Keeping landfill leachate contained is an easier process when it’s planned for—during the construction of each cell.
Most garbage contains moisture; few people wring the last drops of milk from the carton, and moldy, forgotten leftovers are tossed, containers and all. Many trash components manage to decompose to a certain extent, and that can cause moisture, which becomes leachate. In addition, except for in the most arid climates, rainfall and snowmelt seep into landfills, creating and/or adding to leachate. As this mix must be kept onsite, and away from groundwater sources, the best way to contain leachate is to prepare for it from the very beginning; liners, trenches, piping, and pumps are installed into each new cell before the first scrap of trash goes in.
Lots of Rain, High Water Table
The Pasco County, FL, Solid Waste facility takes in a variety of trash, and deals with most of it onsite. “Pasco County operates a Class I landfill, a Class III landfill, a yardwaste processing facility, a tire processing and a waste-to-energy facility,” explains Facility Manager John Power. “The site is 800 acres total. The Class I landfill area is 160 acres—once completely built out; currently only 50 acres have been developed. Thirty acres, or three cells, are dedicated for incinerator ash. Twenty acres, or two cells, have been developed for MSW. The leachate from both is collected separately, as the MSW leachate is processed at a county wastewater treatment facility (WWTF) adjacent to the site. The ash leachate is collected and stored in a 2-million gallon Crom tank onsite, and eventually transported to Tampa to a larger WWTF facility for treatment.”
The ash Power mentions comes from onsite burning: “Pasco processes more than 90% of its MSW through its Waste-To-Energy facility, which is rated at 1,050 tons per day. Tires are shredded and chipped onsite and then hauled off site to a facility for fuel to produce electricity. Yard waste is processed onsite and the resultant mulch is utilized by citizens of Pasco, as well as some businesses.”
Extra steps are taken at the Pasco landfill: “Although we weren’t required to, we lined our Class III C&D landfill, because it was the right thing to do. We also cover our ash cells. As for our leachate piping systems, we double-lined each cell, which has independent primary and secondary piping, which monitors leachate. The leachate then goes to a pump station, either to be treated in our wastewater, or, in the case of the ash, to the collection tank.”
Leachate from the incinerated ash is the only thing moved offsite. “This leachate is transported in tanker trucks to Tampa, which has a greater wastewater capacity. How often it’s taken depends upon how much rainfall we have. During the summer, it’s shipped probably every day, during our rainy season.”
Because of all the planning, Power has seen no leachate spills or problems. “We have 40-plus monitoring wells within the primary and secondary system on each cell, and all have meters. We have not seen any elevated parameters in our monitoring wells; and, in 18 years, I haven’t seen any spills at all. Typically you’d only see that in landfills that haven’t been lined.”
The Tampa firm Camp Dresser & McKee (CDM) designed Pasco’s landfill cells. “I finished the design of a 20-acre Ash Monofill, which is in construction now,” says CDM senior project manager Aamod Sonawane, P.E., BCEE. “Previous cells were designed by another engineer. Pasco County has a waste-to-energy plant; the first thing they do is burn for electricity, and that’s where the ash comes from. Lots of counties around here—burning trash for electricity is their first choice, there’s lots of capacity.”
To prepare for leachate in a landfill design, Sonawane first collects data. “I like to get the rainfall data from the National Weather Service or NOAA. Plus, gathering as much site-specific information as possible, from onsite rain gauges, is important. I’ll then use the EPA’s HELP (Hydraulic Evaluation of Landfill Performance) model to estimate how much leachate can get generated. Knowing the density of a landfill is also essential to predicting leachate.”
Cells are constructed to collect leachate. “The cell has a ‘sawtooth’ design at the bottom, alternating valleys and crests. Every crest has a 2% longitudinal slope; in the trenches are perforated FDEP pipe. Pulled by gravity, the leachate naturally travels to the trenches and the piping. Then we pipe it to the lift station. In Pasco, the ash leachate goes to a tank, then a vendor takes it to the City of Tampa Wastewater Treatment Plant. Magmeters (flow meters) are on the downstream side of the pump station to record the leachate flow.”
