At global scenario, with increase in industrialization, there has been growing concern regarding increase in air pollution. WHO defines air pollution as "Air pollution is contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere[1]." Increase in the level of toxicants in the air, especially in Asian developing countries[2], has rather made the situation gruesome for some parts of the world in recent past like china[3-6] and USA[7].The implications of worsening quality of air have been Lower Birth rate[8], declining life expectancy[9] and increase in respiratory disorder[2, 4], Cardiovascular disorder[5], Cancer[10]. So, Dwindling these changes for improvement in the quality of air is indeed the need of hour.
Review from literature indicates that increase in unregulated industrialization has been the major factor for this worsening situation[11]. A number of methods, listed in Appendix 1, are already available to lessen the emission of harmful industrially spent chemicals such as Sulphur dioxide (SO2), Particulate Matter (PM2.5 and PM10), Nitrogen dioxide (NO2), Carbon monoxide (CO), Volatile Organic Compounds (VOC) etc, as highlighted by National Environmental Agency (NEA), Singapore[12]. Nevertheless, availability of cost effective method that itself causes lesser environmental effects is still desirable.
Toward this, the comparison of already industrially employed techniques indicates that there are still some remaining technical hitches to the existing control methods. There is a need to shift from methods such as Hepa Filters, Ionizers, Regenerative Thermal Oxidizers (RTO) and Regenerative Catalytic Oxidizers (RCO) to a cost and environmental friendly technology. To counteract the present deficiencies and in lieu of using better environmental friendly technique, the use of Bio filters to solve the persistent problems has been indicated[13, 14].
Biofiltration
Biofiltration is the term employed to the process that involves biological removal of contaminants from the off gas. This process is typically similar to other chemical and physical process as the industrially spent chemicals are adsorbed, absorbed and then biologically oxidized to simpler species. This helps to control (1) Odor (2) VOC (3) Toxics in air emissions. This technique has been in use since early 1920's in Germany and Netherlands but has gained lesser attention in rest parts of the world[15]. A modification of this technique making use of soil bed filters have already been patented in early 1960's[16].
Biological reactors used for biofiltration can be classified into three types based on their operation and their design viz.
Biofilters
Biotrickling filters
Bioscrubbers
Biofilters consist of microorganisms immobilized on a moist porous filter medium forming a fixed bed. This filter medium provides large surface area and stable structural matrix to withstand the pressure of passing-by waste gas. Pollutants are at first adsorbed on the immobilized solid particles and then absorbed into the moist biofilm formed by microorganisms where they undergo degradation. Optimum moisture must be maintained to prevent the membrane from drying by continuous replenishment of lost water and nutrients. Adequate nutrient supply, sufficient oxygen availability, maintenance of moisture, optimal temperature and pH must be maintained to extend the shelf life of the filter medium that typically lasts for 3-5 years. Materials used in biofilters must have high water retention capacity and porosity e.g. Activated carbons, ceramics, Soil, compost, peat etc. Solubility of toxics into the biofilm of microorganisms and their biodegradability drives this process. Most commonly open bed biofilters are used but even closed beds may be used depending upon the location and plant design.
A Biotrickling filter makes use of immobilized microorganisms on the support of filtering material. A mobile phase of liquid is continuously passed over the filter bed. Filter bed allows the formation of a biofilm on which the pollutants are degraded. It is relatively new technology as compare to Biofilters. The main advantage of using this method is that it prevents clogging of the filter. Also, as this method involves a liquid media that continuously recirculates, it provides an opportunity to remove the degraded material for recycling. By using this methodology, high inlet loads can be treated. However, this requires periodic cleaning and thus elevates the maintenance cost. Hence their selection is guided by a choice between cost effectiveness and desirable operation output requirements.
