istorically, the focus in water pollution abatement has been on the removal of suspended solids, nutrients and biochemical H oxygen demand. However, there is increasing interest in studying the removal of toxic and persistent compounds. Despite the xenobiotic nature of these compounds, many are at least partially degradable by microorganisms, particularly if the microorganisms have been acclimated to a given wastewater.The most common biological wastewater treatment method is the activated sludge process, which consists of a mixed culture of freely suspended microorganisms that use pollutants both as a carbon source and for energy. In this process, there are limitations on the maximum permissible biomass concentration to allow for floc formation and settling under quiescent conditions, and on the dilution rate (reactor feed rate per unit liquid volume) to avoid washout of the biomass. Additionally, the activated sludge process is generally sensitive to fluctuations in flow, concentration or temperature, arising from either periodic variations in the feed conditions or by shock loads due to spills, malfunctions, etc.(Christiansen and Spraker, 1982).Biofilm processes, in which the microorganisms are attached to a support material, have certain advantages over conventional suspension cell systems such as the activated sludge process as follows: 1) more resistance to process fluctuations (Holladay et al., 1978); 2 ) better protection against toxic or inhibitory compounds (Stevens, 1988); 3) tolerance to higher dilution rates without washout of the biomass (Tang et al., 1987); and 4) biomass concentrations 5 to 10 times higher than in free suspension system are obtained, allowing for higher degradation rates and smaller reactors (Stathis, 1980). Advantages 1 through 3 arise from the diffusional resistance to the flux of substrate into the biofilm. Wanner and Cujer (1986) presented an early model of changes in biofilm thickness, which also could be used to predict the dynamics and spatial distribution of several microbial species present in a biofilm. Their model was not intended to provide a precise numerical description of heterotrophic-autotrophic competition in biofilms, but rather to illustrate the ease by which the model could be solved on a computer.Biofilm systems may occur in a number of configurations, including trickling filters, submerged filters, rotating disks and fluidized beds. Advantages of immobilized-cell fluidized-bed reactors (ICFBRs) are: a significantly larger surface for biofilm formation as compared to other loo, ON N2L 3 G l . CanadaThe dynamic response of an immobilized-cell, fluidized-bed reactor (ICFBR) to step changes in phenol loading was investigated at 10°C for a pure culture of Pseudomonas putida Q5, a psychrotrophic bacterium. A novel dynamic model was developed and tested to simulate the response of all four key process variables: the bulk phenol concentration, the suspended biomass concentration, the concentration profile of the substrate in the biofilm and the biofilm thickness. A...
