May
2006
Re-Engineering
Waste
By Sangamithra Iyer |
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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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© STEALTH TECHNOLOGIES INC. |
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