Putting the squeeze On

Lynn Merrill

Conserving airspace through effective compaction: It's the goal of every landfill operator. And in recent years, the ability of facility operators to achieve greater compaction has improved. Through the management of a compactor fleet and the use of global positioning systems (GPS), alternative daily covers (ADC) and bioreaction, an operator can maximize capacity and extend a landfill's lifespan (in some cases by 20 percent) without having to worry about expanding a site's existing footprint.

Given the difficulties involved with expanding a disposal site, maximizing landfill airspace is paramount and cost-effective. “If you were playing the commodities market, landfill airspace is a commodity that is definitely going up in value so you want to make the most of it,” says Steve Viny, president of Independence, Ohio-based Norton Environmental, which operates landfills.

Good landfill compaction starts with the compactors themselves. In general, the heavier a machine, the denser it makes the trash that it rolls over in a landfill. Therefore, manufacturers recently have focused both on maximizing the weight of compactors and ensuring that the weight is transmitted to the machines' wheels as effectively as possible. It is not unusual for landfills to now use compactors weighing up to 126,000 pounds.

Compactor wheel designs also play a significant role in achieving maximum densities. Earlier wheel designs chopped the waste, and while they provided good traction on the slopes of the landfill, they did not create ideal compaction. “[Earlier designs] tended to fluff the top portion of the waste,” Viny says. “The newer designs, while you loose a little bit of traction, you gain more compaction.”

While the industry has moved toward heavier compactors, landfill compaction is for the most part a function of weight, speed and number of passes. “If you ran a compactor that weighs 80,000 pounds over the waste five times, then ran a 130,000 pound machine and ran it over the same waste five times, you'd get more compaction on the 130,000 pound machine,” Viny says. “Ideally what you want is a heavy machine that's fast and maneuverable and that can get the same number of passes as a smaller machine.”

A general rule of thumb is to drive over a stretch of waste five times, but after the first couple of passes, the amount of compaction achieved per pass begins to diminish.

Working Smarter, Not Harder

While compactors are the heart and soul of compaction, landfill operators have begun using other technologies and techniques in conjunction with the vehicles to preserve airspace. For instance, many site managers have equipped their compactors with GPS units. The devices contain display screens that help compactor operators keep track of the number of passes they've made over solid waste and make sure they have obtained the targeted density.

ADCs also can play a key role in compacting landfill waste. The working face of a landfill must be covered at the end of each day, and the standard cover is a layer of soil. With ADCs — which can consist of tarps, foams, shredded green wastes or construction materials — airspace that was once filled strictly by dirt can be used for waste. Deciding which ADC to use requires considering several factors, including climate, availability and costs.

Finally, bioreaction is another way that landfills can conserve airspace. Bioreactor landfills differ from the traditional dry-tomb sites because leachate is recirculated throughout the waste instead of collected and shipped offsite for treatment. The leachate recirculation causes waste to decompose more quickly than it does in traditional landfills, allowing waste to be compacted in higher densities, experts say.

Density is a function of moisture, says Viny, who uses an analogy to describe the challenge. “If you took flour and a rolling pin and rolled it over dry flour, you're only going to compact it so much and that's it,” he says. “When you add water and mix it with the flour, you can run that rolling pin over the flour and you can roll it until it's paper thin.”

While compaction techniques in dry-tomb landfills often achieve a density between 1,100 pounds and 1,300 pounds per cubic yards, recent studies have indicated that the bioreactor landfills are achieving densities of around 2,000 pounds per cubic yard. And, “that doesn't take into consideration the settlement that occurs over several years of operation,” Viny says. “If you add differential settlement it could be almost double to 2,200 to 2,600 pounds per cubic yard.”

Many of the techniques for maximizing airspace can be used simultaneously, according to Viny. “You can have a bioreactor landfill and also use an alternative daily cover and a very heavy landfill compactor with properly designed teeth,” he says. “You can use all or some of these techniques and each of them have their own advantages and disadvantages.”

Method Mixture

The Chester County Solid Waste Authority uses a variety of methods to achieve maximum compaction at its Lancaster, Pa., landfill. The site is the designated disposal facility for 49 municipalities and receives between 1,100 and 1,400 tons of solid waste per day, including approximately 150 tons per day of construction and demolition (C&D) debris.

The site has had various landfilling operations since the 1960s, according to Robert Watts, executive director of the agency. “Like a lot of the older sites here in the East, we're always crammed for capacity and doing things innovatively,” he says. Instead of expanding the site's footprint, the agency has expanded capacity by constructing new landfill cells on top of older closed cells, and it has used reinforced earth walls to expand the height of cells adjacent to other closed cells.

The authority uses a GPS from Caterpillar, called the Computer-Aided Earthmoving System (CAES), in order to achieve its targeted compaction levels, Watts says. The system features an in-cab screen that allows drivers to keep track of how many passes they've made over material and to what depth they are spreading the material. “We did see over a 10 percent increase in compaction by using GPS,” he says.

The agency has set up the GPS so that it ensures that the agency's compactor operators don't spread the waste at a depth greater than 2.5 feet. “We also have the system dialed in that [the drivers] have to do five passes over the material,” Watts says.

The system tells operators when they have done their job. “It's good feedback for them,” Watts adds. “We even post the compaction on a daily basis on the bulletin board in the lunchroom, and I think that's been very positive feedback.”

The landfill also uses a leachate recirculation system consisting of trenches and infiltration cells, also called blankets. Each of the site's four blankets consist of a 200-foot by 200-foot area that was excavated and then covered with geotextile and a foot of tire chips. “Our plan is we're going to be pumping raw leachate into the blanket to allow it to seep back into the landfill,” Watts says. “This way, we enhance the decomposition, settlement and gas production. We just finished construction of the blankets so this is something new for us, but we've had a lot of history with the trenches.”

Finally, the agency uses what Watts calls a “hierarchy of cover schemes” to help the landfill achieve the maximum capacity. The authority first uses tarps as daily cover. “Our preferred cover is tarps and that's been great,” he says.

The authority also sends its C&D debris to a facility in Philadelphia that processes the material. The agency then takes back some of the fine material that has been approved for use as a daily and intermediate cover. Some months, the landfill sends between 2,000 and 3,000 tons of C&D debris, which doesn't compact much. “We save our space by sending it out for recycling and then getting a product back that we use as cover,” Watts says.

In the past, the authority achieved 1,100 pounds per cubic yard, which wasn't good, Watts says. But through the techniques described above, the agency's landfill now is achieving 1,300 to 1,400 pounds per cubic yard.

Conclusion

Conserving landfill airspace is always a challenge. However, with heavier compactors reaching the market and a widening variety of technology to help maximize capacity, making the most of a site's existing footprint is more feasible than ever. By using a variety of methods, landfill managers can achieve maximum compaction and get the most out of their site.

Lynn Merrill is a contributing writer based in San Bernardino, Calif.