This landfill reclamation will pay for itself over time—and the project will also reap non-monetary benefits.
The City of Clovis, CA, owns and operates a municipal solid waste landfill that has been in operation since 1955. In the early 1990s the city was faced with several problems at the facility. The landfill was projected to reach capacity in less than 25 years, volatile organic compounds had been detected in the groundwater, and the facility was running out of soil necessary to support the operation. The older, unlined portion of the landfill was determined to be a source of groundwater contamination, and due to the configuration of the site, it was an obstacle to expanding the facility.
In 1996 the city contracted with a consultant to prepare a feasibility study to determine how the city should dispose of its solid waste and mitigate the problems at the landfill. The alternatives for disposing of the city’s waste were to improve, expand, and continue to operate the existing landfill, or to close the landfill and haul the city’s garbage to the county landfill, which is located 35 miles away. Numerous alternatives for mitigating the groundwater issues were evaluated, and potential sources of soil were investigated. The study recommended a project that would entail excavating and sorting the waste that had been buried in unlined areas of the landfill, placing the sorted waste in the lined portion of the facility, and stockpiling the sorted soil for future use. The study determined that such a project would provide space for the construction of lined waste management units in the future, would mitigate the groundwater contamination by placing all of the waste material on a modern liner system, and would provide the soil necessary for continued operation of the facility.
The city prepared the necessary environmental documentation and received approval for the project from the regulatory agencies. The project began in August 1998, and is projected to be completed in late 2008. When completed, the project will have excavated and sorted approximately 2.5 million cubic yards of waste and soil.
Process and Equipment
The project consists of three basic operations: excavating the buried waste and moving it to the sorting equipment, sorting the material to separate the trash from the soil, and transporting the soil to the stockpile and the trash to the active face of the landfill for disposal. Different combinations of equipment and methods have been utilized for each operation at various times during the project as the overall process has evolved and been refined.
Three different equipment/methods have been used to excavate the material and move it to the sorting equipment. First, an excavator (Caterpillar 225) was used to excavate the waste and place it directly into the sorting equipment. The excavator worked well at excavating and placing the waste in the sorting equipment, but it is an inefficient way to move material any distance. This required very frequent repositioning of the sorting equipment, which resulted in significant downtime and an overall loss of productivity. Next, a bulldozer (Komatsu D155) was used to excavate and push the material into a combination trap/belt loader (Kolberg 404) that moved the material to the sorting equipment. The dozer was efficient at excavating and moving the material, but the trap and belt loader frequently clogged or broke due to the wide variety in size and weight of the material that was being moved. Since 2005, the project has used a larger bulldozer (Caterpillar D9) to excavate and push the material to a stationary excavator (Link Belt LS-4300) that picks up the material and places it in the sorting equipment. This combination has been found to provide the greatest productivity and reliability, even though it requires two expensive pieces of equipment and two operators. The excavation slope is maintained at 3:1 (horizontal:vertical), and the excavator only moves when the sorting equipment is repositioned each year.
Two different types of sorting equipment have been tried, shaker screens and rotary trommels. Two different shaker screens were used; one that included two stages with different size screens, and another that incorporated a long train of screens of equal size. Of the two, the second worked better, but neither was really satisfactory. The first was prone to frequent clogging of the space between the two screens, and both allowed too much soil to remain with the trash. Three rotary trommels have been used on the project. The first (Powerscreen) was prone to excessive clogging at the hopper. The second trommel (ReTech Magnum) had a better hopper design and proved much more reliable. The current trommel (Re-Tech Olympian II) is made by the same manufacturer as the second, but it is larger and was selected to increase the production rate. The trommel utilizes a 2-inch-square screen size, and incorporates two belt loaders for the sorted material, one to lift the trash and another to lift the soil.
Excavation and Sorting Operation
Several methods have been used to transport the sorted trash and soil. Originally, trucks (40-cubic-yard roll-off box trucks) were used to transport the trash to the active face of the landfill, and scrapers (Caterpillar 623) were used to transport the soil to a stockpile, but the scrapers proved to be inefficient. Conveyors (various 30-inch and 36-inch wide units) were then tried for transporting and stockpiling the soil. The conveyors proved successful, but the desired height and slope of the stockpile led to frequent breakdowns of the conveyors. The conveyors are currently used to transport the soil a distance of approximately 800 feet to the stockpile area, where a bulldozer (Komatsu D155) pushes the soil into a higher and steeper mound. The “soil” still contains waste material that passed through the 2-inch screen, so it is still classified as a waste. It has been approved for use as cover material, but it must be stockpiled in an area approved to accept waste. The soil is stockpiled in the area where waste was previously excavated. Even though this area is unlined, the regulatory agencies allow the material to be placed there because it is a historical waste area.
