Removal of hydrogen sulfide by immobilized Thiobacillus thioparus in a biotrickling filter packed with polyurethane foam
Introduction
Hydrogen sulfide (H2S) is a colourless, toxic and flammable gas that has a characteristic odour of rotten eggs.
Both natural and anthropogenic sources contribute to the total emission of hydrogen sulfide. Hydrogen sulfide occurs naturally in the gases from volcanoes, sulfur springs, undersea vents, swamps, stagnant bodies of water in crude petroleum and natural gas and as a product of the biological degradation of organic matter (Lomans et al., 2002). Considerable amounts of hydrogen sulfide are also emitted from industrial activities such as petroleum refining, pulp and paper manufacturing, wastewater treatment, food processing, livestock farming, and natural gas processing.
The concentrations of hydrogen sulfide in gas emissions are usually very dilute and traditional physical–chemical technologies such as incineration, adsorption or chemical scrubbing tend to be costly and are associated with their own pollution problems. As a result, based on the cost of the equipment and operation, biological treatment is believed to be the most economical option for the removal of hydrogen sulfide.
Many microorganisms have been used for H2S removal, principally Acidithiobacillus and Thiobacillus. In these groups are acidophilic bacteria such as Acidithiobacillus thiooxidans (Aroca et al., 2007, Sercu et al., 2005), neutrophilic bacteria such as Thiobacillus novellus (Cha et al., 1999), Thiobacillus thioparus (Chung et al., 2000, Cox and Deshusses, 2002, Oyarzún et al., 2003) and Thiobacillus denitrificans (Ma et al., 2006). Other bacteria such as Pseudomonas putida CH11 (Chung et al., 2001), Hyphomicrobium sp. (Sercu et al., 2005) and haloalkaliphilic consortium (Gonzalez-Sanchez et al., 2008) have been used for the removal of H2S.
The major biological reactors for the treatment of dilute gases are biofilters, biotrickling filters, and bioscrubbers. These systems differ in the presence or absence of a carrier material, the phase of the biomass (suspended or fixed), and the state of the liquid phase (flowing or stationary).
In biofilters the most commonly used carriers are compost and peat, although some authors add other materials such as perlite and/or wood chips in an effort to avoid compaction of the bed (Wani et al., 1999). Activated carbons have also been used to remove H2S and these give very good performance (Chung et al., 2005, Ma et al., 2006). The active carbon allows the combination of adsorption and biological degradation.
The use of bioscrubbers to remove H2S is very unusual because the solubility of H2S in water is very low.
In biotrickling filters the most commonly used carriers are propylene rings (Jin et al., 2005), ceramics (Ruokojärvi et al., 2001) and lava rocks (Chitwood et al., 1999). However, some investigations have been carried out with polyurethane foam (Gabriel et al., 2004, Gabriel and Deshusses, 2003), but using active sludge as inoculum.
The objective of the work described here was to study the feasibility of treating air contaminated with H2S using a biotrickling filter packed with cubes of polyurethane foam inoculated with pure culture (T. thioparus). T. thioparus was selected because this bacteria can oxidize other sulfur compounds and the optimal pH is neutral and therefore the hydrogen sulfide solubility will be greater.
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Microorganism and cultivation medium
A pure culture of T. thioparus (ATCC 23645) was obtained from the American Type Culture Collection. The composition of the ATCC290:S6 mineral medium was (in grams per litre): 1.2 g of Na2HPO4, 1.4 g of KH2PO4, 0.1 g of MgSO4·7H2O, 0.1 g of (NH4)2SO4, 0.03 g of CaCl2, 0.02 g of FeCl3, 0.02 g of MnSO4 and 10.0 g of Na2S2O3 and the final pH was adjusted to 7.0 using 2.0 N NaOH. Prior to preparation of the medium the iron solution was filtered (0.22 μm filter) and the basalt salt solutions were autoclaved at
Growth kinetics
As can be observed in Fig. 2, the initial biomass concentration was 1.6 × 107 cells/mL, with a growth maximum of 5.7 × 108 cells/mL at 46 h. The substrate concentration decreased to zero at 70 h with a decrease in pH from 7.2 to 3.8. When the sulfate concentration was above 4.0 g/L, the elemental sulfur concentration increased (elemental sulfur measured by mass balance). A maximum specific growth rate (μ) of 0.0971 h−1 with a linear regression coefficient of 0.993 was obtained.
Biomass immobilization
A total of 11
Conclusions
The results obtained lead us to conclude that the parameters with the most influence on the performance of the biotrickling filter are the pH (optimal pH between 7.0 and 7.5) and the sulfate concentration (optimum < 5 g/L).
The critical EC value was 14.9 g S/m3/h (removal efficiency of 99.8%) and ECmax was 55.0 g S/m3/h (removal efficiency of 79.8%) for an EBRT of 150 s. For loadings of 2.89 ± 0.05 and 11.5 ± 0.1 g S/m3/h, the removal efficiency was higher than 99% for an EBRT over 90 s.
The performance can be
Acknowledgement
The authors wish to thank the Ministry of Science and Technology for financing received under Project PPQ2002-0217.
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