This paper presents the results of a re-rating study for the activated sludge system at the Municipality of Metropolitan Seattle’s East Division Reclamation Plant at Renton, Washington. Using mathematical simulation models for the biological process and for the secondary clarifiers, it was found that a 27 percent increase in average dry weather flow capacity could be achieved by 1) replacing exiting side channel swing diffuser units with full floor coverage fine bubble diffuser grids, and 2) using chlorination of return sludge or an anaerobic biological selector to control solids settling characteristics. This permitted the system to be expanded to accommodate over a 50 percent increase in flow with only an 33 percent increase in aeration tank volume.