Environmental Toxicants, Pollution, and Sustainable Development
Environmental Toxicants
Any toxic material or substance is termed as a toxicant. They are hazardous and poisonous. Toxicants are generally man-made and artificial products introduced into the environment due to human activity. They include bisphenol, insecticides and a number of industrial chemicals.
Types and Sources of Environmental Toxicants
Toxic agents can be chemical, physical, or biological in nature and produce toxic effects on the body. The different toxic agents include: chemical (cyanide), physical (radiation) and biological (snake venom). There exist a number of toxicants and they can be classified by various means. Classification may be by exposure classes and by user classes. The different toxicants include the following:
Natural Pollutants
Toxic pollutants can also be released through natural processes. For example, volcanoes emit particulate matter, sulfur dioxide, hydrogen sulfide, and methane. Forest fires release smoke, unburned hydrocarbons, carbon monoxide, nitrogen oxides, and ash. These can be harmful to human health when inhaled. Dust storms release particulate matter and oceans release aerosols in the form of salt particles. Plants produce pollen and spores, which cause respiratory problems and allergic reactions.
Anthropogenic Pollutants
These are pollutants introduced due to human activity/ man-made activities. These substances come primarily from three sources: (1) combustion; (2) industrial; (3) mining and drilling processes.
Some important man-made sources of toxicants are as follows:
Air Pollutants
Humans have been polluting the air and there are also significant natural pollutants such as terpenes from plants, smoke from forest fires, and fumes and smoke from volcanoes. Among air pollutants there are gaseous pollutants like carbon dioxide, carbon monoxide, hydrocarbons, hydrogen sulfide, nitrogen oxides, ozone and other oxidants, sulfur oxides. There are also fine particulates in the air. The particulates include dust (coal, ash, sawdust, cement) that come from chemical processes, mist droplets, smoke resulting from incomplete combustion of fossil fuels and aerosols.
Indoor Pollutants
These are produced from heating, cooking, pesticides, tobacco smoking, radon, gases, microbes from people and animals. Materials used for construction of buildings can give out gaseous indoor chemicals that have serious health concerns. Carbon monoxide and polycyclic aromatic hydrocarbons released from wood, crop residues, animal dung used for cooking cause acute respiratory infections in poorly ventilated areas.
Water Pollutants
Surface waters may be polluted from point and nonpoint sources. Industrial wastes discharged into waters contain organic and inorganic wastes including hazardous chemicals. Toxic effects are seen in humans when they consume this contaminated water.
Soil Pollutants
When wastes are not properly disposed off then soil also gets polluted. Soil contaminants include: domestic waste, solid wastes, electronic wastes, municipal wastes, agricultural wastes that contain a number of chemicals harmful to life. Further agricultural toxicants like persistent pesticides that do not biodegrade remain in the soil of many years and move into the food chain causing greater health impacts. The most toxic hazardous pesticides are the organochlorine compounds such as DDT, aldrin, dieldrin, and chlordane.
Heavy Metals
Metals released from industrial activities cause toxicity. For example, the heavy metals lead and arsenic are highly toxic and is found in potable water in certain areas. Lead induces neurological damage and can penetrate the placental barrier and induce birth defects among children. Arsenic toxicity is also a serious cause of concern especially in West Bengal in India. It can leach into water from pesticide sprays, arsenic-containing fossil fuels, and leaching of mine tailings and smelter runoff. Cadmium enters the food chain through industrial activities. It can accumulate in the tissues of aquatic organisms. Cadmium contaminated rice in Japan caused the disease ItaiItai. The disease was characterized by severe kidney damage, painful bone and joint problems.
Nitrates and Phosphate
These arise from contamination due to fertilizers, discharge from sewage treatment plants, leachate from septic tanks, manure and detergents which are hazardous. They leach into the soil and drinking water. Nitrates in drinking water cause adverse health effects. It occurs due to the formation of: (1) nitrosamine and (2) methemoglobinemia. The nitrates are converted to nitrites by bacteria in the intestine. Thereafter nitrite ions combine with hemoglobin to form methemoglobin. This reduces the oxygen-carrying capacity of the blood and leads to the blue-baby syndrome. This is seen in young or new born children who have ingested nitrate containing water or milk foods.
