Chem321:Membrane bioreactors

This is the 2012 paper by Kelly Balbian.

The Future for Wastewater Sustainability: Membrane Bioreactors

Membrane bioreactor(MBR) technology is the amalgamation of activated sludge, which originates from the process of the treatment of waste water, cure with a separation of the biological sludge by micro-or ultra-filtration membranes to produce the particle-free effluent[1]. This system is used to filter and clean water streams that are produced from many places, such as sewage plants, textile factories and chemical plants. MBR equipment has had much advancement in the past years but is still a new developing tool. It will become a new innovative tool to make water treatment more efficient and effective. Membrane bioreactors are currently being used by some companies but are still connected with higher total life cost and risk[1].

Wastewaters' history

The development of MBRs started in the late 1960s. The first commercial reactors used were in the 70’s and 80’s with sidestream configurations. A sidestream configuration is when the membrane separation step is employed in an external sludge recirculation loop with mostly in-to-out flow through organic or ceramic tubular membranes, but has a high energy demand. During the 90’s Japan started a 6-year R&D plan, which produced important technological and industrial breakthroughs of the membrane bioreactor system, called the submerged membrane module. This worked with low negative pressure and membrane aeration to reduce fouling. With this new advancement the MBR process was able to produce reductions in capital and operation costs because of the reduction and simplification of the equipment and loss of the energy demanding sludge recirculation loop[1].

A conventional activated sludge process(CASP) was what was and still is used to treat most wastewater streams for over 100 years before MBR methods were starting to appear. The CASP is what could be called the “dirty” system throughout this paper, due to the fact that it is less energy efficient than the membrane bioreactor system. The conventional activated sludge process is operated by the natural biodegradation of the pollutants by heterotrophic bacteria, which is activated sludge, in aerated bioreactors. When activated sludge is used for wastewater treatment gravitational settling separates it. This conventional process is limited to improvements due to the difficulty in separating the suspended solids and being effective when high sludge concentrations in streams need to be filtered[11]. The CASP is still an effective method in wastewater treatment. In a study done in 1996, it was shown that the CASP examined over 10 months in a full scale plant showed that no excess sludge needed to be withdrawn and no significant accumulation of inorganic solids was found in the aeration tank[12]. MBRs have developed to make the wastewater treatment more efficient.

MBR innovations began by a combination of membrane technology and activated sludge where the solid separation is filtered instead of using gravity to settle the material. In the conventional gravity settling, the energy loss comes from hydraulic headloss across the settling tank, resulting with a pressure drop of 1.5kPa. When a MBR is used the pressure drop is between 40 and 60 kPa, providing less hydraulic headloss[8]. The MBR technology that came from the membrane separation process has an absolute retention of all the micro-organisms that insures that the sludge concentration will increase and the disinfection of the treated water will be completed. The sludge retention time (SRT) and hydraulic retention time (HRT) allow complete separation and therefore a higher sludge concentration can be maintained with the MBR and high-strength wastewater can be treated more effectively[11].

What do we use today?

Two different types of submerged membrane units are most commonly used today. Through innovations and new technology both modules have out-to-in permeate filtration and contain the flat-sheet (or plate and frame) membrane module and the hollow fiber membrane module. More alternative systems are appearing and the market is expected to expand significantly[1]. Many countries are investing in projects dedicated to improve MBR technology, such as Japan and Switzerland.

As mentioned earlier, there are two predominant MBR systems widely used in today’s market. In this essay we will look at a plate membrane system manufactured by Kubota and a hollow fiber membrane manufactured by Zenon.

Flat-sheet MBR

The flat sheet technology incorporated into MBRs uses cross flow filtration, which has mixed liquor flowing parallel to the membrane surface, while water permeates through the surface. This cross flow helps prevent the membrane surface from fouling[10]. The Kubota model has a nominal pore size of 0.4μm of the membrane sheet that can prevent activated sludge and coliform bacteria and virus from penetrating. It also removes substances difficult to biodegrade, which is said to be why it has a longer sludge retention time (SRT). This system is designed to conserve energy and uses aeration to save energy by using oxygen supply for biological treatment and cleaning of membrane surface with turbulent flow. The Kubota MBR performs high-concentration activated sludge treatment and therefore replaces settling tanks and sludge thickening tanks. This system assures stable treated water with a more compact simple system[9].

