Hybrid MBR-Based Design: Cost Effective Performance Optimization for Peak Flows and Loads

The Brightwater Treatment Plant (Brightwater) is a new wastewater treatment and reclamation facility owned and operated by the King County Wastewater Treatment Division. The project is under construction and is scheduled to begin service in late 2010. The plant is configured with an innovative, hybrid, split-flow treatment process, incorporating conventional and chemically enhanced primary clarification (CEPC) processes ahead of a membrane bioreactor (MBR). This design uses the CEPC process to reduce peak flow to the MBR by treating portions of extreme wet weather flows, expected to occur on less than 10 percent of the days in any year. CEPC effluent will flow either to the MBR or blend with MBR effluent prior to disinfection and discharge as dictated by available MBR capacity.The hybrid MBR design was originally conceived to allow MBR technology to be applied to a treatment plant that receives high peak flows during wet weather events. Sizing the liquid stream process for highly efficient treatment of non-peak flows and providing another means of treatment for storm-influenced flows offers opportunities to reduce pollutant loads, capital costs, and operating and maintenance costs. In addition to reducing peak flows to the MBR, the CEPC process is available as a tool to reduce peak biochemical oxygen demand (BOD) loads to the MBR process. For example, if aeration capacity or process volumes prove inadequate for peak BOD loadings, CEPC could be used to reduce the BOD load to the MBR.With the hybrid MBR split-flow design, the total BOD and total suspended solids (TSS) percent removals will be greater than 88% and 92% respectively for the projected peak month flow and load. Additionally, effluent from the hybrid MBR process provides a significant net environmental benefit when compared to conventional activated sludge (CAS), reducing the discharge of BOD and TSS by 73% and 79% respectively on an average annual basis. This paper discusses configuration and development of the hybrid MBR process, system optimization through a separate membrane system procurement, the value of the process in facility phasing, and regulatory approval of the project.