R.P. Beaven

Developments to a landfill processes model following its application to two landfill modelling challenges.

The landfill model LDAT simulates the transport and bio-chemical behaviour of the solid, liquid and gas phases of waste contained in a landfill. LDAT was applied to the LMC1 and LMC2 landfill modelling challenges held in 2009 and 2011. These were blind modelling challenges with the model acting in a predictive mode based on limited early time sections of full datasets. The LMC1 challenge dataset was from a 0.34m deep 0.48m diameter laboratory test cell, and the LMC2 dataset was from a 55m×80m 8m deep landfill test cell which formed part of the Dutch sustainable landfill research programme at Landgraaf in the Netherlands. The paper describes developments in LDAT arising directly from the experience of responding to the two challenges, and discusses the model input and output data obtained from a calibration using the full datasets. The developments include the modularisation of the model into a set of linked sub-models, the strategy for converting conventional waste characteristics into model input parameters, the identification of flexible degradation pathways to control the CO2:CH4 ratio, and the application of a chemical equilibrium model that includes a stage in which the solid waste components dissolve into the leachate.

Leachate recirculation in a landfill: Some insights obtained from the development of a simple 1-D model

The re-introduction of leachate back into the waste can play an important part in landfill management. It can encourage biodegradation by raising the water content and transporting bacteria, nutrients and waste products. It also enables leachate to be stored within the body of the landfill, for example to help minimise temporal variations in the load on a leachate treatment plant. It is helpful for a landfill operator to be able to estimate the rate at which the landfill can accept leachate (the maximum infiltration or injection rate), the storage capacity of the landfill and the leachate retention time. This paper discusses some of the insights obtained from the development and application of a simple conceptual model of leachate recirculation that can be used to estimate key parameter values on the basis of the hydraulic properties of the waste. The model is described, partly validated against a more rigorous numerical analysis, and then used to interpret data obtained from field tests on a real site. The shortcomings of the model in its current form are discussed, and suggestions are made as to how these might be addressed in the context of developing the model as a design tool.

Clogging of landfill tyre and aggregate drainage layers by methanogenic leachate and implications for practice

This paper reports the results of pilot scale tests carried out to investigate the clogging of shredded and baled tyres in comparison with aggregates when percolated by leachates representative of those generated by methanogenic stage landfills. Realistic lifetime loading rates of methanogenic leachate were applied, and clogging was not generally apparent in any of the drainage media studied. This is in apparent contrast to many other studies that have demonstrated the susceptibility of all forms of drainage media to biological and chemical clogging when percolated with high strength organic and calcium rich leachates. The reasons for this difference are identified, the implications for landfill drainage system design are discussed and some suggestions for operational practice are presented for discussion.

Hydraulic properties of MSW

The distribution and movement of water within a waste landfill are important for two main reasons. First, water represents a major potential pathway for pollution of the surrounding environment. As liquid passes through the waste to emerge as leachate, it dissolves or carries in suspension substances that could cause contamination through interaction with the natural groundwater. Artificial and natural barriers help to prevent this, but it is usually also necessary to control leachate pressures (levels) – usually by pumping from wells or drainage blankets – so as to minimise head differences acting on a sealing layer or a natural barrier boundary. Leachate movement through the waste mass in response to pumping is controlled by the hydraulic conductivity or permeability of the waste. The distribution of permeability within the waste, which will vary as a result of anisotropy, heterogeneity, partial saturation and changes in waste density or effective stress, is also of vital importance. The total and drainable porosity of waste will control the rate of build up of leachate levels in response to infiltration or other water inputs, and the rate of decline when dewatering a site. It is also necessary to understand and control the impact of pore pressures on mechanical stability. Secondly, water and water flow are essential to achieving landfill completion, i.e. bringing it to a stable, non-polluting state in a controlled way. Municipal solid wastes (MSW) have for the past 50 years contained a high proportion of biodegradable components: water is essential to the biochemical decomposition of organic substances (Pohland 1975; Leckie and Pacey 1979; Klink and Ham 1982). Water flow is also needed for the leaching out of soluble compounds, even in an inert or organically stabilised waste.

A new economic instrument for financing accelerated landfill aftercare

The key aspects of landfill operation that remain unresolved are the extended timescale and uncertain funding of the post-closure period. This paper reviews the topic and proposes an economic instrument to resolve the unsustainable nature of the current situation. Unsustainability arises from the sluggish degradation of organic material and also the slow flushing of potential pollutants that is exacerbated by low-permeability capping. A landfill tax or aftercare provision rebate is proposed as an economic instrument to encourage operators to actively advance the stabilization of landfilled waste. The rebate could be accommodated within existing regulatory and tax regimes and would be paid for: (i) every tonne of nitrogen (or other agreed leachate marker) whose removal is advanced via the accelerated production and extraction of leachate; (ii) every tonne of non-commercially viable carbon removed via landfill gas collection and treatment. The rebates would be set at a level that would make it financially attractive to operators and would encourage measures such as leachate recirculation, in situ aeration, and enhanced flushing. Illustrative calculations suggest that a maximum rebate of up to ∼€50/tonne MSW would provide an adequate incentive.

