Air Treatment Technologies: Bioscrubbers and Biofilters.

Introduction

The condition in which the air is contaminated by foreign substances interfering with the human welfare like agriculture and health also causes environment damage is called air pollution.

There are many pollutants which shows adverse effects on agriculture. These include 

• Carbon Dioxide

• Methane

• Nitrous Oxide or Ammonia

• Hydrogen Sulfide

As agriculture and some industries are well known for contributing the green-house emissions like methane and carbon dioxide, these lands are usually equipped with the air treatment technology systems. These air pollutants are also by-products of farm animal’s digestion process. Some of the emerging biological technologies which helps in air treatment include.

• Biofilters

• Bioscrubbers

• Biotrickling Systems.

Biofilters

Bio-filtration is the process in which the harmful emissions are breakdown by the micro-organisms grown as a bio film on the nutrient media provided in the biofilter systems. Hence biofilter is one of the pollution control technique which is used for the elimination of the malodorous gases like H2S, methane and some of the low volatile organic compounds (VOCs) of the air

Structure

If the waste gas with tremendous organic compounds are present which can also give a foul odor, biotechnology systems has potential to remove the pollutants and remove the odor from the biological waste by treating it in a biological way which is by using the micro-organisms. So this odorous emission of gases results in the serious problems on health in the residential areas.

Biofilter systems are cylindrical systems which has central part has media and the layer of biofilm on media formed by the growth of the micro-organisms. Here the biofilm consists of a layer of hydrogen peroxide degrading micro-organisms. These systems can be reversible where the pollutant free air or treated air is passed out from the top or bottom of the systems.

The design mainly depends on the type of gases to be treated. Here the contaminated air is allowed to pass into the system from the top which is then passed down to microbial film layer These organisms then break down the H₂S or other contaminants allowing the treated air to pass through the bottom layer along with CO₂ and H₂O.

These biofilter systems technique is helpful in the chimneys of many industries.

How does it Work?

Biofilter system uses the micro-organisms for the absorption and oxidation of the volatile organic compounds and the oxidisable inorganic gases and vapors to produce water, salts, biomass and carbon dioxide.

Media used in the biofilters can include peat, heather, bark and composted sewage sludge, granular carbon or other suitable substances. This media is generally used to provide nutrients for the microbial growth and should be capable of minimizing the pressure drop.

In addition to the media, the moisture content must be maintained in the system which can support the microbial growth. In addition to the humidifying the airflow, sprinklers can also be used for frequent maintenance of filter bed moisture.

Advantages of the Biofilters

Well managed biofilters should be able to show below performance

• Reduce odors by 95%

• About 40-70% ammonia is removed

• Can reduce about 60-80% of hydrogen sulfide from the air.

Biofilters are low cost operating systems, these use less chemical and requires no combustion source.

Biofiltration is good enough to treat odors, toxic compounds and VOCs.

Dis-advantages

Biofilter systems cannot treat successfully some of the compounds with low absorption or degradation rates like volatile organic compounds VOCs. Larger biological systems are required to treat the contaminant air rich in high chemical emissions.

Initial acclimatization time required for the microbial growth is more than that of other biological systems.

Bioscrubbers

Bioscrubbers are another type of air pollution control systems which mainly used to remove harmful materials from the industrial exhaust gases before they are released into the environment. This technique uses mostly Acidithiobacillus thioxidans which is a natural occuring bacteria.

These are mainly used in 2 ways to scrub the pollutants.

Wet Scrubbing

Here the liquid or solid harmful pollutant particles are removed by spraying the liquid substance through the gases. The liquid used here is mostly water however if the gaseous pollutants are also being removed, it may be an aqueous solution that contains chemicals which can react with the absorbed contaminants.

Most of these wet scrubbing systems works with a particulate collection efficiency of over 95%.

Dry Scrubbing

Here the system is introduced with the solid substances in powdered form to the gaseous contaminants for the removal of harmful components from the exhaust fuel gases.

Design and Working of Bioscrubbers

These bioscrubbers consists of the washing towers in which the circulating fluid corresponds to the mixing liquor coming from the activated sludge biological plants or industries.

Here the compound to be first degraded by the micro-organisms is absorbed in the liquid phase of absorption tower.

The process involves the flowing of gases through the liquid where the contaminants and the oxygen is absorbed. Then the liquid is subsequently fed to the reactor packed with the inert material as medium and is covered with the bio film which is mainly responsible for degradation of pollutants.

Here the selection of the system depends on the characteristics of the gas stream being treated, expected removal efficiency and the costs involved.

In general, higher pressure drops indicate more aggressive contact between the liquid and gas stream causing smaller particles to be collected with greater efficiency. Scrubbers with pressure drops less than about 5 inches of water are capable of efficiently removing the particles of greater than about 5-110microns in diameter. These referred to as low energy wet scrubbers.

Advantages

• These biological systems are mostly suitable for the treatments of highly water soluble compounds.

• Advantages of these bioscrubbers with respect to the biofilters are that these do not accumulate the harmful products that have an adverse effect on the micro-organisms.

• Here these systems facilitate control of the biological process through the composition of the liquid medium.

• Here the initial time taken by micro-organisms for its growth is less compared to that of biofilters.

Dis-advantages

These bioscrubbers are most costly systems as these have to maintain 2 systems one for absorption and one for degradation of the contaminants which makes these less economical when compared to the biofilters.

Conclusion

Biofilters are mainly used for treating air-borne emissions or live-stock feeding operations. Airborne emissions from the animal feeding operations include odor, ammonia, hydrogen sulfide, VOCs, particulate matter and biological aerosols.

Many chemical compounds are emitted in most of the extremely low concentrations. Biofilters are used for waste water treatment plants and food processing plants. Bioscrubbers are another air controlling devices used for treating the exhaust gases in the industries and for removal of particulates from the gaseous environment.

Here 2 systems are needed one for the absorption and another for the biodegradation of the pollutants by micro-organisms. Biofilters are proven to be economical when compared to the bioscrubbers.