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The substrate concentration is usually much higher than the KS and Ki value for the

majority of the biofilter operation and hence the biodegradation reaction is often zero order over a large range of substrate concentrations (Ottengraf and Vandenoever, 1983).

4.3 Nutrients

The bed material in a biofilter is typically soil, peat, bark, compost or other materials that contain a large variety of indigenous microorganisms (Alahari and Apte, 2004; Madigan et al., 2009). In addition to providing a physical support for the microorganisms, these materials also provide some amount of minor and trace nutrients. The presence of sufficient nutrients in the biofiltration medium is required for the maintenance of

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microbial activity and the consequent degradation of pollutants. However, the continuous supply of supplemental nutrients may also lead to undesirable problems such as biomass overgrowth with eventual clogging (Delhoménie and Heitz, 2005).

Microorganisms present in the biofilter bed use the carbon present in the pollutant (e.g. VOCs) as a carbon source for cell material, as carbon is the most important building block in any cell (Kennes and Veiga, 2001). Biofilters which treat non-carbon containing pollutants (e.g. NH3, H2S etc.,) are mostly autotrophs and hence occasionally need to be

supplemented with additional carbon sources. After carbon, nitrogen is the most essential nutrient for microbial growth. It makes up about 15% of the dry cell mass and is a major constituent of nucleic acids and proteins. The bulk of available nitrogen in nature is in inorganic form, with most microorganisms able to use ammonia, and some can also use nitrate. A large fraction of the nitrogen2 used by microorganisms is recycled after organisms die and lyse (Madigan et al., 2009).

The effect of nitrogen concentration and its chemical form on biofilter performance has been often reported (Delhomenie et al., 2001; Song and Kinney, 2005). An increase in removal efficiency by 59% is reported in a biofilter treating hexane after the addition of potassium nitrate to the biofilter bed (Morgenroth et al., 1996). There are also reports on the application of gaseous ammonia as a nitrogen source to the biofilter bed increasing the EC by 10 times (Kibazohi et al., 2004; Morales et al., 1998).

Next to nitrogen, phosphorus, sulphur and potassium are considered essential for many intracellular processes in a microorganism (ATP production, disulphate bond formation, maintenance of cellular pH, etc.) (Alahari and Apte, 2004; du Plessis et al., 1998; Sorial et al., 1998). Addition of phosphorus increased the VOC removal rate by 70% in a compost biofilter (Morgenroth et al., 1996). However, there are no reports that sulphur or potassium addition increased the performance of a biofilter (Beuger and Gostomski, 2009). However, it is necessary to maintain a threshold concentration for all these macronutrients in a biofilter in order to maintain the normal microbial metabolism (Prado et al., 2002). In addition to the macronutrients, cells require micronutrients like vitamins, magnesium, iron, calcium, copper, zinc and molybdenum in the form of trace elements for maintaining various metabolic pathways. For this reason, micronutrients are

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often added along with macronutrients to biofilters in trace levels (Delhoménie and Heitz, 2005).

Though both macro- and micronutrients are required in biofilter media for adequate biofilter performance, there is no consensus on the optimal concentration required for each of these nutrients. Routine top up of these nutrients is often needed to maintain good biofilter performance during a long run or higher inlet loads (Cherry and Thompson, 1997; Gribbins and Loehr, 1998; Morgenroth et al., 1996). Conversely, there are also few biofilters reported to be used for extended time without nutrient addition (Cárdenas‐González et al., 1999; Devinny et al., 1999; Weckhuysen et al., 1993). Therefore, the concentration, frequency and type of nutrients needed for treating different gaseous pollutants with various biofilter bed media remains highly empirical.

4.4 Temperature

Maintaining an optimum temperature in biofiltration is very crucial as the microorganisms involved in the biodegradation reaction can show maximum activity only over certain temperature ranges. The pollutant degradation rate usually increases with a rise in biofilter bed temperature until an optimum is reached. On the other hand, for the majority of gases, the solubility of the gas in the aqueous phase decreases, which in turn makes the contaminant less readily available for microbes (Yoon and Park, 2002). Since the biodegradation taking place in a biofilter is an exothermic process, it will add heat to the biofilter bed, which will also contribute to the overall temperature in the biofiltration process. The temperature of the biofilter bed increases when the cells are most active, which usually happens in a temperature range of 30-40 oC for toluene degraders (Kiared et al., 1997). There are a few reports which recommend 40 oC as the optimum operating temperature for biofilters (Leson and Winer, 1991; Ottengraf and Vandenoever, 1983). However, based on the type of microorganisms involved in degrading the particular pollutant, their optimum temperature range will change for attaining maximum degradation rate.

Hence, the major objective of this chapter is to understand the effects of toluene (substrate) concentration, different nutrients and temperature on the toluene degradation rate. These studies will help further to select a threshold toluene concentration and working temperature to attain high EC. Moreover, it will also help to understand about

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the limiting nutrients, so that the use of chemicals containing those limiting nutrient can be avoided.