Geosynthetica’s Director, Elizabeth Peggs, interviewed Dr. Craig Benson (University of Virginia) and engineer John Allen (CETCO) about coal combustion residuals (CCRs) and new barrier system regulations governing CCR handling and storage. Both Benson and Allen will be at the Coal Ash Management Forum on July 21 and 22 in Charlotte, North Carolina. Benson is a speaker at the event.
In their discussion, Peggs, Benson, and Allen focus on characterizing coal combustion residuals, alternative lining/cover systems, and properly selecting geosynthetic clay liner (GCL) materials for optimal performance with particular CCR leachates.
The transcript of the interview is included below.
ELIZABETH PEGGS: Three years ago the volume of coal ash generated by coal-fired plants was estimated to be around 130 million tons annually…. Craig, the EPA document is substantial in its length; but there are some key points in the rule to discuss today. Can you give us your perspective on those key points?
CRAIG BENSON: Sure, Elizabeth. The document is really very extensive. It covers a lot of different issues which relates to disposal and management of coal combustion residuals. But for this particular discussion today, the key thing is the shift from wet disposal to dry disposal of CCRs.
Wet disposal largely being the entrainment of coal combustion residuals in water streams and disposal in ponds and shifting that to dry management strategy where we manage to dry particulate in a traditional landfill type of scenario.
The aspect that is particularly important is we’re taking technology that is well-established and has a track record of environmental protection for municipal solid waste, hazardous waste, low-level radioactive waste and applying that same technology with its track record for safe environmental containment of coal combustion residuals—taking a technology that we know works in other areas and applying it to this area as well. The key aspect of that is we are designing a landfill for coal combustion residual, and the key aspect of that is a composite liner that’s at the base of the facility and overlain by a leachate collection system. A composite liner is a barrier system that consists of a polymeric component—a geomembrane—and an earthen component. That earthen component may be a compacted clay liner that we would construct out of natural clay materials from a borrow source nearby or might be a geosynthetic clay liner, or GCL, that we purchase from a factory-manufactured scenario where we have bentonite between two geotextiles. The composite liner is a key aspect of the new regulations to provide safe containment of CCRs.
ELIZABETH PEGGS: John, what about the performance system requirements? And how do they perform and what are the requirements under the rule for the two systems?
JOHN ALLEN: Well, the rule is pretty clear about the composite liner being the standard for implementation. If someone chooses to go with the alternate composite liner, they have to demonstrate a few things to make sure that the alternate composite liner is equivalent to the composite. There are some key words we look at in the rule. One is “appropriate chemical properties.” That is, the alternate has to have appropriate chemical properties to be equivalent to the composite lining system. It also needs to have the appropriate shear stress, so that whatever materials are used in the alternate demonstrate the strengths needed for stability. The alternate system also needs to have a liquid flow rate that is no greater than the 2 ft. or 600 ml of compacted clay that’s describe in the composite system. So when we switch to the alternate system to use geosynthetics or geosynthetic clay liner, we have to work through these equivalency numbers to demonstrate that we are going to meet the same requirements as the composite liner system. This is all done relatively easily in design but you do need some supporting data to demonstrate that equivalency.
ELIZABETH PEGGS: So we have an idea about the options and performance requirements, but how does an engineer figure out what’s the best choice for their scenario? Craig, when an engineer is approach design of a CCR landfill, what are the reasons to choose one or the other engineering solution?
CRAIG BENSON: Well, there are a variety of different options and factors to consider. Perhaps one, and perhaps most important, is how effective is it going to be? We know that with the right materials we can design systems that have very low leakage rates and be highly protective of the environment. If we can do that with either type of scenario, then we start to look at other issues. We look at using GCLs versus compacted clay liners. [GCL] is a highly reproducible, highly uniform product, in contrast to compacted clay, which we’re excavating out of borrow source, and which has spatial and temporal varability. It requires a great deal of care and oversight to ensure we get the product we want. When we use a GCL, it’s highly reproducible, it’s a factory-managed, consistent product.
ELIZABETH PEGGS: You know what you’re getting.