Hollow fiber MBR

The Zenon Zeeweed membrane bioreactor is a reinforced hollow fiber membrane that combines ultrafiltration technology with biological treatment for municipal, commercial and industrial wastewater treatment and water reuse applications. This system replaces the conventional wastewater treatment and is shown below on the left. When compared to the flat sheet membrane on the right it is more difficult to clean because the flat sheets can simply be removed when necessary.

The Zeeweed MBR system can be used for plant retrofits, cold climate operation, bio-phosphorus removal, bio-nitrogen removal, reverse osmosis pretreatment, direct use, direct discharge, aquifer recharge and small, medium and large plants[2]. The MBR process that Zeeweed uses consists of a suspended growth biological reactor integrated with an ultrafiltration membrane system that replaces the solids separation function of secondary clarifiers and sand filters that are used in a conventional activated sludge system. The Zeeweed system is immersed in an aeration tank that is in direct contact with mixed liquor. By using a permeate pump, a vacuum is applied to a header connected to the membranes and draws the treated water through the hollow fiber ultrafiltration membranes implemented into the system. Permeate is then sent to disinfection or discharge facilities. The airflow in the system becomes turbulent on the bottom of the membrane and then rubs the exterior of the hollow fibers. As the turbulent airflow rubs the bottom of the membrane solid particles are then discarded away from its surface. The Zeeweed system is effective in solving problems associated with poor settling of sludge in the conventional activated sludge processes. The Zeeweed membrane bioreactor has higher mixed liquor solids concentrations than conventional sludge systems that are limited by sludge settling. With the elevated biomass concentrations, the removal of soluble and particulate biodegradable material in the waste stream is highly effective. The hollow fiber membrane module shown by the Zeeweed system produces a high quality effluent, simplifies operations and reduces space[2].

How are they different

When the Zenon fiber membrane bioreactor and the flat sheet membrane module are compared they are very similar and both provide high quality effluent. Although, the Zeeweed prototype has a pore size of 0.1 micron, whereas the Kubota has a pore size of 0.4 micron, giving Zenon a better removal efficiency than the Kubota system. The hollow fiber system can also operate at a higher suction pressure than the plate system and allows the Zenon system to be pushed more when it is fouled. While the looser membrane in the Kubota system allows a higher flux rate than the Zenon system, allowing more gallons per day per square foot of membrane. The fiber membrane bioreactor requires an automatic backpulse system, making it more complex than the flat plate system[5].

Problems with MBR technology

A problem that still arises with membrane bioreactors is when some of the material that should be filtered out of the stream is not and gets through to the output stream, called fouling. In a study done on different fouling on hollow-fiber and flat-sheet membranes, they found that both of the MBR modules needed necessary improvements related with fouling and more development was needed. Both of the MBR systems were exposed to recalcitrant industrial wastewater conditions. 99% color and 97% total organic carbon (TOC) was removed by both types of membranes in the study. The hollow-fiber membrane was found to be susceptible to fouling by a cake layer, while the critical flux dependent pore blocking of the flat sheet was detected. During separate tests the streams containing only starch and only dye induced a negligible increase in TMP. The flux-dependent pore blocking emerged for both the hollow fiber and flat sheet membranes for the mixture stream. The MBR systems still need to make more investments in the technology that will reduce the fouling in all of the different modules[7].

