Wastewater treatment, microbiology, biochemistry, and engineering are the major subject areas covered in this preliminary thesaurus designed for use in a private information retrieval system. The thesaurus was developed through meetings where each descriptor was discussed, necessary scope notes were written, definition and cross references were added, and the final term format was compared with other thesauri. Included are directions for retrieval procedures, guidelines for indexing, an explanation of the main entry format of the descriptors, and the 220 descriptors. (JAB)