ABSTRACT:
In situ bioremediation (ISB) technologies are gaining wide acceptance as viable and economic
remediation technologies for contaminated groundwater. The name itself suggests that the
contaminated ground water is remediated with the help of biological agents i.e. microorganisms.
So before the process of bioremediation the ease or difficulty of degrading contaminants have to
be evolved, the ability to achieve total mineralization and the environmental conditions
necessary to implement the process. In situ groundwater Bioremediation is a technology that
encourages growth and reproduction of indigenous microorganisms to enhance the
biodegradation of organic constituents of in the saturated zone. This process can effectively
degrade the organic constituents which are dissolved in groundwater and adsorbed onto the
aquifer matrix. The effectiveness of In situ bioremediation can be affected by various physical,
Chemical and biological factors. These factors include Soil pH, Temperature, Moisture content,
Chemical composition, Pressure, Soil conditions and soil nutrients. Every process has its own
advantages as well as disadvantages, In the same way the in situ bioremediation process has
certain advantages and disadvantages. Hence the in situ bioremediation is a site specific
technology which establishes a hydrostatic gradient through the contaminated area by flooding it
with water carrying nutrients and possible organisms adapted to the contaminants.
INTRODUCTION:
Waste products resulting from human life have always been a serious problem. Today these
waste products range from raw sewage to nuclear waste. In the past disposal of these wastes
meant digging a hole, dumping the waste material in, then filling it all in. Out of sight, out of
mind. But lately this method has become insufficient. The toxic materials from these “dig and
dump” sites have begun to leak into water sources and into areas that sustain human life. This
problem has led to modern-day bioremediation.
Bioremediation is the application of biological treatment to the cleanup of contaminants of
concern. It requires the control and manipulation of microbial processes in surface reactors or in
the subsurface for in situ treatment.
Basically, bioremediation melds an understanding of microbiology, chemistry, hydrogeology,
and engineering into a cohesive strategy for controlled microbial degradation of specific classes
of organic compounds and, in certain instances, inorganic compounds as well. This assemblage
of science and engineering requires a rigorous degree of data evaluation to determine the effect
and efficiency of bioremediation.
ISB entails the creation of subsurface environmental conditions conducive to the degradation of
chemicals (i.e., the target chemical) via microbial catalyzed biochemical reactions. This is a
technical way of saying that certain microbes can degrade specific chemicals in the subsurface
by optimizing their environmental conditions (which causes them to grow and reproduce). In
turn, the microbes produce enzymes that are utilized to derive energy and that are instrumental in
the degradation of target chemicals. In order to accomplish this chain of events, several crucial
aspects must converge, according to the National Research Council (NRC,1993):
• The type of microorganisms,
• The type of contaminant, and
• The geological conditions at the site.
Once converged, such conditions accelerate microbial activity that, in turn, cause target chemical
“biological” destruction. This bioremediation solution yields an elegant and cost-effective way to
attack chemicals in the environment using naturally occurring microbes.
Summary:
Enormous quantities of organic & inorganic compounds are released into the environment each
year as a result of human activities.
The release may be:
Deliberate and well regulated (industrial emissions)
Accidental and largely unavoidable (chemical/oil spills)
Acc. to US EPA the waste can be categorized into three general groups:
Heavy metal, Pb, Hg, Cd, Ni and Be can accumulate in various organs, interfere with normal
enzymatic reactions and cause disease including cancer.
Chlorinated hydrocarbons, also known as organochlorides including pesticides and other organic
compounds such as PCB (polychlorinated biphenyls)
Research proven a positive correlation between cancer in lab animals and organochlorides.
Nuclear waste including radioactive material such as plutonium which are dangerous for
thousands of years.
Microorganisms (or microbes) -
The basic premise of bioremediation is to accelerate microbial activity using nutrients (i.e.,
phosphorus, nitrogen) and substrate (i.e., food) to create conditions conducive to biodegradation
of a target chemical or contaminant. This is not new. Sanitary engineers understood the
implications of bioremediation as early as the turn of the 20th century when the first vestiges of
the common sewage treatment plant were first recognized, applied, and utilized for treatment of
raw human excrement (i.e., sewage). These engineers recognized that controlled aeration of
sewage would cause a decrease in odor and offensiveness. They also observed that the effluent
from such treatment could be easily settled (i.e., clarified) and then discharged to a watercourse
without the detrimental effects of the original raw sewage.
This was one of the first applications of engineered bioremediation systems to enhance
environmental conditions . Microbes can use a variety of organic chemicals for their own growth
and propagation. These organic chemicals may serve various functions but primarily may be
used as either a carbon source for growth or as a source of electrons for energy.
TYPES OF BIOREMEDIATION
The two main types of bioremediation are in situ bioremediation and ex situ bioremediation. In
addition, another offshoot of bioremediation is phytoremediation.
However here we are concerned with the Insitu Bioremediation of ground water.
