B M Taylor
BSc Hons (Zoology)
BSc Hons (Chemistry)
? Mark Taylor, ECSOL Limited, 9-2007
The contemporary remediation approaches are defined, and bioremediation is discussed in detail.
The basic characteristics and environmental problems associated with hydrocarbons resulting from spillage/leakage are defined.
The theoretical EA (UK) best practice for bioremediation process is presented, and the process in practice, utilising enhancement technology, is proposed as a Best Practicable Environmental Option to remediate hydrocarbon contaminated soil.
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Remediation of contaminated land is necessary when the results of risk assessment define the land as harmful to the environment media air, land and water media; harm is damage or destruction to receptors - humans, fauna, flora, and the built and natural environment.
Remediation approaches typically include:
; containment, and
; treatment-based technologies:
； Physical processes
； Biological methods;
； Natural attenuation (monitored);
； Chemical processes;
； Permeable reactive barrier installation;
； Solidification/stabilisation processes; and
； Thermal processes
Remediation may be affected by use of a single, or a combination of approaches.
This paper focuses on the application of biological methods to remediate oil contaminated land, to promote health and safety of the working and broader environment.
1.1 Biological methods utilised for the contaminated land remediation depend on one or more of the four basic processes:
; Biological Transformation (biotransformation)
; Biological Accumulation (bioaccumulation)
; Biological mobilisation
1.1.1 Biodegradation is a complex series of metabolic processes that effect
the decomposition of organic compounds into smaller, simpler
chemical subunits, catalysed largely through the action of
microorganisms - bacteria and/or fungi. Since the organic compounds
are converted into different forms, the process is also known as
bioconversion. Plants also can cause biodegradation reactions
(universally termed phytoremediation), but they are more suited for
uptake and accumulation reactions.
Inorganic contaminants (metals, non-metals, metal oxyanions, and
radionuclides) cannot be biodegraded, but their environmental mobility
can be altered through oxidation-reduction, sorption, methylation and
precipitation reactions mediated by microorganisms or plants.
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1.1.2 Biotransformation is the conversion of a contaminant to a less toxic
and/or less mobile form by the biodegradation process directly, or as a
consequence. For example, direct decontamination conversion of
chloroalkanes into alkane and chloride ion; and the exemplar
consequential decontamination of water-soluble heavy metals, by
precipitation as virtual insoluble sulphide forms, the sulphide having
been generated as a result of microbial reduction of sulphate.
1.1.3 Bioaccumulation is the accretion of contaminants within the tissues of
biological organisms; this mechanism may be exploited to concentrate
contaminants into harvestable biomass.
1.1.4 Mobilisation is the bioconversion of contaminants into more readily
accessible varieties, such as water soluble forms or gases, which
facilitates subsequent removal and recovery or destruction.
1.2 These processes are the basis for potential site cleanup technology; thus, bioremediation is the intentional use of biodegradation or contaminant accumulation processes to eliminate environmental pollutants from sites where they have been released.
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2 CHARACTERISTICS OF CRUDE-OIL DERIVED PRODUCTS
Crude Oil is a complex mixture of thousands of organic chemicals. Practical limitations restrict assessment of the impact of crude oil release to the environment to a limited subset of key components. It is necessary to have a basic understanding of crude oil composition and the physical and chemical properties of some the key or "indicator" chemicals.
2.2 Basics of Crude Oil
Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency from water to tar-like solids, and in colour from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1%-3% sulphur, and less than 1% each of nitrogen, oxygen, metals, and salts. Crude oils are generally classified as paraffinic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules.
TABLE 1. TYPICAL APPROXIMATE CHARACTERISTICS AND
PROPERTIES AND GASOLINE POTENTIAL OF VARIOUS CRUDES
(Representative average numbers)
Crude source Paraffins Aromatics Naphthenes Sulphur
(% vol) (% vol) (% vol) (% wt)
37 9 54 0.2 Nigerian - Light
63 19 18 2 Saudi - Light
60 15 25 2.1 Saudi - Heavy
35 12 53 2.3 Venezuela - Heavy
52 14 34 1.5 Venezuela - Light
- - - 0.4 USA - Midcont. Sweet
46 22 32 1.9 USA - W. Texas Sour
50 16 34 0.4 North Sea - Brent
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3 IMPACT OF CRUDE OIL ON THE ENVIRONMENT
3.1 Examples of the impact of crude oil in the environment
; Toxic to humans/fauna/flora by ingestion, inhalation, and transport across
; Groundwater contamination ;
; Physical impact, e.g. soil structure denaturisation, water ingress
prevention, increased toxicity levels;
; Physical impact on biota, e.g. coating of avian plumage, blockage of
invertebrate respiratory and feeding mechanisms, blockage of sunlight on
; Prevention of use of amenities;
; Consequential economic impacts;
; Consequential social impacts.
