Landfill Engineering: Gas and Leachate Management

That energy production is a significant byproduct of an effective landfill, says Gary Hater, senior director of Bioreactor, BioSite, and New Technologies for Waste Management Inc.

“You collect gas much earlier and are able to collect that gas and beneficially use it at a significantly different rate,” he says. “The gas normally flared on the tail end of the project is much shorter, so it makes a significant difference.”

Other benefits include a gain in landfill space and reduced post-closure care.

But the biggest roadblock to bioreactor projects getting off of the ground is getting regulators—at the state and federal levels alike—to establish permitting standards.

“The EPA is currently in the data collection phase and no decision has been made yet to develop any new standards for bioreactors,” says Tisha Petteway of the agency’s public affairs office.

In the many years bioreactor technology has evolved, it has become embraced more by the solid waste industry as an acceptable landfill practice, says Hater.

“It’s a matter now of letting it go mainstream, which should go forward based on the energy needs,” he says.

A bioreactor is defined by the Solid Waste Association of North America (SWANA) as “any permitted Subtitle D landfill or landfill cell where liquid or air is injected in a controlled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste.” That’s in contrast to the slower process in the traditional “dry tomb” municipal landfill.

The EPA’s information-gathering process encompasses studying existing landfills, pilot projects and additional data to formulate standards and recommend operating parameters.

For now, bioreactor landfill design and operation is subject to the EPA’s 2004 research, development, and demonstration (RD&D) rule. The rule permits states to allow the addition of bulk liquids into landfills constructed with an approved alternative liner design and leachate collection system but does not allow variance to post-closure care requirements, although state regulations can be more flexible.

States can issue an RD&D permit for three years with three-year renewals up to 12 years. The system must demonstrate no increased risk to human health and the environment.

Bioreactor landfill design generally fits into one of three operational processes. They are, by EPA definition:

• Aerobic—Leachate is removed from the bottom layer, piped to liquid storage tanks, and recirculated into the landfill in a controlled manner. Air injected into the waste mass through vertical or horizontal wells promotes aerobic activity and accelerates waste stabilization.

 

• Anaerobic—Moisture is added to the waste mass through recirculated leachate and other sources for optimal moisture levels. In the absence of oxygen (anaerobic), biodegradation occurs, producing landfill gas.
• Hybrid—The hybrid aerobic-anaerobic bioreactor landfill accelerates waste degradation through sequential aerobic-anaerobic treatment. Organics in the landfill’s upper sections rapidly degrade and collect gas from lower sections. This model produces an earlier onset of methanogenesis as compared with aerobic landfills.

 

Such liquids as stormwater, wastewater, and wastewater treatment plant sludges often supplement leachate to enhance the microbiological process by focused moisture content control in contrast to a landfill that recirculates leachate for liquids management.

Leachate recirculation was a precursor to contemporary bioreactor technology. Liquids are removed from landfills and sumps, collected, and put back on top of the waste mass until the moisture content is optimal for biological activity to degrade the waste.

Landfills that merely recirculate leachate or have gas generation may not necessarily operate as optimized bioreactors. The percentage of moisture content—40% by volume—is the defining factor.

A bioreactor introduces moisture or liquids from outside sources beyond mere leachate recirculation in order to achieve that goal, says Bob Gardner, senior vice president for SCS Engineers and vice director of SWANA’s landfill management division.

That takes a long time to do, leaving many sites to be interpreted as leachate recirculation, says Chris Gabel, solid waste discipline leader with Camp Dresser & McKee. One such company that does leachate recirculation but does not have bioreactors by strict definition is Republic Industries. However, Republic is noting a lot of sediment and gas generation at a high level similar to that of a true bioreactor, says Clark Lundell, the company’s director of engineering.

