The start-up and step-up operation of two 16-liter, continuously operated, upflow anaerobic reactors receiving baker's yeast wastewater is presented in this thesis. The two reactors (A and B) were almost identical in construction. Reactor A was a conventional upflow anaerobic sludge blanket (UASB) reactor, and reactor B was a hybrid reactor. In addition to all the features of a UASB reactor, a fixed-film structure was installed in the mid section of the reactor B. Both reactors were operated at 35 °C and at a constant hydraulic retention time of 7 days. The waste strength, expressed in chemical oxygen demand (COD), was varied from 8 g COD liter⁻¹ (during the start-up) to 58 g COD liter⁻¹. The organic loading rate ranged from 1.1 to 9.4 g COD liter⁻¹ day⁻¹. The start-up lasted for the first 46 days. Towards the end of the start-up, methane production rates of 0.23 and 0.28 liter CH₄ liter⁻¹ day⁻¹ and COD reductions of 62.2% and 67.2% were achieved at organic loading rates of 1.1 and 1.3 g COD liter⁻¹ day⁻¹ for reactors A and B respectively. During the step-up operation, maximum methane production rates were, for reactors A and B respectively, 0.91 and 0.95 liter CH₄ liter⁻¹ day⁻¹ at organic loading rates of 5.8 and 6.4 g COD liter⁻¹ day⁻¹. In addition, reactor profiles for sludge concentration, pH, volatile fatty acids, and COD are also presented. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate