Saturday, 25 October 2014

BIODEGRADATION: Introduction, sources of pollution, types of biodegradation - aerobic and anaerobic degradation.



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. 

 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.
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 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.
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.
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.
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.
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.
The paper industry uses a large number of chlorine compounds to bleach the pulp. These can damage the environment if left untreated.
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.   


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.  
 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.
 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.  


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.  

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.  
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.  

 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.
 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. 
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.   

  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.
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. 
 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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