Sonawane ticks through a “shopping list” of the items needed to enclose and collect leachate: “You need Subtitle D liners, a geosynthetic clay liner, a double HDPE liner, a SGP layer Geocomposite, a protective sand layer on top of the geocomposite, and perforated pipes at the bottom of the liner. Then of course you need pumping systems, pumps, pump stations, and control stuff to monitor what you’re pumping.”
Although every precaution is taken, one must always be on the lookout for leakage. “Within the cell, the Florida Department of Environmental Protection requires an annual videotaping through the leachate system to see if it’s working fine. I’d think a line break would be a unique case; if you’re in an earthquake zone, of course, you have to plan for it.”
Avoiding a Gas Attack
“Leachate itself is typically not corrosive; it has a neutral pH, 6 to 8. However, the gas that comes along with it can be very corrosive,” explains Dennis Davis, solid waste engineer at Gainesville, FL’s Jones Edmunds & Associates Inc. “Sometimes landfill gas gets into the groundwater, and that can contaminate. Taking care of leachate can help solve that problem.”
Davis’s first steps involve installing a liner, then sloping and piping the landfill. “The liner system helps keep out leachate. What if the pressure from the groundwater is greater than the leachate? We use two 60-mil thickness HDPE liners.
“The sheets are 20 feet wide, so we have to ‘weld’ them together to make them large enough. That’s why you use two thicknesses—to protect the weld areas. Slopes on a landfill are typically at the edge; we’ll use sand, pipe, and rock there, building a ‘burrito’ of leachate collection. The installed perforated pipe is wrapped with rock and geotextile; this pipe may run through the length of the landfill. After pumping the leachate in the edge, the liquid is then pumped out to a leachate storage tank, which can be steel, glass-lined, or plastic. We periodically test the leachate to see what’s in there, but it’s not technically hazardous. You might have to pretreat it, but typically it goes to a water treatment plant.”
What does one do with leachate, once it’s collected? “Sometimes you load it on truck and haul it to a water treatment plant. You’d like to be able to pump it one more time—right to the water treatment plant, but sometimes the nearest water treatment plant can’t, or won’t, take it and you have to truck it to someone who will. If a wastewater treatment plant is too far away, other ways to deal with leachate then become economically feasible. You can use a bioreactor. Sometimes we re-inject the leachate into landfill, which is like a big sponge. If that proves cost-effective, you can do this—but not forever. Sometimes landfills have to build their own pretreatment plant onsite. These aren’t ‘full’ water treatment plants; the facility merely reduces constituents that test too high for ‘regular’ water treatment plants to accept. For example, a water treatment plant might say, ‘We cant take any more than 500 parts per million of ammonia, so you have to take that out before sending it to us.’”
Leachate isn’t forever; capping usually ends the process. “When you close a landfill, you’re required to put a liner on the top of it. Since rainwater is no longer getting in, your leachate curve is goes down near to zero.
Monitoring for leakage starts with the liners. “We have two liner systems. In the lower liner, the leak detection system has its own pipe. We monitor quantity of leachate that comes out of that system. Ten to 100 gallons per acre per day is about normal. If we were producing much more, we would have to evaluate the system; why is the primary (the higher) liner not working? The only way we truly know if it’s getting out, is if it’s getting into the environment. You must install groundwater monitoring wells no more than 50 feet from the landfill, all properly spaced. We monitor our wells two to four times a year and evaluate those water samples. If we see any contamination, there’s a set of rules of what must be done. You have to evaluate why it’s there, what’s the cause. You may be required to put in additional wells, do more sampling. However, I haven’t seen any Subtitle D liners fail yet.”
The Rust Belt: Rains and Snowmelt
Midwestern and Northeastern landfills can be designed much like those in Florida; however, in northern states, annual snowmelt adds to the equation.
