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#119 - Leachate Collection Systems: The Achilles' Heel Of Landfills, 06-Mar-1989

[See PDF format version for diagram of landfill.]

A landfill is a bathtub in the ground, and a bathtub can leak two ways:
it can leak through a hole in the bottom (failure of its bottom liner),
or it can fill up with fluid and spill over its sides. Either way, it's
bad news. The basic problem is the fluid. If a landfill begins to fill
up with fluid, the weight of the fluid puts pressure on the bottom of
the landfill, increasing the likelihood of bottom liner failure, so any
fluid inside a landfill is a potential source of trouble.

To prevent fluid from causing problems, every modern landfill has a
system for draining liquids out of the landfill. This is called a
leachate collection system. What is leachate? Think of a landfill as
being like a drip coffee maker. The dry coffee is the garbage, the
water you pour in the top is rainwater, and the dark, brewed coffee
dripping out the bottom is leachate. You might want to drink coffee,
but you definitely do not want to drink leachate: it has many toxic and
dangerous characteristics. It is badly polluted with chemicals and with
micro-organisms (bacteria and viruses) that would make you sick.

The picture below represents a closed landfill; the heavy dark line
represents the plastic baggie (bottom liner and top cover) that is
supposed to keep leachate from entering the environment. The round
circles between the two bottom liners represent collection pipes which
have many holes drilled along their length (making these pipes resemble
a swiss cheese); they are supposed to collect any leachate that flows
to the bottom of the landfill. In theory, these pipes carry off the
leachate to a wastewater treatment plant, where the leachate is
processed to remove the toxic chemicals. (At the wastewater treatment
plant, some of the chemicals are released into the air, and the
remaining ones are collected [they're now in a mud-like sludge] and
they are sent to another landfill somewhere.)

One of the least-studied aspects of landfill design is how to make a
leachate collection system that will work for many decades (much less
many hundreds of years). The fact is, leachate collection systems can
clog up in less than a decade and, when that happens, fluids begin to
build up inside the landfill--a dangerous situation, as we have noted
above.

Leachate collection systems fail in several known ways. First, they can
clog up from silt or mud. Second, they can clog up because of the
growth of microorganisms in the pipes. Third, they can clog because of
a chemical reaction leading to the precipitation of minerals in the
pipes; anyone who has boiled a pot of "hard" water and seen the whitish
crusty residue in the bottom of the pot knows what "precipitated
chemicals" look like. Fourth, the pipes themselves can be weakened by
chemical attack (acids, solvents, oxidizing agents, or corrosion) and
may then be crushed by the tons of garbage piled above them.

The book, AVOIDING FAILURE OF LEACHATE COLLECTION AND CAP DRAINAGE
SYSTEMS, by Jeffrey Bass, discusses these four failure mechanisms. The
first problem (silt) can sometimes be avoided, or at least reduced, by
installing a "filter layer" above the leachate collection system. The
filter layer may be made up of gravel or of a rug-like plastic material
called "geotextile." Since the oldest leachate collection systems date
from the early 1970s, humans have very little experience with the long-
term performance of leachate collection systems. The hope is that
a "filter layer" will solve the siltclogging problem, but after many
decades the entire filter layer itself may clog. Only time will tell.

The growth of microorganisms seems to be an uncontrollable problem. The
conditions for growth of slime-forming microorganisms are not well
understood. Even if they were understood, we could not control chemical
and physical conditions (temperature, pH, etc.) at the bottom of a
landfill because of the thousands of tons of wastes heaped up in the
landfill.

The problem of chemical precipitation also appears to be
uncontrollable. The chemical conditions that lead to precipitation may
be knowable, but again the conditions in the leachate collection system
cannot be controlled because the system is not accessible once wastes
have begun to be dumped into the landfill.

The last problem--chemical attack on the leachate collection pipes,
leading to destruction of the pipes themselves--also appears to be an
unsolvable problem. Mr. Bass suggests, in best ivory tower fashion,
that the way to control chemical attack on the pipes is to select pipes
that are resistant to the chemicals that you know will make their way
into the landfill. In principal, this is a good idea. But in the real
world, how do you know what's going to be put into your landfill next
week? Next year? With 1000 brand new chemicals being put into
commercial use each year, over the next 10 years, today's leachate
collection pipes may come into contact with 10,000 new chemicals that
don't even exist today. Any of those chemicals may attack the pipes. In
addition, chemicals mixing together inside a landfill will create new
chemical combinations that may produce heat or may otherwise attack the
pipes.

Mr. Bass's book is misnamed because it seems to suggest that the
failure of leachate collection systems can be avoided. However, as the
text of Mr. Bass's book makes abundantly clear, if such failures were
to be avoided, it would be by dumb luck, not by engineering design.
Only a fool trusts dumb luck.

Mr. Bass's book is overpriced at $36.00 from: Noyes Data Corporation,
Mill Road, Park Ridge, NJ 07656. No telephone orders accepted.

--Peter Montague

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Descriptor terms: landfilling; landfill failure mechanisms; leachate
collection systems; msw;