Biological treatment of air pollutants is now attempted in many countries.
The gases must be treated at the end of pipe to prevent release to the atmosphere. The treatment is done by oxidation.
The volatile organic compounds (VOCs) are oxidized to CO2, H2S and SO2 to sulphate and Nitrogen oxides (NOx) to nitrate:
Voc + O2→ CO2 + H2O
The bio-reactions require no fuel, or chemicals create no secondary pollutants and the reaction rates increase with pollutant emission rates. The microorganisms in soil and water carry out the reaction shown above for their metabolic energy. The following biological methods of air pollution control are used (Fig. 9.5).
Bio-filter:
Bio-filteration may be defined as the oxidation of air pollutants as contaminated air flows slowly through porous media such as soils, peat, compost, activated carbon, plastic packing material and ceramic packing material that supports a degradative microbial population attached to the walls of the pores. Air pollutants adsorb on the pore walls and flow more slowly than the air. The high O2 concentration and the intimate mixing of oxygen with the VOCs ensure complete oxidation.
However, the various media have both advantages and disadvantages. Soil is inexpensive, neutralizes acids well, its moisture content may be easily controlled. Bio-filtration does not pollute the soil because soils cannot accumulate gases or their metabolic protects. In soil, organic gases are converted to CO and water; inorganic gases are converted to their calcium oxy salts, e.g., SO2 and H2S to Ca SO4, and NOx to Ca (NO3)2.
Among other media, compost is also not expensive. It is lighter than soil and requires less area. It neutralizes acids poorly, is hydrophobic when dry and has to be replaced within 2-5 years. The inorganic media such as plastic packing materials, ceramics, and activated carbon offer promise of smaller size because uniform air flow is easier to attain.
Overcoming the size disadvantage of bio filters means increasing the VOC loading rate. Bio-filtration requires only a fan and some means of maintaining a high moisture content in the bio filter bed. An advantage of bio-filtration is that it is quite inexpensive compared to physico- chemical air pollution control methods.
Bio-scrubber is a modified water scrubber. The spray into the gas stream is a sludge of suspended microbes which increases the aqueous solubility of the VOCs and removes them more effectively than water alone. The sludge then flows to a water treatment plant to biodegrade the dissolved VOCs and generate a new sludge.
This method appears best suited for large air flows because of their low backpressure and small size. Their disadvantage is operating a wastewater treatment plant in addition to one that destroys the VOCs, rather than just stripping VOCs from the water, generating a non-odorous sludge.
Thus, bio-scrubbing is helpful to deal with toxic and odorous gases. Microbiological methods for scrubbing of waste effluents are available, although, in general, their performance at the point of waste discharge remains to be determined.
Bio-Trickle Filter:
Bio-trickle filter consists of a sheet of a plastic or other microbial support medium hung in the contaminated air stream. The sheet is bathed continuously by a recirculating stream of water containing the nutrients and vitamins required by the microbes. Bio-oxidation rates per unit volume are high so bio-trickle filters can be as small as physico- chemical units.
Their disadvantages are that at high loading rates, biological reactions approach zero order kinetics and are incapable of responding to peak loads, accidental discharges and explosion incidents. Bio-trickle filters would seem to be susceptible to microbially produced bactericides and to invasion of less-desirable microbial populations.
Biological air pollution control is related to wastewater treatment and is only slowly being accepted on an industrial scale. Wastewater usually contains a. suitable mixture of nitrogen, phosphate and other chemicals as microbial growth factors. In contrast, industrial waste air might contain only a single carbon-, hydrogen molecule. Therefore, the bio-filter medium must supply the other growth factors including water so that the microorganisms may metabolize the “the empty calories” in the contaminated air.