A 6-week Recovery of a Major High Purity Oxygen Activated Sludge Plant to Full Capacity Following Flooding and Plant Shutdown

The West Point Treatment Plant (WPTP) of King County, WA, has a rated capacity of 215 mgd under maximum month condition and 440 mgd at peak. It provides secondary treatment of flows up to 300 mgd using a high-purity oxygen (HPO) activated sludge process. The plant recently suffered a power failure and subsequent effluent pumping and control system failures, leading to the flooding of the subterranean pipe and equipment galleries and eventually a complete plant shutdown. The plant managed to quickly return to half of its hydraulic capacity and limited treatment with screenings, partial primary treatment, and final effluent disinfection, but grit removal and primary sludge pumps and digester boilers were damaged, disabling the solids stabilization process. To minimize the production of waste activate sludge (WAS) and protect the secondary process from impacts due to degraded primary sedimentation performance and absence of grit removal, flows to the secondary process had been essentially stopped for a couple of months, resulting in significant loss of biomass, its activity, and settleability. Coordinated efforts were made by the County staff and process experts from the consulting industry and academia to bring the plant back to its full capacity as quickly as possible. A secondary treatment recovery plan was prepared to guide the recovery by evaluating various strategies and establishing recovery procedures and process control set points, based on dynamic modeling. During the restart, meetings were held regularly to review plant operations and refine restart activities. This paper highlights the HPO process recovery planning, strategies, and key observations and activities.The HPO process at the WPTP proved robust and resilient. It was fully recovered in about 6 weeks following a complete plant shutdown, despite primary treatment degradation and interruptions from biomass loss due to over-wasting caused by instrumentation malfunctioning, foaming, and required corrective measures. A combination of F:M ratio and solids retention time (SRT) controls, assisted with a recovery planning considering plant specific conditions and historical performance, effectively restarted and restored the HPO process to its capacity and routine operations.