Efficiency And Mechanism Of Partial Nitritation-anammox Coupled With Iron-based Denitrification For Nitrogen And Phosphorus Removal

With the change of people’s lifestyle and the improvement of living standards,the carbon to nitrogen ratio C/N ratio of municipal sewage has been gradually decreasing.In addition,livestock wastewater,anaerobic digestion liquid,and landfill leachate also exhibit more prominent low C/N characteristics.The lack of organic carbon sources greatly limits the nitrogen removal of traditional nitrification and denitrification processes.Oxygen-limited activated sludge technology can achieve partial nitrification and anammox（PN/A）process by controlling the dissolved oxygen level（<0.5 mg/L）,it has the advantages of cost effective and without additional carbon sources.However,the nitrate generated by the PN/A process restricts the total nitrogen（TN）removal of the system,and phosphate removal efficiency of the system is poor due to the low C/N.In view of the above problems,this study proposes an idea to enhance the nitrogen and phosphorus removal of oxygen-limited activated sludge system by zero valent iron（ZVI）.With the addition of external ZVI,a mixed nitrogen removal pathway was established in an upflow oxygen-limited activated sludge system,coupling PN/A with iron-based denitrification（PNAID）,and the phosphate removal was also enhanced by the chemical precipitation of Fe（II）/Fe（III）.The effect of ZVI addition on the metabolic activity of ammonia oxidizing bacteria（AOB）,nitrite oxidizing bacteria（NOB）,anaerobic ammonia oxidizing bacteria（ANAMMOX）and denitrification bacteria（DNB）was first explored through the cultivation experiment of biological nitrogen removal functional bacteria.The results indicate that there is a significant"dose effect"relationship between ZVI dosage and microbial metabolic activity.An appropriate amount of ZVI can stimulate the activity of key enzymes,accelerating the utilization and transformation of target substrates by functional flora.On this basis,two sets of upflow oxygen-limited activated sludge systems,R1（without ZVI）and R2（with ZVI added was 300 mg/L calculated by working volume,once every 4 days）,were constructed and operated in parallel.Under the conditions of HRT 8 h,28°C,and influent ammonium（NH₄⁺-N）and total phosphorus（TP）about 320 mg/L and 18 mg/L,respectively,the TN and TP removal of R1 were about 80.1%and 13.9%,while the average removal of R2 were as high as 90.1%and 80.2%,respectively,increasing by 10.0%and 66.3%compared to R1.This result indicates that using ZVI to enhance the nitrogen and phosphorus removal of low C/N wastewater oxygen-limited biological treatment is feasible and effective.After R2 successful constructed the PNAID process,the influence of temperature（gradually decreasing from 30°C to 12°C）on the nitrogen and phosphorus removal efficiency of the PNAID system was further investigated,especially the long-term operating characteristics and stability at low temperatures（12°C）.The results showed that temperature had little effect on the TP removal of R1and R2,but had a significant impact on the TN removal.During the continuous operation（421 days）,the TP removal of R1 and R2 were maintained about 10.7%and74.1%,respectively.When the temperature drops to 12°C,the NH₄⁺-N and TN removal of R2 are still about 84.3%and 82.2%,while R1 is only about 63.1%and45.3%.Analysis found that exogenous ZVI can not only effectively promote the granulation of oxygen-limited activated sludge and improve the characteristics of sludge particles,but also significantly reduce the temperature correlation coefficient and apparent activation energy of AOB,ANAMMOX,and DNB functional flora,improving their low-temperature tolerance.The analysis of microbial community structure shows that the relative abundance of AOB（Nitrosomonas and Nitrosococus）,ANAMMOX（Candidtus Brocadia and Candidtus Kuenenia）,and DNB（Pseudomonas and Thauera）in R2 is 1.94,1.24,and 1.98 times higher than that in R1,respectively.The above results indicate that exogenous ZVI can effectively enhance the nitrogen and phosphorus removal of the PNAID system at low temperatures by improving the characteristics of sludge particles,inducing the enrichment of nitrogen removal functional flora,stimulating the growth and metabolic activity of functional flora,and enhancing their low-temperature toleranceTo explore the biological nitrogen and phosphorus removal efficiency of PNAID process for low NH₄⁺-N and low C/N municipal wastewater,R2 was restarted with the similar control conditions to the initial start-up.After 87 days of continuous operation,the PNAID process was successfully established again in the reactor.On this basis,adopt by reducing ammonium concentration in steps（300,200,100,and60 mg/L）during the long-term operation,and nitrogen and phosphorus removal efficiency of the PNAID system were investigated especially under low NH₄⁺-N stages.The results showed that when the influent NH₄⁺-N and TP were about 58.2mg/L and 6.0 mg/L,respectively,the TN and TP removal of the PNAID system were as high as 97.4%and 92.0%,respectively.It was found that with the decrease of NH₄⁺-N concentration,the dominant ANAMMOX strain Candidatus Brocadia coupled with iron-based autotrophic denitrification bacteria was gradually replaced by Candidatus Kuenenia,and the activities of the key denitrification enzymes of NAR and NIR were also significantly improved due to the participation of ZVI.There are multiple mechanisms by which ZVI enhances the nitrogen removal efficiency of oxygen-limited activated sludge systems.Under the participation of ZVI,the secretion ability of N-acylated homoserine lactones（AHLs）signaling molecule secreted by oxygen-limited activated sludge was significantly improved,and effectively mediating the EPS synthesis,secretion,and aggregation behavior of the microbial community.The adsorption and"skeleton"effect of iron oxides made the structure and properties of granular sludge more stable,strengthening the synergistic metabolism of functional flora.The uptake behavior of microorganism increases the intracellular Fe²⁺/Fe³⁺concentration,enhances dehydrogenase activity,promotes intracellular electron transfer and ATP synthesis,improved intracellular redox metabolism.The addition of ZVI stimulated the expression of nitrogen removal functional genes amo,hzs,hdh,nar,and cold shock proteins（Csp family）and transporters,strengthening the biosynthesis of amino acids,carbohydrates,fatty acids,nucleosides,and nucleotides in related metabolic pathways,and improving metabolic flux.In summary,guided by the technical approach of enhancing low C/N wastewater treatment with ZVI,this study successfully established the coupled PNAID biological nitrogen removal pathway in an upflow oxygen-limited activated sludge system.In the novel system,TN and TP removal were both significantly improved.Additionally,the mechanism of ZVI-enhanced nitrogen removal in oxygen-limited activated sludge systems was revealed from multiple perspectives.This study provides technical reference and theoretical guidance for the development of cost-effective and efficient nitrogen and phosphorus removal technology for low C/N wastewater.