If you have ever watched someone load a moving van, you've noticed that they pack as much material as possible into a limited space. Every nook and cranny is full by the time the vehicle is ready to go because a partially full van costs the moving company money. This “airspace” philosophy also holds true for landfill operations.
From the moment a company decides to build a landfill, it incurs high capital costs for acquisition, permitting, development, closure, capping and monitoring. A site owner can recover some of those costs by filling the landfill's airspace with waste. Profitability depends on how much tonnage can be disposed of in the airspace.
How do you successfully pack a variety of garbage into a finite area using people, heavy equipment and technology to get a return on your investment? The answer: effective compaction.
It can be said that the landfill mantra is “push, spread, compact and cover.” How much waste you push, what kind of waste you push, what you push it with and what you cover it with affects compaction.
Inadequate compaction shortens the life of a landfill and increases operation costs. Reduced site life means the waste must be taken elsewhere or perhaps that a new facility must be built.
Here are some key issues to examine when addressing landfill density. Each landfill is different, so how you apply these tips may vary slightly.
You've probably heard the expression, “What weighs more: a pound of feathers or a pound of lead?” They both weigh the same but consume different volumes (airspace). You can't compact lead, but you can compact feathers. Such is the strategic thinking process of the landfill operator.
Waste mix is one of the bigger factors affecting density. Knowing that each load of waste is different from the previous one, operators must learn methods to handle the different types of waste to get the best compaction. Republic Services generally mixes the waste together as it comes into the facility but some pre-processing aids compaction.
For instance, bulky items can present a challenge. Pre-crushing them in the turn-around area can make them easier to compact. Municipal solid waste (MSW) tends to compact tighter and rebound less due to its moisture content. Some landfill operators don't mix bulky items with MSW and instead place the bulky items in a different area. In the end, though, all types of waste consume airspace and compacting them requires careful attention. Material like concrete and large pieces of furniture can get caught up on the bottom of the compactor, hanging it up. Pre-crushing the bulky items and placing them in with the MSW can lessen the likelihood of hang-ups.
Thickness of Spread
Once the waste load is deposited at the working face, it is the operator's responsibility to push the load to the designated area and feather it so that it is spread at a thickness of 18 inches to 24 inches. If the waste is just pushed and deposited in a thicker mass, compaction is hindered. And spreading it any thinner is an inefficient use of time. Waste spread at the recommended depth should allow for sufficient penetration by the compactor cleats.
Slope of the Face
A slope ratio of 5:1 or flatter is most effective. Weight shifts on steeper slopes, forcing compactors to work harder. As they work harder, the wear and tear on the machine increases, translating into increased maintenance costs. Steeper slopes also are more difficult to place waste against.
Size of the Compactor
The weight of compactors ranges from around 50,000 pounds to nearly 125,000 pounds. It's a pretty simple formula: the heavier the machine, the more horsepower needed.
In order to maintain a steady flow of customers, select the right sized compactors and the correct number of them to match the waste flow. Your estimated daily tonnage will dictate the type and number of compactors.
An underweight compactor will not transmit enough weight to its wheels to compact effectively. When you have the right compactor, you must provide your crew with training. A high-end piece of equipment that is not used effectively and efficiently has a negative impact on production.
Compactors have not changed in general concept in the last 30 years. What has changed is the size, dependability, efficiency, serviceability and operator comfort. The machines that a site selects are driven by a number of factors such as dealer support, national accounts, parts availability, purchase price, resale value and quality.
Processing the waste can involve more than just the compactor. A small site can push, spread and compact with a single compactor. Medium and large sites with more trucks should use the bulldozer for pushing and spreading to allow the compactor to spend more time making the machine passes. When the compactor has to stop packing to come into the turn around area and push waste piles, there is no compaction at the working face.
Compactor teeth/cleats perform two functions. They assure that the machine gets reasonable traction in the waste, and they chop/break/cut up the waste into smaller pieces and weave the wastes together. This contributes to the compaction and densification of the material.
A cleat length of 7 inches to 8 inches is standard. It's time for new wheels when cleat height is five inches or less. Wheels must be replaced four at a time, and a new set costs about $35,000. Furthermore, a set lasts only two to three years.
It's important to measure wheels and cleats after every 1,000 hours of operation and to rotate the wheels from front to back. The cleats all wear differently.
Maintaining and extending cleat life also is accomplished by reducing constant travel over hard dirt, concrete and gravel surfaces as much as is practical. The cleats will wear out quickly enough while traveling over solid waste, construction and demolition debris, sludge, and other common softer landfill materials.
Along with saving time and money on maintenance, clean wheels are a primary factor in assuring traction at landfills. Clogged wheels reduce traction and prevent compactors from operating effectively.
There has been much debate about the number of passes over the waste that a compactor should make. At Republic, our guidance is to make four to six passes before adding additional lifts of waste over an area and to repeat this process until the predetermined fill area plan, desired grade or daily receipts are completed. We have some sayings in the business: “Walk the cleats out of the waste.” This means compacting the waste so tight that the cleats ride on top of the waste like soccer/football cleats do on concrete.
Quantity of Daily Cover
Minimize use of soil as a daily cover to the extent that you can. Six inches of dirt is required by Subtitle D. However, alternative daily covers (ADC), such as tarps, foams and certain kinds of waste material, can be used instead to maximize density. The airspace consumed by six inches of dirt over a working face, when added up over the years, can drastically reduce the life of a landfill.
However, not all ADCs are appropriate for all landfills. They have their limitations based on different climates, levels of rainfall, wind conditions and so forth. All ADC needs to be analyzed for its particular application at each site.
Achieving good compaction can considerably extend the landfill's life and save the landfill operator a significant amount of money. Each landfill is unique in its tonnage rates, equipment fleet, tipping fees and many other factors that impact its profitability. The cost of the space is tremendous and extending the life of the landfill a couple years can save a lot of money.
Compressing waste material to fit the maximum amount into limited airspace may sound easy. But to do it successfully and increase the life span of a landfill takes a balance of man, machine and science. Often overlooked is the importance of a trained and seasoned team of professional operators who constantly challenge themselves and their teams to improve the process of “push, spread, compact and cover” on a daily basis.
Dave Hildreth is the senior manager of operations support for Phoenix-based Republic Services.