Effects of leachate recirculation on geotechnical properties of municipal solid waste in landfills

Geotechnical properties of municipal solid waste (MSW) play an important role in the integrity and stability design of landfills. Due to accelerated degradation Change in the MSW characteristics results in the increased waste compressibility and settlement rates. Change in the physical and compositional characteristics of the field fresh and landfilled waste subjected to leachate recirculation were studied on the field using MODECOM protocol. Evaluation of geotechnical properties on these waste resulted in understanding the effect of recirculation. There was no predominant change in the geotechnical properties to establish the recirculation effects. However observed changes were attributed due to presence of more fines in the landfilled waste. Effect of long term recirculation and its influence on geotechnical properties of field fresh MSW were studied in the laboratory bioreactor cells. Based on the percentage of gas composition and methane production rate, samples were exhumed and tested for the geotechnical properties. Due to enhanced degradation reduction in particle size was observed. Hydraulic conductivity of the degraded fresh MSW decreased with increase in confining pressure and dry unit weight. Compressibility of the degraded MSW does not exhibit any trend with stages. Decrease in friction angle and increase in cohesion was observed with degradation. However change in the field MSW due to degradation was difficult to quantify due to heterogeneity of the waste, hence controlled laboratory degradation testing was performed on the synthetic MSW (SMSW) which stimulates the composition of MSW generated in the United States. Due to enhanced degradation, more than 73% reduction in organic content and 86% degree of decomposition was observed. Reduction in particle size and decrease in hydraulic conductivity by three to four orders with degradation. Compression ratio was reduced from 0.34 to 0.15 with degradation. Compressibility of the degraded SMSW was lesser than the fresh SMSW due to the presence of more fine. Drained direct shear test resulted decrease in friction angle and increase in cohesion with increase in degradation. Overall, this study showed that waste degradation is enhanced by leachate recirculation and these changes should be properly accounted in geotechnical analysis and design of safe and effective bioreactor landfills.