Gas purification techniques

Biofilter

The polluted gas flow is purified with biofiltration by conducting the gas flow upward through a filterbed, that consists from biological material, e.g. compost, tree bark or peat. The filter material is a carrier of a thin water film in which micro-organism live. The pollution in the gas flow is held back by ad -and absorption on the filter material, and then broken by present micro-organism. The filter material serves as a supplier of necessary nutrients. The products of the conversion are carbon-dioxide, sulfate, nitrate. The dry weight of the filter varies typically from 40 to 60 %. To reduce desiccation of the bed the gas flow must has to saturated with water. For this reason polluted gas flow is moistened before it goes threw the biofilter. This is possible with a pre-scrubber. The relative humidity of the gas must be 95%. In practice it is always better to apply a moistener to protect the biofilter against dehydration.

For easily degradable substances the filter material contains by nature sufficiently several types micro-organism. For difficult degradable substances the filter with special cultures can be grafted faster to obtain faster start up of the filter.
Mainly mixtures of easy and difficult degradable substances are with difficulty to be broken in the biofilter because in the first place the easily degradable substances are removed and the more complex substances are not broken.
With biofiltration it is important that the filter material has a pH between 7 and 8 for decomposition of organic components. At pH smaller than 6.5 the speed of decompositioning is less.
The stay time of the gas by the filter must have a minimum 30 - 45 seconds
to get a good removal of odor and solvents.

Some variants in biofilters are:

Several layers of filter material: Sometimes several layers are used to obtain several bacteria cultures. In first layer the easily degradable components are removed. In second layer the difficult degradable components will sit in offgas. Here the specialized (generally slowly growing) bacteria can survive.
Open or closed: The biofilter can be open or closed at the topside. A open biofilter is liable to rain, sun, searches, … . This can cause a increase of plant-growth, too wet biofilter, at sun a too dry biofilter. A closed biofilter is screened of the external influences and can operate better.
Air-inlet above or under bed: The biological decomposition at the inlet under the bed and the irrigation in several areas. In the filter moist accumulation can occur because of the air drain. The bed will have a constant humidity when the unsaturated air and the wetting will be at the same place under the bed. Because of this the danger for local moist accumulation is smaller
Biofilter with bacteria or moulds: The traditional biofilters function generally with bacteria. A newer development is the use of moulds. These are better against dehydration, acidification, temporary stop of the filter.
Type of filter material: Depending on the availability and requirements of the biofilter the composition of the biofiltermaterial can be adapted. Examples of filter material are compost, tree barks, heath land, peat, worth, root wood, coconut material and expanded clay material.

The advantages of a biofilter are:

Low investment- and functioning costs
Simple construction
Good disposal of biologically degradable components (also poor hydrogensolvent components such as xylenes, styreen)
Low pressure drop
Little effluentwater
Little waste material (only replace filter material)

The disadvantages are:

Large surface necessary (this can be solved by placing the biofilter on the container so that the ground-area becomes smaller; this raises however the cost)
Filter material must be replaced periodically
Humidity and pH of the filter material are difficult to check
Few parameters to improve the efficiency
Stoppage danger by substance
Fluctuations in concentration have a large influence on the working
efficiency
Bed must be ventilated continuously to avoid conditions anaerobic conditions

Biofilters are applied in the water purification installations, fertilizer industry, foundries, chemistry industry, polymer production, feeding industry and flesh and fish industry.

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