Nitrogen Removal By Partial Denitrification Coupled With Anaerobic Ammonium Oxidation In Biofilter

With the increasingly strict requirements for secondary effluent discharge from urban sewage treatment plants in China,deep denitrification has become an important problem faced by existing or new urban sewage treatment plants.The traditional denitrification filter can achieve deep nitrification,however,it has some drawbacks such as consuming a large amount of carbon source,having a high sludge yield,being prone to clogging,and spending a high operation cost.The discovery of various denitrification processes with anammox as the core provides a new choice for the deep denitrification of wastewater treatment plant.If anammox is combined with partial denitrification to realize partial denitrification coupled with anaerobic ammonia oxidation（PD/A filter）in the denitrification filter,it can not only reduce carbon source consumption,but also alleviate the blockage of the biological filter and small floor area.In this study,the artificial water distribution was used to simulate the secondary biological treatment effluent of urban sewage treatment plant,and the downward flow biofilter reactor was used to start and operate the one-stage partial denitrification coupled anaerobic ammonia oxidation process.The water quality change characteristics and NO₂^--N accumulation characteristics in the reactor were systematically analyzed,and the microbial population structure in the biofilm of the filter was observed and analyzed.Finally,the effects of temperature changes on the activity of anammox bacteria with different sludge forms were discussed by using cultured mature anammox granular sludge and biofilm.The main conclusions are as follows:（1）In the 300 days of biofilter startup and operation,under the conditions of influent NH₄⁺-N and NO₃^--N concentrations of 10mg·L^-1 and 10 mg·L^-1 respectively,C/N ratio of 3.5 and HRT of 8 h,the average removal rates of NH₄⁺-N and NO₃^--N were57%and 71%respectively,the effluent NH₄⁺-N concentration was 4.22 mg·L^-1,and the effluent NO₃^--N concentration was 2.94 mg·L^-1.In PD/A biofilter process,TN was effectively removed,and the average TN removal rate was 54%.（2）In PD/A biofilter,the anaerobic ammoxidation activity（calculated by NH₄⁺-N）reaches 28.51 mg N·（L·d）^-1,and the maximum partial denitrification activity（calculated by NO₃^--N）reaches 488.01 mg N·（L·d）^-1.The partial denitrification activity is greater than the anaerobic ammoxidation activity,which is the rate control step of PD/A process.（3）According to the results of high-throughput sequencing,ten phyla were detected in PD/A biofilter,mainly including Proteobacteria,Chloroflexi,Bacteroidetes,actinobacteria and planctomycetes.The functional microorganisms with high abundance in biofilm after enrichment in biofilter were mainly Candida＿Brocadia（0.70%）and Thauera（0.26%）.The microbial community composition in PD/A biofilter in this paper is consistent with that in PD/A bioreactor with sodium acetate as carbon source for domestic sewage treatment,which indicates that PD/A biofilter process has the potential of long-term treatment of actual wastewater.PD/A biofilter process can realize mainstream anaerobic ammonia oxidation while reducing energy consumption of sewage treatment plant.（4）The activation energy of anammox reaction in the form of granular sludge and biofilm is different between 15-25℃and 25-35℃.The E_a values of granular sludge and biofilm were 105.60 k J·mol^-1 and 88.25 k J·mol^-1 between 15-25℃and 62.15k J·mol^-1 and 56.78 k J·mol^-1 between 25-35℃,respectively,which were lower than that of anammox existed in suspended state.Anammox bacteria in granular sludge and biofilm have a better ability to resist temperature changes compered to anammox bacteria in suspended state.Notably,in the range of 15-25℃and 25-35℃,the temperature coefficient（θ）of anammox bacteria in granular sludge and biofilm were1.14,1.12 and 1.09,1.08,respectively.Compared to nitrifying bacteria or denitrifying bacteria,the obtainedθof anammox bacteria were relatively higher,indicating that anammox bacteria were more sensitive to temperature changes.Therefore,anammox process will be the limiting step when the temperature drops.