After reading this article you will learn about the control of odour pollution from area and point sources.
Odour Control from Area Sources:
There are a large number of chemicals and proprietary products that claim to reduce odour when they are applied to area sources. Atmospheric odours that are contained in a restricted area can be oxidized by atomization of chlorine dioxide. Odour from sources such as holding ponds, lagoons and sewage pre- or post- treatment effluent can be controlled by atomized spray of chlorine dioxide.
To reduce odour, chemicals have to be applied over very large area, the cost of materials and labour would be very high. The large quantity of these compounds required could itself cause pollution. For large area sources following methods can be used to reduce odour complaints.
(i) Excluding Development Close to the Site:
The development of “buffer zone” around the area sources is the simplest method to reduce odour.
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 odour emissions, the actual process being undertaken, the topography of the site, the weather conditions that prevail at the site.
Green belt development in the buffer zone may help at least partially to obfuscate the odour.
(ii) Best management practices (BMP) will vary according to the industry producing the odour.
(iii) 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.
Rotary atomizer is one of such techniques widely recommended for adoption for effective control of odour in case of area sources. The Atomizer uses centrifugal action by a spinning inner mesh to force droplets on to an outer mesh which “cuts” water into atoms (Fig. 2).
The rotary atomizer produces millions or microscopic droplets of water-up to 238 billion from a single litre droplet that are thinner than a human hair and a fine spray which covers up to 30 linear metres. This creates a fine mist, which is more effective with minimal use of water and electricity.
The spray or atomiser techniques are also used to control odour from buildings and fugative sources.
Odour Control from Point Source:
In case of point sources such as that of industries, the odour-causing gas stream can be collected through piping and ventilation system and made available for treatment. Dispersion method is the simplest of the methods that can be adopted for odour abatement. This is nothing but to release odorous gases from tall stack.
It results in normal dispersion in the atmosphere and consequent decrease in ground-level concentration below the threshold value. A number of technologies are available for control of odour from gas streams collected through process ventilation systems.
(i) Mist filteration
(ii) Thermal oxidation/incineration
(iii) Catalytic oxidation
(vi) Wet scrubbing/Absorption
(vii) Chemical treatment and
(i) Mist Filtration:
The odorous air has high concentration of moisture. If these vapours are cooled to <40°C a substantial quantity of water vapour will be condensed and so volume of gases (to be incinerated) will reduce. Mist filter works on this principle. Mist filters can also remove odour producing liquid and solid particles.
(ii) Thermal Oxidation/Incineration:
The oxidation of odour into CO2 and H2O by the combustion of the odour with fuel and air is called thermal oxidation. The reaction takes place at temperature ranging from 750°C-850°C in a thermal oxidiser. 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 oxidizers:
(a) Direct fired thermal oxidizer
(b) Recuperative thermal oxidizer
(c) Regenarative thermal oxidizer
(a) Direct fired thermal oxidizer:
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).
(b) Recuperative thermal oxidizer:
These are a simple, cost effective, means of destroying volatile organic chemicals where the inlet concentration is relatively high or particularly where heat can be usefully recovered for other processes.
(c) Regenerative thermal oxidizer:
It is the most commonly used thermal oxidizer because of its robust performance and its ability to operate at high thermal efficiency.
(iii) Catalytic Oxidation:
These reactions proceed at much lower temperatures (e.g. 200°C) in the presence of a catalyst. Catalytic systems are, therefore, more favourable where auto-thermal operation is not practicalike and heat cannot be economically used elsewhere.
A number of transitions and precious metal catalysts can be used in catalytic oxidizer to destroy various VOCs over a wide range of process conditions.
It is a method of reducing odours by biological process. Biofilteration 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 vapours into non-malodorous compounds such as water and CO2.
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.
Adsorption is applicable when the odorous gases are soluble or emulsifiable in a liquid or react chemically in solution. This is a method that can work perfectly even at low temperature. In this process the substances causing odour are absorbed on to activated carbon. For effectivity, the contaminated air stream must be free from dust and particulates that might clog the carbon particles.
Regeneration of carbon for re-use will produce either waste water or a concentrated vapour 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 sulphur dioxide, depending on their composition. The alumina bed is replaced progressively as the permanganate is exhausted.
(vi) Wet Scrubbing/Absorption:
Wet scrubbing is a useful process to handle acid gas streams, ammonia or streams with solids that might foul other equipment. In wet scrubbing of odorous gases is either absorbed in a suitable solvent or chemically treated with a suitable reagent.
Wet scrubbing or absorption system can be either ventury system or packed tower system, Ventury 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. Packed Towers are typically counter current scrubbers that utilise high surface area medium as a contact zone for the gas stream with suitable scrubbing liquor.
When the odour is caused by the presence of unsaturated organic compounds it becomes necessary to use an oxidizing agent such as chlorine, diluted sulphuric acid and sodium hydroxide to treat odour.
(vii) Chemical Treatment:
Injecting controlled quantities of chemicals such as chlorine or ozone into process-gas stream is called chemical treatment of odour. In “odour control” treatments, chlorine dioxide is used to destroy the odour. Chlorine dioxide is several times more effective than chlorine and other commonly used treatments, and will not form hazardous by products.
Odours arising from water bodies can generally be eliminated by adding chlorine dioxide solution directly to the odoriferous fluid. The first action of chlorine dioxide is to rapidly oxidize the vapour gases dissolved in the fluid to their oxide form.
As the dissolved gases are oxidized and the amount of the chlorine dioxide will increase, next action of chlorine dioxide is the oxidation of small molecular material (micro-organisms), and as the amount of chlorine 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.