CRAIG BENSON: We know what we’re getting. The other thing that is important in using geosynthetics in lieu of compacted clays, we can have more rapid construction. That’s particularly important in regions where construction seasons are short. We can place liner more quickly. We also use less air space, and this is a really important consideration, not only from a cost scenario but a sustainability scenario as well. When we look at building a thinner but equally effective liner, we’re using that land mass in a lot more efficient way. We’re able to place more waste per unit area of land. Finally, we can actually have lower cost. When we look at materials, we want to look at both are they effective—Can we build them in a timeframe we have at hand?—but what are the relative costs using different types of materials. In many cases, we can construct composite barriers with GCLs more cost-effectively than we can with compacted clays.
ELIZABETH PEGGS: That all sounds fairly straight forward until we get to chemical compatibility. This is where it gets kind of interesting. We know that leachate chemistry isn’t consistent. We know it can be affected by the various processes a facility uses to manage air quality under another set of rules. When we say “coal combustion residuals,” we are talking about more than fly ash. How do you begin to understand and confirm that the system you are designing is going to meet that chemical compatibility requirement?
CRAIG BENSON: There are a lot of waste streams that are classified as coal combustion residuals. Many of them are fly ash, but there are bottom ashes, flue gas desulfurization residuals, a variety of materials, but in essence their processes are essentially the same. We have a particulate material, a dry material, that gets moistened from precipitation, and when it gets moistened some of the constituents on the surface of those materials dissolve into the water. Actually, that same process is what happens in a municipal solid waste landfill and it happens in a hazardous waste landfill—the same leaching process that drives constituents in the waste into the water to create leachate. What’s different is the characteristics of the chemistry. When we deal with CCRs, we are really dealing with inorganic type of solutions. We end up with something that’s largely salt, analogous to what you would have if you took table salt and dissolved it in a glass of water, just with a variety of different salts beside sodium chloride. Fortunately we are able to characterize that relatively easily. If we have the leachate chemistry, if we a sample from the field, or we do a leaching test in a laboratory, we can send that leachate off to the laboratory and get the chemical characteristics reported back, the major cations and major anions, and do a relatively straightforward analysis with that data.
There are really two things we need to think about, in that chemical context, when we’re looking at geosynthetic clay liners, or clay liners more broadly—when we deal with the bentonite in GCLs, the key aspect of bentonite that creates a GCL with very low hydraulic conductivity is the ability of the bentonite to swell. When you put bentonite in water, for example in a beaker of deionized water on a bench, you’ll see it swell in an almost miraculous way. It’s really remarkable. The amount of the swelling is tied directly to the chemistry of the leachate.
There are two key aspects: one, something called the ionic strength. That’s a pretty complicated sounding term, but all it is is an aggregate or total concentration of all the different ions in solution. Adding all the concentrations up for the different species. So we have ionic strength. The other component is what we call RMD. It’s just a relative abundance of monovalent cations to polyvalent cations. That ratio. Monovalent is like sodium, which has a +1 charge, the sodium ion from table salt, and in other types of salt like we might normally see, like calcium and magnesium which have a +2 charge—so the ratio of the monovalent to polyvalent is important. Those two variables control how bentonite swells. If the solution gets really concentrated or has high ionic strength, the swelling diminishes. Or if the solution becomes really divalent, tends to be dominated by divalent or trivalent cations, it will also suppress the swelling. So you can imagine there is a range in which bentonite is going to swell really well, and if we get it too concentrated or too divalent the swelling properties will be suppressed and we won’t have the low hydraulic conductivity we want.
So we can look at the chemistry of that leachate and in a very simple spreadsheet calculation, calculate the ionic strength, calculate the RMD, and get a sense immediately whether a traditional bentonite for a geosynthetic clay liner is going to function well. If we are outside that range—and that’s one of the things we find with CCRS: they tend to create stronger and more divalent leachates than the other waste we manage in our industry—when we get outside that range where traditional bentonites work, then we need to look at other materials.
The materials we’ve been working with over the last decade are what we call bentonie-polymer composites, where we take largely bentonite but add some polymer to it as well that helps bridge between the bentonite particles and help fill pores. That extends our range. Depending on the polymer used and the amount of polymer that’s added, we can get geosynthetic clay liners that work in very extreme leachate environments.
ELIZABETH PEGGS: That sounds very promising. I’m a little nervous about the bentonite-swelling issue, but it sounds like the industry has recognized that for some time and tackled it well. John, with all of these considerations in play, what’s the process for identification, selection, and success with those polymer-added geosynthetic clay liners? How does an engineer address it?
