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Ammonia gas dynamics in four Vancouver area landfills

University of British Columbia

A nine month field and laboratory study was undertaken to measure, predict and model the variation of detected ammonia concentrations in landfill gas. An additional side study attempted to characterize organic trace contaminants found in landfill gas. The field project consisted of biweekly sampling of gas extraction wells from four Vancouver-area landfills for the analysis of NH₃-N in the gas and leachate. Methane and other common landfill gases were also analyzed. The wet chemical boric-acid sampling technique used in this study was estimated to have a ammonia gas recovery efficiency of 50 %. Other than a low recovery efficiency, problems encountered with this sampling technique was the high humidity and negative interferences inherent in the landfill gas. Laboratory analysis of the collected NH₃-N gas samples was by the automated phenate method, which could detect NH₃-N gas concentrations greater than 10 ppb. The NH₃-N concentrations in gas were found to exceed 600 ppb, but were more commonly in the 50 to 200 ppb range. In the statistical and graphical analysis, gas temperature and precipitation were found to correlate the most to the variation in ammonia gas concentration, while leachate ionic strength correlated strongest with most CH₄ % analysis. Prediction of both NH₃-N gas and CH₄ % by regression analysis was found to be suspect due to low R² values and non-normality of some data. Four different Henry's Law constants of ammonia gas were evaluated to help predict the concentration of NH₃-N in the gas phase. The combination of already measured NH₃-N leachate concentrations and Henry's Law constants yielded results that over and underpredicted measured NH₃-N gas data by 2000 fold or more. This leads the author to believe Henry's Law may not be applicable in a landfill environment due to non-equilibrium conditions coupling with various other reaction mechanisms. Comparison of landfill ammonia gas flux rates with total ammonia leachate flux rates in two of the four landfills yielded an insignificant gas flux rate of less than 0.03 % of the total leachate NH₃-N fluxes. The NH₃-N gas flux results were calculated from a spreadsheet emission model employing both convection and diffusion flow through the landfill cover. A comparison of the emission model results for the 20 ha Richmond landfill study area (3.862 kg/yr) compared favorably to the mass flux results determined from a simple gas generation mass balance model.

Text

2010-09-16

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http://hdl.handle.net/2429/28506

Civil Engineering

University of British Columbia

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