Long-term performance of contaminant barrier systems

Abstract

This lecture describes the latest findings with respect to the long-term performance of modern municipal solid waste (MSW) landfill barrier systems. Field data relating to the clogging of leachate collection systems and the latest techniques for predicting their performance are examined. It is indicated that the primary leachate collection systems may have service lives that range from less than a decade to more than a century, depending on the design details, waste characteristics and mode of operation. Recent data indicate that landfill liner temperatures can be expected to reach at least 30–40°C for normal landfill operations. With recirculation of leachate the liner temperature increases faster than under normal operating conditions, and may be expected to exceed 40°C. Temperatures (up to 40–60°C) may occur at the base of landfills where there is a significant leachate mound. Temperature is shown to have a significant impact on both contaminant migration and the service life of the liner system. Field measurements and theoretical calculations show that composite liners are substantially better than single liners in terms of controlling leakage from landfills. Also, the leakage rates with a composite liner are very small, and diffusion will dominate as a transport mechanism for contaminants that can readily diffuse through a geomembrane (GM). Composite liners involving a GM over a geosynthetic clay liner (GCL) gave rise to substantially less leakage than those involving a compacted clay liner (CCL). The observed leakage through composite liners can be explained by the holes in, or adjacent to, wrinkles/waves in the GM, and this leakage can be calculated using simple equations. High-density polyethylene (HDPE) GMs provide an excellent diffusive barrier to ions. However, some organic compounds readily diffuse through HDPE GMs, and a combination of GM and an adequate thickness of liner and attenuation layer are required to control impact to negligible levels. The long-term performance of HDPE GMs is discussed. Based on the currently available data, the service life for HDPE GM in MSW landfill is estimated to be about 160 years for a primary liner at 35°C and greater than 600 years for a secondary GM provided it is at a temperature of less than 20°C. Clay liners are susceptible to both shrinkage and cracking during construction (due to heating by solar radiation or freezing) and after placement of the waste (due to temperature gradients generated by the waste). The former can be controlled by quickly covering the liner with a suitable protection layer. The latter can be controlled by appropriate design. The use of numerical models for predicting the service lives of engineered systems and long-term contaminant transport is demonstrated.

Evaluating landfill aftercare strategies: A life cycle assessment approach

“…The process of liner system deterioration and failure is complex and involves multiple interacting chemical and physical processes that, over time, adversely affect the material properties of the GM, such as its stress-crack resistance and crystallinity (Rowe, 2005). Liner temperature has been identified as a key controlling factor in liner longevity, with higher temperatures equating to a shorter service life (Rowe, 2005;Rowe et al, 2010).…”

“…Liner temperature has been identified as a key controlling factor in liner longevity, with higher temperatures equating to a shorter service life (Rowe, 2005;Rowe et al, 2010). The default values used in this paper's model are based on the work of Rowe et al, 2010 and a liner temperature of 30°C.…”

“…The rate at which the hydraulic conductivity of a liner increases as it degrades is somewhat uncertain: an exponential form is indicated in Rowe (2005), whereas Drury et al (2003) assume a linear decline. For simplicity, the model developed here has adopted a linear increase in liner hydraulic conductivity between its initial value (Kc), the value at the start of degradation (Kd) and the value at the end of the GM service life (KL).…”

“…This contradicts one of the core principles of sustainable development, namely that the problems of today should not be passed on to future generations (United Nations, 1987). Furthermore, it is broadly recognised that the efficacy of landfill engineering systems will deteriorate in the long term (Drury et al, 2003;Rowe, 2005). The deterioration or failure/shut down of these control systems may increase the impact on future generations through the release of untreated leachates and landfill gases into the natural environment.…”

Laboratory investigation of self-healing properties on geosynthetic clay liners with flaw

“…The hydraulic conductivity of the sodium bentonite used in most GCLs is on the order of 10 -9 cm/s when permeated with deionized water at stresses typical of cover applications (Rowe 2005).…”