BIODEGRADATION
INTRODUCTION
Environmental pollution is caused by toxic industrial byproducts, gases, and exhaust fumes from vehicles, chemicals, oil spillage, animal and human wastes. It is a major concern because of harmful effects like depletion of the ozone layer, greenhouse effect, acid rain, global warming, contamination of water sources, health problems to humans, adverse effects on agriculture, and many more. Industrialization is the main cause of environmental pollution; however it is difficult to curtail it since it plays an important role in the economic development of countries. It is important, then, to take measures to minimize environmental pollution. Several countries have set up agencies to help protect humans and reduce the extent of pollution. The United States Environmental Protection Agency (EPA) was created in the year 1970 with an aim to establish and enforce environmental protection standards in order to protect human health and the environment. The EPA sets standards through intensive research to help the government make policy changes that ensure public safety from the hazards of environmental pollution. Similar organizations are being established by various governments across the world.
ECOSYSTEM-NATURAL
DEGRADATION
The ecosystem (the physical, climatic features of living and nonliving beings present in a given area) is responsible for natural biodegradation. An ecosystem consists of a chain of events which involves energy transfer from one organism to another, commonly known as the “food chain”. It consists of the sun, green plants and algae called producers which utilize energy from the sun, carbon dioxide to produce organic compounds, herbivorous animals called primary consumers which feed on the green plants and algae, carnivorous animals called secondary consumers which feed on herbivorous animals, bacteria, and fungi, and other microorganisms called decomposers which survive on dead bodies of plants and animals. Decomposers play the important role of regenerating essential minerals so that they can be utilized by the producers, thus, maintaining the food chain.
The rate and
extent of biodegradation depends on various factors like the presence of
microorganisms, climate, humidity, temperature, nutrition, and other
environmental factors. Due to industrialization the amount of wastes produced
exceeds the capacity of naturally occurring decomposers. Recent technological
advances have enabled scientists to use microorganisms effectively to reduce
environmental pollution through biodegradation.
SOURCES OF POLLUTION
Pollutions sources include industrial, radioactive, domestic, and animal waste.
Industrial Waste
In industry types of waste include
petrochemical, pharmaceutical and cosmetic, plastic, pesticides, paint,
textile, paper, and metal.
Petrochemical
Petrochemical
industries produce large amounts of chemicals such as gasoline aromatic
hydrocarbons like benzene and toluene, bulk chemicals like alcohols, organic
acids, and aldehydes. These chemicals are widely used for various purposes in
the petrochemical industry. When released as waste they pose great danger to
the environment.
Pharmaceutical and
Cosmetic
Due to rapid
strides in research and development in the pharmaceutical and cosmetic
industry, many drugs and cosmetics are being discovered leading to production
of these compounds on a very large scale. Waste from these compounds cause
damage to the environment when released.
Plastic
In this
industry, chemicals like aniline, toluene, benzene, anti-oxidants,
plasticizers, and polymerizing agents are used.
Such chemicals are harmful to the environment when released. Finished products like plastic bags and
containers are not biodegradable, hence, pose threat to environment.
Pesticides
A wide range
of chemicals are used in large amounts in the pesticide industry, hence, the
amount of wastes released into the environment is very high. Pesticides can be
hazardous to organisms other than the target organisms including humans since
they are highly toxic. Hence there is need for biodegradation of the pesticides
that are released into the environment.
Paint
Solvent
preservatives are the commonly used chemicals within the paint industry. They
can be harmful to the environment when released as waste. Recently nontoxic
alternatives to these chemicals have been created.
Textile
Chemicals
are used in the textile industry for “finishing” the textiles in fabric
cleansers and in the making of synthetic textiles. These chemicals have the
potential to damage the environment when released.
Paper
The paper
industry uses a large number of chlorine compounds to bleach the pulp. These
can damage the environment if left untreated.
Metal
Different
chemicals are used in metal industries for finishing and cleaning. Traces of
metals such as mercury, lead, and arsenic can be hazardous to the
environment.
