North America's First Full-scale OxyMem-based MABR: Lessons and Performance Results

Introduction
Membrane aerated biofilm reactor (MABR) is a process intensification technology offering benefits of increased process capacity, improved nitrification and nutrient removal performance, as well as reduced energy consumption and greenhouse gas footprint. The Regional Municipality of Waterloo (the Region) faced challenges at Elmira WWTP due to high strength industrial wastewater and high peak flows. The Region selected OxyMem's (Dupont) OxyFAS technology following a competitive equipment preselection in 2021. Eight MABR modules were retrofitted into existing unaerated zones at the head end of the two parallel bioreactors and commissioned in June 2023, making it the first full-scale OxyMem-based MABR in North America. This paper discusses performance testing, first year of operation and lessons learned during the design, construction, and commissioning phases. Thus, showcasing the challenges and opportunities that may arise when implementing a full-scale MABR system, as this emerging technology continues to gather interest across North America.

Performance Results
Fall and winter performance tests were conducted to evaluate the system's ability to achieve the designed nitrification rate (NR) of 20.3 kg N/d. The fall performance test had two sampling campaigns over two weeks, while the winter test had four sampling campaigns over the same period. NR was calculated by measuring the ammonia concentration from the mixed liquor upstream and downstream of the MABR zone. Average NR from both performance tests are shown in Figures 1 and 2.

Figure 1 and Figure 2 show that NR is higher in the morning and decreases in the afternoon. NR peaks tend to correlate with the typical ammonia loads observed in municipal wastewater diurnal profiles. The higher the influent ammonia the greater the driving force for nitrification, hence why MABRs are typically installed in the first anoxic zones in combination with suspended growth activated sludge system. This allows the MABR to target ammonia, while the activated sludge provides supplemental treatment of residual ammonia and organics. A tabulated summary of the nitrification performance from both tests are shown in Table 1. Measured Oxygen Transfer Rate (OTR) from the fall performance test was also included in the data set. Due to issues with the oxygen analyzer OTR data was not available during the winter performance test.

The MABR system at Elmira WWTP exceeded the designed NR targets in both fall and winter performance tests. Average wastewater temperatures between the two tests were 18.5 °C and 10.5 °C, respectively. Higher wastewater temperature, which leads to higher biological activity likely contributed to the higher NR observed in the fall performance test. Of note, based on the stoichiometric oxygen consumption for conventional nitrification (4.57 gO2/gN) and the measured OTR 189.65 kgO2/d, the observed oxygen demand for nitrification during the fall performance test was approximately 89%. This means that 89% of the oxygen transferred across the membrane was utilized for nitrification, the remainder 11% was likely used to oxidize other compounds such as COD and/or hydrogen sulfide.

Lessons Learned
Mixing Importance: adequate mixing ensures the even distribution of substrates and nutrients into the biofilm for optimal performance, while also preventing potential sludge accumulation within the MABR zone. Computational fluid dynamics (CFD) modelling was conducted to validate the mixer selection, identify dead zones, short circuiting and additional tank modifications required to provide even flow distribution into the cassettes.

Understanding Hydraulic Gradeline: a specific range of water surface elevations within the MABR cell must be maintained when operating OxyMem modules. A hydraulic model was developed during design and later adjusted per site conditions to confirm the MABR installation elevations and bioreactor effluent weir height adjustments.

Heat tracing: cold climate MABR applications with off-gas analysis requires heat tracing and insulation of the MABR off-gas manifold. From a design and procurement perspective, including the heat tracing and insulation as part of the vendor's scope would prompt the vendor to provide a better tailored solution to their system.

Back-flow in MABR zone: Bulk liquid density differences between the MABR zone and the downstream aerated zone caused back-flow into the MABR zone, presenting a challenge for MABR performance evaluation based on ammonia concentrations. Bulk liquid differences between zones adjacent to the MABR system should be considered along with a thorough review of plant hydraulics.

Challenges with analytical instruments: ease of operation, maintenance, and overall instrumentation reliability are critical for MABR systems, since the process is fully automated, and these instruments provide a live snapshot of the MABR performance.

Conclusions
MABR technology continues to gather interest from global utilities due to its ability to increase process treatment capacity, and ease to retrofit into existing infrastructure. The presented lessons learned throughout the design, construction, commissioning and operation phases should be taken into consideration for future MABR installations. Despite some of the challenges faced, the MABR system at Elmira WWTP has shown promising results exceeding the design NR targets as shown by the results from both performance tests.