1. Excluding development close to the site

Development close to the site is to be excluded. A reasonable "buffer zone" around the area sources has to be determined. The actual size of this zone will depend upon a number of factors, including the size of the area from which odours emanate, the intensity of the odours being emitted, the duration and frequency of the odour emissions, the actual process being undertaken, the topography of the site, the weather conditions that prevails at the site. Green belt development in the buffer zone may help at least partially to obfuscate the odour.

2. Ensuring that the operation is carried out under the best management practice

Best management practices (BMP) will vary according to the industry producing the odour. However, for all new developments, BMPs will start with the site selection and the building of the facilities.

3. Nozzles, sprayers and atomizers that spray ultra-fine particles of water or chemicals can be used along the boundary lines of area sources to suppress odours.

Figure 4. A typical atomizer installation for odour control

6.2.1 Mist filtration

While gases cause most odour, problems may also result from aerosols in the fumes. Odorous air streams frequently contain high concentration of moisture. If these vapour discharge can be cooled to less than 40° C, a substantial quantity of the water vapour will be condensed and so reduce the volume of gases to be incinerated. Mist filters can be used for this purpose. Mist filters can also remove solids and liquids from gas stream; if the odour is caused by these particles, then it will result in odour reduction.

Thermal oxidation/ incineration is the oxidation of the odour into carbon dioxide and water by the combustion of the odour with fuel and air. The reaction takes place at temperatures ranging from 750^oC to 850^oC. This is generally above the auto-ignition temperature of most solvents and other VOCs and is a reflection of the heat required to maintain the reaction at dilute concentrations with additional process heat losses. In this regime, the destruction efficiency is almost 100%, assuming adequate oxygen supply. In some cases, other compounds may be formed depending on the mixture of fuel and air used, the flame temperature and the composition of the odour. These compounds may include carbon monoxide, oxide of nitrogen and sulfur oxides. Thermal oxidizer is a refractory-lined furnace fitted with one or more burners. The furnace consists of two chambers- mixing chamber and combustion chamber. There are 3 types of thermal oxidizer:

Direct-fired thermal oxidizer is effectively a combustion chamber with a burner and the appropriate control system. The exhaust from a direct-fired unit is typically at the combustion temperature with no primary or secondary heat recovery. This is used where heat recovery is not required (e.g. when fuel for the burner is free or very cheap). In many cases the fuel cost of heating a process stream to the combustion temperature leads to the inclusion of some sort of heat recovery mechanism. Where the level of VOC is significant, then the heat release from the VOC can be recovered to improve the cost effectiveness of the system. Both recuperative and regenerative thermal oxidizer technologies include heat recovery systems to recover heat as a utility for other energy requirements.

Recuperative systems are basic thermal oxidisers with built-in primary shell and tube heat exchangers. A primary heat exchanger can recover up to 70% of the heat input by the burner or released during the oxidation process by heating up the inlet stream thus reducing the required burner load to maintain the required oxidation temperature (typically 750^oC-800^oC). These are a simple, cost effective, means of destroying VOC where the inlet concentration is relatively high or particularly where heat can be usefully recovered for other processes.

Regenerative thermal oxidizer (RTO) is the most often used type of thermal oxidizer because of its robust performance and its ability to operate at high thermal efficiency. The RTO utilizes beds of ceramic media to provide the thermal efficiency. Two or more beds are used in a controlled cycle and alternatively operate to heat incoming air and to cool exit air. The unit can operate at thermal efficiencies of between 80% and 98% and can handle most types of fume. This means that where an exhaust stream contains a significant level of VOC, then auto-thermal burning (without the use of burners) is possible. At lower concentrations also, the RTO often provides cost-effective operation because of its very high thermal efficiency.

6.2.3 Catalytic oxidation Catalytic oxidation reaction can be forced to proceed at much lower temperatures (e.g. 200^oC) in the presence of a catalyst (Figure 5). Thus, the advantage of this process over thermal oxidation is the reduction in required energy input. Catalytic systems are therefore more favourable where auto-thermal operation is not practical and heat cannot be economically used elsewhere     

            


Figure 5. A catalytic oxidizer system

A number of transition and precious metal catalysts can be used in catalytic oxidizer to destroy various VOCs over a wide range of process conditions.