Radioactive Waste
Nuclear
fission is used for many purposes such as preparing nuclear weapons, power
generation, and nuclear medicine. Nuclear waste includes protective clothing
used in the plants, water used in the nuclear industry, used equipment, nuclear
fuel, heavy radioactive isotopes, and mail tailings from the uranium extraction
process. These wastes emit radiation
like alpha-rays, beta-rays, and gamma-rays which are high energy radiations and
are extremely dangerous to animals, plant life, and humans. Radioactive wastes
emit radiations till naturally decayed; therefore, it cannot be treated as
other industrial wastes. The need exists for a process to be designed to
effectively and safely dispose of such harmful wastes.
Domestic Waste
Domestic waste forms one of the major
components that cause environmental pollution. Such waste or trash includes the
full range of the by-products of daily living.
Animal Waste
Animal waste includes wastes from slaughter
and farm houses and from aquatic animals. Animal waste poses severe problems in
urban areas since the disposal is difficult resulting in pollution in the form
of a noxious, pungent odor. Such waste when processed properly can be used as
organic manure.
TYPES OF BIODEGRADATION
Aerobic Degradation
This process is used to treat waste water. It is carried out in two tanks, one aeration, the other decantation. The aeration tank is a large fermentation vessel in which the waste water is aerated to provide oxygen needed for microorganism survival to perform the function of converting organic matter to biomass, carbon dioxide, water, and other substances. The decantation tank allows settling of biomass that is formed as a result of microbial action on waste water. The activated sludge (mixture of bacteria, yeast, fungi, protozoa, and other microorganisms which perform biodegradation) utilizes organic matter and converts the wastes into biomass, water, and carbon dioxide by aerobic metabolism.
The Process
Waste water
enters the aeration tank where it is oxidized, then along with microorganisms
enters the decantation tank where the microorganisms grow to form flocs and
settle at the bottom. The supernatant is removed and the microbial flocs along
with the biomass produced are collected from the bottom of the tank.
Products of Aerobic
Degradation
Water is one
of the products obtained from aerobic degradation which can be reused for different
purposes. Single cell proteins (SCP) are obtained from the sugar industry.
Slaughter house waste water treatment can be used as fish meal. Activated
sludge obtained from treatment of waste water from breweries can be used as
animal feed. Sludge can also be used as organic manure.
Biofiltration
This technology is used for the treatment of
wastewater from chemical industries, solid waste processing plants, and
composting operations. It breaks down these organic compounds into carbon
dioxide and water. Biofilters break down
organic compounds using microorganisms like bacteria, fungi, protozoa, and
algae. Biofilters can be used to remove both dissolved and suspended organic
matter. As the waste water comes in
contact with the biofilter, soluble organic compounds diffuse into it and are
absorbed on the surface of the microorganisms.
Nitrification/Denitrification
Nitrogen exists in many forms. It is present
in waste water in the form of ammonia, which has to be removed during
purification. This can be done first by oxidizing ammonia to nitrate, then
denitrification of nitrate to nitrogen which separates from waste water in the
form of gas.
TYPES OF BIOFILTERS
Biofilters remove un-dissolved organic matter during nitrification/denitrification. Five types of filters are discussed next.
Trickling Filters
Trickling filters are natural systems using
sand and rocks as filtering media and microorganisms as agents to purify the
water. Trickling filters are the most commonly used biofilters. They consist of
a bed of highly permeable material like coarse rocks (commonly used in the
past), or synthetic or plastic material in a large cylindrical vessel. Waste
water is evenly distributed by a rotating distributor which consists of radial
arms with a large number of orifices to release waste water evenly over the
bed. The waste water then trickles through the filtering media and is collected
at the bottom. A continuous air supply is provided to ensure proper growth of
the aerobic microorganisms.
Advantages of using
trickling filters are that they:
·
Are
simply designed, hence easy to use,
·
are
cheap, consume very little power to operate,
·
are
robust, therefore can tolerate shock loads,
·
can
be used to treat waste water with high concentrations of pollutants,
·
Have
good reliability and durability.
