The Performance And Mechanism Of Completely Autotrophic Nitrogen Removal Process Based On Formic Acid Aided Partial Nitrification

Biological nitrogen removal(BNR)is one of the most important part in wastewater treatment,which greatly reduces water eutrophication.Currently,conventional BNR based on nitrification and denitrification is still the most widely used process.However,large energy consumption of aeration and external organic carbon in conventional BNR not only increases the cost,but also leads to huge energy consumption and carbon dioxide emissions,which is not conducive to the realization of the national strategy of "Carbon Neutralization" and "Carbon Peak".Due to its advantages of lower energy consumption,no additional organic carbon and lower sludge yield,anammox process has become a research hotspot in the field of BNR.The substrate of anammox is ammonia and nitrite,while the wastewater is generally rich in ammonia but nitrite.Therefore,partial nitrification,especially partial nitritation,is regarded as the most ideal pre-process.Although many partial nitrification regulation strategies have been proposed,the stability of its long-term operation seriously limits the practical application of anammox.In this study,formic acid aided partial nitrification was developed,and the completely autotrophic nitrogen removal process integrating formic acid aided partial nitritation,anammox and sulfur-based denitrification was established.The nitrogen removal performance and microbial characteristics of the above process were investigated,and its robustness and economy were also evaluated.The main content is as follow:1.The partial nitrification using formic acid as inhibor in mainstream was established,and the selective inhibition mechanism of nitrite oxidizing bacteria(NOB)by formic acid was revealed.On this basis,the kinetics model of functional microorganisms when exposure to formic acid was constructed.The results showed that:(1)Formic acid can rapidly inhibit the oxidation of nitrite in biological nitrification,and the inhibition effect was positively correlated with formic acid concentration.When formic acid concentration reached 30 mM,ammonia removal efficiency and nitrite accumulation ratio were above 97%and 80%,respectively.In the presence of formic acid,the accumulation of nitrite was not caused by denitrification.Formic acid specifically inhibited the transcription of nxrB gene encoding nitrite oxidoreductase,thereby reducing the substrate oxidation activity of NOB.(2)Adding 30 mM formic acid to SBR nitrification process can quickly realize the start-up and stable operation of partial nitrification.After formic acid acclimation,the specific ammonia oxidation activity of the system was significantly enhanced,and was 1.3 times that of the control experiment.During the long-term operation of the process,Nitrosococcus and Nitrosomonas gradually became the dominant ammonia oxidizing bacteria(AOB),while the abundance of Nitrospira,the main NOB,continued to decrease.The succession of functional microorganisms was the key factor to maintain the long-term stability of the process.(3)The inhibition type of formic acid on AOB and NOB was mixed inhibition,and AOB had higher competitive inhibition constant and noncompetitive inhibition constant than NOB.At the same time,AOB and NOB showed logarithmic decay and recovery in the process of formic acid continuous acclimation,and AOB had lower decay rate and higher recovery rate than NOB.On this basis,the estabilished kinetics model including the activity inhibition,inactivation and recovery of functional microorganisms can accurately simulate the metabolism in the partial nitrification process.2.The partial nitrification with formic acid side-stream sludge treatment was established,in which part of the sludge was transferred to the side-stream treatment reactor regularly,and 30 mM formic acid was added to it.After 2 days of treatment,the sludge was returned to the main-stream reactor.The process performance,microbial community structure and operation parameters of the process were studied:(1)The nitrite accumulation ratio and ammonia removal efficiency of the process were more than 96.5%and 97.4%,respectively.Compared with the partial nitrification adding formic acid in influent,it saveed about 98%formic acid consumption and increased the specific ammonia oxidation rate by 8%.Further analysis showed that the microbial community structure of dominant functional microorganisms were not significantly different from those in the formic acid mainstream inhibition process.(2)Based on the kinetics model,it was found that the ammonia oxidation rate was related to sludge retention time(SRT)and hydraulic retention time(HRT),while the nitrite accumulation ratio and ammonia removal efficiency were related to SRT and HRT,respectively.It is beneficial to set the SRT at 10 d to maintain the stability of ammonia oxidation rate and accelerate the wash out of NOB.Shortening SRT to 5 d can reduce 35%of the start-up time,but losed 28.6%of the ammonia oxidation rate.Shortened HRT from 10 h to 6.7 h can increase the ammonia oxidation rate by 49.9%,and further shortened to 5 h can realize the stable operation of the partial nitritation for more than 200 days.The ratio of nitrite and ammonia in the effluent maintaind at 1.0-1.1.3.The partial nitritation process with formic acid side-stream sludge treatment was established,and meanwhile coupled it with anammox and sulfur-based autotrophic denitrification.The performance,robustness,economy,microorganism community structure and function of the integrated process were analyzed:(1)The partial nitritation process with formic acid side-stream sludge treatment was successfully started within 15 days,and ammonia removal efficiency and nitrite accumulation rate were 50.5%and 92.4%,respectively.The ratio of ammonia and nitrite in the effluent was maintained at 0.99?1.24.After coupling with anammox process,the nitrogen removal performance of anammox process was not significantly affected,and the total nitrogen removal efficiency was maintained at 78.0-79.9%.In the sulfur-based autotrophic denitrification,the removal efficiency of ammonia and total nitrogen were 42.4%and 72.9%,respectively,and the concentrations of ammonia,nitrite and nitrate in effluent were 1.9±1.8 mg·L-1,2.6±2.1 mg·L-1,and 4.0±2.5 mg·L-1,respectively.(2)AOB showed strong tolerance when exposure to formic acid,and Nitrosomonas was the dominant genus with an abundance of 10.2%.The abundance of Nitrobacter and Nitrospira was only 0.04%and 0.01%,respectively.In anammox process,Candida kuenenia was the dominant genus with an abundance of 5.3%.The strong formic acid anabolism and catabolism in the system indicated that the residual formic acid in the influent would not adversely affect the anammox,but might be conducive to the growth of anammox bacteria.In sulfur-based denitrification,although Thauera,a typical denitrifying microorganism with relative abundance of 2.3%,is the dominant genus,sulfuricella,with relative abundance less than 0.01%,showed greater denitrification activity in the system.(3)According to model evaluation results of the integrated process,when the influent ammonia decreased or increased by 50%,the partial nitritation will gradually enter the unstable state,the total nitrogen removal efficiency and contribution of anammox in the integrated process decreased significantly(>20%),while that of the sulfur-based autotrophic denitrification changed slightly(<10%),and the its contribution to total nitrogen removal increased significantly(>20%).Ammonia fluctuation in the influent did not seriously affect the effluent quality of the integrated process.Economic analysis showed that the novel BNR process based on formic acid aided partial nitrification can save about 95%of reagent cost compared with the conventional nitrification-denitrification process,and significantly improved the economy of BNR.