CEE 328 Solid and Hazardous Wastes Engineering

Jae K. (Jim) Park

Dept. of Civil and Environmental Engineering

University of Wisconsin-Madison

1

Leachate Collection System

Leachate Collection System (1)

Designed as containment facilities due to concern with the environment impact of landfills

Needed to prevent landfill gas and leachate from migrating from the site in significant quantities

Purpose: to collect leachate for treatment or alternative disposal and to reduce the depths of leachate buildup or level of saturation over the low-permeability liner.

Underdrain system: constructed prior to landfilling and consists of a drainage system that remove the leachate from the base of the fill.

Peripheral system: installed after landfilling, constructed around the edge of the disposal area, and used to control leachate seeps through the face of the landfill.

2

Leachate Collection System (2)

Refuse

Drainage layer

Low permeability

barrier

Undisturbed

native material

Simple collection system

Refuse

Undisturbed

native material

Low permeability

barrier

Drainage layer

Double liner system

Drainage tile

3

Drainage tile

Leachate Collection Systemwith Graded Terraces

4

Leachate collection pipe (see detail below)

Sloped intercepting leachate collection pipe

Sloped terraces

Leachate movement

Liner

Perforated leachate collection pipe

Protective soil layer

Geotextile filter fabric

Sand drainage layer

Extra geomembrane (optional)

Geomembrane liner

Compacted clay layer

Washed gravel (1~2 in.)

Geotextile filter fabric

Schematic of Various Leachate Discharge Pathways

Infiltration

Optional

toe drain

Leachate

collection

tiles

Toe

seepage

Leachate to

groundwater

Toe

seepage

Leachate

seep

through

face

5

Leachate Seep Remediation

6

Landfill cover

Granular toe-drainage collection

Peripheral toe-drainage collection

Refuse

Components of LCS

French drain

Tile drain

Refuse

Drainage layer

Low permeable

liner

Undisturbed

native material

K of drainage layer: min. 10-3 cm/sec; 10-2 desirable

Drainage layer gravel should be washed to remove fines; no limestone-based aggregate

French drain: used in the event of pipe failure or clogging; gravel pack

Additional containment and/or leak detection system

7

8

Leachate Collection System Layouts

Clean-out access point

1200 ft

130 ft

S = 1~5%

S = 2~5%

Min. 2%

Schematic of Clean-Out System

9

Access manhole

Final grade

Perforated pipe

Solid pipe

Drainage blanket

Refuse

Leachate Collection System

10

Slotted leachate collection pipe

Clay berm

First cell to be developed

Slotted pipe connected to leachate removal system

Leachate collection line

Stormwater collection line

Solid waste

Clay berm (2 ft)

Sand layer

Geomembrane

Clay liner (3 ft)

Slotted leachate collection pipe

Storm Water Management in Area Type Landfill

11

Leachate Removal System

Pipe passed through

side of landfill

Leachate removed

with a pump

12

Potential leakage:

Not recommended

Most widely used

Leachate Collection Facilities

Leachate collection and transmission vault

Leachate holding tank

13

14

Leachate Collection Facilities

Above grade

Below grade

Used in

cold regions

Role of LCS Components (1)

Barrier layer: a very low-permeability synthetic or natural soil liner to restrict and control the rate of vertical downward flow of liquids

Drainage layer: a high permeability gravel drainage layer to laterally drain the liquid to the collector drain pipes; at least 30 cm thick with a min. K of 10-3 cm/sec

Slope: to encourage lateral migration; min. 2% bottom final slope after long-term settling

15

French drains and tiles: maximize the amount of leachate diverted to, and collected by the tile drains; subangular gravel with UC < 4 and max. of 2 in.; two or more rows of holes at the 2 and 10 oclock positions; min. slope of 0.5% and min. of 6 in.

Filter layer: granular or synthetic, used above the drainage layer to reduce the potential for migration of fines into the drainage layer

Fine soil or refuse: K of 10-4 cm/sec; 2 ft (0.7 m) thick layer to cushion the engineered system against damage and act as a filter

16

Role of LCS Components (2)

UC: Uniformity coefficient = d60/d10

Design Considerations for Tile Spacing

Why? To control the height of a mound of leachate

Design considerations

Flow rate or flux of leachate impinging on the barrier layer

Spacing between the tiles

Slope of the liner

Thickness and hydraulic conductivity of the drainage layer

If the tiles are separated by too large a distance, the leachate mound will penetrate back up into the refuse, resulting in increase in the hydraulic gradient and consequently increase in leachate seepage.

