Hydrogen sulfide (H2S) is being emitted by many industrial activities such as petroleum refining, natural gas and petrochemical plants, craft pulp manufacturing, viscose rayon manufacturing, food processing, tanneries, aerobic and anaerobic wastewater treatments. H2S is a colourless, flammable and highly toxic gas, and heavier than air. Its value of Henry's Law constant for the air-water-H2S system at 25ºC is 0,41 (H2S concentration in air/H2S concentration in water). Usually H2S is found in mixture with other organic sulfur compounds such as methanethiol, dimethylsulfide and dimethyldisulfide. This mixture is known as the Total Reduced Sulfur Compounds (TRS) (Ruokojarvi et al. 2000).
The physicochemical methods used at present for the treatment of gaseous emissions containing H2S and reduced sulfur compounds, such as adsorption, absorption, incineration, have relatively high energy requirements and/or high chemical and disposal cost. Several biological processes used to treat gaseous emissions contaminated with TRS, and specifically with H2S, have been reported (Jensen and Webb, 1995; Smet et al. 1998).
Biological treatments using biotrickling filters have been proposed as convenient alternatives for treating air streams containing low concentration of contaminants (Deshusses, 1997; Kennes and Thalasso, 1998; Gabriel and Deshusses, 2003). These are packed columns where a bacterial biofilm is formed on the surface of an inert packing material. The contaminated air stream is concurrently or counter-currently contacted with a liquid phase that provides nutrients and conditions to keep the viability and activity of the biofilm. The gas is absorbed in the liquid phase and biologically oxidized, thus converting H2S compounds into oxidized sulfur compounds such as sulfur and sulfate, chemicals that will go out of the bioreactor in the liquid phase (Alonso et al. 1997).
Although it is possible to establish a complex microbial population with the ability to oxidize H2S, i.e.: by inoculating the biofilter with biological complex inoculums such as active sludge, a dominating microbial population exhibiting the highest degradation activity develops after a period of adaptation (Hirai et al. 1990; Cho et al. 1992). However in some cases, the efficiency of these systems is limited and generally its removing capacity is not constant (Wani et al. 1997). The use of biofilms generated with single bacterial cultures has been proposed for improving the efficiency and removal capacities of the biotrickling filters. Chemoheterotrophic microorganisms such as Pseudomonas acidovorans DMR-11 and Pseudomonas putida show degrading activity of H2S and organic sulfur compounds. Bacteria from the genus Acidithiobacillus, such as A. thiooxidans that use H2S as energy source, seem to be appropriate because of their low nutritional requirements (Cho et al. 2000; Sercu et al. 2005). The inoculation of biotrickling systems with single cultures leads to the shortening and even the absence of bacterial lag phase, as well as to the increase in the efficiency for removing H2S and other organosulfur compounds during a stable operation. T. thioparus strains CH11 (Chung et al. 1997) and T. thioparus DW44 (Cho et al. 1992) shown high efficiency for removing H2S, being also able to degrade methanethiol, dimethylsulfide and dimethyldisulfide, and to use carbon disulfide as energy source (Smet et al. 1998). Other bacterial species that have been evaluated for the removal of sulfur reduced compounds are T. denitrificans (Sublette and Sylvester, 1987), A. ferrooxidans (Pagella and De Faveri, 2000), T. novellus (Cha et al. 1999), and A. thioxidans (Oprime et al. 2001). There is no comparison in the literature about the performance of biotrickling filters inoculated with different microorganism for the bioxidation of H2S.
Various materials have been used to support biofilms of Thiobacillus, among them polypropylene rings (Tanji et al. 1989), polystyrene foam, diatomaceous earth, ceramics, polystyrene mixed with active carbon, pellets of synthetic materials and perlite (Cox et al. 1997). In general, porous and non hydrophobic surfaces with high specific surface seem to facilitate or promote colonization by microorganisms and the subsequent formation of biofilms (Chitwood and Devinny, 2001). Since these materials offer different characteristics for bacterial growth and for the operating conditions, it is important to develop analytical techniques to evaluate and compare their behaviour.
This article reports the performance in the bioxidation of H2S using biotrickling filters inoculated with T. thioparus operated at neutral pH conditions, and a biotrickling filter inoculated with A. thiooxidans in an acid environment. The selection of the supporting material was made by using respirometry as an index of the sulfur oxidizing capacity of the biofilm formed on three materials: (i) volcanic stones (tezontle (TZ)); (ii) polypropylene rings and (iii) polyvinilclorure (PVC).