Air Pollution Abatement
Air pollution is the presence in larger than normal concentrations of chemicals, particulate matter and biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment, or built environment in the atmosphere.
The composition of normal free air in the atmosphere is Nitrogen (N2) 78.084%, Oxygen (O2) 20.9476%, Argon (Ar) 0.934%, Carbon Dioxide (CO2) 0.0314% and Neon (Ne) 0.001818%.
Substantial change in this proportions or infiltration of other particles including disease causing materials constitutes pollution. There are several major types of pollutants in air causing different types of effects on the environment and human health.
These include smog, acid rain, the greenhouse effect, and “holes” in the ozone layer. Each of these problems has serious implications for our health and well-being as well as for the whole environment.
One type of air pollution is the release of particles into the air from several human activities including burning fuel for energy. Diesel smoke is a good example of this particulate matter loading.
The particles are very small pieces of matter measuring between 2.5 (PM2.5) and 10 (PM10) microns that form black carbon pollution in the air. Other sources include exhaust from burning fuels in automobiles, homes, and industries as well as the burning of biomass materials such as wood and charcoal.
Another type of pollution is the release of noxious gases, such as sulfur dioxide, carbon monoxide, nitrogen oxides, and chemical vapors. These can take part in further chemical reactions once they are in the atmosphere, forming smog and acid rain.
Pollution is also created in our homes, offices and schools by some activities we perform, e.g. smoking and cooking. Exposure to harmful indoor pollutants can be serious and increases as the number of hours we spend inside built up environment increases.
It is therefore important to consider both indoor and outdoor air pollution in any mitigation and control programmes.
Conventional Methods
Conventional methods of air pollution control are based mainly on things we as individuals can do to prevent air pollution and make our environment cleaner and safer.
Recommended clean air practice include carpooling, walking, riding bicycles and using public transportation systems to help reduce the number of cars on the road, thus, air pollution from exhaust pipes.
On a larger scale, there are many different types of equipment available for businesses and factories to cut down or even prevent air pollution. These equipment include baghouse filters, activated carbon absorbers and gas absorption towers.
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Baghouse filter can be used in many areas like coal, power, steel, chemical, and even mining industries. They have the capacity to filter huge gas volumes and remove most particulate matter from air before it is released into the atmosphere.
The carbon absorbers remove things like organic acids, hydrogen sulfide and aldeheydes. Chemical absorption towers are designed to remove chemicals such as hydrogen sulfide, ammonia, sulfuric acid, nitrogen oxides, sulfur dioxide and many more. Sometimes it may be necessary to customize any of these equipment to meet specific needs.
Cigarette smoke is also a major contributor to air pollution and one of the best control practice is reduction in the number of smokers and the rate at which individuals smoke.
Indoor air pollution is best controlled by appropriate ventilation and reduction in rate of activities that generate smoke or polluted air indoors. Such practices include indoor smoking, cooking etc.
Biofiltration
Biofiltration is a pollution control technique that uses living materials to capture and biologically degrade process pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface run-off and micro-biotic oxidation of contaminants in air.
Examples of biofiltration include bioswales, biostrips, biotrickling, biofilters, constructed and natural wetlands, slow sand filters, treatment ponds, green belts and the living walls. The most commonly used filters for the removal of odour and particulate matter are the biofilters and bioscrubbers in air.
A biofilter is simply a bed of organic material (medium), typically a mixture of compost and wood chips or shreds, about 25-46 cm deep.
As air passes through the biofilter the microbes on the organic material convert odorous gases to carbon dioxide and water. The effectiveness of the biofilter is primarily a function of the amount of time the odorous air spends in the biofilter (contact time) and the moisture content of the filter material.
Contact time is part of the biofilter design while moisture content is a function of good management. The size (footprint) of the biofilter depends primarily on the amount of air needing treatment.
A typical biofilter will require 4.65 – 7.90 m2/28.3 m3/minute of airflow. Biofilters are also categorised by their configuration (open or closed), flow sequence (up-flow, down-flow or horizontal flow).
A bioscrubber has a circulated scrubbing liquid which contains water and microorganisms for degradation of the substances to be separated off from the dirty gas.
The gas enters at an inlet, moves through a mass- transfer zone, where it undergoes a phase change from the gas phase to the liquid phase, and the clean gas exits through an outlet.