17

Analytical Formulations for Tile Spacing

Mathematical models to examine a series of design considerations including:

Depth, hydraulic conductivity, and slope of the drainage layer

Thickness of the low-permeability barrier layer

Two measures of hydraulic performance: max. saturated depth over the barrier and amount of leakage through the barrier

Leachate mounding: function of liner slope, leachate infiltration rate, permeability of drainage and barrier layers, and drainage tile spacing

Assumptions in mathematical formulation

Flow is one direction (lateral).

Saturated steady-state flow conditions exist.

The drainage media are homogeneous and isotropic.

18

Continuous-Slope Formulation (1)

19

L

D - L

D

A

p

e

x

A

p

e

x

D

x

y

(

x

)

y

o

D

r

a

i

n

L

i

n

e

r

D

r

a

i

n

L

S

Z

z

(

x

)

P

Continuous-Slope Formulation (2)

20

zx = sx + yx (10.1)

where:zx = static head at location x (m);

s = slope of the liner (radians);

x = horizontal distance (m); and

yx = depth of flow at location x (m).

where:K = hydraulic conductivity of the media (m/sec)

A = cross-sectional area of flow (m2);

W = width (m); and

dz/dx = gradient of static head (m/m).

At steady state, Qx = (L - x)pW(10.3)

where:p = rate of infiltration of moisture (m/sec).

(10.2)

Continuous-Slope Formulation (3)

21

Assuming a unit width of aquifer and combining Eqs. 10.2 and 10.3 yields:

where = p/K, w = L - x, and y = vw.

Solving the preceding equation and invoking the boundary condition y(0) = yo, yields three conditional cases:

Apex

Case I: 4 > s2

Case II: 4 = s2

Case III: 4 < s2

Low permeable liner

Drain tile

(10.4)

(10.5)

Continuous-Slope Formulation (4)

22

Case I: 4 > s2

Case II: 4 = s2

Case III: 4 < s2

(10.6)

(10.7)

(10.8)

Example

p = 15.2 cm/yr (6 inches/yr); K = 10-3 cm/sec; max. allowable mound depth = 0.3 m; drainage tile spacing 30 m; min. slope of the liner?

2.5 cm/yr

7.6 cm/yr

15.2 cm/yr

30 cm/yr

61 cm/yr

23

Flat-Slope Configuration (Worst Scenario)

When the slope of the liner system equals zero, Eq. 10.6 becomes:

ymax occurs at x = D/2. From Eq. 10.9, ymax becomes:

Ex. Determine ymax using Eq. 10.10 for a 30 m tile drain spacing, a drainage layer hydraulic conductivity of 10-3 cm/sec, a percolation rate of 7.6 cm/yr, and zero liner slope.

Solution:

(10.9)

(10.10)

= 0.23 m

24

*

07/16/96

*

##

Sawtooth Formulation (1)

25

P

x

Q(x)

Drain

d

Liner

s

L=D/2

Z

Apex

L=D/2

L=D/2

D

Based on the Dupuit assumption for unconfined flow, the differential equation governing the steady drainage on a sloping barrier is:

This is equivalent to Eq. 10.4 with transformation of the origin (i.e., xsawtooth = L - xcontinuous). Transforming Eq. 10.11 by substituting the expressions xo = x/L, yo = y/L, and yo* = yo/L, defining u* = yo/xo, substituting u*x* for y*, and then separating variables leads to:

(10.11)

(10.12)

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Sawtooth Formulation (2)

Case I

Case II

Case III

Alternative mathematical eqs. for determining ymax

(Moore, 1983)

(Richardson and

Koerner, 1987)

27

Sawtooth Formulation (3)

Case I: 4 > s2

Case II: 4 = s2

Case III: 4 < s2

(10.13)

(10.14)

(10.15)

28

Sawtooth Formulation (4)

