THERE'S MORE TO CONSERVING landfill space than just driving over the trash. Compaction is a word that is often used in the landfill industry, but it is rarely clearly defined. The loose interpretation of the word leads many industry professionals to confuse compaction with compression. Even Webster's Dictionary provides little clarification, with its definition of a compactor as “a device that compresses trash into small bundles.”
The difference between compression and compaction is the ability of the affected material to maintain its reduced volume when not restricted by devices such as ropes or bands. Compression temporarily reduces the volume of material by eliminating air voids, but the retention of mass requires work from an outside force. Once the outside force is removed, the trash will spring back to nearly its original size.
For many years, compression was used to achieve a certain amount of density in landfills. However, research on soil and asphalt compaction indicated that compression techniques do not work effectively in landfills because the space trash consumes must be reduced without permanent devices. On the other hand, compaction breaks down material and manipulates particles into a configuration in which the reduced volume is maintained. Proper compaction techniques help to maximize landfill space.
Finding the Solution
Many of the concepts applied to maximizing landfill space have resulted from the soil and asphalt compaction research. Soil and asphalt compaction was performed for many years using static rollers, until it was discovered that no matter how heavy the roller, the lack of deep penetration of compression forces into the material prevented the particles below the surface from forming the optimal density configuration. Even with proper soil and asphalt mix designs and lubricants, the application of static weight, regardless of how heavy, did not create the desired compaction. The industry discovered that rather than pressing particles together, the particles must be manipulated to fit together to use the least amount of space.
When exploring material sizing, material lubrication and particle rearranging for soil and asphalt compaction, vibratory rollers provided a solution. However, unlike soil and asphalt, waste absorbs vibration. Thus, the compaction method for landfills had to be altered and a unique machine evolved.
To properly size material, trash must be broken down into smaller particles. The effect this has on landfill density can be seen in what is sometimes referred to as the “potato chip theory.” When you buy a bag of potato chips, the bag is full. Smash the bag a few times, and the chips only take up about a third of the space they originally occupied. Breaking materials into smaller particles allows the pieces to fit together more closely by reducing the air voids between them. This concept is key in landfills.
Trash particles must be broken down to reduce air voids and allow the particles to fit together in a tighter configuration. A machine wheel with the appropriate tooth pattern and design is critical to reducing particle size in landfills. A wheel with an aggressive tooth design shreds material and breaks it down into smaller particles, maximizing the amount of material in a limited space.
In addition to proper equipment, waste spreading techniques affect particle breakdown. Material should be spread as thinly as possible, preferably 2 feet or less in depth. Each layer should be compacted with at least three to four passes, more if time allows. The combination of material spreading, tooth design and repeated passes ensures proper sizing and maximizes space.
Waste's moisture content affects its compaction. Proper material lubrication helps particles slide together and eliminates air voids. Most landfill professionals can attest that dry conditions inhibit compaction, while a light rain can have the opposite effect. Municipal solid waste usually has a fair to good moisture content, but the addition of a very dry material can make waste difficult to compact.
On the other hand, too much moisture poses a challenge for some landfill equipment. Because trash compaction is a daily necessity in landfills regardless of the weather, a machine that can operate — and effectively compact — in all conditions is essential.
A machine's wheel design should include an optimum number of teeth to afford the best tractive effort in heavy rain or snow. Also, mechanisms to keep the wheels clean, such as scraper bars, can prevent the wheels from becoming clogged and reducing traction.
Various methods for introducing moisture in dry conditions are available. Recirculating leachate through the newly applied layers of trash is one option. While this may not be allowed in some areas for environmental reasons, the technique reduces the leachate requiring treatment and significantly increases daily compaction densities.
Sludge also introduces moisture. When working with sludge, it is important to use a compactor that can operate in the sludge and mix it in. Simply pushing dry material over the sludge causes “bridging” of layers of dry material and does not use the sludge to fill and lubricate the voids, which thus reduces the overall compaction.
Once particles are demolished to the smallest possible size and lubricated properly, moving them together into the least amount of space becomes easier. As the compactor teeth continuously go over processed material, effective manipulation and particle arrangement occur to ensure good compaction.
Again, the more passes that are made over the trash deposits, the better the final result. More passes give the machine a better chance to work the material into a tighter configuration.
Good compaction is a goal that operators must strive for. Proper operating techniques, material preparation and appropriate equipment all are necessary to manipulate trash particles to achieve the best possible compaction.
Mickey Cereoli is national sales support manager for landfill and stabilization for Bomag Americas, Kewanee, Ill.