An estimated 40 million tonnes of farmland soil across Australia is contaminated with fuel and oil from leaking underground storage tanks. While remediating this soil has not been economically justified, political pressure to clean it up is increasing. Bioremediation, which uses microorganisms to break down soil contaminants, is one technique for remediating this contaminated soil. Bioremediation can be done in-situ or by excavating soil for ex-situ treatment. Case studies demonstrate bioremediation has successfully cleaned soil contaminated with hydrocarbons like diesel and tar.

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How to Clean
40 Million Tonnes of
Contaminated Soil
By Turlough Guerin
• Workshop maintenance wastes.
These wastes can move extensively from
their points of original release into soil,
water and air. Si.nce Minenco first
implemented Australia's first commercial
soil bioremediation project in Sydney in
the late 1980s, they have invested
substantially in developing and refining
bioremediation techniques which are
robust and reliable. Farming soil that has
or groundwater. For example, in-situ
bioventing is the process of introducing
air into a soil that has hydrocarbon
contamination above the watertable. This
process has been employed where
contamination has resulted from fuel
spills, either from above ground or from
shallow leaking USTs, or where the
watertable has been so low that ground-
water contamination has not occurred.
Where contamination has reached the
groundwater, biosparging has been used.
Biosparging is an in-situ process that
treats both the contaminated groundwater
and above-lying soil by introducing air
and nutrients below the watertable.
Ex-si~ processes are more amenable to
greater process control. As a result clean-
up times and final concentrations of
contaminants can be predicted quite
accurately. Composting is another ex-situ
process which has been shown to be suit-
able for th~ destruction of a range of high I
molecular weight (or 'heavy')
hydrocarbons and chlorinated compounds.
Composting is not a new technology and
has been used for a long time by farmers
to manage woody, paper and household
wastes. However, the concept of
composting to t.reat heavy oil wastes is
relatively new.
One of the major sources 01 fuel and oil contamination is above lround spills.
How Does Bioremediation Work?
Bioremediation processes can be
categorised into those that are
implemented in place, where the
conta.mination was originally present (or .
in-situ), and those that involve excavation
or removal of the contaminated soil or
water and treatment on the same site or at
another location (or ex-situ).
In-situ processes involve introducing air
and nutrients in to the contaminated soil
become contaminated with either fuel or
oil is usually not remediated because it is
more profitable to change land-use or put
it out of production. Technically, Minenco
could clean up this 40 million tonnes of
contaminated soil.
Above ground diesel and waste oil
spills;
Leaking underground storage tanks
(USTs); and
•
•
A
cross Australia, 40 million tonnes
of fann soil are estimated to be
contaminated with fuel and oil from
leaking underground storage tanks (USTs).
This is a 'back-of-the-envelope' estimate,
based on the knowledge that 80 per cent
of USTs are leaking and there are usually
2 to 3 of these on each farm. This
estimate does not include contamination
from other sources. While no one has
come up with a convincing economic case
for remediating this soil, how long before
our government yields to political pressure
[0 legislate for their compulsory clean up?
Given the effectiveness of the anti-
pollution lobby in other Australian
industries, and throughout the world, it is
surely only a maner of time. Minenco is
one company in the environmental
management industry which is taking a
proactive approach to this potentially
massive problem by developing and
refining a process for soil clean up called
bioremediation. Biorernediation relies on
soil micro-organisms to remove wastes
from soil and water and requires close
monitoring and manipulation to control
the process.
The major sources of fuel and oil
contamination are:
38 AGRICULTURAL SCIENCE NOVEMBER - DECEMBER 1995

2. LA N D REMEDIATION
Bioremediation of fuels and oils. whether
in-situ or ex-situ. usually involves the
stimulation of indigenous microbial
populations. referred to as biostimulanon.
This is in contrast to the
'bioaugmentation'. which is the addition
of specialised contaminant-degrading
micro-organisms. that was conunonly
employed during our developmental stage
of bioremediation. This recognition of the
benefits of biostimulation over
bioaugmentation has come from the better
understanding of the processes we have
gained through our extensive field work.
When Do You Use
Bioremediation?
Before bioremediation is be commissioned
for clean-up. all relevant technologies for
remediating a site should be evaluated.
