Converting HPO to Nitrogen Removal: Closing the Oxygen Gap Through Electrolysis

APPLICABILITY Major US water recovery facilities (WRRFs) operate as High Purity Oxygen Activated Sludge (HPOAS) systems, originally built for BOD removal. Stringent nutrient limits now require lowering effluent nutrients, leading HPOAS facilities to consider expanding or upgrading for nitrification/denitrification processes (NDN). Upgrading presents challenges, including the need for extra oxygen to nitrify ammonia, requiring expansion of the existing oxygen supply system. In parallel, there has been global recognition of green hydrogen as a way to decarbonize hard-to-electrify sectors , with several facilities commissioned in the US. This study explores co-locating electrolytic green hydrogen generation at HPOAS facilities, producing green hydrogen and co-valorizing byproduct oxygen to increase oxygenation capacity. This strategy interfaces with two trends: expanding oxygen capacities of large HPOAS transitioning to NDN and decarbonizing hard-to-electrify industries through green hydrogen. Converting HPO Facilities to Nitrogen Removal and Its Implications Adapting Large High Purity Oxygen Activated Sludge (HPOAS) systems for nitrogen removal is gaining attention (Pitt et al. 2023). Coastal facilities, traditionally exempt from strict nitrogen regulations, are anticipated to upgrade their system to nutrient removal due water reuse and ocean discharge requirements. Retrofitting for nitrogen removal poses challenges in meeting increased oxygen demands, potentially surpassing existing oxygen demand by over 60%. Thorough evaluation of the oxygenation system is crucial for HPOAS upgrades to an NDN process, considering both immediate and future oxygen requirements. Energy Transition and Green Hydrogen An emerging solution for decarbonizing hard-to-electrify industries is green hydrogen production, which enables the storage of renewable energy in hydrogen form, addressing reliability issues during low renewable energy periods. Green hydrogen benefits energy storage and aids in decarbonizing sectors like long-distance transportation, industrial chemical synthesis, and various fuel applications (Davis et al., 2018; Saeedmanesh et al., 2018). The Case for Green Oxygen Recovery at HPO Plants Green hydrogen generation yields a valuable by-product: pure oxygen. In the context of Water Resource Recovery Facilities (WRRFs), this by-product oxygen, holds the potential for beneficial re-use in the bio-reactors. Given WRRFs' frequent co-location with industries (i.e., airports, ports, chemical industries, etc.) there is an opportunity to integrate renewable hydrogen generation systems at WRRFs. This enables the supply of green hydrogen for local industry decarbonization while recovering pure green oxygen for aeration. Since HPOAS systems operate at high purity oxygen levels (greater than 95%) the oxygen produced by the electrolyzer can be easily blended into the pure oxygen line of the plant. The ability to leverage the existing oxygen equipment allows to minimize the need for modification to the existing oxygenation system. Figure 1 illustrates a process schematic of an HPOAS system incorporating green oxygen from electrolysis. Case Study: City of Cedar Rapids The Cedar Rapids Water Pollution Control Facility (CRWPCF) shows the application of this approach. Operating on a UNOX configuration (45 MGD), with an oxygen generation capacity of 80-ton/day. Available renewable energy sources (RES) were assessed when developing hydrogen production curves. Figure 2 shows the specific solar power generation for the geographic coordinates of the CRWPCF (PVWatts, NREL). Various electrolyzer sizes are considered to project dynamic green oxygen generation curves. These are compared with the estimated oxygen demand from the hetero- and auto-trophic process and to assess oxygen storage needs. Overall, to supply the near-term oxygen needs of the CRWPCF approximately 35MW are required. As part of the ongoing work, this study will further explore different RES blends and assess the costs/performance/constructability/ implications of retrofitting the existing UNOX equipment to receive electrolytic oxygen. Conclusions This case study explores integrating green hydrogen production at HPOAS facilities at WRRFs. Co-valorizing oxygen enables concurrent green hydrogen production, supporting the acceleration of global decarbonization. Re-using by-product oxygen for nitrification meets NDN transition needs of HPOAS systems, expanding capacity and generating renewable fuel. This approach allows to leverage ongoing decarbonization funding opportunities, offsetting costs, and expanding oxygen supply operations at HPOAS. The City of Cedar Rapids provides practical insights on infrastructure, power, and equipment considerations, offering a sustainable solution for wastewater utilities. RELEVANCE TO AUDIENCE This research introduces an innovative approach for HPOAS plants aiming for NDN and sustainability. Valuable insights for plant operators, engineers, and decision-makers for upgrading HPOAS systems will be addressed. The study also highlights the potential synergy between wastewater treatment and green hydrogen production. The proposed co-location of electrolytic green hydrogen generation and HPOAS facilities not only addresses environmental concerns but also creates economic opportunities for nearby industrial users. This research contributes to a holistic understanding of sustainable practices at WRRF.