Landfill Gases and Leachate: Changing the Schedule
Achieving biodegradation of landfills sooner rather than later is a strategy gaining momentum today.
"Solve the problems now!" is a practical and genuinely concerned attitude of many professionals associated with landfill design and operation, according to Charles Fiedler, an engineer with HDR Inc. and an active member of the Solid Waste Association of North America (SWANA). HDR can claim good experience in solid waste management; its completed projects have involved more than 10% of this country’s solid waste management system, including more than 300 landfills, 35 recycling facilities, 80 transfer stations, and 36 waste-to-energy facilities. "Solid waste problems are finally being addressed with the same enthusiasm and intensity as we have seen for clean air and water," notes Fiedler. In the 1970s and ’80s, federal regulations primarily addressed the problems caused by leaks of gas and leachate in landfills, and stabilization was perceived as the solution. Keep everything out!
"In the last 20 years we have realized that there is a new problem with stabilized landfills," states Fiedler. "Everything stays there forever and is inert to the environment. People in engineering firms and professionals in municipalities and counties (with good leadership from SWANA) have been questioning if this is the wisest and kindest heritage for our next generations." At a site in west Texas, some solid wastes have been mummified for about 50 years. The people who planned that, believing that the landfill would never again be a problem, would be surprised that there is a new anxiety at the site: The trees won’t grow. This site and others like it are not reasons for blaming any person, company, or public authority. They are demonstrating that at any given point in time, we just don’t know everything. The natural process for the gradual disappearance of landfill contents would be biodegradation with natural materials achieving the goal, and that process is looking more attractive and environmentally wise as we try to emulate nature’s efficiency and schedule.
Most people agree that increased regulation of the solid waste industry in the last decade has brought needed changes to protect the environment and has advanced the level of engineering technologies for that industry. The individuality of each landfill is something that professionals with years of experience in the industry recognize, and it is a compelling reason why there is so much caution about regulations mandated as if all landfills were the same and each county in the United States had the same soils, vegetation, and groundwater characteristics. "A point that cannot be made too often in considering landfill design and operation is that everything done on or to a landfill or its subsystems (such as landfill gas, leachate, surface drainage, cover, and groundwater protection) should be done with consideration for the other components that make up the total landfill system," stresses Jim Wheeler of Wheeler Environmental Management, who has practical experience in landfill design, operation, and maintenance. "The interdependency of those systems is a vital consideration. A general trend to compartmentalize each activity in landfill design and operation (and view it as an independent activity) seems to be ignoring the whole picture. A precipitate plan focusing on one aspect, especially under some sort of regulatory compliance order, might place the landfill operator in a difficult situation when it is realized, too late, that something else should have been considered because one system now affects another and has produced a counterproductive result."
"We are suggesting that a landfill be treated more like a wastewater treatment plant," says Fiedler. "We would monitor temperature and gas flows for the optimum microbial atmosphere. A landfill would be considered a treatment facility, and the microorganisms are there already." There are many examples in which natural processes have resolved problems and brought the environment back to a balanced condition - examples found in all kinds of climates and terrain. Forest fires and floods are the obvious catastrophes that have been overcome naturally, over periods of time that are - in the history of our planet - amazingly short. You could probably find nature doing its remedial work around your own community - in gardens, alleys, and new housing developments - so the suggestion that more attention be paid to, and more credit be given to, Mother Nature is neither impractical nor unfounded. Today’s proposals for encouraging and accelerating of natural biodegradation will not suit every landfill or every community, because ground conditions vary from state to state (or even neighborhood to neighborhood) and the cost of this "short shutdown" could prove more expensive than some landfill owners can afford.
"What the regulators created was a dry tomb," Dick Sprague, also of HDR, points out. "It could be many years before some sites are stabilized, but with rapid settlement of the wastes, a community may be looking at five to 20 years instead of 100 - or even 300 - years." In programs that manage decomposition quickly, the liner is critical because there will be more pressure on it, and it is no consolation to anybody associated with landfills to learn that up to 25% of current landfills have been badly lined. Problems have been caused, according to reports from independent investigators, more by the inefficiency or ignorance of people handling the liners at installation than the liners themselves. (The word "ignorance" is used here not with any sense of culpability or carelessness but in its pure sense of "not knowing.")
Wherever household and commercial waste is disposed of in an engineered landfill, there will be landfill gas (LFG) because as it decomposes the buried waste creates a biogas rich in methane. The composition of the LFG is usually 45% carbon dioxide and 55% methane, with the latter being valuable as a source of power and heat. Liners surround and cap areas of solid waste to prevent any gas from escaping, and useful LFG is produced as quickly as several months after the initial disposals were made.
|Gannett Fleming's long-term plan for management of groundwater and leachate.|
To the public, most gases may just be nasty smells and a drag on residential property values, but environmentally there is more to them than that. One of the dangers of not controlling or using LFG is that it is a prolific contributor to greenhouse gases (30 times more so than carbon dioxide) if it is allowed to escape freely. How much gas does a landfill produce? Dutch engineer Dick van Faassen, who was head of the engineering department at COGAS, an energy distribution company, states that in theory, 1 kg (2.2 lb.) of biomass will generate approximately 0.74 m3 (0.97 yd.3) of gas. The production and practical use of LFG are being encouraged worldwide, usually with tax incentives and other governmental inspirations.
