Evaluation of Blue Plains PdNA full-scale demonstration pilot: Key Lessons and Future Directions

[b]Abstract[/b] A full-scale post-anoxic PdNA demonstration was implemented at Blue Plains to evaluate the interaction between high-density IFAS media, screen hydraulics, and mixer operation in a 30-ft deep reactor. Rapid head loss developed across both influent and effluent screens due to debris accumulation and insufficient media scouring, leading to hydraulic and mixing challenges. The findings highlight the need for screen designs that increase interaction between screen and media, continuous or adaptive mixing strategies, and operational strategies to ensure stable operation under high flow conditions. [b]Introduction[/b] PdNA has progressed from concept to practice, with full-scale implementations showing reliable performance. Remaining challenges are largely design-driven, including hydraulic management, screen capacity, and media handling in deep reactors. At Blue Plains, the design evaluation focused on selecting media suitable for a 30-ft deep reactor and compatible with existing Invent® mixers. Media size governs allowable screen openings, making available screen area the key factor controlling flux and head loss. This paper presents the first use of heavy IFAS media in a deep post-anoxic application and outlines the associated hydraulic, screening, and mixing challenges, and design recommendations. [b]Materials and Methods[/b] A five-stage, 30-ft deep reactor (≈20 MGD equivalent) was configured with Stage 4 as the IFAS PdNA zone (Figures 1 & 2). MicroC was supplied through feedforward/feedback control. Hyperboloid mixers with Cyberprop elements were installed to maintain media suspension; mixers were shut off during wet-weather events. Influent expanded-metal screens and effluent wedge-wire screens retained media, with periodic air sparging for cleaning. Selection of a high-density media (SG 1.04–1.06) was made on the expectation that the increased specific gravity would allow the media to settle at high flows, reducing the potential for media stacking on screens while maintaining sufficient scouring during normal operation. [b]Results and Discussion[/b] [i]Influent Screen Performance[/i] A rapid head loss developed across the influent screen within the first day, creating a ~0.26 m water level difference between stages 3B & 4A (Figure 3) and elevating upstream levels relative to the adjacent non-PdNA reactor. The upstream expanded-metal screen accumulated fibrous debris (Figure 4), reducing effective open area, while limited media contact and lack of active cleaning prevented effective scouring. These factors restricted flow into the PdNA zone. Potential mitigation includes hydraulic controls (e.g., submersible weir), replacing expanded metal with a perforated plate screen on the IFAS side to promote media scouring, and implementing a defined cleaning and maintenance protocol. [i]Effluent Screen Performance[/i] Initially stable, even during short-wet weather mixer shutdowns, head loss over the effluent screens increased after several days (Figures 3&5), indicating onset of effluent screen fouling. Following heavy rainfall and extended mixer shutdowns, hydraulic capacity could not be recovered despite continuous air sparging. Media lodged on both effluent screens, confirming insufficient scouring velocities and contributing to MLSS concentrations exceeding 3,500 mg/L (Figure 6). Recommended improvements include increasing hydraulic openings between stages 4B & 5A, removing or perforating the long-wall shelf to create an open channel, repositioning the short-wall screen to improve approach velocities, enhancing air sparging with higher-intensity and large-bubble systems, and implementing level-based control strategies to actively manage head loss, mixing, and screen cleaning. [i]Mixer Performance[/i] As part of the wet-weather strategy, mixers were shut off when influent flow exceeded 330 MGD. While short shutdowns during early rain events allowed successful restart, extended shutdowns later caused mixers to fail to restart due to settled dense media, requiring diffuser air to resuspend the media or full reactor shutdown. These events revealed a critical interaction between media density, screen fouling, and mixer operation. Allowing media to settle eliminated continuous scouring, accelerated screen fouling, increased head loss and MLSS, and hindered mixer restart. The results indicate that fully shutting off mixers during wet weather is ineffective; continuous operation with adaptive speed control is needed to maintain media suspension, screen cleaning, and hydraulic stability, although higher mixer power may be required. [b]Conclusion[/b] This PdNA demonstration showed the critical interaction between media properties, screen hydraulics, and mixer operation in deep post-anoxic IFAS systems. Rapid head loss on both influent and effluent screens was driven by debris accumulation and insufficient media scouring, causing hydraulic constraints, elevated MLSS, and mixing challenges. While high-density media were expected to reduce the potential for media stacking at high flows, the results confirmed that continuous media scouring is the dominant mechanism for maintaining screen cleanliness and hydraulic capacity. Wet-weather mixer shutdowns eliminated scouring, accelerated fouling, and increased head loss, underscoring the need for screen designs that enhance media interaction, continuous mixing, and adaptive control strategies for stable high-flow operation.