Aggressive Compaction: Where the Wheel Meets the Waste

Lynn Merrill

The difference between achieving a density of 1,200 pounds and 1,500 pounds of compaction may not seem like much at first glance, but over the long haul it is. Smart landfill operators who aggressively manage their compaction program daily will be adding years to the landfill's operating life and revenue to the bottom line.

Landfill capacity includes two physical measures: volumetric capacity and weight. Volumetric capacity represents the cubic yards available, including height. Weight establishes a value (tipping fee) for the landfill use. Because a cubic yard of feathers and a cubic yard of lead both take up the same airspace, "density" is used to reconcile weight and volume into a single measure. Density simply is the number of pounds per cubic yard, but it has significance for the landfill engineer and manager.

In reality, landfills receive materials of different densities, volumes and weights. With some exceptions, these disparate materials all arrive at the tipping face simultaneously, creating a challenge for the landfill manager who must spread, compact and cover it quickly and cost efficiently.

While trash is pre-compacted in the collection equipment, most space-saving compaction depends on the equipment used at the landfill, and the skill and techniques of the operators.

Size Does Matter Landfill compaction is accomplished with two machine types - one with tracks to move it, such as a bulldozer, and the other with wheels. These are physically designed to exert the maximum downward pressure where the wheel meets the waste.

The challenge, of course, is to buy equipment that maximizes this downward pressure within the constraints of maintainability, efficiency and cost. While bulldozers can perform this function to some degree, the wheeled compactor has become the standard for achieving maximum compaction at the tipping face.

Selecting the right compactor requires understanding the landfill's needs. "A compactor exists for one reason and one reason only - to save airspace," says Kenneth H. Pratt, president of the solid waste division of Al-Jon Inc., Ottumwa, Iowa. "The first question I always ask is what do you want out of the machine?"

Pratt typically looks at the customer's overall operation, including maximum daily tonnage, peak delivery times at the landfill and traffic patterns.

A compactor's ability to maximize compaction is a function of its weight. The heavier the machine, the more pressure at the wheel tread - and the more expensive the machine likely will be. For that reason, most compactor manufacturers produce machines of different weights.

Airspace value determines whether a landfill should purchase a larger machine. "Machines that achieve higher densities deserve some serious consideration," says Chris Klinck, vice president of compaction equipment for CMI, Brookfield, Wis. "If you project over the landfill's life to see what a 5 to 15 percent density increase will mean as far as additional revenue, it becomes a no-brainer."

However, if cash flow makes a larger machine cost prohibitive, a landfill owner may have to opt for a smaller machine. "But when you're spending high dollars to construct a landfill cell to Subtitle D requirements, you're making a considerable investment," Klinck says. "The compactor is a relatively small part of that investment, yet it's going to enhance the airspace value."

Packing It In The city of Arlington, Texas, operates a 500-acre landfill that receives approximately 1,300 tons per day (tpd). Over the past year, the city has increased its in-place density to 1,500 pounds per cubic yard, up from 1,200 pounds per cubic yard.

Several techniques are involved in managing airspace to maximum capacity, according to Allen Jones, the city's solid waste manager.

Compaction management starts by keeping certain commodities out of the landfill. The city has on-site contractors that crush concrete and chip brush, two materials that can consume space through the air voids in the compacted material. In addition, the city has banned grass clippings and currently is considering a leaf composting program. The city also uses foam as an alternative daily cover, which not only preserves airspace, but also avoids imported dirt costs.

Jones operates two tipping faces: one for haulers and the other primarily used by the public. The split face method more efficiently manages the compactors, which work the larger waste volumes into the face the haulers use, avoiding the conflicting movements of the public with their smaller volumes.

Jones attributes the density increase to heavier compaction equipment and better supervision. "I now have two field supervisors instead of one," he says, noting that one supervisor's primary job is to ensure maximum compaction. Also, he says that workers are less inclined to take short cuts with increased supervision. "We may extend the time that we're out working the face, but we're saving valuable airspace," Jones says.

Having a Choice In Chesapeake, Va., the Southeast Public Service Authority operates an integrated waste system handling 1 million tons of waste per year. Of this, 600,000 tons are processed through a refuse-derived fuel plant. The resulting ash and the remaining 400,000 tons are landfilled. According to Dan Miles, director of field services, the Authority uses the ash as an alternative daily cover.

