Odour is an important pollution agent. An odorant generally originates from a solid, a liquid, or a concentrated gas. Odour sources may be confined in space, like emission from ducts, or they may be unconfined, like drainage ditches, and settling lagoons.
Hydrogen sulphide, carbon disulphide, mercaptans, products of decomposition of proteins (especially those of animal origin), phenols, and some petroleum hydrocarbon are the malodours which are very common.
Odourous compounds are also generated due to various human activities. Garbage dumps, sewage works and agricultural activities are typical examples. Bad smell is also produced by decaying vegetation. The exhaust from motor vehicle is also a common source of malodour.
The sources of odour are so many, that it is almost impossible to prepare a complete list of them. Table indicates the various odourous industrial operations.
Odourous Industrial Operations
||Pulp and paper
||Hides, flesh, hair
||Ammonia, nitrogen compounds
||Sulphur, compounds from crude oil, cresols
||Ammonia, phenols, mercaptans, hydrogen sulphide, chlorine, organic products
||Biological extracts and wastes, fermented
||Cannery waste, dairy waste, meat products, packing house wastes, fish, cooking odours, coffee roaster effluents
||Burning rubber, solvents, incinerator smoke
An important problem of air pollution is the control of objectionable odours, which may be gases, mists, or solids discharged into the atmosphere from industrial, commercial, and municipal operations. Source control is the most effective means of abating odour. In many cases, this can be achieved by good sanitation practices, as the most persistent and offensive odours arise due to putrefaction.
Following are the methods which may be employed alone or in various combinations to eliminate or diminish odours.
- Modification of the process
- Dilution by ventilation or dispersal
- Combustion or oxidation
- Direct incineration
- Catalytic incineration
- Odour masking
- Odour counteraction of neutralization
- Injection of a reactive substance
Modification of the Process :
In some cases a change in the process, either by way in the composition of process materials or removal of impurities may help in odour control. Methods includes substitution of low-odour solvents for highly odourous ones, adjustment of process variables like temperature, residence time etc. If such alteration makes the resultant source less intense or more tolerable from the point of odour and if such modification is technically and economically feasible, then it, merits first consideration.
Dilution by Ventilation or Dispersal
Odour intensity being a function of the odoraqnt concentration, proper well designed ventilation is the most common method for removal of odours from enclosed spaces.
A method sometimes used for odour abatement outdoors is to release odourous gases from tall stacks. It results in normal dispersion in the atmosphere and consequent decrease in ground-level concentrations below the threshold value. Dispersal by stacks requires careful consideration of the location, meteorological parameters, etc.
Absorption is applicable when the odourous gases are soluble or emulsifiable in a liquid or react chemically in solution. Liquid scrubbing of the gases in a suitable absorption unit is an important methods of odour control.
Adsorption, particularly on activated carbon, has been widely used in odour control because activated carbon has a preferential attraction and high retentivity for organic vapours. Activated carbon is highly porous and has great adsorptive power due to very large surface area. The highly porous structure permits the carbon to remove and hold the organic vapours, hydrogen sulphide, and other odour-producing substances. Furthermore, the retained material may be disrobed comparatively easily, and the carbon reactivates and is used again. But, if the concentration of the odourous material is high, the method may not be economical.
Combustion or Oxidation
In this process, the odour generating gases are made to pass through a combustion chamber at a temperature of the order of 650-815oC, in the presence of excess oxygen. The main demerit of this method is the cost of the fuel to produce the required temperature. Sometimes heat exchangers may be employed to recover the heat from the hot incinerated gases. The heat recovered may be used for other purposes like preheating the effluent gases, generating steam etc. The optimum residence time of the gases in the chamber should be found out by trial, since the various gas constituent to be burnt react at different rates. Normally this residence time should not be less than 0.3 sec.
When the cost of heating the gas stream to 650oC is very high, catalytic combustion may be the choice. In a catalytic unit, oxidation takes place at a much lower temperature than necessary for direct incineration. In this method, the process gases, pass through specially designed units containing catalyst elements, on the surface of which oxidation occurs. During the catalytic oxidation, the constituents in the gas stream such as hydrocarbons and other organic malodours, react with oxygen to form carbon dioxide and water vapour. In general, complete oxidation should take place to solve the odour problem. Catalytic combustion of most of the organic constituents that occur in the effluent as gas stream is initiated and self sustaining at about 260oC. In case the gas stream is colder than this before coming into contact with the catalytic, it must be pre-heated. However, the fuel cost there for pre-heating will be very much less, when compared to direct incineration.