Petroleum and Oil Pollutants
Shore animals, such as crabs, shrimp, mussels, and barnacles, are also affected by the toxic hydrocarbons (oil and petroleum compounds) ingested by them.
Volatile Organic Compounds (VOCs)
They include halogenated solvents and petroleum products. They are used in industries involving degreasing, dry cleaning, paint, and in the military.
Therapeutic Drugs
Generally, all therapeutic drugs are toxic and produce hazardous effects at some dose. This depends on many factors like: dose, nature of the drug, individual (genetic) variation, diet, age, etc. The side effect of chloroquinol, an antidiarrhea drug used in Japan in 1960 caused stiffness of the joints accompanied by damage to the optic nerve.
Biological Toxicants
Some naturally occurring substances that cause toxicity include plant, animal, algal, fungal and microbial toxins. They include many phytotoxins and mycotoxins. For example, Aflatoxins are products of Aspergillus flavus. It is fungus that contaminates grain, maize, peanuts, and so on. Aflatoxin B1 is the most toxic and is reported to have carcinogenic effects.
Cosmetics
Cosmetics induce allergies and contact dermatitis. Lipsticks contain lead at varying concentrations. Hair dyes contain resorcinol which is toxic. Paraffin wax is used in some lotions which hinders skin breathing.
Physiological Response to Toxicants: Mutagenesis
Many agents (physical, chemical and environmental) have the mutagenic properties to cause mutations. They are known as mutagens. Mutagenic agents induce mutation in either of the following ways:
- They may replace in the DNA.
- They may alter the base in such a way that it specifically mispairs with another base.
- They may damage the base so much that it can no longer pair with any base.
- They may intercalate themselves in the DNA paving way for addition or deletion of bases.
Physical Mutagens
These consist of high energy radiations which could penetrate living cells and affect the genetic material. The effect of radiations on living cells and tissues is directly proportional to the degree of penetration of the radiation. Radiations are of two types viz. electro-magnetic radiations and particulate radiations. X-rays, gama-rays and UV rays are short wavelength electromagnetic radiations which penetrate cells and tissues strongly.
The physical mutagens are also divided as high energy ionizing radiations which include cosmic rays, X-rays, gamma-rays and particulate radiations and low energy non-ionizing radiations which include ultraviolet light. The high energy radiations create ionization in the living cells. Non-ionizing radiations such as UV rays have major effect in the formation of dimers whereby adjacent pyrimidine bases become linked to one another by carbon to carbon bonds.
Chemical Mutagens
Chemical mutagens are classified into four major groups on the basis of their specific reaction with DNA.
Base Analogs
Base analogues are the chemicals that have molecular structure that are extremely similar to bases of DNA. These chemicals act as mutagens and during DNA replication get incorporated so as to form base pairs with usual bases. One such chemical is 5- bromouracil.
Deaminating Agents
Many chemicals [e.g. nitrous acid, hydroxylamine] are known to change the base sequence in DNA. Nitrous acid (-HNO2) and hydroxylamine replace amino group (-NH2) by hydroxyl group (-OH) which leads to deamination of nitrogenous bases.
For example, deamination of cytosine produces uracil.
Alkylating Agents
Such as ethyl methane sulfonate (EMS), ethyl ethane sulfonate (EES) and diethyl sulphate (DES) act on DNA by adding alkyl group (ethyl or methyl) to all four bases. However, these agents show a strong preference for base guanine. This results either in mispairing of affected base or its loss entirely, creating a gap thus causing mutations.
Intercalating Agents
This type of mutagen includes ethidium bromide and Acridine dyes (proflavin and acridine orange). Intercalating agents can mimic base pairs and slip between the base pairs in double helix and open the helix which leads to increase in distance between base pairs. This results in deletion or addition of base pairs during DNA replication.
