The Process of Aerobic Digestion in Landfills
The US generates 251 million tons of solid waste annually, ranking it fifth in the developed world for waste production (behind New Zealand, Ireland, Norway, and Switzerland). Of the solid waste produced by the US, 32.5% is recycled or composted, 12.5% is burned, and the rest—roughly 55%—is buried in landfills. According to the Environmental Protection Agency, the US has 3,091 active landfills and more than 10,000 closed municipal landfills.
In addition to the space consumed by landfills, the environmental perils that result are well-documented. Three of the most significant issues are pollution and contamination of the surrounding soil by toxins, groundwater by leachate, and air by greenhouse or poisonous gases.
With a population continuing to rise, the US can take strides towards sustainability by changing the way landfills are managed and reducing the number of them or extending the life of them.
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Aerobic Digestion: Patented Solution
Engineered Waste Systems has introduced a patented and innovative biological process called hydrodigestion that provides a cost-effective and environmentally safe solution. EWS, based in Michigan, was formed in 2005 with the intention to discover applications for and to distribute advanced environmental technology, focusing specifically on aerobic digestion and biological processing for the waste industry.
Hydrodigestion accelerates aerobic digestion to quickly reduce the volume of waste. In landfills, this is done by the creation of a supercharged biofilm in selected areas of the waste mass that processes organic material with a minimum of fossil fuels and equipment—and without turning of the material. The end product is a mixture of rich humus, biologically inert plastics, glass, and metals that are biologically stable and incapable of producing methane. Contamination, toxins, and compounds are processed to their elemental components.
Benefits of aerobic digestion include doubling the life of a landfill (depending on its organic content), minimizing the cost and related liability of leachate, eliminating methane gas, and improved safety—all at a lower cost than the typical landfill. Another advantage of using the biological process is that it is flexible enough to adapt to virtually any size application.
Typical landfills contain 60% organics and 30% recyclables. Optimized recycling can still take place. “This is not an enemy of recycling or manufacturing,” insists Rick Aho, principal, EWS. It’s possible to increase recovery of metals and plastics or optimize the efficiency of waste-to-energy systems.
The system doesn’t require enzymes or bacteria that must be continually fed into the system. Hydrodigestion is not a forced-air bioreactor or an anaerobic bioreactor.
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New aerobic digestion cell
In fact, unlike an anaerobic bioreactor, this process produces none of the high-strength greenhouse gases normally associated with landfills. As MSW decomposes in an anaerobic landfill, it releases methane, a greenhouse gas 25 times more potent than carbon dioxide, and other greenhouse gases, which entails the majority of the solid waste industry’s contribution to climate change. By adding aerobic organisms and giving them an opportunity to thrive, says Aho, no high-strength gases are produced. “It’s nature optimized through control of conditions,” he says. “Hydrodigestion gives aerobic microorganisms the components necessary to optimize the microbiological population in the controlled environment of the modern Subtitle D landfill.”
But it begins by modifying operating procedures to take biological control of the system, exposing the waste to enable the microorganisms to do their job. When organic material such as food scraps and green waste is put into a landfill, it is usually bagged and compacted. This causes the waste to slowly to break down over decades. The anaerobic process produces odors and releases methane.
Compaction is not necessary when using hydrodigestion; in fact, it isn’t even advantageous. “Instead of compacting the material, your goal is to break open the bags to expose it,” indicates Aho. The compactor’s job is placement and exposure of the material to digestion.
There’s no need to purchase new equipment to do so. A regular compactor can be fitted with a modified cleat to help open the bags instead of compacting them. Aho explains that a chopper cleat does a great job of breaking up items like wood and shredding bags. It is inexpensive and durable. The compactor can be operated to process waste to aid the biological organisms in beginning digestion and eliminating odors.
