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The task is complex and perhaps difficult—but certainly not impossible. By Neal Bolton “Protect human health and the environment.” These six words form the underlying principle for virtually all landfill regulations, so simple—but, “oh, so complex.” How simple is it? Just ask those who haven’t had the responsibility to design, regulate, construct, operate, support, close, or monitor a landfill ... and they’ll tell you. But ask any of those other folks—the ones with the dirty boots and calloused hands, the ones who are out there making it happen through the practical application of that phrase—and they might tell you something else. Complex is the word. Complex and perhaps difficult—but certainly not impossible. At all landfills, there are many opportunities to protect human health and the environment, but in most cases the most obvious and the most serious are in relation to prevention and control of leachate, gas, and runoff. That’s because while they may not always be the most visible (remember we didn’t mention birds or litter), those issues pose the greatest potential for harm. We might be able to survive a fence plastered with litter, but pollute our air or our water, and we’re all in big trouble. A landfill operator’s ability to prevent and control those three things will often determine his or her ability to succeed, perhaps even to survive. So let’s take a look at how some of the more successful landfill operators are handling them. The Wide-Angle View In basic terms, landfills are seen by our culture as depositories—places where trash goes in and, if they’re working properly, nothing comes out. This is wrong thinking. Step back and switch to the wide-angle lens. You’ll see that landfills are not really the final resting point for our trash but just a pause along the way. Natural forces are at work here. Basic forces like conservation of mass. The Law of Conservation of Mass states that the total mass of the products of a chemical reaction is equal to the total mass of the original substances. In other words, a ton of trash comes in, and after decomposition there’s still a ton. Only some of it is no longer in the landfill. Take landfill gas for example. A 1-ton block of trash inside a typical landfill might measure 3.5 feet cubed. If the organic material in that ton of trash was allowed to decompose completely, it would generate approximately 12,000 cubic feet of gas. The gas would consist of approximately 50% methane and 50% carbon dioxide, with small quantities of other constituents, depending on what we started with. According to Brian Guzzone, with the EPA’s Landfill Methane Outreach Program (LMOP), landfill-gas-to-energy (LFGTE) projects in the US delivered 75 billion cubic feet of gas in 2006. Another 250 billion cubic feet per year may be available from other potential LFGTE projects that have yet to be developed. What does 325 billion cubic feet of gas look like? Well, uh ... it’s a lot of gas. It would cover the state of Massachusetts to a depth of nearly 6,000 feet! The same concept also applies to landfill leachate. Most landfills generate leachate. Of course, the amount can vary dramatically, depending on many factors, of which climate is the most significant. Landfills in very arid climates may generate no detectable leachate under normal conditions, while those in wet climates may generate millions of gallons per year. With lined landfills, the goal is to contain any leachate generated and send it to a treatment plant or evaporation pond, inject it into the trash, or, in some cases, simply spray it on the landfill roads for dust control. In other words: Do something with it that will not contribute to groundwater or surface-water contamination. Just remember: Despite our best efforts, most landfills do not operate as closed systems. Materials go into a landfill; a variety of physical, biological, and chemical reactions occur; and then a group of different materials comes out. The rate and magnitude of those reactions also depend on many factors, with the most significant being moisture content. Wet trash decomposes faster than dry waste, which means it creates more gas and often more leachate. Reduce the amount of water entering the landfill, and the reactions slow down. Controlling gas and leachate at landfills often means controlling the water—primarily runoff. Runoff is the last and most visible aspect of our environmental triple-threat. If not managed properly, runoff can cause erosion, ponding, and—most importantly—leachate. No doubt you’ve been hearing about storm water pollution prevention plans (SWPPPs) and National Pollutant Discharge Elimination System (NPDES) permits. It’s likely you are using best management practices (BMPs). If you operate a landfill, you haven’t just heard about these things—you’ve developed and followed them. Why the big deal? It’s a big deal because an incredible quantity of water, in the form of rain and snow, falls on landfills every year. Even at landfills located in what might be classified as an arid climate (less than 25 inches of precipitation per year), the amount of moisture can be staggering. In terms of actual volume, 1 inch of rain represents approximately 27,000 gallons of water per acre. This means a 50-acre landfill, receiving 25 inches of rainfall per year, would actually receive 34 million gallons of water. That’s 34,000,000 gallons per year. Leachate generation can also be determined through the use of a water-balance equation. Simply stated, a water-balance equation is like a bank account. The initial moisture content of the waste is the starting balance. Additions to the balance are made for rainfall, snowmelt, and any offsite water that flows into the landfill (say, from an improperly constructed drainage ditch). There are subtractions for water that runs off, evaporates, or is removed by plants through the process of evapotranspiration. Some moisture is held temporarily in the landfill. The amount of moisture that can be held depends on many factors. The maximum amount of moisture a landfill can hold is referred to as field capacity. If the field capacity is exceeded, leachate will be generated. Think of this a dry sponge. You can drip water on the sponge until it reaches its field capacity ... then water will begin dripping out the bottom. Probably the most common method used for calculating leachate generation rates is the hydrologic evaluation of landfill performance (HELP) model. The bottom line here is that everything we put into a landfill may not necessarily stay there. Some of it is just passing through. Keep in mind: It’s not just the quantity of runoff that creates problems, but a landfill’s ability to handle it. During a recent class, one of the participants commented, “We received 9 inches of rain in a seven-day period last year, but because we expect and prepare for heavy storms, it wasn’t a serious problem.” Someone else from a southern California landfill laughed and said, “Man, if we got 9 inches of rain in a year it would cause serious problems for us.” It’s always relative, but runoff control is always important, even though it may take on a different look for each landfill.
