Nitrogen Removal By Partial Nitrification And Denitrification Process For Anaerobically Treated Leachate From Municipal Solid Waste Incineration Plant

The leachate from municipal solid waste incineration plant（incineration leachate）is anaerobically treated and the effluent of digester contains high ammonium concentrations as 1400～1800 mg NH₄⁺-N/L but with a low C/N,which is suitably treated by partial nitrification and denitrification process.The free ammonia（FA）in effluent of digester and the free nitrous acid（FNA）during partial nitrification are biological inhibitory factors.The ammonia oxidizing bacteria（AOB）will be noticeably inhibited when the biological inhibitory factors are at high concentrations.Meanwhile,the anaerobically treated Fv A in effluent（AFv A）is toxic to microbes,which also impairs the performance of biological nitrogen removal.Thus,biological nitrogen removal process in practical engineering is difficult to stably operate.The partial biological nitrogen removal process was used to treat the anaerobically treated incineration leachate in this study.The inhibition thresholds of biological inhibitory factors were determined and the operation was optimized.Subsequently the influence mechanism of biological inhibitory factors and Fv A were further analyzed.Firstly,biological inhibitory factors and optimal operation conditions were investigated in sequencing partial biological nitrogen removal system.The partial nitrification sludge was acclimated by controlling DO and FA.Under low nitrogen concentrations,the optimized alkalinity was 60%～80%of theoretical amount and optimized DO was 0.4～0.8 mg/L in the partial nitrification phase.The FA and FNA inhibition thresholds on AOB were determined as 110～120 mg/L and～0.1 mg/L respectively during elevated influent ammonium concentrations.The FA and FNA concentrations could be adjusted simultaneously via several times of alkalinity addition and thus anaerobically treated incineration leachate was steadily treated（0.56 kg N/（m³·d））.Heterotrophic ammonia oxidizing bacteria（HAOB）were enriched in system（30%～35%,mainly as Paracoccus）.The expression of heterotrophic amo A was consistently normal when FA concentrations were below inhibition threshold.But expression of autotrophic amo A was inhibited until FA concentrations dropped to low level（<45 mg/L）.When the FNA was above inhibition threshold,the expression of heterotrophic amo A was highly decreased.Secondly,the influence of Fv A on partial biological nitrogen removal was investigated in this study.The raw Fv A（RFv A）in leachate converted to AFv A during anaerobic treatment which had higher molecular weight,aromatic and humification degree.The hydroquinone structure was destructed in AFv A.Meanwhile,biotoxicity of AFv A impaired both nitritation and denitrification.AFv A could be degraded and converted in the partial biological nitrogen removal process.The nitrogen removal treated AFv A（NAFv A）in effluent had higher aromatic and humification degree compared to AFv A.The RFv A（0.5g/L）from raw incineration leachate functioned as redox mediators which could significantly improve the partial denitrification rates（147.3%）.RFv A could not be degraded and converted in the partial biological nitrogen removal process.The biotoxicity orderly increased among RFv A,AFv A and NAFv A.RFv A could accelerate microbial electron transfer process while AFv A and NAFv A inhibited the electron transfer process and NAFv A had more negative effects.Finally,based on the above results,the nitrogen removal performance was further improved via using MBBR continuous partial biological nitrogen removal system.The optimized operation conditions in partial nitrification tank were FA<40 mg/L and DO of～1.2 mg/L.The optimized C/N in partial denitrification tank was 3～4.The optimized reflux ratio combined system was 4.The anaerobically treated incineration leachate could be directly treated in optimized system with desirable nitrogen removal performance（ACE>99%,TN removal efficiency>86%）and partial nitrification stability（NAR>96%）.The AAOB（15%～20%）was significantly enriched in partial nitrification tank,which improved ammonium oxidizing efficiency.HAOB contributed to maintain the ammonium conversion at higher FA（>25 mg/L）.The enrichment of Hydrogenophaga at excessive FA（>40 mg/L）caused instability of partial nitrification.The dominant denitrifying bacteria in anoxic tank were Rhodobacter（28.1%）and Hydrogenophaga（14.0%）.In this study,based on the optimized operation conditions,the anaerobically treated incineration leachate could be directly treated via partial biological nitrogen removal process.The threshold concentrations of biological inhibitory factors were determined and the dominant functional microbes for nitrogen removal under different operation conditions were revealed.The related mechanisms were further analyzed.This study can provide theoretical reference and technical support for efficient and stable nitrogen removal in treatment of leachate as well as wastewater with high-strength ammonia.