The digested material provides all the requirements for daily cover. Using digested MSW as daily cover speeds the processing of the landfill and further reduces costs because the increase in density reduces per-unit capital expenditures associated with the landfill. In addition to reducing costs and eliminating hauling soil, the digested material kills odors and speeds the digestion of incoming material. A bonus of the digestion process is that it eliminates the possibilities of fire in the processed material and minimizes the severity and likelihood of fire in newly delivered material—a beneficial safety aspect for landfill personnel.
Landfills can pollute the environment: air, water, and soil. In addition to GHG, landfills produce leachate, the liquid that passes through the landfill and contains organic and inorganic materials, heavy metals, and pathogens. Because it can pollute ground and surface water, it poses a health risk. Normally, landfill leachate must be processed in treatment facilities to meet minimum discharge requirements. This represents a large portion of the operating budget at many landfills.
However, hydrodigestion can reduce that expense by eliminating or minimizing leachate liability and costs through onsite leachate treatment with aggressive contamination processing. Onsite leachate treatment requires an understanding of the discharge regulations, in addition to the biological functions of the treatment system. Treatment is not dilution; it evolves, based on the feedstock and contamination. “Landfill owners have to modify their leachate collection system and develop or modify their recirculation system,” states Aho. Leachate is utilized to process the MSW; it is distributed back into the pile and manipulated to optimize and change the biological population in the waste mass.
After closing a landfill, Aho says the goal is to produce leachate suitable for discharge into the groundwater because the contaminants have been digested or rendered inert.
The difference between an anaerobic perpetual storage facility and an aerobic liability minimization system is biological control. Using the latter, EWS helps landfill owners reduce the volume of MSW and conserve landfill space while recovering previously used landfill space.
Every landfill has an internal structure. As organic material is processed biologically into humus, the internal structure of the landfill collapses as it consolidates due to reductions in particle size. New material is digested as it arrives. In addition, the landfill digests existing material, freeing up space for the reduced volume of the incoming materials. An efficient operation would digest enough landfilled material to result in a net of zero airspace used for new material.
An increase in material density subsequently increases compaction of the total waste mass. A short digestion cycle results in real-time availability of recovered landfill space and operational efficiency.
The first step is a preliminary evaluation of the landfill, checking the characteristics of the liner, waste material, leachate collection components, leachate storage, and how it’s operated. “We look at the facility’s assets and build a transition plan based on existing resources,” explains Aho.
Next, EWS provides a template of each state’s permits. The company provides the training required to change the function and value of the landfill, including training operators to monitor and operate the digestion system. “There will be some operational changes,” says Aho. However, the current collection system doesn’t change. Nor does pre-landfill handling. But once material arrives at the landfill, things proceed differently. “Smart operators who understand the system [and realize that they] are operating a digester are very valuable. You just don’t pound on the garbage with your expensive equipment anymore; compaction isn’t necessary.”
EWS supplies the procedures the operators will follow to discharge leachate, use digested MSW as daily cover, and document the elimination of methane at the facility. The standard operating procedures that have been reviewed and evaluated by state and federal regulators will be furnished to landfill management. These new operating requirements produce dramatic ecological and financial returns.
In the typical composting operation, oxygen is provided through the mechanical turning of the materials. Because hydrodigestion requires no turning or grinding to digest the landfill, operators learn a different use of the same resources already onsite. Optimizing operations minimizes costs, protects employees and the public, and gives operators better control of the facility.
Return on investment depends on the individual application. Savings are based on equipment efficiency, reduced equipment cost, and reduced equipment maintenance, but in general, operating costs decrease due to superior utilization of existing resources. EWS is paid based on the net savings. “We keep the customer informed of optimization and developments and provide general regulatory assistance associated with hydrodigestion,” adds Aho.
One of the most significant results of using hydrodigestion is environmental enhancement through both the elimination of GHG and the reduction of leachate, which can be treated onsite to surface water discharge standard and beyond. Both methane and odor are eliminated by aerobic processing, removing the threat of poison gas and deadly bacteria.
