Increasingly, landfills are taking in waste types requiring special handling, such as those with high-liquid content; and those that contain PFAS or metals. Some treatments to ensure their safe disposal include solidification, stabilization and bulking. These approaches have their known purpose and merits, though there is still investigation underway to learn more about how effective some of them are. Bryan Staley, President and CEO of Environmental Research & Education Foundation (EREF), explains how these techniques work and applications for each of them. And he provides an update on ongoing research at the University of Texas-Austin, funded by EREF.

Waste360: What are some forms of stabilization and solidification, and what problems do they curb?

Bryan Staley: Solidification and stabilization are treatments that entail mixing wet waste with amendments to get it out of the water phase and solidify or strengthen it so it can be safely disposed. Among problems these techniques mitigate are potential for contaminants to flow into leachate or for impact on the structural integrity of the landfill.

One form of stabilization is applied to aqueous waste that has low pH, which is potentially problematic because low pH interferes with the microbes needed to break down waste to make biogas from landfill methane. Low pH also causes problems for leachate treatment. So, the approach in this case would entail adjusting pH to ensure the microbes are not killed.  You would add a stabilization agent like lime to elevate pH, but adding lime along with a solidification agent also facilitates making the material less mobile and easier to control.

An example of how this is done in practice, which applies to any solidification strategy, involves mixing aqueous waste with the solidification or bulking agent and mixing it with a backhoe. This could be done in a pit, roll-off box or something similar.  Once the moisture content is sufficiently low enough, then the solidified material would be moved to the working face of the landfill. 

Waste360: What is bulking and when is it an appropriate technique? How is it different than solidification?

Bryan Staley: Bulking is a strategy used to reduce the moisture content but does not necessarily solidify the waste into a monolith structure or mitigate other attributes that may need to be dealt with (e.g. low pH).  The reason reducing moisture content is important is that applying high moisture content wastes directly to the working face can result in instability of the waste mass or inconsistency in compaction of waste. Operators would use bulking agents such as construction and demolition fines, sawdust and wood chips or similar materials (whatever can serve the purposes most economically).

It’s like solidification but what differentiates it is with solidification you use different materials that tend to bind the aqueous waste more into a monolithic form rather than just reduce moisture content (e.g. ash, kiln dust). 

So basically, with solidification, you combine materials with aqueous waste to create a hard, binding material. But with bulking agents you are just adding something to soak up water, and the water is used to create a reaction to result in a cement-like material to immobilize waste and create structural integrity.

Waste360: How can solidification and stabilization address waste from the power industries, including from coal-fired plants?

Bryan Staley: These processes are actually commonly applied in the electric and power industries, specifically for coal-fired plants that, through their combustion process, generate flue gas materials or brines with heavy metals.

Stabilization is of value here because it sequesters material that you don’t want to be mobile, such as these concentrated flue gas brines. This approach is especially practical at landfills that receive this ash; it’s important for them to be able to solidify it to minimize mobility of these materials once placed in the landfill.  

Waste360: Tell us about the ongoing research EREF funded at the University of Texas-Austin (UTA) to determine if solidification effectively deals with heavy metals.

Bryan Staley: As I explained, you do not want heavy metals in leachate, so if you solidify materials with metal you want to know if the solidified material will leach over time and how much. So, we funded the UTA study to assess different test methods for evaluating methods to evaluate leaching from solidified material. There is a standard test method called the LEAF method, but it has limitations and may not accurately assess leaching behavior from waste materials.  

So, we were evaluating tests to be better informed as to how effective solidification strategies are.  This understanding is critical. You do not want more contaminants coming out that you have to treat aggressively in order to remove them. 

The study has another year left, but the researcher team, led by Dr. Maria Juenger and Dr. Lynn Katz, are making great progress in evaluating leaching method as well as different mixtures of different solidification and stabilization agents. As part of the latter effort, they are trying to determine the optimal range of percentages of lime or other agents in order to result in lower leaching levels.

Waste360: Can solidification manage PFAS-containing waste?

Bryan Staley: Solidification techniques are now being explored to use in conjunction with current PFAS treatment methods, such as reverse osmosis and granular activated carbon. The biggest limitation of these treatment strategies is they generate a residual that must be managed. The question is, do you place concentrate residual in the landfill without further treatment? It’s a question that needs further research but use of solidification techniques as a way to sequester PFAS is currently being looked at.

Given that solidification and stabilization are shown effective for flue gas brines and other aqueous wastes, I am optimistic that these strategies could be effective in sequestering PFAS, especially over the short term since complete destruction strategies for PFAS are being explored but haven’t been fully evaluated.

Waste360: How does a landfill operator implement solidification with cost in mind?

Bryan Staley: Because aqueous wastes are extremely variable in their properties, there is no ‘one size fits all’.  In most situations, the bench tests should be done to confirm effectiveness of the bulking or stabilization agents at specific mixing ratios before they should be used in practice.  Economics play a role in that in many situations the landfill operator may be able to use materials that are readily available and inexpensive so long as the waste material to be solidified doesn’t pose unique challenges (e.g. it is reactive). 

If they can get material for free to use as a solidifier or bulking agent that’s always attractive. For example: sawdust from a facility with chicken waste can be used as a bulking agent. If it is something reactive or leaching needs to be minimized they may need to go with a solidification strategy that uses something like lime and fly ash, which may increase the cost but may be necessary to ensure appropriate disposal.

Waste360: What are the main remaining research questions around these strategies? And what are the concerns?

Bryan Staley: All the solidification, stabilization, and bulking methods I mentioned are being used today (though with PFAS it’s in a pilot phase), so they are practical strategies. I think the primary research question that remains is, how do we evaluate the effectiveness of these strategies to minimize the need for additional leachate management?

As we see more aqueous waste from landfills, ensuring such strategies mitigate any reactive materials before placement and maintaining appropriate structural integrity are aspects that are currently evolving. This is because the waste stream is always evolving.  How do we manage and place it while ensuring worker safety?

Research can come into play in optimizing these kinds of strategies.