Waste Degradation: What are the Facts?

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Waste Degradation and the Idea of Acceptable Release Rate
By Steve D Evans

Every landfill is an engineering project with an engineered leachate management system. Any landfill liner is part of the system. The system should be designed to accommodate a range of seepage rates from very low (the best containment that can be achieved) to high (a controlled form of dilute and attenuate) depending on the degree of environmental protection needed at that site.

Many guidance documents or statutory assessment methods use absolute terms such as "total containment", "prevention of leachate and landfill gas migration", or "impermeable". These terms can lead to the erroneous belief that such concepts are achievable, and that failure to do so is indicative of poor materials or workmanship.

Consideration of material properties demonstrates that, as all materials have a finite permeability, some finite seepage is inevitable.

For example, the frequently quoted requirements for one metre of clay with a maximum permeability of 10-9 m/s and a maximum leachate head of 1 metre implies, using Darcy's Law, a seepage rate of 1.7 m^/d/ha, and yet it is commonly referred to as the provision of "total containment".

Actual flow rates are affected by a range of ameliorating and aggregating factors. Calculation of actual seepage rates is complex, and should be carried out using risk assessment methodology.

For gases, viscosity and diffusion characteristics must be taken into account. In general, mineral liners on their own are not usually effective barriers against gas migration, though they can be useful components of a multi-barrier system.

Measurements of hydraulic conductivity are of little relevance in respect to gas, as gas is able to pass through a barrier at a rate several orders of magnitude greater than that which may be measured for water.

All materials will allow the passage of liquids to an extent determined by their permeability. Risk assessment methodology will quantify the probability distribution for a derived seepage rate for a given liner/landfill situation.

This should be used either to assess the probable impact on the receiving source, or to determine the performance specification for the liner and landfill operational methods. In this way, an appropriate liner specification can be derived, incorporating a suitable safety margin.

Excessive over-engineering should be avoided, as this can itself may be considered to contravene the principles of sustainable development.

The statement we have made regarding the fact that flow through landfill containment systems will never be absolutely zero and will be a quantifiable small amount, cannot be refuted. It is a rigorously scientific approach to adopt this principle.

However, the concept of the fact that a seepage through a liner is acceptable has been resisted by some, and in particular can appear to be in conflict with the EC Groundwater Directive.

However, in certain circumstances, the discharge into the unsaturated zone of leachates containing List II substances (as defined by the EC Groundwater Directive) is permissible, provided that prior investigation has shown that this will be satisfactory and not significantly impact upon the environment.

Around the globe the primary concern will be to ensure the protection of groundwater, so for example in the EU landfill acceptability is classified by type within zones of travel time to a water source, and within resource protection areas.

Landfills are accepted as suitable for development when subject to adequate risk assessed engineered containment and operational safeguards.

Author Steve Evans has been writing about landfill daily cover and many other aspects of practical waste and secondary resource management since 2006. As you have been reading this article, there is a fair chance that you may enjoy his blog. We recommend a visit now!

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