TABLE OF CONTENTS In fixed-film reactors, the microbial biomass is present as a film, which grows over the surface of an inert and solid medium. Purification is achieved when the wastewater is brought into contact with this microbial film. Because the active biomass is largely retained within the reactor, there is no need to recirculate any displaced biomass to maintain a sufficient density of micro-organisms, as is the case with completely mixed systems (Section 15.1). The required contact between the film and wastewater is achieved in most fixed-film reactors by allowing the wastewater to pass over the stationary medium on which the film has developed. However, it is not essential for the medium to be stationary, and in more recently developed reactor systems (e.g. rotating biological contactors (RBCs), fluidized beds, etc.) the medium itself moves through the wastewater (Section 16.4.1). Fixed-film reactors can be designed as secondary treatment processes to partially treat (high-rate systems) or fully treat (low-rate systems) either screened or more generally settled wastewater (Bruce and Hawkes, 1983). While they are normally aerobic, for carbonaceous removal and nitrification, anoxic and anaerobic filters are used for denitrification (Section 18.4.1) and for treating moderately strong organic wastewaters, respectively (Section 19.3). While anoxic and anaerobic filters are submerged reactors, that is, the media is permanently submerged under the wastewater, submerged aerobic filters are less common as they require a diffused aeration system to maintain a sufficient oxygen concentration within the reactor. However, submerged aerobic filters are particularly useful...
