Study On N₂O Accumulation Characteristics And Influencing Factors In Denitrification Process

In order to prevent the eutrophication of water caused by the direct discharge of nitrogen-containing wastewater into the river,most countries require the removal of nitrogen and adopt biological nutrient removal technology?BNR?.However,with the intensification of greenhouse effect and frequent occurrence of extreme weather in recent years,the emission reduction of greenhouse gas N₂O in BNR process also shows an urgent need,and the N₂O produced in denitrification process can not be ignored.In this paper,methanol and sodium acetate,two most popular external carbon sources are used to acquire denitrifying sludge.Under different conditions,N₂O reductase activity,nitrogen transfer characteristics and N₂O accumulation characteristics in denitrification process are studied.The purpose is to explore the optimal environmental conditions for denitrification process and achieve the goal of N₂O emission reduction in sewage treatment plants.Firstly,methanol and sodium acetate denitrification sludge were successfully enriched in SBR reactor.After 120 days of stable operation,real denitrifying bacteria such as Methylotenera,Hyphomicrobium and Melioribacter gradually dominated the methanol denitrification sludge,and incomplete denitrifying bacteria such as Thauera played an assistant role in denitrification.However,in the sodium acetate denitrification sludge,the incomplete denitrifying bacteria Thauera accounted for 63.7%,which was absolutely dominant in the denitrification process,while the other denitrifying bacteria accounted for less than 3%.Therefore,the accumulation of NO₂^--N in sodium acetate denitrification system is more than 10 times that in methanol denitrification system,and the maximum accumulation rate is as high as 41.9%.The effects of temperature,pH,stirring rate,salinity and Fe³⁺on N₂O reductase activity of methanol and sodium acetate denitrification sludge were studied by using denitrification sludge successfully enriched.It was found that N₂O reductase had the best activity at 35?,pH=8,400 rpm,salinity 0%,Fe³⁺=0mg/L.The N₂O reductase activity of denitrifying sludge using sodium acetate as carbon source is more than 50%higher than that of methanol denitrifying sludge,but the environmental impact is greater.In the study of denitrification system with C/N ratio,it was found that denitrification sludge with sodium acetate as an additional carbon source could easily cause very high N₂O accumulation when the carbon source was insufficient,and under the same conditions,the accumulation of N₂O was more than 10 times of that of methanol denitrification sludge.When the C/N ratio is 1,the maximum N₂O accumulation of sodium acetate denitrifying sludge can reach 14580.4?g/L,the instant conversion rate can reach 26.7%,while methanol is only 483.1?g/L,the conversion rate is less than 0.1%.For methanol denitrification sludge,when the C/N ratio is more than 3,the nitrogen in influent can be completely removed,and the accumulation of N₂O is less than 60?g/L,and the release of N₂O into the environment is less than 20 ppm,so the appropriate C/N ratio of methanol denitrification system is 3:1.For sodium acetate denitrification sludge,when C/N<4,N₂O accumulated more than 200?g/L,so the optimum C/N ratio was above5:1.Nitrite concentration in influent nitrogen source has direct influence on N₂O accumulation.Whether methanol or sodium acetate denitrification sludge NO₂^--N>50mg/L,N₂O reductase was significantly inhibited in the system,and the denitrification process would be significantly delayed when NO₂^--N was further increased.In the denitrification system of methanol and sodium acetate,NO₂-N was 100 mg/L.The removal rates of nitrogen were only 40.9 and 39.6%respectively.The maximum accumulations of N₂O were 15490.6 and 24590.6?g/L.The instantaneous conversion rates were 27.6%and 77.6%.However,low concentration of NO₂^--N?<25mg/L?could increase the reaction process by more than 20%,and could not cause a large accumulation of N₂O.The effect of Fe³⁺on denitrification of methanol and sodium acetate was consistent.When the concentration of Fe³⁺was 10-75 mg/L,the activities of nitrate,nitrite and nitrous oxide reductase in denitrification system decreased with the increase of Fe³⁺and the accumulation of N₂O in denitrification system increased.When the concentration of Fe³⁺was 0 mg/L,nitrite accumulation was37.4 mg/L in sodium acetate denitrification system and N₂O accumulation was4612.5?g/L in the system.When the concentration of Fe³⁺was more than 25 mg/L,all three reductases were inhibited to some extent,and the inhibition increased with the increase of Fe³⁺concentration.When the concentration of Fe³⁺was 75mg/L,the accumulation of N₂O in denitrification system was the highest?9670.6?g/L?.Whether the external electrons are sufficient or not,there exists electronic competition among various reductases in denitrification.When using external carbon sources,the electronic consumption capacity of each reductase in methanol denitrification sludge is Nos>Nir>Nos,so the denitrification system is not easy to cause the accumulation of N₂O and NO₂^--N.For sodium acetate denitrification sludge,the reductase is Nar>Nir>Nos,so the denitrification system is easy to cause nitrite to increase.After the reactor was stable,the dominant species in methanol system were Methylotenera and Hyphomicrobium,in which Methylotenera was a typical incomplete denitrifying bacteria and did not possess N₂O reduction function;Hyphomicrobium was a real denitrifying bacteria;and in sodium acetate system,the dominant species was Thauera,with relative abundance of 69.02%in 90 days.Thauera was an incomplete denitrifying bacteria and its various reductases were among them.There are great differences between them,so it is easy to cause nitrite accumulation in denitrification system,so the accumulation of N₂O in methanol denitrification system is less than that of sodium acetate.