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May 2006
Re-Engineering Waste
By Sangamithra Iyer


Anatamy of a Landfil

Growing up, I never thought much about dirt or garbage and never imagined I would pursue degrees in those subjects. I practiced civil engineering, a profession responsible for most of the infrastructure everyone uses, but often takes for granted—buildings, transportation, water supply and waste management. My interest was in soil behavior, more specifically, how the earth reacts to the pressures put on it. At first, for me, this was a technical question, but later it became a social one.

I consulted with municipalities who were seismically retrofitting their aging water infrastructure—reservoir embankments, pipelines and water tanks. This left a sense of social responsibility in that by rehabilitating these aging structures, public safety risks were mitigated and future access to water was ensured. But as I read more about water worldwide, and traveled to places where water was beyond scarce, I was haunted by one thing: why do we allow so much water to be wasted? In the U.S., about 40 percent of water used indoors is flushed down the toilet, more than 30 percent flows down the drain, laundry and dishwashing consume about 15 percent, leaks claim about five percent, leaving only about 10 percent for consumption. But our domestic usage is nothing compared to industrial and agricultural consumption, where water is vastly depleted and polluted. Why was it so difficult to get water in some places, yet so easy to waste it here?

I contemplated waste more when I became involved in the seismic stability of municipal solid waste landfills. I came to understand the regulatory framework of waste disposal. Prior to the Resource Conservation and Recovery Act of 1976, most landfills operated as mere dumping grounds. But with increasing federal environmental regulations and the adoption of stricter guidelines in 1991 outlined in the Code of Federal Regulations, Title 40 Subtitle D, municipal solid waste landfills were engineered to address the environmental and health concerns affiliated with them.

Anatomy of a Landfill
Subtitle D placed location restrictions on landfills near airports, floodplains, wetlands, fault areas and seismic impact zones, but didn’t consider the social impact of landfills on surrounding communities. Much of the public concern over landfills is about leachate, the liquid ooze that passes through or emerges from the decomposing waste. The liquids can be from percolating rainwater or the wastes themselves, but as they pass through the landfill, many organic and inorganic compounds like heavy metals may be transported with it. The leachate can contain dissolved or suspended material, and the health and environmental risks of the leachate migrating to soil and groundwater are due to the potential presence of pathonogenic microorganisms and toxic substances like ammonia.

To address this concern, design criteria requires the use of a composite liner system on the base and sides of the landfill to contain waste and minimize the transport of leachate to underlying soil and groundwater. These liner systems are typically comprised of a minimum of two feet of a low permeability soil like clay overlain by a synthetic geomembrane (think thick plastic bag). High-density polyethylene, HDPE, is most commonly used due to its strength and high chemical resistance. Having a composite lining system adds redundancy, but even with double measures, these lining systems are not perfect. Geomembranes are subject to defects during manufacturing, damage during construction, and long-term degradation. Leaking through underlying clay is possible through fissures and cracks in these soils.

Addressing leachate goes beyond lining systems, and incorporates the installation of leachate collection and removal systems to minimize the liquid accumulating on top of the liners. This usually entails a drainage layer of granular soils above the liner systems containing perforated pipes sloped toward a sump, so leachate migrating its way down the landfill would drain to a specified location where it could be pumped out and treated. To minimize the liquid in the landfill, good storm water drainage is imperative to prevent rainwater infiltration.

In addition to liquid byproducts, anaerobic decomposition of organic solid wastes produces landfill gas, which mainly comprise of methane and carbon dioxide. Both are greenhouse gases, but methane presents an additional problem since it’s an explosive gas that could pose a threat if it migrates to an enclosed structure. Monitoring, collecting and processing landfill gas is important to prevent fire hazards and improve air quality. The methane may also be reclaimed as a fuel source.

In landfill daily operations, garbage is compacted and placed in the cells of a landfill. At the end of each day, a daily cover of six inches of soil is placed over the waste to serve as a deterrent to vermin and minimize odors. When a landfill is complete, a final cover system caps the waste to minimize the long-term migration of liquids through the closed landfill. This cap typically consists of a soil layer serving as a hydraulic barrier over the wastes, overlain by a geomembrane and covered with an erosion layer of vegetative soil. Once the landfill is properly capped and closed, it can be reclaimed as a park and public space, but groundwater and landfill gas should continue to be monitored.

The Ultimate Design
At first, garbage to me was just this input parameter in my analyses. What’s the unit weight? How do I calculate its shear strength? How does the liner system affect the slope stability? But soon, I began to question what garbage really was. How did it get there? Why is there so much of it? As landfill space and cells were filling up across the country, it didn’t seem like the best long-term sustainable solution for managing our waste, despite stricter environmental controls.

I realized that despite our best efforts to mitigate the problems associated with garbage disposal, it didn’t get rid of the problem in the first place. Eighty percent of U.S. products are used once and discarded. Thirty percent of landfills is packaging. Sixty percent is compostable. Only five percent of plastic is recycled, while two-thirds of glass and half of aluminum cans get trashed. Why are we throwing so much out, and why is it all mixed together?

We live in a disposable society paying little attention to the fate of our discards. As engineers, we create systems that accommodate this level of consumption in the short-term, but our consumption habits and discarded products are being exported to places with no such minimal environmental controls. Manufacturers get to flood the markets with disposable, toxic products, as citizens bear the environmental, health and financial burdens of waste.

Landfilling is an evolving yet imperfect science. While there is a place for technical solutions, we need to recognize that garbage is not a technical problem, it’s an ethical one. Just because we can design systems to contain our waste for now doesn’t mean we can continue to generate so much of it indefinitely.

We need to redesign our lives to eliminate waste before it happens.

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