Effect Of S/N Ratio On Nitrogen Removal And Pathways Of Electrolysis-constructed Wetland For Secondary Effluent Of Municipal Wastewater Treatment Plants

To solve the problem that the nitrogen removal mechanism was unclear in the electrolysis-constructed wetland（ECW）in secondary effluent of municipal wastewater treatment plants（MWTPs）with high SO₄^2--S concentration.This study explored the effects of S/N on the construction and efficiency in the ECW.The effects of current intensity and hydrolic residence time（HRT）on the efficiency of advanced nitrogen and phosphorus removal in different S/N systems were investigated.The shock resistance of different S/N systems was obtained.Through X-ray photoelectron spectroscopy（XPS）analysis,isolation and identification of functional bacteria,and 16S r RNA Amplicom sequencing technology,the pathways of nitrogen and phosphorus removal,sulfur transformation and its effects on nitrogen removal,as well as the microorganisms in different S/N systems were studied.The main conclusions are as follows:S/N had a significant effect on the construction of nitrogen removal systerm,but had no significant effect on the construction of phosphorus removal systerm in the ECW.When the S/N were low（0.6）,medium（4.7）,and high（9.4）,the systems need 30 d,20 d,and 20 d to achive construction,and the average removal rates of NO₃^--N,TN,PO₄^3--P and SO₄^2--S were 79.69%,90.70%,91.12%,68.51%,75.36%,75.92%,98.06%,97.86%,97.68%and 4.18%,17.60%,12.95%,respectively.The medium and high S/N systems had higher nitrogen removal efficiency.Current intensity and HRT had significant effects on nitrogen and phosphorus removal efficiency of secondary effluent in the ECW.With the decrease of current intensity and HRT,the nitrogen and phosphorus removal rates of different S/N systems decreased.Because of the dynamic storage and release of electrons,the medium and high S/N systems had a certain impact resistance.Under the most suitable working conditions of different S/N systems,the average removal rates of TN and PO₄^3--P were 80.40%,89.93%,88.15%and 96.99%,97.53%and 96.77%,respectively.S/N had a significant effect on the pathway of sulfur conversion and nitrogen removal in the ECW.The cathodes of medium and high S/N systems not only generated S⁰,Fe S and Fe S₂,in situ,but also had the S-driven autotrophic denitrification process.The systerms used the deposited S⁰,Fe S and Fe S₂ as electron donors,resupplying electrons and improving the nitrogen removal efficiency.S/N had no significant effect on the removal pathway of phosphorus in the ECW.The phosphorus removal was realized by coprecipitation and adsorption with iron ions produced by electrolysis to form iron-phosphorus polymers such as Fe PO₄,Fe^（n+）OH-PO₄,Fe OOH-PO₄ and Fe（OH）₃-PO₄.One strain of sulfate-reducing bacteria using H₂ as electron donor to reduce SO₄^2--S was screened and isolated from different S/N reactors seperately.They were Pseudoxanthomonas mexicana、Pseudomonas aeruginosa、Pseudomonas stutzeri,separately.There types of sulfate-reducing bacteria screened and purified by different S/N systems were different.But,the strains of medium and high S/N systems belonged to the same Pseudomonas genus.The current intensity had no significant effect on the microbial functional bacteria species related to nitrogen transformation and sulfur cycle,but had a significant effect on its relative abundance.Under different current intensity,the functional bacteria of different S/N systerms were Bacillus,Exiguobacterium,Desulfovibrio,Pseudolabrys and Clostridium＿sensu＿stricto＿1.The medium and high S/N systems could reduce sulfate through sulfate reducing bacteria（Desulfovibrio）for electron storage under high current intensity.When the current intensity decreased,the deposited S⁰,Fe S and Fe S₂could supply electrons to sulfur autotrophic denitrifying bacteria（Exiguobacterium）for denitrification.HRT had a significant effect on the microbial functional bacteria species and relative abundance related to nitrogen transformation and sulfur cycle in systems.When the HRT was 24 h,the functional bacteria of different S/N systems were Bacillus,Exiguobacterium,Desulfovibrio,Pseudolabrys and Clostridium＿sensu＿stricto＿1.When the HRT was 6 h,the functional bacteria of different S/N systems were Bacillus,Exiguobacterium,Desulfotomaculum,Azoarcus and Clostridium＿sensu＿stricto＿1.With the decrease of HRT,the Desulfotomaculum and Exiguobacterium could use the deposited sulfides and elemental sulfur as electron donors to reduce nitrate,achieving advanced nitrogen removal in medium and high S/N systems.