JOHN ALLEN: It’s an interactive design process, between the engineering design team and the manufacture that supplies the product to the project. Starting with the baseline information from the water quality analysis on the leachate, the manufacturer can than make decisions on which product will be most applicable for solving the problem for that given site. But it takes several steps to get through that, and working back and forth to get a proof of concept that will be satisfactory to the regulatory body and prove that equivalency and long-term performance of the products. There can be problems too, if you get too much polymer or the wrong polymer. It needs to be well vetted before it’s implemented. That can be done and has been many times.
ELIZABETH PEGGS: What kind of a timeline does an engineer or project manager need to understand leachate and GCL compatibility?
JOHN ALLEN: The facility owner and their engineering consultant need to be thinking way in advance if they are going to go down the alternative design route. The water quality analysis part happens rather quickly. One has to get to the site, get a sample of the leachate or ash, get it back to the laboratory, and get the water quality analysis results back. Then, after that, the next step is going to be running the calculations for the ratio of monovalent to divalent cations and calculating ionic strength. All of this happens in the first five to 10 days, to get some kind of idea of initial product polymer modified GCL that will solve the site’s problems. Once that happens, the next step is proof of concept. This is the equivalency test that will look at the permeability of that material with time and say that the GCL is equivalent and has the suitable chemical properties that the composite liner requires. We’re going to prove equivalency. This process can take up to two to nine months to see long-term stable reaction of the product with the leachate. Once it’s through that, it’s relatively quick to finalize the manufactured product and move forward with the design.
ELIZABETH PEGGS: What do you do at the build phase to make sure that what arrives in the truck on site is the same as what was tested?
JOHN ALLEN: There is a strategy. It’s actually a parallel process to the equivalency testing for permeability and chemical properties. When we set out on a program to approve a product for a particular site, we’re also going to look at things like the peel strength of the GCL, the slope stability interface friction properties of that product, and we’re going to want to measure the polymer that’s present in the product as well, so that the manufacturer knows how to produce the product with the correct amount of polymer presence and that the owner and design engineer have some way to know the polymer is present and evenly distributed across the role. Right now we do that through two tests: one is viscosity and one is the loss on ignition test.
Together, those tests prove the polymer is there and in the right amount to satisfy the needs for chemical capatibility.
ELIZABETH PEGGS: Sort of conformance testing for delivered materials against tested materials. That’s another process everyone employing geosynthetics should be in touch with. You mentioned shear strength. Are there considerations there to talk about?
JOHN ALLEN: Sure. The polymers, in my experience, tend to be slightly weaker than the bentonite strength, and bentonite is a weak material. That’s why we needlepunch these two textiles together to encapsulate the bentonite, which is the hydraulic barrier. If the wrong textiles are used in process, or if the polymer is allowed to escape from the textiles, it can cause slope stability problems. This is why we vet the products during the design phase. … It’s something to do as the project moves forward.
These processes are not new. I should say that. Four projects come to mind readily that have gone in the recent month where we’ve walked through all these different steps, from design through construction in vetting the products. These are not new concepts for engineers or the owners.
ELIZABETH PEGGS: Craig, that still sounds pretty complicated. How does an engineer navigate all of that with confidence?
CRAIG BENSON: It is more complicated than just implementing the traditional, standard design. But, the benefits outweigh the complexities. Really, the complexities are not that hard to navigate. We’ve been working on these issues with traditional bentonites for 20 years and bentonite polymer composite materials for a decade. We’ve been able to distill a lot of these issues of chemistry and hydraulic properties and mechanical properties into straightforward engineering design procedures and methods. So we have a process by which you go about looking at the leachate. You do the calculations. You evaluate the materials that you have available. And you select the appropriate material for the project.
We have standardized test methods as well. ASTM established methods. High-quality, reproducible laboratory procedures to demonstrate and validate that the materials we’ve selected will function properly. So we have that experience of engineering methods that we step through and test procedures to validate design that are standardized in a traditional engineering context. So while there are a number of steps to take, the steps themselves are well defined. Really, the key aspect is planning—making sure we have the information we need and have the timeline available to do the analysis and conducts the tests to make sure the products are appropriate for that job site. So planning ahead is perhaps the most important aspect of the project, and then stepping through the process that has been developed.
ELIZABETH PEGGS: John Allen and Craig Benson will be at the Coal Ash Management Forum July 21 and 22 in Charlotte, North Carolina.
Geosynthetica is a media partner to the Coal Ash Management Forum and will be at the event.0