Performance Of Enhanced Nitrogen And Phosphorus Removal In Sulfur Autotrophic Denitrification By Fe²⁺/Fe⁰

The water quality standard of secondary effluent from urban sewage treatment plants is increasingly strict,which puts forward higher requirements for the advanced nitrogen and phosphorus removal process of secondary effluent.Sulfur autotrophic denitrification has attracted wide attention because it does not need additional organic carbon source,and has good denitrification effect and low cost.However,its application is limited due to high alkalinity consumption,high by-product SO₄^2-production and inability to remove phosphorus simultaneously.Based on the theory of sulfur autotrophic denitrification,this thesis added Fe²⁺and Fe⁰ respectively to enhance the denitrification and phosphorus removal effect of sulfur autotrophic reactor,and at the same time alleviated the shortcoming of high sulfate concentration in effluent during SAD process to some extent.The experiment was divided into two stages.In the first stage,two reactors B1（S⁰）and B2（S⁰+Fe⁰）were built.The effects of HRT and different influent NO₃^-N concentrations on the reaction system were investigated,and the removal effects of N and P in the two reactors were compared.In the second stage,Fe²⁺was added to B1（S⁰）reactor to explore the enhanced effects of different influent Fe²⁺concentrations on nitrogen and phosphorus removal in sulfur autotrophic reactor,and the long-term operation of the two reactors was compared.The main research results are as follows:（1）The optimal HRT of B1 and B2 is 6 h.Under the same conditions,the nitrogen removal capacity of the two reactors is similar.The addition of sponge iron in B2 has no obvious enhancement effect on the nitrogen removal effect in the reaction system,and the whole HRT stage is dominated by sulfur denitrification,with the effect ratio greater than 60%.In the HRT stage,the B1 reactor did not show the effect of phosphorus removal,while the sponge iron in B2 enhanced phosphorus removal by releasing iron ions,and always showed high-efficiency phosphorus removal characteristics,almost completely removing PO₄^3--P in the influent,and the synergistic phosphorus removal performance of the reactor was B2>B1.NH₄⁺-N and NO₂^-N accumulated in different degrees in the two reactors,and the effluent content gradually decreased with the shortening of HRT.The SO₄^2-in the effluent of B2 is lower than that of B1 on the whole,and the addition of sponge iron alleviates the disadvantage of excessive SO₄^2-in autotrophic denitrification.（2）When the HRT is controlled for 6 hours,the concentration of NO₃^--N in influent is increased from 20 mg/L to 100 mg/L,the NO₃^--N removal rate of sulfur autotrophic reactor and sulfur-iron composite reactor is reduced from 99%to 66.54%and 81.01%respectively,and the maximum NO₃^--N removal load of the reactor is divided into 0.243kg/（m³?d）and sulfur-iron composite reactor.The content of PO₄^3--P in the effluent of sulfur-iron composite system is always lower than 0.2mg/L,and the effluent SO₄^2-is still lower than that of sulfur autotrophic reactor.（3）The influent NO₃^--N is controlled at 80mg/L,and the influent Fe²⁺concentration of B1 sulfur autotrophic reactor is continuously increased.The optimal influent Fe²⁺concentration of the reactor is 90 mg/L.At this time,the maximum NO₃^--N removal load of the system is 0.287 kg/（m³?d）,which is 0.044 kg/（m³?d）higher than that without Fe²⁺,and the effluent PO₄^3--P.Under the same operating conditions,the NO₃^--N removal efficiency of B2 sulfur-iron composite reactor is always higher than that of B1 reactor,and the enhanced denitrification effect of Fe²⁺is better than that of Fe²⁺,and the ratio of zero-valent iron autotrophic denitrification is higher than that of bivalent iron autotrophic denitrification,so Fe²⁺is easier to use as an electron donor.The SO₄^2-content in the effluent of sulfur-iron composite system is lower than that of B1,and the SO₄^2-content in the effluent of B1 is lower than that of sulfur autotrophic reactor under the same conditions.（4）High-throughput sequencing showed that Proteobacteria was always the dominant bacteria in the two reaction systems,and Thiobacillus ferrooxidans was the main functional bacteria.At the end of HRT stage,the abundance of Thiobacillus in B1and B2 was 13.03%and 4.38%respectively,and the abundance of ferrooxidans was3.24%and 13.52%respectively.After the Fe²⁺stage,the richness and diversity of communities in B1 and B2 decreased.The dominant bacteria,Thiobacillus and Ferritrophicum,accounted for 7.54%,17.02%,58.34%and 4.12%in B1 and B2respectively.Compared with HRT stage,the addition of ferrous iron in B1 system resulted in the gradual enrichment of iron-oxidizing bacteria,which greatly increased its abundance and enhanced its phosphorus removal performance.B2 Due to the increase of nitrogen load in influent,the growth of sulfur autotrophic bacteria was stimulated,and the proportion of Thiobacillus increased.With the extension of operation time,the release of iron ions decreased,and the proportion of iron-oxidizing bacteria in the sample decreased relatively.