Bioremediation: Panacea or fad?
Abstract
All things in nature ultimately succumb to decay. Much of
this is a natural consequence of the laws of thermodynamics.
Researchers developed bioremediation as one feasible way to accelerate
or encourage the degradation of pollutants. The basis of
bioremediation is that all organisms remove substances from the
environment to carry out growth and metabolism. Bioremediation does
not involve only the degradation of pollutants. Bioremediation can be
used to clean unwanted substances from air, soil, water and raw
materials for industrial processing. Technologies using
bioremediation treatment include: Bioaugmentation, a general term
describing the addition of organisms or enzymes to a material to
remove unwanted chemicals. Biofilters, removal or organic gases by
passing air through compost or soil containing microorganisms capable
of degrading the gases. Bioreactors, treatment of a contaminated
substance in a large tank containing organisms or enzymes.
Biostimulation, the use of nutrients or substrates to stimulate the
naturally occurring organisms that can perform bioremediation.
Bioventing, involves the venting of oxygen through soil to stimulate
the growth or natural and introduced bioremediation organisms.
Composting, involves mixing contaminated materials with compost
containing bioremediation organisms. Landfarming, the use of farming
tilling and soil amendment techniques to encourage the growth of
bioremediation organisms in a contaminated area. These technologies
are classified as either in situ or ex situ. There are several
limitations for using of bioremediation. One major limitation has to
do with the nature of the organisms. Two other limitations concern
cost and the benefits versus overall environmental impact. The
question in the mind of many environmental scientists is, are the
merits of bioremediation over-rated?
Introduction
All things in nature ultimately succumb to decay.
Much of this is a
natural consequence of the laws of thermodynamics. Many molecules
degrade by the action of oxygen, halogens and radicals naturally found
in the environment. A large proportion of materials degrade because
their components are subject to the action of enzymes. Unfortunately
for humans, many of the wastes that we produce do not decay as fast as
we would like. They end up polluting the air, land and water. Two
major factors prevent our wastes from decaying rapidly. One is that
we produce so much at one time that the rate of natural decay is
insignificant compared to the amount present. Another factor is that
most wastes end up in areas not conducive to rapid degradation.
Researchers developed bioremediation as a way to accelerate
or encourage the degradation of pollutants. Bioremediation is a term
that describes the use of organisms to remove or reduce human-made
pollution. The term bioremediation is new in the scientific
vocabulary. In fact, the Environmental Protection Agency had to
develop an absolute definition of the term for their purposes. The
principles of modern bioremediation are not new, dating back to the
1960's. Some of the first studies on using microbes to degrade oil in
sea water were conducted in 1972 although sewage treatment using rock
filtration was the earliest application of bioremediation. Bacteria
were encouraged to grow in these filters which broke down and absorbed
organic matter before it reached a lake, ocean or river. At first
bioremediation was a curiosity, today it is seen as a cure-all for up
all types of pollution, including radioactive isotopes.
Principles of Bioremediation
Bioremediation is based on the idea
that all organisms remove substances from the environment to carry out
growth and metabolism. Bacteria, protista and fungi are very good at
degrading complex molecules and incorporating the breakdown products
into their metabolisms. The resultant metabolic wastes that they
produce are generally safe and somehow recycled into other organisms.
Fungi are especially good at digesting complex organic compounds that
are normally not degraded by other organisms. The ability to degrade
a pollutant is dependent on enzymes produced by the organism.
Petroleum can be degraded only by bacteria with the ability to produce
enzymes that select petroleum as a substrate.
Bioremediation does not involve only the degradation of
pollutants. Sometimes it is sufficient to remove the pollutant from
the environment without degrading it. Bacteria in particular take up
large amounts of metals and minerals to ensure adequate resources for
binary fission. Algae and plants are very good at absorbing nitrogen,
phosphorus, sulfur and many minerals and metals from the environments.
For example, plants like locoweed remove large amounts of the toxic
element selenium. The selenium is stored in plant tissues where it
poses no harm until/unless the plant is eaten. Many algae and
bacteria produce secretions that attract metals that are toxic in high
levels. The metals are in effect removed from the food chain by being
bound to the secretions.
Technology of Bioremediation
Bioremediation can be used to clean
unwanted substances from air, soil, water and raw materials from
industrial processing. Living organisms or just their enzymes can be
used to accomplish this task. One unusual application involves the
use of fungi to remove excess lignin from paper pulp. Lignin is not
one compound, but, a group of polyaromatic chemicals that harden wood.
It is normally difficult to extract from pulp and ends up a pollutant
once it is removed. Most bioremediation technology is designed to
remove a pollutant once it is generated or released into the
environment although some types of bioremediation remove chemicals
before they become pollutants.
Technologies using bioremediation treatment include
bioaugmentation, biofilters, bioreactors, biostimulation, bioventing,
composting and landfarming.
