Rebooting a WRRF Site for Resiliency, Capacity, and Nutrient Removal

PROBLEM AND OBJECTIVES The Des Moines Wastewater Reclamation Authority (WRA) is at a crossroads at their Wastewater Reclamation Facility (WRF). It has performed well for decades providing a high-quality, nitrified effluent protective of receiving waters with a facility bult on a tightly constrained site, but it is due for renewal. A planning and modeling effort, oriented toward answering challenging questions about the facility, was completed. Drivers and questions for the future:

• Aging Infrastructure – Parts of the WRF have not been replaced or rehabilitated in over 40 years are approaching the end of useful life. What is the best path forward for this infrastructure?
• Resiliency of WRF – Wet Weather and flooding caused problems historically. Can future work increase flood resiliency with appropriate building elevations and setbacks while minimizing failure points?
• Nutrient Removal – Nutrient removal is a future requirement. Can the process be modified to provide for total nitrogen (TN) and total phosphorus (TP) of 10 mg-N/L and 1 mg-P/L; and can it tighter future limits?
• Capacity and Wet Weather Needs – The existing WRF hydraulic capacity has been 200 MGD since 1982. Can the existing, constrained site be adapted to treat 400 MG?
• Sustainability – Sustainability is a key value of the WRA. Are there opportunities that grow energy efficiency and resource recovery?
• High-risk infrastructure – What is the best way to address:
° Headworks with critical pumping and electrical constraints,
° Yard piping, stretching for miles, showing signs of stress failures at joints,
° Equipment (e.g. blowers) reaching 40+ years of age?

OVERVIEW OF WRF The WRA serves approximately 550,000 people in 18 member communities with an average flow of 70 MGD through 125 miles of conveyance piping. It treats industrial hauled waste from 90 customers and produces 500,000 MMBTU of RNG yearly. The configuration of the WRF from the 1980s has a capacity of roughly 200 MGD with primary clarifiers, secondary nitrification, and anaerobic digestion (Figure 1). The existing WRF site is constrained with the Des Moines River on the south, a railroad on the north, and wetlands on the east and west sides (Figure 2). Sewers stretch across the WRF routing flow to the headworks and influent pumping from all directions. Limitations and opportunities persist at the WRF. Processes no longer in use should be removed. During the 1980s, the WRA committed to a centralized treatment facility. More recently, the WRA has invested heavily into conveyance providing a route for flow to the treatment facility. This planning cycle focuses on treatment facilities, and reviews the challenges with a centralized facility on a constrained site. METHODOLOGY A unique approach to facility planning was conducted. For this plan, the evaluation was conducted as a fully coordinated effort. WRA staff worked hand in hand with the treatment and conveyance planning teams as a unified effort that included frequent check-ins. A key team decision was that everything is on the table; no process or infrastructure is sacred. FINDINGS Several alternatives were considered for WRA to meet their goals. One is building EQ basins in the conveyance system. Another alternative is in-kind expansion that builds on the baseline configurations. Satellite treatment was reviewed as well. These alternatives were found to be complex, cost prohibitive, and challenging. The preferred alternative is to [b]re-configure the WRRF[/b] at the existing site. While the WRF is site constrained and does not support in-kind expansion, an in-depth analysis with a novel reconfiguration of processes showed that the site can support WRA's goals and initiatives while solving treatment challenges, unlocking capacity and providing nutrient removal as shown in the modeled scheme (Figure 3). A site reset was proposed (Figure 4) that relocates and reconfigures infrastructure increasing capacity from 200 MGD to over 400 MGD by implementing novel intensification concepts. Examples of efficiencies are weaved throughout the plan. The hydraulic profile of the site is reset with new influent pumping to support future capacity while eliminating intermediate pumping. Primary clarifiers are relocated adjacent to secondary treatment [b]eliminating single points of failure[/b] associated with a pipeline in a levee. Circular clarifiers are replaced with rectangular tanks using baffling. This approach is primed for surface overflow rates exceeding 4,000 lb/ft[sup]2[/sup]/d (Gerges et al., 2024). The cost evaluation shows savings compared to satellite treatment, and a shift from conveyance to the treatment system (Figure 5). As with others, the WRA seeks the highest level of fiscal responsibility. Therefore, a sensible phasing plan was developed that supports the improvements based on appropriate trigger points (Figure 3). SUMMARY The Des Moines WRF underwent planning to support a number of drivers ranging from water quality to capacity while overcoming challenges ranging from site constraints to aged infrastructure with reliability concerns. Staff employed a collaborative planning effort oriented toward unlocking capacity, improving resilience, and identifying optimizations and synergies. The result is a reset of the WRRF with a new approach to treatment.