Landfill has been the principal method of disposal for residual wastes in the UK and many other countries for over a century. Sixty years ago, much of our domestic waste was biologically inert (primarily ash and cinders) and its use to fill inconvenient holes in the ground probably seemed logical. Over the years, however, changes in technology, legislation and lifestyle have led to corresponding major changes in the composition of domestic waste. By the early 21st century, the biodegradable fraction (paper, kitchen and garden waste) had grown to almost 60%. As a result, predicting and dealing with the consequences of the settlement of landfilled waste has come to represent a major challenge to those responsible for the operation and aftercare of domestic waste landfills. Settlement of the waste will affect the useful life of a landfill, and the levels to which it must be filled prior to closure if the required long-term landform contours are to be achieved. Waste settlement during the operational stage and after closure may affect the integrity and performance of engineered features such as gas and leachate extraction and control wells, side liners and capping layers.
Initial attempts to predict landfill settlements and settlement rates were largely empirical, and based on the concept of simple mechanical creep. This approach ignored the fact that settlements arising from the biodegradation of the waste will take place at a rate dependent on the nature of the biodegradable material, its water content and the leachate management regime within the landfill. In recent years, as awareness of the need to understand and manage landfills and landfill processes more closely has grown, the requirement for a more scientific approach to predicting biologically induced settlements of landfills has been increasingly recognised. This is essential if the effects of potential variations in waste composition, waste type, waste structure and landfill operational regime—in some cases in response to the EU Landfill Directive—are to be assessed and taken into account in landfill management and aftercare. The situation is further complicated by the mutual interdependence of biological activity, fluid flow and the hydro-mechanical properties of wastes. Water content and leachate flow encourage biodegradation, while biodegradation generates gas and causes settlement of the waste and collapse of its pore structure—both of which potentially inhibit liquid flow.
Over the past five years or so, various groups around the world have turned their attention to developing better models of landfill processes and obtaining high quality experimental data. The paper by Ivanova et al.1 describes the results of a 919 day laboratory experiment, in which the degradation and settlement of 160 litre samples of shredded waste in consolidating anaerobic reactors (CARs) were closely monitored. The initial part of this dataset formed the basis of the challenge issued to the landfill process modelling community described by Beaven et al.,2 which was in essence to predict the behaviour of the waste in the CARs over the full 919 days of the experiment, given details of the waste and the experimental set-up and leachate quality data from the first two months. The papers by Lobo et al.,3 Clewes et al.4 and Reichel and Haarstrick5 describe the responses to the challenge by three groups of landfill process modellers in Spain, the UK and Germany, using models of varying complexity. In the November issue of Waste and Resource Management, we will publish papers detailing the responses of other groups, together with some further experimental results linking changes in the chemical composition of the waste and leachate to gas production and a commentary on the comparative performance of the various modelling approaches adopted. The authors of the prediction papers will have the opportunity to respond to the commentary paper and make further observations on the fit between their model and the experimental data in future issues of Waste and Resource Management.
At the start of the challenge, the modellers were given only the information contained in the paper by Beaven et al.2 The behaviour they were trying to predict (reported by Ivanova et al.1) was not revealed to them until after their predictions had been submitted to the challenge organisers. Thus to have entered the challenge was in a professional sense quite brave, and my thanks goes to all of the groups who have participated, as well as to the organisers of the challenge. I believe that these two issues of Waste and Resource Management will serve three major purposes. First, they will represent a definitive statement of the current state of the art in understanding and modelling landfill processes. Secondly, they will provide an archival record of the progress that has been achieved in this area over the past six years or so. Thirdly, and most importantly, they will provide the starting point for the further developments needed if we are to meet the real challenge of returning residual wastes to the environment sustainably. This will involve the whole-life management of landfills, including during the remediation and aftercare periods, in ways that do not damage the environment or compromise natural resources. The timescale for historic landfills to reach ‘completion’ (i.e., a stable, non-polluting state in equilibrium with the surrounding environment) under current management regimes is very long, and cannot be regarded as sustainable. The wastes that will be landfilled in the future will include incinerator, air pollution control, filter cake and mechanical-biological treatment residues. These will have very different chemical, biological and mechanical properties from the wastes we are used to dealing with, and we will need tools and models of the type described in these volumes if we are to manage them safely and responsibly.
Waste and Resource Management is part of the Proceedings of the Institution of Civil Engineers. The journal aims to publish original contributions on research and practice relating to all civil engineering and construction aspects of the resource management cycle, from waste minimisation through the reuse and processing of waste materials to the management and disposal of residual wastes. Articles covering relevant legislation, standards, socio-economic and sustainability matters are welcomed.
