Grantee Research Project Results
Final Report: The Feasibility of Electrophoretic Repair of Impoundment Leaks
EPA Grant Number: R828598C774Subproject: this is subproject number 774 , established and managed by the Center Director under grant R828598
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: Organotypic Culture Models For Predictive Toxicology Center
Center Director: Rusyn, Ivan
Title: The Feasibility of Electrophoretic Repair of Impoundment Leaks
Investigators: Corapcioglu, Yavuz , Herbert, Bruce
Institution: Texas A & M University
EPA Project Officer: Aja, Hayley
Project Period: September 1, 2000 through August 31, 2004
RFA: Gulf Coast Hazardous Substance Research Center (Lamar University) (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Targeted Research
Objective:
The objectives of this study were to evaluate the effects of solution chemistry and clay particle properties on the performance of electrophoretic repair of clay liners leaks through modeling and experiment.
Summary/Accomplishments (Outputs/Outcomes):
Synthetic geomembranes liners installed in liquid waste impoundments may develop leaks due to damage, imperfections, or improper installation. Conventional repair techniques, which involve locating the leaks and removing the liquid from the impoundment, are prohibitively expensive, dangerous to workers, and often cause additional damage to aged liners. Electrophoretic repair of clay liners offers a cost-effective and safe in-situ method of repair of leaking clay liners at impoundments. The technique relies on deposition of a clay sealant in the leak by electrophoresis. Electrophoresis uses a DC current to direct charged clay particles in a suspension to move to leaks in the clay liner. This project, which focused on development of a validated model, showed that bentonite slurries in a range of solutions of varying chemistry, are effective in controlling leaks in laboratory systems. Because the laboratory studies and model incorporate dominant processes that act at the micron scale, it is anticipated that the laboratory tests are adequate to guide field-scale applications.
Conclusions:
We have developed, tested and modeled an in situ technique to repair leaky clay liners. Our final report is available online. Our code will also be available online. We are working on submitting peer-reviewed paper on the research to disseminate our findings.
Development of an Axisymmetric Model: The numerical models conducted in this study were designed to (a) illuminate the processes occurring during clay cake formation (b) mimic the laboratory experiments, and (c) explore the relevant parameter space that controls the feasibility and efficient of leak sealing by electrophoretic clay cake formation. The layout of the problem (Figure 1) and the equations governing the potential field, and the velocity of clay particles (1-6) closely follow Corapcioglu et al. (1998).
Consider a radially symmetric pool, centered on a circular leak of radius Rl. The pool diameter is R, and the water depth is H. A circular electrode with radius RE is placed directly above the leak. The voltage in electrode is set to be VE, and the voltage in the ground beneath the leak is VL. The water surface and horizontal and vertical pool liners are insulating, so that there are no normal voltage gradients across these boundaries. The voltage in the region of the leak is insensitive to the choice of R, providing it is at least 2 times the water depth, H, and at least 5 RL (i.e., the leak is far from the sides of the tank).
Figure 1. Schematic layout of the axisymmetric electrophoretic sealing problem. The two axes are radial (r) and vertical (z). The boundary conditions on voltage, V, for each section of the boundaries.
The voltage is governed by
∇2V=0
The boundary conditions consist of fixed voltage at the leak and at the electrode, and no normal gradient elsewhere:
(2a,b)
(3a,b)
(4a,b)
This equation is solved for V(r,z). The electrophoretic velocity of the clay particles is then given by
(5)
where m is electrophoretic mobility. The total velocity of clay particles, u⃗, is then
(6)
where ws is the vertical settling velocity of the particles, assumed to be constant, and z⃗ is the vertical unit vector (positive upward).
We define the concentration of clay particles in the slurry to be governed by
(7)
The initial distribution of clay particles is specified, (t=0)=0(r,z). Boundary conditions on are no flux into the top (no more input of clay at the water surface), and no flux through the liner (including the leak), or through the clay cake. Finally, cannot exceed a maximum value of m, the concentration of the settled clay on the bottom. Velocities within the clay cake are taken to be zero. In practice, the concentration of settled clay will increase over time, as the clay compacts under its own weight, expelling fluid from the top of the cake. Models that include compaction of the settled clay (Kambham et al., 1995) show that this effect causes only small (~10%) changes to the final thickness and shape of the clay cake, and therefore it is neglected in this study.
Model Results: Uniform suspension: Figure 4 shows this evolution for the case of a uniform initial distribution of clay throughout the fluid (= 0.1 max ), and a moderate induced flow, =1. A clay cake forms very rapidly due to the voltage gradients around the leak (Figure 4b). Voltage gradients near the electrode drive clay away, “clearing” the fluid near electrode. As the clay settles out, a front of clay-free fluid propagates downward from the surface (since no more clay is being added), and the area initially cleared by the electrode causes a depression in this otherwise horizontal surface. The clay cake continues to grow until the depressed part of the clay-free front intersects the cake (Figure 4d). After this happens, the cake over the leak widens slightly, but most of the remaining suspended clay contributes to surrounding horizontal layer. Away from the effect of the applied voltage, the final thickness of the settled clay layer is 0.1H, which is determined by the total amount of clay added to the pool, and the concentration of the settled clay.
Figure 4. Snapshots showing the evolution through time of the clay concentration within the fluid, for the case of =1, Rleak=0.1 and Relectrode=0.1. The shading represents the logarithm of the concentration. In the initial state (a), the concentration is uniform, at 0.1 times the maximum settled value. The clay settles most rapidly over the leak, in this case building a cake with a maximum height of 0.2, while far from the induced voltage, the eventual thickness of the settled clay layer will be 0.1.
