Start-up Characteristics Of Complete Ammonia Oxidation Biofilter Inoculated With Sand-filter Attachment

Drinking water sources in China have been often threatened by ammonia micro-pollutants.The presence of high concentrations of ammonia in the water source will lead to the proliferation of algae in the drinking water treatment process,thereby blocking the filter and reducing the water purification effect.While excess ammonia in the effluent of drinking water treatment plants will increase the risk of nitrite generation,which is well-known as human toxicants and carcinogenic precursors.Compared with physical and chemical removal methods,biological nitrogen removal by biofiltration has the advantages of low costs,less secondary pollution and simultaneous removal of multiplate pollutants.However,the system is vulnerable to varying pollutants loading and surrounding environment change,and thus lead to unstable performances.Due to its high ammonia and oxygen affinity,complete ammonia oxidation（Complete ammonia oxidation,Comammox）bacteria have been expected to play critical role in biological ammonia pre-treatment or deep-treatment in drinking water purification system.In this paper,the sand filter attachment containing Comammox bacteria were inoculated to start-up the biofilter for treating ammonia micro-polluted water source.The reactor operation characteristics and microbial mechanisms of complete ammonia oxidation were further elaborated under different water source component and filter media conditions.Microbial attachments containing ammonia oxidizing bacteria（AOB）,nitrate oxidizing bacteria（NOB）and Comammox bacteria were successfully extracted and characterized from the sand filter in drinking water treatment plant.The microorganisms were mostly present in the form of cellular aggregates and elliptical/elongated cells.The results of sequential batch incubation at different ammonia concentrations（0.5-5.0 mg-N/L）showed that the sand-attached microorganisms exhibited a long lag growth period at the beginning of the reactor incubation（<40 d）.Thereafter,the reactor displayed a high rate of ammonia oxidation（0.024-0.358 mg N/（L·h））.In addition,the increase in ammonia concentration enhanced the transient accumulation level of NO₂^--N in the reactor.It is therefore inferred that the increase in ammonia-nitrogen concentration is not conducive to the complete ammonia oxidation in the sequencing batch reactor.Six biofilter reactors were successfully started-up and operated using batch cultivated cultures,and the effects of water source components,filter media type and layer height on the system ammonia degradation performance were investigated.（1）The reactors all showed high removal rates（>96.9%）of NH₄⁺-N at a hydraulic retention time of 2 d.The activated carbon filter not only had higher NH₄⁺-N removal rate（about 1.5-3.0%higher）than the quartz sand filter,but also had higher total nitrogen removal effect（>90.0%）.（2）Different ammonia concentrations（0.5-2.5 mg-N/L）did not significantly affect the ammonia removal in the biofilter,while the addition of organic matter（5 mg-BOD/L）led to a significant reduction in ammonia removal in the quartz sand biofilter,but not significantly affect that in the activated carbon filter.（3）Filter media type significantly influenced the conversion and removal of ammonia from the reactor,and the removal rate of ammonia in the quartz sand biofilter was significantly lower than that in the activated carbon biofilter.The adsorption-biological oxidation-reabsorption process in the activated carbon biofilter might probably be an important reason for its higher ammonia and total nitrogen removal effect.（4）The increase in layer height in the quartz sand filter might lead to other nitrogen removal processes(e.g.denitrification and reduction of ammonia nitrogen,while the different layer heights in the activated carbon filter had no significant effect on ammonia degradation.The 16S rRNA gene sequencing results showed that（1）Only three genera of nitrifying bacteria,Nitrospira,Nitrosospira,and Nitrosomonas,were found in sequential batch cultures with different ammonia nitrogen concentrations.With the decrease of ammonia concentrations,Nitrospira gradually competed over AOB,and Comammox Nitrospira was expected to mainly contribute to ammonia oxidation.In addition,Nitrospira-Nitrosomonas symbiosis had stronger ammonia competition potential than Nitrospira-Nitrosospira symbiosis.（2）The increase of ammonia concentrations led that Nitrosomonas compete over Nitrosospira in the quartz sand biofilter,but did not significantly change the proportion of AOB and Nitrospira in nitrifying bacteria.It can be therefore inferred that both Comammox bacteria and AOB were the main contributors to complete ammonia oxidation in the quartz sand filter.（3）Although the increase of ammonia nitrogen concentration favored the growth of AOB in the activated carbon biofilter,Nitrospira bacteria in the activated carbon biofilter still absolutely dominated among the genus of nitrifying bacteria.It can be therefore inferred that Comammox Nitrospira might be the main contributor to ammonia oxidation in the activated carbon biofilter.（4）Comammox Nitrospira may be more suitable for hanging film growth in activated carbon biofilter than in quartz sand biofilter.Besides,the lower layer of both types of upward flow biofilters is more suitable for Nitrospira enrichment growth（about 1.4-4.3 times more abundant than the upper layer）.（5）PICRUSt2 predictions showed that the relative abundances of genes encoding for ammonia oxidation（e.g.ammonia oxidase and hydroxylamine oxidase）was higher in the activated carbon filter than in the quartz sand filter.The addition of organic matter led to a decrease in the abundance of ammonia oxidase and hydroxylamine oxidase encoding genes.The metabolic pathway of ammonia nitrogen oxidation via NH₂OH→NO→NO₃^-process might play an important role in the complete ammonia oxidation in the biofilter.In this paper,six biofilter reactors were successfully started up using batch cultivated cultures containing Comammox Nitrospira,and they all achieved efficient and stable ammonia degradation performances,which provided a theoretical basis for subsequent experiments and practical engineering applications.The results of this paper will help to achieve pollution and carbon reduction in the treatment of micro-polluted water sources,and provide theoretical support and technical support to further ensure the quality and safety of drinking water and build a beautiful China.