It is not only the fact that methane is explosive that demands its control at landfills. The regulations of the federal government and most state governments mandate the safe control and collection of LFG. The management of an acceptable gas-collection system would include adjustment and balancing of the LFG wellfield, plus the operation of an LFG flare facility or equipment for the recovery of energy. If the landfill is large enough to justify the recovery of its gas, it can be a profitable and practical operation, and there are some 500 sites of that nature in North America and Europe, with economic life expectancies of 10-30 years. These facilities benefit their communities by providing a source of energy that would otherwise be lost - energy in the form of natural gas, electricity, medium Btu gas, and compressed natural gas for powering equipment and vehicles. The relationship between the amounts of waste and gas can be seen in statistics from the Greengairs Landfill Site in Scotland. It opened in 1990 and has an expected site life of more than 30 years. The input of waste is about 840,000 tpy, and there are more than 3.9 million tons in place. The gas flow rate is approximately 4,050 yd.3/hr. (with a normal methane concentration of 55%). The current electrical generation capacity is 4.78 MW, and the contract price for the electricity produced is about 1.75 times its cost. "To make such a venture economically feasible, the landfill must have a sufficient volume of energy to market, and the owners must be prepared to look at the program as a long-term investment," agreed several professionals when presented with figures like those of Greengairs.
Jim Wheeler mentions a seldom-heard cause for concern: landfill fires. He is concerned that some New Source Performance Standards (NSPS) regulations might promote overly aggressive LFG extraction from landfills. "This could not be considered a good LFG operating practice," he says. "I think there may be more landfill subsurface fires than we have seen so far. Such fires can be most persistent and difficult to treat, but I have noticed more interest from fire fighting, fire safety, and emergency services in dealing with landfill fire events. The NSPS prohibits landfill fires but does not say how to prevent them or what to do if one is discovered at your landfill."
Liquids in Waste Stabilization
Moisture is a significant factor in most landfills with an LFG system, with high moisture generally necessary and beneficial to LFG generation and production, as long as it does not interfere with LFG extraction and transportation in collection piping. "Too much moisture can be a problem," notes Wheeler. "Positive results from moisture are the rate of waste decomposition and gas generation, but there may be a negative aspect when there is landfill saturation or liquid interference in the horizontal collection system. Excess moisture may reduce the effectiveness of the LFG extraction well and gas-collection transport piping system. Managing moisture and leachate in the landfill becomes more important for sites that have LFG-to-energy recovery systems or that are experimenting with the landfill as a bioreactor."
"The successful management of liquids plays a central role in the ability of a landfill to operate economically, especially with the trend toward bioreactor landfills and leachate recirculation," asserts Chris Campman of Gannett Fleming Inc., a leading engineering firm in this field. "In contrast to conventional landfills, bioreactor landfills seek to stabilize waste rapidly by adding leachate, water, and possibly air to create the best conditions for waste degradation. Bioreactor landfills, however, typically require more leachate than is available to effectively wet the waste. Gannett Fleming has developed a comprehensive Total Liquids Management philosophy that seeks to minimize discharge from landfill facilities and stabilize waste more rapidly."
Chester County Solid Waste Authority (CCSWA) is pursuing this approach at its Lanchester Landfill located in the counties of Lancaster and Chester, PA. CCSWA is committed to reducing or eliminating effluent discharge through an onsite leachate treatment plant. Gannett Fleming developed a long-term leachate and groundwater management plan. It included a leachate recirculation system in the active Municipal Solid Waste Overfill (MSOF) area (a lined Resource Conservation and Recovery Act–compliant piggyback portion of the landfill) and a spray irrigation system for the managed groundwater sources on capped portions of completed disposal areas.
currently leachate and LFG condensate are treated at an onsite plant that is expensive to operate and maintain. The Lanchester Landfill facility is also required to pump and sample groundwater from various sources, including groundwater seepage along an adjacent road, a secondary containment underdrain system, and a monitoring well. When combined, these sources represent a significant component of the total managed liquids. While not routinely treated, these flows combine with leachate treatment plant effluent and are then discharged to a nearby river. To meet the goal of minimizing discharge and reducing treatment costs, alternatives were sought and evaluated for the management of the leachate and the near-surface water. The Gannett Fleming long-term plan for management of groundwater and leachate includes:
- spray irrigation and dust control, using near-surface water,
- leachate recirculation within the active disposal area,
- assessment of capacity to handle seasonal variation in flows and maximum flow rates,
- evaluation of the feasibility of decommissioning the leachate treatment plant.
