Victoria Kippax
Energy requirements are of primary interest in membrane bioreactor systems. Aeration energy is the largest consumer of energy in an MBR system. This comes in two forms: to provide oxygen for biological nutrient removal, and to provide scouring of the membrane to keep it clean. As the air required for biological nutrient removal is calculated in terms of lbs or kg of oxygen, limited optimization can be done. However, MBR suppliers and universities are avidly researching the optimization of membrane air scour energy.
The hydrodynamic conditions and system configuration in an MBR system can greatly affect membrane performance. A system's performance improved by 20-60% when a two-phase (air and liquid) cross-flow was applied rather than just a single-phase (liquid only) cross-flow (Kang et al, 2008). Air scour energy in an MBR system can provide high levels of turbulence and surface contact to remove solids particles that attach to the surface of the membrane and to guard against irreversible membrane fouling. Membrane fouling can cause reduced production capacity, shortens membrane life, and increases operational cost.
Various technological innovations have been brought to market over the past 10 years to decrease the air scour energy required in an MBR system, where the industry has seen air rates drop by 75% from 1.2 m3air/m3 filtered to 0.3 m3air/m3 filtered.
Two Aeration Configurations
Different types of aeration configurations affect the use of air scour in MBR systems. Depending on the air and liquid flow rates and the properties of the liquid, the mixture of air-liquid can adopt a wide spectrum of flow patterns. However, in MBR systems where the applied air flow rates are relatively low, the most likely flow regimes are bubbly flow and slug flow (also known as plug flow).
Traditionally, bubbly flow has been applied in various different forms, most commonly through use of a coarse-bubble diffuser. This type of diffuser can be as simple as a pipe with holes drilled into it, individual tubes of the membrane modules, or some form of diffusion grid.
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| Figure 1: Air-liquid two-phase flow patterns. In MBR systems where the applied air flow rates are relatively low, the most likely flow regimes are bubbly flow and slug flow (also known as plug flow). |
The main drawback with the bubbly flow regime is its relative inefficiency. The small bubbles created in the bubbly flow regime cannot provide enough energy to shear and shake the membrane surface and, as a result, a large amount of energy must be expelled to provide the air scour needed to keep the membranes clean.
In contrast, the slug or plug flow provides maximum efficiency due to three major factors (Cui et al, 2003). The moving bubbles generate secondary flows behind the initial bubble that assist in breaking up cake layer and subsequently promote local mixing near the membrane surface. A liquid film around the outside of the bubble can be a high shear region, promoting the movement of solids away from the membrane surface. The moving slugs result in pulsing pressure in the liquid around it, causing instability and disturbance near the membrane surface.
The outcome: by using slug or plug flow, the MBR system can provide significantly less air for the same scouring efficiency.
Slug/Plug Flow Improvements
One application that now uses the slug or plug flow is the MemPulse™ MBR system from Siemens Water Technologies. The system is supplied with a continuous air supply that is accumulated in the base of the module. It periodically releases irregular pulses of air to the MBR module, creating plug flow.
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| Figure 2: A MemPulse MBR rack installed in a membrane tank. The simple design of the accumulation device can be seen at the base of the module. |
To create the plug flow, the MemPulse system acts as an airlift pump, drawing mixed liquor from below the device through a suction pipe and into the module. The liquid helps to prevent solids accumulation as the mixed liquor on the membrane is continuously refreshed with new liquor.
The major difference between the MemPulse system over traditional MBR systems is that the air supply to the system is up to 60% lower. Compared to a similar system with bubbly flow regime, a comparable performance could be achieved with half of the air flow rate. Equating this to a system design, the energy required for a typical 5 mgd system was reduced by 48% over a traditional MBR system.
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| Figure 3: MBR systems come in a wide range of configurations and designs, but they all produce the same high-quality effluent in a small footprint. |
In addition to providing an improvement in aeration energy, the MemPulse MBR system has a secondary advantage over traditional MBR systems. Apart from the accumulation device on the base of the module, no additional equipment is required to produce the necessary air flow.
Traditional MBR systems often provide valves or additional equipment to reduce the amount of bubbly flow supplied to the membrane modules while still maintaining a certain scouring efficiency. This equipment can require increased maintenance and care over the accumulation device, which has no moving parts.
References
Cui Z.F., Chang S. and Fane A.G., 2003, "The Use of Gas Bubbling to Enhance Membrane Processes," Journal of Membrane Science, 221, pp. 1-35.
Kang C.W., Hua J.S., Lou J., Liu W. and Jordan E., 2008, "Bridging the Gap between Membrane Bio-reactor (MBR) Pilot and Plant Studies," Journal of Membrane Science, 325, pp. 861–871.
About the Author
Victoria Kippax is the MBR product manager at Siemens Water Technologies Corp. She is located in Waukesha, Wis., and can be reached at 262-521-8487 or at Victoria.kippax@siemens.com.
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