Disadvantages of using trickling filters are that:
·
The
degree of purification is less than other filters,
·
they
require a large area for installation, the end product requires further
treatment and proper disposal,
·
the
process produces foul odor,
·
They
are highly susceptible to clogging.
Activated
Biofilters
This is a two-step process including a biocell
and activated sludge processing. The biocell is a tower of redwood slat medium
which is made up of horizontal racks of wooden lath attached to supporting
rails. Primary effluent along with the activated sludge from the final
clarifier is pumped to the top of the redwood slat medium tower where it is
sprayed evenly through fixed orifices. As the waste water trickles down the
tower the microorganisms present in the tower and in the sludge perform
biodegradation. Oxygen is provided by aerators or by simply splashing the waste
water in the redwood slat tower. The effluent is collected in an aeration
vessel and then sent to the final clarifier where remaining organic matter is
removed by the activated sludge process.
Submerged Filters
These filters, also called contact aerators
and biological aerated filters remain submerged in the wastewater. Submerged
filters were widely used in the early twentieth century but lost their
popularity during the latter part of the century. Recently, these filters are regaining their
popularity due to the discovery of synthetic media. Contact media in submerged
filters consists of coarse rocks, ceramic material, or plastic media. Aeration is provided by pumps from the bottom
to provide oxygen to microorganisms by mixing the contents. The most important
advantage of this process is the high efficiency of nitrification of the
wastewater.
Biological Fluidized
Beds
These consist of tall towers partially filled
with silica sand, plastic beads, white quartz, or other fine material as the
media. As the fluid flows upward, the bed expands and is fluidized. The media
allows the fluid to pass through it freely while the microorganisms present and
distributed in the media perform biodegradation of the organic compounds and
nitrification.
Advantages
of biological fluidized beds are that:
·
Reactor
size is very small,
·
there
is high efficiency of filtration, the process time is short,
·
there
is a large surface area available for microbial growth,
·
The
beds do not clog.
Rotating Biological
Contractors
Rotating biological contractor treatment
plants consist of a series of rotating biological contractor units also called
shaft trains. As the waste water passes through each of the sets, the degree of
biodegradation increases. Microorganisms grow on the wet surface of the disks
to form a layer of 1 to 2 mm thickness and perform biodegradation. As the disks
come out of the waste water and are exposed to air the microorganisms absorb
atmospheric oxygen to sustain growth and remain aerobic. Advantages of
rotating
Biological
contractors are that they:
·
Are
simply designed and are easy to operate,
·
require
less land area,
·
can
withstand shock loads,
·
Provide
high degree of purification and nitrification.
BIODEGRADATION OF PETROCHEMICALS
CONTAMINATION
Petroleum products are also known as hydrocarbons since they are mainly made up of hydrogen and carbon atoms. Some petroleum products contain other atoms like halogens but they are also called hydrocarbons. Microorganisms can degrade hydrocarbons to carbon dioxide, water, and biomass .This process also involves partial oxidation of harmful materials present in the petroleum products. Soil contains a wide variety of microorganisms like bacteria, algae, fungi, protozoa, and yeasts which play an important role in biodegradation of the hydrocarbons. Geological, physical, chemical, nutritional status and microbiological properties of soil and type, size, and structure of the hydrocarbons affect the rate and extent of the biodegradation. Soil reduces the toxic effects of petroleum products by binding onto of these substances. Microorganisms of soil utilize the carbon present in the petroleum products as food and grow rapidly thus breaking (degrading) up the hydrocarbons and making them nontoxic.
In addition,
the microorganisms also require other nutrients like trace metals, nitrogen,
and phosphorous for proper growth and effective biodegradation. Surfactants are used to solubilize the
hydrocarbons so that they come in contact with soil since they are hydrophobic
and since the microorganisms are present in the aqueous phase.
