CADDAS (Computer Aided Design of Diffused Aeration Systems) is a microcomputer-based program that analyzes the cost and performance of diffused aeration used in activated sludge wastewater treatment systems. The program can analyze both coarse bubble and fine pore diffusers as well as detailed blower configurations and operational strategies. For a given set of hourly oxygen demands throughout the year, diffuser performance characteristics, and blower capacity/operational capability, CADDAS computes hourly dissolved oxygen levels, air flow requirements, and blower energy usage over each month of a multi-year planning period. Special attention is paid to the effects of fine pore diffuser fouling on oxygen transfer efficiency and its restoration through cleaning. CADDAS also computes the present worth costs of initial equipment installation, monthly energy usage, annual routine maintenance, and periodic diffuser cleaning.