A3.10.5 Control at point of effect – End of Pipe Technology


Where it is not possible to eliminate pollution completely, the pollution should be captured, or controlled or treated before emission. There are a number of technological options available.


A3.10.5.1        Air Pollution



Adsorption is a process of removing a gaseous contaminate, by physically or chemically adhering it to the surface of a solid.

Activated Carbon is often used as the solid. Activated Carbon is charcoal that has been chemically or thermally treated to produce an open pore structure. The gases or vapours are passed over the activated carbon filter and become trapped into the surface. The filters must be periodically replaced to remain effective.



Absorption is a process where gas is captured by dissolving it into a liquid. Water is most often used as the liquid in absorption because it is able to dissolve a wide range of contaminants; it is cheap and freely available.

The gas is bought into contact with the liquid under controlled conditions. A common technique is to allow the liquid to flow down a tower packed with pieces of inert material whilst hot gas is forced up it.

Absorbers (or scrubbers) are commonly used for the control of inorganic materials as many organic materials are insoluble or have very low solubility in water.

Scrubbers are normally very large and require plenty of space. The effluent will require treatment and / or specialist disposal, converting an air pollution problem into a land and or water pollution problem.



A process in which volatile gases are removed from a contaminated stream and changed into a liquid. This technique is used for solvent recovery and as preliminary air pollution control prior to incineration.



Flare Stacks may be used to dispose of waste gases during unusual process events such as start up or shut-down and in emergencies. It consists of a vent to atmosphere via which flammable gases are burnt.


Thermal Incinerators

Thermal oxidation or incineration is a high temperature operation. It can be used to control a variety of continuous waste streams containing dilute mixtures of organic compounds.

The controlling factors are:

  • Temperature
  • Resistance time
  • Turbulence (mixing of the gases)

Heat may be recovered to pre-heat incoming gases, reducing the need for additional fuel. This is not normally an option where the waste gases are highly variable in nature and /or quantity.


Catalytic Incinerators

Generally operate at lower temperatures than thermal incinerators due to the addition of a catalyst to speed up the rate of combustion.

The technique is limited as pollutants such as phosphorus compounds, lead, iron oxide and sulphur can ‘poison’ the catalyst and dust and liquid particles can ‘blind it’ (coat it) impairing performance.



Odorous gases and other volatile organic compounds may be oxidised (broken down anaerobically) by microbial activity to less objectionable products. The microbes exist in an aqueous medium, which is bought into contact with the contaminated air-stream.



A biofilter is simply a bed of organic material, often a 10 to 18 inches deep mixture of compost and wood chips or shreds. As air passes through the biofilter microorganisms (bacteria or fungi) on the organic material degrade the pollutant.

The effectiveness of the biofilter is primarily a function of the amount of time the odorous air spends in the biofilter (contact time) and the moisture content of the filter material. Contact time is part of the biofilter design while moisture content is a function of good management.

The technology finds greatest application in treating malodorous compounds and water-soluble volatile organic compounds (VOCs). Industries employing the technology include food and animal products, off-gas from wastewater treatment facilities, pharmaceuticals, wood products manufacturing, paint and coatings application and manufacturing and resin manufacturing and application.

Very large airflows may be treated, but require a very large area. This is one of the principal drawbacks of the technology.

One of the main challenges to optimum biofilter operation is maintaining proper moisture throughout the system. The air is normally humidified before it enters the bed with a watering (spray) system, humidification chamber, bioscrubber, or biotrickling filter. Properly maintained, a natural, organic packing media like peat, vegetable mulch, bark or wood chips may last for several years.


These work in a similar way to many chemical scrubbers. Contaminated gas is passed up a tower against a flow of water containing a population of microbes suitable for oxidising the noxious components of the gas. The tower contains packing material on which the microbes adhere to form a biological ‘mat’. They are less common than biofilters as they are generally more expensive. However, they are highly effective with relatively water soluble solvents such as alcohols, esters, aldehydes, ketones and amines.


Inorganic Materials

Flue Gas Desulphurisation technique (FGD).

In the variation of this technique, which is usually most cost effective, the sulphur dioxide is adsorbed by a limestone slurry spray to form calcium sulphite. This is then oxidised to form calcium sulphate, which forms water and gypsum.