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19 April 2017

Coal combustion residuals – what should you consider before accepting them?

grahlherr-don_photoUnless you live under a rock, you know about the new CCR Rule, a RCRA Subtitle D Non-Hazardous Waste Rule that covers coal combustion residuals (CCRs). These include fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) waste.

I thought it would be interesting to share my thoughts on 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.

Photos courtesy of American Coal Ash Association

Photos courtesy of American Coal Ash Association

How will the new rule affect disposal by plants?

  • Added location restrictions, design criteria, including a composite liner, stability restrictions, and groundwater monitoring/corrective action from the CCR Rule – not to mention the new Effluent Limit Guidelines (ELGs) – will 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.
  • Most impoundments will be closed within 5 years.

So what should a 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/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/operate a CCR landfill at a generating station.

Before doing so, here are some key areas to keep in mind:

1.     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!

2.     Chemical precipitation (calcium-based) in the leachate drainage system – CCR materials contain high levels of calcium and leachate is usually highly alkaline. But it is difficult to come up with a definitive prediction of scaling potential due to calcium precipitation. 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.

3.     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 may also likely solve the compatibility issue, but I really recommend testing to confirm.

4.     Odor/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 WWTP sludge will exacerbate this issue. This presents a strong case to not comingle CCRs and MSW.

5.     Leachate/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 USEPA 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.

6.     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.

7.     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. Then there’s additional personnel and equipment that may be needed to handle larger volumes.

You may need to segregate materials, which would require additional equipment to handle and compact the CCR. There are also geotechnical considerations, because CCR may create saturated and/or lower permeability zones that can lead to leachate breakout and slip planes. This is even more support for the case that it may be best not to comingle CCR with other waste without good design/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.

Are there other considerations you are aware of for accepting CCR at landfills?

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/permit documents for solid waste landfill and disposal pond design, primarily for coal combustion residuals (CCR), as well as site/civil design, geotechnical investigations and recommendations for industrial foundation applications.

Don will be presenting Structural Considerations for In-Place Closure of CCR Impoundments at 2017 World of Coal Ash Conference, to be held May 8-11, 2017. The presentation is based on a paper that was co-written by Christopher Lewis, PE.

Categories: Air Quality, Alternative Energy, Biogas and Landfill Gas, Environmental Planning & Compliance, Environmental Remediation, Solid Waste
Posted By Don Grahlherr, PE at 11:30 AM  |  No Comments on Coal combustion residuals – what should you consider before accepting them?

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