USFilter MEMCOR® introduced the first drinking water system in 1987. In the years since, technology has evolved and more communities are relying on membrane operating systems to solve drinking and wastewater treatment challenges. With this growth and discovery stems questions about specific uses and maintenance. The following article in Membrane Technology answers frequently asked membrane-related questions.
Q: Why would I choose low-pressure membranes over media filtration for a drinking water application?
A: Low-pressure membranes are now an accepted alternative to media filtration and are fully compliant with the Long–Term 2 Enhanced Surface Water Treatment Rule. The installed cost of membranes is very comparable to that of granular media filters. However, membranes provide the advantage of an absolute barrier proven to be integral. Therefore, the question today isn’t why choose membranes; the question is why not?
Q: Can membranes be applied to challenging water sources such as those requiring enhanced coagulation for total organic carbon (TOC) control or seasonal powdered activated carbon (PAC) addition for taste and odor mitigation?
A: First generation membranes were designed for turbidity and pathogen removal, but were not well suited to high solids or chemically coagulated feed waters. In the last few years, new membranes and system configurations have been developed to address more challenging water sources, including those containing high solids, chemical addition, or even PAC addition. Today, MEMCOR® plants treat any feed water from raw sewage and high TOC surface waters to “clean” groundwater.
Q: What if I pre-chlorinate, add permanganate, or feed other oxidants to my source water. Can I still use membranes?
A: With MEMCOR®, yes, because the membranes are made of oxidant tolerant polymers designed to withstand these conditions.
Q: How does membrane fiber packing density relate to water quality?
A: Because MEMCOR® membrane systems operate as a continuous batch process (like a rapid sand filter), the cycle between backwashes depends on the system’s ability to retain solids before terminal headloss (maximum transmembrane pressure) is reached. The standard MEMCOR® membrane system module has 20,000 fibers and is ideal for relatively clean supplies. For a solids concentration of approximately 20 mg/l or more, MEMCOR® offers intermediate packing density (IPD) modules with fewer fibers and more interstices to increase solids handling capacity.
The tradeoff with lower packing density is less filtration surface area, which normally indicates lower filtrate production. However, extensive high solids testing conducted by MEMCOR® has confirmed that the reduction of membrane surface area is more than offset by the higher flux achieved by the reduced number of fibers in the IPD module. This means for waters with a higher suspended solids concentration or chemically conditioned waters, the filtrate production from an IPD module can be higher than a standard MEMCOR® module.
How do you know if IPD modules are right for your source water? Pilot testing is the best way to confirm the benefits of fiber packing density.
As solids loadings approach and exceed 100 mg/l, pre-clarification is normally a more economical approach to reduce the stress on the membranes and to optimize the membrane area required.
Q: My community is considering reclaiming wastewater. How do membranes apply?
A: Membranes now play a prominent role in wastewater reclamation, primarily in two ways. First, membrane filtration can be added to a secondary wastewater treatment plant to provide tertiary effluent for reclamation. Membranes provide a higher, more reliable standard of treatment than media or cloth filters. This is especially important when the reclaimed water is planned for unrestricted use where potential for human contact exists (e.g., irrigation of golf courses, parks or schools). Under those circumstances, it is reassuring to know that there is a positive barrier between pathogens and the public.
Second, membrane bioreactors (MBR) are taking on an increasing role in wastewater reclamation for good reason. The MBR process uses membranes to replace secondary clarifiers. Following the activated sludge process, hollow fiber membranes are immersed in mixed liquor. Vacuum pumps create suction on the membranes which separates the treated effluent from the mixed liquor. The MBR process operates under the same principles as the continuous microfiltration process, except that the system is optimized to operate in mixed liquor.
Because of the higher water quality produced with membranes, the options for reuse are expanded and include:
- Golf course water features
and irrigation;
- Irrigation of public and private spaces such as parks, greenbelts and schools;
- Indirect potable reuse;
- Aquifer storage and recovery;
- Salt intrusion barriers in coastal communities; and
- Industrial applications, such as cooling towers and boiler feed water.
Q: Why should I consider MBR over secondary effluent filtration?
A: There are several instances where MBR may meet your needs better than secondary effluent filtration. First, if space for expansion is limited at a facility, then MBR technology offers some significant advantages. Existing clarifiers can be replaced or retrofitted with membranes. Because membrane filtration is unaffected by sludge settleability, the activated sludge process can be optimized for biological activity. Typical MBR mixed liquor suspended solids (MLSS) concentrations are 10,000 to 15,000 mg/l, allowing the activated sludge process to treat a higher flow and load in the same tankage.
Second, when the potential users of reclaimed water are not located near the main wastewater treatment plant(s), or installing distribution systems to convey the reclaimed water to the reuse sites is difficult or expensive, MBR technology offers a unique solution. By locating remote MBR facilities near the reclaimed water users, these problems are avoided. The “satellite” or “scalping” plants can extract or “mine” sewage from large trunkline or interceptor sewers, then deliver the treated effluent directly to the users and return the waste activated sludge to the sewer.
Q: Can I expect better quality effluent with MBR technology?
A: Yes. In general, MBR systems can provide higher quality effluent than settled activated sludge followed by membrane filtration. By removing the operational constraint of sludge settleability, the activated sludge process can be operated at very high MLSS concentrations. This allows for higher solids retention time and leads to more complete biological oxidation. MBR effluent looks, smells, and simply is better than conventional secondary effluent. New data suggests that the very high mean cell residence time of the MBR process results in higher removals of difficult-to-treat compounds such as endocrine disrupters and biopersistent pharmaceuticals.
Q: Is an MBR system difficult to operate and maintain?
A: No. An MBR system is a fully automated process. An operator only needs to enter a few key parameters. The computer monitors the process and provides data trends that tell the operator when routine maintenance is needed.
Membrane cleaning is the primary maintenance needed to keep the system operating at peak form. Cleaning the membranes is recommended every four to six months for a short duration (about four to six hours). This task is performed within the membrane tanks (no need to move the membranes to a designated tank) and is fully automated by the push of a button. The MBR process can easily be observed, diagnosed, and controlled from a distance using proven, reliable remote telemetry systems.
Q: How will I know my system is functioning properly?
A: To be effective, all parts of the membrane system must maintain a physical barrier between feed water and filtrate.
A fully automated pressure decay test (PDT) provides online verification of membrane integrity. And independent testing has proven the PDT as more accurate than turbidimeters or particle counters in finding defects. The PDT works by applying backpressure to the permeate (clean water) side of the membrane system and holding it for a prescribed time period. If the pressure loss is low, you know the system is integral and the barrier is valid. If not, a diagnostic test can be conducted to locate and isolate the integrity breach.
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