Study On The Production Characteristics And Reduction Control Of N₂O In The Novel Nitrogen And Phosphorus Removal System Of Static/Oxic/Anoxic

The biological nitrogen removal processes of wastewater will emit a large number of strong greenhouse gases nitrous oxide?N₂O?,which will easily aggravate global warming.In the previous study of our research group,a static/oxic/anoxic?SOA?biological nitrogen and phosphorus removal system was proposed,but the N₂O production characteristics were not clear,which restricts the promotion and application of this technique.In this paper,N₂O emission amounts and factors in the SOA reactor were determined to explore the N₂O production characteristics in the SOA system,and were compared with those in the conventional anaerobic/oxic/anoxic?AOA?system.Then,the effect of static time on N₂O emissions in the SOA system and its mechanism were explored by shortening the static time from 60 to 30 and 15 minutes,and the optimum static time was obtained.At last,the effect of DO concentration on N₂O production characteristics in SOA system was studied by comparing the changes of N₂O emissions under different dissolved oxygen?DO?concentrations?4.10,2.03,0.99,0.52 mg/L?,and the influence mechanism was discussed from the aspects of free nitrous acid?FNA?accumulation,microbial activity,electron transfer,and so on.The results show that the SOA system can achieve a good effect of nitrogen and phosphorus removal,and the removal efficiencies of total nitrogen and phosphorus were 82.0%and 85.6%respectively,which are slightly lower than the values of 82.3%and 88.5%in the conventional AOA system.However,the N₂O emission amounts and factors in the SOA system are 10.8 mg and 7.32%respectively,which are significantly higher than the values of 7.88 mg and 5.32%in the AOA system.This shows that the N₂O emissions in the SOA system are significantly higher than those in the traditional nitrogen and phosphorus removal system.Further study shows that N₂O emissions in the SOA system can be effectively reduced by shortening the static time.As the static time was shortened from 60 min to 30 and 15 min,the N₂O emissions decreased from 10.8 to 5.51 and 5.30 mg,respectively,and the emission factors decreased from 7.32%to 3.69%and 3.58%,respectively.The mechanism analysis shows that shortening the static time was beneficial for the microorganisms to more fully absorb and use the external carbon sources in the oxic phase to ensure that the subsequent denitrification process could be carried out thoroughly,thereby reducing theproduction.Phosphorus removal was significantly inhibited,although shortening static time didn't have a significant impact on the nitrogen removal of the system.When the static time was shortened to 15 min,the phosphorus removal efficiency decreased to 77.1%.Considering the effect of nitrogen and phosphorus removal andemissions,a static time of 30 min is the best choice for the SOA system.At the same time,the change of DO concentration also has a significant effect on the N₂O production in the SOA system,and the hypoxia environment is beneficial to N₂O production.As the DO decreased from 4.10 to 0.52 mg/L,N₂O emission factors increased from 1.51%to4.32%.The proportion of N₂O produced by ammonia-oxidizing bacteria?AOB?in the total production increased from 87.0%to 92.6%.The analysis shows that the hypoxia environment reduced the capacity of aerobic metabolism and electron transport,which enhances the electron competition among reductases.The decrease of aeration intensity and the cytotoxicity of FNA had more effect on nitrite-oxidizing bacteria than on AOB,which further increased the potential of FNA accumulation thus inducing N₂O production.In consideration of the nutrient removal and N₂O mitigation,the optimum DO concentration in the oxic phase for the SOA system is 2.0 mg/L.