Conference Agenda

The Southwest portion of the City of Tacoma’s (City) Central Treatment Plant (CTP) collection system is experiencing ongoing redevelopment that includes both large proposed projects and smaller projects from continued steady population growth. This portion of the collection system flows by gravity to the South Tacoma Pump Station (STPS) and is then conveyed to the CTP. STPS is due for rehabilitation based on equipment age in the coming years.

This presentation summarizes updates of Tacoma’s collection system hydraulic model and describes model use to answer questions regarding large redevelopment projects and pump station design optimization. Several possible development scenarios were projected and evaluated in the hydraulic model. A sensitivity analysis for different development types was conducted with respect to system performance criteria balancing flows, piping improvements, pump station capacity and operations.

The City provided the most recent field verified system GIS data, pump station settings, and local flow monitoring in the key basins. This information was added to Tacoma’s CTP model, making the model completely up to date. Projected flow scenarios were developed for an old Airfield that is being re-developed along South Tacoma Way. Re-development of a high density 400-unit development on 6th Avenue, and the Mall at South 48th Street were also included in the analysis. The model was used as a tool to recommend future pipe upsize and help size the new planned conveyance system to meet the City’s performance criteria.

Additionally, the hydraulic model was used to evaluate the STPS operations during both a large rain on snow storm in January 2022 and the City’s collection system design storm. Model results were compared against the system performance criteria when either 4 or 5 pumps are operating at the current STPS. This was completed for the existing system configuration and for the proposed piping changes identified for the Airfield development. Operational changes with the current pumps were tested to improve system performance during storms. Lastly, pumping capacity for pump replacement were recommended that balanced system performance both upstream and downstream of the STPS, based on the recent large January 2022 storm and the City’s design storm.

The Gothenburg region was able to address emerging challenges within its sewerage system by developing a digital twin. Gothenburg, located on Sweden’s west coast, faces many of the same challenges faced in the Pacific Northwest. Gothenburg, one of the rainiest cities in Sweden, experiences heavy rainfall causing large loading variations to its central Rya water resource recovery facility (WRRF). Approximately 25% of the sewers in the City are combined, increasing the risk of flooding and discharge of untreated wastewater into the surrounding ecosystem.

Gryaab AB, the regional wastewater utility serving Gothenburg and surrounding municipalities, owns and operates the Rya WRRF and the tunnel system transporting wastewater to Rya WRRF. Gryaab saw the potential and developed a digital twin to help better manage its sewerage system, including:

• Developing a detailed H&H model of the tunnel system and catchment area
• Incorporating all controllable devices (gates, pump, etc.) in the digital twin
• Incorporating weather forecasting in the simulation
• Development of Forecast-on-Demand features
• Development of operational strategies to mitigate the risk of urban flooding and minimize CSO discharges

Deployment of the digital twin allowed Gryaab to get better real-time information about events in the tunnels and accurate predictions of potential issues and peak pressure on the system. This allowed them to optimize the overall system performance. Simulations indicate that yearly CSOs may, ideally, be reduced by 65%, and bypass volume at the WRRF by 85% through dynamic operation of CSO sites and increased utilization of tunnel volumes. Simulations for years 2035, 2050 with various climate and population scenarios helped understand how and when to plan for expanding WRRF capacity.

The digital twin allows Gryaab to act proactively. Staff can make decisions based on comprehensive, real-time data and prognosis and even allow direct implementation of the digital twin’s suggested setpoints in the SCADA-system. The digital twin is used for training in various scenarios so that staff is better prepared for future critical situations and for making better informed decisions. In short, the digital twin is used for gaining better real-time control and improving the operation of the sewer system, both today and in the future.