April 29, 2024

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In Conversation: Turning off the Flame

How Biofilters Reduce Cost and Raise the Value of Post-Closure Landfills

Owners and operators guiding landfills through post-closure face rising costs and tightening regulations. Now more than ever, finding solutions that can reduce the financial burden while also meeting greenhouse-gas emission regulations is paramount.

And what if such a solution could also complement the transition of these sites to public spaces?

In this conversation, Geosyntec’s Justin Lottig and Peter Bannister, PE (Washington), from Aspect, a Geosyntec Company, discuss how landfill covers mitigate fugitive greenhouse-gas emissions and how biofilters could provide additional mitigation.

About Speakers

Justin is a senior scientist and former landfill operator with more than 25 years of experience improving solid-waste operations and environmental compliance in the US Pacific Northwest, Alaska, and Hawaii. He specializes in solid-waste-services master planning, construction management, landfill-gas management, landfill and transfer station capital improvements, and environmental monitoring and investigations.

Peter is a principal environmental engineer with more than 20 years of experience leading work to optimize engineering controls at landfills, simulate complex groundwater resource systems, and clean up sites with groundwater contamination. He has worked with landfill owners, municipalities, and tribal entities to help understand the effects of landfill gas on groundwater quality, reduce greenhouse-gas emissions, and end post-closure care requirements.

Q&A

Justin Lottig

There’s a renewed focus on closed landfills and their methane emissions with Washington’s new greenhouse-gas emissions regulation, WAC 173-408.
You’ve recently worked with Washington’s King County to study this issue. What did you discover about post-closure landfills and greenhouse gases?

Peter Bannister

There were several questions King County was trying to answer. First, how much emissions do we have at old, closed landfills with different cover types? Second, can we reduce these greenhouse-gas emissions? And third, can we take advantage of the organics that we're diverting from landfilling and use the compost that we’re creating to help mitigate emissions?

We investigated fugitive emissions through three different styles of covers, including a soil cover designed and constructed under the old landfill regulations, a composite cover with a geomembrane at an unlined landfill, and a composite cover with geomembrane at a lined landfill.

With all three, our major finding was that older, closed landfills don't contribute a lot of fugitive emissions to the atmosphere through their cover systems.

Biofilter blankets, which are constructed of locally-sourced media composed of woody compost, are better than flare systems to control fugitive emissions at the soil-covered landfills and landfill-gas migration from old, closed landfills.

Justin Lottig

I want to ask you more about biofilters versus flare systems. But first, how did you assess the emissions through their cover systems?

Peter Bannister

Each month in spring, summer, and fall, we investigated baseline emissions using traditional surface-emission monitoring and drones. We worked with Sniffer Robotics to evaluate the efficacy of the drone technology. Sniffer has since received USEPA approval for compliance-level surface emissions monitoring.

We also conducted traditional walking monitoring; we walked back and forth across the landfill to measure methane concentrations at ground surface. To avoid missing anything, we used 25-foot spacing instead of the normal 100-foot spacing. Those were long days.

With drones and our feet, we found no methane at landfills with composite covers. We had been concerned there might have been deficiencies in cover construction. You know, open seams in the geomembrane or imperfections around the well boots. But we didn't find any methane above background. It was very impressive.

At the landfill with the soil cover, the County had planted poplars about 20 years ago to reduce leachate generation by increasing evapotranspiration during the growing season. They ended up with deep roots in their soil cover and some of the trees died back.

With this soil cover, we were concerned there might be fugitive emissions from root holes or soil cracks due to differential settlement. The trees restricted the drone from flying close enough to the ground to measure methane effectively. However, we found reproducible areas of higher methane concentrations during walking surveys.

Justin Lottig

What do you mean by higher methane concentrations?