Insitu Bioremediation
In-situ groundwater bioremediation is a technology that encourages growth and reproduction of
indigenous microorganisms to enhance biodegradation of organic constituents in the saturated
zone. In-situ bioremediation can effectively degrade organic constituents which are dissolved in
groundwater and adsorbed onto the aquifer matrix. Depends on:-
Hydrology of the subsurface area
The extend of the contaminated area
Nature (type) of contamination
This method is effective only when the subsurface soils are highly permeable, the soil horizon to
be treated falls within a depth of 8-10 m.
The average time frame for an in situ bioremediation project can be in the order of 12-24 months
depending on the levels of contamination and depth of contaminated soil.
This mechanism is usually a delivery system for providing one or more of the following:
An electron acceptor (oxygen, nitrate)
Nutrients (nitrogen, phosphorus)
An energy source (carbon).
FACTORS AFFECTING THE EFFECTIVENESS OF INSITU BIOREMEEDIATION
Contaminant Intrinsic Biodegradability.
Oxygen requirements
Soil pH conditions
Soil Nutrients
Chemical composition and Hydrocarbon Concentration
Temperature
pH
Moisture
Pressure .
Contaminant Intrinsic Biodegradability:-
The intrinsic biodegradability of a chemical compound is significantly related to its chemical
structure.This parameter is the ratio of the 5-day biochemical oxygen demand (BOD5) to the
chemical oxygen demand (COD). In general, the following designations are made:
BOD5 > 0.01 Biodegradable.
COD
BOD5 <0.01Non-biodegradable.
COD
The refractory index (RI), defined as the ratio of the ultimate biochemical oxygen demand.
(BODu) to the ultimate oxygen demand (UOD).
RI < 0.5 Biodegradable
RI > 0.5 Non-biodegradable.
Oxygen Requirements:-
Oxygen can be supplied to microorganisms during bioreclamation through any of the following
methods: aeration, oxygenation, or use of compounds that can supply oxygen molecules.
About 10 mg/ litre of dissolved oxygen at 15°C is the maximum concentration that can
be attained through injection of air.Sawyer and McCarty (1967)
C0 =fPH
C0 = concentration of oxygen in water (mg/L)
f =volume fraction of oxygen in air (0.21)
P =air pressure (atm)
H =Henry’s law constant for oxygen 43.8 (mg/L) • atm at 68°F (20°C)
Soil pH Conditions:-
A soil pH range of about 6–8 generally produces the greatest growth rate of
microorganisms, although most bacteria favour neutral pH conditions.
Soil Nutrients:-
The typical composition of bacterial cells (on a dry-weight basis) are carbon (50%),
oxygen (20%), nitrogen (14%), hydrogen (8%), phosphorus (3%), sulfur (1%), potassium (1%),
sodium (1%), and calcium, magnesium, chlorine, iron, and other trace elements in percentages
that are each less than 1%.
Chemical Composition and Hydrocarbon Concentration
Susceptibility of hydrocarbons to microbial degradation has following order:
n-alkanes > branched alkanes > low-molecular-weight aromatics > cyclic alkenes
The simpler aliphatics and monocyclic aromatics are readily degradable, but more complex
compounds such as PAHs are not easily degraded and may persist for some time.
The more complex and less soluble oil components will be degraded much more slowly than the
lighter oils.
The presence of some functional groups in organic compounds enhances
their biodegradability. For example, alcohols, aldehydes, acids, esters,
amides, and amino acids are generally biodegradable.
Temperature:-
As temperature increases, the rates of chemical as well as biochemical reactions generally
increase. This phenomenon is referred to as Arrhenius behavior. The same phenomenon also
occurs with microorganisms and the myriad of chemical and biochemical reactions that
constitute “microbial activity,” but only to a point. While the rates of abiotic chemical reactions
might increase in an unbounded fashion with increasing temperature, this is not the case with
microbial activity. Beyond some optimum temperature, the activity of any organism declines
precipitously. At the lower end of the temperature range, most bacteria stop metabolic activities
at temperatures just above the freezing point of water.
Found that the optimum temperature for biodegradation of mineral oil hydrocarbons
under temperate climates is in the range of 20-30 °C.
At low temperatures, rate of biodegradation of oil is discouraged as a result of the decreased rate
of enzymatic activities
pH:-
The pH affects the microorganism’s ability to conduct cellular functions, cell membrane
transport, and the equilibrium of catalyzed reactions by having an impact on the three
dimensional conformation of enzymes and transport proteins of microbial cells. It also affects
the protonmotive forces responsible for energy production within the cell.
Most natural environments possess pH values in the range between 5 and 9. Therefore, it is not
surprising to find that most microorganisms have evolved with pH tolerances within this range.
Most bacteria tolerate pH 5 to 9 but prefer pH 6.5 to 7.5. There are acidophilic bacteria such as
Thiobacillus thioxidans, which have pH optimum near 2.5. Also, there are alkalophilic bacteria
that can function at pH 10 to 12. The overall biodegradation rate of hydrocarbons is generally
higher under slightly alkaline conditions. pH range of 7.0-7.5. The pH of the soil is an important
factor for anthracene and pyrene degradation activity of introduced bacteria (Sphingomonas
paucimobilis BA 2 and strain BP 9). A shift of the pH from 5.2 to 7.0 enhanced anthracene
degradation by S. paucimobilis
Moisture:-
Moisture is a very important variable relative to bioremediation. Moisture content of soil
affects the bioavailability of contaminants, the transfer of gases, the effective toxicity level
of contaminants, the movement and growth stage of microorganisms, and species distribution.2
Soil moisture is frequently measured as a gravimetric percentage or reported as field capacity.