The system has a device for irrigating the mass-transfer zone with the scrubbing liquid, and a tank for collecting the scrubbing liquid and for activating the microorganisms.
The bioscrubber has particularly high separation rates and particularly low risk of blockage because, in at least one mass- transfer zone of the bioscrubber, there may be provided a package of adjacent vertical tubes and a cleaning device for cleaning the tubes. A mass-transfer zone can be formed by a spray tower.
Water Pollution Abatement
Water pollution is an undesirable change in the state of water, contaminated with harmful substances. It is the second most important environmental issue next to air pollution. Any change in the physical, chemical and biological properties of water that has a harmful effect on living things is water pollution.
It affects all the major water bodies of the world such as lakes, rivers, oceans and groundwater. Pollution of the water bodies disturbs the ecosystem as a whole.
Polluted water is not only unsafe for drinking and other consumption purposes, but it is also unsuitable for agricultural and industrial uses. The effects of water pollution are detrimental to human beings, plants, animals, fish and birds.
Polluted water also contains virus, bacteria, intestinal parasites and other pathogenic microorganisms. Using it for drinking purpose is the prime cause for waterborne diseases such as diarrhoea, dysentery and typhoid.
The important sources of water pollution are domestic wastes, industrial effluents and agricultural wastes. Other sources include oil spills, atmospheric deposition, marine dumping, radioactive waste, global warming and eutrophication.
Among these, domestic waste (domestic sewage) and industrial waste generate most pollutants, which make their way to groundwater and surface water bodies.
In order to reduce the level of pollution of water and effectively utilize water resources, it is important to control not only existing pollution in water but also the rate of pollution in the future.
Although 71% of earth’s surface is covered with water bodies, we don’t have enough water to drink. Many researches have been done on water purification systems in order to have safe drinking water.
However, there are about 1 billion people, who don’t have proper access to drinking water. Therefore, water needs to be conserved and prevented from pollution in order to make it safe for drinking and other consumption purposes.
The major water treatment procedures available to achieve this goal are broadly divided into chemical and biological methods. There are three major types of biological treatment methods defined on the basis of oxygen demand.
All involves the activities of microorganisms in the presence of oxygen (aerobic), absence of oxygen (anaerobic) or in oxygen deficient environment (anoxic).
Aerobic Biological Treatment
Aerobic biological treatment which may follow some form of pretreatment such as oil removal, involves exposing wastewater to microbes and oxygen in a reactor or pond to optimise the growth and efficiency of the biomass.
The microorganisms act to catalyse the oxidation of biodegradable organics and other contaminants such as ammonia, generating harmless by- products such as carbon dioxide, water, and excess biomass (sludge).
This is a bioprocess activity in which microorganisms (aerobes) utilise dissolved oxygen supplied naturally or artificially from aerators to degrade organic wastes.
The microorganisms may consist of naturally occurring bacteria, fungi, protozoa, rotifers or other microbes usually present in most wastewaters or may be genetically-engineered to optimise their activities.
Population dynamics of the microbes depend on environmental factors such as pH, temperature, type and concentration of the substrate, hydrogen acceptor, concentration of essential nutrients e.g. nitrogen, phosphorus, sulfur, etc.
The microorganisms feed on the organic materials in the process degrading them to simpler organic or inorganic compounds. Typical organic materials that are found in residential wastewater include carbohydrates, fats, proteins, urea, soaps and detergents.
These are degraded into simple organics like CO2 or biologically transformed from organic forms to mineralised forms (i.e., NH3, NH4, NO3, SO4, and PO4). The primary mechanism of action used by both the aerobic and anaerobic microorganisms is fermentation process in two lines.
The first line involves heterotrophic microorganisms that use organic carbon for the formation of new biomass. These organisms are consumers and decomposers that depend on a readily available source of organic carbon for respiration and growth.
They primarily reduce soluble BOD in wastewater treatment. The second line are the autotrophic microorganisms that utilise simple forms of carbon (such as carbon dioxide) to remove nitrogen from wastewater.
Design of treatment facilities such as bioreactors provide the microbes with optimal conditions for rapid degradation of wastewaters. This includes excess dissolved oxygen to enable the aerobic and facultative microbes rapidly oxidise soluble, bioavailable organic and nitrogenous compounds in wastewater.
When dissolved oxygen is available, the aerobic decomposition of organic compounds consumes dissolved oxygen in the water. If the rate of re-aeration is not equal to the rate of consumption, the dissolved oxygen concentration will fall below the level needed to sustain a viable aquatic system.
