Mikio Kitagawa

Long term competition between sulfate-reducing and methane-producing bacteria for acetate in anaerobic biofilm

Abstract A long term competition between sulfate-reducing bacteria (SRB) and methane-producing bacteria (MPB) for acetate was investigated using a laboratory scale anaerobic fluidized bed. When the synthetic wastewater composed of acetate and sulfate was fed at a low organic loading rate, averages of the remaining acetate and sulfate concentrations were 1.7 mg Cl −1 and 78.5 mg l −1 , respectively. During several months of this acetate limited operation the methane production rate as well as the microbial mass of MPB declined gradually, whereas the amount of reduced sulfate along with the microbial mass of SRB increased, which apparently indicates that SRB out-compete MPB in the biofilm at lower acetate concentrations. On the other hand, MPB were able to form a biofilm faster than SRB at higher acetate concentrations presumably due to MPBs higher ability to adhere carrier surfaces compared with SRB. Kinetic constants for both species in the biofilm were determined and compared with those reported for pure MPB and SRB cultures. Based on the kinetic mechanism of this competition, operational conditions which would support methanogenesis by suppressing sulfate reduction were identified.

Granular sludge formation in the anaerobic expanded micro-carrier bed process

The anaerobic expanded micro-carrier bed (MCB) process, which utilizes fine (50-100 microns) support materials as expanded bed media, was found to have the ability to cultivate granular sludge similar to that formed in the upflow anaerobic sludge blanket (UASB) process. Two laboratory-scale MCB reactors were studied with VFA and glucose wastewaters to clarify the role of the micro-carrier and the influence of substrates on granular sludge formation. Based on these results, a scale-up model with a reactor volume of 800 1 was successfully operated using molasses wastewater to demonstrate the feasibility of granular sludge formation in the MCB process.