WITH THE HIGH COST to develop landfills, it makes considerable sense to save as much room for solid waste as possible. Daily cover material can take up precious space. “The space that landfills are selling is worth about $20 to $32 per cubic yard,” says Donald Breaux, president of Landfill Solutions, Baton Rouge, La. “If a landfill applies 6 inches of dirt using 1,000 yards of dirt to do that and the space is worth $25 per cubic yard, it actually wastes $25,000 dollars of space.” Fortunately, there are several alternative daily cover (ADC) options that provide a less bulky alternative to the 6 inches of dirt that is required to be placed on the landfill at the end of each working day.
According to the American Society for Testing and Materials Standard D 6523-00, there are four primary classifications of ADCs: foams, slurries, geosynthetics and indigenous materials. Each material must “control disease, vectors, fires, odors, blowing litter and scavenging without presenting a threat to human health and the environment,” the West Conshohocken, Pa.-based ASTM says. So how do you choose the right type of ADC for your landfill?
According to landfill operators, you should consider the value of the airspace being conserved, the availability of and distance from dirt sources, the cost for moving dirt versus applying another ADC, the climate found at the site and whether the local regulatory agency has approved the proposed ADC.
Slurries and Foams
Slurries are usually paper-based materials mixed with water and other components, then sprayed onto the working face of the landfill to create a hard crust or shell over the top of the landfill's working face. The materials usually are applied with hydroseeding equipment that has been modified to handle the type of slurry being used. No additional work is required to remove the material from the landfill the next morning because the slurry is broken up by the placement of new waste. Slurries also provide an alternate use for some waste paper products that cannot be traditionally recycled.
Foams are usually a synthetic material that are combined with air when sprayed onto the landfill to form closed-cell air pockets. Foams require specialized equipment capable of mixing and discharging the product onto the landfill's face. They may consist of hardening or non-hardening compositions. Foams are not recommended for application during rainfall, and may require cleanup and post application maintenance.
The Bi-County Solid Waste Authority landfill in Clarksville, Tenn., for example, uses a slurry at its 550-ton per day (tpd) Class-1 balefill. The authority creates a 2½ cubic yard bale using two onsite balers, prior to stacking the bales on the site. Because of the landfill's steep face, the authority found slurries worked best in its operations.
“Our vertical face is our biggest concern,” says Pete Reed, authority director. “We used plastic, and that took a lot of manpower. If you get a little tear in it, which we always did, we'd have to rip it off and have to redo it. We've been using our [slurry] alternate daily cover here for a couple of years now, and we are very happy with it.”
Reed says application is fairly straightforward. The authority has a self-contained spray rig that is pulled by a bulldozer. “We [mix] 2,000 gallons of the material with water and a small amount of plastic fiber as an additional binder. We can spray close-up, or we can spray across a large area. It's got a great pump on it that will pump this cementatious material through it. We can load, spray and clean it out in less than an hour, depending on the distance we have to travel.”
Additionally, Reed says the product withstands the rain. “We had another product in here that worked okay for the vertical face,” he says, “but on any flat surfaces that we covered, if it was raining or got very wet, the product would not hold up.” Consequently, Reed says the authority searched for something more long-term that would simultaneously help with erosion control.
Further down into Louisiana, the Tangipahoa Parish Regional Solid Waste Facility wanted to save airspace at its 400 tpd landfill. Buddy Till, solid waste superintendent, says choosing a slurry for an ADC saved “a tremendous amount of air space due to the lack of having to use the cover material.” But in addition to saving airspace, the slurry allowed the landfill to divert manpower to other areas.
“It allows us to go a number of days without having to actually haul cover material,” Till says. “That allows us to concentrate on other projects that we may have going on within the landfill, such as drainage issues, erosion control issues or any other projects.”
Till's operation uses a hydromulch ADC consisting of ground paper mixed with gypsum and applies it with a 1,200-gallon hydromulch machine. “One of the reasons that we liked the hydromulch is because it's easy for us to mix on our own,” he says. “It doesn't really require a large, expensive amount of equipment. The material that we purchase is easily stored out of the way, and it's easy for my employees to learn to use.”
