Hydrogen sulfide removal utilizing immobilized Thiobacillus sp. IW with Ca-alginate bead

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Abstract

H2S removal by immobilized cell bioreactor system was investigated using continuous glass column bioreactor with diameter of 90 mm and with a height of 1000 mm. Thiobacillus sp. IW was encapsulated inside the Ca-alginate bead in a continuous H2S removal bioreactor system and an optimum bioreactor performance was achieved at 1.5 l media volume, under the following conditions: operating temperature of 30 °C, pH 8, bead volume of 30%, and bead diameter of 2 mm. The most active cell-immobilized bead was made in a continuous bead maker at 3% Na-alginate, 3% CaCl2, and 2.0 g/l of cell concentration. We could remove H2S in the inlet stream completely up to 600 ppm of influent concentration. It could be confirmed by scanning electron microscope (SEM) that not only did the cells grow well inside the pores of the bead but they also grew out to the surface of the bead after the H2S removal reaction. It has so far successfully run the continuous H2S removal bioreactor for up to 240 h without pH control.

Introduction

Pulping site, petroleum refinery plant, drug manufacturing process, sewage treatment facility, and livestock raising farm emit very offensive odors and most of them are composed of sulfur compounds. Among the many sulfur-containing compounds, hydrogen sulfide (H2S), methyl mercaptane (CH3SH), dimethyl sulfide ((CH3)2S), and dimethyl disulfide ((CH3)2S2) are notorious compounds, which are produced in large quantity. Hydrogen sulfide, which is a very toxic and corrosive gas, can be the standard indicator among the obnoxious odors and therefore the amount released into the air is required to be regulated strictly [1].

For the economical and efficient removal of hydrogen sulfide, it is very important to select the proper microorganism which can grow fast at room temperature and neutral pH. After the extensive preliminary screening test, Thiobacillus sp. IW separated by Cha and Lee [2] was selected for this study, which is a very effective sulfur-oxidative bacteria.

For economical purposes, to remove H2S it is very essential to immobilize the bacterium inside the carrier. We can reduce the loss of biomass and run the bioreactor in a more stable condition through the immobilization method. The common methods for immobilization include carrier-bonding, cross-linking, and encapsulation [3]. The materials for the immobilization of microorganism can be natural or synthetic polymers and the most common material in alginate [4], [5].

So far many studies on the removal of malodorous gases using immobilized microorganism have been carried out by simple adsorption method and encapsulation method was rarely used especially in a continuous process [6]. Fluidized-bed type deodoring bioreactor has many advantages which are: reduced reactor volume, high throughput capacity, ideal mixing condition, and reduced secondary pollution problem [7], [8].

Therefore, in this study, we used Thiobacillus sp. IW encapsulated inside Ca-alginate beads for efficient removal of hydrogen sulfide and investigated various immobilization and bioreactor operating conditions to achieve the maximum performance.

Section snippets

Cultivation of microorganism

Thiobacillus sp. IW which was separated by Cha and Lee [2] was donated and used in this study. Thiosulfate media [9] was prepared by dissolving 2 g of K2HPO4, 2 g of KH2PO4, 0.4 g of NH4Cl, 0.2 g of MgCl2·6H2O, 0.01 g of FeSO4·7H2O, 8 g of Na2S2O3·5H2O, and 2 g of yeast extract in 1 l of distilled water. The pH was adjusted to 7.0 before sterilizing by an autoclave. Two hundred and fifty milliliters of inoculum was prepared in a shake flask incubated for 3 days at 30 °C. Three milliliters (0.1%) of

Removal of H2S by free and immobilized cell system

Fig. 2 shows the comparison of efficiency of H2S removal in the bioreactor. Two grams per liter of Thiobacillus sp. IW free cell was introduced into 1.5 l volume of bioreactor and 50 ppm concentration of H2S gas was fed. After 9 h of operation, the output concentration of H2S went back to 50 ppm and H2S removal rate became zero. So it was concluded that the free cell system could not be used for the H2S removal. When we immobilized 2.0 g/l of cells in 300 ml of 3.0% Ca-alginate bead of diameter 2 mm

Acknowledgements

This work was supported by the Korea Research Foundation Grant in the program year of 1996.

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