New capital cost information for conventional and some advanced wastewater treating processes has been collected and correlated in terms of capacity or physical size. Operating costs have been calculated for some of the processes. Conventional processes considered include comminution, grit removal, primary settling, biological treatment by activated sludge and trickling filter, final settling, and sludge treatment and disposal. Advanced processes include phosphate extraction, chemical clarification and sand filtration, carbon adsorption, and membrane ultrafiltration. Most of the cost information was obtained from sales information of Dorr-Oliver, Incorporated. Supplementary data from other sources were utilized, however, when necessary.