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Biological nitrification and dentrification of high ammonia landfill leachate using pre- and post-dentrification systems and methanol as supplementary source of organic carbon Ilies, Poesis

Abstract

Landfill leachate discharges characterized by high nitrogen concentrations are detrimental to the environment. This research investigated nitrogen removal capabilities of a nitrification with pre- and post denitrification biological process, when treating a landfill leachate characterized by high ammonia concentrations and low levels of biodegradable organics. The process configuration is generally referred to as 4-Stage Bardenpho process, and consists of a sequence of anoxic and aerobic zones. Two 4-Stage Bardenpho systems with first anoxic reactor actual hydraulic retention times of 1.5 and 1.7 hours, respectively, were operated in parallel. The first system had a first aerobic reactor actual hydraulic retention time of 3 hours, while the second system had one of 3.4 hours. Complete nitrification and denitrification of the "base" landfill leachate, with an average ammonia concentration of 200 mg N/L and organic carbon levels of less than 50 mg N/L, was achieved during the Base Leachate Phase. Higher concentrations were simulated by artificially increasing influent ammonia concentration to about 2200 mg N/L during the Ammonia Loading phase. The pH Phase examined the acclimatization of the bacterial populations to decreased reactor pH levels, as a means to improve system overall denitrification performance and decrease effluent residual NOx concentrations. The overall process performance, at progressively decreased ambient temperature from 20 °C to 10 °C, was investigated during the Temperature Phase. Both systems experienced nitrification inhibition during the first attempt at incrementally increasing influent ammonia concentrations to over 2200 mg N/L. Methanol loadings increased concomitantly with ammonia loadings, to match expected aerobic NOx production and using CH3OH:NOx ratios of about 20:1; this resulted in methanol bleeding into the first aerobic zone, enhanced aerobic heterotrophic growth, and further inhibition of the mtrifying population, already inhibited by the recycling through the elevated "free" ammonia levels of the first anoxic zone. When the systems were allowed to acclimatize to each incremental ammonia increase, and with methanol loadings changed to yield CH₃OH:NOx removed ratios of only 5:1, the desired influent ammonia concentration was reached within 88 days from the start of the second attempt. The systems generated ammonia free effluents with NOx concentrations of 78 mg N/L and 250 mg N/L, respectively, treating simulated landfill leachate containing over 2200 mg N/L of ammonia. The systems started to respond positively to decreased reactor pH values by producing effluents with lower NOx concentrations; However, subsequently, due to some inhibitory constituents in the natural landfill leachate, percentage denitrification rates decreased to about 50% and the systems produced effluent residual NOx concentrations of about 170 mg N/L, When ambient temperature decreased from 20 °G to 17 °C, and, subsequently, to 14 °C, there seemed to be no negative influence on the nitrification processes of the systems, while denitrification inhibition was observed starting with the ambient temperature of 17 °C. Nevertheless, at ambient temperatures of 10 °C, the percentage nitrification rates of the systems decreased from about 100% to 10% and 30%, respectively, while percentage denitrification rates decreased to less than 5%. Decreased bacterial growth rates and nitrifiers inhibition at low temperatures were suspected to be the principal factors that determined the failure of the processes. However, percentage total ammonia removal of about 50% was maintained by both systems. Except for the decreased ambient temperature and system failure periods, the overall performance of the system with first anoxic reactor actual hydraulic retention time of 1.5 hours, and first aerobic reactor actual hydraulic retention time of 3 hours, was more effective than the performance of the system with actual hydraulic retention times of 1.7 hours, and 3.4 hours, respectively, as the system usually generated ammonia free effluents with lower NOx concentrations.

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