The EPA and other industry experts cite several advantages of bioreactor landfills. Among them:

• A potential decrease in long-term environmental risks and landfill operating and postclosure costs, with reduced postclosure care due to the inert mass left at the end

 

• Quicker decomposition and biological stabilization of waste mass—in terms of years versus the decades typical for “dry tombs”
•   Higher-quality leachate and lower waste toxicity and mobility as the result of aerobic and anaerobic conditions alike
• Reduced leachate disposal costs as the result of improved leachate quality
• A 15%–30% gain in landfill space as the result of an increase in density of waste mass

 

A bioreactor produces a much higher rate of landfill gas in an anaerobic unit at an earlier stage in the landfill’s life. Landfill gas consists mostly of methane and carbon dioxide with lesser amounts of volatile organic chemicals or hazardous air pollutants. When captured, it can be sold or used onsite for energy use. It is estimated that energy applications of landfill gas have only reached 10% of potential use. The US Department of Energy estimates that if bioreactor technology were to be applied to 50% of the waste currently being landfilled, it would produce more than 270 billion cubic feet of methane annually, enough to provide 1% of US electrical needs.

However, there are drawbacks to bioreactor technology. The EPA identifies them as higher initial capital costs, additional control and monitoring during operational life, increased gas emissions and odors, physical instability of waste mass because of increased moisture and density, instability of liner systems, surface seeps, and the potential for landfill fires.

Tony Maxson is a client manager with the Cornerstone Environmental Group, an engineering consulting, and field service company serving the solid waste industry. He also chairs a bioreactor committee for SWANA.

Maxson believes the perceived drawbacks to bioreactors are generally minor concerns but can become major when such potential issues as an out-of-control chemical reaction are not given forethought.

“There are instances where surface seeps and leachate management become a very large issue, because you’re basically getting a very moist waste mass and you’ve got to deal with water that runs out everywhere—even the side slopes of the landfill,” he says. “Dealing with additional issues caused by the bioreactor, such as gas production and moisture management, is absolutely critical. It’s not something you can rush into. There are plenty of ways to design around it. I see people getting into this and ending up reacting instead of being proactive. That’s not a good situation to be in when you have landfill gas odor problems.”

Engineering precautions, such as doing a stability analysis on the conceptual design, should be undertaken before engaging in bioreactor technology, says Hater.

Maxson, who does such analyses, says that because moisture is heavier when placed into waste, “there’s more of a tendency for gravity to cause slope instability.”

Getting moisture into the waste mass in a way that doesn’t create operational challenges, especially in an established landfill that’s been compacted, is an issue under research at universities throughout the United States, says Gardner.

“The general sense is if you can get moisture in while you’re putting in waste, that’s the most effective way of doing it rather than just trying to inject it,” says Gardner. “Both techniques will continue to be used, but it’s very difficult.”

In designing for an injection system, “You try to stay away from the slopes to minimize what type of outcrops you get on the slide slopes,” explains Gardner. “When you have an intermediate cover, liquids tend to try to find a place to weep out. That’s an operational challenge.”

A larger concern is landfill gas, says Maxson. Increased gas production without an appropriate control system will result in increased odors and unhappy area residents.

Stormwater runoff, leakage issues, and fire potential are other challenges, Lundell says. “In the past, there was a tendency to get overly aggressive with trying to extract gas out of collection systems that are fairly shallow in depth,” he says. “If it is not done right or there’s not much cover, you end up with more potential for oxygen to come into the system quicker and a potential fire problem.”

Fulfilling both water demands and gas collection can be another challenge. “The two are largely incompatible with the nature of what you’re doing,” Gabel says. “On one side, you’re adding water. On the other side, you’re trying to collect gas. How do you prevent your gas system from being compromised? Often the water from the wetting operations is wetting out the wells and dramatically decreasing your ability to collect gas.”

Applying water without compromising the gas collection can be effectively accomplished through sequencing or separating the processes within the landfill. “At some point you’re wetting in one area and collecting gas in the other and progressively shifting your operations so you’re not trying always to wet and collect gas in the same location,” says Gabel.

There are now design advancements in the infrastructure of the conduit used to introduce water into the landfill or collect gas. “You really see two different approaches: one being vertical systems more or less designed after the vertical collection well that gas collections come from. Horizontal systems are more easily installed when the landfill is active,” Gabel says. “It allows the process to get up and running when the landfill is still being built, as opposed to waiting for it to close out.

