Remote Wet Weather Treatment Facility Operational Insights and Treatment Success

BACKGROUND
The Georgetown Wet Weather Treatment Station (GWWTS) is one of 14 Consent Decree projects in King County's Combined Sewer Overflow (CSO) Long Term Control Plan to reduce CSOs into local water bodies and protect public health and the environment. The GWWTS improves the resiliency of the regional King County wastewater system by capturing and treating the CSOs at two CSO outfalls into the Lower Duwamish Waterway. The remote high-rate treatment facility has a capacity of 265 MLD (70 mgd) and began operations in December 2022.

Operating a remote high-rate treatment facility and meeting permit limits is challenging due to the complexity of the treatment process, its intermittent use, and the dynamic nature of wet weather events. The facility's discharge permit is unique as it includes monitoring and reporting requirements for compounds in its treated effluent that aren't typically regulated at remote facilities. Through two years of operation, King County staff and the engineer found several facility design features to be especially useful, identified and implemented measures to improve operations, and determined improvements for the design of future facilities.

OVERVIEW
The location of the GWWTS is shown in Figure 1. Land use in the area is primarily industrial and commercial but also includes residential and neighborhood business areas. Figure 2 shows a rendering of the facility and identifies major treatment processes, buildings, and water conservation and aesthetic features. Figure 3 shows the GWWTS process flow diagram. Flow enters the facility through a flow regulator. Two influent screens remove oversized materials protecting downstream pumps and process equipment from clogging. A dual-purpose equalization basin attenuates the peak influent flow to a 265 MLD (70 mgd) treatment station flow while also doubling as a wet well for the facility's six influent pumps. Two ballasted sedimentation trains are followed by two UV disinfection batteries. An onsite solids storage and thickening tank stores solids onsite during the treatment event. A distinctive feature of the GWWTS is an internal recycle loop that allows for full-scale testing of the facility under dry weather conditions (Figure 4).

TREATMENT PERFORMANCE
Table 1 shows selected monitoring, discharge limits, and reporting requirements in the GWWTS discharge permit. The four parameters with permit limits are typical for remote wet weather treatment facilities. Over 20 other parameters have effluent monitoring requirements that are atypical for wet weather treatment facilities. Figure 5 shows influent flow and effluent flow volumes and the peak influent flow during six wet weather treatment events. All events had some flow that remained in the equalization basin that was returned to the collection system for treatment at King County's downstream West Point Treatment Plant following the storm event. The peak treatment flows were all relatively high even though treatment volumes were typically low, demonstrating the variability of wet weather events.
Figure 6 shows the influent and effluent TSS concentration from 24-hour flow proportional composite samples and percent TSS removal during each event. The figure indicates excellent TSS removal by the ballasted sedimentation process. Figure 7 shows the effluent fecal coliform counts from grab samples taken during each wet weather event.

Table 2 shows selected effluent priority pollutant concentrations for events when sampled. The facility's discharge permit was reissued on 6/1/24 and contains additional monitoring parameters such as ammonia. These parameters were not sampled under the previous permit. They were sampled during the 12/25/24 event, but the results aren't available yet. The results will be included in the paper along with data from any additional events.

BENEFICIAL DESIGN FEATURES, OPERATIONAL IMPROVEMENTS, AND FUTURE FACILITY DESIGN IMPROVEMENTS
Effluent recirculation piping: Operational staff found the effluent recirculation piping to be very helpful as the GWWTS can be operated at full capacity during dry weather to enable testing, training, and troubleshooting without discharging effluent.
Uniform operator interface panels: Graphics were standardized across control packages so operators had similar interfaces throughout the station. This is especially beneficial for infrequently used facilities that are operated under stressful conditions.
Cycling optimization: During wet weather events, the facility cycles between storage and treatment and between discharging and recirculating flow depending on the influent flow to the facility. The process engineer and control engineer can work with operational staff during smaller wet weather events to optimize station controls and process parameters.
Chemical recirculation piping for future facilities: Add piping to recirculate chemicals while calibrating/troubleshooting feed pumps to keep excess chemicals out of the process. High effluent pH observed initially during a treatment event was attributed to caustic lingering in the process after operation and maintenance activity.
Rock and grit removal for future facilities: The extent of grit accumulation was greater than anticipated and large volumes of grit settled out in the GWWTS influent pipe and screening channel during two wet weather events. This has led to operational suggestions to include rock and grit removal in future facilities.