Membrane Separation—Things to Know
Greg Reyneke, MWS
In the water quality improvement industry, membranes are simply defined as physical barriers that separate solutions and allow passage of contaminants within a certain range of size, molecular mass or even charge polarity and strength. When driving pressure is applied, certain contaminants will be selectively rejected or concentrated by the membrane (depending on membrane material[s], pore size and electrical charge), while water and unrejected contaminants will pass through as a permeate stream. Membrane separation technology is an invaluable resource for residential, commercial and industrial water treatment applications.
Separation membranes are typically operated in dead-end or cross-flow configuration. In dead-end configuration, the rejected contaminants concentrate into the influent stream and eventually accumulate against the surface and pores of the membrane. Naturally, the concentrated contaminants will inevitably clog the membrane pores entirely, so this process is reserved for applications where the cost/inconvenience of replacing fouled membrane elements (or modules) is less important than losing any of the raw fluid or where the engineered process-flow design specifically calls for it. A growing segment of dead-end configuration is back-washable tubular or hollow-fiber construction, which is significantly less sensitive to fouling than spiral-wound elements. Look for increasing market penetration of this technology as the manufacture of these tubes becomes more reliable and cost-effective.
In cross-flow filtration, the membrane geometry is designed for contaminants to be scrubbed away from the membrane surface when the concentrated discharge stream is passed to drain or a secondary process. Leveraging the principles of Fick’s laws of diffusion, designers can manipulate macromolecule concentration molecules at the membrane surface as a function of the velocity of fluid that is flowing parallel to it.
Membrane materials and element construction
Cross-flow technology is cost-effective and practical due to durable organic polymeric materials. Those who have been around the industry for a few years will remember cellulose acetate and cellulose triacetate (CTA) RO membranes that were once ubiquitous. A game-changer popularized in the mid-1990s was thin film composite (TFC) RO elements, which lowered total cost of ownership and increased flux at lower driving pressures. Composite membranes can be made from a number of materials, such as polyethersulfone (PES), polysulfone (PSU), polyphenylsulfone (PPSU), polytetrafluoroethylene (PTFE), polyvinyldene fluoride (PVDF) and even polypropylene (PP). The vast majority of installations these days utilize composite membranes, while other materials such as ceramics made from silica, aluminum, titanium and other materials are only used where pH, temperature, abrasiveness, cleaning chemistry or other operational parameters prohibit the use of polymers.
Polymeric membranes can be manufactured symmetrically or asymmetrically, depending on the intended use. Contrary to popular belief, there are many ways to build a cross-flow membrane, including type of polymer, length of membrane leaves, membrane support configuration and membrane density. These configuration options are significant in mission-critical operations and also important when selecting regular water filtration membranes upon which you may stake your reputation.
Reverse osmosis (RO)
Sometimes called hyperfiltration, RO is the finest form of filtration used today. The membrane pores are small enough to enable the reversal of osmotic pressure through ionic diffusion when sufficient external energy (pumping pressure) is applied. This reversal of osmotic pressure actually drives pure water away from molecular contaminants and enables processes like seawater desalination (where sodium ions are physically removed from water), greening the desert and bringing clean, safe drinking water to places where it was previously impractical. RO is also used industrially in many innovative applications, such as concentrating fruit juice, concentrating whey protein and of course, wastewater sludge dewatering.
Developed as an extension of RO, NF functions according to the same principles of ionic diffusion as RO, but with a pore-size configuration and slight surface charge that rejects multivalent salts and larger organics. Monovalent ions (such as sodium and potassium) pass right through an NF membrane, allowing it to be used as a highly effective salt-free softening technology without the complications of RO. NF is also highly effective at addressing semi-volatile organics (such as pesticides) and removing color from water.
Ultrafiltration is a true physical exclusion process and doesn’t rely on osmotic principles. UF membranes are categorized by their molecular weight cut-off rating (MWCO). The typical range of MWCOs for UF is from 1,000 to 1,000,000 Dalton, which correlates to approximately 0.005-0.1 micron (µm). UF is extremely effective in removing suspended solids, colloids, bacteria, virus, cysts and high molecular weight organics like tannins. UF membranes are operated in dead-end configuration, occasional flush (forward flush and/or backflush) or cross-flow configuration. Membrane configuration can vary between manufacturers, but the hollow-fiber type is the most commonly used. Membranes of the hollow-fiber type are made into small diameter tubes or straws. Thousands of these straws are bundled together and the ends are bonded/potted into an epoxy bulkhead. The bundles are then sealed into a housing, which is usually PVC, fiberglass or stainless steel. The sealed potting creates a separate, sealed space that isolates access to the inside of the fibers from the outside. This membrane and housing combination is called a module. Many UF membrane assemblies on the market are certified for log reduction of pathogens in drinking water (such as bacteria and viruses), enabling dealers to provide safe drinking water more cost-effectively and efficiently than ever before.
Microfiltration technology has been successfully employed in both cross-filtration spiral-wound, occasional-flush hollow-fiber (forward-flush and/or back-flush) and dead-end plate and frame configurations, depending on the nature of the application and space available. This membrane technology typically has an exclusion size of 0.2-1 µm and is very well suited for the removal of particulates, turbidity, suspended solids, as well as certain pathogens such as Cryptosporidium and Giardia. MF has an established industrial track record for sterile clarification of wine and beer, whey concentration and fruit-juice sterilization. In the wastewater treatment field, microfiltration is invaluable for dewatering flocculant sludge and economically lowering BOD and COD in discharge streams. Microfiltration is also extremely effective in protecting other downstream membrane separators.
