A study of the structure in solid wastes and some implications for fluid flow in landfills

Abstract

The search for alternative landfill operation and management strategies has triggered the development of the concept of a landfill as a bioreactor. The application of the concept requires the recirculation of liquids and hence a better understanding of fluid flow and transport processes that are strongly controlled by the physical structure of the media. It is generally accepted that as a result of the deposition in progressive layers, compaction and heterogeneity; solid waste develops a strong and anisotropic structure. Also, that due to their flat shape and orientation, materials such as plastics and textiles can influence flow behavior. The aim of this research was to provide a better understanding of the structure that develops when solid wastes are landfilled and the influence of this structure on fluid flow. The research included a characterization and description of specimens of raw household waste (MSW) and pretreated wastes (MBT) using PSD mathematical models, an study of the changes caused to particle size and shape by degradation processes, a study of the effect that flat shaped particles have on the fluid flow characteristics of a porous medium, and an investigation of the structure of a MSW specimen applying invasive and non-invasive radiographic techniques. The study revealed that the characteristics of particle size and shape differ between waste materials and also change with degradation. An MBT specimen that had gone a dual anaerobic aerobic treatment showed statistically insignificant changes in particle size and shape with degradation, whilst partially treated MBT and MSW specimens showed significant changes in the particle size and in the content of flat shaped materials. PSD models were successfully fitted to the different specimens investigated suggesting that analytical expressions can be incorporated into existing waste behavioural mathematical models to characterise the particle size. Flat shaped particles that comply to be at least 15 times larger than the matrix particles and constitute at least 7.3% by dry mass were found to reduce the hydraulic conductivity by a factor of more than 30%. The reduction factor is controlled by the relative content and size of the intrusive particles and it is always within one order of magnitude. The use of dye tracer visualization, thin sectioning and ?CT techniques were pioneered during this research for the study of preferential flow and the structure in solid waste. This study evidenced that the presence of high content of inert coarse flat shaped materials in a specimen of MSW resulted in the development of a strongly layered structure, with large pores horizontally connected and that favoured preferential flow.

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ORCID for W. Powrie: orcid.org/0000-0002-2271-0826

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