The conveyors were also investigated for transporting the trash to the active face of the landfill, but they were not suitable for the wide variety of material, and the string of conveyors disrupted traffic patterns for the incoming refuse trucks. The 40-cubic-yard rolloff trucks (Peterbilt 320) have proven to be the most feasible means of transporting the sorted trash; the boxes remain on the trucks, so they are essentially used as 40-yard dump trucks. Currently, two trucks are used to transport the sorted trash approximately 1,500 feet to the active face of the landfill where it is dumped and compacted.
There are two other pieces of equipment that support this operation. A skid-steer loader is used to pick up the trash and soil that “spills” around the trommel; and a diesel-powered generator provides the power for the string of conveyors.
Personnel
A total of seven personnel work on the project. Three equipment operators operate the bulldozers and excavator, one person operates the skid steer loader, two persons drive the trucks transporting the waste to the active face of the landfill, and one person is assigned to litter control around the excavation site.
Productivity and Costs
At the time this report is being written, the project has been in operation for more than 9 years, and has excavated and sorted a total of approximately 2.3 million cubic yards of buried waste and soil to date. However, production has not occurred at a steady rate throughout the project’s duration; changes in equipment occurred during the period that affected the output, and issues with contractors resulted in a few interruptions in the work.
At the time this project began, there was very little information available regarding similar projects that involved excavating and sorting buried waste. The City of Clovis had no experience in this type of work, nor did the regulatory agencies, consultants or contractors. The productivity expectations and projections were largely based on more common mining operations. Waste material has proved much harder to sort, and the actual productivity has been much lower than originally estimated in the feasibility study. The study anticipated that 4,500 cubic yards of material would be excavated and sorted per day, whereas the actual sustainable production rate has proven to be less than one third of that figure with the current operation.
The current combination of equipment and methods, which has been in place since 2005, has provided the most efficiency and consistent productivity of the different processes that have been utilized on the project. This operation has excavated an average of 1,300 cubic yards per day of operation, and has operated 60% of the working days. This has resulted in an annual production of approximately 200,000 cubic yards of excavated and sorted material.
Maintaining consistent production has proven to be a significant challenge; downtime has exceeded projections. Downtime has been primarily due to three factors: weather, equipment malfunctions, and the need to reposition the trommel and conveyors as the excavation progresses. During significant rain events, the operation is shut down because the wet material clogs the trommel. Earlier in the rainy season, the operation is shut down only while it is actually raining, and maybe for one day after it stops. As the season progresses, the material on the excavation face absorbs more and more water and takes longer and longer to dry out. Late in the season, the operation has been shut down for up to two weeks after a few days rain.
Sorting the waste is hard on the equipment due to the wide variety of materials that must be handled, which includes everything from concrete rubble to the ubiquitous polyethylene grocery bags. The heavier and harder debris damages the screens on the trommel, requiring that they be replaced every one to two years. It is also necessary to closely monitor and control the rate at which material is fed into the trommel to prevent it from clogging.
The trommel and conveyors must be repositioned periodically as the excavation progresses. Clovis has found it beneficial to reposition the equipment on an annual basis at the end of the rainy season, giving the site time to dry out before restarting the operation. The city also takes advantage of this period of downtime to perform significant repairs and preventative maintenance on the equipment.
The unit cost with the current operation is approximately $5.10 per cubic yard of material that is excavated. This cost includes the excavation and sorting of the waste, transporting the soil to the stockpile, and transporting the trash to the active face of the landfill. It does not include the disposal cost of the sorted trash.
Health and Safety
A health and safety plan was developed prior to beginning the project to ensure that the personnel are not exposed to unnecessary risks or hazards. The risks associated with this project include those that are commonly associated with any similar excavation project, plus certain other hazards that are associated with the excavation of partially decomposed municipal solid waste. Most notably, these include exposure to asbestos and volatile organic compounds (VOCs), the potential for methane gas at explosive levels, the potential for uncovering hazardous materials, and the potential for spontaneous combustion of decomposing waste once it is uncovered.
All personnel working on the project have been trained regarding these potential hazards, and they regularly monitor for their presence. The operators of the bulldozer and the excavator monitor the waste they excavate for any visual or odorous indications of fuel, chemical wastes, friable asbestos, or other hazardous materials. Work is suspended if any suspicious materials are encountered until the materials are identified and removed in an appropriate manner. Work is also suspended if any significant amount of dust is produced and water is applied to the excavation area before resuming. The excavation face is monitored every two hours using a photoionization detector (PID) and an oxygen/lower explosive level (LEL) meter. Work is stopped if the PID shows that VOCs are present at levels greater than 5 ppm, or if the LEL meter detects methane at a concentration of more than 1% or the oxygen level is below 16%.
