William A. Worrell, P. Aarne Vesilind-Solid Waste Engineering , Second Edition -CL-Engineering (2011)

4-3 Landfill Design 125

required by RCRA Subtitle D, consists of two layers: The bottom is a clay material and
the top layer is a geomembrane. The two layers of a composite liner are in intimate
contact to minimize leakage. A double liner may be either two single liners or two
composite liners (or even one of each). Figure 4-10 shows a synthetic liner on a side
slope, ready for the earth cover. The clay layer already has been installed under this
synthetic liner.

Each liner is provided with a leachate collection system. The collection system
separating the two liners is a leak detection system—a series of pipes placed between
the liners to collect and monitor any leachate that leaks through the top liner.
Recently, the geosynthetic clay liner has been introduced for use as the top compo-
nent in the double liner system. This liner is composed of a thin clay layer (usually
sodium bentonite) supported by geotextiles (a geosynthetic filter) or geomembranes.
The geosynthetic clay liner is easily placed in the field and uses up less volume,
allowing for more volume to be used for waste deposition.

Clearly, the more layers that are included, the more protective the liner system
will be. The costs, however, increase dramatically. A composite liner can cost as
much as $250,000 per acre. Because the liner is so critical to groundwater protec-
tion, an exhaustive quality control/quality assurance program is required during
liner installation.

4-3-2 Leachate Collection, Treatment, and Disposal

Leachate is directed to low points at the bottom of the landfill through the use of
an efficient drainage layer composed of sand, gravel, or a geosynthetic material. Per-
forated pipes are placed at low points to collect leachate and are sloped to allow the
moisture to move out of the landfill.

Figure 4-10 Synthetic liner on slope, ready for earth cover. (Courtesy William A. Worrell)

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126 Chapter 4 Landfills

Leachate Collection and Storage

The primary purpose of lining a landfill cell is to minimize the potential for
groundwater contamination. The liner serves as a barrier between the buried
waste and the groundwater and forms a catch basin for leachate produced by the
landfill. The leachate that is collected within the cell must be removed from
above the liner as quickly as possible, since the RCRA Subtitle D regulations
restrict the head of leachate (free liquid depth) on a liner system to 30 cm.
Leachate is typically removed by two means: gravity flow or pumping. The
various components of a leachate collection system for an MSW landfill typi-
cally include the following.

• Protective and drainage layers
• Perforated collection lateral and header pipes
• Pump station sump
• Leachate pumps
• Pump controls
• Pump station appurtenances
• Force main or gravity sewer line

Table 4-10 provides general guidelines for leachate collection system compo-
nents based on a survey of landfill design engineers.23

Leachate removed from the landfill cell(s) is temporarily stored on site until it
can be treated, recirculated, or transported off site for final treatment and disposal.
Storage of leachate is also important for equalization of flow quantities and
constituent quality to protect downstream treatment facilities. The typical leachate
storage alternatives are surface impoundments and tanks.

Leachate Collection System Design Equations and Techniques

Because of federal regulations24 that restrict leachate head to 12 in. (30 cm) on top
of the liner, much attention has been devoted to predicting this value. The drainage
length, drainage slope, permeability of the drainage materials, and the leachate
impingement rate control this depth on the liner.

Darcy’s law (in conjunction with the law of continuity) can be used to develop
an equation to predict the leachate depth on the liner based on anticipated infiltration

Table 4-10 Design Guidance for Leachate-Collection System Components

Parameter Range Median

Leachate loading rate (gpd/ac) 600–1000 750
Maximum leachate head (in.) 9–12 11
Pipe spacing (ft) 60–400 180
Collection pipe dia. (in.) 6–8 8
Collection pipe material HDPE
Pipe slope (%) PVC or HDPE 1
Drainage slope (%) 0.5–2 1
0.2–2

Source: Reinhart, D. R., and T. Townsend. 1998. “Assessment of Leachate Collection System Clogging at Florida
Municipal Solid Waste Landfills.” Report to the Florida Center for Solid and Hazardous Waste Management (April).

Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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4-3 Landfill Design 127

rates, drainage material permeability, distance from the drain pipe, and slope of the
collection system.25, 26

For R Ͻ 1/4,

Ymax = (R - RS + R2S2)1/2 c (1 - A - 2R)(1 + A - 2RS) 1/2.4
(1 + A - 2R)(1 - A - 2RS) d

For R ϭ 1/4,

Ymax = (R - RS + R2S2)1/2 exp c 1 tan-1 a 2RS - 1b - 1 tan - 1 a 2R - 1b d
B B B B

For R Ͼ 1/4,

R(1 - 2RS) 2R(S - 1)
Ymax = 1 - 2R exp c (1 - 2RS)(1 - 2R) d

where
R ϭ q/(K sin2␣), unitless
A ϭ (1 Ϫ 4R)2, unitless
B ϭ (4R Ϫ 1)2, unitless

S ϭ tan ␣, slope of liner, unitless

Ymax ϭ maximum head on liner, ft
L ϭ horizontal drainage distance, ft

␣ ϭ inclination of liner from horizontal, degrees

q ϭ vertical inflow (infiltration) per unit of horizontal area, ft/day

K ϭ hydraulic conductivity of the drainage layer, ft/day

These equations are obviously cumbersome to use, and a more conservative but
far-easier-to-use equation has been proposed:27

Ymax P a q b c K tan2a 1 K tana a tan2a q 1/2
2 K q q Kb d
= + - +

where
Ymax ϭ maximum saturated depth over the liner, ft
P ϭ distance between collection pipes, ft
q ϭ vertical inflow (infiltration), defined in this equation as from a
25-year, 24-hour storm, ft/day

This equation can be used to calculate the maximum allowable pipe spacing based
on the maximum allowable design head, anticipated leachate impingement rate,
slope of the liner, and permeability of the drainage materials. The previous equation
suggests that, holding all other parameters constant, the closer together the pipes are
placed (at greater construction cost), the lower the head will be. A reduced head on
the liner results in a lower hydraulic driving force through the liner, and the conse-
quence of a puncture in the liner is likewise reduced.

EXAMPLE Determine the spacing between pipes in a leachate collection system using granular
4-5 drainage material and the following properties. Assume that in the most conservative
design all stormwater from a 25-year, 24-hour storm enters the leachate collection system.

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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.