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 .