3.2 Effect of Hydrocarbons on Receptors,
3.2.1 Health and Safety Issues.
Humans can be exposed to hydrocarbon contamination by ingestion,
inhalation, and dermal contact; effects can be either acute and/or
Acute effects arise from short-term exposure, effects include contact
dermatitis, respiratory difficulties, anaphylactic shock
Chronic? Chronic effects build up over extended periods e.g. kidney
damage, neurological conditions or carcinogenic effects.
Also, there are risks such as fire, explosion and/or asphyxiation.
EPA90 Part IIA defines the receptors for the purposes of contaminated
land regulation, and details what effects constitute significant harm or
significant possibility of harm.
EPA90 Part IIA defines two classes of property:
Live Property Livestock, crops, timber, allotments, wild animals
covered by shooting rights.
Buildings, Structures and Services Hydrocarbons can reduce
concrete strength and other structural materials. Hydrocarbon vapours
may mean that a building or area cannot be used. Buried services can
also be affected; PVC pipes are permeable to some hydrocarbons, and
water supplies may be tainted or power lines penetrated, leading to a
potential source of ignition.
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3.3 Hydrocarbon Behaviour in the Sub-Surface
Hydrocarbons that escape into the environment behave differently depending upon their chemical constituents and the environment they encounter.
3.3.1 Residual/Adsorbed Hydrocarbons
Free product migrates through strata by ‘smearing’, leaving product in
the pore spaces, which frequently gets either trapped or binds to the
surface of the strata it passes through. This can act a source of
continued contamination in the event of groundwater level fluctuations,
or rainfall percolation.
3.3.2 Volatilised Constituents
A proportion of the more volatile fractions of any hydrocarbon escape
may migrate away in the gas phase, and even reach to the surface as
part of a vapour plume.
3.3.3 Hydrocarbons and Water
When free product encounters water, a proportion of the hydrocarbons
will, after a while, dissolve, float or sink, dependent upon factors such
as solubility and the hydrocarbon type.
3.3.4 Dissolved Phase
Hydrocarbons with a high relative solubility are likely to dissolve in the
water and be more mobile than other, heavier hydrocarbons.
Parameters of interest are the solubility and partition coefficient (i.e. a
measure of how readily and to what extent hydrocarbons will dissolve
3.3.5 LNAPLs - Light Non-Aqueous Phase Liquids
These refer to free phase hydrocarbons that float on water. Although
less mobile than the dissolved phase hydrocarbons, they can act as a
further source of mobile hydrocarbon in a contaminant plume.
3.3.6 DNAPLS - Dense Non-Aqueous Phase Liquids
These represent heavier compounds that readily sink in water and are
the least mobile of all the hydrocarbon groups (e.g. tar, heavy oils, etc).
They can break down over time to sustain an elevated concentration of
the lighter more mobile hydrocarbon fractions. They are very persistent
in the environment, bioaccumulate in living tissue, and frequently
contain toxic compounds.
3.3.7 Hydrocarbon Vapours
Many hydrocarbon mixtures in the aqueous environment can still
contain volatile fractions, which can return to the gas phase at a
distance from the source.
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These can occur as naturally occurring components of crude (e.g.
3.4 Factors Affecting Hydrocarbon Concentration and Mobility The persistence of the contaminant in the environment is dependent upon the initial composition and concentration of the hydrocarbon contamination and other environmental parameters in processes known collectively as Natural Attenuation. Natural Attenuation involves the physical processes, the biological action (biodegradation), and any combination of these processes.
