It’s not cheap to run a landfill. The costs vary by location, but estimates by the Environmental Protection Agency and various state agencies range from $300,000 to $800,000 per acre for the application, construction, and design and engineering, with as much as $100,000 per acre for overhead and quality control—costs that extend indefinitely in the post-closure care period.
When landfills are the responsibility of the local government, fees, taxes, or municipal bonds typically cover the expenses that are incurred. The ability to reduce construction and operations costs is the primary advantage provided by aerobic digestion.
Other benefits of the aerobic digestion system offered by Engineered Waste Systems include:
- Doubling the life of a landfill because digested materials take up less volume
- Reducing the post-closure care period
- Eliminating long-term liability by processing materials
- Increasing stability of the processed waste mass
- Eliminating the need for compaction because the internal bridging and organics are digested
- Providing potential for perpetual revenue through mining of digested MSW
Daily cover soils can take up as much as 10% of the landfill’s entire volume. Digested MSW has been approved to be used as daily cover, which constitutes a significant cost-saving, says Rick Aho, principal at EWS, the Michigan-based company providing advanced environmental technology applications such as aerobic digestion and biological processing for the waste industry. He claims that their system improves how landfills operate, reduces operating costs, saves landfill space, and protects the environment.
Environmental Impact: Air Quality
Traditional landfills pose a potential threat to the environment. According to the US Geological Survey, chemicals and gases permeate the liner and plastic piping, which become brittle, swell, and break down, leading to leakage. That threat extends to human health because leakages have been tied to an increased risk of cancer and birth defects, due to the airborne release of chemicals in active and inactive landfills, according to a report by G. Fred Lee & Associates, an environmental consulting firm.
To combat airborne hazards, the Clean Air Act requires a gas collection system if capacity meets a specified threshold, says Phil Roycraft, district supervisor for the northern region of Michigan, Department of Environmental Quality, Waste Management Division. “Typically, it affects large sites, but even below the threshold, a gas collection system is installed to control odors if there are issues.” Those systems are generally less complex—and may be flares—but big sites have to invest in piping and a gas engine to convert the gas to electricity. “The costs can be overwhelming.”
In 2016, the Environmental Protection Agency updated its New Source Performance Standards to reduce emissions of methane gas from new, modified, and reconstructed MSW landfills. It also issued guidelines for reducing emissions from existing landfills. Nevertheless, Roycraft notes, large sites can have nuisance odors even when they’re collecting 75% of emissions.
The cost of building and operating gas collection systems is measured in the millions, Aho states. In addition, he says it is very difficult to operate a gas collection system in the northern climate, where freezing temperatures are normal. In contrast, hydrodigestion doesn’t require a gas collection system because it inhibits the ability to produce methane, thus leading to millions of dollars in savings. “Hydrodigestion’s air quality efficiency is documented by the federal eGGRT system.”
Aho explains that the use of digested MSW material as daily cover can save 10% of the volume of the landfill and reduce its ability to create emissions (both methane and non-methane). “The savings involved [includes] the cost of cover and the cost of the gas collection system. The soft benefits include being able to tell your customers that you’re not producing methane, a very strong greenhouse gas, while the landfill is in operation.” He goes on to add that landfills that do not utilize hydrodigestion continuously emit large tonnages of methane and other gases while the landfill is not capped.
The digested material provides a landfill with a very large increase in capacity, which is important because air quality requirements are based on landfill capacity. “The systematic documentation of the characteristics of the digested materials provides proof that the landfill cannot produce emissions,” states Aho, explaining that it is similar to landfills that contain materials such as power plant ash. “They cannot produce emissions also, and do not require gas collection systems.”
Landfills that utilize hydrodigestion may be more beneficial to the environment in comparison with today’s traditional landfills because they provide a decrease in long-term environmental risks. They also alleviate landfill operating and post-closure costs by reducing the post-closure care period and eliminating long-term care.
Hydrodigestion eliminates gas and odor production without the need for a gas collection system or flare. Whether aerobic processing is considered a gas collection system or it eliminates the need for one, the inevitable result is that processed organics cannot produce gas. Financial assurance will be reduced because of the inability to produce gas emissions from the landfill when it’s closed, Aho says.
Leachate
Like the air, water is also susceptible to contamination from traditional landfills. The generation of leachate—a liquid that passes through a landfill carrying dissolved and suspended matter—can be problematic for landfills and poses a serious threat to surface water and groundwater. “The contaminant levels in leachate are low. It’s roughly 99.5% water with only trace contaminants,” states Roycraft, “but the threat to groundwater is too high for discharge.”
Most sites pump and haul rather than treat onsite, Roycraft observes. “Some have a sewer connection to the wastewater treatment plant. That’s ideal, but they pay per gallon. Some pump and haul to a treatment plant—that’s the most common practice, but it requires dedicated trucks, a dedicated driver, transportation costs, and paying the treatment plant. If you have too much of some constituent, such as ammonia, you have to pay extra or pre-treat it before hauling.”
