In this article we will discuss about:- 1. Definition of Biofilters 2. Microbial Potential of Biofilters 3. Biofilter Media 4. Mechanism.
Biofiltration is a new technology used to purify contaminated air evolved from volatile organic and inorganic compounds by involving microorganisms. It is a low cost technology gradually becoming popular due to simple operational and waste-removal efficiencies.
Biofiltration is the oldest biotechnological method for removal of undesired foul gas components from air. Since 1920, biofilters were used to remove odorous compounds from wastewater treatment plants or animal farming. It could be achieved by digging trenches, laying an air distribution system and refilling the trenches with permeable soil, wood chips and compost.
In the 1960s, the first biofilters were built in the USA. Between 1980s and 1990s, about 30 large and full-scale systems of about 1,000 m3 capacity have been constructed. Biofiltration has more industrial success in Europe and Japan where over 500 biofilters are in operation.
Moreover, biofiltration is not suitable for highly haloginated compounds (e.g. trichloroethylene, trichloroethane and carbon tetrachloride) due to its low aerobic degradation. Also the size of a biofiltration is inversely proportional to the degradation rate.
Definition of Biofilters:
Biofiltration is done by using biofilters. Biofilters are the packed-bed units in which gas is blown through bed of compost or soil covered by an active biofilm made by the natural microorganisms. Diagram of a biofilter is given in Fig. 33.15.
The microorganisms consume the gaseous organic pollutants and use as source of carbon and energy. Instead, it may contain an inner support where a special pool of microorganisms is cultivated. The harmful compounds are degraded by an active biofilm covering the bed.
The unwanted odorous organic compounds from gaseous phase are removed. They are absorbed or adsorbed on porous solid base of the biofilter, or dissolved into liquid phase and then oxidised by the microorganisms. Biofiltration is beneficial because it does not require large amount of energy operation.
Microbial Potential of Biofilters::
Different aspects have been studied regarding the microbial potential of biofilters.
They are:
(a) isolation and characterisation,
(b) use of pure cultures of bacteria or fungi,
(c) mixed microbial population,
(d) effect of enrichment culture including application of special strains, types of microorganisms and their metabolic activities,
(e) effect of external conditions on microbial activity,
(f) release of microorganisms from biofilters.
Microorganisms present in biofiltration are mainly aerobic ones. Most of them present in biofilters are bacteria (mostly coryneforms and endospore formers), occasionally pseudomonads, protozoa, invertebrates and few actinomycetes (mainly Streptomyces spp.) and some fungi, (mostly Alternaria, Aspergillus, Botrytis, Cladosporium, Fusarium, Mortierella, Rhizopus, Penicillium, Trichoderma). Fungi from a large specific area which remain in direct contact of air flowing through filter.
Microorganisms are the most critical components of biofilters. Because they transform or degrade the contaminants. Naturally occurring microorganisms are available for the process because they get adapted to the contaminants. In some cases a specific microbe or genetically engineered microorganisms may be used.
Biofilter Media:
The filter media must have some characteristics for performance of the biofilter. Because all the filter media allow the polluted air to interact closely with the degradative microorganisms, oxygen and water.
Constitution of the physical media provides fine porous, large surface area and distribution of a uniform pore size which strongly defines the efficiency of biofilm. Inorganic bed material has a good flow properties and consists of a variety of metal oxides, glass or ceramics beads; PVC is commonly used as packing material.
The active microbial biofilm will adhere onto the biofilter media. The amount of microorganisms present depends on the availability of surface which in turn increases efficiency of biofilters. Thus the suitable biofilter media have large surface area for both adsorption of contaminants and support for microbial growth. Synthetic or inert media must be inoculated with soil, compost or sewage sludge.
Because these materials have a big and complex population of microorganisms available to develop the proper microbial culture for the process. Pure culture can also be tested as inoculum.
Suitable biofilter media have the ability to retain moisture to sustain biofilm layer and retain capacity of nutrient supply to microbes that form active biofilm. Some of the materials used as biofilter media are compost, peat, soil, activated carbon, wood chips or bark, perlite, vermiculite, lava rock and inert plastic material.
Mechanism of Biofilter:
Removal of contaminated material is multistep process. The contaminants are converted into liquid phase and transported to bacterial cell in the biofilm and transferred across the cell membrane, where the compound is degraded and used in cell metabolism.
The treatment process depends on two mechanisms:
(a) Direct adsorption in biofilm and degradation,
(b) Adsorption on organic media and biodegradation, and
(c) Dissolution in aqueous phase and degradation.
After biodegradation the contaminants are exhausted from the biofilter.
The process can be expressed as below:
Pollutants + O2 + Microorganism → Microbial cells + CO2 + H2O
Immobilisation of microorganisms to the bedding materials in biofilters:
Immobilisation of microorganisms consists of two processes:
(a) The self-attachment of cells to the filter bedding material, and
(b) The artificial immobilisation of microorganisms to the bedding material.
Self- attachment of microorganisms to a surface depends on the microbial culture i.e. secretion of glycocalyx (extracellular polysaccharide) and several forces such as electrostatic interaction covalent bond formation, hydrophobic interaction, and partial covalent bond between microorganisms and hydroxyl groups on surfaces.
Immobilisation of microorganisms at filter bedding is done by five methods such as: carrier bonding, cross-linking, entrapment, micro capsulation and membrane methods.