Miles operates two compactors, two light units and one heavier machine. "We operate the smaller machines for their speed and access, particularly in wet weather conditions," he says. "The heavier unit, even though it's a little bit slower, typically has more pushing power and better compaction."

Miles uses the smaller units to push and spread the garbage, then he uses the heavier unit for compaction. "You have to have a spread of equipment to be able to keep things moving in all weather conditions," he says.

Miles monitors his airspace annually. He starts with an aerial photograph, which shows the landfill's current contours. This photograph then is scanned into a computer aided drafting (CAD) system and overlaid onto the previous year's map. The system calculates the change in volume.

From this, he determines "what our in-place densities are," he says. "That way we know whether our compaction efforts are meeting our objectives. If they're not, we try to find out exactly what's happening. We typically shoot for around 1,400 pounds per cubic yard."

Miles cautions that aerial photography should be combined with planning and common sense. "You can end up with 1,900 to 2,000 pounds per cubic yard if you don't account for the natural consolidation that's going to be taking place from the vehicle traffic and garbage's biodegradation."

Compaction Technology In King County, Wash., Nigel White, manager of landfill operations, oversees a landfill that receives 3,200 tons per day Monday through Friday, 1,400 tons on Saturday and 600 tons on Sunday. Managing this waste volume requires the precision of a military operation to ensure maximum compaction.

"We make sure we are spreading the refuse into 2-foot lifts and use four passes to get compaction," he says. "One of the reasons we added a third compactor was to allow each operator a specific lane to work in. With the third compactor, we felt we got better coverage.

"The idea is that we keep the compactors moving," he continues. "Typically, the bulldozers spread and the compactors compact."

White is studying the use of Global Positioning Systems (GPS) to monitor daily compaction. So far, the cost does not justify its potential value, he says, but this may change as landfill space costs increase.

The GPS system would provide real-time data on the volume of waste placed against a certain landfill contour. Information about the landfill lifts is entered into the GPS system, which feeds into a unit on the compactor. As the garbage is spread, the machine senses the difference in elevation between the original lift and the new lift. Through a monitor in the cab, the GPS system could be used to advise the operator that additional passes are required or that the optimum density has been achieved.

Although White does not currently use GPS technology, he thinks it would save money through higher compaction, but only when the value of the airspace increases enough to offset the cost. "It's all about airspace," White says. "And compaction is a function of how well you do your job."

Landfill compactor selection is a subject that may have more questions than answers. Many buyers are loyal to a manufacturer, while others may prefer a certain engine or drive type. A more general approach to landfill compactor selection can allow buyers to take a complete look at what each machine offers.

* Machine Size: Landfills should address machine size in terms of their tons per day (tpd). Many small sites (300 tpd or less) consider machines in the 50,000- to 60,000-pound class. However, such a selection often is based on the flow of trash coming in evenly during the day. In reality, most landfills experience two peak hours of delivery each day - one in the morning and one in the afternoon - during which approximately half of the landfill's daily tonnage (150 tons) will be delivered. A machine in the 50,000- to 60,000-pound class may be too small to properly spread, place and compact at a delivery rate of 75 tons per hour. Therefore, small landfills may choose to consider machines in the 70,000- to 80,000-pound class. The 100,000-pound class machines also are used on high-volume sites.

* Wheel Design: Primarily, buyers want a wheel that offers good wear life and minimal maintenance. While these are important factors, buyers also should consider the following questions:

1. How deep do the teeth penetrate? Depth of penetration allows a larger lift to be manipulated and compacted.

2. Will the wheel stay clean in my application? A packed up wheel limits a machine's ability to compact and impedes its tractive effort.

3. Does the wheel create an aggressive action? Aggressive action allows more trash to be compacted into each cubic yard of placed trash.

* Blade Choice: Buyers should consider what type of movement is required at their landfill. Most landfills use a straight, open-ended blade.

* Serviceability: Ease of access to components and the machine's ability to stay clean in its working areas are key considerations. Frequently, cleaning can cause more downtime than service and repair.

* Operator Comfort: A buyer should look for a comfortable and operator-friendly cab.

* Fuel Economy: Saving even two or three gallons per hour can save a landfill considerable money. Three gallons per hour over a machine's possible life cycle of 10,000 hours can save a landfill $30,000.

* Overall Design: Buyers should look for a machine that is designed to work specifically in the landfill environment. Is the design protective of components and yet serviceable? Will the machine's performance meet the level needed for the landfill?

Finally, ask for a demonstration, which really is the only way to see if a machine will work at your landfill.