Intensification on the Clock: How MABR Delivers

[b]Abstract Summary[/b] Faced with rapid growth, TRA's DCRWS plant needed interim capacity before its major expansion would be completed in 2028. This presentation explores how Membrane Aerated Biofilm Reactor (MABR) technology was selected and integrated to increase treatment capacity. Specifically, challenges related to design and construction within active basins in less than 14 months' time. [b]Introduction and Background[/b] The Denton Creek Regional Wastewater System (DCRWS), operated by the Trinity River Authority (TRA), has been experiencing rapid growth. Originally designed to accommodate a flow of 11.5 million gallons per day (mgd), the plant has experienced unprecedented growth in recent years necessitating an expansion starting in 2022. In response to this growth trajectory, TRA initiated a phased expansion plan, with Phase 1 targeting a capacity increase to 16.5 mgd. Construction for this phase started in 2024, but was just underway when the plant saw a sharp rise in flows that was higher than anticipated – approximately 1 mgd in less than a 6 months. With construction of the major expansion phase not completing until 2028 concerns were focused on maintaining treatment performance and remaining within existing permitted capacity. Recent data was used to update projections and review capacity each year until construction was completed. The secondary treatment was the primary focus with increased organic loadings resulting in a significant peak flow capacity restriction, Figure 1. To mitigate this risk, TRA authorized development of a rapid solution to increase treatment capacity ahead of the major expansion project. Due to the increasing flows, the solution would have to be implemented without taking any existing treatment units offline. [b]The Solution[/b] The preliminary assessment indicated that intensification would be the quickest way to increase capacity. Intensification in the wastewater treatment realm often refers to increasing the capacity of existing infrastructure. The intensification technology selected was the membrane aerated biofilm reactor (MABR) system. MABR is an advanced biological wastewater treatment technology that integrates gas-selective membranes and attached growth biofilm processes. The biofilm develops on the outer surface of the membrane and is exposed to the bulk wastewater on one side and to oxygen on the other, enabling stratified microbial zones: (a) Aerobic/Nitrifying Zone (b) Anoxic/Denitrifying Zone. Because a portion of the biomass is retained in the biofilm and not subject to washout, the total sludge inventory in the bioreactor can be increased without increasing the mixed liquor suspended solids (MLSS) load to the clarifier. This increased biomass retention enhanced the overall treatment capacity of the secondary treatment process, Figure 2, without requiring major changes to the clarifier or aeration volumes. [b]Design, Construction, and Installation[/b] The facility consists of three aeration basins with Aeration Basins 1 and 2 being mirror images and Aeration Basin 3 double the size with a slightly different configuration. The strategic placement of MABR cassettes in Aeration Basins 1, 2, and 3 enabled enhanced biological activity without the need for extensive downtime or reconfiguration of major unit processes. The MABR system at the DCRWS Plant consists of 14 cassettes each in Aeration Basin No. 1 and 2 and 36 cassettes in Aeration Basin No. 3, making it the largest MABR installation in the US at this time. Retrofitting new treatment technologies into existing treatment trains can be difficult. The installation at the DCRWS Plant had the additional complexities of expedited timelines and the requirement of keeping the plant online without shutdowns. The MABR technology consists of cassettes similar to MBRs but are placed within the anoxic zone and provide aeration to a biofilm rather than filtration like the MBR. These cassettes typically hang into the zone, which created a significant challenge considering 10,000 lb cassettes and spans of up to 52 feet. The requirement to not shutdown any treatment trains led to a complex structural design. The team attempted to avoid this complex design by looking at alternative solutions, such as floating the cassettes or supporting cassettes in a cage from the floor. However, these alternative methods all proved to have flaws that would be prohibitive to implementation. Therefore, all structural supports had to be designed to span the basins without any intermediate supports. A significant focus will be on the special design and construction considerations for varying basin dimensions. The site included aeration basins with spans less than 20 ft (two MABR cassettes) up to 52 ft (six MABR cassettes). The longer span required significantly more structural evaluation with the end results a bridge truss type of support structure to enable installation while the basins are in service, Figure 3 and Figure 4. To meet the increasing demands the BV team, along with the manufacturer, designed and prepared bid documents in less than 3 months. This was successful due to a collaborative design between TRA, the consultant, and contractor. The MABR is currently onsite with the first cassettes scheduled to be installed in February and the final by March, Figure 5.