Evaluating moisture by these methods provides little information on the “water availability” for
microbial metabolism. Water availability is defined by biologists in terms
of a parameter called water activity(aw):
At soil saturation, all pore spaces are filled with water.
At a 10% moisture level in soil the osmotic and matrix forces may reduce metabolic activity to
marginal levels.Soil moisture levels in the range of 20%- 80% of saturation generally allow
suitable biodegradation to take place.
100% saturation inhibits aerobic biodegradation because of lack of oxygen.
Pressure:-
The importance of pressure is confined to the deep-ocean environment where the oil that
reaches there will be degraded very slowly by microbial populations. Thus, certain recalcitrant
fractions of the oil could persist for decades .
Schwarz et al. (1974, 1975) monitored the degradation of hydrocarbons by a mixed culture of
deep-sea sediment bacteria under 1 atm and 495 or 500 atm at 4 °C. After a 40-week high-
pressure incubation, 94% of the hexadecane was degraded, the same amount that occurred after 8
weeks at 1 atm.
In-situ groundwater bioremediation is a technology that encourages growth and reproduction of
indigenous microorganisms to enhance biodegradation of organic constituents in the saturated
zone. In-situ bioremediation can effectively degrade organic constituents which are dissolved in
groundwater and adsorbed onto the aquifer matrix.
Degradation of organic molecules and adsorption onto the aquifer matrix depends on:-
Hydrology of the subsurface area
The extend of the contaminated area
Nature (type) of contamination
This method is effective only when the subsurface soils are highly permeable, the soil horizon to
be treated falls within a depth of 8-10 m.
Advantages and Disadvantages of Bioremediation
Each and every mechanism in this universe has its own advantages along with certain
disadvantages. In the same way, the process of Bioremediation has both advantages as well as
disadvantages. But here in this case the advantages are predominant compared to the
disadvantages of the technology. They are discussed here under.
Advantages of In situ Bioremediation:-
a) There is limited or minimal disruption to the site of activity.
b) This method ensures minimal exposure to the public and site personnel.
c) It is cost effective.
d) The simultaneous treatment of water and soil is possible.
e) There is no waste production from this technique which needs to be disposed.
f) Remediates contaminants that are adsorbed onto or trapped within the geological
materials of which the aquifier is made of along with contaminants in the ground water.
g) Instead of transferring contaminants from one environmental medium to another i.e. from
land to water or air, The complete destruction of target pollutants is possible within the
site.
h) Bioremediation can often be carried out onsite without causing any disturbance to the
normal activities.
Disadvantages of Insitu Bioremediation:-
a) The sites are directly exposed to environmental factors like Temperature and Oxygen
supply.
b) Problematic application of treatment additives like nutrients, surfactants and oxygen etc.
c) The process is very tedious and time consuming.
d) Difficult to implement in low permeability aquifiers.
e) The seasonal variation of microbial activity occurs.
f) Injection wells or infiltration galleries may be plugged by microbial growth or mineral
precipitation.
g) It requires continuous monitoring and maintenance.
h) Bioremediation is limited to certain compounds which are biodegradable only. Not all
compounds are susceptible for rapid and complete degradation.
CONCLUSION :
Biological processes, which take place in the natural environment, can modify organic
contaminant molecules at the spill location or during their transport in the subsurface. Such
biological transformations, which involve enzymes as catalysts, frequently bring about extensive
modification in the structure and toxicological properties of the contaminants. These biotic
processes may result in the complete conversion of the organic molecule to innocuous inorganic
end products, cause major changes that result in new organic products, or occasionally lead to
only minor modifications. The available body of information suggests that the major agents
causing the biological transformations in soil, sediment, surface water, and groundwater are the
indigenous microorganisms that inhabit these environments.Among these Biological processes
Bioremediation is the one which is widely in use.
In situ bioremediation is a very site specific technology that involves establishing a hydrostatic
gradient through the contaminated area by flooding it with water carrying nutrients and possibly
organisms adapted to the contaminants. Water is continuously circulated through the site until it
is determined to be clean.often in-situ bioremediation is applied to the degradation of
contaminants in saturated soils and groundwater.
This method is less costly, more effective alternative to the standard pump-and-treat methods
used to clean up aquifers and soils contaminated with chlorinated solvents, fuel hydrocarbons,
explosives, nitrates, and toxic metals.
ISB has the potential to provide advantages such as complete destruction of the contaminant(s),
lower risk to site workers, and lower equipment/operating costs.This technique is relatively
passive and is a natural attenuation process, it is also used in treatment of contaminated soil as
well as water.
Though this technology has certain limitations such as environmental constraints, extended
treatment time and monitoring difficulties it is better chosen for treating the contaminated soil .
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