Sulfidogenesis in Pretreatment of High-Sulfate Acidic Wastewater Using Anaerobic Sequencing Batch Reactor and Upflow Anaerobic Sludge Blanket Reactor Sumate Chaiprapat, 1,2, * Pireeporn Preechalertmit, 3 Piyarat Boonsawang, 4 and Seni Karnchanawong 5 1 Green Technology Research Unit, Department of Civil Engineering, Faculty of Engineering; 2 National Center of Excellence for Environmental and Hazardous Waste Management, PSU Satellite Center; 3 Faculty of Environmental Management; and 4 Department of Industrial Biotechnology, Faculty of Agro-Industry; Prince of Songkla University, Songkhla, Thailand. 5 Department of Environmental Engineering, Faculty of Engineering, Chiang Mai University, Chiang Mai, Thailand. Received: December 28, 2010 Accepted in revised form: May 2, 2011 Abstract In rubber skim block production, use of sulfuric acid is a general practice, which generates acidic wastewater (pH 2.4 – 0.5) with high chemical oxygen demand (COD) (14,911 – 1,819 mg/L) and sulfate (6,506 – 1,038 mg/L). Our work aimed to pretreat this wastewater separately prior to combining with the other wastewaters to facilitate efficient anaerobic digesters that follow. Results indicated that sulfate removal efficiency of the an- aerobic sequencing batch reactor (ASBR) pretreating raw rubber skim wastewater (pH 2.43 – 0.50) was only 7.25% – 4.01% and gradually improved with increasing influent pH to 45.45% – 3.75% at pH 7. In the second part, upflow anaerobic sludge blanket (UASB) and ASBR fed with the rubber skim wastewater at pH 7 faced system failure after 97 days of operation. When the influent pH was adjusted to 8, sulfate removals of UASB and ASBR at 27.37% – 2.55% and 46.58% – 1.98%, respectively, could be sustained. A distinctive ASBR operation that allows mixing and settling phases was deemed as key to enable, respectively, better substrate dispersion and retention of the poor granulating sulfate-reducing bacteria. Key words: high sulfate; biogas; hydrogen sulfide; pretreatment; rubber Introduction R ubber (Hevea brasiliensis Muell. Arg) is one of the most important economic crops of Southeast Asian countries, especially Thailand, the largest natural rubber producer of the world (The Thai Rubber Association, 2010). In year 2009, Thailand had exported rubber over 2.7 million tons (Office of Agricultural Economics, 2010), approximately one third of the total world’s rubber. Of this amount, concentrated rubber latex accounted for 26% (Rubber Research Institute of Thailand, 2010). Unfortunately, concentrated rubber latex industry is a significant source of air and water pollution because of an improper management of its wastewater and has often drawn complaints from nearby communities (Agamuthu, 1999; Boonreongkaow et al., 2002; Bunyakan et al., 2004). The treatments that have been widely adopted are natural pond system and aerated lagoon, both of which are open systems. The former generates malodorous off-gas, comprising of hydrogen sulfide (H 2 S), and the latter con- sumes great amount of energy. Therefore, an anaerobic closed system or biogas system becomes an attractive choice, be- cause odor emission is prevented while requiring low energy input for operation and generating methane, which can be used as fuel. Some works have been done regarding the fea- sibility of anaerobic reactors for this wastewater (Khetanan, 2008; Saritpongteeraka and Chaiprapat, 2008). The typical process in a concentrated latex factory that consists of, first, rubber centrifugation to remove liquid, to concentrate the field rubber latex from 30% to 60% in dry rubber content, and second, rubber skimming, which adds sulfuric acid to the rejected liquid from centrifugation to capture the left-over rubber particles to make a byproduct of rubber skim block. Wastewater from rubber skim process is highly acidic, pH 2.0–4.5, and high in chemical oxygen de- mand (COD) and sulfate, 32,560 and 5,411 mg/L, respectively ( Jakaew, 2003). In contrast, wastewater from the other parts of this concentrated latex factory is slightly alkali, pH 7.9, and has low COD and sulfate at 500 – 126 and 275 – 82 mg/L, re- spectively (Wongchana, 2010). In general, the two streams are combined, causing the dilution of COD and sulfate. The re- sultant larger wastewater volume will reduce the hydraulic retention time of the anaerobic treatment system, resulting in lesser degradation of the organics and biogas production. *Corresponding author: Green Technology Research Unit, Depart- ment of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai Campus, Hat Yai, Songkhla 90110, Thailand. Phone: 66815403037; Fax: 6674459396; E-mail: sumate.ch@psu.ac.th ENVIRONMENTAL ENGINEERING SCIENCE Volume 28, Number 8, 2011 ª Mary Ann Liebert, Inc. DOI: 10.1089/ees.2010.0492 597