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3 Discussion

These results show that improvements have been made in landfill design and operation compared to previous years. The number of landfills continues to decrease, down 48 per cent from 2002. The proportion of landfills collecting leachate and requiring documentation for hazardous waste disposal has increased substantially. Only four landfills do not measure and charge for the disposal of waste.

Improvements can still be made in the siting and design of landfills in New Zealand, particularly in relation to underlying material, engineered liners and landfill gas collection. The following discusses these areas for improvement.

3.1 Underlying material

Because engineered liner systems have a finite lifetime, the ability of the underlying materials to minimise the potential for liquids to migrate out of the landfills into the environment if the liner degrades, tears or cracks needs careful consideration (Centre for Advanced Engineering, 2000).

Having low-permeability underlying material is important at landfill sites because it ensures a high level of natural leachate containment. This reduces the reliance on engineered liner systems (which have a finite lifetime), allows for more efficient and effective site management, and can result in savings in development and operating costs (Centre for Advanced Engineering, 2000).

However, since the 1998/99 survey the percentage of landfills with low-permeability underlying material, as recommended in the landfill guidelines, has remained at between 10 and 15 per cent. Progress has been hindered by the following factors.

  • Many landfills sited on high-permeability underlying material have consent to operate until past 2020.

  • In more remote locations, small landfills are often built on local sites to avoid high transfer/transport costs, and these local sites may be on high-permeability underlying material.

  • Suitable geology can be absent or rare in some areas of New Zealand.

  • When a low-permeability site is found, community resistance (the NIMBY effect) can make it unworkable.

3.2 Engineered liners

The migration of leachate into aquifer systems is a major cause of concern for landfills. To prevent this, landfills need to ensure there is an appropriate level of retention of leachate. At some sites the underlying material provides this retention, but as we have seen many sites do not have appropriate underlying material. For these sites, the alternative is to use an engineered liner (Centre for Advanced Engineering, 2000).

Progress in this area has been positive, with a 50 per cent increase since 1998/99. It is important that this trend continues, especially if new landfills cannot be built on sites with low-permeability underlying material. However, there are issues slowing progress, including:

  • some sites without engineered liner systems have consent to operate until past 2020

  • where landfilling has already commenced, the cost of retrofitting an engineered liner is often considered too high (especially when it is not required by the consent) and impractical.

3.3 Landfill gas collection

Landfill gas is produced in almost all landfills as an end product of biological decomposition. In the latter stages of this decomposition phase the gas has a high methane content, which can be generated in commercial quantities. The problems with these gases include risks to human health, odour nuisance, and detrimental effects on soils and vegetation in the immediate vicinity (Centre for Advanced Engineering, 2000). In addition, methane gas is a powerful greenhouse gas.

In 1998, 5 per cent of landfills had a landfill gas collection system. This has now increased to 22 per cent in 2006/07 as a result of the National Environmental Standard, which requires landfills of over 1 million tonnes to collect and flare or reuse landfill methane gas. However, as with landfill siting and engineered liners, there are a number of barriers to progress. The cost of retrofitting with a landfill gas collection system when landfilling has already commenced is often considered too high, especially when it is not required by the consent. This is particularly the case at smaller landfills, where the quantities of landfill gas are simply not economic to collect and reuse. It can also be practically difficult to collect very small quantities of landfill gas at small, newer landfills.