Membrane Separation—More than Just RO
By Greg Reyneke, MWS
Membrane separation is finally reaching a level of general acceptance as the powerful tool that it is for residential, commercial and industrial water treatment. Most dealers and users know about reverse osmosis separation technology, but there are other technologies that you need to know more about. Membranes are physical barriers that selectively allow passage of contaminants up to a certain size, molecular mass or even charge polarity and strength. Depending on the material, pore size and electrical charge of the membrane, certain waterborne contaminants will be selectively rejected or concentrated by the membrane, while water and unrejected contaminants will pass through as the permeate stream.
Membranes are either operated in dead-end or crossflow configuration. In dead-end configuration, the rejected contaminants concentrate into the influent stream and eventually accumulate against the pores of the membrane. Naturally, the concentrated contaminants can potentially clog the membrane pores entirely, so this process is reserved for applications where the cost/inconvenience of replacing fouled membrane sheets is less important than losing any of the raw fluid, or where the process flow design specifically calls for it.
In crossflow filtration, the membrane geometry is such that contaminants are scrubbed away from the membrane surface when the concentrated discharge stream is passed to drain or a secondary recovery/reuse process. Leveraging the principles of Fick’s laws of diffusion, designers are able to manipulate the concentration of macromolecules at the membrane surface as a function of the velocity of fluid that is flowing parallel to it. Crossflow technology is cost-effective and practical due to durable organic polymeric materials. The vast majority of crossflow installations these days utilize polymer-based membranes, while inorganic materials (such as ceramics) are only specified in unique circumstances where pH, temperature or cleaning chemistry prohibit the use of polymeric membranes. Contrary to popular belief, there are many ways to build a crossflow membrane, including type of polymer, length of membrane leaves, membrane support configuration and membrane density. These configuration options are critical in mission-critical operations, but also important when selecting regular water-filtration membranes on which you are willing to 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 recently as an extension of RO, NF functions according to the same principles of ionic diffusion as RO, but with a pore configuration that allows passage of all contaminants except divalent and larger ions. 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 and removing color from water.
Ultrafiltration is a true physical exclusion process and not reliant upon 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 Daltons, which correlates to approximately 0.005 to 0.1 micron (µm). UF is extremely effective in removing suspended solids, colloids, bacteria, viruses, cysts and high molecular weight organics like tannins. UF membranes are operated in dead-end configuration, occasional flush (forward flush and/or backflush) or crossflow configuration. Membrane configuration can vary between manufacturers, but the hollow-fiber type is the most commonly used. Membranes in this category are cast 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, usually PVC 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. A number of UF membrane assemblies on the market are BioVir-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 is deployed in both crossfiltration spiral-wound, occasional flush hollow-fiber (forward flush and/or backflush), and dead-end plate-and-frame configurations to great success, depending on the nature of the application. This membrane technology typically has an exclusion size of 0.2µm to 1 µm and is very well suited for the removal of particulates, turbidity, suspended solids and 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 extremely effective in protecting other downstream membrane separators and should be used frequently.
It is very important to select the appropriate pretreatment for any membrane separation process being used. Polymeric membranes are sensitive to oxidative damage, so special care should be taken to ensure that chlorine and other oxidative disinfectants are not present in the water being processed. Careful consideration should also be given to macro particles 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 or pretreatment required to ensure long run times and economic operation.
Regardless of the membrane pore size, operational fouling is almost inevitable, even with adequate pretreatment. The types and amounts of fouling are dependent on many different factors, such as feed-water quality, membrane type, membrane materials and process design and control. Scale precipitation and biofouling are the most common types of membrane fouling. Fouling causes a decrease in flux, which in turn requires greater pressure against the membrane to produce a satisfactory permeate flowrate. As fouling becomes worse, the increased pressure (energy) requirement will cause the operating cost to increase significantly and possibly even blind the membrane completely.
If you haven’t realized by now, 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 you consider the options before getting yourself in trouble through an uninformed decision. Some well-meaning but misinformed people denigrate membrane separation systems as 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 akin to saying that an apple tree dropping its unpicked fruit is wasteful. The fruit returns nutrients to the earth and feeds the tree, which then grows more fruit.
Discharge water from a potable water membrane separation system is also not lost forever; it will return through a building’s drainage system to a municipal plant or back to the earth in an off-grid application. We naturally can’t be blind to the opportunity cost of the water though, since it has to 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), the opportunity cost of the discharge can be recovered through innovative reuse techniques, such as graywater recovery, blending with harvested rainwater, re-purposing in secondary process or used as landscape irrigation. Rest assured that membrane separations are an environmentally friendly technology and another valuable tool in lowering our environmental impact.
The future is bright for membrane separations; costs are falling, flux rates are improving and there are many manufacturers hoping to earn your business. Talk to your OEM or business coach about how you can start incorporating more membrane separation technologies into your dealership this year.
About the author
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 Committe. You can follow him on his blog at www.gregknowswater.com