Tertiary Membrane Aerated Biofilm Reactor/Partial Denitrification-Anammox Process

INTRODUCTION AND OBJECTIVES
The A.K. Warren Water Resource Facility (Warren Facility) is owned and operated by the Los Angeles County Sanitation Districts (LACSD). The facility has an average daily dry-weather flow capacity of 400 million gallons per day (mgd) and employs a high-purity oxygen activated sludge (HPOAS) process (Figure 1). Non-nitrified secondary effluent (SE) is chlorinated and discharged to the ocean.

LACSD has been investigating nitrogen management options that may be implemented at the Warren Facility to address two drivers: (1) effluent reuse for the Pure Water Southern California (PWSC) program which will produce 150 mgd of purified water and (2) total inorganic nitrogen (TIN) load reduction for ocean discharge. PWSC processes that will be implemented are shown in Figure 2.

Implementing nitrification-denitrification (NdN) within PWSC is projected to result in a net reduction in the TIN load discharged to the ocean (~ 49%) relative to continued operation with HPOAS. However, LACSD is proactively investigating alternative approaches to enhance load reduction beyond that achieved by implementing PWSC. One approach developed by LACSD is to target treatment of the HPOAS SE that is not directed to reuse (Figure 2, Stream 7) utilizing an energy efficient membrane aerated biofilm reactor (MABR) process coupled with a carbon efficient partial denitrification-anammox (PdNA) process (Figure 3). In this novel approach, a tertiary MABR (tMABR) provides pre-nitrification to produce a desired NH4-N/NOx-N feed ratio to a PdNA process where supplemental carbon is added to drive TIN removal. Producing a consistent feed NH4-N/NOx-N ratio is a key element for PdNA processes and has been achieved with aeration control strategies such as NH4-N versus NOx-N (AvN) and/or by a step-feed configuration (Le et al., 2019; Fofana et al., 2022; Macmanus et al., 2022; Klaus et al., 2023, Sun et al., 2024). The potential benefit of using an energy efficient tMABR for pre-nitrification is that the target feed ratio may be achieved without the need for sophisticated operational controls.

The objective of this study was to conduct proof-of-concept testing of the tMABR/PdNA process to (1) demonstrate the ability to produce a consistent effluent NH4-N/NOx-N ratio, (2) demonstrate oxygen transfer efficiency, (3) demonstrate PdNA treatment of tMABR effluent, and (4) develop design criteria that can be used for planning level full-scale process sizing and cost estimation.

PILOT SYSTEM AND RESULTS
The tMABR pilot system included an SE feed pump, a tMABR reactor with three ZeeLung Model LS membrane modules and a mixer, and a feed air delivery and control system (Figure 4, Table 1). The tMABR reactor was started up in July 2024 and has included two major phases. The goal for Phase 1 was biofilm development and for Phase 2 was to demonstrate the ability to produce a consistent effluent NH4-N/NOx-N ratio. Key tMABR performance metrics (i.e., oxygen transfer efficiency [OTE, %], oxygen transfer rate [OTR, g O2/m2-day], theoretical nitrification rate [TNR, g NH4-N/m2-day]) were calculated as shown in Equations 1 - 3.

During Phase 1, a blend of seed biomass (from a 0.5 mgd tertiary membrane bioreactor) and SE was added to the reactor (target MLSS ~ 2,500 mg/L) and mixed, with the feed air supply on, for approximately three days to allow for biofilm attachment. The remaining suspended biomass was washed out by turning on the SE feed and TNR was used to assess the progression of biofilm development (Figure 5). The TNR increased from ~ 0.60 to 0.93 g NH4-N/m2-day during the first month of operation (Phase 1A) and continued to increase over the next month (Phase 1B) to ~ 1.1 g NH4-N/m2-day. During Phases 1C and 1D, the impact of O2 partial pressure was demonstrated. At the end of Phase 1, an in-situ batch nitrification rate test (Figure 6) was conducted to validate the TNR performance metric. Results of the batch test (actual nitrification rate = 0.936 g NH4-N/m2-day, TNR = 0.931 g NH4-N/m2-day) demonstrate the simplicity and utility of TNR for process performance monitoring, only requiring exhaust air O2 content and air flow rate data as shown in Equations 1 — 3.

Based on Phase 1 results, target Phase 2 operating conditions were established (Table 2) with the goal of maintaining an NH4-N/NOx-N ratio of ~ 0.8 - 1. Nitrification performance results are summarized in Figures 7 - 8 and Table 3. Average NH4 removal, TNR, and NH4-N/NOx-N ratio were 51.9 ± 3.9%, 0.90 ± 0.04 g NH4-N/m2-day, and 0.91 ± 0.15, respectively. Due to the relatively stable TNR, more precise control can be achieved with a simple strategy of maintaining a target SE NH4-N load/TNR ratio as illustrated in Figure 9.

OTE results are summarized in Table 3 and Figure 10. The OTE (Avg = 47.3 ± 2.2%) observed during this study demonstrates the ability of the tMABR process to reduce aeration energy requirements and associated costs due to efficient bubble-less oxygen transfer through membranes as opposed to fine bubble diffusers which is typically 10 - 20% at LACSD facilities depending on the degree of diffuser fouling (Wong et al., 2018).

To demonstrate PdNA treatment batch tests were conducted with a novel approach that uses Microvi Denitrovi™ biocatalysts for PdN and moving bed biofilm reactor (MBBR) media for anammox. Biocatalysts were taken from a full denitrification (FdN) pilot treating reverse osmosis concentrate at the Warren Facility.