6.2.4 Biofiltration
This method is becoming an acceptable and successful way of reducing odours from biological process. Biofiltration is a natural process that occurs in the soil that has been adopted for commercial use. Bio-filters contain micro–organisms that break down VOC’s and oxidize inorganic gases and vapors into non–malodorous compounds such as water and CO₂. The bacteria grow on inert supports, allowing intimate contact between the odorous gases and the bacteria. The process is self-sustaining. Bio-filters constructed of various materials including compost, straw, wood chips, peat, soil, and other inexpensive biologically active materials. Two typical arrangements of biofilters for odour control is shown in Figure 6.

 

Figure 6. Biofilters for odour control

Another type of bio-filter is the soil-bed filter. Here the odorous gas stream is allowed to flow through a porous soil with a typical depth of 60 cm. The bacteria in the soil are responsible for the destruction of the odorous compounds.

6.2.5 Adsorption

A method that is suitable for controlling odorous substances, even at low concentrations, is adsorption on to activated carbon. For effectivity, the contaminated air stream must be free from dusts and particulates that might clog the carbon particles. Regeneration of carbon for re-use will produce either waste water, which will require further treatment before disposal, or a concentrated vapour stream, which can be incinerated more cheaply than the original air stream.There are also systems that use activated alumina impregnated with potassium permanganate for adsorption. The alumina absorbs the odorous substances so that the permanganate can oxidize them, usually to carbon dioxide, water, nitrogen and sulfur dioxide, depending on their composition. The alumina bed is replaced progressively as the permanganate is exhausted. This has an advantage over carbon because no further treatment is needed; this may offset the cost of alumina.

Wet scrubbing of gases to remove odour involve either absorption in a suitable solvent or chemical treatment with a suitable reagent. It is important that hot, moist streams are cooled before they contact scrubbing solutions. If this is not done the scrubbing solution will be heated and become less efficient, the scrubbing medium will become diluted from condensation of water vapour.

Wet scrubbing or absorption systems can be either ventury systems or packed tower systems. Venturi systems are co-current scrubbers that accelerate the gas stream into a high density liquor spray. The aqueous droplets then impinge or impact at high relative velocity with solids in the gas stream. The resulting conglomerated particle is then separated from the gas stream in a disengagement tower by virtue of inertial forces. The high density spray also provides reasonable mass transfer to the absorption of gaseous contaminants. Packed Towers are typically counter current scrubbers that utilise high surface area media as a contact zone for the gas stream with suitable scrubbing liquor. The media facilitates high efficiency mass transfer to provide >99.9% removal of gaseous contaminants.

When the odour is caused by the presence of unsaturated organic compounds, it may be necessary to use an oxidizing agent such as chlorine, diluted sulfuric acid and sodium hydroxide to treat odour.

Absorption is applicable when the odorous gases are soluble or emulsifiable in a liquid or react chemically in solution. Wet scrubbing is a useful process to handle acid gas streams, ammonia or streams with solids that might foul other equipment. It has been suggested that liquid scrubbing becomes economically attractive compared to incineration and adsorption on activated carbon when the volume of odorous gas to be treated is greater than 5000 cubic meters per hour.

6.2.7 Chemical treatment

Injecting controlled quantities of chemicals such as chlorine or ozone into process-gas stream can control odour. Similarly, unlike various other "odour control" treatments, chlorine dioxide will destroy the odour at source. Chlorine dioxide is several times more effective than chlorine and other commonly used treatments, and will not form hazardous by products, such as chlorinated organic, which can cause more problems than the original odour itself.

Odours arising from water bodies can generally be eliminated by adding the chlorine dioxide solution directly to the odoriferous fluid. The first action of chlorine dioxide is to rapidly oxidize the vapor gases dissolved in the fluid to their oxide form. As the dissolved gases are oxidized and the amount of chlorine dioxide will increase, next action of chlorine dioxide is the oxidation of small molecular material (micro-organisms), and, as the amount of chlorine dioxide will further increase, the larger molecules and compounds are oxidized.

Due to this versatility, chlorine dioxide can be used in all aspects of the odour control process, from air scrubbers and wastewater treatment with stabilized chlorine dioxide solutions.

6.2.8 Irradiation

Ultra-violet irradiation can be used to control of odour. Here, the action is probably due to ozone formation or bactericidal effect.