This manual contains the textual material for a three-lesson unit which introduces students to the basic concepts applicable to all biological treatment systems. The general topic areas addressed in the lessons are: (1) the microorganisms found in biological systems; (2) the factors that affect the growth and health of biological systems; and (3) the interrelationships between groups of microorganisms, their competition, predominance, and symbiosis. A list of objectives, glossary of key terms, list of references, and student worksheets (one for each lesson) are included. (JN)