3.4.1 Physical Degradation (or conversion).
This includes numerous processes:
; Volatilisation and dissolution tends to remove low molecular
weight aromatics and aliphatics
; Hydrodynamic dispersion - relates to aqueous redistribution of
; Dissolution is very important for soluble contaminants which
breakdown in the presence of water (hydrolysis)
; Sorption - reduction of contaminant availability and mobility due to
chemical and physical binding within the soil environment. A given
volume of strata can adsorb a given amount of contaminants; hence
with very concentrated hydrocarbon spills this process can be
overwhelmed as the ground exceeds its "sorption capacity"
; Dilution - reduction of concentration although increased mobility
; Abiotic degradation or chemical transformation involves the
breakdown of contaminant molecules by physiochemical processes
(e.g. cation exchange)
Initially, biodegradation favours the removal of n-alkanes, low
molecular weight cycloalkanes and light aromatics since they are more
chemically/physically susceptible to metabolism by soil organisms. The
action of biodegradation is more pronounced at the periphery of
contaminant plumes where sufficient Redox (electron acceptor)
compounds (oxygen, nitrate, iron, sulphate and carbon dioxide) are
present. The more concentrated a hydrocarbon plume the less the
impact of biodegradation.
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BIOREMEDIATION IN THEORY – EA (UK) GUIDANCE TO BEST PRACTICE
4 Bioremediation Processes
4.1 Biopile and Windrow Processes
Treatment of contaminated soils in static biopiles is a controlled
process that involves constructing soil piles above ground, and
promoting aerobic microbial degradation of organic contaminants.
Static biopiles are ex-situ engineered treatment systems, whereby
contaminated soils are placed within a bunded area. Their size and
shape is largely influenced by the practical limitations of effectively
aerating the soil. Generally they do not exceed 2.4m in height, although
they may be of any length with a proportional width. Biopiles are
aerated using air injection or vacuum extraction to push or draw air
through the soil respectively to optimise the transfer of oxygen within
soils in order to promote aerobic biodegradation.
4.1.2 The Windrow Process
Treatment of contaminated soils in windrows is a controlled process
that involves constructing and turning soil piles as a means of
promoting aerobic biodegradation. Windrows are similar to soil
composting systems. Contaminated soils are mixed with composting
materials and loosely placed in windrows. Their size and shape is
largely influenced by the practical limitations of effectively aerating the
soil. Generally they do not exceed approximately 2m in height and 2-
4m in width, although they may be of any length. Windrows are aerated
periodically by mechanically rotavating the soil pile. This optimises the
transfer of oxygen into contaminated soils and promotes aerobic
degradation of organic contaminants.
4.1.3 The main principles to consider when remediating contaminated soils
by biopile or windrow include:
; Stimulation of microbial degradation within contaminated soils;
; Controlled application of bioremediation; and
; Containment of process emissions.
4.2 Stimulation of microbial degradation
Although the process uses naturally occurring micro-organisms, contaminated soils do not always have suitably active microbial populations and supplementary microbial inocula may be necessary.
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4.3 Controlled application of bioremediation
Biodegradation is optimised by controlling a number of key environmental parameters, of which oxygen is the most critical. Other environmental parameters important to process performance include; soil moisture, nutrient levels, pH and temperature.
4.4 Containment of process emissions
Biopiles/windrows should be constructed upon an impermeable base, individually bunded and covered to prevent the ingress of rainwater, whilst allowing air flow, to contain leachates, and other emissions.
4.5 Considerations for Effectiveness
4.5.1 Contaminant types
The processes are proven to be effective for treating soils with, e.g.:
; Polycyclic Aromatic hydrocarbons
; Petroleum hydrocarbons (e.g. diesels, lubricating oils, crude oil)
; Herbicides / Pesticides (e.g. atrazine)
4.5.2 Contaminant chemical properties
Contaminant properties that should be considered when determining
the suitability of the process include:
; Hydrocarbons composition;
; Soluble components;
; Variability in concentration range;
; High concentrations of heavy metals, cyanides, etc that may inhibit
4.5.3 Site conditions that influence effectiveness
The treatment area must provide:
; Adequate space for constructing biopiles/windrows to treat the
volume of contaminated soil on site;
; Utilities such as water and electricity when pre-treating and
; Suitable climatic conditions:
； temperature on site during treatment should ideally be in the
range of 10-25?C;
； cover soil to protect from the ingress of heavy rainfall and retain
； soil pH (typically in the range of 6 to 8).
; Treatment area available to complete remediation.
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