Biodegradation
can take place both in aerobic and anaerobic conditions but most commonly in
the former. This treatment is used for hydrocarbon contaminated soils (like
soil from petroleum storage places or soil contaminated with leakage of
petroleum products). Similarly, microorganisms are used for the biodegradation
of petroleum contaminated water like the ground water, lakes, rivers, seas, and
oceans. The most common causes of petroleum contamination of water are
accidents involving the tankers and pipes carrying oil on rivers and oceans.
Such accidents result in the formation of oil slicks that pose a threat to the
aquatic life, thus, adversely affecting the environment. It is therefore
important to remove oil slicks as soon as possible to minimize the
environmental threat. Bioremediation is cheap and is the most effective way of
treating petroleum contaminated waters.
BIODEGRADATION OF INDUSTRIAL WASTES
Industrial wastes include effluents mainly from chemical, petroleum, and pharmaceutical industries. Waste treatment of effluents from the chemical industry is presented here. Waste waters (effluents) from the chemical industry contain high concentrations of suspended solids and toxic chemicals. Treatment of such waste waters involves initial pre-treatment and final biological treatment. Pre-treatment can be performed by using different techniques such as ion-exchange, extraction, and centrifugation to remove suspended solid particles and the use of adsorbing resins and flocculation. Pre-treatment also includes neutralization of the effluents with acids or bases depending on the nature of the effluent, and passing the waste water through screens and grit chambers to separate suspended solids and through mixing basins with aeration to remove settling solids. The next step is biological treatment which is carried out in an activated sludge tank. Aeration is provided by pumps or turbines to maintain aerobic conditions.
TREATMENT OF WASTE
GASES
There are two types of waste gases: toxic and malodorous gases. Toxic gases are produced by chemical, pharmaceutical, paint, paper, and the petroleum industries, among others; whereas malodorous gases are produced by food, beverage, sugar, animal feed, biotechnology, and animal slaughter industries. These waste gases emitted by various industries not only cause inconvenience but are also harmful to human beings and animals. Waste gases contain volatile organic compounds which can be utilized as an energy source by the microorganisms, thus, degrading the toxic compounds present in the waste gases, rendering them non-toxic. Microorganisms can perform this function in aqueous phase so the waste gases are converted to liquid phase before treatment.
Bioscrubbers
This technique is carried out in a plant with
spray and activated sludge compartments. Gases are converted to liquid phase in
the spray compartment. Waste gases are introduced from the bottom of the spray
compartment and water is sprayed from the top through fine nozzles. The water
soluble compounds present in the gases are converted to liquid and are
collected at the bottom of the compartment. The residual gases escape through
the vent on the top of the compartment. The liquid phase is then transferred to
the activated sludge tank where the microorganisms are present. Oxidation of
organic compounds takes place in the tank removing the toxic compounds. Oxygen
can be supplied to the activated sludge tank and favorable conditions should be
provided including optimum temperature, pH, and nutrients. The contents of the
tank are mixed continuously by mechanical stirrers to ensure uniform action of
microbes on the sludge.
Trickling Tilters
These consist of a column filled with packing
material that has a large diameter so that gas can pass freely through the
column. Microorganisms grow in the column using packing material as support.
Nutrients are dissolved in water and introduced into the column from the top so
that it wets the packing material, thus, coming in contact with the
microorganisms. Waste gas is introduced from the bottom through a column which
rises to the top, thus, coming in contact with packing material and microbes.
Water soluble compounds present in the gases are dissolved and the organic
compounds present in the gas are biodegraded by the microorganisms.
Biofilters
Microorganisms
are suspended as flocs in fluid to form a biofilter by using a filter bed as
support. Nutrients are supplied to the microorganisms through a layer which is
in contact with the biofilter. Waste gases that come in contact with the
biofilter are oxidized and biodegraded by the microorganism present in the
biofilter.
