throughput sequencing was used to analyze the microbial community at phylum and genus levels,which the relative abundance was dominant in Campilobacterota（>80.26%）and Sulfurovum（>79.46%）.3)The influence of substrate ratio on the performance of simultaneous nitrate and sulfide removal process was revealed,as well as the characteristics of microbial community.Taking substrate removal performance and high elemental sulfur conversion into account,the optimal substrate ratio for simultaneous nitrate and sulfide removal process was 5:2 within the tested substrate ratio range（S/N=5:2、5:1.5 and 5:1）and fixed hydraulic retention time（20h）.When the substrate concentration was low（the influent sulfide concentration was 120～240 mg S/L）,there was only a significant difference of sulfate production in the simultaneous nitrate and sulfide removal process at different substrate ratios（p<0.05）,while at high substrate concentration（the influent sulfide concentration was 480～720 mg S/L）,there were significant differences in nitrogen removal performance（effluent nitrate concentration,nitrite production,and nitrogen production）and sulfur removal performance（effluent sulfide concentration,total element yield）with different substrate ratios（p<0.05）.Through high-throughput sequencing,the microbial communities were analyzed at the taxonomic level of phylum and genus,and the relative abundance was dominant in Campilobacterota（>79.83%）and Sulfurovum（>79%）4)The characteristics of elemental sulfur produced by the simultaneous nitrate and sulfide process had been proved.Elemental sulfur was mostly spherical particles or irregular shapes with non-smooth surfaces.The main elements of elemental sulfur in the effluent were S（53.32%）,C（25.52%）,and O（5.05%）,while the main elements of elemental sulfur in the sludge were S（41.79%）,C（28.52%）,and O（7.94%）.Elemental sulfur mainly existed in the form of orthorhombic sulfur（S₈）,and the particle size of the elemental sulfur was normally distributed in the effluent.When the influent sulfide concentration was 120～480 mg S/L,the average particle size of elemental sulfur particles was 23.18～28.8μm;while the influent concentration rises to 720 mg S/L,the average particle size reduced to 7.52μm.Within the tested concentration range,the average Zeta potential ranged from-15.7 m V to-18.92 m V,indicating that the biological elemental sulfur particles were negatively charged.