Environmental Mutagens
These mutagens include air and water pollutants, agricultural chemicals, pesticides, cigarette and industrial smoke which includes chemicals like Benzidine, Vinyl Chloride etc.
Some environmental mutagens are carcinogenic. Some of the biological agents may also act as mutagens which are listed below:
- Transposons.
- Virus – Inserted virus DNA into genome may change genetic function.
- Bacteria – Some bacteria like Helicobacter pylori cause inflammation and DNA damage which may lead to mutation.
- Parasitic Fungi of Field Crops.
Physiological Response to Toxicants: Carcinogenesis
A carcinogen is any agent that directly increases the incidence of cancer. Most, but not all carcinogens are mutagens. Carcinogens that do not directly damage DNA include substances that accelerate cell division, thereby leaving less opportunity for cell to repair induced mutations, or errors in replication. Carcinogens that act as mutagens may be biological, physical, or chemical in nature, although the term is most often used in relation to chemical substances.
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The transformation of a normal cell into a cancer cell is a multi-step process that involves initiation, promotion, progression and finally malignancy. This process takes years and starts with a single cell in which the right genes are mutated so the cell does not appropriately die and begins to proliferate abnormally. Then, additional mutations occur that select for more rapidly growing cells within this population leading to a tumor with rapid growth and malignancy. By the time the cells are cancerous, proto-oncogenes have been activated and tumor suppressor genes inactivated. Even within the same tumor type, like colon cancer, the specific genes mutated can vary from person to person making cancer a unique disease for each individual.
Chemical Carcinogens
Chemical carcinogens can be either natural or synthetic compounds that, based on animal feeding trials or epidemiological (i.e. human population) studies, increase the incidence of cancer. The definition of a chemical as a carcinogen is problematic for several reasons. Some chemicals become carcinogenic only after they are metabolized into another compound in the body; not all species or individuals may metabolize chemicals in the same way. Also, the carcinogenic properties of a compound are usually dependent on its dose.
Physiological Response to Toxicants: Teratogenesis
Birth Defects:
Teratogens: A teratogen is a compound that permanently deforms the function or structure of a developing embryo or fetus in utero. In general, the degree of teratogenicity depends on:
- The potency of the drug as a mutagen.
- The susceptibility of the fetus to teratogenesis.
- The dose of the teratogen. The duration of teratogen exposure.
- The time of exposure.
- The degree of transfer from maternal to fetal circulation.
The global average of all live births complicated by malformation is 6% (Environmental Health Perspectives, (NIH), October 2009). The majority of these complications are due to unknown factors. The vast majority of recognized etiologies are genetic, with only 10% being attributed to environmental etiologies such as maternal health, infection, and toxicants. In general, the central nervous and skeletal systems are the most affected.
Thalidomide (a sedative previously marketed in Europe to prevent morning sickness) is a classic teratogen that caused limb defects in babies born to women who took this drug in the 1960s.
Case Studies of Toxic Events and Responses
Carcinogenesis:
An article concluded result from National Cancer Registry Programme, aims to provide an update on the cancer incidence estimates in India by sex, age groups and anatomical sites for the year 2022. It was found that, the cancer incidence burden is continuing to increase in India.
The estimated number of incident cases of cancer in India for the year 2022 was found to be 14,61,427 (crude rate:100.4 per 100,000). In India, one in nine people are likely to develop cancer in his/her lifetime. Lung and breast cancers were the leading sites of cancer in males and females, respectively. Among the childhood (0-14 yr) cancers, lymphoid leukaemia (boys: 29.2% and girls: 24.2%) was the leading site. The incidence of cancer cases is estimated to increase by 12.8 per cent in 2025 as compared to 2020.
Types of Wastes
Municipal Solid Waste
Municipal solid waste consists of household waste, construction and demolition debris, sanitation residue, and waste from streets. This garbage is generated mainly from residential and commercial complexes. They may be categorised as:
Garbage: Refers to the putrescible solid waste constituents produced during the preparation or storage of meat, fruit, vegetable etc. These wastes have a moisture content of about 70%.