TREATMENT OF DOMESTIC AND ANIMAL
WASTES
Composting
Composting is the process of microbial
decomposition of dead plants and/or animals. During composting the temperature
rises, increasing the metabolism of the microorganism which results in
decomposition of the plant or animal debris. This technique is used effectively
for the decomposition of domestic waste, agricultural and food wastes and
sewage. The essential components of composting are substrate, aeration, and
moisture. Substrate This is the organic matter that is being
composted and used by the microbes as a food source. Substrate should provide
nutrients for proper growth of the microorganisms.
Aeration
Microorganisms
involved in composting are aerobic so proper aeration is required for effective
composting process. Aeration should be provided to the center of the pile since
this area will be oxygen deficient. This can be done by mixing the composting
pile at regular intervals so it is exposed to the atmospheric air.
Moisture Microbial activity requires 40-50%
moisture content because they can utilize organic compounds and nutrients when
they are in aqueous form.
Anaerobic Digestion/Biomethanation
Solid wastes are decomposed by microorganisms
under anaerobic conditions to produce biogas (a mixture of methane and carbon
dioxide). Solid residue is settled at
the bottom of the reactor that may be further used as fertilizer for
composting. Bacteria used in the process are called methanogenic. Solid wastes
and other plant wastes are shredded and placed in an airtight reactor along
with microorganisms and allowed to ferment for a specified period of time at
temperature around 35° C. The organic compounds are first broken down into
sugars which are converted to organic acids and finally to methane and carbon
dioxide. Biogas has applications as an alternative energy source. It is used as
cooking gas, lighting, and fuel.
Anaerobic digestion is a cheap method of degrading toxic industrial
wastes.
FUELS FROM BIOMASS
Population explosion results in increased
energy demands and, as a result, fossil (natural/conventional) fuels are being
used. If natural fuels are used at the same rate they will be exhausted within
a very short period of time, hence, there is a need to look towards alternate
sources of energy. Biomass can be used to produce alcohol fuels that can be
used effectively. Biomass contains domestic, agricultural, animal, and
industrial wastes, along with sewage. Thus, it is an excellent source of
organic matter which can undergo microbial decay to produce alcohol fuels.
Methanol and ethanol are the most important alcohol fuels produced from
biomass.
Methanol (Methyl Alcohol)
Methanol is also called wood alcohol as it was
produced by wood distillation in the past. Methanol is a clear, colorless,
volatile liquid most commonly used as a solvent. It is also used in the
chemical industry in the production of various compounds. Methanol is
considered as the potential fuel for automobiles in the future. Natural gas is
the main source for the production of methanol. It can also be produced from
any gas that can be decomposed into hydrogen and carbon dioxide/carbon
monoxide.
C + H2O --> CO + H2
C + 2H2O --> CO2 + 2H2
CH4 + H2O -->
CO + 3H2
CH4 +2H2O --> CO2 + 4H2
Natural gas is passed over a catalyst
at high temperature and high pressure then treated with steam. CO2 + 3H2 -->
CH3OH + H2O
CO + 2H2 -->
CH3OH
Methanol can
be produced from direct oxidation of the hydrocarbons.
2CH4 + O2 -->
2CH3OH
Organic
compounds like biomass produced from wastes are also used for production of
methanol. In specially designed plants biomass is partially oxidized or burned
to produce a gas containing hydrogen, carbon dioxide, and carbon monoxide which
then passes through different stages to form methanol.
Ethanol (Ethyl Alcohol)
From ancient times ethanol was used primarily
as a beverage. It is produced through fermentation of carbohydrates (grains,
sugars, and starches). Large scale ethanol is produced from ethylene (petroleum
derivative). Ethanol has a wide range of uses in the chemical, paint, and
pharmaceutical industries. In addition
to this, alcohol can be used as fuel when combined with gasoline --called gasohol. Biomass from plant and vegetable wastes
contain cellulose, an excellent source of carbon which is pretreated with an
alkali at high temperature. Specific strains of bacteria, yeasts, and fungi
that promote fermentation are added to this along with concentrated sugar
solutions with enzymes. Fermentation is carried out to produce alcohol and is
recovered by distillation method.
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