Performance And Mechanism Of Autotrophic Nitrogen Removal From Digested Effluent Of Swine Wastewater Mediated By Zero-valent Iron(ZVI)

The digested swine wastewater contains a high concentration of ammonia and low concentration of organic carbon.In the region where digested effluent cannot be returned to the farmland,it needs to be treated further for nitrogen removal.The autotrophic nitrogen removal mediated with zero-valent iron（ZVI）has broad prospects,but its influencing factor and mechanism is still unclear.This paper studied the factors of autotrophic nitrogen removal mediated with ZVI treating digested effluent of swine wastewater and explored the microbial mechanism.The main research results are as follows:（1）The effect of ZVI addition（0,5,10,20,30,40 g/L）on autotrophic nitrogen removal system mediated with ZVI were studied.The results showed when the amount of ZVI added was less than 40g/L,ZVI could improve the nitrogen removal performance.The nitrogen effect was the best in the reactors with 30 g/L ZVI added,which the removal efficiency of NH₄⁺-N and TIN are 69.68%and50.12%respectively.Excessive ZVI had a toxic effect on microorganisms and deteriorated the systems’operating performance.ZVI could promote the enrichment of Anammox bacteria（Candidatus Brocadia）and Feammox bacteria（Ignavibacterium）and change the nitrogen removal routes of the systems.The main nitrogen removal route in the reactor without ZVI addition was traditional nitrification-heterotrophic denitrification.However the nitrogen removal routes in the reactor with ZVI addition were by ammonia oxidation,Anammox and Feammox with ZVI addition.（2）The effect of inorganic carbon source（0,0.5,1,2 g of KHCO₃,systems were namend R0,R0.5,R1,R2,respectively）on autotrophic nitrogen removal system mediated with ZVI were studied.The results showed that KHCO₃ can significantly increased the removal efficiency of NH₄⁺-N.Without the addition of ZVI,the removal efficiency of NH₄⁺-N of R2,R1,R0.5 were 98.39%,65.18%,and 44.56%,which increased by 86.5%,53.29%,and 32.67%respectively,compared with R0.After adding ZVI,the NH₄⁺-N removal efficiency of R2,R1,R0.5 was increased by 73.03%,47.07%and 24.02%respectively,compared to R0.The addition of ZVI could reduce the promotion of KHCO₃ on NH₄⁺-N oxidation.KHCO₃ could obviously improve the removal efficiency of TIN.The average TIN removal efficiency of R2,R1,R0.5 were 19.01%,32.04%,27.62%,which increased by 10.60%,23.63%,and 19.21%compared with R0.KHCO₃ could buffer the reaction system and promote the activity of ammonia-oxidizing bacteria.When the addition dosage was less than 2 g,the larger the addition amount of KHCO₃,the higher the AOM activity.The removal of TIN in the reactors were the results of the combined action of Anammox and heterotrophic denitrification（DN）.（3）The effect of organic carbon source on autotrophic nitrogen removal system mediated with ZVI were discussed.The results showed that the addition of organic carbon source（sodium acetate,influent C/N ratio=2）could significantly increased the NH₄⁺-N and TIN removal.The average NH₄⁺-N and TIN removal efficnecy of the treatment group were 91.09%and 84.1%,respectively,which increased by 60.06%and 75%compared with the control group（C/N ratio<0.1）.Adding organic carbon source could improve ammonia oxidation activity,ANAMMOX activity and denitrification activity.The main ways of nitrogen removal were DN and ANAMMOX.Adding an organic carbon source can also stimulate microbes to secrete extracellular polymers,promoting the flocculation of sludge,and hinder the attachment of iron oxide on the cell surface,thereby reducing cell surface passivation,mitigating the toxicity of iron to microbial cells and keeping them high activity.（4）The autotrophic nitrogen removal system mediated with zero-valent iron for the treatment of digested effluent of swine wastwatwer was conducted.The results showed that the average NH₄⁺-N removal efficincey reached 98%,but the TIN removal efficincy was only 43.7%.The removal efficincy of NH₄⁺-N and TIN were higher than that in the control group（simulated wastewater without organic carbon）,but lower than the treatment effect in the tests treating simulated nitrogen-containing wastewater which added organic carbon source.It is speculated that the actual digested effluent of swine wastewater had complex components and the existed inhibitors such as antibiotics and heavy metals.