Landfill gas, leachate, and runoff are individual issues, yet they are closely tied together. Each affects, even as it is affected by, the other two. Runoff, if not controlled and effectively allowed to run off, will contribute to the creation of leachate and gas. Leachate can and does affect the creation of gas and, if allowed to exit the landfill, may also cause problems by contaminating what would otherwise be clean stormwater runoff. And finally, landfill gas can affect the quality of leachate. And in some cases, gas that comes into contact with clean groundwater can impart contaminants to it, thus creating groundwater-monitoring hits that have nothing at all to do with leachate. Controlling one often means controlling the others, but if we had to focus our effort on one area, a logical place to start is with runoff. Of the three, we can most often point to inadequate runoff as a contributing factor. Runoff One of the most common problems is related to the establishment of proper slope on interim or final landfill surfaces. Some states require a minimum slope of, say, 3%. Of course, the goal here is to provide enough slope for good runoff—even after the landfill has settled—without creating an erosion problem. Long-term landfill settlement rates are affected by many factors, including depth of waste, age of waste, moisture content, type of waste, and the amount of compactive effort applied when the trash was placed. Coming up with an accurate estimate of landfill settlement is difficult, so designers often design landfills with relatively steep slopes and then design drainage systems that will resist erosion. Here are some examples: Berms—Many landfills use soil berms, sandbags or straw bales to concentrate runoff to specific points on top of the landfill. From there, it can be safely directed into a downdrain or lined swale and transported to the bottom of the slope. Downdrains—Downdrains are often placed on the surface of the landfill and held in place by metal stakes. Some may be partially or entirely covered with soil. This type of system looks impressive and may work well initially, but if not designed properly it can be problematic. Over time, landfill slopes will settle and move. This can create significant problems with downdrains—especially inflexible metal pipe. Rigid plastic downdrains have a bit more flexibility but may also be affected by landfill settlement. Flexible plastic pipe can easily move as the landfill settles. Plastic pipe also has the benefit of being lightweight, which makes for easy handling and installation. Lined Swales—Using a drainage swale in lieu of a downdrain can be a good long-term solution, especially in an area where lots of settlement is expected. Swales can be lined with riprap, interlocking concrete blocks, or in some cases simply with a strip of HDPE left over from the last liner project. The goal is to create a secure flowline that is flexible enough to move as the landfill settles but also protected against erosion. Flow Interrupters—As runoff velocity increases, so does its ability to cause erosion. For that reason, many landfills incorporate some type of flow interrupter to slow the runoff. These may include straw bales, soil berms, or sand bags. These devices will also provide an area where small amounts of sediment can drop out. A number of these small sediment traps located around the landfill can take a significant load off the sedimentation basins. Contact Water—Contact water refers to otherwise clean surface water that has come into contact with waste and may, in fact, be contaminated. Landfills go to great effort to minimize the potential for surface-water contamination by keeping clean stormwater away from exposed waste. To accomplish this, landfill managers must micromanage wet-weather drainage at every step. Randy Wall, senior project engineer with Shaw Environmental, notes that “managing contact water is an important issue for landfill managers.” This is especially important during the initial filling of lined landfill areas. Most landfill managers can describe the personal lessons learned during the wet season immediately following their first big liner project. It’s easy to forget that the liner will hold stormwater just as effectively as the leachate it is designed to contain. Landfills may use straw bales, soil, or even a flap of plastic welded or glued to the liner to help separate and direct clean water away from leachate.