Rubbish: Refers to non-putrescible solid waste constitute, either combustible or non-combustible waste. Combustible wastes would include paper, wood, scrap, rubber, leather etc., while non-combustible waste are metals, glass, ceramics etc. These wastes contain a moisture content of about 25%.
Hazardous Waste
USEPA defines Hazardous Waste as waste that is dangerous or potentially harmful to our health or environment. Hazardous wastes can be liquid, solid, gaseous or sludge. They can be discarded commercial products, byproducts from industries, or from households. Hazardous Waste includes many different toxic chemicals organic compounds as well as metals). They require complex treatment processes. Some of the commonly known priority chemicals are polychlorinated biphenyls (PCBs), furans, polyaromatic hydrocarbons (PAHs); and toxic metals include lead, cadmium, chromium, mercury. These pose serious health hazards and their migration must be contained. Wastes considered hazardous are those which are ignitable, corrosive, reactive, and toxic. Hazardous waste can be categorized into waste from non-specific sources and waste from specific sources.
Industrial Waste
They include chemicals, paints, sand, metal ore processing, fly ash, sewage treatment sludge etc. Manufacturing industries produce wastes which are solid or semi-solid. This waste can be self-igniting, explosive, toxic or radioactive. Chemical process industries generate a variety of waste, both organic and inorganic, which are mixtures with wide range of component concentration.
E-waste
It is a term used to cover items of electrical and electronic equipment and their parts that have been discarded by the owner as waste without the intention of reuse.
E-waste includes almost any household or business item containing circuitry or electrical components with either power, or battery supply—items such as TV appliances, Computers, laptops, tablets, mobile phone, white goods (fridges, washing machines, dryers, etc.), home entertainment and stereo systems, toys, toasters and kettles
The industries that make such products thrive on obsolescence. New gadgets and new models appear almost daily, prices keep dropping and consumption is soaring. The older models are discarded as junk, even when they are in working condition, all this has resulted in becoming one of the fastest growing waste streams globally. From 19.5 m tonnes in 1990 global e-waste grew to 57.4 m tonnes in 2010 and is set to reach 75 m tonnes by 2030.
E-waste contains many hazardous materials like lead, copper. zinc, and aluminium, flame retardants, plastic casings, cables etc. If e-waste is disposed in landfills, burnt outdoors, or recycled in other appropriate ways, the toxic substances can contaminate the air, water, and soil, affecting all living creatures.
Bio-medical Wastes
The wastes generated from hospitals, nursing homes, health centres, medical and dental colleges, veterinary institutes, laboratories and other associated areas constitute bio-medical wastes.
Examples of bio-medical wastes are – biological cultures; blood, cells and tissues; chemotherapy wastes; pathological wastes; waste from surgery and autopsy; used syringes, gloves, blades, instruments and empty gas containers; stocks (samples) of infectious agents; etc. These wastes are potentially dangerous and infectious.
is generated during the diagnosis, treatment or immunization of human beings or animals or in research activities in these fields or in the production or testing of biologicals. They can pose serious environmental and health problems, if they are not properly handled.
Waste Management Strategies (e.g., recycling, treatment, disposal)
The principle objectives of solid waste management include collection, processing and disposal of solid wastes in an economical manner consistent with the protection of public health: Reduce the generation of the waste: The first principle for the solid waste management is- we should try our best to reduce waste.
Collection:
It refers to the gathering of solid wastes from places such as residences, commercial, institutional and industrial establishments and other public places. Generally, there are two methods of collection- Hauled-container System and stationary-container system. In the hauled-container system, the container is hauled from the collection point to the final point of disposal, processing facility, or transfer station. In the stationary-container system, the container is emptied into collection vehicles at the point of collection.
Handling and Separation
Waste handling and separation involves the activities associated with management of waste until they are placed in a storage container for collection. Handling includes the movement of loaded containers to the point of collection. Separation of waste components is an important step in the handling and storage of solid waste at the source.
