Biofilters

Biofiltration can be done in aerobic/anaerobic bioreactors, in which a contaminated air stream is passed through a porous packed medium that supports a thriving population of microbes (Fig. 3.6). The principles governing this are similar to the common biofilm process. The contaminants of gas-phase (VOCs) are first absorbed from the air-phase to the water of the biofilm inter-phase. Then, this contaminant diffuses through the biofilm that holds the microorganisms. The microorganisms play a major role in converting them into CO2, water and other end-products. On the contrary, most hydrogen sulphide (H2S) degraders are autotrophic organisms, which has the ability to use carbon dioxide as its carbon source. However, in heterotrophic co-metabolic degradation processes, another carbon source may be required besides the pollutant itself. The microorganisms obtain sufficient energy from the oxidation of the chemical, while utilizing them as a primary substrate via specific enzymes. The degree of treatment depends on the type and composition of the waste-gas stream (solubility factor), activity of the microbes, pH, temperature, moisture content of the filter bed, nutrient and O2 availability and other suitable conditions needed for biodegradation. The design of conventional biofilters have varied and improved over the past 50 years. Also a number of extensive reviews and studies about the technical aspects of biofiltration have been published in the past few years (Kennes and Thalasso 1998; Rene et al. 2004; Estevez et al. 2005; Mohammad et al. 2007; Jin et al. 2006a, 2008; Rene et al. 2009a; Kennes et al. 2009b).

Table 3.1 List of individual pollutants treated in biofilter and the corresponding elimination capacity values, reported from literature for biofilters

Pollutant Packing material Microorganism ECmax (g/m3.h) References

Table 3.1 List of individual pollutants treated in biofilter and the corresponding elimination capacity values, reported from literature for biofilters

Pollutant Packing material Microorganism ECmax (g/m3.h) References

Dichloromethane

Compost, perlite,

Activated sludge,

>98%,

Ergas et al. (1994)

(50%v/v)

Pseudomonas putida

50 ppmv

Ethanol

Compost, polystyrene (1:1)

Mixed culture

195

Arulneyam and Swaminathan (2000)

Methanol

Wood chips

Mixed culture

127

Sagastume et al. (2001)

Methanol

Compost, polystyrene (6:4)

Mixed culture

85

Arulneyam and Swaminathan (2003)

Toluene

GAC, compost

Mixed culture

97

Hwang and Tang (1997)

Xylene

Conditioned peat balls

Activated consortium

61

Jorio et al. (2000)

Methanol

Lava rock

Activated sludge

185

Prado et al. (2005)

a-Pinene

Lava rock

Ophiostoma stenoceras

143

Jin et al. (2006a, b)

Styrene

Perlite

Mixed culture

382

Rene et al. (2009a)

Toluene

Perlite

Exophiala oligosperma and

Paecliomyces variotii

164

Estévez et al. (2005)

Styrene

Perlite

Sporothrix variecibatus

336

Rene et al. (2010a)

EC elimination capacity, GAC granular activated carbon, v/v volume to volume ratio

EC elimination capacity, GAC granular activated carbon, v/v volume to volume ratio

A wide variety of both organic and inorganic compounds such as alcohols, aldehydes, ketones, hydrocarbons, carboxylic acids, H2 S and NH3 have been treated in biofilters (Leson and Winer 1991; Jin et al. 2005; Prado et al. 2008; Kennes et al. 2009a). Table 3.1 shows typical examples of gas-phase pollutants treated in biofilters and their corresponding elimination capacity values.

The main advantages of using biofiltration over other conventional control methods are their low capital costs, low operating costs, use of less chemicals and low energy requirements. Biofiltration units can be designed to physically fit into any industrial setting. It can be also designed as an open field with the piping and delivery system placed underground. In addition, biofilters can be designed with stacked beds to minimize space requirements and multiple units can be run in parallel. Biofiltration is versatile enough to treat odors, toxic compounds and VOCs. The treatment efficiencies of these contaminants are above 90% for low concentrations of contaminants (<1,000 ppm3). Different media, microbes and operating conditions can be used to tailor a biofilter system for many emission points.

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