King County is designing a new wastewater treatment plant, known as Brightwater. Phase I of this project includes a 36-mgd average wet weather flow activated sludge system, which will incorporate a membrane bioreactor (MBR) in order to meet site constraints, improve effluent quality, and produce a source of reclaimed water. The unique challenges faced by large MBR plant designers are described as are approaches to address them. They include optimization of membrane capacity sizing (to minimize capital cost) by base loading the MBR and directing higher flows to enhanced primary clarification, defining the split flow threshold, accommodating different vendor designs for MBRs, membrane tank sizing and operational flexibility, and the interactions between SRT, oxygen demand, oxygen transfer constraints, RAS rate and its impact on anoxic selectors.