Fortunately, the Clovis Landfill has only accepted waste from within the city limits, and there are not any large industrial operations in the city that produce significant quantities of hazardous wastes. Almost all of the waste that is being excavated originated from residential or commercial sources, so the types and quantities of hazardous materials that have been encountered are limited to what is typical of household hazardous waste. Since the project’s inception, the VOCs have never exceeded 5 ppm, nor has the methane ever exceeded 1% or the oxygen concentration been below 16%. The material also has sufficient moisture content to prevent the generation of significant dust. Periodically, a smoldering fire has developed due to the spontaneous combustion of waste that has recently been uncovered, but these have been easily addressed by using the bulldozer to spread the material and a water truck to extinguish the fire.
When the regulatory agencies set conditions for the project, they did not require that the excavation face be covered at the end of each day. Currently, the open excavation face measures approximately 12 acres. It is notable that the open excavation of this landfill has not led to significant nuisances, probably due to the climate. The area receives approximately 12 inches of rain per year, almost all of which occurs during the colder winter months, and the summers are hot and dry. When the material’s moisture content is conducive to nuisances and vectors, it is too cold, and when the temperature is conducive, it is too dry.
Odors are noticeable immediately adjacent to the excavation, but they are not severe and do not usually migrate very far. Birds are not a significant problem at the facility, and any that are present tend to congregate on the active face of the landfill and not on the excavation face. Other vectors, such as flies and small mammals, also appear to be much less attracted to the excavation face than they are to the active face where the new trash is placed. Litter is periodically blown from the excavation face, but the quantity is easily collected by the one person assigned to the task except during periods of very high winds.
Costs Versus Benefits
The feasibility study identified the benefits of the project to be the ability to reutilize and expand the landfill to serve the city for a much longer period of time, the mitigation of the groundwater contamination, and the recovery of soil, which eliminates the need to import soil for ongoing operations. The study quantified the value of these benefits and found that they exceeded the estimated cost of the project. As the project is nearing completion, it is possible to re-evaluate the project based on the actual results and costs.
We find that the project has in fact achieved the goals for expansion, groundwater mitigation and soil production. At the end of 2006, the city received a new solid waste facility permit that provides for the expansion of the facility through the redevelopment of the old unlined portion of the facility. The city will realize almost 30 years of additional lifespan at the facility by reconstructing this area and operating it as a modern landfill. The VOC levels in the underlying groundwater have steadily decreased as the project has progressed, and projections show that the contamination will continue to attenuate, thereby eliminating the need for the city to perform costly remediation. Finally, the soil that has been recovered from the project, which equals approximately one-third of the material that has been excavated, is adequate to meet the facility’s operational needs for more than 20 years.
Unfortunately, we also find that the project has cost substantially more than was estimated in the feasibility study. In 1998, the project was expected to cost $3.8 million, whereas the actual cost of the project is going to be approximately $9 million. This increase in the cost is primarily the result of two factors; the quantity of waste to be excavated was underestimated and the daily productivity was overestimated.
The estimate for the quantity of in-place waste was based on operating records and exploratory drilling, and was originally 1.9 million cubic yards. In reality, the bottom surface of the waste differed significantly from the estimate, and the actual quantity of material that will be excavated and sorted is 2.5 million cubic yards, 32% more than the original estimate.
The feasibility study also estimated the daily productivity at 4,500 cubic yards, whereas the actual productivity only averages 1,300 cubic yards per day. This discrepancy has greatly increased both the time to complete the project and the overall cost.
Re-evaluation of the project using these actual costs shows that the project is just above the “break-even” point; the projected value of the benefits is only slightly higher than the actual cost. However, even this comparison is based on assumptions of future conditions and costs that could change significantly. If future regulations to control greenhouse gas emissions or energy prices were to make waste-to-energy more feasible during the next 40 years, it could render the expanded capacity at the Clovis Landfill unnecessary thereby reducing the value of that benefit.
On the other hand, it is just as likely that future deviations from the assumptions will actually improve the cost-benefit ratio. For example, one of the surprising findings of this project was how slowly the buried waste was decomposing in the landfill; partially burned newspapers from the 1950s were still readable.
It may become beneficial some day to implement a process to accelerate the decomposition of this waste, such as a bioreactor landfill, in order to greatly shorten the post-closure monitoring period. Having all of the waste on a liner with a leachate collection and removal system may make such a process much more feasible.
One of the challenges of managing any capital improvement like a landfill is the need to make sound decisions that are based on long-term assumptions and projections about the future. The actual cost of a project may be significantly different than the estimate, and the actual value of the project may not be known or realized for years, or even decades. Clovis’ landfill reclamation project has been a relatively long and costly endeavor that is expected to pay for itself over time, but the project will also reap non-monetary benefits. The value of public perception, environmental responsibility, and peace of mind are not easily quantified. Regardless of what the future holds, it is doubtful that 40 years from now anyone that has been involved with this project will look back and regret the decision to move all of the waste at this landfill onto a liner.