This student manual contains textual material for a two-lesson unit on activated bio-filters (ABF). The first lesson (the sewage treatment plant) examines those process units that are unique to the ABF system. The lesson includes a review of the structural components of the ABF system and their functions and a discussion of several operational modes and the conditions under which they might be used. The second lesson covers the operation of ABF systems. The laboratory tests recommended for influent and effluent monitoring are presented and related to the factors affecting biomass growth. Calculations regarding food-to-microorganism (F/M) ratio and mean cell residence time (MCRT) are presented and worked out. Plant observations and monitoring are discussed with an emphasis on awareness and identification of existing and potential problems. Some operational problems are also presented with recommended corrective measures. The manual includes a list of objectives, a glossary of key terms, a list of references, and worksheets for each lesson. (JN)