This unit provides an introduction to basic concepts which are applicable to all biological treatment systems. It consists of three lessons which focus on: the microorganisms found in biological systems; the factors that affect the growth and health of biological systems; and the interrelationship between groups of microorganisms, their competition, predominance, and symbiosis. The instructor's guide for the unit includes: (1) an overview of the unit; (2) lesson plans; (3) lecture outlines (keyed to a set of slides designed for use with the lessons); (4) a student worksheet for each lesson (with answers); and (5) two copies of a final quiz (with and without answers). (JN)