MABR-DAS: Coupling MABR & Densification for Enhanced Biological Selection

Introduction Membrane Aerated Biofilm Reactor (MABR) and Densified Activated Sludge (DAS) employ different mechanisms to deliver process intensification, both of which are applied independently at full-scale (Astrand 2023). Coupling MABR and DAS (MABR-DAS) is an area of opportunity to bring more value for WRRFs. The first opportunity is to combine the individual features of the technologies to de-bottleneck plant operations where MABR intensifies the biological reactor and DAS intensifies secondary clarification (Donnaz 2020; Roche 2021) and uncouples SRT -- floc and granules which helps with even further intensification of BNR. The second opportunity is to exploit synergies between the technologies to enable intensification that is greater than the sum of the parts and expand the range of WRRF plant configurations that can benefit from intensification. MABR and DAS Process synergies The hybrid MABR-DAS process offers several unique features that enhance biological selection compared to conventional BNR. These features promote a virtuous cycle with densification. Maximize Substrate Gradient in the Feast Zone A high substrate gradient in the feast zone favors the growth of densified biomass. MABR maximizes substrate gradient by reducing recycle flows that dilute substrate concentration. SND in the anoxic zone of an MABR enables nitrate recycle to be eliminated or reduced. Densified sludge enables RAS rates to be reduced. Enhance Anaerobic Conditions and Bio-P MABR enables and improves bio-P and the biological selection of PAOs and GAOs by enhancing or creating anaerobic conditions. Elimination or reduction of nitrate recycle by MABR increases anaerobic HRT. SND and complete denitrification in the MABR zone reduces NOx return in the RAS. The MABR anoxic zone can serve as an extension of the anaerobic volume due to: An ORP gradient along the length of the MABR zone enabled by the attached biofilm, Use of the unaerated volume below the MABR cassettes for fermentation, BNR intensification by MABR enables aerobic volume to be cannibalized for anaerobic and/or anoxic fractions. This allows CAS plants with no or small unaerated volumes to increase the unaerated volume fraction necessary for biological selection. Reduce rbCOD Bleed into the Famine Zone SND in MABR consumes rbCOD for denitrification, preventing it from entering the famine zone. MABR enables increased F/M compared to conventional BNR systems. The seeding effect of the nitrifying MABR biofilm enables operation at lower suspended growth Aerobic SRT and thus higher total F/M for the system. Full-Scale Implementation at the Yorkville-Bristol Sanitary District A full-scale implementation of MABR-DAS has been in operation at the Yorkville-Bristol Sanitary District (YBSD) since 2023. The YBSD MABR-DAS upgrade has occurred in the following stages (Figure 1). (1) Nitrifying CAS. Prior to 2017 the plant was a nitrifying CAS system rated for 3.62 MGD with the biological reactor arranged as ten aerobic tanks in series. (2) MABR. In 2017 the plant was upgraded to MABR and Enhanced Biological Phosphorus Removal (EBPR) to increase the biological capacity by 25% and meet a new phosphorus limit. The biological process was reconfigured to A2O with one reactor anaerobic and one reactor anoxic with MABR cassettes. The plant does not have nitrate recycle. (3) MABR-DAS. In January 2023 densification was implemented to increase the hydraulic capacity by 40% to 5 MGD and improve bio-P performance. In the future, MABR cassettes could be added to increase biological capacity by an additional 20% (145% total). Results from 11 months of operation of the MABR-DAS system are provided below. Sludge Settleability During the first 11 months of MABR-DAS operation, SVI30 decreased from 100 mL/g to 60 mL/g and SVI5 decreased from 250 ml/g to < 100 mL/g. The granule fraction (GF, fraction of solids > 250 µm in size) increased from 35% to 55%, see Figure 3. Figure 4 summarizes the correlation between SVI and GF. The gray data points are from before implementation of MABR-DAS and the blue points are after implementation. The data suggests two phases in the transition of the plant. Phase 1 indicates densification, with selective wasting and washout of filaments when SVI30 decreased from 110 mL/g to 60-75 ml/g with no corresponding change in GF. During Phase 2, GF increased from 35% to around 55%, and this was coincident with a further decrease in SVI. Figure 5 summarizes phosphorus removal performance before and after operation of the MABR-DAS system. Phosphorus removal is achieved by a combination of bio-P and chem-P with bio-P limited by a 10% anaerobic mass fraction. A decrease in coagulant dose since the start of MABR-DAS indicates that bio-P performance has improved with MABR-DAS. Phosphorus release profiles from a kinetic & CFD model demonstrate that the MABR anoxic zone serves as an extension of the anaerobic volume (Figure 6). Alternative mixing strategies in the anaerobic zone improve phosphorus release by a further 15-30% (Duchi 2023). Knowledge Gaps & Opportunities Physical Selection of Nitrifiers Sloughed from MABR Biofilm Adding a Third Dimension to SRT Uncoupling MABR introduces another form of biomass (biofilm) in the activated sludge process. This brings an additional lever to the principle of SRT uncoupling, control the SRT of the biofilm independently from the SRT of the suspended growth