Wong, JM, Hess, RJ and Wang, LK (2019). Operation and performance of the AquaDAF® process system for water purification. In: “Evolutionary Progress in Science, Technology, Engineering, Arts, and Mathematics (STEAM)”, Wang, Lawrence K. and Tsao, Hung-ping (editors). Volume 1, Number 7, July 2019; 49 pages. Lenox Institute Press, Newtonville, NY, 12128-0405, USA. No. STEAM-VOL1-NUM7-JUL2019; ISBN 978-0-9890870-3-2. This book chapter introduces the high-rate AquaDAF clarification system which was developed in the mid 1990s for water treatment. The process is much more compact than conventional clarification systems with surface loading rate as high as 20 gpm/ft2. The presentation includes the history of process development, application, system description and design criteria, pilot and full-scale case studies, and marketing outlook. Since 2000 more than 55 full-scale AquaDAF plants have been installed worldwide with a total capacity of 1.4 billion gallons per day.