| This thesis is based on an experiment to investigate how low percentages of different modifying sorbents affect the overall hydraulic conductivities and attenuation efficiency of a clayey soil sample. The most widely used of the three standard methods of municipal solid waste disposal is landfilling of waste in engineered landfills.The main intention for using liners at the bottom of landfills is to slow down the rate of pollutant transport into groundwater resources whilst the same materials can be used to build a cap over a closed landfill to prevent the entry of surface water into the landfill. Natural mineral liners; mainly clayey soils are widely used for these purposes.However, there have been some problems, which make the mineral liners not quite efficient. For example, the clay mineral liners can be dissolved by some chemicals present in leachate, can be affected by freezing and thawing during the seasons, uneven settlement, causing shear failure and puncturing by heavy landfill machinery. Moreover, the current design concept of a lining system based on advection may not offer protection to our groundwater resources forever. There is therefore the need to for ways to enhance the performance of the mineral liners.To investigate how different modifying sorbents would affect the overall hydraulic conductivities and attenuation efficiency of the sample clayey soil, compacted clay liners, each of thickness 3 cm were modified with bentonite, zeolites, and lime added in low percentages as shown in Table 3-1.The liners labeled as in Columns A, B, C, D, E, F, G and H were used to evaluate how the modification with sorbents would affect the hydraulic conductivity, pH, electrical conductivity, hardness, COD, BOD5, ammonium-nitrogen, nitrate-nitrogen and nitrite-nitrogen. Column A, which was 100% clay, was used as the control.In the experiment, the compacted clay liner systems in Columns A, (100% clay) Column C (10% zeolite +90% clay), achieved hydraulic conductivity values of the order 10-7 cm/s which is the maximum limit permitted by most countries. Columns B, D, F, G and H had hydraulic conductivity values in the order of 10-8 cm/s. Based on the experimental results, it can be deduced that Column B, (15% bentonite +85% clay), Column D, (10% lime+90% clay) Column F, (5% zeolites+10% bentonite+85% clay) Column G (5% lime +5% zeolites +90% clay) and Column H (5% bentonite +5% lime +5% zeolites + 85% clay) put up better performance in terms of lower hydraulic conductivity amongst the others. The liner in Column E, (5% lime +10% bentonite +85% clay) was so permeable that its results cannot be of any significance in this research, thus it has not been discussed in the thesis. Though Columns A, and C, had hydraulic conductivities at the maximum threshold and the liners in Columns B, D, F, G and H had lower hydraulic conductivities, economic considerations may not make the use of Columns F, G and H practicable to be used as materials for the bottom liner and the top caps of landfills since they consist of mixtures of modifiers as compared to Columns B and C which had only single modifiers. With such low levels of hydraulic conductivity, the compacted clay liners in Columns B and D may constitute more viable choices as materials for municipal landfill liner system.Considering the low percentages of sorbents added to the clay, the liners provided good results, in terms of low hydraulic conductivity values, but did not perform consistently in terms of attenuation. Results of the experiment also suggest that the liners may not be able to effectively remove microbes or natural organics for very long periods, thus the sustainability of the liners in the removal of some contaminants may have to be further evaluated. The findings of this research give us the understanding of the physico-chemical properties of bentonites, zeolites and lime when used to modify clays for liners. The research also points out that the utilization of these modified compacted clays as components in landfill lining systems, may... |
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