Transfer and Transport
The transfer of wastes from the smaller collection vehicle to the larger transport equipment.
The subsequent transport of the wastes, usually over long distances, to a processing or disposal site.
Recycling
Treatment Facilities
The importance of increased interest in mechanized facilities for waste processing is that, as MSW disposal costs rise, recycling seems to come with incentives. Hence convenient and rapid methods of separation and processing should be developed.
- Shredding: for size reduction; done using hammer mills, flail mills, shear shredder, glass crusher and wood grinder
- Screening: for separation of over and under-sized material – using trammel
- Cyclone separator: for separation of light combustible materials from air stream
- Air classification: for separation of light combustible materials from air stream
- Magnetic separation: for separation of ferrous metal from miscellaneous waste
- Densification: for compaction and flattening; using balers and can crushers
- Weighing and handling facilities
Incineration:
Incineration means ‘reduction to ashes’. It is a process of burning of the solid Waste at high temperature to form ash in properly constructed hearth of furnaces. Incineration is used to destroy combustible household waste, chemical waste and biological waste. The burning of solid wastes is carried out at a temperature of 1000℃ or more so as to incinerate all the combustible organic matter and oxidize the foul smelling gases. In case the moisture content of the solid wastes is high, then auxiliary fuels (like wood, coal or oil) may be used along with the solid wastes for complete burning at high temperature. The ashes are disposed by dumping in low lying areas, while the clinkers can be used as aggregate for low grade concrete or as road material. The heat produced during incineration can also be used to produce electricity.
Advantages of Incineration:
- This method is hygienic. as all the pathogens and insects are destroyed.
- Incineration is free from odour and dust nuisance.
- Incineration substantially reduces the volume of waste to be disposed of in a landfill.
- Some revenue can be generated by raising steam/electricity and selling of the clinkers.
- Clinkers produced during incineration can be used as aggregate for low grade concrete or as road material.
Disadvantages of Incineration:
- High initial cost.
- Nuisance of smoke, odour and ash during the improper functioning of incinerators.
- Toxic substances like dioxin, mercury, ozone, etc. may be emitted during incineration.
Composting:
Composting utilises natural degradation process in which biodegradable materials are decomposed by micro-organisms (bacteria, fungi and actinomycetes) and converted into humus and stable mineral compounds. It is a hygienic method which converts the solid wastes into manure. It not only reduces the volume of solid waste considerably but also makes it free from most of the pathogenic organisms.
Composting is a biological process. It can be either anaerobic (oxygen not required) or aerobic (oxygen required).
Anaerobic Composting:
Anaerobic composting is prevalent in Indian villages on a small scale, for combined disposal of solid waste and cattle dung. Anaerobic composting is characterized by slow degradation of biodegradable material extending over a period of 4 to 12 months, low temperature process, destruction of pathogens by their exposure to unfavourable environment for long period, and production of valuable biogas.
In anaerobic composting, trenches 4 to 10 m long, 2 to 3 m wide and 0.7 to 1.0 m deep are excavated. Biodegradable solid waste is disposed of into these trenches till the heaps so formed rise about 30 cm above the original ground level. A layer of 5 to 7.5 cm of good earth is than spread on top of these heaps. Within 2-3 days, intensive biological action starts and the organic matter begins to decompose. After about 4 to 5 months, the decomposing mass gets fully stabilized and changes into brown odourless powdery humus (known as compost) having high fertilizing value. The compost is then removed from trenches, sieved to remove coarse inert materials (like brick bats, stones, bracken glass, etc.). The sieved compost is then used as manure.