An efficient sealing will create a thick clay cake over the leak, while not draping a thick layer of clay over the entire bottom of the pool. We can define a sealing efficiency, SE, as the maximum thickness of the clay cake relative to the thickness of settled clay that would be achieved by gravitational settling alone. This measure of relative thickness will be determined by the strength of lateral flow of the clay (controlled by ), and the initial distribution of clay. Figure 5 shows the dependence of SE on , when the clay is initially uniformly distributed throughout the layer. Enhanced thickness of clay at the leak becomes noticeable for ≥ 0.03, and SE reaches a maximum of 2.35 for by about ≥ 30.
As would be expected, the rate of cake formation is nearly constant in the very low cases, and increases when electrophoretic forcing becomes important (F). Note that the final thickness does not change for values of > 100, but the rate of formation will continue to increase linearly with .
Figure 5 & 6. (left) The sealing factor, defined as the maximum thickness of the clay cake scaled by the thickness of the clay layer in the absence of electrophoretic flow, as a function of , the ratio of the electrophoretic induced particle velocity to the settling velocity of the clay particles. For < 0.01, the applied voltage has very little effect. The maximum thickness of clay, a factor of about 2.8, is achieved by =50. (Right) The maximum thickness of the clay cake (at r=0) as a function of dimensionless time for the runs shown in Figure 5. A time of 1 corresponds to the time required for a clay particle to sink through the entire water column at the settling velocity.
Model Results: Layered suspension A model that may be more relevant to the situation that might be encountered in an actual waste pool uses a much smaller amount of clay initially distributed on the surface of the pool, instead of uniformly distributed throughout the layer. Numerical models were run in which the initial distribution of clay was uniform at 0.1max throughout a layer that was 0.25H thick. When this layer is placed at the surface of the pool (so clay is suspended throughout the upper 1/4 of the water column), the sealing efficiency is essentially the same as in the case of uniform distribution throughout the layer, with a maximum SE of about 2.3(white symbols in Figure 7). As with the case shown in Figure 4, the clay is initially driven away from the electrode, and then gathered together by the voltage field around the leak.
To enhance the efficiency, the method could be modified to eliminate the initial driving of the fluid away from the area of the leak by the field surrounding the electrode. One method would be to allow the clay to sink some distance before turning on the current. When the clay layer is centered in the middle of the fluid layer (by allowing it to settle to that level, or by some other method of introducing it below the level of the electrode), where the voltage gradients are primarily vertical, there is no lateral motion of the clay until it begins to be drawn in by the field surrounding the leak. This method is much more efficient at concentrating the clay at the leak (black symbols in Figure 7). The disadvantage of this method is that, if settling is slow (corresponding to large ), the time required to form the seal will be significantly increased.
Figure 7. The maximum scaled thickness of clay cake as a function of , when the initial distribution of clay is in a horizontal layer of thickness 0.25H, positioned along the top of the pool (open symbols) or centered at a depth of 0.5 H (filled symbols).
A similar effect might be achieved by moving the electrode so that it is not vertically in line with the leak. However, as the distance from the leak increases, the voltage gradients, and therefore the strength of the electrophoretic forcing, decrease, which would slow down the sealing process. Another solution would be to use as large an electrode as possible, since this increases the area of vertical voltage gradients that drive the clay downward, as opposed to laterally away from the electrode (Figure 2).
The flow patterns of the clay in the fluid layer suggest a modification to the electrode design to better concentrate the suspended clay, and take full advantage of the electrophoretic forcing. A design configuration that both maximizes the sealing efficiency, while minimizing the sealing time uses a ring-shaped electrode centered over the leak. The diameter of the ring would need to be on the order the water depth. Using this design, all suspended clay within the vertical cylinder defined by the ring is concentrated toward the leak, even near the surface (Figure 8). In practice clay need only be added to the water inside the ring electrode, since suspended clay placed outside the electrode is forced away from the leak.
The result is a much thicker and more tightly constrained clay cake than that produced by disk shaped electrodes (Figure 9), reaching a peak sealing efficiency of 10 when =10.. Further, because the voltage can be turned on as soon as the clay is added to the surface, the time required is significantly shorter. (Figure 10)
Figure 8 & 9. (Left) The shading shows the normalized voltage in the fluid, for RL=0.1 and a ring-shaped electrode centered over the leak, with inner radius of 0.4 and outer radius of 0.5. The final shape of the cake for the initial condition of a layer of suspended clay at the water surface, =10, and (a) disk-shaped electrode, turned on at t=0, (b) disk-shaped electrode, turned on t=0.375, (c) ring-shaped electrode, turned on at t=0. (Right) The clay layer thickness with no electrophoretic forcing in this case is 0.02, so the sealing efficiency in case (c) is 10. The steps in these figures are due to the discontinuous nature of the definition of the top of the clay cake, and the vertical resolution of 0.005.
Figure 10. The development of maximum cake thickness (at r=0) as a function of dimensionless for the initial condition of a layer of suspended clay at the water surface, =10, and (a) disk-shaped electrode, turned on at t=0, (b) disk-shaped electrode, turned on t=0.375, (c) ring-shaped electrode, turned on at t=0. The clay layer thickness with no electrophoretic forcing in this case is 0.02, so the sealing efficiency in case (c) is 10.
Supplemental Keywords:
Geomembranes, leaks, electrophoresis, in situ repair,, RFA, Scientific Discipline, WASTES, Waste, Environmental Chemistry, Hazardous Waste, Engineering, Hazardous, Waste Disposal, hazardous waste disposal, hazardous waste management, hazardous waste treatment, subsurface, hazardous waste storage, geomembrane liners, electrophoretic repair, environmental engineering, liners, hazardous chemicals, waste management, high density polyethylene, mobility of contaminants, disposalProgress and Final Reports:
Original AbstractMain Center Abstract and Reports:
R828598 Organotypic Culture Models For Predictive Toxicology Center Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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