These may include non-biological
processes such as incineration or
landfilling. The final choice of the
technology should then incorporate a cost
comparison and a realistic assessment of
the time-frame required to clean up the
contamination to concentrations accept-
able to the land-owner. Naturally, the
chosen technology must be cost-effective.
To properly evaluate technologies for
treating a contaminated site, an
understanding of each technology is
required including. how, when. and where
the technology should be employed.
Typically. where bioremediation has been
unsuccessful, technology evaluation has
been ignored.
What Does Bioremediation Cost?
The choice of process determines the time
required for clean-up and the types of
contaminants that can be degraded. While
the operating costs are similar between in-
situ and ex-situ processes. the capital costs
for ex-situ processes is high because of
excavation and materials' handling.
Where has Bioremediation
Been Used?
Bioremediation has been applied by
Minenco to the commercial clean-up of
contaminated soil and groundw3ter across
a range of industries. These have included
the management of diesel and oil spills
and sites conlaminated with more
recalcitrant hydrocarbons such as larS.
Typically, more than 80 per cent of all
underground storage tanks leak.
Depending on the size of the tank, its
contents, and the bedding soil type, the
extent of the problem will vary. At one
particular operation. approximately
800 000 litres of diesel escaped from a
ruptured underground pipe. This led to
extensive soil contamination and, because
the underlying subsurface was comprised
of pea gravel, the underlying aquifer
became heavily contaminated.
Biosparging has been employed at the site
and the diesel concentrations have been
substantially reduced. Most importantly,
the operation was able to continue
production because bioremediation was
conducted in-situ and the soil and
underlying aquifer were not substantially
disturbed.
Soil from a range of sites contaminated
with hydrocarbons has been treated using
land treatment. Leakage of fuel oil from
an underground storage lank resulted in
120mJ of soil contaminated with
8000mglkg of petroleum hydrocarbons.
The oil residue was significantly
weathered. A simple high density
polyethylene cell (HOPE) lined treatment
cell was designed and constructed. The
contaminated soil was then screened and
placed in the treatment cell. A bacterial
culture, selected for growth on the residual
fuel oil hydrocarbons, was applied to the
soil over a period of six weeks. Nutrients
and water were added as required, and the
soil was mixed at weekly intervals. The
treatment reduced petroleum hydrocarbons
to less than lOOmglkg in 8 weeks.
At another site, land treatment was
employed to clean 150mJ
of sandy soil
that was contaminated with up to
4500mglkg petroleum hydrocarbons from
a leaking pipe. The biotreatment process
design study showed that the soil was
amenable to bioremediation, using a very
simple land treatment process, with
stimulation of the indigenous micr-
organisms. Treatment was successfully
completed to less than lOOmglkg.of
residual petroleum hydrocarbons.
Although land treatment has been
implemented with considerable success
for bioremediating contaminated soil,
bioventing has become the more favoured
option for treatment. This can be used for
Soil tram a ranre of sites contaminated with
hydrocarbons has been treated.
treating small or large batches of
contaminated soil both in-situ or ex-situ.
Bioventing provides the advantage of
minimising losses of hydrocarbons due to
volatilisation and allowing improved
estimates of the clean-up time to be made.
Heavy hydrocarbons, including grease and
tar, have caused substantial soil and
groundwater contamination in a range of
industries. Tars contain relatively stable,
complex cyclic hydrocarbon compounds
which require very active microbial
populations for their degradation.
A composting trial at a site in rural New
South Wales has demonstrated that
composting of tar contaminated soil can
reduce initial polycyclic aromatic
hydrocarbon (the major contaminants in
tar), at concentrations of 2500mglkg, by
more than 80 per cent in 6 months.
These types of examples indicate the
potential for bioremediation in agriculture.
Whatever way the political winds blow on
the issue of fuel and oil contamination on-
farm, it is reassuring to know that the
technology exists to rehabilitate the 40
million tonnes of contaminated soil in
question.•
Dr Thrlough Guerin is a Senior
Environmental Scientist with Minenco
Pty Lld Environmental Services -
a CRA Cnmpany, 1 Research Avenue,
Bundoora 3083,
He is contactable by E-mail on:
T_Guerin@am.atd.cra.com.au
AGRICULTURAL SCIENCE NOVEMBER - DECEMBER 1995
39