THE THREE KEY INGREDIENTS to achieving optimal landfill compaction are simple and unchanging: impact force plus confinement plus moisture. It's not exactly rocket science. Rockets, however, are beginning to play an indirect role in landfill compaction — at least those rockets responsible for launch global positioning system (GPS) satellites.
In recent years, a combination of heavy machinery, high-tech GPS measuring tools and low-tech space-saving methods, such as alternative daily covers (ADCs), waste reclamation and leachate recirculation, have reinvigorated the age-old question of how to squeeze as much waste as possible into today's rapidly filling landfills. Some of the more progressive landfills are extending their operational lifespans by more than 15 percent, and in some cases by nearly 40 percent.
Without the use of the above methods, Dave Bonus, operations manager for Lycoming County Resource Management Services (LCRMS), Montgomery, Pa., says his 1,100-ton per day (tpd) regional municipal solid waste (MSW) landfill would not be active today. “This landfill was scheduled to close in 1998,” Bonus says. “Here we are in 2005, and we still have five or so years left.”
On the most basic level, size matters most in the world of landfill density. In recent years, compactor manufacturers have begun offering compacting machines in excess of 50 tons.
Todd Watermolen, vice president of engineering and compliance for Onyx Waste Services, says the weight of the machine is the most crucial factor for increasing compaction vs. wheel tooth configurations. “Teeth wear down,” he explains.
Over the past six to 10 years, Watermolen says, the 23 Midwestern and Eastern U.S. landfills operated by Milwaukee-based Onyx began replacing their 80,000-pound machines with 100,000-pound-plus compactors, including Caterpillar 836s, Terex 3-90s and Al-jon 525s. “Since then, we've seen at least a 15 percent increase in compaction at all of our landfills, with 25 percent at some sites,” he says.
Johnny Poore, executive director of the Lamar County Regional Solid Waste Authority, Barnesville, Ga., says his crews have enjoyed using his 125,000-pound Al-jon 600 compactor. “In 2003, we had just under six years of capacity left,” Poore says. “After one year of using the [machine], capacity increased to more than eight years.”
Although this capacity figure was skewed slightly by a drop in waste tonnage over that period, Poore says even his most conservative estimates of density increases have been dramatic, rising from about 1,200 pounds per cubic yard to around 1,660 pounds — a 38 percent boost.
Eye in the Sky
These 50- and 60-ton behemoths, of course, tend to use more fuel to make it up and down landfill slopes each day. With fuel costs at an all-time high in many regions, landfills are looking at ways to make each diesel-guzzling pass more efficient. For some, the answer has been GPS.
While GPS tools cannot immediately increase waste density like a compactor can, the precise measurements they make on cell heights and slope angles can give landfill operators a much more accurate view of how well their compacting equipment is doing its job.
About four years ago, Watermolen says surveyors at the Onyx FCR Landfill in Buffalo, Minn., began using backpack-mounted Radian Survey System GPS units, made by Sokkia Corp., to more quickly determine waste density. The units bounce signals off of two orbiting GPS satellites, which instantly triangulate the operator's exact position and elevation down to the centimeter. Once this data is recorded, he says, it can be compared to the gate tonnage for the day to create an extremely accurate calculation of density at any desired spot in far less time than it used to take by physically surveying the site with stakes.
Taking the technology one step further, Caterpillar has introduced a GPS measuring tool called the Computer-Aided Earthmoving System (CAES), which can be mounted on individual landfill compactors to track their movements. A screen located within each cab displays the compactor's orientation on a map of the landfill face, which is color-coded depending on the number of passes that have been made.
Once a compactor has gone over a particular spot for a preset number of times and has spread the waste in the proper 1.5- to 2-foot depth, the spot turns green, meaning the driver can move elsewhere.
“Knowing how well he's doing his job makes the operator try even harder to achieve highest compaction by trying to get his screen as ‘green’ as possible,” says Lycoming County's Bonus, who has a CAES unit on his facility's 836G.
“It's almost like ‘Big Brother’ technology,” says Scott Bost, landfill operations supervisor for the municipal White Street Landfill, Greensboro, N.C. “You can know exactly what everyone's doing in real time. Anything more than four or five passes is just wasting fuel.”
The 1,000-acre landfill, which accepts 2,000 to 2,400 tpd of MSW, construction and demolition (C&D) debris and yard waste, has used the technology for the past two years on its two Cat 836s, one Al-jon 525 and a Cat D6 dozer, Bost says. In that time, waste density at the facility has increased from an average of 1,250-1,300 pounds per cubic yard to between 1,350 and 1,500 pounds.
Dan Harris, engineering and compliance manager for the Sprint Landfill, Fort Bend County, Texas, says machine time at his C&D facility has been reduced by 60 percent since the system was added to his Cat 836 in January 2004. “We used to need two guys working the landfill face each day. Now we just need one,” he says.
The cost for installing the technology was about $150,000, Harris says. “The price seemed high, but it paid for itself in six months,” he says. Compaction ratios for the 2,500 to 4,000 cubic yards the site accepts each day have climbed from an average of 2.5 or 2.6:1 to 3:1, he adds.
The GPS unit also can be used to mark “trouble spots” to avoid on the landfill face, Harris says. “If a load of asbestos comes in, we can pinpoint it and mark the exact time it came in,” he says.
