Spring 2021
SEARCH

HWEA will be the organization of dedicated and knowledgeable professionals
recognized for preserving and enhancing the water environment in the Pacific Island Region.

Wastewater treatment and resource recovery has been driven by the activated sludge process for more than 100 years. From the research in 1882 that began to investigate blowing air into sewage, to the research by Arden and Lockett that developed the concept of activated sludge, through our most innovative wastewater treatment technologies, the majority of advancements in the field have focused on supplying oxygen to microorganisms for wastewater treatment (Alleman and T. B. S. Prakasam, 1983).

Activated sludge has thus relied historically on forcing oxygen into water through various mechanisms (large bubbles of air, small bubbles of air, mechanical mixing of pure oxygen to name a few). The membrane aerated biofilm reactor (MABR) is a truly transformational technology, as it breaks this reliance on oxygen transfer from gas to liquid. Instead, the MABR transfers oxygen via a gas permeable membrane into a biofilm, potentially achieving 100% oxygen transfer efficiency while enabling simultaneous nitrification and denitrification (SND). This bubble-free oxygen transfer achieves higher rate biological nutrient removal in a smaller footprint. 

What is the MABR?

The MABR has no relationship to the MBR. The membrane bioreactor (MBR) was also a transformational technology, but it was focused on changing the way we think of solids removal in activated sludge by filtering mixed liquor suspended solids (MLSS) with membrane rather than settling the solids in a clarifier. While the MABR also uses membranes, they are a completely different material and the membranes are used to transfer a gas into liquid, rather than removing solids from liquid. As shown in Figure 1, a membrane is provided with air, pressurized to a relatively low pressure (two to seven pounds per square inch (psi)), and oxygen permeates through the membrane material to the wastewater. There are no bubbles associated with this gas transfer. The bubble-free gas transfer provides a surface for bacteria to colonize and form a biofilm. This biofilm has an aerobic base that supports nitrification. An anoxic exterior of the biofilm, as well as an anoxic bulk liquid, provides nitrification and denitrification in a single tank. This simplifies the process flow diagram for biological nitrogen removal by combining anoxic and aerobic volume into a single tank, and also minimizes the need for internal mixed liquor recycle (IMLR) pumping. 

How Long has the MABR Been in Development?

For any new technology, there is often a large leap that is required to move from pilot scale testing to full-scale application. The development of new technologies has typically followed an “S-Curve” in the wastewater industry, as discussed by Parker (2011). As discussed in the recent Water Resources Utility of the Future Annual Report (2015), this acceleration of the S-Curve has led to the emergence of an Innovation Ecosystem. Within this ecosystem, technology developers, consulting engineers, utilities, and universities are collaborating to better define risks associated with innovative technology. This collaboration results in a deeper understanding of technology risks and benefits, and increased implementation of innovative technologies that provide clear resource reduction and recovery benefit for a utility.

The MABR technology development has actually had an extended bench scale and pilot scale period. Initial attempts to incorporate membrane aeration into biological processes focused on using the membranes solely for gas transfer and not as a support structure for biofilms. However, gas transfer efficiency decreased rapidly due to biofouling of the membranes (Weiss et al. 1998). Timberlake et al. (1988) were the first to design a system to take advantage of the aeration membranes as a support for bacteria. By pressurizing hollow fiber membranes with air, Timberlake et al. found that a significant amount of total nitrogetn (TN) removal was achievable with a biofilm grown directly on the membrane surface. Additional studies focused on achieving nitrification and denitrification in a stratified biofilm for TN removal (summarized by Syron and Casey, 2008). The thickness and density of the biofilm led to mass transfer and biofilm management concerns. Research began to examine a hybrid system, where a nitrifying biofilm was supported by the MABR, but suspended growth was maintained under anoxic conditions (Downing and Nerenberg, 2008). Pilot scale studies indicated that this hybrid system could achieve a high TN removal while maintaining a thinner biofilm (Downing et al, 2010). This essentially resulted in a ‘supercharged’ integrated fixed film activated sludge (IFAS) process, where an aerobic biofilm was present in an anoxic tank. Even with all of the research investment, it has only been in the past five years that commercially available MABR technologies have come to the market from multiple manufacturers. Beginning in 2015, the installations of the MABR technology as a stand-alone processes, as well as for retrofits of the activated sludge process, have been growing exponentially. 

What are the key design metrics for the MABR technology?

The fundamental understanding of the competition of nitrifying bacteria, denitrifying bacteria, and oxygen transfer in an MABR biofilm has been studied for nearly 30 years. This provides a solid fundamental understanding of how the MABR works from a physical, chemical, and biological perspective. The challenge has been the development of a commercially viable technology. There have been two main paths for commercial technology development: package MABR systems that rely on the MABR to achieve the majority of nitrification in the system and activated sludge retrofit applications where a portion of the overall nitrification (30% to 50%) is achieved in the MABR while the activated sludge process is relied on for the remaining treatment. For both of these technology packages, there are several key design metrics that have to be considered:

Influent screening – given the high potential for clogging, manufacturers are currently recommending influent screening of 1 to 3 mm.

