A Flexible Design Concept for a 180 MGD MBR Facility

Background The Metropolitan Water District of Southern California (MWD) and Los Angeles County Sanitation Districts (LACSD) are jointly developing the Pure Water Southern California (PWSC) program to produce up to 150 MGD of purified water at LACSD's A.K. Warren Water Resource Facility (Warren Facility) in Carson, California. The Warren Facility (Figure 1) is a 400 MGD capacity high purity oxygen activated sludge (HPOAS) facility that discharges treated (non-nitrified) secondary effluent (SE) to the Pacific Ocean. Product water from the PWSC Advanced Water Treatment (AWT) facility would be used for indirect potable reuse via recharge of local groundwater basins and possibly direct potable reuse through raw water augmentation of surface water treatment plants (Figure 2). Nitrogen management is a key component of PWSC and a flexible design approach that allows the facility to be optimized for current treatment needs, while preserving expandability to meet future requirements, is needed. As such, a series of Nitrogen Management Studies (Liang et al., 2019, Fitzgerald et al., 2021; Danker, 2022, LACSD 2023) were conducted to evaluate alternative process trains to manage nitrogen in the AWT feed. Based on these studies, the proposed process train for PWSC includes biological nitrogen removal followed by reverse osmosis and an ultraviolet advanced oxidation. The primary process includes a 180 MGD capacity flexible membrane bioreactor (Flex MBR, Figure 3) that would be fed Warren Facility SE. Additional processes that will be integrated into PWSC will include operation of two existing HPOAS reactors to achieve nitrogen removal and implementation of sidestream deammonification. When constructed, the MBR facility will be one of the largest in world. The Flex MBR includes a modular bioreactor design concept (Figure 4) that allows for the ability to operate in either nitrification only or nitrification-denitrification (NdN) mode. NdN can be achieved by addition of supplemental carbon (glycerol) and/or by blending primary effluent (PE) with SE, referred to as hybrid operation. Hybrid operation is intended to reduce supplemental carbon requirements. To further enhance reductions in supplemental carbon requirements, provisions have been included in the design concept to implement partial-denitrification/anammox (PdNA). Additionally, since the basins will be identically designed, the Flex MBR could also be converted to a traditional secondary MBR with PE feed (Figure 5). Pilot-Scale Testing To inform the Flex MBR design concept, pilot testing was conducted (Deco et al., 2023) for approximately one year to validate a pre-anoxic NdN concept (Figures 6 and 7). This concept was developed to mitigate partial NdN (i.e., nitrite accumulation) that was observed during previous demonstration-scale testing in a post-anoxic configuration (Liang et al., 2020) . In addition, initial proof-of-concept testing of the hybrid operating approach was completed. The study concluded that: (1) hybrid operation could result in significant savings in supplemental carbon addition and thus project life-cycle costs; (2) partial NdN can be effectively mitigated with a pre-anoxic configuration designed to control anoxic zone nitrate loading and associated nitrate residual (Figures 8 and 9); and (3) the post-aerobic zone provides a buffer to prevent high effluent nitrite concentrations in the event that anoxic zone nitrite accumulation does occur. Full-Scale Design Concept and Process Modeling Building on the pilot testing, the full-scale Flex MBR concept was developed and refined. A preliminary site plan at the Warren Facility is shown in Figure 5. Major infrastructure will include an influent pump station, fine screen facility, covered bioreactors, membrane tanks and an equipment building, chemical storage and feed facilities, and an odor control facility. The site layout was developed based on a PWSC program phased implementation plan of 115 mgd (Phase 1) and 150 mgd (Phase 2) product water flow. To facilitate the desired operational flexibility, the modular bioreactor design is partitioned into 12 equal zones (Figure 4). The modular concept utilizes a two-pass bioreactor with a multi-purpose central channel and targeted swing zones. The proposed configuration allows for half of a bioreactor train (one pass) to be taken out of service to reduce the out of service volume for maintenance activities. Bioreactor process simulations were completed utilizing BioWin. Design criteria considered for process simulations are shown in Table 1. Simulations were completed considering both the supplemental carbon-based and hybrid operating approaches. Modeling results are shown in Table 2 and Table 3. Key observations from the process simulations include:

*Nitrogen targets can be met without the use of an internal nitrate recycle

*Hybrid operation requires slightly higher return activated sludge rates, as compared to carbon based operation, to stay below the maximum design membrane tank MLSS concentration of 10,000 mg/L

*To meet the Phase 1 nitrogen target under current average day loading conditions, NdN, and thus carbon or PE feed, is projected to be needed on only 40% of days

*Hybrid operation is projected to result in significant savings in supplemental carbon

*Carbon-based operation simulations indicate that anoxic zone nitrate loading is predicted to align with pilot-scale testing