This document is an instructional module package prepared in objective form for use by an instructor familiar with operation and maintenance of a rotating biological surface (RBS) wastewater treatment system. Included are objectives, instructor guides, student handouts, and transparency masters. This is the third level of a three module series and considers the biology and biochemistry of biochemical oxygen demand (BOD), reduction and nitrification in the RBS process, system applications, process performance and measures to assure proper plant performance. (Author/RH)