A multi-component two-phase flow algorithm for use in landfill processes modelling.

This paper describes the finite difference algorithm that has been developed for the flow sub-model of the University of Southampton landfill degradation and transport model LDAT. The liquid and gas phase flow components are first decoupled from the solid phase of the full multi-phase, multi-component landfill process constitutive equations and are then rearranged into a format that can be applied as a calculation procedure within the framework of a three dimensional array of finite difference rectangular elements. The algorithm contains a source term which accommodates the non-flow landfill processes of degradation, gas solubility, and leachate chemical equilibrium, sub-models that have been described in White and Beaven (2013). The paper includes an illustration of the application of the flow sub-model in the context of the leachate recirculation tests carried out at the Beddington landfill project. This illustration demonstrates the ability of the sub-model to track movement in the gas phase as well as the liquid phase, and to simulate multi-directional flow patterns that are different in each of the phases.

The influence of landfill gas on the hydraulic conductivity of waste

Knowledge of the hydraulic conductivity of waste is essential to the efficient and effective management of leachate in landfills. Over the past few years it has become apparent that the presence of gas in a waste landfill will affect the hydraulic conductivity. This paper reports the results of tests carried out in a large scale (2 m diameter) compression cell to investigate the influence of gas on the hydraulic conductivity of municipal solid wastes compressed to different initial stresses, at different pore water pressures.

Enhanced biodegradation at the Landgraaf bioreactor test-cell

From 2001 to 2011, a bioreactor demonstration was performed in a 25,000m(3) (8m deep, 3500m(2) surface) test-cell. In this bioreactor, biodegradation was enhanced by premixing and homogenizing of waste, recirculation of leachate and aeration. Anaerobic biodegradation was completed within four years and was followed by two years of aeration. Ultimately a residue was obtained that had lost approximately 95% of its biogas potential. Biodegradation resulted in a significantly reduced leaching potential for dissolved organic carbon (DOC) and specific heavy metals. For other inorganic components, less progress was achieved. Increased flushing would be required for further reduction of the leaching potential. A significant reduction in chemical oxygen demand (COD) and ammonia (NH4(+)) in leachate was not demonstrated during the relative short-term aeration: COD concentrations actually increased slightly and there was no effect on NH4(+). During the project, it became clear that moisture flow through the waste followed preferential flow paths. Therefore, attention was also paid to gain better understanding of leachate flows. From a tracer test, it was concluded that part of the waste contaminants are held in immobile blocks and are to a large extent unaffected by flow occurring in the surrounding preferential flow paths.

Sorption of Mecoprop by two clay landfill liner materials: Oxford Clay and Mercia Mudstone

Abstract As part of a programme of research into natural attenuation of contaminants in landfill liners, the sorption and desorption of Mecoprop (MCPP) by mineral liner materials was investigated. Mercia Mudstone and Oxford Clay have the potential to attenuate MCPP by sorption processes, which were observed to be rapid in both materials. Sorption to iron hydroxides was thought to be the main mechanism operating in Mercia Mudstone and may be responsible for some of the sorption occurring in Oxford Clay. Additional sorption to Oxford Clay may be related to its kerogen content, as this type of organic matter has been shown to be highly sorbent for hydrophobic organic compounds, but further research is needed to assess whether kerogen has a high affinity for polar compounds such as MCPP. Desorption tests demonstrated that desorption was rapid and that sorption was potentially reversible. There was some evidence that tannic acid in the synthetic leachate did not affect MCPP sorption, but the high ionic strength leachate reduced sorption of MCPP in Oxford Clay. For regulatory risk assessments, this result emphasizes the importance of conducting sorption tests in a matrix similar to that of the environment under investigation to obtain realistic results for sorption coefficients.

Multiple-tracer tests for contaminant transport process identification in saturated municipal solid waste

Two column tests were performed in conditions emulating vertical flow beneath the leachate table in a biologically active landfill to determine dominant transport mechanisms occurring in landfills. An improved understanding of contaminant transport process in wastes is required for developing better predictions about potential length of the long term aftercare of landfills, currently measured in timescales of centuries. Three tracers (lithium, bromide and deuterium) were used. Lithium did not behave conservatively. Given that lithium has been used extensively for tracing in landfill wastes, the tracer itself and the findings of previous tests which assume that it has behaved conservatively may need revisiting. The smaller column test could not be fitted with continuum models, probably because the volume of waste was below a representative elemental volume. Modelling compared advection-dispersion (AD), dual porosity (DP) and hybrid AD-DP models. Of these models, the DP model was found to be the most suitable. Although there is good evidence to suggest that diffusion is an important transport mechanism, the breakthrough curves of the different tracers did not differ from each other as would be predicted based on the free-water diffusion coefficients. This suggested that solute diffusion in wastes requires further study.