Start-up And Influential Factors Of PN-Anammox Process In A Single-stage Packed Bed Bioreactor

The single-stage partial nitritation-anammox?PN/A?autotrophic denitrification process has the advantages of saving carbon sources and low energy consumption.It is a new type of denitrification technology with broad prospects,especially suitable for treatment of wastewater with high ammonia nitrogen and low C/N ratio.However,due to the different growth environment requirements of AOB and AnAOB,and the competitive effect of NOB,AOB and AnAOB on nutrients,the start-up time of this process is long and the denitrification efficiency is not stable,which restricts the application of single-stage PN/A autotrophic denitrification process in sewage treatment.In view of the above problems,this paper applies the packed bed reactor to the start-up of the PN/A process,and explores the feasibility of starting the reactor quickly by controlling the nitrogen load and the dissolved amount of the inlet water.The microbial community structure,nitrogen removal potential and denitrification pathway in the stable operation of the reactor were studied by means of macro genome and macro transcriptome.Finally,the influences of three factors,such as influent ammonia nitrogen load,pH value and temperature,on the stable operation of the reactor were investigated to determine the optimal operation conditions of the system.It is expected to provide new ideas for the rapid start-up of the single-stage PN/A process,provide reliable reference data for the actual operation of the process,and finally achieve the purpose of efficient treatment of ammonia nitrogen wastewater.The main results are as follows:?1?PN/A was developed within 67 days in a novel packed bed equipped with porous bio-carriers by gradually increasing influent nitrogen loading rate and controlling dissolved oxygen.Average ammonium nitrogen removal efficiency?ARE?and total nitrogen removal efficiency?TNR?of 87.01%and 72.41%,respectively,were obtained.Nitrogen analysis along the reactor revealed that the removal of NH₄⁺-N and TN mainly occurred in the height range of 0³0 cm from the bottom.The study of reaction kinetics shows that the degradation process of ammonia nitrogen in the reactor is a second-order reaction.?2?The results of microbial community structure showed that,Proteobacteria and Planctomycetes,with which AOB and AnAOB were affiliated,accounted for 38.8%,representing the dominant phylum in the system after acclimation.The relative abundance of Nitrospirae,with which NOB was affiliated,decreased from 8.6%to0.05%.At the genus level,the abundance of Nitrosomonas and Candidatus Brocadia increased by 16%and 1.79%,respectively.The relative abundance of Nitrospira decreased from 2.80%to 0.01%after acclimation,indicating that the NOB was successfully eluted out of the PN/A system.?3?The macrogenomic results showed that heterotrophic denitrifiers CHB1 and CHB2,which were detected in the system,completed the incomplete denitrification process.Since only the denitrification genes nar/nap were encoded,no genes encoding nitrite reductase?nirS,nirK?.In addition,the relative abundance and expression of key enzyme functional genes,such as nitrification,denitrification and anammox,were analyzed.The results showed that there are abundant genes encoding various nitrogen transformation related enzymes in the system.Among them,the relative expression of the genes encoding the key enzyme of anammox?hzs and hzo?and the genes encoding the key enzyme of anammox?amo and hao?was dominant.On the contrary,although nxr,a functional gene of enzymes related to nitrite oxidation process,was detected,its relative abundance was relatively small and almost no expression.The study shows that the nitrite oxidation process in nitrogen metabolism is inhibited,and the process has been dominated by partial nitritation-anammox.?4?For the fluctuation of influent nitrogen load,the system shows a strong anti-shock load capacity.When the influent nitrogen loading rate was increased to1.172 kg-N?m^-3?d^-1,the nitrogen removal efficiency was reduced because the FA value reached the inhibitory concentrations of AOB and AnAOB.Immediately after reducing the influent nitrogen loading rate,the efficiency rebounded in about a week.In addition,the system has great potential for the treatment of wastewater with low ammonia nitrogen concentration.When the influent nitrogen loading rate was reduced to 0.147 kg-N?m^-3?d^-1,the average removal efficiency of ammonia nitrogen and total nitrogen are 98%and 85.4%,respectively.At the same time,AOB and AnAOB have high activity.?5?The optimum pH of the reactor was determined to be 8.0.At this time,the activity of AnAOB and AOB was the strongest,and the highest ARE and TNR could be obtained.It was determined that the optimal temperature range for the reactor operation is 25³0°C,during which the NO₃^--N_eff/?NH₄⁺-N values are stable around the theoretical value of 0.11,and the nitrogen removal reaction is dominated by PN/A.When the temperature was 30°C,the average ARE and TNR were the highest,88.4%and 78.1%respectively.