Peter Bannister

When I say higher, it was still relatively low compared to federal regulations. For large landfills, the USEPA has specified an action level of 500 parts per million. For this project, we decided to lower that level to 200 parts per million, which is the value in the proposed language for WAC 173-408. While most of the soil cover had methane concentrations below 200, we found pockets that exceeded 1000 parts per million.

We believe we can mitigate these concentrations by applying a biofilter cover, like a blanket over the existing surface.

This ”bioblanket,” created using the County’s compost material, would improve the habitat for microbes that digest methane and create biomass and carbon dioxide. We call these “methanotrophs.”

Justin Lottig

What are the advantages of biofilters, compared with traditional flare systems for managing landfill-gas emissions?

Peter Bannister

Biofilters help owners stay in compliance and reduce costs at these old, closed landfills, compared to intermittently operating flares.

This is a challenging spot for landfill owners and operators to be in. The regulations they are trying to satisfy, about controlling landfill-gas migration and treating landfill gas, were written in the 1980s and 90s. And there weren’t many examples about what the end of post-closure would look like. That’s what everyone’s struggling with now.

Operators responsible for keeping a flare running to control landfill-gas migration might charge overtime to respond at night to a flare shutdown. But the reason the flare shut down could be because of low methane generation compared to the flare design.

They might spend hours trying to get a flare going, when there just isn’t enough methane. In comparison, a biofilter can be used to treat landfill gas collected using a blower or just from a passive system. It’s always on and never shuts down. Operators get their sleep, and owners don’t have to pay overtime.

A lot of closed landfills have gone beyond their financial assurance planning horizon and, as a result, have become a liability to their owners. Their owners are seeking ways to reduce costs by not needing to conduct monthly, quarterly, and annual post-closure activities. As I understand it, a lot of regulators are also interested in ending post-closure care.

Justin Lottig

“How do we end post-closure care?”

Peter Bannister

Exactly. The original solid-waste regulators scripted a 30-year planning horizon for post-closure care. Thirty years later, a new generation of regulators has provided guidance to help demonstrate “functionally stable” landfills with little to no settlement, leachate generation, or landfill-gas generation.

For landfill gas, we end post-closure care when we don’t need power to run a blower or flare system and have demonstrated little to no landfill gas migration. Landfill-gas generation is highest shortly after you close a landfill and decreases over time. Eventually, there comes a point when a flare system, designed to control migration based on higher generations rates, is no longer reliable at lower generation rates.

When a flare stops operating, there is usually an actuated valve that prevents venting. This results in pressure buildup and increases the risk of fugitive emissions through the cover. Biofilters can treat passive venting landfill-gas methane levels to below 1.25 percent to meet regulatory guidance for explosive gas control, and they likely offer a solution to address the newer, tighter greenhouse-gas regulations.

Justin Lottig

So, can a biofilter be seen as a replacement for existing combustion technology?

Peter Bannister

Absolutely. Flares have been the standard at smaller landfills where beneficial use of methane is not cost effective. At older landfills, where methane content is no longer high enough to support a flame, we’ve helped owners transition to a biofilter treatment system for active or passive collection of landfill gas.

Where active collection is necessary to control landfill-gas migration, or where passive collection using a centralized system still provides migration control, we replace the flare with a larger biobed. Where decentralized venting can occur, we decommission the flare and landfill-gas conveyance system and hook up small biofilters right at the venting wellhead.

The flare at the Jefferson County landfill ran from 1993 until 2020, when owners, regulators, and Aspect partnered to turn it off. This is a rare milestone in the life cycle of closed landfill operations).


There are cost and labor savings here, too, especially if you consider the potential for intermittent flare operations and how difficult it is to get a flare restarted, like when it's freezing out. A biofilter system can operate 24 hours a day, 7 days a week. You can run a blower into a biofiler constantly, if necessary, and you don't need a control system.

Biofilters work because these microbes live to digest methane. There may be some seasonal performance changes during a dry summer or a cold winter, and there is some replacement cost. We currently replace biofilter media every 5 years. But who knows? Maybe within 5 years you won’t have this issue to deal with anymore.