Aerobic Composting:
Aerobic composting is characterized by rapid degradation of biodegradable material, and speedy destruction of pathogens due to high temperature attained during the process. In aerobic composting (also called Open Window Composting), the coarse inert matter is first removed from solid wastes. Then it is dumped on the ground in the form of 5 to 10 m long, 1 to 2 m wide and 0.5 to 1.0 m high piles at about 60% moisture content. The pile is then covered with night soil/animal dung. Biological activity starts through aerobic bacteria and heat starts developing upto about 75℃ in the piles. After few days, the pile is termed up for cooling and aeration to avoid anaerobic reactions. The pile temperature again rises and the process of turning, cooling and aeration is repeated. The complete process may take about 4 to 6 weeks. After this, the compost is ready for use as manure. Though aerobic composting is faster, its practical application is hindered due to following reasons—
- It requires mixing and/or aeration facilities.
- The process is dependent on availability of air as well as moisture. The moisture content should be maintained always above 40%.
- Offensive smells (odour) will start if aerobic system is not properly maintained.
- Aerobic composting is relatively expensive and thus not economical.
Benefits of Composting:
- Conversion of biodegradable solid waste into valuable organic fertilizer (manure) for crops.
- Considerable reduction of the quantity of waste to be disposed.
- Production of biogas that can be used directly for heating, cooking or electric power generation.
- It recycles organic materials and nutrients back into the soil.
- Less requirement of space for landfilling.
Disposal:
It refers to the placing of solid waste in its ultimate resting place.
Dumping or Land Filling:
Dumping is a method of controlled final disposal of solid waste into the low-lying land areas. The improved form of open dumping is termed as Land-filling.
In this method of disposal of waste, solid wastes are carried and dumped at landfall sites. The refuse is filled-up or dumped in layers of 1.5 m or so and each layer is covered by good earth of at least 20 cm thickness, so that refuse is not directly exposed. Each layer is left out for at least seven days and then compaction by trucks is carried out for its settlement before starting filling the next layer. Insecticides like DDT should be sprayed on top to prevent breeding of flies and mosquitoes.
Advantages of Land-filling:
- The method is simple and economical.
- NO costly plant and equipment is required.
- Skilled labour is not required.
- Separation of different kinds of solid-wastes is not required.
- No residue or by product; hence no further disposal.
- Low-lying areas can be reclaimed and put to better use.
Disadvantages of Land-filling:
- Larger land area requirement.
- Continuous evolution of foul smell near the site of disposal.
- Disposal landfill sites along the highways give an aesthetically unpleasant view
- Use of insecticides is required.
- These landfill sites may prove to be good breeding grounds for insects, rodents and birds.
- Liquid formed due to seepage Of rainy water in the landfill may dissolve the toxic compounds present in the refuse. When such polluted water contaminates the ground water. it may lead to diseases like cholera, typhoid, polio, etc.
Disposal into Sea:
This method of solid waste disposal can be used in coastal areas having deep sea water (> 30m) at reasonable distance (<16 to 20 km) and with strong forward currents- This is quite a simple and cheap method but it has following disadvantages:
- The bulky and tighter components of solid waste float, spread, and tend to return to the shores during high tides.
- During monsoons or stormy weather solid waste has to be either stored or disposed of by some other methods.
- Some portion of the solid waste may return and spoil the beaches, despite all the necessary precautions.
Three-Rs of Solid Waste Management
In solid waste management, the stress should be on 3R’s — Reduce, Reuse and Recycle.
Reduce
Reduction in the use of raw materials is one of the fundamental ways to decrease the production of Solid waste. This can be achieved by using fewer raw materials when making a product, reuse of products on site, designing of products or packaging to reduce their quantity.
Reuse
Reuse of the materials in their original form is the next fundamental way to minimize the generation of solid waste. The reuse of materials, instead of throwing them away, not only reduces waste but also save money. Some of the examples of reuse of materials are as under —
- Reuse the refillable containers after washing.
- Wash and reuse the disposable items like plastic bags, plastic utensils, etc. as most of them can last for a long time with many uses.
- Rubber rings made from discarded cycle tubes can be used by vendors instead of new rubber bands.
Recycling
is the reprocessing of discarded materials that may have some economic value into new useful products. Recycling will not only make materials available to future generations but will also save energy and environment. It is important for the recovery of reusable products from waste before its final disposal. This can be done either at separation point or the waste can be brought to Material Recovery Facility (MRF). It is important to have the wastes homogeneous and free of contamination before going to MRF.
