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A look at machines and techniques, management and technologies.
The United States Army is generally credited with being the inventor, during World War II, of the modern sanitary landfill. Historically, large armies encamped for extended periods of time had alarmingly high casualty rates (even when compared to combat troops) due to poor sanitation. The US Army addressed the issue by designating sites, remote from where the troops lived, as “dump sites” to which waste was transported, dumped, spread out, and covered with earth. A lot has changed since then. It is the nature of gradual change that there appears to be no change at all. We still take our waste away from the urban centers to a remote site where it is dumped, spread, and covered with earth—or at least that’s how we dispose of 57% of the waste we create in North America. It may sound similar to Army practices during World War II—and from a distance it may even look similar, though the differences are profound. Many factors contribute to successful landfill management: the geological make-up of a site, landfill design, liner technology, type of waste, and water content, to name a few. But one of the more common misperceptions is the general idea that solid waste management is a simple, low-tech, unskilled industry. In reality nothing could be further from the truth. There are many issues to be resolved in running an efficient, modern sanitary landfill. This article deals with four of them:
High Densities The “onionskin” approach—Spreading the waste in thin layers improves the crushing action of the compactor and permits increased machine travel speeds due to reduced rolling resistance. Studies have demonstrated up to an 8% increase in machine coverage on a 2-foot layer versus a 3-foot layer. Thin layers benefit the site operator in four direct ways:
Multiple compactor passes—Waste becomes compacted when a force (the weight and motion of the landfill compactor) moves over it, crushing out the air voids, shredding the material, and binding it to other waste. By running over the waste in one direction (one machine pass) and returning along the same tracks (a second machine pass), waste rebound is reduced as the structure of the waste breaks down. Studies have shown that three to five landfill compactor passes are required to break down the structure of North American waste. Operating the landfill compactor in straight-line passes is a good practice because it’s easier to keep track of where the machine has already been and it reduces the likelihood of wheel spin. Wheel spin occurs most often when the compactor is turning, trying to gain traction. As a wheel spins it fluffs up the waste, defeating the desired compaction results. Operating in patterns—To achieve uniform compaction, operators should run the machines in a structured “pattern,” which is defined as a “set sequence of operations.” This set sequence, either individually or as part of a multiple machine system, should cover the entire area before a new layer is added. Start compacting on one side of the working face—the left side, let’s say—making one machine pass straight forward and a second straight back, moving over one wheel width and going straight forward and reverse, then moving over one more wheel width and repeating this process until you reach the right side of the working face. This technique ensures total coverage of the waste layer (except the first two wheel widths) with four machine passes. It’s an easy method to keep track of where you’ve been, and it avoids making deep wheel ruts that could cause the machine underbelly to drag. This type of organized approach reduces confusion and cycle times while yielding more complete compaction and fewer soft spots. Soft spots are a bad thing for a number of reasons: They are a safety hazard for machinery; they rob the landfill of airspace; and they cause differential settling later on. It is particularly bothersome to take on these additional expenses at a time when the site is generating no revenue. Compactor wheels and cleats—The work of a landfill compactor is achieved by repeatedly running over the waste, allowing the cleats on the wheels to rip up, tear, and shred the waste mass, making it more homogeneous in size. The cleats serve three primary functions: They provide traction to power the machine through the waste; they apply a tearing/ripping action to the waste; and they provide side=slope stability to the machine. It is the weight of the landfill compactor that crushes out the airspaces and fills the voids, making the waste mass denser. When it comes to selecting landfill compactor wheels and cleats, it helps to remember a couple of things. One is that the cleats are on the wheel so the compactor can get traction and power through the waste underfoot. It is the weight and technology of the machine and repetitive passes that cause the waste to become dense, not cleat penetrations. Therefore, the cleats should be patterned on the wheel to provide traction but spaced far enough apart so as not to clog and lose traction. And, often, there are large cleat holes on the final layer of waste. Many site operators are instructing their tractor operators to make one final pass over the compacted waste to “track it in” before applying cover. The tractor’s final pass will fill in the cleat holes, smoothing out the top surface, helping to minimize the amount of cover materials used. Management Framework The financial landscape—The up-front infrastructure costs of a modern sanitary landfill quickly get into the millions of dollars. Once the landfill is constructed, the largest single variable cost is the procurement and operation of a sizeable fleet of equipment. The main types of equipment used in landfill management are as follows:
Waste applications are extremely hard on heavy equipment. One management tool that has grown in popularity over the past decade is the total maintenance and repair (TM&R) contract. TM&R contracts with the equipment provider have become common, if not the norm, for the industry. This allows the landfill management team to focus its talents on the primary goal of an environmentally compliant operation and let the experts worry about routine maintenance, oil sampling, scheduled machine rebuilds, and warehousing of spare parts. It also means maintenance costs can be managed through negotiation and budgeted well in advance. Fill it to the brim—The fill sequence is extremely important and a key management requirement. Occupying available airspace in the most efficient manner involves multiple variables. The fill sequence should minimize costs related to dozing distances and to hauling distances for cover material. The fill sequence should provide for the construction of high areas for use during rainy seasons, while ensuring that internal slopes are maintained. In this way, surface water flows away from the waste mass, thereby avoiding the drama of machines and men working in a standing pool of polluted water, which is hard on both. Minimize the need to handle material twice. The fill sequence should be managed in such a way as to maximize the distance between the active working face and the property boundaries. Create as big a “buffer” between operations and neighbors as quickly as possible. Constant management vigilance is required to avoid costly mistakes like the rehandling of waste or inefficient use of cover material. Technology Tools The Caterpillar CAES system is accurate to plus or minus two centimeters. The machine operator has a display screen in the cab that tells him when the machine has achieved maximum practical compaction. In a track-type tractor, CAES enables the operator to know when he has spread the cover soil to the exact specification. Methane headers, flare vents, drainage pipes, and other obstacles are clearly identified on the screen, eliminating the risk of accidentally running over and destroying them. Operators can also use the system to survey special waste loads disposed of onsite to meet contractual obligations. This system virtually eliminates the need for a surveyor (either staff or contracted), as site management can now complete most survey tasks using existing personnel. Machine Selection Standard machines will not survive on the landfill. Because it is such a destructive application, machines going into waste should be specially guarded. Some of this guarding may be invisible to the naked eye but is essential to machine life. Specialty guarding—such as Kevlar seals on final drives and idlers, heat shields on the engine manifold, reversible fans, and heavy-duty handles and latches—adds to the initial price but pays off by keeping machines running effectively. Track-type tractors are for dozing; landfill compactors are for compacting. The more this principle can be applied in daily operations, the better the results will be from both a production and cost standpoint. Working together, tractors and compactors can accomplish more as a team than as independently working units. Type loaders are extremely versatile and can be used in many varied waste applications. Landfills of 300 metric tons per day or less should seriously consider a track loader, which could be the only machine needed. Versatility and work-alone capability also make track loaders attractive machines for larger operations. A final good tip is to visit other landfills, observe the equipment being used (and that not being used because it’s inoperable) and ask the manager for an honest appraisal of machine performance. Learning from our own mistakes is a sign of intelligence; learning from mistakes of others is a sign of wisdom. Be wise: It’s less costly! William DeBord is senior waste market specialist with Peoria, IL–based Caterpillar Inc. MSW - March/April 2007
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