Lanchester Landfill will use leachate recirculation in the MSOF area as part of its long-term goal to minimize effluent discharges from the facility. Leachate, LFG condensate, and groundwater from an extraction well under the leachate treatment plant will be recirculated through a horizontal injection system, the prewetting of waste, and daily injection trenches. The MSOF liner system comprises a composite secondary liner system meeting Subtitle D design criteria and a geomembrane primary liner, and design calculations indicate that the MSOF waste contains adequate space to absorb the maximum amount of leachate generated monthly. "Anticipated additional benefits from leachate recirculation include enhanced degradation of wastes, increased landfill airspace, and gas production," states Campman. "There is also the possibility of decommissioning the treatment plant and increasing site life and revenue potential."
The managed groundwater sources will be pumped into an existing, 750,000-gal. storage pond, where they will serve as dust control or spray irrigation on the slopes of the previously capped disposal areas. The pond will be used for storage during the colder winter months and wet-weather periods when spray irrigation is not practical. "Although the goal of spray irrigation is to minimize effluent discharges," explains Campman, "the authority also expects to enhance the vitality of the vegetative cover and reduce the potential for erosion of the closed landfill slopes." Another innovative approach to the bioreactor landfill concept is to inoculate the leachate with anaerobic bacteria prior to reintroduction to the landfill. Alfred M. Yates, also of Gannett Fleming, has researched the use of an inexpensive external, high-rate anaerobic reactor within a leachate recirculation loop to seed the landfill waste with bacteria to accelerate methane generation. Such a system would jump-start the decomposition of the waste and accelerate the production of methane. It would also be possible to move this type of reactor from cell to cell of a landfill as it is sequentially filled. There seems to be little doubt that the engineering and technologies required to achieve success in new approaches will be available. "Local conditions, however, will dictate the feasibility of one method or another," caution experts, "and those will include a community’s budget."
CH2M Hill offers guidance to clients who are considering "going bioreactor."
"There’s no doubt that recirculating leachate and/or adding water to a landfill will promote much more rapid decomposition and a higher rate of gas production than otherwise," remarks Tom Kraemer, an engineer with CH2M Hill with landfill design and operation expertise. "Predicting how much of an increase will occur at any given site is guesswork. Studies show a definite increase, but the range varies considerably from site to site." Recirculation of leachate - the most effective thing a landfill operator can do to promote anaerobic bioactivity - can help in managing leachate treatment because one can control the amount of leachate bleeding off for treatment or disposal at a steady rate. After an initial surge, the recirculated leachate is treated to a certain extent in the landfill and emerges with fewer pollutants than uncirculated leachate, as demonstrated in large-scale lysimeter studies.
"The principle is great, but the mechanics of recirculating leachate are difficult," adds Kraemer. "Unless you’re very careful about how the leachate is reintroduced, it may concentrate in localized areas of the landfill. This will cause rapid decomposition in some areas (where the leachate goes) and slow decomposition in others, leading to some drastic differential settlement. This can cause havoc with the leachate piping itself." Kraemer also points out that leachate recirculation may lead to water saturation of parts of the landfill that have not been saturated previously and that water saturation could lead to leachate breakouts on sideslopes and stability problems with the refuse mass and lining systems beneath it, unless the design included the possibility of completely saturated waste. All of these considerations can be taken into account with prudent engineering, but they were often not considered when the landfill was conceived.
Again, we detect an unwillingness to blame anybody for miscalculations made through insufficient training or knowledge of local conditions, but a concern that was expressed too frequently is that some landfill owners tend to not perform maintenance once they believe they have met regulations. An example of this is a site where artificially seeded vegetation for a final landfill cover has been allowed to proliferate naturally until overgrown. Workers trying to reach monitoring equipment have frequently complained about the poor access. At one site they were attacked by "large black and yellow spiders." Such events seem petty perhaps, but they do reinforce the importance of the practical aspect of landfill closures and maintenance.
Remote monitoring of these sites, as a way to reduce human inconvenience and errors, was something we expected greater enthusiasm for. Secure remote monitoring of LFG-flaring facilities via computer through embedded control and the Internet is now possible, as is remote visual surveillance. The costs of such systems may be justified by the reduction in call-outs of personnel and in the more accurate and timely control and status information, but there does not seem to have been the anticipated surge of enthusiasm from authorities. It was suggested by some that they are waiting for the next inevitable advance in remote monitoring technologies and are wary of committing to any solution that might be out of date by the time it is requisitioned, installed, and paid for. Along with that caution goes the "sneaking suspicion" that unfunded new mandates will be given to communities and they had better conserve their funds to satisfy those before anything else. From a technical aspect, then, the best methods for solving landfill problems are available or under development at the country’s leading engineering firms and manufacturers, but there is evidence of confusion among owners as to what is best for their particular landfills (and communities) and what is ordered by regulation.
Author's Bio: Paul Hull is a frequent contributor to Forester Media publications.