The San Diego County Water Authority (Water Authority) is the regional water wholesaler responsible for ensuring a safe and reliable water supply to support San Diego's $130 billion economy and three million residents. The Water Authority imports 75 to 95 percent of the water used in the region. The Water Authority's 2030 Regional Water Facilities Master Plan indicated an immediate regional need for an additional 50 to 100 million gallons per day (mgd) of treatment capacity. The Water Authority does not own or operate water treatment facilities and relies on Metropolitan Water District of Southern California (MWD) and its member agencies to supply all of the region's treated water demands. Approximately 45 percent of all imported water is treated and supplied by MWD and the remaining is supplied by member agency water treatment plants (WTP). Multiple studies indicated the most effective option for additional regional treatment capacity is to build a new WTP at the Water Authority's Twin Oaks Valley (TOV) site in northern San Diego county. The Water Authority plans to implement the TOV-WTP using the design-build-operate (DBO) procurement method. The type of WTP being implemented, conventional or membrane, will not be specified in the request for proposals for the project. Therefore, the Water Authority developed conceptual designs for the WTP to serve as a basis for the California Environmental Quality Act permitting and included development of budgetary cost opinions for comparison to DBO bids. They will also serve as a resource for the DBO entities bidding for the project. Proposed conceptual designs using both conventional and low-pressure membrane technologies were prepared for the 50 mgd expandable to 100 mgd WTP. To provide adequate information to potential bidders of this project, the conceptual designs were proposed to satisfy the target water quality goals for the facility. The conventional design included ozone for primary disinfection and taste and odor removal. The membrane design included granular activated carbon for taste and odor removal. The proposed designs indicate that the processes are very comparable with respect to cost and can meet equivalent water quality goals. However, the membrane treatment process may be capable of providing a better barrier against microbial pathogens. The Water Authority is conducting pilot-scale testing of low-pressure membrane treatment processes, with and without pretreatment, to evaluate its feasibility for this project. This paper, based on the information developed for the conceptual designs, will present a comparison of cost, ability to meet water quality goals, potential for future expansion, and other key criteria. Includes tables, figures.