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Every landfill operator in America is striving to extend the operational life of the landfill. By Joe Giebelhaus Intuitively, we all know that maximum site life is achieved by placing waste in the landfill at the maximum density. With a little experience, a little research, and no small amount of work, the City of Albany found that shredding MSW and construction-and-demolition (C&D) waste at the working face not only is feasible but also significantly extends the operational life of the landfill beyond what anyone could have hoped. One of the more popular methods we in the industry have been hearing about is the process referred to as biostabilization or operating the landfill as a bioreactor. The intent of a bioreactor landfill is to reverse all efforts undertaken to promote dry entombment of waste and thereby develop an environment where aggressive in-vessel, anaerobic decomposition can occur. The industry has several fine examples of the bioreactor process that concentrate on the careful application of liquids to the wastemass. The City of Albany, just like every other landfill owner, wanted to maximize the operational life of its current facility and began to look at the concept of bioreactors. Given that the basic premise is to decompose the waste buried in the landfill, the City of Albany believes the first step is to address not the issue of moisture content but rather the size of the material being decomposed. Falling back on firsthand experience with yardwaste composting, we know that volume reduction is the key to achieving rapid decomposition. The means to achieve volume reduction, just as with any yardwaste composting facility, is to shred waste as it is received at the working face. The facility is municipally owned and operated and has been receiving waste in one form or another since 1969. The facility was originally used as a large-scale, fixed-waste shredding operation. The shredded waste was used as a fuel source for an incinerator that in turn provided heat to the buildings of the state capitol. The landfill accepted ash, shredded waste, and unshredded waste as necessary. The incinerator was closed in the mid-1990s, thereby negating any need to shred waste. With the closure of the incinerator, the city has come to rely on landfilling as the primary means of waste disposal. Since 1991, 11 double-lined cells totaling over 46 acres have been installed. The facility accepts, on average, 1,000 tons per day of solid waste. Cover materials include approximately 215 tons per day of alternative daily cover (ADC) and 250 tons per day of petroleum-contaminated soil (PCS) and the facility expends approximately 3,000 gallons of a slurry-based spray-on daily cover (Posi-Shell) per day. On an annual basis, the facility accepts 330,000 tons of material. The city does not have additional acreage readily available for a horizontal expansion. Leachate is managed by a series of pump stations and is disposed of via a direct discharge agreement with the local sewer district. There is no cost for the disposal of the leachate to the city. The city has an active gas-collection system that supplies landfill gas to a small, private gas-to-energy operation. Approximately 3,000 cubic feet per minute of landfill gas are collected. Standard heavy equipment at the working face includes a CMI 35-C, a Caterpillar 836, a Caterpillar 973, and a Caterpillar D6. Supporting equipment includes a John Deere 200 LC excavator, a backup 826, and various wheel-loaders and site pick-ups.
In June 2003, the city was nearing a crisis of capacity. Given our historical waste densities of 1,700 pounds per cubic yard and our remaining capacity, the landfill would have reached its maximum capacity at the conclusion of that year. The next set of cells was not going to be ready to receive waste, but because of our commitments, the city could not turn waste away. We knew that applying leachate or water to the waste in place was not going to meet our timeline. We needed something else, and we needed it in a hurry. Falling back on institutional knowledge of large-scale, fixed shredders and yardwaste composting, the city tried to simply reduce the volume of waste via a track-mounted, low-speed, high-torque shredder. The city began by leasing a Doppstat Mammoth in October 2003. The unit is diesel-powered and simply top-loaded with an excavator. The shedder was radio-controlled so the operator in the excavator could increase or decrease mill speed, move the unit around the working face, or stop the engine in an emergency. The unit proved to be very reliable, with a minimum number of breakdowns. It is, however, a slow-speed shredder. The unit relies on torque to shred waste and is quite tolerant of fugitive wastes. Time trials had revealed that the Doppstat’s throughput was 94 tons per hour. But because of the cyclical nature of waste inputs throughout the day, the unit could not come close to shredding 100% of the wastestream. We found that minimal stockpiling of waste throughout the day was acceptable, but it was not getting us to our goal of 