Odour Masking :
Odour masking is based on the principle that, when two odours are mixed, the stronger one will predominate. In other words, strong odours tend to mask weaker ones. Thus a strong, pleasant odour can be used to ‘mask’ or ‘cover’ weak unpleasant odour. However, care must be taken to see that the odorant used for superimposing the pleasant odour is not flammable, corrosive, or allergic.
Following are the methods used for odour masking :
- Spraying, vaporizing, or atomizing the odorant chosen, into air-gas streams in stacks.
- Adding directly to a process wherever possible.
- Adding to scrubbing liquors.
In cases where the masking odorants are directly added to the process, care should be taken to se that they will not affect the quality of the products.
Odour masking can also be used to control odours in outdoor places like refuse dumps or waste lagoons. Here, the masking compound must vaporize rapidly enough to overcome the unpleasant odour and slow enough to last for a reasonable length of time.
Due to various practical problems involved on odour masking the selection of the suitable odorant and its quantity should be left to specialists in the field.
Certain pairs of odours of relative concentrations are antagonistic. Therefore, when they are mixed together, the intensity of each odour is diminished. This effect is known as counteraction or neutralization. Examples of such odour pairs are musk and bitter almond; rubber and cedar wood.
Selection of the proper counteractant for a given odour is even more difficult than the selection of masking compounds, and therefore the selection should be left to experts.
Control of Air Pollution by process Changes
In controlling air pollution by process changes, the method to be employed naturally depends upon the particular process involved. As a result, no fixed set of rules can be applied on a universal basis.
Generally, four methods are available to control the pollutants by process changes. They are :
- Substitution of raw materials or fuels
- Modification of the process itself
- Modification or replacement of the process equipment
- Changes in operational practices
Substitution of Raw Materials or Fuels
This method has been used successfully in many cases for controlling atmospheric pollution. For example use of low-volatile coals in place of high-volatile coals has proved quite effective in eliminating smoke and soot in many industrial and commercial heating applications. Similarly, substitution of low sulphur fuels for high sulphur fuels has reduced considerably the sulphur dioxide discharge into the atmosphere. Another method of decreasing emission of air pollution is substituting bauxite flux for fluorspar in an open health furnace.
Modification of the Process
While modifying a given process, a unit operation may be eliminated or altered, or other unit operations may be substituted or added. For example, in disposing combustible refuse, the practice of incineration may be discontinued in favour of sanitary land fill. Another example is in Brass Foundry Practice, where an additional operational step has been used to reduce air pollution. Here, a fluxing material is applied to the surface of the molten brass which serves as an evaporation barrier and consequently reduces the emission of brass fumes. This additional step has been included strictly as an air pollution control measure. A third example of process modification is substitution of oxygen for air in gas manufacture and in blast furnaces. The vent gases produced as a result are lower in volume.
Modification or Replacement of the Process Equipment
This may include (a) modification of one or more items of the process equipment, (b) replacement or repair of faulty or repair of faulty or malfunctioning equipment, or (c) substitution of one type of equipment for another type. For example
- Use of vapour recovery systems to control vapour losses in handling volatile materials-absorbers, condensers, and compressors may be used.
- In cast iron founders, substitution of reverberatory furnaces for cupolas has resulted in reducing atmospheric pollution from this type of operation.
Changes in Operational Practices
A good example for this is a thermal power plant. Hereby using a low-sulphur fuel in place of high-sulphur fuel, the rate of release of sulphur dioxide from coal burning operations can be reduced, especially during the periods of adverse meteorological conditions, and consequently reduce the air pollution problem.
Investigations have shown that fuel additives are effective in reducing smoke gas turbines. Hence, probably they may be used to reduce emission of smoke, soot, carbon monoxide, and hydrocarbons, associated with incomplete combustion, and thus reducing the air pollution problem.