Page 331 - Materials Chemistry, Second Edition

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302 Waste Management Practices: Municipal, Hazardous, and Industrial

TABLE 10.5
Water Vapor Transmission for Different Geomembranes

Thickness Vapor Transmission Rate
2
Geomembrane mm mil g/m /day gal/acre/day
PVC 0.75 30 1.9 2.03
CPE 1.0 40 0.4 0.43
CSPE 1.0 40 0.4 0.43
HDPE 0.75 30 0.02 0.021
HDPE 2.45 98 0.006 0.0064

Source: U.S. EPA, 1988.
CPE chlorinated polyethylene

It follows that Fick’s first law controls leakage through a synthetic liner. The diffusion
process is similar to the rate of flow governed by Darcy’s law, except that the former is driven by
concentration gradients as opposed to hydraulic head. Diffusion rates in membranes are very low
in comparison with hydraulic flow rates, even in clays. Of course, if a synthetic liner is installed
with incomplete seams or small holes, the amount of leachate that leaks through will increase
substantially.
Data revealing problems with synthetic membranes have been documented in the water indus-
try, where contamination of drinking water due to permeation of trace organic contaminants from
soil through plastic pipes has occurred. Laboratory studies have also demonstrated the transport of
solvents through membranes. Haxo and Lahey (1988) demonstrated the transport of trichloroethyl-
ene and toluene through a membrane. Park and Nibras (1993) measured diffusion parameters for a
range of volatile organic compounds (VOCs) in HDPE liner materials, and demonstrated that this
might be a significant source of releases from lined landfills. Diffusive mass transport could, in the-
ory, have a significant environmental impact by allowing the release of organic solvents though
intact membranes at rates comparable with those of leakage through a defect (Butler et al., 1995).
Also of possible concern is solvent gas transmission through the membrane. Very light gases, such
as methane (CH ), will rise from the waste cell and contact the membrane. Methane gas transmis-
4
sion rates for several geomembranes are shown in Table 10.6.
Factors in a landfill cell that might affect leachate diffusion rate through an intact liner include
temperature, pressure, and elongation due to tensile stress. As the temperature increases, diffusion
will increase due to greater thermal motion in the polymer chains, thus producing more voids
through which leachate can escape (Butler et al., 1995).
Of more practical importance, however, is the occurrence of holes in the liner caused by
improper placement and positioning over sharp stones. Giroud and Bonaparte (1989) state that with
good quality control, 2.5 holes/ ha of geomembrane (1 hole/acre) is a fairly typical occurrence. With
poor quality control, we can expect 75 holes/ha (30 holes/acre). Most defects tend to be small ( 0.1
2
cm ), but larger holes do occasionally occur (Qian et al., 2002). Table 10.7 provides data for esti-
mated losses from geomembranes having holes.
The Bernoulii equation can be used to estimate the flow rates through holes in geomembranes,
assuming that the size and shape of the holes are known:

Q C a (2gh) 0.5 (10.3)
b
3
where Q is the flow rate through a geomembrane (cm /sec), C the flow coefficient with a value of about
b
2
0.6 for a circular hole, a the area of circular hole (cm ), g acceleration due to gravity (981 cm/sec), and
h the liquid head acting on the liner (cm).