1319 † To whom correspondence should be addressed. E-mail: sschoi@semyung.ac.kr Korean J. Chem. Eng., 26(5), 1319-1322 (2009) DOI: 10.1007/s11814-009-0229-0 RAPID COMMUNICATION Analysis of methane production inhibition for treatment of sewage sludge containing sulfate using an anaerobic continuous degradation process Tae-Young Jeong*, Hyung-Keun Chung*, Sung Ho Yeom**, and Suk Soon Choi*** ,† *YIEST, Division of Environmental Engineering, Yonsei University, Wonju 220-710, Korea **Department of Environmental & Applied Chemical Engineering, Kangnung National University, Kangneung 210-702, Korea ***Department of Biological and Environmental Engineering, Semyung University, Jecheon 390-711, Korea (Received 10 April 2009  accepted 7 May 2009) Abstract −The inhibition of methane production in the continuous anaerobic degradation process for the treatment of sewage sludge containing sulfate was investigated. Also, the competition between sulfate-reducing bacteria (SRB) and methane-producing bacteria (MPB) with COD/sulfate ratio was explained in terms of electron flow. The methane production rate was 0.07, 0.13, 0.24, 0.31 and 0.33 l-CH 4 g-COD −1 when the initial COD/sulfate ratio was 3.3, 5.0, 6.7, 10 and 20, respectively. The numbers of SRB and MPB were counted after the continuous reactor reached steady state and the two bacteria showed opposite growth behaviors with COD/sulfate ratio. The inhibition by sulfate com- pounds was found to follow the uncompetitive model and inhibition constants were 24.57 and 87.99 mg l −1 for SRB and MPB, respectively. These results can be useful data for the efficient treatment of sewage sludge in a continuous anaerobic degradation process. Key words: Continuous Anaerobic Degradation, Inhibition of Methane Production, Uncompetitive Model INTRODUCTION It is generally known that the reduction of sulfate during the di- gestion of sludge reduces bioavailable organic contents, which in turn decreases methane production. In particular, unionized hydro- gen sulfide (H 2 S) was reported to impose toxic effect on various anaerobic bacteria [1-3]. The extent of inhibition over methane pro- ducing bacteria (MPB) was related to the concentration of sulfur compounds produced by sulfate-reducing bacteria (SRB) [4]. Ac- cording to Parkin et al. [5], methane production was inhibited under as low as 50 mg S 2− l −1 of sulfur compounds in a batch anaerobic digestion. They also reported that methane production was not af- fected by the sulfur compounds up to 400 mg S 2− l −1 in a sub- merged anaerobic filtering process, while it was reduced by 30% at 800 mg l −1 of sulfur compounds. Sulfur compounds produced by SRB usually exist as the form of H 2 S, HS − , S 2− or gaseous H 2 S in aqueous solution and 20% of the total sulfur compounds at pH 7.0 is free H 2 S [6]. Kroiss and Wanbnegg suggested that the level of free H 2 S in aqueous solution was related to the toxicity toward MPB [7]. Acetoclastic MPB was inhibited by about 50% at 50 mg l −1 of free H 2 S and was completely inhibited at 200 mg l −1 of free H 2 S [8]. Recently, pretreatment methods such as thermal sludge treatment have been tried in many researches in order to enhance the efficiency of methane production [9-11]. In a previous study [12], methane production was reduced by 50 % in a batch experiment at the COD/sulfate ratio of 11.6. So far, few researches have been per- formed to analyze methane production in terms of electron flow. In this study, the concept of “electron flow” was introduced to explain the inhibition of methane production and analyze quantita- tively the inhibition by SRB and MPB. The results were applied to the continuous anaerobic degradation containing sulfate for the actual treatment of sludge. Also, the inhibition by sulfate on the two dif- ferent bacteria was evaluated by kinetic models. MATERIALS AND METHODS The waste activated sludge (WAS) serving as a substrate in this study was collected at a sewage treatment facility located at the city of Wonju. After being treated at 120 o C for 30 minutes, the sludge was used in this study. The cell concentration of mixed liquor volatile suspended solid (MLVSS) was adjusted to 8,200 (±200) mg l −1 and divided equally to several portions. Sulfate with different concen- trations was added to each portion and the inhibition of methane production by SRB and MPB was investigated. The influent COD of waste sewage sludge was about 10,000 mg l −1 and initial sulfate concentrations were 500, 1,000, 1,500, 2,000, 3,000 or 5,000 mg l −1 , which corresponded to 20, 10, 6.7, 5, 3.3 or 2.0 of COD/sulfate ratio, respectively. The schematic of the reactor system is depicted in Fig. 1. And the digestion gas from the reactor was circulated into the reactor itself, which continuously mixed and discharged digesting sludge solution. To maintain temperature in the reactor at 35±1 o C, a thermo controller was installed in a thermostat vessel. Operation condition of the reactor is described in Table 1. The composition of emitting gas was analyzed by gas chromatography (GC-14A, Shimadzu, Japan) with a TCD detector, and volatile fatty acid (VFA) was done with another gas chromatography (GC-8A, Shimadzu, Japan) with an FID detector. Total sulfide (TS) and dissolved sulfide (DS) pro- duced by SRB were analyzed according to iodometric method [13]. Sulfate ion (SO 4 2− ) was analyzed by ion chromatography (DX-120, Dionex, USA). The number of bacteria was counted at steady state ac-