Some sites use leachate evaporation. “They use landfill gas to burn off the leachate,” explains Roycraft. “That’s a couple million dollars in capital costs, though.” He mentions a “low-cost version” in the Neptune aerator (which he compares with a snow machine) to vaporize the leachate by using high-pressure nozzles to evaporate it. “You must spray over an active area, which can be tough to do with trucks coming in, and you need low humidity for it to work properly.”
Evaporation is costly, and there’s some maintenance required, Roycraft adds. “Aerators are less costly but are more difficult for operators to use. Every option has its ups and downs.” Another tactic is “deep well” injection. One particular site he’s familiar with has applied for a permit to dig down 5,000 feet. “It will be very costly.”
For landfills that want to treat leachate onsite, the Department of Environmental Quality requires a permit, which must be applied for 180 days prior to use in order to allow for time to process it. A National Pollutant Discharge Elimination System permit allows the landfill to discharge leachate into a river, stream, or lake. The NPDES Industrial annual permit fee depends on the amount being discharged, according to Jay Parent, P.E., environmental engineer, MDEQ—WRD—UP District Office, with years of regulatory involvement with the discharge of hydrodigestion leachate.
EWS will work with landfills that opt for the hydrodigestion process to apply for and obtain permits from the applicable regulatory agency that meets the needs of the specific site, Aho points out. “EWS will write your general permit template at no cost. It is part of the program.”
The costs of processing leachate onsite can be minimized while still achieving standards with the landfill collection system that meet stringent surface water discharge requirements. The cost of testing and processing is a fraction of the cost of trucking, in most cases. As Parent notes, the costs for processing leachate at municipal wastewater treatment plants are determined by the individual facilities. “The DEQ does not get involved with pricing,” he states. However, he says that a quick survey of local WWTPs reveals that they charge 2–5 cents per gallon.
Landfills that practice surface water discharge have to go through additional required testing, as specified in their NPDES permits. Parent says he does not know how much the labs charge to run the tests required for treated leachate surface water discharges in his region.
Treating leachate onsite can be an even better option than hauling it. According to Aho, wastewater treatment plants are designed to a BOD of 200, but landfills can easily average a BOD of 5,000, with peaks over 10,000 ppm. “The plant can’t handle that much without precautions and procedures or untreated waste ends up in the effluent.”
In effect, it “shocks” the plant. “There is no manual on handling landfill leachate, but we rely on biology to digest landfills.” It also processes contaminants. Aho explains that the standards for discharge are stringent and that the leachate from an active landfill must be isolated in order to clean it to meet those standards. Digesting landfills reduces the strength of leachate in the landfill by 90% or more. Once leachate is removed from the landfill and polished to surface water standards, it can be discharged.
Aho says that hydrodigestion provides superior processing when compared with a wastewater treatment plant—and allows the landfill to avoid the associated costs. He calculates the costs of hydrodigestion treatment that have been achieved at $0.005 per gallon and says financial assurance reductions are possible with onsite treatment. “Testing for landfill leachate will always be extensive because of the nature of the material and the possibilities of the source.”
Post-Closure Financial Assurance
When leachate is treated onsite, the costs associated with financial assurance and the actual cost of leachate treatment are significantly lower. “Treated leachate in the closed landfill should be able to be released into the environment, so monitoring costs are minimized,” elaborates Aho.
The same monitoring practices required while a site is active must be continued post-closure. “You have to continue the same process post-closure,” confirms Roycraft. “You have to maintain [the site] for 30 years after closure.” Financial assurance and federal regulations last 30 years in order to achieve “stabilization and functionality.”
Roycraft says that Michigan is considering assessing the functional stability period. “It may not necessarily be 30 years.” He says it could be changed in order to encourage innovation and promote compaction. “The volume should decrease over time. Maximum gas production declines as the landfill dries out and material decomposes.”
Reducing the volume of the materials within the landfill more quickly through hydrodigestion changes the economics of a landfill. “Twice the capacity means twice the return on the investment,” states Aho. This also results in the reduction in financial assurance because:
- A reduction in post-closure care costs and the term (the number of years of post-closure care) can be reduced by a petition of the state.
- There’s no subsiding of the digested material, so the integrity of the cap is significantly improved.
- The liability of the digested material of the landfill may be sufficiently documented so that the landfill may not require a cap.
- A digested landfill can be easily processed for the value of the materials inside. This would allow the land to be redeveloped or facilitate a perpetual hydrodigestion landfill option.
According to Roycraft, the property can be converted to beneficial use, such as a site for solar or wind power, a park, or just an open space. However, due to the settlement potential, he says, “you can’t put a building on it without structural reinforcement.”
