By Don Grahlherr |
Wed Apr 19 21:37:43 UTC 2017
Courtesy of ACAA and Boral Resources
Don Grahlherr, vice president of Tetra Tech’s Coal Combustion Residuals (CCR) Practice, discusses how the new CCR rule, RCRA Subtitle D Non-Hazardous Waste Rule, is likely to affect municipal solid waste landfills (MSWLFs) that may receive CCRs. This blog post originally appeared on CornerPost. All opinions are the author’s own.
Unless you live under a rock, you know about the new CCR Rule, a RCRA Subtitle D Non-Hazardous Waste Rule that covers CCRs including fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) waste.
Many waste managers are wondering how the rules are likely to affect municipal solid waste landfills (MSWLFs) that may receive CCRs, and what they should consider when deciding whether to allow disposal in their facilities.
How will the new rule affect disposal by plants?
While these rules are under reconsideration by the U.S. Environmental Protection Agency (EPA), the added location restrictions, design criteria, including a composite liner, stability restrictions, and groundwater monitoring and corrective action from the CCR Rule—not to mention the new Effluent Limit Guidelines (ELGs)—will eventually make impoundments impractical. Plants are converting to dry ash handling, and will continue to do so. Some will likely shut down—this will depend on the power market. Disposal will shift to landfills over the next 5 years, and most impoundments will be closed within 5 to 7 years.
So what should an MSWLF that may receive CCRs keep in mind?
The rule actually offers several interesting opportunities to municipal solid waste companies. They can choose to accept CCR at an existing MSW landfill; build and operate a new CCR landfill (monofill) at an existing MSW landfill site; build a dedicated cell for CCR (monofill) within an MSW landfill; and finally, they could build and operate a CCR landfill at a generating station.
Before doing so, here are some key areas to keep in mind:
Filter compatibility of geotextiles with CCRs—Fly ash or FGD is very fine-grained and difficult to demonstrate filter compatibility with any geotextile. Non-cohesive material may migrate into nonwoven geotextile and clog or pass through. For geosynthetics, woven geotextile is better for long-term filter performance for CCR due to the physical nature of the material (more sieve-like). Manufacturers have responded by developing a combination of nonwoven/woven geotextile called composite geotextiles. Bottom ash may be used as a graded filter component above the leachate collection system (LCS), but you may not have an opportunity for a separate stream of bottom ash to help with this issue at an MSWLF. (A soil operations layer may help here as well.) Most importantly, you need to do the required testing and analyses!
Chemical precipitation (calcium-based) in the leachate drainage system—CCR materials contain high levels of calcium and leachate is usually highly alkaline. But coming up with a definitive prediction of scaling potential due to calcium precipitation is difficult. Be sure to use a conservative factor of safety for clogging, use redundant design features where practical, and make sure to include cleanouts in the design, as well as planned maintenance for pumps.
Geosynthetic clay liner (GCL) compatibility with CCRs—Through ion exchange, calcium substitutes for sodium in the sodium montmorillonite minerals in the bentonite clays used in the production of GCLs and reduces swell and increases permeability of the GCL. Manufacturers are now producing CCR-resistant bentonite, which will help this issue. The timing of hydration also may likely solve the compatibility issue, but I really recommend testing to confirm.
Odor and gas generation—FGD can include calcium sulfite, magnesium sulfite, and calcium sulfate, among others. Under MSWLF conditions, bacteria can convert the sulfates and sulfites into hydrogen sulfide, which can cause odor problems or even potential health and safety issues. Also, the likely increase in pH may kill the bugs, affecting gas generation and extraction. Adding any wastewater treatment plant sludge will exacerbate this issue. This presents a strong case to not comingle CCRs and MSW.
Leachate and water quality—Begin by anticipating the potential leachate quality changes and the impacts on leachate management strategies. Since pH will likely increase, other contaminant levels may change. Mercury could become an issue in ash/leachate. The EPA expects state regulatory agencies to require a MSWLF accepting CCR to include inorganic indicators known to be associated with CCRs in the groundwater detection monitoring plan.
Pozzolanic reaction—Fly ash is a pozzolan—a siliceous material that reacts with calcium hydroxide in the presence of water to form cementitious compounds. The reaction creates heat, which you will have to consider in your landfill gas temperature monitoring. I recommend considering these effects and advising regulators before changes become an issue.
Operations—Power plants typically generate large quantities of CCR 24/7/365, so you have to evaluate your daily permit tonnage limits. Other operational factors to be considered include dusting potential and control methods, odors, and additional truck traffic. You also may need additional personnel and equipment to handle larger volumes.You may need to segregate materials, which would require additional equipment to handle and compact the CCR. There also are geotechnical considerations, because CCR may create saturated or lower permeability zones that can lead to leachate breakout and slip planes. This is even more support for the case that comingling CCR with other waste may not work without good design and controls in place. When segregating or monofilling with CCR, the MSW landfill cell’s basis for design must be reviewed to confirm the change in materials are compatible with overall design parameters (for example, shear strength).
Based on recommendations in the CCR Rule, state regulatory agencies will likely require a MSWLF accepting CCR to prepare a CCR Acceptance Plan that addresses the physical and chemical characteristics, as well as dust and structural integrity. The plan also has to show that the CCR will not compromise the leachate and gas collection systems.
About the Author
Don Grahlherr, PE, is vice president of Tetra Tech’s National CCR Practice. He has 28 years of experience on a range of geotechnical and environmental projects including design, analysis, and preparation of contract and permit documents for solid waste landfill and disposal pond design, primarily for coal combustion residuals (CCR), as well as site and civil design, geotechnical investigations, and recommendations for industrial foundation applications.