The Air Up There

AS ORGANIC LANDFILL MATERIALS decompose, landfill waste shrinks and settles, adding valuable new airspace that can extend a disposal facility's life while generating millions of dollars in unanticipated revenue. Until recently, no one could figure out how many years and how many millions could be gained.

But a study conducted by SCS Engineers, Long Beach, Calif., and Essentia Management Inc., also in Long Beach, has begun swapping data for rough estimates.

According to the companies, six mechanisms cause landfills to settle: primary compression, organic materials biodegradation, physical creep, metals corrosion, the interaction of acids produced by decomposition with corrosive materials, and consolidation caused by pressure that squeezes water from pore spaces in certain soils.

The first two are the most important. “Primary compaction and biodegradation account for almost all shrinkage in a landfill,” says Michael Leonard Sr., senior technical manager with SCS. Primary compression can range from 10 percent to 20 percent of the volume of new materials added to a landfill, Leonard says. Over time, biodegradation can double or triple shrinkage obtained from primary compaction.

While biodegradation causes the most shrinkage, it is also the most variable and difficult to calculate. Measuring decomposition always has been easy, but computing how much will occur in coming years has flummoxed engineers.

“Biodegradation can cause a landfill to lose 30 percent to 60 percent of its volume,” Leonard says. “The low end is when you have a lot of construction debris that won't degrade. The high end applies to landfills with a lot of green waste, food and other organic matter.”

In the early 1990s, Leonard and Kenneth J. Floom Jr., a project manager with Essentia, set out to find a way to predict decomposition by studying shrinkage at a closed Superfund site in Monterey Park, Calif., owned by Operating Industries Inc. (OII).

The two engineers began by comparing topographical maps of the surface and the bottom of the facility to estimate the depth of the waste mass. Next, they calculated a curve representing the volume of methane gas that the site would produce during the next 40 years. From records, they established a percentage representing decomposable organics by weight within the waste mass. Then, they created a settlement factor from the ratio of future gas generation to total gas generation.

Using this data, they posited that future settlement could be determined by multiplying the thickness of the trash by the percentage of decomposable organics by the settlement factor.

They carried out the math for each of the OII landfill's grids and checked their conclusions against actual OII shrinkage. The results were encouraging. On average, actual settlement totaled 10 percent less than the projections. Leonard and Floom attribute the difference to something they call a bridging factor. “In a landfill, there are pieces of metal and stone that do not decompose,” Leonard says. “These materials support surrounding waste like a bridge and reduce shrinkage.”

The cost of setting up the tool, including mapping the facility and collecting the data, came to approximately $30,000. Once set up is complete, Leonard says it costs approximately $5,000 to apply the tool.

Leonard and Floom tested the model at the El Sobrante Landfill in Riverside County, Calif. The landfill is operated under a public-private partnership agreement between the county and Houston-based Waste Management Inc.

El Sobrante's permit specified a capacity of 8 million tons, a number that will be reached by 2004. In 1995, the owner asked Leonard to project remaining airspace. As a first step, Leonard checked the state regulations and found that landfill volume estimates were permitted to incorporate settlement estimates.

Like many states, California requires landfill owners to conduct annual aerial surveys and produce topographic maps illustrating remaining airspace. Using that data and the settlement formula, Leonard estimated that the facility could handle 10 million tons of waste, two million more tons than its permitted capacity.

Because the landfill was taking in 250,000 tons per year, the numbers suggested that shrinkage might ultimately extend El Sobrante's useful life for an additional eight years. Equally important, the additional 2 million tons of waste potentially would generate 2 million tons worth of revenue.

However, that's not quite how it worked out. In 1997, Riverside County and Waste Management applied for a permit to expand El Sobrante to a 90-million-ton facility. Leonard suggested using the settlement model to check the new capacity estimate. The owners agreed, and the calculations suggested the new landfill space could in fact accommodate 110 million tons of waste. At $40 per ton for 20 million additional tons, the new specification raised the revenue prospects of the expanded El Sobrante by $800 million.

While attention has been largely focused on El Sobrante's expansion, Leonard says that the original landfill cells, which have been filled and closed, did indeed accommodate a total of 10 million tons of waste, two million tons more than the original permit projected and about what the settlement model had predicted.

According to Leonard, the tool can tighten up forecasts for existing landfills, project more accurate life spans for new landfills, and even assist in the timing of opening new cells.

Leonard and Floom presented their paper, “Estimating Method and Use of Landfill Settlement,” at SWANA's WASTECON trade show, held Oct. 13-16, 2003.