Managing Leachate Uncontrolled leachate is bad. Remember the goal: Protect human health and the environment. But an ever-increasing number of landfills are recirculating leachate back into the waste to accelerate decomposition, settlement, and gas production. For many landfills, leachate production rates result in an excess of leachate. Recirculation, if possible, may provide temporary storage for some of the leachate, but often is not a good long-term solution. Here are some ways landfills deal with excess leachate. Evaporation Pond—Installing a lined leachate storage pond can provide a relatively simple way to get rid of excess leachate. Such a pond may require some type of netting to protect birds and other wildlife from the potential risk of harm. In order for an evaporation pond to work, it must provide a net loss when annual precipitation and evaporation are compared. Too, it must have adequate capacity to handle the peak leachate flows, which may occur during or just after the wet season. Of course this is when evaporation may also be at its lowest point. The pond design will also have an impact on the evaporation rate. A shallow pond, or one with relatively flat slopes, will tend to heat up more during the summer, thus increasing evaporation. Finally, in order to keep rain out, some landfills have installed pond covers that can be removed during the dry season or simply act as storage, with the evaporation occurring outside via a sprinkler system. Dust Control—With regulatory approval, some landfills use leachate for dust control during the dry season. While offering a low-cost alternative to leachate treatment, care must be taken to avoid excessive runoff that could contaminate surface water. Most of these facilities also have policies preventing leachate from being sprayed near people or vehicles. Injection—A number of landfills, ones with approved liner systems, are extracting leachate from the base liner and injecting it back into the landfill. This can help increase the moisture content of the landfill, but because a landfill is not homogenous, certain areas may become saturated while others are bypassed. This can create isolated areas that settle faster and more than the rest of the landfill. These factors should be considered before injecting leachate back into the landfill. Pipe or Truck to Wastewater Treatment Plant—For one reason or another, many landfills are not able to dispose of leachate onsite through evaporation. These sites may be forced to send their leachate to a wastewater treatment facility. If the landfill is fortunate enough to be near a sewer line, great. Otherwise, a line may have to be extended to the landfill. If the volume of leachate is not excessive, tanker trucks may be an option. But with increasing fuel prices this option is becoming less attractive. Depending on the characteristics of the leachate, it may be practical, even for small landfills, to construct an onsite treatment facility. Another alternative for disposal of small quantities of leachate may be to set up a system that utilizes some of the heat generated by the gas flare to evaporate leachate. Managing Landfill Gas Increased gas production is a good thing then, right? Well, in terms of developing an LFGTE system, generally the answer is yes. But don’t forget, these are complex issues. While increased gas production makes justifying an LFGTE project easier, it may also cause problems if the additional gas can’t be controlled effectively. Two examples follow. Example 1—At an active landfill, overall gas production rates have increased due to an increasing waste mass and the recent practice of injecting leachate back into the landfill. The increased flow has made an LFGTE project viable. That’s good. But even with an active gas extraction system, the increased production has pushed several perimeter gas-monitoring wells out of compliance. That’s bad. Historically, a simple solution was for landfills to purchase more land, extend the buffer around the landfill, and then relocate the gas-monitoring wells farther from the edge of waste. This practice is becoming more difficult as regulatory agencies continue to limit the ability for landfills to maintain compliance through an expanding boundary.
However, in old, unlined landfills where waste was placed to within a few feet of the property line, establishing a reasonable buffer makes sense. Otherwise, expecting compliance would be unreasonable. At California landfills, he affirms, “the ability to maintain compliance through boundary expansion is becoming more difficult.” Example 2—An active landfill has sufficient gas production to justify an LFGTE system. So it contracts with a company that purchases the rights to the gas and agrees to build and operate the system. But the agreement is geared more toward gas running the LFGTE system than controlling gas. As a result, when some of the perimeter gas-monitoring wells showed hits, the landfill owner asked the contractor to install additional wells along the perimeter and help bring the site back into compliance. But the contractor refused to install additional extraction wells in that area because the gas production rate wasn’t high enough. In other words, additional wells didn’t make economic sense for the LFGTE system. And because the contractor owned the rights to the gas, the owner was unable to control gas at the landfill in order to stay in compliance. Talk about a catch-22. There are many challenges to running a landfill safely and efficiently. Managing runoff, leachate, and landfill gas are some of the most important steps toward protecting human health and the environment. By understanding how these three things interact, landfill designers and managers can get these things working for them, not against. Neal Bolton is a consultant specializing in landfill operations and management. MSW - September/October 2007
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