King County's West Point Treatment Plant A 90-Day Recovery to NPDES Permit Compliance Following Catastrophic Flooding and Plant Shutdown Due to Effluent Pump Station and Equipment Failures

摘要

The West Point Treatment Plant (WPTP) in Seattle, WA is part of the King County Wastewater Treatment Division (WTD) and is the largest wastewater treatment plant treating domestic, commercial, and industrial wastewater and significant combined sewer flows in the greater Seattle area. The WPTP has a rated capacity of 215 MGD under maximum month condition and 440 MGD at peak. It uses a high purity oxygen activated sludge process followed by chlorine disinfection before discharging the treated effluent to central Puget Sound. After the original construction in 1965 as a primary treatment plant, the plant was expanded in 1996 to include secondary treatment for flows up to 300 MGD. Through the use of three pump stations and two flow diversion/control structures, influent flows in excess of 300 MGD receive only primary treatment. The plant hydraulic profile is presented in Figure 1. The three pump stations are driven by electric motors or gas engines and thus respond differently to power failures. On February 9, 2017, the WPTP suffered a power failure and subsequent effluent pumping and control system failures, which led to the flooding of a mile of subterranean pipe and equipment galleries with stormwater, raw sewage, and sludge. The flooding contaminated, damaged or destroyed millions of dollars of plant equipment including pumps, motors, boilers, electrical panels, wiring, transformers, lighting, switches, and motor control centers. The flood contaminated the entire surface area of the galleries, over two miles of pipe insulation, and destroyed WTD staff locker/shower facilities leading to a complete shutdown of the WPTP. WTS staff responded immediately to initiate emergency response efforts and within 18 hours returned the plant to half of its hydraulic capacity including limited treatment with screenings, partial primary treatment, and final effluent disinfection and dechlorination. Over the following 75 days, a joint effort from up to 46 WTD staff and 115 contract personnel performed the required refurbishments and replacements of critical plant equipment to allow for the full startup of all hydraulic processes damaged by the flood. Following the equipment repair and replacement effort, coordinated efforts were made by the WTD staff and process experts from the consulting industry and academia to achieve full capacity for both the secondary treatment and solids digestion and handling as quickly as possible. These combined efforts led to success in the WPTP achieving 100% NPDES permit compliance within 90 days of the event with fully functioning equipment, secondary treatment and solids digestion. Recovery efforts continued for another six months concluding with the full recovery of the digesters, the commissioning of three fully refurbished boilers, installation of new motor control centers (MCCs), and return to permanent power.