Presentation on theme: "Leachate Collection System"— Presentation transcript:
1 Leachate Collection System Jae K. (Jim) ParkDept. of Civil and Environmental EngineeringUniversity of Wisconsin-Madison
2 Leachate Collection System (1) Designed as containment facilities due to concern with the environment impact of landfillsNeeded to prevent landfill gas and leachate from migrating from the site in significant quantitiesPurpose: 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.
3 Leachate Collection System (2) RefuseDrainage tileDrainage layerLow permeabilitybarrierUndisturbednative materialSimple collection systemRefuseDrainage tileDrainage layerLow permeabilitybarrierUndisturbednative materialDouble liner system
7 Components of LCS French drain Refuse Tile drain Drainage layer Low permeablelinerUndisturbednative materialK of drainage layer: min cm/sec; 10-2 desirableDrainage layer gravel should be washed to remove fines; no limestone-based aggregateFrench drain: used in the event of pipe failure or clogging; gravel packAdditional containment and/or leak detection system
12 Leachate Removal System Pipe passed throughside of landfillPotential leakage:Not recommendedLeachate removedwith a pumpMost widely used
13 Leachate Collection Facilities Leachate collection and transmission vaultLeachate holding tank
14 Leachate Collection Facilities Above gradeBelow gradeUsed incold regions
15 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 liquidsDrainage 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/secSlope: to encourage lateral migration; min. 2% bottom final slope after long-term settling
16 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 layerFine 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 filterUC: Uniformity coefficient = d60/d10
17 Design Considerations for Tile Spacing Why? To control the height of a mound of leachateDesign considerationsFlow rate or flux of leachate impinging on the barrier layerSpacing between the tilesSlope of the linerThickness and hydraulic conductivity of the drainage layerIf 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.
18 Analytical Formulations for Tile Spacing Mathematical models to examine a series of design considerations including:Depth, hydraulic conductivity, and slope of the drainage layerThickness of the low-permeability barrier layerTwo measures of hydraulic performance: max. saturated depth over the barrier and amount of leakage through the barrierLeachate mounding: function of liner slope, leachate infiltration rate, permeability of drainage and barrier layers, and drainage tile spacingAssumptions in mathematical formulationFlow is one direction (lateral).Saturated steady-state flow conditions exist.The drainage media are homogeneous and isotropic.
20 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); andyx = 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); anddz/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)
21 Continuous-Slope Formulation (3) Assuming a unit width of aquifer and combining Eqs 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)ApexCase I: 4 > s2Case II: 4 = s2Low permeable linerDrain tileCase III: 4 < s2
23 Examplep = 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/yr30 cm/yr15.2 cm/yr7.6 cm/yr2.5 cm/yr
24 Flat-Slope Configuration (Worst Scenario) *07/16/96Flat-Slope Configuration (Worst Scenario)When the slope of the liner system equals zero, Eq becomes:ymax occurs at x = D/2. From Eq. 10.9, ymax becomes:Ex. Determine ymax using Eq 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*
26 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 with transformation of the origin (i.e., xsawtooth = L - xcontinuous). Transforming Eq 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)
27 Sawtooth Formulation (3) Case IICase ICase IIIAlternative mathematical eqs. for determining ymax(Moore, 1983)(Richardson andKoerner, 1987)
33 Wisconsin Regulations NR (5)(a) Wisconsin Administrative Code (WAC): 12 inches of average leachate head over the liner< 130 ft drain spacingNR (3) WAC:Open conditions: p = 6 inches/yr = 0.5 inch/monthClosed conditions: p = 1 inch/yr = inch/monthFactors affecting the leachate mount heightPercolation rate into the drainage layerHydraulic conductivity of the drainage layerLeachate flow distance from the upstream boundary to the leachate collection pipeSlope of the landfill liner
34 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); andymax = 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):McEnroe, B.M. (1993). “Maximum Saturated Depth over Landfill Liner,” Jour. of Envion. Eng., ASCE, 119(2):
35 Performance Measures Residence Time, T where s’ = slope approximated by the bottom slope, m/m.Efficiency of Captureymax: Max. height of leachate moundd: Thickness of low permeable layerUndisturbed native material
36 Breakthrough Time K = permeability coefficient, L/T; dK = permeability coefficient, L/T;ne = effective porosity;d = liner thickness, L; andh = leachate mound height.Example: ne = 0.4; d = 4 ft; h = 1 ft;K = 1ⅹ10-7 cm/sec = ft/yr
37 Clogging ProblemsOccur in agricultural irrigation, weeping tile systems, sanitary landfills, septic system leachate fields, and the like.Remedial measuresSmaller-diameter lines (15~30 cm): cables> 30 cm lines: rodding equipmentMax. 300 m between access ports or manholesRemoval mechanismsMechanical procedures: roto-routers, pigs, sewer balls, snakes, and bucketsLow-pressure jets: 70 to 140 psi at nozzleHigh-pressure jets: 410 to 1300 psi at nozzleChemical methods: such as SO2 gas; some danger
42 Other Design Considerations Collector sizing and type: at least 15 cm diameter; min cm, preferably 30 cm to reduce the effects of silting and to facilitate inspection and cleaning; schedule 80 PVC or HDPECollector slope: 2% if practical but not < 0.5%Collector perforations: at 2 and 10 o’clock positionsFrench drain around the collector pipe: 38 to 50 mm washed stoneAttention 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
44 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.
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