Some of the examples of recycling and waste utilization are as under:
- Recycling of metals, paper, glass and plastics- Mining of metals (like aluminium, iron, copper, tin, etc.) is expensive and hence recycling of metals is economically important, Recycling of paper helps in preserving forests, as it takes about 17 trees to make one ton of paper.
- Utilising fly ash (a waste material from coal fired thermal power plants)
- Conversion of agricultural wastes (like rice husk and groundnut shells) into cheap and efficient fuel.
- Paper from agricultural wastes
- Energy from urban wastes
- Utilisation of slaughter house waste- Blood is used in pharmaceutical industry; and hides and skins are used for leather production.
- Recycling of metals, paper, glass and plastics- Mining of metals (like aluminium, iron, copper, tin, etc.) is expensive and hence recycling of metals is economically important, Recycling of paper helps in preserving forests, as it takes about 17 trees to make one ton of paper.
Advantages of Recycling and Waste Utilization:
Recycling is an integral part of solid waste management. It also makes economic sense. By recycling and proper utilization of waste, particularly; in developing countries like India, many advantage can be availed.
These include—
- Directly or indirectly, recycling and waste utilization contributes to economic development.
- Recycling is helpful in conservation of natural resources.
- Recycling can reduce/control environmental pollution substantially
- Employment opportunities are also generated.
Bioremediation
Bioremediation is the utilization of microorganisms to break down organic contaminants present in soil, groundwater, and sludge. To stimulate microbial activity, bioaugmentation or biostimulation is done.
Bioaugmentation is the introduction of microorganisms to the contaminated site, if the existing concentration of microorganisms is too low to be effective. Biostimulation is the addition of nutrient media or electron donors/ acceptors so as to favour microbial growth.
Bioremediation may be performed ex-situ or in-situ. In-situ processes treat the contaminants at the site where they are present, without removal to a different site. Ex-situ processes involve relocation of contaminated site to a designated treatment area.
Biological processes are usually implemented at a lower cost as compared to physicochemical treatment processes. Contaminants are destroyed completely in most cases. Sometimes, more toxic by-products are generated (TCE to vinyl chloride). These contaminants may become mobilized, especially in ground water. To remediate such a site, bioremediation will be performed above a low permeability soil layer, and groundwater monitoring wells will be placed downgradient of the remediation area.
Remediation with the help of microbes can be of two types: anabolic and catabolic. Catabolism is the generation of energy from the degradation of organic contaminants. Bonds which are easily broken contribute to more energy being released. Anabolism is the synthesis of new microbial cells.
In-situ bioremediation is the remediation of soils and/or groundwater utilizing naturally occurring microorganisms in order to biologically break down contaminants present. The media is not removed from its location. The development of microbial culture within the site can be brought about with oxygen (aerobic) or without oxygen (anaerobic or anoxic).
The advantages of an in-situ treatment system are: ideal for small operational sites, minimal intrusion to above-ground structures. However, it is not suitable for sites with free phase contaminants.
In-situ bioremediation of soil involves supplying of oxygen and nutrients to the soil. Two such methods are bioventing and injection of hydrogen peroxide. Bioventing systems deliver air from the atmosphere to the soil above the water table through injection wells placed in the contaminated area. Injection of H2O2 on the other hand, delivers H2O2 which in turn stimulates microbial activity and helps speed up the biodegradation process. Injection of H2O2 is done only in instances when the groundwater is already contaminated.
Aerobic bioremediation
Aerobic bioremediation is the oxidation of waste using O2 as the electron acceptor.
For example: HCHO + O2 → CO2 + H2 O
It is effective for hydrocarbons (such as mid-weight petroleum products like diesel and jet fuel). Lighter products like gasoline volatilize readily, and it is more effective to remove them using soil vapour extraction or air sparging.
Oxygen is the most favoured electron acceptor, followed by nitrate, manganese, iron and so on. Most of the municipal waste components serve as electron
donors.