Aerobic treatment has many advantages and disadvantages. The advantages include:
Production of minimum odour effect when properly loaded and maintained
Removal of large biochemical oxygen demand (BOD) providing a good quality effluent
High rate treatment allowing smaller scale systems, e.g., less land required
The final discharge may contain dissolved oxygen which reduces the immediate oxygen demand on a receiving water; and
The aerobic environment eliminates many pathogens present in agricultural wastes.
Aerobic treatment also has main disadvantage which include:
High energy cost of aeration that must be maintained to achieve adequate rate of dissolved oxygen levels needed to maintain aerobic conditions in the treated wastewater for aerobic growth
Some organics cannot be efficiently decomposed aerobically because they are biologically non-reactive and may constitute about 70% of the chemical oxygen demand (COD)
Rapid production of sludge may affect the storage capacity of the ponds.
Anaerobic Biological Treatment
Anaerobic (without oxygen) and anoxic (oxygen deficient) treatments are similar to aerobic treatment, but use microorganisms that do not require the addition of oxygen. These microorganisms use the compounds other than oxygen to catalyse the oxidation of biodegradable organics and other contaminants, resulting in harmless by-products.
Since the organic pollutants are degraded by anaerobic microorganisms in the absence of oxygen the gas produced contain predominantly methane and carbon dioxide. This is known as “biogas”.
Aeration and Loading Techniques
Regardless of the type of system selected (aerobic, anaerobic or anoxic), there are two major contact design options that could be adopted to maintain the required population of microbes in a bioreactor. These are:
Fixed film processes — microorganisms are held on a surface, the fixed film, which may be mobile or stationary with wastewater flowing past the surface/media. These processes are designed to maintain active contact between the biofilm, wastewater and oxygen (where necessary).
Suspended growth processes — biomass is freely suspended in the wastewater and is mixed and can be aerated by a variety of devices that transfer oxygen to the bioreactor contents
It is also possible to combine both methods in a single reactor for more effective treatment.
There are many types of fixed film process, as described below:
Biotrickling filters: Also known as biotowers, is the one of the most commonly used fixed film process. It consists of a basin or tower filled with support media such as stones, plastic shapes, or wooden slats.
Wastewater is applied intermittently, or sometimes continuously, over the media. The water then trickles downward through the bed. Air circulates upward through the media as treated water is removed by an under drain system.
As the wastewater trickles downward through the bed, a biological slime of microbes develops on the surface of the media. Continuous flow provides the needed contact between the microbes and the organics.
Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolised.
Oxygen is normally supplied to the film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air.
Forced air can also be supplied by blowers but this is rarely necessary. The thickness of the bio-film increases as new organisms grow.
Rotating biological contactor(RBC): This consists of vertically arranged, plastic media on a horizontal, rotating shaft.
The biomass coated media are alternately exposed to wastewater and atmospheric oxygen as the shaft slowly rotates at 1–1.5 rpm, with about 40% of the media submerged.
High surface area allows a large, stable biomass population to develop, with excess growth continuously and automatically shed and removed in a downstream clarifier.
RBC systems are particularly used in the in the petroleum industry because of their ability to quickly recover from upset conditions and because it can easily be expanded if need arises.
Submerged biological contactors (SBC): This is similar to RBC, but operates at nearly 90% (RBC is about 40%) submergence with coarse- bubble diffused aeration providing a means of both aeration and motive force for rotation. Because of greater submergence, the load on the shaft is significantly less than that of an RBC.
The SBC also provides nearly three times the surface area of a conventional RBC per foot of shaft length. With its compact design, the SBC is very easy to cover for VOC and odor containment.
Unlike the RBC, the SBC system is driven completely by air, making it one of the lowest maintenance and lowest operation-intensive biological- treatment systems available. Like the RBC, the SBC is modular and can easily be expanded
Examples of the suspended-growth processes include:
Diffused aeration: Here air is added to wastewater artificially, increasing dissolved oxygen content and supplying microorganisms with oxygen necessary for aerobic biological treatment.
Fine-bubble diffused-aeration systems are available in various types including ceramic and membranes, and are highly efficient.
More reliable, but less efficient, coarse-bubble aeration systems are also available, and are normally manufactured of corrosion- resistant, stainless-steel components.
Both systems are compatible with new installations and replacement of existing gas-aeration equipment.