“We take a couple different steps to plan for leachate in our designs,” says senior project manager Amy Knight. Knight works for Cornerstone Environmental Group LLC’s Middletown, NY, office. “Landfills must have a leachate collection system, based on which area of the country we’re building. If we’re adding to an existing site, we look at historical data; for a new landfill, we use the EPA’s HELP model. We’ll work up a couple different scenarios, depending on the thickness of waste. We’ll look at open cell, which maybe contains 10 to 20 feet of waste, then look at cap conditions. After collecting that data, we’ll run ‘sequencing,’ doing a time line of how much leachate can be expected. We’ll then use the peak volume number to figure what the landfill needs.”
This data helps determine the design. “We’ll space your collection pipes and drainage layer, knowing the depth of a cell can vary. If you’re only on cell two, its depth may be 60 feet, and you can build to your maximum height. Since it’s stormwater that infiltrates, becoming leachate, you’d like to design to minimize stormwater; to control the water, to reduce leachate. Of course, moisture also increases the rate of methane production. The wetter your waste is, the more work, pumps, et cetera, you might need. It’s a balance.”
Piping is essential. “Leachate is collected with a pipe system in the landfill’s base, buried in drainage sand. The leachate drains to one low point in the cell. This can be accomplished with gravity penetration to a sewer system, or you might have to pump it up to tanks or the sewer system.
“Some sites have pretreatment plants on site,” Knight goes on. “Then the leachate is discharged to the municipal sewer system—you have to meet guidelines, of course. Some leachate can go directly to the sewers, depending on the area’s wastewater treatment capacity. Some landfills collect leachate in tanks and truck it off to another treatment facility.”
Monitoring activity depends on the state. “Some require double liners, and you monitor what you get from the secondary system. If something’s there, you must have a leak in the primary system. There’s also continual groundwater monitoring. If you see something, you will suspect a leak. Even landfills that have a secondary system monitor groundwater—usually through a sunk well, sampled quarterly. There are minimum Federal requirements, and the states can make other rules from there.”
Knight hasn’t seen spills. “The collection tanks have a secondary system too, so the site shouldn’t be compromised. However, not every state requires a secondary system.”
Recirculation—a Good Idea Going Around
Fred Doran, senior environmental engineer and solid waste practice leader for R.W. Beck’s Minnesota office gives a detailed account of building a leachate system.
“Leachate collection system design relies on a good estimate of leachate generation, both under average and peak conditions. The HELP model, created by the US Army Corps of Engineers, has been used for 20 years for this estimation. The HELP model predicts leachate generation based on many factors: historic precipitation in the area, the amount of waste in place, and the type of geometry of the liner collection—meaning slope and flow distance. For peak conditions, you look at the worst case, which is an empty cell or one that contains about 10 feet of waste. In the permits we complete for our local clients, the geometry is fixed by regulations, with standard slopes and fixed slope distances. A typical cell is comprised of drainage trenches or basins connected to a sump.”
Cell sizes can vary from state to state, site to site. “Larger landfills can have cells greater than 25 acres; our projects are typically 3 to 8 acres in size, with individual collection trenches about 200 feet apart,” he goes on. “Maybe a cell will have multiple trenches or basins which tie into one sump, at the low point of that cell, where the leachate is then pumped out.”
Doran describes his usual liner “profile.” “First, there’s 2 feet of clay, then an HDPE membrane, and then a drainage material—we usually use one or two feet of sand—for leachate collection. The perforated pipe within the trench is surrounded by gravel; we typically avoid limestone. In the upper Midwest, we use river rock with a low limestone content from glacial outwash deposits. A landfill has phases of waste degradation. By the time leachate reaches the collection system, it’s pretty neutral, and dissolution of the drainage media may not be prevalent.
“Here in Minnesota, and likely in most locations, leachate collection design needs to be flexible,” he goes on. “Peak generation can occur during high intensity rain events, during snow melt, and in the spring, during both together. You need to consider designing the pump and piping system for the worst case rain event but be flexible to handle regular situations too. Cells can last from two to 10 years, depending upon how much trash goes in, and the rate of compaction.”