100% processing while maintaining some semblance of order at the working face. Roughly two-thirds of the waste accepted was being bypassed because the unit could not keep pace during a “rush” at the working face. Simply speaking, if three or four live-floor trailer loads arrived simultaneously, that volume of waste could keep the unit tied up for an hour. Were we successful? Yes. The city’s engineering consultant routinely conducts quarterly volume surveys. As such, any gains in capacity from changes in settlement rates, efficiencies of employees, or changes in waste characteristics are realized in that number. The city was historically achieving average waste densities of approximately 1,700 pounds per cubic yard. The density for the fourth quarter of 2003 jumped from 1,898 pounds per cubic yard to 2,566 pounds per cubic yard. Of course, that change in density was not solely attributed to waste shredding but the process did allow the city to bridge a “capacity gap,” allowing time while certification issues with the new cells were being resolved. Moving forward, we have been able to rely on an average density of 2,400 pounds per cubic yard, which yields a projected closure date of November 2009. That change in density allows nearly two additional years of operating in the same landfill cells. Obviously, with limited success, a dearth of hands-on knowledge, an acknowledgement of the difficulties that lie ahead with any onsite landfill expansion, the city wanted to look at expanding the process. Faced with another crisis of capacity at the lower compaction densities, the city had a projected closure date of January 2008. The slow-speed unit was in service for an entire year, slowly pushing the closure date into the future at a ratio of three to one. In plain terms, for every three months of limited shredding, the city yielded another month of capacity. In one calendar year, the closure date went from January 2008 to early May 2008. Although it provided excellent results, the city needed more time to develop more landfill capacity. The answer seemed to be found in a Diamond Z 1600 SWG. The unit was designed to be mobile and shred waste at a rate greater than 200 tons per hour. It definitely falls into the category of ‘high-speed’ shredders. It relies on rapid, repeated percussion of the waste to reduce its volume. It is a traditional tub grinder constructed to withstand the abuse that garbage causes on steel. The unit weighs over 80 tons. It produces over 1,200 horsepower from its two Detroit Diesel engines. The unit is track-mounted, radio-controlled, and built with easy access to essential compartments. During its acceptance trials, the unit processed over 217 tons down to 10 inches-minus in one hour with no downtime. Were we successful? Yes, again. We have continued to use quarterly surveys to track progress. Our average density has continued to demonstrate an increase from 1,700 pounds per cubic yard to 2,400 pounds per cubic yard. This once again pushed our projected closure date to November 2009. The survey data does reveal a unique anomaly. The second and third quarters of 2006 revealed a reduction of projected landfill life at the facility, yet the calculated density increased. Waste-processing tonnage remained constant throughout that period, although the application of some 30,000 additional tons of contaminated-soil cover materials consumed extra airspace. Because of its high density, it also served to significantly boost calculated overall density for that period. The areas where the additional cover was placed will be stripped in the coming months to recapture the airspace. Operations Obviously, an 80-ton piece of equipment spinning 26 hammers weighing 118 pounds each is going to produce some vibration. The vibration is not a huge problem for the individual components of the unit but rather for the entire unit itself. Without significant tonnage placed under the tracks, the unit will literally vibrate itself into the waste to the point where it can no longer function. Fugitive wastes can be a problem. Fire hydrants, pallet jacks, tires (with rims), propane cylinders, hazardous wastes, and Kevlar vests, to name only a few, have passed through the unit. The largest amount of damage from fugitive wastes resulted from a concrete-encased bollard that significantly damaged mill components. Largely, fugitive materials will pass through the grinding mill without serious damage to major components. The drive train is protected by a flex-coupler arrangement that will absorb a sudden stop caused by a jam. However, the conveyor belts that take the finished product away from the mill are prone to damage. More than once, a belt had to be replaced because a long (12 feet or more) piece of steel pipe plunged through the mill and pierced the belt. In terms of items specifically bypassed, any loads known to originate from hospitals, medical facilities, or nursing homes are routinely rejected. No matter the quality of waste control, the city does not want to endanger the health of an employee by taking an unnecessary risk of exposure to a pathogen by shredding the waste.
