The objectives of this study are to develop a model which can express the characteristics of anaerobic treatment in a methanogenic fluidized bed reactor in non-steady states, and evaluate the usefulness of the model in which “active” biomass concentration is used as an index of bacteria concentration instead of traditional ones such as volatile suspended solids.
The treatment in the fluidized bed reactor started with biocoated synthetic zeolite as support material. The reactor was supplied with synthetic wastewater, a mixture of volatile fatty acids. An experiment on response was conducted according to changing stepwise the influent concentration. Batch experiments on activity were carried out under several steady state conditions, using a kind of volatile fatty acid as a substrate. Transient responses of water quality are investigated in relation to the growth of bacterial groups which decompose volatile fatty acids. Based on the results, a dynamic model is proposed, in which attached biomass is categorized into “active” biomass and inert organic matter. The transient response of substrate concentrations was clearly expressed by considering the growth of “active” biomass. It was impossible to explain the responses when VSS was regarded as the biomass concentration in the model.