Iron-mediated Sulfur Autotrophic Denitrification System For Improving Deep Nitrogen Removal From Landfill Leachate

Landfill leachate is a typical of high concentration nitrogen-containing organic wastewater produced in the process of waste domestic landfill,featuring high concentration of ammonia,poor biology,and many undegradable organics etc.,which will cause great damage to surrounding environment if discharged without treatment to meet the emission standards.The partial nitrification and denitrification（PNDN）process is a new technique of nitrogen removal process,which can effectively reduce carbon source addition and aeration compared with the conventional nitrification and denitrification.However,the PNDN process has bottlenecks in nitrogen removal efficiency,and unstable operation leading to the accumulation of nitrite nitrogen in the effluent,which is unable to meet the landfill leachate discharge standards（GB 16889-2008,TN≤40 mg/L and NH₄⁺-N≤25 mg/L）.Therefore,a novel process is needed to search for an advanced nitrogen removal treatment for the effluent of PNDN process to meet the emission standards of landfill leachate.In this study,the iron-mediated sulfur autotrophic denitrification（IS⁰AD）process was used for advanced nitrogen removal of the PNDN process effluent landfill leachate.To evaluate nitrogen and phosphorus removal capacity of the IS⁰AD process by operating the IS⁰AD reactors over a long period and optimizing the operating parameters.Meanwhile,the advanced nitrogen removal mechanisms of the IS⁰AD system were revealed by microbial community structure analysis,material balance analysis,reactivity activity and denitrification kinetics analysis,EPS characteristics of electron transferring active substances and iron ion concentration measurement.Furthermore,the results of the small-scale research were then expanded applied to the pilot-scale to investigate the performance of IS⁰AD system for advanced nitrogen removal treatment of landfill leachate and changes in microbial community structure.The main findings of this study are as follows:（1）In this study,five different ratios of filler made of pure S⁰,pure FeS₂,S⁰:FeS₂=1:1,S⁰:FeS₂=3:1 and S⁰:FeS₂=1:3,with diameters of 10-15 mm.The BET analysis showed that the specific surface area of the FeS₂-S⁰composite fillers with different mass ratios were higher than that of pure S⁰or FeS₂.Composite fillers have mesoporous structure,which is favorable for microbes to attach and grow on the surface of fillers.Five kinds of fillers were applied to packed bed reactor of small-scale,the reactor of R5 with S⁰:FeS₂=1:3 operated best,maintaining good nitrogen removal effect even under high influent NO₂^--N（300 mg/L）concentration with an average TN removal rate of 90.8%,and achieving stable and efficient nitrogen and phosphorus removal capacity under low HRT conditions.When the HRT was 6 h（nitrogen load 0.80 kg-N/（m³·d））,the average TN removal rate and the effluent TN concentration of R5 reactor was 88.6%and 27.22 mg/L,which met the landfill leachate discharge standard,and the average total phosphorus removal rate was 70.4%.（2）Additionally,the microbial community structure analysis showed that the external iron-mediated effect of the IS⁰AD reactor could stimulate the enrichment growth of sulfur autotrophic functional bacterias such as Thiobacillus and Sulfurimonas and iron autotrophic bacteria of Truepera.The iron-mediated effects further stimulated the secretion of the EPS of electron transfer-active substance,resulting in enriched a significant amount of iron ions in EPS,which improved the electron transfer capacity and enhanced the nitrogen removal activity of biofilm.The batch experiments revealed that the iron-mediated effect could significantly increase denitrification rate of the reactor,which increased from 82.79 mg-N/h to228.66 mg-N/h for R5 reactor as the influent NO₂^--N concentration increased from 100 to 300mg/L.The denitrification kinetic analysis indicated that the denitrification reaction of the IS⁰AD reactors were consistent with the half-stage reaction and the R5 reactor had the highest fit,with highest reaction rate constant K_（1/2）vthan the other reactors.（3）The optimum ratio of FeS₂-S⁰composite filler（S⁰:FeS₂=1:1）was applied to the pilot-scale work,the IS⁰AD reactor is capable of removing NO₂^--N in depth during the long-term treatment of PNDN process effluent.Under optimal operating conditions,the average TN removal rate of above 78.5%and the effluent TN below 39.35 mg/L,which met the discharge standards.Even under the condition of high influent NO₂^--N concentration（above 500 mg/L）,a higher denitrification efficiency of over 79.5%could be maintained.Moreover,pilot-scale IS⁰AD reactor also had an efficient phosphorus removal capacity with average total phosphorus removal rate was above 68.1%.The microbial community analysis for pilot-scale IS⁰AD reactor showed that Thiobacillus and Sulfurimonas were dominant populations and also enriched with iron autotrophic bacteria of Truepera.In summary,the exogenous iron-mediated effect significantly enhanced the deep denitrification and phosphorus removal capacity of the sulphur autotrophic denitrification system for waste leachate,and was further validated in a pilot project for landfill leachate treatment,providing theoretical and technical support for the technical application and engineering practice of deep denitrification and phosphorus removal from landfill leachate.