Feasibility study, Design, Construction and Operation of Landfills.

What is Landfill?

Worldwide, the disposal of waste materials is an increasing problem. A landfill is a living organism, bringing constantly new engineering challenges. This applies not only to landfills but also to situations where there is a need to protect native ground from contamination by external sources, such as mining leachate lagoons or heap-leach pads, storm water attenuation ponds next to highways and slurry storage cells in agriculture.

We offer clients our expertise regarding the development of integrated design and construction of landfills and the protection of polluted areas. Flexibility and extensive manufacturing capability allow us to customize products to suit specific project requirements. We care about safety and the environment: the presence of our personnel during the preparation of EU landfill regulations, together with the CE markings, led us to offer products and solutions that meet appropriate legislative requirements.

Construction of Landfills.

Cells

Cells are sub-divisions of phases. The number of cells in a phase and cell size should be based on water balance calculations. Consideration should be given to a combination of the factors discussed under ‘phasing’ and from constraints on vehicular maneuvering. Minimising the cell size facilitates the landfilling operation through reducing leachate generation, minimising the area of exposed waste thereby reducing cover requirements, and assisting in the control of windblown litter. For each cell the designer should indicate estimated void space volume, active lifespan, and development sequence.

Surface Water Systems

The design of surface water drains is usually based on storm events with specified return period and duration of rainfall. Common return periods for design purposes are 1, 5, 10, 25 and 50 years. The return period may be selected based on site characteristics, the risk of failure and the consequences of failure of the drainage system. It should be noted that longer return periods will lead to systems with greater capacities but at a higher cost. The peak discharge rate and run off volume during peak discharge should be determined. Design methods used include.

LINING SYSTEMS

The lining system protects the surrounding environment including soil, groundwater and surface water by containing leachate generated within the landfill, controlling ingress of groundwater, and assisting in the control of the migration of landfill gas. The selected liner system must achieve consistent performance and be compatible with the expected leachate for the design life of the facility.

Natural clays of low hydraulic conductivity, such as clays, silty clays and clayey silts, have the potential to make good liners. The continuity and hydraulic conductivity of in situ natural liner materials are difficult to predict and expensive to prove and for this reason engineered liners are recommended.

GEOMEMBRANES (FLEXIBLE MEMBRANE LINERS)

There are many types of geomembranes, or flexible membrane liners as they are often called, available. A geomembrane for use as a component in a basal landfill liner should have a low permeability, a physical strength capable of withstanding mechanical stresses and strains, and be chemically compatible with the waste contained by the liner.

LEACHATE MANAGEMENT

Leachate produced in a landfill is a liquid which has percolated through the waste, picking up suspended and soluble materials that originate from or are products of the degradation of the waste. Before considering the design of any leachate management system it is important to consider the objectives that are to be achieved. Leachate needs to be controlled in a landfill for the following reasons:

• to reduce the potential for seepage out of the landfill through the sides or the base either by exploiting weaknesses in the liner or by flow through its matrix;
• to prevent liquid levels rising to such an extent that they can spill over and cause uncontrolled pollution to ditches, drains, watercourses etc.;
• to influence the processes leading to the formation of landfill gas, chemical and biological stabilisation of the landfill;
• to minimise the interaction between the leachate and the liner;
• in the case of above ground landfill, to ensure the stability of the waste.

LEACHATE TREATMENT

The main constituents of leachate requiring treatment are the ammoniacal content and the organic constituent of the leachates. Treatment methods may be divided into four categories:

• Physical/chemical pre-treatment;
• Biological treatment;
• Combination of 1 and 2 in one system;
• Advanced Treatment

Physical-chemical pretreatment methods are particularly useful in treating leachate from older/closed landfills that have lower biodegradable organic carbon, or as a polishing step for biologically treated leachate.

Biological treatment methods are classified as aerobic and anaerobic according to microbial metabolism. In aerobic processes, micro-organisms generate energy by enzyme mediated electron transport using molecular oxygen as electron acceptor. In anaerobic processes, inorganic compounds such as nitrate, sulphate and carbon dioxide are used as electron acceptor.

The filtration process consists of a fixed or moving bed of media that traps and removes suspended solids from leachate passing through the media. Monomedia filters usually contain sand, while multimedia filters include sand, anthracite and possibly activated carbon. In the filtration process leachate flows downward through the filter media. Particles are removed primarily by straining, adsorption, and microbiological action.

LANDFILL GAS MANAGEMENT

Landfill gas (LFG) results from the biodegradation of wastes. Gas production within the landfill takes place at elevated temperatures and the gas will invariably be saturated with water vapour. Undiluted landfill gas can be expected to have a calorific value of 15 to 21 MJ/m3 (half that of natural gas).

The purpose of a landfill gas management system is to:

• minimise the impact on air quality and the effect of greenhouse gases on the global climate;
• minimise the risk of migration of LFG beyond the perimeter of the site;
• minimise the risk of migration of LFG into services and buildings on site;
• avoid unnecessary ingress of air into the landfill and thereby minimise the risk of landfill fires;
• minimise the damage to soils and vegetation within the restored landfill area;
• permit effective control of gas emissions;
• where practicable permit energy recovery.

Landfill gas may migrate by diffusion, convection or by water transport. These modes of transport of gases are independent of each other but may occur simultaneously so that migration control measures may mitigate one without removing the risk presented by the others. Diffusion results in gas moving from an area of high concentration to an area of low concentration. Convection results in gas moving from areas of high pressure to areas of low pressure. Gas pressure depends on factors such as changes in atmospheric pressure, changes in water table and changes in bacterial activity. The distance the gas travels is influenced by the ease of the migration path and by the pressure of the gas in the landfill.

Power generation

Normally the size of power generation schemes are in the range 1MW to 5MW. Typically some 600/700m3 of landfill gas (containing 50% methane) are required to generate 1 MW of electricity. The type of plant for power generation will depend on the quantity and quality of gas generated. Power generation is feasible at a landfill when the methane content range is 28% to 65%.

Power generation plants used include gas turbines, dual-fuel engines and spark ignition engines. Minimum methane content for these plants vary but typically around 35% methane content is required. Gas turbines start at about 3MW and are suitable for larger schemes with gas flow rates in excess of 2500m3/hr.

LANDFILL GAS MONITORING

Minimum requirements for monitoring are outlined in the Agency’s Manual ‘Landfill Monitoring’. A typical monitoring borehole is presented in Figure 9.13. It is essential that monitoring points be established on the perimeter of the site and between the site and locations such as buildings that may be at risk from landfill gas migration. Investigations should identify likely monitoring point locations. To establish if there are any other sources of gas around the site monitoring should be undertaken prior to waste emplacement, in accordance with the Monitoring Manual.

CAPPING CONSTRUCTION

The capping system comprises engineering and restoration layers. The makeup of the restoration layers must be consistent with the proposed afteruse of the facility. Further guidance on restoration and aftercare is provided in the Agency’s Manual ‘Landfill Restoration and Aftercare’.

The main objectives in designing a capping system are to:

• minimise infiltration of water into the waste;
• promote surface drainage and maximise run off;
• control gas migration; and
• provide a physical separation between waste and plant and animal life.

The capping system normally includes a number of components which are selected to meet the above objectives. The principal function of the capping system is to minimise infiltration into the waste and consequently reduce the amount of leachate being generated.