Section 2: Waste management technologies

2.1  Introduction

There are many ways of dealing with wastes. Some methods are suitable for the entire waste stream and others can only treat certain types of waste. Each technology has its advantages and disadvantages, and all technologies have some impacts on public health and the environment. One of the tasks of the environmental professional is to select the combination of waste treatment processes that gives the greatest health and environmental benefits with the smallest disbenefits.

As a first stage in the decision-making process, this section discusses the main waste treatment options, how the technologies work and the environmental impacts of the technologies. I have concentrated on the technologies in the context of general municipal waste, but the basic principles can also be used to treat any specialised waste stream.

2.2  Landfill

Landfill is firmly at the bottom of most waste hierarchies (i.e. it is usually considered the least desirable option for managing waste), and virtually all international and national waste policies focus on minimising landfill. Having said that, there will always be a need for landfill. In some areas, there is simply insufficient waste for other options to be practicable. In other areas, a lack of finance dictates that landfill is the only option. Even in highly developed affluent economies with large amounts of waste, there will be some waste materials that cannot be used as resources (either in reuse, through recycling or as energy sources).

This subsection will concentrate on the landfill of biodegradable waste – the waste with the greatest potential to cause environmental pollution.

2.2.1  Regulation

The practice of landfill varies from country to country, depending on the regulations in place. For example, amongst other provisions, all EU member states are obliged to do the following (European Commission, 1999):

• restrict the types of waste that may be landfilled
• install landfill liners and caps to a specified standard
• collect any leachate and gas.

This means that European landfills are highly engineered structures with several pollution control measures in place. In contrast, landfills in developing countries are often simple holes that go some way towards separating people from their wastes. As an example, Figures 3 and 4 show contrasting landfills in the UK and Uganda respectively.

Figure 3  UK landfill site (note the completed cell in the background, the prepared and lined cell in the centre, and the leachate/run-off pond in the foreground)

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Copyright © Stephen Burnley

Figure 4  Ugandan landfill site

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2.2.2  Site operation

The operation of a landfill site is influenced by:

• the type and quantity of wastes to be landfilled
• whether or not the wastes are to be pretreated
• the need to maximise site life
• the type of delivery vehicles to be used
• the use of compacting equipment to increase the density of the waste
• the expected current and future rates of filling
• the type and availability of cover material
• the site location (whether it is close to habitation or environmentally sensitive areas).

Most engineered landfill sites operate using the cell method, in which waste is deposited in pre-constructed bunded areas. This method encourages progressive filling and restoration of the site, which helps to minimise the formation and impacts of gas and leachate.

Figure 5 shows a typical operational plan for a landfill site using the cell method. Leachate generation can be minimised by paying careful attention to cell design. A number of factors have to be considered in deciding the optimum size of each cell:

• quantity and variation of rainfall
• absorptive capacity of the waste
• rate of waste input
• number of incoming vehicles expected per day
• the need to ensure sufficient working space for safe vehicle turn-round.

Figure 5  Landfill operation: the cell method

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At some sites, daily cells may be constructed within the larger main cells.

The cell walls are constructed either by pushing material up from the base of the site (on the initial lift) or from waste material. In either case, care needs to be taken to ensure their structural stability. In addition to concealing the operation, the cell walls help to reduce the incidence of windblown litter.

The working face should be of a size that allows safe manoeuvring of vehicles and site plant (equipment), yet minimises infiltration, cover requirements and litter problems. The size of the working face needs to be reviewed regularly to ensure that it is at its optimum.

Incoming waste should be placed in such a way as to achieve a high degree of compaction. The UK’s Department of the Environment (DoE, 1997) lists the following advantages of compaction:

• An increase in waste density leads to an extension of the life of the site.
• A uniform, well-compacted layer of waste reduces the volume of daily cover required.
• A well-compacted site is visually more acceptable and carries less risk of litter blowing across the site.
• Compaction reduces the incidence of fly infestations and colonisation by vermin.
• Voids are eliminated, thus largely preventing underground fires, while surface fires become much easier to control.
• Well-compacted waste provides a more stable base for delivery vehicles during discharge of loads. This reduces vehicle wear and tear and the risk of machinery becoming bogged down during wet weather.
• A high degree of compaction reduces the degree of settlement whilst ensuring that it takes place more evenly.

Good compaction is helped by placing the waste in thin layers (approximately 0.3 m thick) and running over each layer several times using a steel-wheeled compactor. Initial densities of more than 1 t m?3 are achievable using compactors – considerably higher than the 0.5 t m?3 that can be achieved with a tracked vehicle pushing waste from the top of the working face. At the end of each working day, all faces should be covered to a depth of not less than 0.15 m using a suitable covering material. Daily cover is essential for good site operation, since it reduces odours, inhibits pests and flies, helps to control infiltration, minimises the risk of fires and improves the general appearance of the site.

One disadvantage of the cell method is that the walls take up a lot of space that could otherwise be used to house waste. This can, however, be overcome by using one of two strategies:

1. making the cell walls from suitable incoming waste
2. selectively removing walls at the end of each day for use as cover.

When the final level is reached, the cell is capped to reduce leachate generation.

Throughout its operational life, the site should be monitored to ensure that should difficulties arise, the necessary remedial action can be taken.

2.2.3  Site restoration and after-use

Restoration is site-specific and the restoration plan will have been produced as part of the original site design and operational plan. The responsibility for a site can continue long after site closure. For example, in the EU, operators are responsible for the care and maintenance of a site until the national regulator concludes that the site no longer has any potential to cause environmental pollution.

The after-use of a site should also be considered at the early planning stages, in order to ensure that the best environmental and amenity benefits can be obtained from the project. Generally, land is returned to agricultural use (for grazing or arable crops), forestry or general amenity use. There is, however, scope for a more imaginative approach, particularly where the site is close to urban areas. Other options that may be considered...