Scrubbing is the process of removal of impurities from a gaseous phase. Bioscrubbers thus involve a contact between gaseous phase and a mobile liquid phase that scrubs the impurities out of the gaseous phase into the liquid. Thus the use of liquid depends upon the type of impurities to be separated. In general effective diffusivity is a function of molecular weight and solvent solubility. Thus higher separation is achieved for impurities with low molecular weight and high solvent solubility. To compartmentalize, a bioscrubber has typically 2 comparments
Scrubbing tower: Here a counter flow between liquid and gaseous phase allows the diffusion of impurities into the liquid phase. This liquid phase from this compartment is then fed into the second compartment.
Bioreactor: Microorganisms are suspended in aqueous phase that depollute the inlet liquid phase by consuming the pollutants and degrading them into simpler components.
Their use is typically advantageous in processes that involve use of fungi as they form mycelium network and generate high pressure drop in reactors (Biofilters) that involve use of a filter membrane. However, their use is restricted due to dependence on diffusivity of pollutants into the liquid stream and also it roots another environmental problem by releasing the wastewater that usually requires further treatment.
Microorganisms gain nutrition by consuming organic matter and this makes them an important and cost effective target for use to remove toxic waste from air pollutuin.
1. WHO, Air pollution, in Health topics. 2013: http://www.who.int/topics/air_pollution/en/.
2. Chen, B. and H. Kan, Air pollution and population health: a global challenge. Environ Health Prev Med, 2008. 13(2): p. 94-101.
3. Fischer, M., The most shocking photo of Beijing air pollution I've ever seen, in The Washington Post. 2013, World views: http://www.washingtonpost.com/blogs/worldviews/wp/2013/02/28/the-most-shocking-photo-of-beijing-air-pollution-ive-ever-seen/.
4. Chen, R., et al., Communicating air pollution-related health risks to the public: an application of the Air Quality Health Index in Shanghai, China. Environ Int, 2013. 51: p. 168-73.
5. Liu, L., et al., Size-fractioned particulate air pollution and cardiovascular emergency room visits in Beijing, China. Environ Res, 2013. 121: p. 52-63.
6. Xu, L., et al., Spatial distribution and sources identification of elements in PM(2).(5) among the coastal city group in the Western Taiwan Strait region, China. Sci Total Environ, 2013. 442: p. 77-85.
7. Altman, P., Killer Summer Heat: Projected Death Toll from Rising Temperatures in America Due to Climate Change. 2012: NRDC ISSUE BRIEF.
8. Kelishadi, R. and P. Poursafa, Air pollution and non-respiratory health hazards for children. Arch Med Sci, 2010. 6(4): p. 483-95.
9. Correia, A.W., et al., Effect of Air Pollution Control on Life Expectancy in the United States: An Analysis of 545 U.S. Counties for the Period from 2000 to 2007. Epidemiology, 2013. 24(1): p. 23-31 10.1097/EDE.0b013e3182770237.
10. Mu, L., et al., Indoor air pollution and risk of lung cancer among Chinese female non-smokers. Cancer causes & control : CCC, 2013. 24(3): p. 439-450.
11. Fang, Y., et al., Air pollution and associated human mortality: the role of air pollutant emissions, climate change and methane concentration increases from the preindustrial period to present. Atmos. Chem. Phys., 2013. 13(3): p. 1377-1394.
12. Code of practice for pollution control, N.E. Agency, Editor.
13. Warren J. Swanson, R.C.L., Biofiltration: Fundamentals, Design and Operations Principles, and Applications. Journal of Environmental Engineering, 1997. 123(6): p. 538 - 546.
14. Soccol, C.R., et al., Biofiltration: An Emerging Technology. Indian Journal of Biotechnology, 2003. 2(3): p. 396-410.
15. Leson, G. and A.M. Winer, Biofiltration: An Innovative Air Pollution Control Technology For VOC Emissions. Journal of the Air & Waste Management Association, 1991. 41(8): p. 1045-1054.
16. Pomeroy, R.D., Controlling sewage plant odors. Consulting Engineer, 1963. 20(101).
