Leachate Collection System Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison

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.

Leachate Collection System (2) Refuse Drainage tile Drainage layer Low permeability barrier Undisturbed native material Simple collection system Refuse Drainage tile Drainage layer Low permeability barrier Undisturbed native material Double liner system

Leachate Collection System with Graded Terraces Sloped intercepting leachate collection pipe Leachate collection pipe (see detail below) Sloped terraces Leachate movement Liner Protective soil layer Perforated leachate collection pipe Geotextile filter fabric Sand drainage layer Geomembrane liner Geotextile filter fabric Extra geomembrane (optional) Washed gravel (1½~2 in.) Compacted clay layer

Schematic of Various Leachate Discharge Pathways Infiltration Optional toe drain Leachate seep through face Toe seepage Leachate collection tiles Toe seepage Leachate to groundwater

Leachate Seep Remediation Landfill cover Granular toe-drainage collection Refuse Peripheral toe-drainage collection

Components of LCS French drain Refuse Tile drain 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

Leachate Collection System Layouts  130 ft Clean-out access point S = 1~5%  1200 ft S = 2~5% Min. 2%

Schematic of Clean-Out System Access manhole Final grade Refuse Drainage blanket Solid pipe Perforated pipe

Leachate Collection System Slotted leachate collection pipe Clay berm First cell to be developed Leachate collection line Slotted pipe connected to leachate removal system 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

Leachate Removal System Pipe passed through side of landfill Potential leakage: Not recommended Leachate removed with a pump Most widely used

Leachate Collection Facilities Leachate collection and transmission vault Leachate holding tank

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

Role of LCS Components (2) 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 o’clock 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 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.

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.

Continuous-Slope Formulation (1) x L D - L D P Z L A p e x z ( x ) y ( x ) L i n e r D r a i n S y o x D r a i n D

Continuous-Slope Formulation (2) 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)·p·W (10.3) where: p = rate of infiltration of moisture (m/sec). (10.2)

Continuous-Slope Formulation (3) 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: (10.4) (10.5) Apex Case I: 4 > s2 Case II: 4 = s2 Low permeable liner Drain tile Case III: 4 < s2

Continuous-Slope Formulation (4) Case I: 4 > s2 Case II: 4 = s2 (10.6) (10.7) Case III: 4 < s2 (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? 61 cm/yr 30 cm/yr 15.2 cm/yr 7.6 cm/yr 2.5 cm/yr

Flat-Slope Configuration (Worst Scenario) * 07/16/96 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 *

Sawtooth Formulation (1) Apex L=D/2 L=D/2 D P Z x Q(x) d Drain Liner s L=D/2

Sawtooth Formulation (2) 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)

Sawtooth Formulation (3) Case II Case I Case III Alternative mathematical eqs. for determining ymax (Moore, 1983) (Richardson and Koerner, 1987)

Sawtooth Formulation (4) Case I: 4 > s2 (10.13) Case II: 4 = s2 (10.14) Case III: 4 < s2 (10.15)

Calculated Max. Mound Depth Tile spacing, m Slope, % McEnroe, 1989 Moore, 1983 Richardson and Koerner, 1987 100 1 2 3 4 5 1.545 1.225 1.010 0.855 0.740 0.650 1.542 1.121 0.838 0.651 0.526 0.437 1.179 0.999 0.909 0.861 0.833 50 1 2 3 4 5 0.772 0.612 0.505 0.427 0.370 0.325 0.771 0.561 0.419 0.326 0.263 0.219 0.589 0.499 0.454 0.430 0.417 P = 30 cm/yr; K = 10-3 cm/sec; yo = 0

Max. Mound Depth vs. Slope Continuous-slope configuration Lower mound depth Saw-tooth configuration Better p = 15.2 cm/yr; K = 10-3 cm/sec

Impact of Drain Tile Failure Continuous-slope configuration Greater mound depth: more problem Saw-tooth configuration

Max. Mound Depth vs. Slope

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

McEnroe Method R = p/Ksin2α < 1/4 R = 1/4 R > 1/4 p = percolation rate per unit surface area (cm3/sec/cm2); S = tan α = slope of liner (ft/ft); α = 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 tanα) = maximum saturated depth (ft). 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 ymax: Max. height of leachate mound d: Thickness of low permeable layer Undisturbed native material

Breakthrough Time K = permeability coefficient, L/T; d K = permeability coefficient, L/T; ne = effective porosity; d = liner thickness, L; and h = leachate mound height. Example: ne = 0.4; d = 4 ft; h = 1 ft; K = 1ⅹ10-7 cm/sec = 0.103 ft/yr

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

Weeping Tile Two types Helical profile Annular profile

Pipe Cleaning Method Rodding equipment

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.

Sewer ball Snakes

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 o’clock 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

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