Till says it takes two men to apply the material, one operating the tractor that tows the spray rig, and one who sprays the material. “Once they start spraying, depending on the size and accessibility of the area, they can usually spray it in five to 10 minutes. Once they spray, they come back down to the shop, flush [the sprayer] out with water and they go home. The whole process may take anywhere from 30 to 45 minutes,” he says.
Geosynthetics can be divided into two classes, reusable and non-reusable. Reusable systems allow landfill operators to place fabric over a landfill at the end of the working day and retrieve the fabric to uncover the working face the next morning. Non-reusable fabrics are designed to be laid out and anchored in place. The fabrics may contain additives that accelerate the decomposition of the fabric after waste has been placed on it.
At the Capital Regional District's Hartland Landfill in Victoria, British Columbia, Canada, the landfill uses tarps for ADC. “We were aware of these covers back as early as 1992 or 1993, which is when we started using it,” says Chris Riddell, superintendent. “We've actually used different products and, just through experience, we settled on a reinforced polyethylene that is 9 ounces per square yard.”
Riddell says the landfill uses about three or four tarps to cover the site's active face each day, totaling about 12 tarps per year. Part of the reason that Riddell opted for a tarp system was because of the cost savings. After considering the dirt cover material isn't being used and the equipment time to place it, he says the tarps are saving the landfill $700,000 per year. “If you actually include the air space on top of that, that's another probably couple of million dollars. The saving are enormous, so [switching to an ADC] was really a no brainer,” he says.
Riddell tested a variety of tarps before settling on a polyethylene fabric that he says resists tears or damage. “The edges of the tarps are folded over, and we have a rope that's hemmed-in to the edge of the tarp,” he explains. To unroll the tarp over the landfill, a bucket loader has a long spreader bar attached to the front. When workers are ready to apply the tarps, a driver gets on the bucket loader, drives to where the tarp is, and a fellow worker hooks it onto a bar. Then, the driver drags the tarp over the active face every night. Once the tarp is deployed onto the landfill's face, workers unhook one end and then weigh down the edges of the tarp with tires or fill material.
Indigenous waste products that are available locally in sufficient quantity and form can be an inexpensive ADC. These materials can include green wastes, automobile recycling fluff, and ground construction and demolition wastes. Most states have developed regulations establishing the acceptable uses and limitations of applying indigenous materials as ADC.
In San Bernardino County, Calif., the county's solid waste management department operates six regional landfills that serve the 20,000 square-mile county and its 24 cities. The county handled approximately 1.4 million tons in 2003, and used 236,781 tons of processed green material as ADC, as well as an additional 41,000 tons of mulching for other beneficial uses.
“We have two sites, Mid-Valley located in Fontana, Calif., and San Timoteo, located in Redlands, Calif., that make extensive use of alternative daily cover,” says Rex Richardson, spokesman for the division. “We use ADC in varying amounts at the Victorville landfill in the Victor Valley in the High Desert and the Colton landfill in the city of Colton.”
The processed green material used at the county's sites is processed offsite and then brought into the site as ADC. “We treat it as a construction material, and it's not charged [a tipping fee] when it comes across the gate by the county,” Richardson says. The county is conducting onsite diversion of wood and green waste loads, and processes the material onsite.
“Ultimately, we will probably get most of our ADC needs met through our onsite recovery programs,” he says.
In addition to using green waste, the county is looking at using C&D materials as ADC. “We're looking at the issues of having drywall in our cover material in terms of what happens when it decomposes down the way in terms of the gas that's created,” Richardson says.
However, placing green waste or other materials requires the landfill's working face to be relatively smooth to reduce intrusions through the ADC, Richardson says. “Before you put down your ADC, you want to make sure that the trash is fully compacted. You want to make sure things like mattresses and cushions are not in the top layer of the waste before you put down the ADC because they don't compact well. If you're using processed green, it's a matter of just making sure that you meet the cover specs.”
To stay on top of your ADC material — no matter which type you choose — Richardson says, landfill operators must be in tune with local and federal regulations. “You want to make sure that it's applied properly,” he says.
Lynn Merrill is a Waste Age contributing writer based in San Bernardino, Calif.