Pretreatment and maintenance
It is vitally important to select the appropriate pretreatment for the membrane separation process being used. Composite polymeric membranes are very sensitive to oxidative damage, so special care should be taken to ensure that chlorine, ozone and other oxidative disinfectants are removed from the raw water before being processed by the membrane. Careful consideration should also be given to macroparticles and organic/inorganic contaminants in the water stream that could affect proper membrane function. As a good general rule, the smaller the pore size, the greater amount of physical pretreatment required to ensure long runtimes and economic operation.
Regardless of the membrane pore size, operational fouling is almost inevitable, even with pretreatment. The types and amounts of fouling are dependent on many different factors, such as feedwater chemistry, membrane type, membrane materials and operational process. The most common types of membrane fouling are scale precipitation and biofouling. Fouling causes a decrease in flux (passage of purified water), which in turn requires greater pressure against the membrane to produce a satisfactory permeate flowrate. As fouling worsens, the increased pressure (energy) requirement will cause the operating cost to increase significantly and possibly even blind the membrane completely, leading to significant damage and operational failure.
Many innovative membrane antiscalant, metal sequestering, microflocculating and clean-in-place (CIP) compounds are available to help keep membrane(s) clean and operating as energy efficiently as possible. Some good questions to ask your supplier before using a pretreatment chemical:
- Is it 100-percent compatible with the membrane(s) in the application?
- Can it react adversely with any other contaminants in the feed water, like iron, heavy metals and other inhibitor chemicals?
- What are the recommended dosing rates and maximum dosing rates?
- What are the projected limits of solubility for individual scaling and fouling components?
- Are there any special discharge/reclamation regulations or concerns?
- Do you offer technical support services, like water testing, chemical analyses and membrane autopsies?
Emerging contaminants have been in our water for a long time and we only now have the equipment and methods for detecting these low levels of contaminants. More than seven million recognized chemical compounds are known to be in existence and approximately 80,000 of them are in common use worldwide. Regulatory agencies have no idea exactly how many specific compounds are used in consumer goods, nor what specific combinations are used in each product. There are also a host of secondary compounds that form when chemicals are exposed to oxidizers and other reagents. In the 21st century, it is safe to say that all water everywhere is contaminated with some kind of man-made chemical at some level.
Near the end of the last century, evidence began to accumulate that chemicals such as pesticides, surfactants (used in detergents) and synthetic birth-control drugs were causing skewed sex ratios, reproductive disorders, as well as population declines in frogs, alligators and fish. Some researchers have suggested that antibiotics and antimicrobials can pose a serious threat to human health by enhancing the antibiotic resistance of disease-causing microorganisms due to their (over)use in products such as soaps, mouthwash, toothpaste and of course, the ubiquitous hand sanitizer. Membrane separations are proving to be invaluable in the fight against emerging contaminants, since RO technology paired with intelligent oxidation and effective absoption/adsoption can be highly effective in rejecting organo-synthetic molecules from water.
As we continue to see more misguided attempts at softener bans and a continued emphasis on minimizing our net environmental impact, the need for more environmentally friendly water quality improvement technologies becomes glaringly apparent. Membrane separations are highly effective in reducing chloride discharge and in chemical requirements. Many dealers are using NF and blended RO systems as salt-free softeners, providing their clients with water that has hardness levels below 17.1 mg/L (one grain per gallon). New innovative materials like synthetic ceramics and polymers as well as enhanced spacers designs and uniform thin-film fabrication techniques allow for continuous improvements in membrane longevity, reduction in driving energy requirements and minimization of fouling, while continuing to lower the initial acquisition cost and total cost of ownership.
Environmentally friendly technology
Some well-meaning but misinformed people accuse membrane separation systems of being wasteful, since water is used to clean the membrane(s) during operation. I disagree with the negative description of drain-concentrate water as wasted water, since it really is not. Saying that a membrane separator wastes water is no different than saying that a tree dropping unpicked fruit is wasteful. The fruit returns nutrients to the earth and feeds the tree, which then grows more fruit. Discharge water from a membrane separation system is also not lost forever; it will return through the building’s drainage system to a municipal plant or back to the earth in an off-grid application or can be repurposed onsite by design.
We obviously can’t ignore the opportunity-cost of water though, since it must be cleaned, stored, treated, pressurized and distributed before it enters the membrane separator. Since the discharge from a potable water membrane separator is also sanitary potable water (this is obviously not considered wastewater, since it is never in contact with soils, dirt or biological contaminants; it is merely concentrated clean water), much of this opportunity-cost can be recovered through innovative reuse techniques such as graywater recovery, blending with harvested rainwater, repurposing in secondary process or used as landscape irrigation. Recent innovations in concentrate energy recovery and storage are also helpful in demonstrating how truly efficient membrane separation technology can be.
Properly implemented membrane separation systems are a truly environmentally friendly method to improve water quality and an invaluable tool for water quality improvement professionals. It is critically important that you either secure the education you need to ensure proper system selection, design and deployment or work with vendors who you can rely on to help before getting yourself into trouble by making uninformed decisions. WQA’s Modular Education Program (MEP) offers a great starting point for learning more about RO and other membrane separation technologies to help you do a better job.
Image credit: The Innovative Water Project
Greg Reyneke, Managing Director at Red Fox Advisors, has two decades of experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial applications, including wastewater treatment. In addition, Reyneke also consults on water conservation and reuse methods, including rainwater harvesting, aquatic ecosystems, greywater reuse and water-efficient design. He is a member of the WC&P Technical Review Committee and currently serves on the PWQA Board of Directors, chairing the Technical and Education Committee. You can follow him on his blog at www.gregknowswater.com