Gaseous WasteTreatment

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 anodour

• Odour: is the perceived effect of the odourants asdetected and interpreted by the olfactory system. as asingle component or a mixture of odorantsirrespective of whether the smell is pleasant or not.

Sample collection

Should be a representative of the originalgas

Collected in plastic bags or steel canisterswhich is not adsorbant of odour

High moisture content of air can affectodour 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) -4very unpleasant.

Odour Measurement

• Threshold value is the only value that can be determinedfrom an odour of unknown constituency. This is theconcentration below which odour is not detectable by anaverage person. ie. 1ou/m3

• Odour Units- Concentration of an odour in the air sample thatcan be perceived by 50% of the observers expressed asOU/m3.

• This is defined as the number of dilution necessary to bringthe sample to threshold level.

• 1ppm = 1 volume of gaseous pollutants106 total volumes

Or 1ppm = 0.0001% by volume

Concentration of pollutants

Odour measurement

Odourcounteraction/masking

• To mask non toxic nauseating odour eg. sewagetreatment plant, storage tanks

• By injecting a chemical with stronger odour whichcould destroy the odourous properties of both gases

• Counteractant usually sprayed as mist at the top ofthe stack. Every odour require its own counteractantand never desensitise the olfactory nerve

Methods of gaseous wastetreatment

• Dispersion with stacks

• Combustion process

• Adsorption

• Absorption and chemical reaction

• Condensation

1. Combustion -

• Incineration -Destruction of waste gas by combustionprocess.

• 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 whenmixed with air. That is the concentration of organic vapours is highenough to provide sufficient heat of combustion to sustain the oxidation.

• Temperature required for oxidation to be completed- ~1000 oC

• >3000KJ/m3 (natural gas- 24,000KJ/m3)

• 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 oforganic 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 verylow.

• Commonly used catalysts are Platinum and Palladium dispersed on someform of support like alumina

• Catalytic poisoning can happen with Si, Pb, Fe, As

• Temperature required -300-500 oC• Fuel saving but higher investment cost

• Heat recovery

Adsorption

Principle: It is a molecular surface phenomenon wheremolecules adhere to the surface of a solid.

• Adsorbents: activated charcol, silica gel, alumina and bauxite

• Carbon can be either discarded or regenerated and various solventsseparated by distillation

• Works efficiently on large surface to volume ratio

• Air stream should be free of Particulates before carbon adsorption as theycan clog the passages between carbon particles

Biofilter

• Microorganisms in the filter break down organiccomponents 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

ApplicationsFood processing, livestock feed plants, water treatment plants, poultry and meat processing, slaughter houses

Absorption

• Water soluble gaseous effluents-absorptionand/or neutralisation

• Equipment - packed or plate columns, spraytowers, venturi scrubbers

• Applications- removal of HCl, formaldehyde,acetone, hydrogen flouride, ammonia, Hydrogensulphide

• Disadvantage: wastewater disposal

Absorption - examples

• Hydrogen chloride gas can be absorbed in water to form weakhydrochloric acid or

• Neutralised with weak solution of sodium hydroxide orcalcium hydroxide to form sodium chloride or calciumchloride

• Oxidation using potassium chloride or chlorine to removenoxious odours.

• Permaganate under alkaline condition result in the formationof 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 organiccomponents can be recovered in cases where waste gas hassubstantial quantities of condensable materials such as wateror 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 needto be treated before emission.

• Explain two methods of odour removal describing theprinciple, operation and their application to odour treatment.What is meant by odour unit and how is threshold leveldetermined?

• What is the significance of odour panel?

References

Sampling and measurement of odours(2003)-IWA scientific and TechnicalReport No.17

Stuetz and Frechen (2001). Odour inwastewater Treatment- Measurement,Modelling and Treatment