Novel Composite Landfill Liners

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NOVEL COMPOSITE LANDFILL LINERS

An ENTRUST project jointly undertaken by Imperial College and Coventry University,

1998 - 2005

PROJECT PERSONNEL

Professor Alan Atkinson Imperial College

Dr Peter Claisse Coventry University

Dr Mark Tyrer Imperial College

Dr Essie Ganjian Coventry University

Sam Dewsnap P S D Associates

INTRODUCTION

This research programme aims to develop alternative types of liner for leachate containment in waste disposal. These new barriers offer substantial technical, economical and environmental benefits relative to types in current use. The programme started in 1998 and the first project (designated phase 1a) was completed in August 2001. The intended duration and objectives of each phase are given in the table below.

Phase	Start Date	End Date	Objectives
1a	1998	2001	Initial investigation of candidate concrete mixes. Construction of trial cells. Laboratory measurements of strength and permeability and investigation of reaction kinetics.
1b	2002	2004	Design of large-scale trial of the barrier system. Obtain approval for its construction. Evaluate further waste materials. Obtain further results from the existing site trials. Measure diffusion and leaching rates in the candidate concrete mixes.
2	2004	2005	Construct large-scale trial. Trial cell will have extensive instrumentation, but will be used for large-scale waste disposal.
3	2005		Full scale commercial use

The project seeks to minimise wastes from the metals and processing industries by re-cycling them as alternative materials for use as landfill liners. To this end, a consortium of waste producers contributes to the work, under the management of the Mineral Industry Research Organisation, MIRO.

PHASE 1A

The objectives of this phase were to analyse the wastes arising from each partner’s operation and to assess their suitability for use as landfill liner materials. Subsequently, these wastes were combined to produce a novel composite landfill liner, which was used in the three field trials at Risley in Cheshire.

The wastes fall into three principle material types and all have been physically and chemically classified:

1) Spent foundry sands. Residues from the castings industry, these materials are principally quartz sands with residues of thermally degraded binders such as clay minerals (green sand) and phenolic resins combined with carbon char (shell sand).

2) Semi-crystalline slags from the metals refining industry such as borate-zinc oxide melts, alkaline sulphates, ferrosilicates and heavy metal–bearing “soda” slags.

3) Liquid raffinates such as alkaline sodium sulphate solution produced during acid neutralisation of processing waste.

The classification has involved particle size analysis of the solids, optical and electron microscopy, physical testing and both classical and instrumental chemical analysis.

Subsequent laboratory work has focussed on examining these materials as cementitious binders in their own right, as cement replacement materials or as chemical activators for other cementitious materials. This has allowed the solids to be grouped into those materials which have cementitious properties, those which are relative chemically inert and would be suitable for use as aggregates and to identify any materials which are not suitable for use as liner materials. In making this classification, extensive physical testing has been undertaken, examining expansion, compressive strength, permeability and resistance to chemical attack by synthetic leachates. Following experiments evaluating permeability and compressive strength, analytical electron microscopy has been used to identify dewatering pathways and modes of failure.

The materials testing programme has identified a number of promising material combinations and these have been used to produce the three experimental landfill liners trial cells as shown in Figure1. The design philosophy is that of a multi-layer barrier in which the performance of each layer compliments the others.

Composition	Layer	Function
Concrete made with, for example, alkali activated slag, spent foundry sand and metallurgical slag aggregate.	Top Layer Concrete 200mm	Mechanical support of vehicles during operational phase. Protection of clay layer
Non-swelling clay	Middle Layer Clay 500mm	Hydraulic barrier and ion exchange medium
Concrete made with, for example, cement kiln dust and crushed demolition waste	Base Layer Concrete 300mm	Chemical conditioning of leachate by alkaline matrix. Suppression of metal solubilities.
	Host Geology

Figure 1. Detail of the new composite landfill liner system used in phase 1a

The role of the base layer is to provide a strong foundation, which will support the hydraulic barrier but also contribute to the chemical conditioning of leachate which will percolate through the structure in the final stages of the post-closure period. The middle layer consists of non-swelling clay (possibly artificial), compacted to provide a hydraulic barrier which will prevent leachate migration for some hundreds of years.

On top of this, is a top layer of concrete which fulfils two functions; in the operational phase of the landfill, it will support vehicles allowing them to drive directly on to the liner and in the post closure phase, will contribute to the physical containment of the leachate but more importantly, chemically condition the leachate, neutralising organic acids and precipitating heavy metals.

Figure 2: Placing and compacting of middle clay layer by machine bucket.

PHASE 1B

The objectives of this phase are:

1. To design a large-scale trial of the novel cementitious barrier system and obtain approval for its construction.

2. To evaluate further different waste materials for use in cementitious liners.

3. To obtain further results from the existing site trials.

4. To measure diffusion and leaching rates in the candidate concrete mixes.

1. The Design of the Large Scale Trial.

The main input to this work will be the requirements of the Environment Agency. Contact has already been established with the Agency and specific approval was given for the construction of the test cells in phase 1a. Further contacts will be made at regional level in the area where the trial is to take place and also at national level (Bristol Headquarters).

The large-scale trial (Phase 2) will form either a major part of, or all of, a disposal cell. The design will include the following elements:

· Geotechnical design - to be carried out in collaboration with the site operator.

· Engineering design of barrier - based on the results of the laboratory and site experimental work.

· Performance assessment - Numerical modelling is seen as the key to predicting the spatial and temporal evolution of the liner system. Using properties measured in that laboratory and in the field in combination with solicited expert judgement, a system model will be developed with which the chemical and physical evolution of the containment system will be simulated. This will involve reactive chemical-transport modelling using a range of codes and data.

At the initial stage potential sites will be investigated and preliminary designs will be developed. As soon as an initial design is prepared it will be presented to the EA and all of their requirements for further design or research work will be implemented. Monitoring systems for the trial will also be designed. These will, at minimum, comply with any EA requirements and include in-situ monitoring and piezometer arrays.

As part of the project, generic design requirements for cementitious barriers will be developed for use at other sites and to contribute to future EA guidelines.

2. Evaluation of Further Waste Materials.

It is important that the range of materials under test should be continually increased. The basis of the new barrier types is concrete made from combinations of waste materials. No single waste stream can be used for this and widening the range of different materials permits the production of better concretes using both the new materials and those already in the programme. This type of investigation will become yet more important as the designs become site specific and transport costs are included in the design evaluations.

The following materials have already been identified for possible inclusion: Glass polishing residues, Demolition fines (from the crushing of demolition waste), Steel converter slags, Glass cullet, Sugar refining wastes, Masonry wastes from furnace re-lining

3. Monitoring of existing cells.

The monitoring of the existing cells will continue throughout the programme if access to them remains possible. While the cells remain accessible pore fluid recovery and analysis will continue.

The development of test methods will continue both to assist with the monitoring of the existing cells and to help with the design of the monitoring methods for phase 2.

4. Measurement of diffusion and leaching.

Two main processes control transport of harmful species through a landfill barrier:

· Pressure driven liquid flow (advection). This depends on permeability which has been measured extensively on the candidate mixes in phase 1a using novel experimental methods developed during phase 1a.

· Diffusion. This is the movement of aqueous species through porous media and is determined by the coefficient of diffusion and their relative activities in solution. Thin discs of the material will be tested with measurements made with ion-specific electrodes and by classical (wet chemistry) methods as appropriate.

Both tests are affected by adsorption and leaching (these have been measured in the permeability tests) and will also be measured in the diffusion tests. Industry standard leaching tests will also be carried out for comparison purposes. It is anticipated that these will be required by the EA.