The piping systems are next. “Leachate collection trenches contain perforated pipe, covered by a sequence of rock—one-and-a-half-inch diameter rocks right around pipe, then three-quarter-inch diameter pea gravel, then sand—to filter leachate. When you have a cell, it will be sloped to a low point where the sump is located; now, pumps are typically inside the landfill within these sumps.
“We use 18-inch diameter pipes down the side slope to access the sump, which sometimes connects two or three cells together. The pumps—6 to 8 inches in diameter, 2 feet long or larger, depending upon the design flow of the pump—are within the landfill inside this pipe. Leachate generation and pumping continues after cell closure, but the flow significantly decreases.” Automatic systems help monitor the leachate. “The sump pump has a transducer, which measures leachate head elevation. Pump on/pump off’ is programmed into the system controls and relies on the transducer reading within the cell.”
There are options for leachate transport and disposal. “Probably the most common option for leachate storage and disposal is a 10,000- to 20,000-gallon underground tank,” Doran explains. “Rural landfills rely on tanker trucks to haul leachate to wastewater treatment. Others can connect directly to a sanitary sewer—the leachate doesn’t have to go overland. I personally like to maintain leachate management onsite, treating it in aerated ponds. Pond size varies from 300,000- to 2.5-mm gallons, providing flexibility for more storage.
“We also promote leachate recirculation at landfills, to cost-effectively manage leachate and accelerate waste degradation,” he goes on. “Recirculation can include spray application of treated leachate directly on the waste; this provides better distribution. We also promote land application of treated leachate. At the Crow Wing County, MN, landfill, where I’ve been working since 1993, leachate is land-applied to a 36-acre field with a grass cover crop next to the landfill. They rarely haul leachate offsite; the last time was 2002. We have had success with denitrification in the treatment ponds. Leachate is typically high in ammonia, at 500 to 800 parts per million. Bacteria will convert ammonia to nitrate (nitrification) initially. Then the aeration is turned off, methanol is added as a carbon source, and the nitrate is reduced to nitrogen gas. During denitrification, the pond surface almost looks like can of soda pop, with the bubbles coming off.”
Leak prevention, and detection, are planned from the start. “Leak detection monitoring begins at the base of landfill—many sites install a full leak detection layer under the clay portion of the main line. This leak detection is typically a geonet geocomposite, which is a geosynthetic product for drainage, a plastic fiber mesh with high permeability. A geomembrane or other liner then underlies this geonet. This detection layer also has a sump where liquids are collected and monitored. In the upper Midwest we have a lot of locations with good glacial soil systems, basically clay. Therefore, the leak detection system can be limited to the cell low point, immediately beneath the leachate collection sump. This smaller basin is called a lysimeter [a secondary leak detention pump under liner].”
Doran points out that leakage is rarely a problem. “I have never worked on a lined leachate collection system for a landfill that has failed and subsequently impacted groundwater. If they occur, leachate impacts may happen during significant rain events from runoff, or spillage around loadout pads, where trucks load leachate for transport. Should leakage occur, the response would be to dig up some soil where it looks stained, and then some soil testing to verify cleanup.”
His “flagship” site, the Crow Wing County Landfill, has implemented everything he mentions. and he ticks off a list of typical equipment needed: “A transducer level, used to operate the pump, as well as measure the leachate head on the liner. By rule, you can’t have more than 12 inches of leachate on liner. Controls for pumps. If you do recirculation you need pump flexibility for varying head requirements. You need a pump to move liquid to where you want it: the bottom of the landfill, which could be 100 feet belowground, and recirculation points could be 100 feet above ground. The mechanism has to be able to pump it up there. You also need flow meters, which provide information on how much leachate is collected, recirculated, land applied and hauled offsite. I have never seen leachate evaporators in operation; they use landfill gas to vaporize the leachate . Another treatment option I haven’t used yet, but like the concept, is constructed wetlands. The natural plants treat and filter the leachate.”