Explosions have occurred in the mill area without damage to the unit. We once had to shut the unit down because the movement of the mill caused an updraft and fed the flames. Also, items can be ejected with serious velocity from the discharge belt; more than one windshield has been replaced. Maintenance is a constant. They have many moving steel parts, bearings, belts, and hydraulic components that need constant attention. We have committed the operator and one mechanic to two hours of prechecking before the receipt of waste. The unit is shut down one half hour before we end operations for the day to review what will be done the following morning. Hammers and tips need to be inspected and tightened daily. Refacing hammers with a welder is another common task. At least one Saturday per month is donated to some maintenance-related activity as a result of the shredder. Diligence is maintained in cleaning belly pans. Shredded waste gets in every crevice of an engine compartment and will cause a fire. Of course, there is downtime. In the two years we have been operating the unit, the longest period for repair was six consecutive weeks (waiting for the repair of an engine component). We have found that the unit will run without major difficulty for five to six week periods, and then we will experience a week or two of troublesome problems causing intermittent shutdowns. As an owner, you must respect the abrasive and heterogeneous nature of solid waste and base your expectations of the shredder accordingly. Many believe that shredding is a waste of time and that compaction ratios will be just as high at the completion of the decomposition. This may prove to be true. However, shredding provides two benefits: greater densities are achieved on day number-one of waste placement by simply compacting a smaller volume of waste, and by providing a feedstock to the microbes in the wastemass that has a greater surface area, a more expedient bioreaction will occur, thereby promoting rapid settlement. The question is, will the settlement that takes place as a result of the expedited bioreaction occur regardless and will it occur in the operational lifetime of the facility? Our best answer is, no, the space is not likely to be recaptured within the operational life of the facility if shredding is not actively employed.
Given that we are trying to promote settlement via biostabilization, we need a means to capitalize on areas of the landfill that have settled. The most efficient means of recapturing that airspace is with a shredder. The processed waste is far more homogeneous and manageable on side slopes than nonprocessed waste. The shredder will be set in a location where the discharge belt will feed waste directly down onto the side slope that has available capacity. Obviously, with greater decomposition, we have greater amounts of landfill gas being generated. This should be a serious concern for anyone contemplating waste shredding. Historically, we could wait four to six weeks after waste placement to open new gas wells. Now, with shredding, they are producing methane at 50% or greater on the day they go into service. Perhaps the greater density does not allow ambient air to affect their performance, but, more likely, the faster bioreaction is driving methane production. We also expect our gas-production curve to be affected by the change in operations. Landfills are known to generate decomposition gases for a significant time period after waste placement. In terms of planning gas-to-energy projects, our gas production curves have traditionally indicated a 20-year life cycle for such purposes. Given the limited data available, the effect is unknown, but most assume the life cycle will be shortened. To illustrate the effect of shredding on gas production, cells that have not received shredded waste are collecting between 78% and 100% of the predicted production rate. We are collecting 274% of the expected volume of gas from cells that have been receiving processed waste. That alone is an indication that the gas-production model cannot account for the increased gas production but anyone considering such a move needs to be prepared for the additional influx of gas.
Economics The monthly revenue goal for the facility is $1 million. Since 2000, the facility’s gross revenue has averaged $11.7 million annually. In basic terms, applying the 3:1 return ratio (3 months shredding yields and additional one month of capacity) we can conclude that the city will have an additional two years of capacity. Although we cannot absolutely conclude that the influence of the shredder was the total cause, we definitely can conclude that, in basic terms, the gross earnings potential for the facility increased by $24 million (two years multiplied by $12 million per year yields $24 million). In a more accurate cost-benefit analysis, as indicated below, the entire capital expenditure (including shredder, excavator, and debt service) was $1,867,117. Total annual operating costs (including labor, fuel, and parts) are $486,955. Annual expenses, with capital expenditures amortized over 5 years, come to $831,139. We know that the volume of the landfill is fixed. Assuming we cease shredding immediately, and we apply a generous 1,900 pounds per cubic yard to the remaining capacity, we will be able to accept 1.2 million tons valued at $46 million. If we continue to shred and apply a 2,400-pounds-per-cubic-yard density, we will be able to accept 1.5 million tons valued at $57 million.
Conclusion The owner must view the landfill as a long-term revenue-earning asset and understand that the earnings potential of the asset is, relatively speaking, fixed. The owner must be realistic. The machine is being tasked with a very difficult assignment. The wastestream being processed often has components that may, in fact, significantly damage the unit. The owner cannot expect the unit to be operational 100% of the time. The owner must be prepared for the downtime and must remain focused on the goal of extending the life of the landfill. Bioreactors do work. The City of Albany’s experience has been that shredding waste as it is received at the working face is plausible despite the problems and effort. The prime indicators of a successful bioreactor—increased landfill gas production and rapid settlement—have been observed here. In the City’s case, shredding has stimulated the bioreactor process, achieving the goal of every landfill operator—that of extending the operational life of the landfill. Joseph C. Giebelhaus is solid waste manager for the City of Albany, NY. MSW - May/June 2007
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