Effect Of Influent Ammonia-nitrogen Concentration On Microbial Community Structure In MBBR Process

At present,the application and research of Moving-bed Biofilm Reactor(MBBR)in ammonia nitrogen wastewater treatment are mostly focused on operation parameters and technical operation,and the research on microorganisms in MBBR is still insufficient.Moving bed MBBR process was used to treat simulated wastewater.The effects of influent ammonia nitrogen concentration(20,30,50,100 and 200 mg/L)on the treatment efficiency of MBBR process were investigated under the conditions of keeping influent COD 300 mg/L,TP 4 mg/L,dissolved oxygen 2.5-3.0 mg/L,HRT 12 h and filler filling rate 40%.Based on high-throughput sequencing technology,16 S rRNA gene sequencing was used to analyze the changes of microbial community structure under different ammonia nitrogen concentrations.Metagenome sequencing was used to analyze the functional genes and their nitrogen metabolic pathways.According to the test results of water quality indicators,with the increase of influent ammonia nitrogen concentration,MBBR process has a better effect on COD removal,and COD removal rates are stable at more than 85%.With the increase of influent ammonia nitrogen concentration,the removal efficiency of ammonia nitrogen and TN is worse,and the average removal rate of total nitrogen is significantly lower than that of ammonia nitrogen.When ammonia nitrogen concentration is 20 and 30 mg/L,the removal rate of ammonia nitrogen is about 60%.When ammonia nitrogen concentration is increased to 100 mg/L,the removal rate of ammonia nitrogen continues to drop to about 10%.After that,the removal rate of ammonia nitrogen continues to increase,and the removal rate of ammonia nitrogen remains low and stable.The removal efficiency of TP by MBBR is poor,and the removal rate of TP is between 25% and 45%.High concentration of ammonia nitrogen has obvious inhibitory effect on the removal of TP.According to the results of 16 S rRNA gene sequencing,there are many denitrification methods in MBBR,such as traditional biological denitrification,anaerobic ammonia oxidation,simultaneous nitrification and denitrification.At the generic classification level,with the increase of ammonia nitrogen concentration,the structure of microbial community changed obviously,species richness decreased gradually,and diversity index increased first,then decreased,then increased and then decreased.The fluctuation of structure may be related to the competition for dominance among various denitrifying bacteria such as Arcobacter,Hydrogenophaga and Ottowia.AOB related to autotrophic nitrification only includes Nitrosospira and Nitrosomonas.The total relative abundance of Nitrosomonas is stable at about 0.3%,and does not decrease with the increase of ammonia concentration.The decrease of denitrification effect may be due to the inhibition of microbial activity of autotrophic nitrifying bacteria by high ammonia concentration.Rhodanobacter is the only NOB in this sequencing.Its relative abundance ranges from 0.005% to 0.170% and shows fluctuating changes.There are many kinds of microorganisms involved in denitrification,including Ottowia,Arcobacter,Hydrogenophaga,Haliangium and other bacteria,and these bacteria show a fluctuating trend.In addition,the species of Prothecobacter capable of heterotrophic nitrification and Thauera capable of aerobic denitrification were also detected,and their relative abundance decreased with the increase of ammonia nitrogen concentration.According to the metagenomics sequencing of MBBR and the statistical analysis of KEGG functional genes,the total number of functional genes in MBBR samples was 81,962,of which the number of functional genes related to conventional metabolism was the largest,followed by the number of functional genes related to carbohydrate metabolism and amino acid metabolism,which confirmed that most of the microorganisms in MBBR samples had the function of COD degradation,and there are a large number of functional genes related to the growth and metabolism of microorganisms in MBBR biofilm and the degradation of pollutants.In nitrogen metabolism,759 functional genes participated in nitrogen metabolism,accounting for 0.93% of all functional genes.Nitrification and denitrification played a dominant role in the process of nitrogen removal.Among them,the NarGHI gene involved in the process of nitrate conversion to nitrite was the most abundant,but the AmoCAB and Hao genes involved in the process of ammonia nitrogen conversion to nitrate nitrogen and nitrite nitrogen are less,this may be one of the reasons why MBBR unit failed to achieve higher ammonia nitrogen removal rate.