Optimizing Dissolved Organic Nutrient Management: Exploring Carbon-Based Advanced Treatment Strategies

Introduction As population growth, urbanization, and climate change intensify pressures on water resources, utilities are faced with an increased urgency for advanced water treatment (AWT) solutions and a need to enhance resilience against evolving climatic conditions. Concurrently, wastewater resource recovery facilities (WRRFs) are faced with increasingly stringent nutrient discharge limits that surpass the capabilities of conventional treatment methods. Consequently, utilities are compelled to adopt innovative approaches to address the quality and quantity of water supplies while ensuring regulatory compliance and sustainability. In this work, we highlight case studies of two forward-thinking utilities, distinguished by geographical contexts and unique drivers, yet united by a commitment to advance climate change adaptation strategies, achieve ultra-low nutrient limits, and integrate holistic one water initiatives. Metro Water Recovery (Denver, CO): Upcoming Colorado water quality regulations are set to impose stringent stream standards, notably 2.01 mg/L for total nitrogen (TN) and 0.10 mg/L for total phosphorus (P) on WRRFs, including Metro Water Recovery's Robert W. Hite Treatment Facility (RHWTF) and Northern Treatment Plant (NTP). AWT technologies like carbon-based advanced treatment (CBAT) schemes (e.g., coagulation/flocculation/sedimentation (floc/sed), ozone (O3), biofiltration (BAF), and granular activated carbon (GAC)) may address stringent nutrient limits while also aligning with Direct Potable Reuse (DPR) requirements. A study was initiated to: 1) assess the treatability of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and soluble non-reactive phosphorus (sNRP) using CBAT technologies; and 2) discern the capabilities of CBAT in concurrently achieving nutrient limits and DPR compliance. Evaluation of nutrient removal through tertiary CBAT processes spanned full-scale applications (floc/sed at NTP and BAF/floc/sed at RHWTF's Denver Water Recycling Plant (DWRP)) and pilot-scale (O3 only, BAF only, and O3/BAF at RHWTF), supplemented by bench-scale floc/sed (jar testing) and GAC evaluations (rapid small-scale column tests). Parameters such as O3 dose and empty bed contact times aligned with potable reuse standards. While full-scale floc/sed/filtration at NTP (Figure 1; Figure 2) and DWRP (Figure 3, Figure 4) (~20 mg/L alum) effectively managed particulate P and sNRP removal, it did not meet DOC and DON targets for Direct Potable Reuse (DPR) applications. CBAT testing showed potential for control of DOC and DON, albeit with the need for multiple potentially costly unit processes. The GAC runtime (up to 3,500 Bed Volumes for DOC and DON targets) was notably extended through enhanced upstream treatment. While floc/sed processes achieved comparable DOC and DON removal rates (up to 40% each) at the expense of elevated coagulant doses (80 mg/L Ferric chloride), O3/BAF treatments realized a modest 26% reduction in DON. Future pilot efforts aim to assess the efficacy of a comprehensive CBAT treatment train, inclusive of tertiary denitrifying BAF, to fulfill TN requirements and optimize DPR suitability. AWT Utility (Midwest): A midwest AWT utility has seen significant population growth in recent years and is estimated to double by 2040. Rapid growth and a desire improve the health of the receiving water body have urged the utility to consider alternative water supplies. Subsequently, the utility commissioned a CBAT pilot consisting of O3, two-stage BAF, GAC, and UV advanced oxidation process (UVAOP). The objective of this study was to pilot CBAT technologies to demonstrate the ability of AWT process trains to produce high quality finished water that meets stringent water quality targets. This study utilized a multi-barrier scheme of O3/two-stage BAF (aerobic/anoxic) to meet low TN limits. Filters were operated in several phases (Table 1) until effluent quality targets were met (TOC < 3 mg/L, TN < 2 mg/L, contaminants of emerging concern (CECs)- meet MCLs) and steady-state was achieved. Select operational parameters were varied during each phase and the resulting water quality surrounding the first anoxic BAF (BAF 3) is shown in Figure 8. Table 2 shows the effluent DO, nitrate (NO3-N) concentrations and corresponding % NO3-N removal achieved across each anoxic BAF. Results indicate that when supplemental carbon and upstream feed DO levels were controlled, anoxic BAF 1 established and maintained anoxic conditions and achieved NO3-N removals up to 60% with anoxic BAF 2 acting as a polishing biofilter (%NO3-N removal ~30%). When supplemental carbon and upstream DO quenching chemical doses were reduced, anoxic BAF 1 turned into a sacrificial BAF, consuming excess DO and displaying poor nitrate removals (~15%), while anoxic BAF 2 became the dominant anoxic biofilter (> 60% NO3-N removal). Piloting efforts demonstrate how a modified CBAT train may simultaneously meet low TN targets (< 2 mg/L) and produce high quality product water. Conclusion Case studies at Metro Water Recovery and a midwest AWT utility demonstrate CBAT's potential for improving finished water quality for reuse and meeting ultra-low nutrient discharge requirements. Project findings provide first guidance on how WRRFs can adjust operational setpoints using CBAT process infrastructure to meet DPR or discharge water quality requirements in ecologically sensitive and drought-prone watersheds.