Page 555 - Materials Chemistry, Second Edition

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526 Waste Management Practices: Municipal, Hazardous, and Industrial
should, therefore, outperform either a geomembrane or clay liner alone. In accordance with Darcy’s
law (see Chapter 10), leachate that ponds directly on top of a clay liner alone will percolate down
through the liner at a rate controlled by the hydraulic conductivity of the liner, the head of the
leachate on top of the liner, and the liner’s total area. With the addition of a geomembrane placed
directly on top of the clay and sealed against its surface, leachate moving down through a hole or
defect in the geomembrane does not spread out significantly between the geomembrane and the clay
liner (see Figure 17.3). The composite liner system allows much less leakage than a clay liner acting
alone because the area of flow through the clay liner is much smaller.
The geomembrane must be placed on top of the clay liner such that leachate liquids do not
migrate along the interface between the two liners and move downward through the entire area of
the clay liner. This is analogous to placing a geomembrane on a bed of highly permeable materials
such as sand. Liquids moving through a small defect in the geomembrane will spread over the
whole area of the clay liner and percolate down as if the geomembrane were not even present
(Figure 17.4).
As discussed above, clay liners, synthetic liners, or combinations of both are required in secure
landfills. Figure 17.1 presents the synthetic composite double-liner system that appears in the U.S.
EPA minimum technology guidance.

17.3.4 COMPATIBILITY OF LINERS WITH WASTES
The chemical compatibility of a geomembrane with waste leachate is a critical consideration
regarding choice of material. A secure landfill is considered a permanent repository for hazardous
wastes; therefore, materials used in its construction must be expected to withstand a wide range of
natural stresses for very long periods. Many materials deteriorate over time when exposed to
chemicals occurring in both hazardous and nonhazardous leachate. Landfill owners and oper-
ators must anticipate the composition of leachate that a site will generate and select the appropri-

Leachate
Leachate Geomembrane

Clay liner Clay liner

(a) (b)
FIGURE 17.3 The FML should be placed directly upon the clay liner (a); this limits leachate infiltration into
the clay (b) (U.S. EPA, 1989).

Clay layer Composite layer

Leachate Leachate
Geomembrane

Clay liner Clay liner

FIGURE 17.4 Leachate infiltration in a clay and a composite liner system (U.S. EPA, 1989).