Liability
Liability—both during active life and post-closure—is a difficult question, Roycraft believes. “It depends on state law. Remember: modern landfill design has only been around since 1990; there are no numbers on the costs post-closure.”
If there are no official numbers, there are at minimum several “soft” benefits of reducing landfill volume through hydrodigestion, such as being able to tell customers, corporations, and municipalities that you are eliminating the liability of the waste, both now (in the case of GHGs and water) and in the future (the inevitable failure of a perpetual storage device).
Short-term and long-term liability center on the instability of a landfill. “Anaerobic organisms control drainage,” says Aho. When drainage is blocked, potential hazards include avalanches, collapse, loss of containment, poison gases, and toxic odors. “It defeats the purpose of a containment system if you have failures.”
If containment fails, it poses a risk to the public. However, if the landfill can’t produce methane because the material has been aerobically digested, there’s no detrimental effect if a liner fails. “Digested material is inert,” explains Aho.
Liability for a bankrupt landfill ultimately fails whatever entity takes it over for taxes. “If there’s a problem facility,” points out Aho, “the host community could be on the hook, especially if they’re taking host fees.” However, he suggests proving to the regulating body that you’re processing the landfill materials with hydrodigestion.
When a landfill uses hydrodigestion, Aho says they can provide “a compelling petition for the elimination of post-closure regulation” because the digested material no longer possesses the characteristics of solid waste. The default post-closure monitoring period is 30 years, but he suggests petitioning for a shorter period. “If you can prove there’s no hazard, there’s no liability.” EWS can assist with documentation about the volume, the reduction of material, and the characteristics of the site.
In a conventional landfill, the material continues to break down and settle years or even decades after closure. That can lead to cap failure resulting from material that is not structurally competent collapsing under the cap. Although the cap is designed to keep water out, it is the most important, most susceptible component. “Fixing something is costly,” says Aho. He considers hydrodigestion to be a safer, cheaper option that offers improved ecological protection for landfills while minimizing liability.
Another aspect hydrodigestion takes care of is internal bridging. Construction debris in particular leaves voids in the pile because much of the internal structure is wood. A compactor breaks up some of the materials, but they spring back up after compaction. “Voids always exist. Compaction isn’t what you want,” explains Aho. “You want to put the material where you want to process it and open the bags.”
Digestion leaves minimal voids and creates a landfill that has less volume. Once settled, the material becomes “reinforced dirt,” says Aho, contributing to the stability of the landfill. That reinforced dirt (humus and plastic) is valuable and can provide a source of income for landfills if they choose to process and sell it. The metals found in a landfill are also lucrative, he adds.
Operations
According to the Environmental Protection Agency, about 32.5% of the 251 million tons of consumer solid waste generated in the US each year is recycled or composted. Approximately 12.5% is burned. The remaining 55% is buried in landfills. The purpose of a landfill is to isolate waste from the surrounding environment to prevent water contamination and contact with the air—but landfills are not originally built to decompose trash efficiently.
The massive biological loading and isolation by high-density polyethylene plastic lining in landfills mean waste decomposes at a very slow rate. The decomposition process extends for decades.
Digesting landfill material is safer for people, but it’s also easier on equipment and mechanized systems. “Pumps, controls, and metal components quickly become corroded in a typical landfill,” explains Aho. However, a digesting system is not acidic and therefore doesn’t cause corrosion. An aerobic environment is a safe environment, he says. “We are aerobic creatures.”
Cost Savings
Hydrodigestion is safe and cost-effective, Aho says. He lists benefits such as the fact that there are no fires in processed materials; minimized roads, better litter control, and other necessary support; reduced system maintenance due to a neutral pH; minimized operational disruption from moving and construction; and efficient treatment of sludges and unstable wet materials.
The cost of a current project to retrofit an existing facility includes the capacity to process the existing cell (nearing capacity) and all future materials that will be landfilled on the site—a total of at least six cells. The cost of new components and installations necessary for processing is just under $72,000. The benefits include another 20 years of operating capacity in the existing cell and new cell at this small landfill. “The new cell would be processed from its beginning. In addition to volume, which in this case would be an approximately 100% increase in density, the minimization of leachate that will be trucked offsite is also a minor focus. It is realistic to expect a 40% reduction in leachate volume trucked.”
The budget cost for the processing system of the new hydrodigestion landfill that is being sited is about the same as the retrofit: approximately $72,000, Aho says. This is the cost, associated with digestion, above the cost of conventional landfill construction. “The actual extension of the landfill life will be slightly less than the retrofit because of the characteristics of the incoming waste at that facility.” Leachate is expected to be treated onsite and discharged to surface waters. “The savings from leachate treatment versus hauling will be very significant—and more important than volume reduction windfall.”
The EWS digestion system meets regulatory compliance, Aho confirms. “Show your regulator you have the best environmental protection on the planet.”