“If you wait until the landfill is closed, you’ve missed a lot of the gas generation life of that landfill. We’re trying to capture as much of the gas as possible as we’re doing the bioreactor process.”

The water demand is an issue in itself, Gabel says. “It depends on the amount of rainfall in your area and your ability to capture some of rainfall. Finding water sources can be challenging, especially for sites that are in more arid regions in the West. In the Northwest, nature does a lot of the work for you. You get a long, low-intensity rainfall that keeps adding water to the landfill. Many landfills in Oregon and Washington would almost meet the water demands without having to do anything.”

Bioreactors can be more complex from an operational perspective. “You have additional costs and additional regulatory oversight,” Gardner says. “It adds an operational complexity, so a lot of people just don’t want to deal with it.”

Gardner says he’s seen a “marked decrease” in the number of papers presented at conferences with respect to bioreactor technology, especially since the introduction of the RD&D rule. “Some people are shying away from it because of some of the difficulties associated with it,” he says. “There are enough people who are interested in the advantages that they’ll do so. Some of the bigger waste companies are proceeding because they are looking at it from a revenue perspective to take in things like off-spec beer and off-spec Pepsi for the bioreactor. They see it as a wise use of their space and from an energy perspective.”

Combining the efforts of the agency’s Offices of Solid Waste; Air and Radiation; Policy, Economics, and Innovation; and Research and Development, current EPA research is focused on the following:

• Assessing state-of-the-practice of bioreactor landfill design, operation, and maintenance

 

• Identifying case study data comparing traditional and bioreactor landfill approaches
• Determining long-term monitoring needs for environmental compliance for groundwater, gas emissions, leachate quality, liner stability, physical stability, and other factors in addressing life cycle integrity and economic viability concerns
• Exchanging technical and implementation concerns regarding pending and planned regulations affecting landfills in general and the development of regulatory framework for bioreactor landfills
• Examining the full-scale economic viability, impacts, and benefits of bioreactor landfill implementation
• Identifying and prioritizing research and regulatory needs

 

To determine the overall benefits of and concerns about bioreactors, the EPA is gathering data regarding these:

• Alternative liner design/materials for leachate recirculation and bioreactor landfills
• Impacts of leachate quality, quantity, and loading on the liner system
• The times and amounts of liquids it takes to reach field capacity and appropriate means for measuring field capacity
• How leachate recirculation affects the rate and extent of landfill stabilization
• Design, operation, and performance specifications for bioreactors
• Rate, quantity, and quality of gas generation
• Interim covers used after placement to accommodate anticipated settlement and daily and final cover performance
• Optimum moisture content and distribution methods
• Monitoring requirements
• Bioreactor technology impacts on capping and current closure and postclosure requirements

Another angle through which the EPA is studying bioreactors is greenhouse gas associated with air regulations, says Maxson.

Bioreactor technology has evolved since its introduction in the early 1970s, Maxson says. Getting control of barrier technology with liners had to be addressed before it could be adopted on a full-scale basis. Now, Subtitle D composite geosynthethic liner systems equip the solid waste industry to put liquids safely into landfills, he says.

Current landfills that have a quality liner system can be retrofitted to bioreactor systems, as long as they have the natural-gas extraction technology to capture the gas generation that occurs on an accelerated basis, says Maxson.

Gabel says many bioreactor system designs are gravity systems “where you are literally pouring water down a hole and allowing it to percolate out into the wastestream. All modern systems are generally pressurized to some extent or using movable drainfields where you establish a short-term area similar to a septic drainfield in which you would set up pipes and allow the leachate to seep down through the waste in that system. Then you’d periodically dismantle it and move it to a new location.

“More sites now are open to using multiple systems and approaches to the wetting—not just pressurized injection,” says Gabel. “They may look at adding water directly as the waste is being placed. A lot of sites are doing that because it’s an easy thing to do and not very expensive.”

Daily cover is one of the barriers to the advancement of bioreactor technology, says Maxson. Typically, clay covers a landfill’s bagged waste, which is not broken down, but compacted to a dense state to maximize airspace capacity. That works against bioreactor technology, because it lessens the ability of water to travel in the waste, Maxson points out.

