Gaseous Waste Treatment
Lecture 3
Major gaseous pollutants • Odour • • • •
Sulphur oxides- wet scrubbers Oxides of nitrogen (NO and NO2)-wet scrubber Carbon monoxide Hydrocarbons
Odour • Odourant: compounds responsible for imparting an odour • Odour: is the perceived effect of the odourants as detected and interpreted by the olfactory system. as a single component or a mixture of odorants irrespective of whether the smell is pleasant or not.
Sample collection Should be a representative of the original gas Collected in plastic bags or steel canisters which is not adsorbant of odour High moisture content of air can affect odour measurement
Odour sampling and measurement
Odour Panel • Selection of panel members• 6-10 people • Members with near normal odour response • Trained to react to odour intensity and not quality • Odour intensity points - 0 to 4 • Hedonic scale – appreciation of smell (4 to -4) -4 very unpleasant.
Odour Measurement • Threshold value is the only value that can be determined from an odour of unknown constituency. This is the concentration below which odour is not detectable by an average person. ie. 1ou/m3 • Odour Units- Concentration of an odour in the air sample that can be perceived by 50% of the observers expressed as OU/m3. • This is defined as the number of dilution necessary to bring the sample to threshold level.
Concentration of pollutants • 1ppm = 1 volume of gaseous pollutants 106 total volumes Or 1ppm = 0.0001% by volume
Odour measurement
Odour counteraction/masking • To mask non toxic nauseating odour eg. sewage treatment plant, storage tanks • By injecting a chemical with stronger odour which could destroy the odourous properties of both gases • Counteractant usually sprayed as mist at the top of the stack. Every odour require its own counteractant and never desensitise the olfactory nerve
Methods of gaseous waste treatment • Dispersion with stacks • Combustion process • Adsorption • Absorption and chemical reaction • Condensation
1. Combustion • Incineration -Destruction of waste gas by combustion process. • Mainly used for the destruction of organic gases (benzene, toluene, xylene etc etc )
Direct flame incineration •
Used when the waste gas is at or near their lower combustible limit when mixed with air. That is the concentration of organic vapours is high enough to provide sufficient heat of combustion to sustain the oxidation.
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Temperature required for oxidation to be completed- ~1000 oC
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>3000KJ/m3 (natural gas- 24,000KJ/m3)
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Also used if the waste gas itself is combustible- H2S, HCN, CO .
Thermal incineration • used for a mixture of organic gas and air when the amount of organic is very small- ie. Small heating value of 30-600KJ/m3 • Three T’s
- Time of contact with flame • Temperature in the chamber • Turbulence of waste air
• Require 500-800 oC
Catalytic incineration • -Used where the concentration of combustible materials and air are very low.
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Commonly used catalysts are Platinum and Palladium dispersed on some form of support like alumina
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Catalytic poisoning can happen with Si, Pb, Fe, As
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Temperature required -300-500 oC • Fuel saving but higher investment cost
• Heat recovery
Adsorption Principle: It is a molecular surface phenomenon where molecules adhere to the surface of a solid. • Adsorbents: activated charcol, silica gel, alumina and bauxite •
Carbon can be either discarded or regenerated and various solvents separated by distillation
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Works efficiently on large surface to volume ratio
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Air stream should be free of Particulates before carbon adsorption as they can clog the passages between carbon particles
Biofilter • Microorganisms in the filter break down organic components to CO2 and water • Biofilter • Compost , Peat, Bark, chipped wood • Operating conditions • Ammonia should not exceed 25mg/m3 • Air temperature should not exceed 35oC, • Humidity near 100%
Biofilter
Biofilter
Applications Food processing, livestock feed plants, water treatment plants, poultry and meat processing, slaughter houses
Absorption • Water soluble gaseous effluents-absorption and/or neutralisation • Equipment - packed or plate columns, spray towers, venturi scrubbers
• Applications- removal of HCl, formaldehyde, acetone, hydrogen flouride, ammonia, Hydrogen sulphide
• Disadvantage: wastewater disposal
Absorption - examples • Hydrogen chloride gas can be absorbed in water to form weak hydrochloric acid or • Neutralised with weak solution of sodium hydroxide or calcium hydroxide to form sodium chloride or calcium chloride • Oxidation using potassium chloride or chlorine to remove noxious odours. • Permaganate under alkaline condition result in the formation of manganese dioxide (colour change from purple to clear)
Chemical scrubber • Applications • Food processing, feed manufacture*, Water treatment plants • *Poultry and Meat processing, • *Chemical industry, like emissions of organic sulphur compounds, acrylates • Large contact area between gas and liquid • Sufficient residence time around 3 seconds • Multistage design to absorb both acid and alkaline component
Condensation • Is a pre treatment to remove organics. Suitable only if organic components can be recovered in cases where waste gas has substantial quantities of condensable materials such as water or vapour • Shell and tube condenser • Tubular air cooled condenser • Direct contact condenser
Woodman point wastewater treatment plant wet scrubber
Revision questions • What are the different components of gaseous waste that need to be treated before emission. • Explain two methods of odour removal describing the principle, operation and their application to odour treatment. What is meant by odour unit and how is threshold level determined? • What is the significance of odour panel?
References Sampling and measurement of odours (2003)-IWA scientific and Technical Report No.17 Stuetz and Frechen (2001). Odour in wastewater Treatment- Measurement, Modelling and Treatment
