Nitrogen And Phosphorus Removal In Mainstream Anammox Continuous Process Treating Municipal Wastewater

As a new type of biological nitrogen removal technology,anammox technology for urban sewage has many advantages,such as reducing aeration energy consumption,saving water treatment cost and reducing greenhouse gas emissions.It is considered to be the most suitable mainstream nitrogen removal technology for building a new type of sewage treatment plant in the future.At present,the feasibility of mainstream anammox process for urban sewage has been verified.However,the sewage water quality such as low temperature and low ammonia nitrogen,and the excessive proliferation of nitrite oxidizing bacteria（NOB）and heterotrophic bacteria（HB）remain restricted the promotion of the anammox technology in practical application.How to suppress and washout NOB and HB while retaining ammonia oxidizing bacteria（AOB）and anaerobic ammonia oxidation（Anammox）bacteria effectively was the key problem to be solved.Firstly,the individual enrichment culture and characteristics of the functional bacteria related to the mainstream anammox technology in urban sewage was studied,and the relevant operation strategy of enriching AOB and anammox bacteria while inhibiting NOB in the continuous flow process was discussed.Then,the feasibility of continuous flow nitritation process for municipal wastewater was investigated through alternating anoxic and aerobic conditions.Finally,a novel continuous flow mainstream anammox denitrification and phosphorus removal process for urban sewage was proposed using a modified integrated fixed-biofilm activated sludge（IFAS）with alternating anoxic and aerobic zones layout,and the start-up and stable maintenance of the process were studied in detail.This study would provide a theoretical guidance and technical support for the popularization and application of mainstream anammox technology in urban sewage in China.To solve the NOB excessive proliferation,the rapid achievement of nitritation in sequencing batch reactor（SBR）treating municipal wastewater was presented by combining aerobic starvation with real-time control.During the reactivation period,the activity recovery ability and functional gene expression level of AOB were faster than that of NOB,which ensured the rapid achievement of nitritation in SBR system and the enrichment of AOB.Then,the control strategy of steady continuous nitritation process in A²O system was studied,including low DO concentration,aerobic starvation treatment and bio-augmentation.Results showed that low DO concentration and aerobic starvation treatment were not suitable for the continuous nitritation process due to the high abundance of Nitrospira spp.in A²O system.Bio-augmenting with nitritation sludge was helpful to achieve and maintain the nitritation in continuous flow reactor for urban sewage.Nitrospira enriched sludge with a ratio of 32.12±0.25%were cultured with low DO/low nitrite concentration,while Nitrobacter enriched sludge with a ratio of59.46±4.71%were cultured with high DO/high nitrite concentration.The activity of Nitrobacter decreased rapidly in the starved environment,and the expression level of functional genes also decreased significantly.However,Nitrospira showed a slow decrease in its activity in the starved environment,and a temporary over-expressed functional gene was observed.This phenomenon further confirmed that Nitrospira adapted to the low substrate/low DO concentration environment had higher ability to resist the starved environment.The higher expression level of functional genes ensured the long-term activity and cell maintenance of Nitrospira in the starved environment.Therefore,the continuous flow system could not achieve nitritation using aerobic starvation treatment.Batch experimental results show that the transient anoxia caused by alternating anoxic and aerobic can effectively inhibit the activity of NOB,but the residual nitrite at the initial stage of the aerobic phase promotes the recovery of NOB activity and inhibits the activity of AOB.Thus,it is infeasible to maintain the continuous flow nitritation for urban sewage just by alternating anoxic and aerobic zones.In addition,a mathematical model in regard to the NOB specific growth rate(μ_NOB)and initial nitrite concentration at the beginning of aerobic phase was established.The modeling demonstrated that theμ_NOB gradually decreased under anoxic condition and the recovery ofμ_NOB increased with increasing initial nitrite concentration at the beginning of aerobic phase.Therefore,thorough nitrite removal through anaerobic ammonium oxidation or denitrification in anoxic zones is critical for maintaining the nitritation in a continuous system.Based on the above research,a continuous flow mainstream anammox denitrification and phosphorus removal process for urban sewage was developed using a modified IFAS reactor.The process was started-up and stabilized by bio-augmentation and hydroxylamine inhibition.In the absence of external carbon source,the average effluent concentrations of sCOD,PO₄^3--P,NH₄⁺-N and TN in this process for municipal wastewater treatment were 20.6±1.7 mg/L,0.4±0.2 mg P/L,2.3±1.7 mg N/L and7.7±1.8 mg N/L,respectively.The effluent of the mIFAS reactor meets the national effluent treatment plant effluent grade A discharge standard.Material balance analysis showed that the nitrogen removal pathways of this process mainly included denitrification（28.1³1.3%）and anammox（21.3³0.3%）in the anoxic zone,simultaneous nitrification and denitrification（9.6¹5.4%）in the aerobic zone,effluent（20.7%）and microbial assimilation（11.3%）.The phosphorus removal pathways of this process mainly include aerobic phosphorus uptake（63.9⁸7.4%）in aerobic zone and denitrifying phosphorus uptake（11.8³3.2%）by phosphorus accumulating bacteria in anoxic zone.Economic analysis showed that the modified IFAS mainstream anammox process in comparison with the traditional A²O process can save 89%of the chemical cost and 41%of the aeration energy consumption.It has the advantages including high nitrogen and phosphorus removal efficiency,energy saving and consumption reduction,and reducing greenhouse gas emissions.