Justin Lottig

It sounds like biofilters are ahead of the regulations now.

Peter Bannister

Well, we measure the biofilter surface with the goal of reducing methane to below 25 percent of the lower explosive limit, which is based on regulatory guidance to end post-closure care. At this point, we have really good evidence-based information showing that biofilters work on the western side of the Cascade Mountain range at a number of landfills. Our long-term monitoring shows that the microbes develop quickly in these wet conditions after the biofilter is constructed and emissions are addressed quickly.

We installed these biofilter systems for landfills where we were already meeting the small quantity emissions rate, the acceptable source impact levels for toxic air pollutants, or both. We didn't have an air quality regulatory constraint with replacing a flare with a biofilter. Any amount of uptake is just ahead of the curve.

But let’s say the regulators come along and say, “nope, the new criteria for landfill venting wells and biofilters is 1 percent of the lower explosive limit (LEL),” that’s 500 parts per million methane. Then, you can expand the biofilter size at low cost, and you're not spending money on restarting a flare in freezing weather.

I’m hopeful that landfill owners will combine passive venting and biofilters to meet the new 200 parts per million methane standard under WAC 173-408. There is a high cost for surface-emission monitoring at 25-foot spacing. Without drone technology, that’s a lot of knees aching at the end of the day (or week).

A biofilter system can operate 24 hours a day, 7 days a week.

For example, at a landfill in Eastern Oregon, their path right now at the 100-foot spacing is around 42 miles. And that entire distance needs to be walked once every 3 months. Not to mention all that walking is done on a potentially hazardous slope. Even with the potential advantages of drone technology, owners are motivated to move away from that cost burden.

Justin Lottig

Can biofilters be used on landfills with existing cover systems, such as those designed for evapotranspiration?

Peter Bannister

They can. One of the issues found in Washington and Oregon is that only planting something like deciduous poplars limits evapotranspiration during the growing season. But planting deep-rooted trees or shrubs in a landfill that is nearly functionally stable may support the next type of land use, which could be a park.

The nice thing is, it doesn’t really matter whether fugitive emissions are due to differential settlement and potential stress fractures in the soil cover or the deep rooting zone offering preferential flow paths to landfill gas. The biofilter blanket is going to address whatever the issue is and support the development of microbial communities to digest methane before it gets to the atmosphere. We don’t have far to go, when you consider that the soil cover provided sufficient habitat to reduce methane concentrations from 50 percent just below the cover to 0.1 percent (or 1000 ppm) at ground surface.

Then, there’s the question of future uses for these landfills. Public greenspaces with walking paths are at a premium. After decades of maintaining site security, landfill owners are looking at opening the gates and allowing public access to old, closed landfills. The key criteria I keep in mind for allowing public access is minimizing risks of landfill gas and presenting vandalism-proof design for legacy infrastructure.

Biofilters are nice because they digest methane and hydrogen sulfide, addressing LEL and odors, and they blend in with the landscape better than a fenced venting well with a 10-foot-tall stack with signs everywhere. Biofilters can be integrated into custodial care, after the regulators have allowed the end of post-closure care.

Learn More

Waste Management Planning, Engineering, and Design Services (geosyntec.com)
Solid-waste advisory and engineering services on our YouTube channel: Market: Solid Waste - YouTube

How Peter and Aspect guided Jefferson County in Washington state through the post-closure care process: How Turning Off The Flare Helps Us Sleep at Night: Landfill Milestones — Aspect Consulting
Geosyntec’s White Paper on managing PFAS in landfill leachate: Geosyntec PFAS White Paper 2024

To discuss post-closure landfill criteria, strategies, and monitoring, email Peter at This email address is being protected from spambots. You need JavaScript enabled to view it..
To discuss landfill design and construction, email Justin at This email address is being protected from spambots. You need JavaScript enabled to view it..