As each cells grow, the information can become increasingly useful with slope construction, Bonus says. “No grades stakes are used at the working face anymore to plot the slopes or benches,” he says. “The topo[graphy] of the landfill is right on the screen for the operator to look at, and the slopes and benches are coming out perfect.”
Lemons to Lemonade
One of the more difficult factors to control in the landfill density equation is moisture content — too much moisture and you have ponding issues; too little and your waste turns to uncompactable popcorn. There are a growing number of landfills, however, that are turning to a practice that until recently was frowned upon in the industry: leachate recirculation.
To solve intermittent moisture problems at Lamar County's two active 10-acre cells, Poore transformed a leftover leachate line into a larger version of a common garden “soaker hose,” applying reclaimed leachate to the top of the site once a week from the landfill's holding tanks.
“The problem is getting it evenly distributed,” he says. “Leachate will tend to find its own way through the garbage, but you want it to seep in slowly.”
Due to this practice, Poore says the county has not had to send a drop of leachate offsite for treatment since 1998, thus avoiding the hauling costs that can range from 2 cents per gallon to 10 cents per gallon.
Not only does the moisture aid in compaction, it also enhances biodegradation of organics within the landfill, thus providing more airspace for future waste. The process creates more landfill gas to be collected and used for energy, or sold.
“I think this will someday be the norm,” Watermolen predicts. “Some Onyx landfills have been recycling liquids and adding sludges since 1996. It significantly increases the density and avoids wastewater treatment costs, so it makes a tremendous amount of sense.”
The landfills that have recirculated leachate, Watermolen says, have resulted in an average a 20 percent gain in airspace for Onyx. “We won't even see the full effect of this practice for up to five years, so this figure may increase,” he adds.
Lycoming County currently sends its leachate to a wastewater treatment plant, but Bonus says that LCRMS plans to begin a leachate recirculation program for the landfill's active cells this summer. The goal is to promote anaerobic digestion of the garbage and to collect and sell the resulting methane gas. “We don't want to produce more methane with recirculation than we have use for,” he says, “[but] methane gas in the landfill is like money in the bank.”
Until the plan goes forward, Bonus says the landfill is kept moist by the addition of sludge to the MSW loads, representing about 12 percent of the waste entering the facility. “It fills in the voids of the working face and helps with the compaction,” he adds.
Recirculation seems to be making so much sense that even those landfills that produce very little leachate are looking for ways to create it. Rich Allen, site environmental engineer for the Salt River Landfill, Scottsdale, Ariz., has filed an application with the state Environmental Protection Agency (EPA) to start a “bioreactor” in its newest cell, using groundwater from an onsite well to make up for the lack of rain in the desert region.
The water usage at the 145-acre, 2,200-tpd landfill, owned by the Salt River Pima-Maricopa Indian Community, would not make a significant dent in the area's drinking water supply, Allen says. In addition to expanding the landfill's usable life beyond the current expected closing date of 2015, the facility plans to find beneficial uses for the increased output of landfill gas from biological activity in the new cell, he says.
Dust to Dust
“The most valuable man-made commodity at a landfill is airspace, but the most valuable natural resource is dirt,” Poore says. For this reason, Lamar County's 250-tpd operation has embarked on an ambitious project to reclaim as many materials as possible from its long-closed landfill cells that were completed during an era that was less enlightened about recycling.
Over the past four years, the project has reclaimed materials from 14 acres of the old landfill that were closed in the 1960s and '70s, pulling out between 5,000 and 7,000 tons of reusable dirt, tires, plastics, metals and organics in 2004 alone, Poore says. “We're reusing space that was thought to be used up forever,” he says. “We've used reclaimed dirt as cover since we started four years ago and haven't had to purchase any other cover material since then.”
Another progressive solution is to do away with dirt altogether. About half of Onyx's landfills use a form of alternative daily cover (ADC) material, such as shredded wood and certain approved industrial waste streams, Watermolen says. Onyx properties in the Chicago area, in particular, are having success with shredded wood ADC because it provides a good working surface as well as added organic material for future bioreactor projects.
Auto shredder fluff, he adds, also shows much promise as an ADC because it “absorbs so much liquid and doesn't become a slimy mess.”
Other facilities, including the Salt River and Lycoming County landfills, have had great success with the Canton, Ohio-based Tarpomatic system, which involves furling and unfurling a reusable tarp with a bulldozer each day in lieu of daily cover.
Both Lycoming County and the White Street landfills also use the foam-based Posi-Shell ADC, made by Landfill Service Corp., Apalachin, N.Y. After being sprayed on at a depth of six inches each day, the foam hardens to form a lightweight protective shell that can be crushed down by the next day's garbage to a mere fraction of its original volume.
While some landfill operators may balk at the added cost of some of these landfill compaction methods, the resulting space savings have been compelling.
Considering current leachate compaction methods, recirculation and reclamation efforts at the Lamar County Landfill, Poore estimates that approximately 25 years of site life will be added. As the landfill pyramid grows higher and the footprint grows wider, however, “by the time you factor in the exponential volume growth through all of the new cells, it will be more like 60 to 70 years,” he says.
“I think you'll see bigger and heavier compactors produced,” Bonus says of the near future for landfill density. “I also think most landfills will move to the GPS systems to better enhance their operations. Who knows what technology will bring to this industry in 10 years, but it will be exciting to find out.”
Randy Woods is a contributing editor based in Seattle.