MBR vs. conventional

So, why should MBR technology be used over other new innovative technologies or the conventional systems? Many sources have already converted to membrane technology for water purification and wastewater treatment and the technology is continuously developing [6]. Membrane reactors are still developing so they are still competing with other systems such as Extended Aeration (EA) and Sequencing Batch Reactor (SBR) systems. MBR systems have become more affordable with advancements in technology but can still be costly to install. Membrane bioreactors do hold advantages over other systems, such that they provide a higher level of treatment and are much more resistant to upsets due to fluctuating influent flows[5]. A MBR system has the hydraulic retention time (HRT) of 4-8 hours compared to a contemporary system that takes 16-24 hours and a solid retention time (SRT) of 15-365 days. The MLSS of the membrane system is 10-15,000 mg/L and has a sludge yield of 20-40% less than a conventional module. The footprint of the MBR is 25% of what a standard plant has. This system has the highest quality effluent and is capable of meeting AWT standards for nutrient removal. It is also causes less odor[5]. In Switzerland membrane technology is already being used to filter out calcium and sulphate ions allowing the community pleasant tasting water and by removing the calcium relieves boilers, washing machines and dishwashers from clogging[3]. The membrane bioreactor technology has many advantages over conventional systems as it can operate at biomass (MLSS) concentrations that are 5 to 10 times higher than what the activated sludge process can[8].

MBRs' affects

The MBR process has developed into a highly efficient wastewater treatment process that has become a substitute for many conventional activated sludge processes where high effluent quality is required. When a process is optimizing its’ energy and raw materials used to lower the negative environmental impact it can be related as a “green” process. The membrane bioreactor system is used to remove pollutants from water streams. This can already seem like a “green” system, but the process can actually be harmful to the environment. The MBR process that has developed over conventional systems, like the activated sludge, are becoming much more efficient and by improving systems to optimize energy the process is moving toward a much more environmentally friendly method. There has been a lot of progress in raising the energy efficiency for MBR but the membrane fouling still needs more innovation[8].

The membrane bioreactor technology has already had a large significant impact on wastewater treatment and filtration worldwide. Around the world there are over 1.5 billion people who do not have access to clean water. With the development of membrane technology, such as the “LifeStraw,” membrane technology and membrane bioreactors will be able to improve water sources in third world countries as well as in wastewater treatment around the world[4]. There are still many adjustments that need to be made to the MBR system to make it more efficient, affordable and sustainable. MBR technology involves different modules that use diverse technology, but there are still faulty components, such as fouling, that occur. Membrane bioreactors will be an important innovation in the wastewater treatment as industry moves toward more sustainable processes.

References

• (1) AnonymousMBR-Network: the cluster of EU projects dedicated to the MBR technology. MBR-Network 2008-2009.
• (2) AnonymousMembrane Bioreactor (MBR). http://www.gewater.com/products/equipment/mf_uf_mbr/mbr.jsp (accessed 08/06, 2012).
• (3) AnonymousClean drinking water thanks to nanotechnology. , Green Technology.
• (4) European Commission Sewage Sludge. 2012.
• (5) Fitzgerald, K. S. Membrane Reactors. TSG Technologies, Inc. 2008.
• (6) G.W.T. Limited Membrane Bio-Reactor. http://www.geyserwatertreatment.co.uk/?page_id=16 (accessed 08/05, 2012).
• (7) Hai, F. I.; Yamamoto, K.; Fukushi, K. Different fouling modes of submerged hollow-fiber and flat-sheet membranes induced by high strength wastewater with concurrent biofouling. Desalination , 180, 89-97.
• (8) Hermanowicz, Slav W. Membrane Bioreactors: Past, Present and Future? Working Papers 2011.
• (9) Kubota Kubota Submerged Membrane Unit (Kubota SMU). http://www.kubota-mbr.com/mbr.html (accessed 08/06, 2012).
• (10) Mazloum, S. K. Hollow Fiber vs. Flat Sheet Technology. Wetico .
• (11) Xing, C.; Tardieu, E.; Qian, Y.; Wen, X. Ultrafiltration membrane bioreactor for urban wastewater reclamation. Journal of Membrane Science 2000, 177, 73-82.
• (12) Yasui, H.; Nakamura, K.; Sakuma, S.; Iwasaki, M.; Sakai, Y. A full-scale operation of a novel activated sludge process without excess sludge production. Water, Science and Technology 1996, 34, 395-404.