Bio trickling filtration is one of the many promising biological techniques forodor and VOC control. Contrary to bio filters that are usually packed with some sort of organic medium and are operated with a minimum of free water(Devinny et al. 1999), bio trickling filters are almost exclusively packed with inorganic or manufactured media, over which a distinct liquid phase is trickled. The trickling liquid in bio trickling filters provides a convenient means to control pH, salt or metabolite concentration, and to supplement nutrients to the process culture. Bio trickling filters have several advantages over other bio treatment technologies for air pollution control (see Table 7.1;Oh and Bartha 1997; Mpanias and Baltz is 1998; Cox et al. 2000; Gabriel andDeshusses 2003a). Some book chapters and reviews published previously contain comprehensive discussions of bio trickling filtration technology (Cox and Deshusses 1998, 2001, 2002a; Deshusses 2004). In the present chapter, selected aspects of bio trickling filtration for air pollution control are presented and discussed. Background information on bio trickling filter is presented in Sect. 7.1, while Sect. 7.2 deals with bio trickling filter design and operation. In Sect. 7.3, the focus is placed on high-performance bio trickling filters for H2 S and odor control - in particular, the conversion of chemical scrubbers to biological trickling filters is presented and discussed. The latter issue has received considerable attention, as it has been shown that bio trickling filters can achieve H2 S elimination rates equivalent to those observed in chemical scrubbers. The basic principle of a bio trickling filter is illustrated in Fig. 7.1. In bio trickling filters, foul or contaminated air is forced through a packed bed, either downflow or up flow. The packed bed is almost always made of an inert material such as a random dump plastic packing, lava rocks, structured plasticpacking, or open-pore synthetic foam. Other materials that have beenused include glass or rock wool, shredded tires, glass beads, or ceramics. The packing provides the necessary surface for bio film attachment and for gas-liquid contact. During treatment, an aqueous phase is recycled over the packing. It provides moisture, mineral nutrients to the process culture, and a means to control basic operating parameters. In most cases, the trickling liquid is continuously supplemented with essential mineral nutrients such asnitrogen, phosphorus, potassium, and trace elements. Nutrient requirements depend on the pollutant being treated, the pollutant loading and concentration, and general bioreactor operation strategy. In general, most of the pollutant is biodegraded in the bio film, but part may also be removed by micro organisms suspended in the recycle liquid (Cox et al. 2000). Possible biodegradation metabolites will leave the system via the liquid purge, along with small amounts of biomass and dissolved pollutant, if any. Usually, the amount of pollutant leaving the bio trickling filter via the purgeis negligible compared to the amount degraded in the system (Cox et al.2000).Bio trickling filters work because of the action of the pollutant-degrading microorganisms. In the case of the removal of hydrocarbon vapors, the primary degraders are aerobic heterotrophic organisms that use the pollutant asa source of carbon and energy (see Chap. 5). ForH2S or ammonia removal, the primary degraders are autotrophs, and they will use the pollutant as a source of energy, and carbon dioxide as a source of carbon for growth. The removal of compounds such as dimethyl sulfide or dimethyl disulfide will require both autotrophs and heterotrophs to be present. In any case, the bio trickling filter will host a wide variety of microorganisms, similar to those encountered inwaste water treatment operations. The microorganisms responsible for pollutant removal in bio trickling filters are usually aerobic, because bio trickling filters are well-aerated systems. However, it has been proposed that the deeper parts of the bio film (see Fig. 7.1), where anaerobic conditions probably prevail, can be utilized to perform anaerobic biodegradation (e.g., reductive dechlorination, or NOx reduction) for the treatment of pollutants that are recalcitrant under aerobic conditions (Devinny et al. 1995). Anaerobic treatment in aerobicbio trickling filters remains a challenge.As illustrated in Fig. 7.1, if the bio film grows to a certain thickness, the fractionof the bio film against the substratum will become inactive due to masstransfer limitations. Thus, in those cases, the active primary degraders constitute only a minor fraction of the total population in the bio film. Secondary degraders feeding on metabolites, biopolymers, or predators feeding on the primary degraders include bacteria, fungi, and higher organisms such as protozoa, rotifers, mites, even mosquito or fly larvae, worms, or small snails. The importance of the secondary degraders and higher organisms should not be under estimated. They have been shown to play an important role in reducingthe rate of biomass accumulation, and in recycling essential inorganic nutrients(Cox and Deshusses 1999; Woertz et al. 2002; Won et al. 2002). In fact, comparison of fermentation broth design with bio trickling filter recycle liquid composition reveals that most bio trickling filters used for VOC control are operated under some degrees of inorganic nutrient limitation, suggesting that nutrient recycle through the action of secondary degraders within the reactoris a common process. © Springer-Verlag Berlin Heidelberg 2005.