Calculated Max. Mound Depth

P = 30 cm/yr; K = 10-3 cm/sec; yo = 0

29

Tile spacing, mSlope, %McEnroe, 1989Moore, 1983Richardson and Koerner, 19871000123451.5451.2251.0100.8550.7400.6501.5421.1210.8380.6510.5260.4371.5421.1790.9990.9090.8610.833500123450.7720.6120.5050.4270.3700.3250.7710.5610.4190.3260.2630.2190.7710.5890.4990.4540.4300.417

p = 15.2 cm/yr; K = 10-3 cm/sec

Continuous-slope configuration

Saw-tooth configuration

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Max. Mound Depth vs. Slope

Lower mound depth

Better

Impact of Drain Tile Failure

31

Continuous-slope configuration

Saw-tooth configuration

Greater mound depth: more problem

Max. Mound Depth vs. Slope

32

Wisconsin Regulations

NR 504.06(5)(a) Wisconsin Administrative Code (WAC):

12 inches of average leachate head over the liner

< 130 ft drain spacing

NR 512.12(3) WAC:

Open conditions: p = 6 inches/yr = 0.5 inch/month

Closed conditions: p = 1 inch/yr = 0.083 inch/month

Factors affecting the leachate mount height

Percolation rate into the drainage layer

Hydraulic conductivity of the drainage layer

Leachate flow distance from the upstream boundary to the leachate collection pipe

Slope of the landfill liner

33

McEnroe Method

R = p/Ksin2 < 1/4

R = 1/4

R > 1/4

p = percolation rate per unit surface area (cm3/sec/cm2);

S = tan = slope of liner (ft/ft); = slope angle;

K = hydraulic conductivity (cm/sec); A = (1-4R)0.5; B = (4R-1)0.5;

L = drainage distance, measured horizontally (ft); and

ymax = Ymax (L tan) = maximum saturated depth (ft).

34

McEnroe, B.M. (1989). Steady Drainage of Landfill Covers and Bottom Liners, Jour. of Envion. Eng., ASCE, 115(6): 1114-1122.

McEnroe, B.M. (1993). Maximum Saturated Depth over Landfill Liner, Jour. of Envion. Eng., ASCE, 119(2): 262-270.

Performance Measures

Residence Time, T

where s = slope approximated by the bottom slope, m/m.

Efficiency of Capture

d: Thickness of low permeable layer

ymax: Max. height of leachate mound

Undisturbed native material

35

Breakthrough Time

K = permeability coefficient, L/T;

ne = effective porosity;

d = liner thickness, L; and

h = leachate mound height.

d

h

Example: ne = 0.4; d = 4 ft; h = 1 ft;

K = 110-7 cm/sec = 0.103 ft/yr

36

Clogging Problems

Occur in agricultural irrigation, weeping tile systems, sanitary landfills, septic system leachate fields, and the like.

Remedial measures

Smaller-diameter lines (15~30 cm): cables

> 30 cm lines: rodding equipment

Max. 300 m between access ports or manholes

Removal mechanisms

Mechanical procedures: roto-routers, pigs, sewer balls, snakes, and buckets

Low-pressure jets: 70 to 140 psi at nozzle

High-pressure jets: 410 to 1300 psi at nozzle

Chemical methods: such as SO2 gas; some danger

37

Weeping Tile

Two types

Helical profile

Annular profile

38

Rodding equipment

39

Pipe Cleaning Method

Bucket Machines - the only sure way to remove sand, solids, or sludge from storm & sanitation pipelines. Needs no water to create a vacuum slurry. Cost-effective.

40

Snakes

Sewer ball

41

Other Design Considerations

Collector sizing and type: at least 15 cm diameter; min. 22.5 cm, preferably 30 cm to reduce the effects of silting and to facilitate inspection and cleaning; schedule 80 PVC or HDPE

Collector slope: 2% if practical but not < 0.5%

Collector perforations: at 2 and 10 oclock positions

French drain around the collector pipe: 38 to 50 mm washed stone

Attention to field construction practices: within pipes, accumulation of deposits may occur in areas of hydraulic perturbation such as where pipe joins have been poorly installed

42

Diameter: 4" ~ 36"

Length: 20"

Leachate Collection Pipe

Drainage Couplers and Fittings

http://www.ads-pipe.com/markets/waste.html

43

AdvanEDGE is a panel shape pipe offered in 12" and 18" heights, and in coils up to 400 ft. The primary benefit of its panel design is quick drainage response after introduction of water, making it ideal for time-critical applications such as high-traffic road and track beds.

44

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