Effluent ammonium concentration low effluent ammonium concentrations (less than 2 mg/L) will require some polishing from activated sludge suspended growth or more membrane surface area. 

Mixing and biofilm management
as with any biofilm process, mixing and scour are critical for mass transfer into the biofilm and managing the overall thickness of the biofilm. 

Influent BOD loading – high influent BOD to nitrogen ratios can create a large amount of aerobic BOD oxidation, impacting the nitrification efficiency of the MABR.

Process airflow rate – 
replenishment of the oxygen inside the MABR membrane requires a constant airflow. Given the high oxygen transfer efficiency, this airflow is typically less than 30% of the equivalent airflow for suspended growth, but still must be considered when evaluating the blower configuration for the MABR. 

Who manufactures the MABR technology?

Currently, three major manufacturers provide the MABR technology commercially in North America: Suez, OxyMem/DuPont, and Fluence. The Suez MABR application has focused on retrofit into the activated sludge process with the ZeeLung technology. Physically, the ZeeLung MABR looks very similar to the ZeeWeed MBR, but the membrane materials are significantly different. The ZeeLung Cassettes are inserted into anoxic reactors in an activated sludge processes, with a focus on retrofitting the activated sludge tanks. The ZeeLung cassettes provide energy efficient nitrification in a biofilm, while the suspended growth in the activated sludge process provides ammonium polishing, denitrification, and biological phosphorus removal. Mixing of the ZeeLung cassettes is provided with offgas from the MABR modules and delivered via large bubble mixing. 

The OxyMem technology, which is now a DuPont product, is similar in structure to the Suez technology, but relies on very different membrane materials and mixing technologies. The membrane material utilized by OxyMem results in a lower backpressure inside the membrane, and potentially a lower specific surface area. Mixing is achieved via offgas from the MABR module, which is utilized as part of an integrated airlift pumping system. OxyMem has developed similar retrofit configurations as Suez, and additionally offers modular package systems for small systems and expansion options. 

A third MABR manufacturer, Fluence, relies on a very different MABR configuration. The Suez and Oxygem MABRs are cord-like membranes grouped in bundles. The Fluence MABR relies on a flat sheet membrane that is spirally wound with spacers to provide volume for water flow and biofilm growth. Fluence offers a packaged MABR system for small systems (commercially available as ASPIRAL™) as well as activated sludge retrofits (commercially available as SUBRE™). Mixing is accomplished in the Fluence MABR systems with cyclic application of airflow into the MABR zone.

A brief summary of each manufacturer’s MABR is provided in Table 1. All three options provide energy efficiency intensification of the activated sludge processes. More information can be found at each manufacturer’s website. 

Has the MABR Been Implemented in North America?

There are several MABR facilities in design and construction in North America. However, the majority of global installations have been in China. The largest single MABR is located in the CSWEA area, at the Yorkville-Bristol Sanitary District in Illinois. This is the Suez technology, and has been featured at several CSWEA conferences and in previous issues of the Central States Water magazine. 

What are the benefits of the MABR Technology?

The MABR is a disruptive technology that brings together two important aspects of resource recovery focused wastewater treatment: process intensification and low energy consumption. Given the benefits of the MABR, it is fair to ask why it is not applied with every activated sludge process expansion. There are two key factors for the success of the MABR, and facilities typically have to be faced with both factors for a successful MABR project:

1. Process intensification required.

2. Reduction in energy consumption. 

Process intensification can take the form of a capacity expansion requirement, new nutrient removal requirements (nitrogen and/or phosphorus), or drivers related to wet weather flows. The intensification aspect is related to achieving these drivers without the addition of new tanks, but rather through the addition of MABR units. Reduction in energy consumption can either be driven by high energy costs, or simply a goal for more sustainable energy use. When both of these drivers are present, the MABR can be a valuable technology for consideration.

References

Alleman and Praksam (1983) Reflections on Seven Decades of Activated Sludge History. Journal Water Pollution Control Federation
55(5): 436-443

Downing and Nerenberg (2008) Sustainable nitrogen removal from wastewater with the hybrid membrane biofilm process (HMBP): bench-scale studies Water Science and Technology 58(9): 1715-1720.

Downing, Bibby, Esposito, Fascianella, Tsuchihashi, and Nerenberg (2010). Nitrogen Removal from Wastewater using the Hybrid Membrane-Biofilm Process (HMBP): Pilot Scale Studies. Water Environment Research 82(3):195-201.

Timberlake D, Strand S, Williamson K. 1988. Combined aerobic heterotrophic oxidatoin, nitrification and denitrification in a permeable-support biofilm. Water research 22(12):1513-1517.

Weiss PT, Gulliver JS, Semmens MJ. 1998. In-Stream Hollow-Fiber Membrane Aeration. Journal of Hydraulic Engineering-ASCE 124(6):579-588. 

VISIT OUR PHOTO
GALLERY


SHOW ME

Contact Info

Hawai‘i Water Environment Association
PO Box 2422
Honolulu, Hawai‘i 96804
General Inquiries: info@hwea.org