This challenge is being addressed by taking the daily cover off at the start of each day and grinding the waste to small particles that are better able to accept moisture, he adds.

Economics can be another challenge. Operational costs are enhanced, and there are significant maintenance issues associated with the acceleration of waste decomposition by the addition of liquids.

“Landfill operators have to invest more money and effort into managing the landfill,” Maxson points out. “There is a more controlled process to manage the chemistry experiment, if you will, as opposed to a pile of dry waste where you put a cover on it.”

However, there’s an offset to those costs in tax credits granted for the accelerated methane production that leads to accessible alternative energy sources. The return on the investment is site-specific but can occur in as little as two to four years, says Maxson. “There are places where it’s very good economics and places where it’s horrible, because there is nothing that’s driving it, because the utilities don’t want to pay for the energy, or [because] there’s not an incentive from the operator’s side to do the bioreactor technology because there’s not enough economic gain for it,” he says. But more sites are finding it economically viable, he adds.

Yet while landfill operators are more willing to embrace the technology, regulators are not so inclined. Permitting can be a challenge.

“At first, regulators were very accepting of it,” says Maxson. “They thought it couldn’t hurt anything to accelerate waste decomposition. Now they’re taking a step back because they realize they have a new process they need to control.”

Case in point is the Ohio EPA’s declaration in 2006 of Countywide Landfill as a “public nuisance” for its foul odors and underground fires.

The state EPA determined the source to be aluminum production wastes reacting with water and producing rapid settlement, heat, and various gases, including hydrogen. The site was issued more than $100,000 in fines and is being monitored by a team of experts for compliance.

Although the case was not related to bioreactors, it stands as an example of taking a different kind of waste into a landfill that accelerated, added heat and created a runaway reaction, says Maxson.

As such, while regulators are not opposed to bioreactors, “they want to make sure they have the right technology and a good review on it,” Maxson says. “They tend to delay the permitting if you want to do a bioreactor. It doesn’t prevent you from getting a permit. You’re just going to have to invest more money in environmental controls to make sure it’s safe.”

Lundell agrees permit challenges are not a bad thing. “One of my fears is that people who aren’t equipped to operate it correctly dive into it and end up with problems that set the industry back,” he says.

Since it’s up to states now as to whether an entity can operate a bioreactor, there are inconsistencies, Hater points out.

“It takes five years to permit a bioreactor and six months to permit a composting facility, and they are largely doing similar things,” Hater says. “There are some states where you still can’t do bioreactor work, and that’s a problem. The EPA has been slow at best at adopting the RD&D rule as a final. They can find funds to modify AP42 [emissions factors], but they regulate us more stringently and can’t find funds to do something beneficial to the energy balance of the country.”

On the other hand, Hater says, out there are some areas of the country, such as Wisconsin, that have enthusiastically embraced bioreactor landfill technology as a contribution to the energy solution—there are seven active permits in the state.

One of those frustrated with the permitting system is Stephanie Hinson, director of the Salt River Landfill in Scottsville, AZ, affiliated with the Salt River Pima-Maricopa Indian Community.

The landlocked community does not allow for landfill expansion; thus, a bioreactor would maximize the landfill space. In early 2008, a gas pipeline was constructed, connecting the landfill gas collection system to the Tri-Cities Generating plant to enable the landfill gas to be converted into power.

Meanwhile, landfill officials await approval from the EPA to operate the landfill’s Phase VI as an anaerobic bioreactor, which would use groundwater for its operation.

“We have our tentative determination, but we are still waiting on them,” Hinson says. The local EPA office has told her it’s being “worked on,” she adds, but it hasn’t given her a firm timeline.

The Pima-Maricopa community has water rights, Hinson explains, and the local utility has an underground storage project where it is banking water. In the six years since the project began, the groundwater elevation under the landfill has risen 75 feet, she says, so by using the groundwater for the bioreactor, it would have the added benefit of stabilizing the groundwater elevation.

The bioreactor—approved by the