Selection of sulfur oxidizing bacterium for sulfide removal in sulfate rich wastewater to enhance biogas production

Abstract

There are numerous latex rubber factories in southern Thailand and anaerobic treatment, particularly in lagoons, is the most commonly used process for treating their wastewater (Kantachote et al. 2005). The anaerobic wastewater system provides many advantages such as, a low cost operation with high efficiency and also with the possibility of producing a useful energy source biogas; however, sulfide is generated during anaerobic treatment of high sulfate wastewater, a characteristic of wastewater from rubber factories. An advanced system such as the sulfate reduction reactor (SRR) has been used to treat sulfate rich rubber wastewater from processes of concentrated latex and skim crape (Tekasakul and Tekasakul, 2006). However, high sulfide levels are produced due to sulfate being used as a terminal electron acceptor in anaerobic respiration. The SRR is a step to reduce sulfate concentration in wastewater prior to transferring the wastewater to an UASB (upflow anaerobic sludge blanket) for obtaining biogas as a byproduct. Nevertheless, biogas produced from both the SRR and the UASB do not meet the standard of biogas composition because of their high contaminations of H 2 S. Therefore, the biogas was burnt to remove the very toxic and corrosive H 2 S gas that has a very low odor threshold of 1 µg L -1 (Oyarzun et al. 2003). Hence, introducing one more step to convert sulfide to sulfur by partial oxidation instead of by completely oxidizing it to sulfate could be an interesting alternative method for solving this problem. It is realized that levels of sulfide oxidation are dependent on oxygen concentration (Gonzalez Sanchez et al. 2005). It has long been recognized that bacteria able to oxidize reduced sulfur compounds can be used to remove contaminating *Corresponding author H 2 S, from either treated wastewater or gaseous systems (Sublette et al. 1998;Cha et al. 1999;Kleerebezem and Mendez, 2002;Chung et al. 2003). Removal of H 2 S from any SRR effluent, would greatly improve the economics of the process, particularly if this could be achieved microbiologically. It is therefore of importance to select a microbe that can grow well at ambient temperatures and neutral pH and oxidize sulfide to sulfur in wastewater. Our previous studies have reported on the potential use of a Thiobacillus sp. for treating latex rubber sheet wastewater (Kantachote and Innuwat, 2004). Hence, this work is focused on the isolation and identification of a bacterium able to oxidize sulfide to sulfur in sulfate rich wastewater to achieve our goal of obtaining a better and more efficient production of biogas during the treatment of wastewater from rubber manufacturing plants. MATERIALS AND METHODS Isolation and selection of a sulfide oxidizing bacteriumStrains of Thiobacillus sp. were isolated from sulfide rich wastewater samples collected from rubber factories in southern Thailand using a thiobacillus isolation medium named thiosulfate mineral salts medium (thiosulfate MSM). The medium composition in 1L of distilled water is 2.0 g KNO 3 , 1.0...

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