Safety is also increased, partly because of the environmental improvements that reduce the danger from acidic/anaerobic conditions, but also because the possibility of landfill fires within digested material is eliminated. Due to increased safety and reduced emissions, long-term liability is minimized.
No matter how impressive the benefits of any new system, the deciding factor in adoption is often cost. EWS’ hydrodigestion requires a low capital investment while simultaneously eliminating capital, operating, and maintenance expenditures related to cell construction and gas collection systems. “Digestion equipment and components are durable, cheap, and available,” points out Aho.
Simple costs, such as landfill composite liner at $4 per square foot and 6-inch, high-density polyethylene pipe at $7.80 per foot, are reduced because the landfill lasts longer and takes up less space. The cost of land for new or expanding landfills is similarly lessened. Achieving savings while reducing long-term liability and eliminating the majority of the carbon footprint of the landfill opens up a wide array of sustainability improvements and recycling options.
The material is processed to densities that are otherwise not achievable. Because of the efficiencies of the system, the life of a landfill can be doubled depending on the organic content of the MSW.
Extending the life of the landfill goes hand-in-hand with additional capacity, which has become an issue as the population grows and landfills fill up. Aho mentions the Keystone, PA, landfill, an older facility that “wants to expand because it was too hard to do a new one,” and another landfill in Gregory Canyon, CA, that spent millions preparing for a new facility but never built because it wasn’t approved due to siting issues.
The benefits of digestion of landfilled materials are numerous, Aho points out:
- minimization of air quality issues
- minimization of cost and hazard due to leachate
- roughly doubling the life of the landfill
- reduction of liability
- increased control and operational flexibility of the facility
- reduced costs
- a partial solution for the China National Sword restrictions
- new opportunities for recycling
Sustainability through the elimination of both waste and the associated high-strength greenhouse gases is the revolution, Aho insists. “All you have to provide is a capacity (volume) that you wish to process to define the benefits.”
There are several assumptions and misconceptions about the process that have resulted in resistance to acceptance. The first involves the functional characteristics of the drainage material in the leachate collection system.
The validity of the specifications of the components of the landfill leachate collection systems is eliminated by anaerobic biological activity in the landfill. Based on the design specifications of the leachate collection system, leachate should come out of the discharge pipe at the bottom of the cell. In an anaerobic landfill, the excess leachate is either stored in the pile or it escapes the landfill containment.
Aggregate or any other drainage material with a defined permeability prior to placement in a landfill does not exhibit that permeability when used in a landfill. Microorganisms quickly cover the rocks, sand, and geotextile with a biofilm—usually a black slime. This biofilm is an effective filtration system that feeds and protects anaerobic organisms. In order to survive, anaerobic organisms restrict the flow of liquids through the landfill by placing a leaky seal on the bottom and on the outside. The landfill in effect becomes a pile of water.
This is why landfill slope failures occur, gas collection wells fill with water, and leachate spills or seeps happen. If too much water is added, leachate flows out the side of the pile wherever the anaerobic seal is weakest.
“What goes through the collection system is determined by the anaerobic organisms. In an anaerobic landfill, the microorganisms are in charge,” emphasizes Aho. “Aerobic hydrodigestion takes control of the landfill by manipulating the microorganisms inside.”
Another common myth holds that the addition of oxygen to a landfill will set it on fire. However, Aho explains that it’s the uncontrolled addition of oxygen that starts landfill fires. Hydrodigestion is a controlled aerobic process, thus eliminating the possibility of fire. Materials such as digested organics, paper, wood, food, and leather don’t burn. Temperatures are safe for all standard liners. The hydrodigestion process uses liquids to cool the waste mass and distribute nutrients within the system.
Control of the biology in the landfill provides control over the operating characteristics of the leachate collection system. This control provides safe and profitable operation of the landfill and digestion of the organic compounds in the landfill.