Bioaugmentation: This is a general term describing the addition of
organisms or enzymes to a material to remove unwanted chemicals.
Bioaugmentation is used to remove byproducts from raw materials and
potential pollutants from waste. Bacteria are the most common
bioaugmentation organisms. Many applications are accomplished using
vegetation to remove excess nutrients, metals and pathogenic bacteria.
Waste water from human and agricultural effluent is cleaned this way
using wetland plants.
Biofilters: The removal of organic gases by passing air through
compost or soil containing microorganisms capable of degrading the
gases. It has been used to remove volatile organic compounds (VOC's)
from air.
Bioreactors: The treatment of a contaminated substance in a large
tank containing organisms or enzymes. Bioreactors are commonly used
to remove toxic pollutants from solid waste and soil.
Biostimulation: The use of nutrients or substrates to stimulate the
naturally occurring organisms that can perform bioremediation.
Fertilizer and growth supplements are the common stimulant. The
presence of small amounts of the pollutant can also act as a stimulant
by turning on operons for the bioremediation enzymes.
Bioventing: This is similar to biostimulation. It involves the
venting of oxygen through soil to stimulate the growth or natural and
introduced bioremediation organisms. This is used predominantly for
soils contaminated with petroleum products. It is not suitable for
removing halogenated gases that contribute to ozone layer damage.
Composting: This involves mixing contaminated materials with compost
containing bioremediation organisms. The mixture incubates under
aerobic and warm conditions. The resultant compost can be used as a
soil augmentation or be placed in a sanitary landfill.
Landfarming: The use of farming tilling and soil amendment
techniques to encourage the growth of bioremediation organisms in a
contaminated area. It has been used successfully to remove large
petroleum spills in soil.
These technologies are classified as either in situ or ex situ. In
situ technologies are the ones commonly seen in the media. They
involve the use of organisms or enzymes to remove pollutants in the
location that is polluted. Ex situ technologies involve the removal
of the contaminated material where it can be treated using
bioremediation.
Limitations of Bioremediation
There are several limitations to
bioremediation. One major limitation has to do with the nature of
the organisms. The removal of pollutants by organisms is not a
benevolent gesture. Rather, it is a strategy for survival. Most
bioremediation organisms do their job under environmental conditions
that suit their needs. Consequently, some type of environmental
modification is needed to encourage the organisms to degrade or take
up the pollutant at an acceptable rate. In many instances the organism
must be presented with low levels of the pollutant over a period of
time. This induces the organism to produce the metabolic pathways
needed to digest the pollutant. When using bacteria and fungi, it is
usually necessary to add fertilizer or oxygen to the material
containing the pollutant. This can be disruptive to other organisms
when done in situ. In situations where simple compounds and metals
are being taken up it is likely that these pollutants are at toxic
levels for the organisms. Overall, the organisms do not always live
as well on the pollutant diet as on other nutrients found more
commonly in their environment. This is problematical when doing in
situ remediation.
Two other limitations concern cost/benefit ratios: cost versus
overall environmental impact. Neither the government nor industry
wants to spend large amounts of money to clean up pollution. Industry
in particular likes to keep costs down. The petroleum industries are
embroiled curretly in a battle with the EPA about the added costs of
maintaining new Clean Air Act standards. Bioremediation is generally
very costly, is labor intensive, and can take several months for the
remediation to achieve acceptable levels. Air bioremediation in
particular is very ineffecient, considering the volume of polluted air
generated by industry. Another problem is that both ex situ and in
situ technologies can cause environmental disruption beyond the damage
done by the pollution. The long-term effects of introducing naturally
occurring non-native bioremediation organisms into an area are not
fully understood. The impact of genetically altered bioremediation
organisms is even less understood.
The Reality of Bioremediation: A Panacea or a Fad? The question in
the mind of many environmental scientists is, are the merits of
bioremediation over-rated? Will bioremediation be a potential panacea
for cleaning up the environment or will it prove to be impractical?
Bioremediation has been proven to work effectively under laboratory
conditions. Short-term studies show that it also works under several
field conditions. Like many technologies with good scientific
foundations its merits are marred by over-optimistic speculations and
fraudulent claims. The Bioremediation Discussion List, hosted by GZA
Environmental, Inc., provides evidence for the degree of specious
claims that bioremediation specialists read in advertisements and
trade journals. Bioremediation specialists argue over the reputed
effectiveness of advertised "superbugs" that do a thorough cleanup in
very little time. In spite of its limitations, bioremediation is
benefitting from the rush to use biotechnology to solve public health
problems. The EPA and the DOE are even investigating the feasibility
of bioremediation to remove radioactive wastes from contaminated soil
and water. Bioremediation's popularity is further enhanced because it
is perceived as being more "green" than other remediation
technologies. Companies and individuals are investing in
biotechnology futures in spite of the high risks. As a result
bioremediation companies have a viable future regardless of its
long-term effectiveness.
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