Fine-bubble aerators offer very low VOC stripping potential, and both fine and coarse diffusers provide good BOD and COD removal efficiency.
Jet aeration: The jet-aeration system is designed to provide required aeration as well as maintain suspension of biological solids, with the flexibility to either aerate or mix independently without the need for additional equipment.
Air flow rates to the system can be varied. When aeration requirements decrease and air is completely shut off, pumps provide the required mixing action to enhance process control and save energy.
The subsurface discharge leads to smooth and quiet operation, with no misting, splashing or spray from the basin. This also translates to low VOC release to the atmosphere.
Since jet aeration requires no moving parts in the basin, the system offers long life with no in-basin routine maintenance required.
Surface aeration: This involves the use of surface aeration from high and low speed floating aerators pumping oxygen by breaking up the wastewater into a spray of droplets.
The large surface area of the spray allows oxygen to enter the wastewater from the atmosphere. At the same time, the oxygen-enriched water is dispersed and mixed, resulting in effective oxygen delivery.
High- and low-speed surface aerators offer excellent oxygen transfer and low operating costs. They are also able to handle environmental extremes such as high temperatures. Another alternative to surface aeration is the use of horizontally mounted aeration discs or rotors.
These disc or rotor aerators can be used in oxidation ditches known as looped, “race track” reactor configurations. They provide stable operation with resulting high- quality effluent.
The aerators are above water for easy maintenance and are energy efficient. Other multichannel processes use a concentric arrangement of looped reactors, which is particularly energy efficient and designed to achieve total nitrogen removal through simultaneous nitrification/denitrification.
Disc and rotor surface aerators offer good BOD and COD removal efficiencies, and are very easy to replace if necessary. Reactors in a vertical-loop configuration are also available for surface aeration.
They are essentially oxidation ditches flipped on their sides. Upper and lower compartments separated by a horizontal baffle run the length of the tank. Surface-mounted discs or rotors provide mixing and deliver oxygen. Typically, two or more basins make up the system.
The first basin operates as an aerated anoxic reactor and the second basin is operated under aerobic conditions. These types of reactors also have high BOD/COD removal efficiency.
Effluent Treatment Using Enzymes and Microbial Cells
A microbial fuel cell (MFC) is a device that converts chemical energy to electrical energy by the catalytic action of microorganisms. The idea that microbial cells could be used to produce electricity was first conceived at the turn of the twentieth century by M. Potter.
However, empirical evidence was not provided until 1931 when Barnet Cohen created a number of microbial half fuel cells that, when connected in series, produced over 35 volts, though only with a current of 2 milliamps of electricity.
Now studies on electricity generation using organic matter from the wastewater as substrate have shown unequivocally that MFCs can be used to produce electricity from water containing glucose, acetate or lactate.
The principle is based on the understanding that bacteria gain energy by transferring electrons from an electron donor, such as glucose or acetate, to an electron acceptor, such as oxygen.
The larger the difference in potential between donor and acceptor, the larger the energetic gain for the bacterium, and generally the higher the growth yield. In a microbial fuel cell, bacteria do not directly transfer their electrons to their characteristic terminal electron acceptor, but these electrons are diverted towards an electrode, i.e. an anode.
The electrons are subsequently conducted over a resistance or power user towards a cathode and thus, bacterial energy is directly converted to electrical energy. To close the cycle, protons migrate through a proton exchange membrane.
In summary, all environmental media air, water and land are susceptible to pollution and contamination. However, though the specific types of pollutants and contaminants may vary from one media to the other and from one locality to another, they are all physical, chemical or biological in nature.
Control of pollution in any of the media is carried out either by conventional methods or by biotechnological methods. The major biotechnological methods for air pollution control are bio-filtration in nature and include bio-swales, bio-strips, bio-trickling, bio-filters, constructed and natural wetlands, slow sand filters, treatment ponds, green belts and the living walls.
These filter different types of materials to expose microorganisms to organic material (medium) for biodegradation. In water and wastewater treatment, different bioreactor facilities provide platforms for biodegradation activities of microorganisms either in the presence (aerobic) or absence (anaerobic) of oxygen.
Decontamination of degraded land is done by any of the various bio- remediation methods. These include windrows, land farming, bio piling and composting, although most are based on the principle of composting. Another method which is fast gaining ground is phyto-remediation which is the application of resistant plants in the re-mineralization of contaminated lands.
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