Rethinking Reverse Osmosis
By Greg S. Reyneke, CWS-VI
At WQA Aquatech 2014, two good friends of mine received well-deserved awards for their long-time and selfless service to the water treatment industry. C.F. ‘Chubb’ Michaud and Robert Slovak have given much to our ever-improving industry. While listening to Slovak’s acceptance speech, I began to think about how far residential reverse osmosis drinking water purification has come in the last 30 years and how we can save the planet by installing RO purifiers.
While small-scale reverse osmosis purifiers had been available since the 1960s, Slovak and his brother introduced affordable, fresh-squeezed RO water to residential consumers in the US during the ‘Me!’ era of the 1980s. During that decade, regular gasoline cost about a dollar per gallon, and few people gave any thought to how important it would be to provide affordable purified water to all homes. The first residential RO systems leveraged proven membrane separation technology that had already been used in industrial and military applications. The axial-wound cellulose tri-acetate (CTA) membrane allowed for a compact form-factor. An automatic shut-off valve reduced waste and a hydro-pneumatic storage tank allowed for simple storage and re-pressurization of purified water. The average system produced a whopping 17 gallons (64 liters) per day, at an average recovery rate of about 10 percent (compared to some of today’s systems that produce 100+ gallons [378 liters] per day at recovery rates in excess of 70 percent, without booster pumps). Competition quickly spurred innovation and many enhancements to the art trickled into the marketplace, such as thin-film composite (TFC) membranes, quick-change filter housings and water-on-water tanks.
Now, as we approach the middle of the second decade of the 21st century, the global water crisis is well underway, with no conceivable end in sight. Whether you choose a pragmatic or emotional view of water scarcity, a clear conclusion is that we must be more careful in preserving precious, clean-water resources.
I’d be remiss if we didn’t take a moment to discuss how a typical residential under-counter RO system operates. The raw water is passed through multiple filtration stages. The filtration stages will typically include gross particulate filtration, activated carbon absorption/adsorption and fine-particle filtration. The goal is to reduce oxidants and also particulates larger than five microns in size to protect the membrane. A current trend among high-efficiency RO designs is to prefilter using an ultrafilter that protects from submicron solids, as well as microorganisms that could foul the membrane.
After prefiltration, the water is forced against a semi-permeable membrane, where impurities as small as 0.0001 micron in diameter are rejected and flushed to drain as part of the cross-flow effect that helps to keep the membrane pores clean. This cross-flow (or axial flow as some call it) allows membranes to stay cleaner longer, by using the pressurized infuent water to scrub impurities off of the membrane pores and are then discharged through carefully sized spacer channels.
A drain restrictor controls the exact flowrate at which cleaning water is discharged. This discharge rate affects the rate of pure-water production, as well as the projected longevity of the membrane. Smaller drain orifice sizes result in higher concentrate pressure. The higher the influent water pressure, the higher the rate of permeate flux through the membrane, since elevated pressure is able to effectively overcome the trans-membrane pressure requirement, which is a function of the cleanliness and temperature of the influent water; cold, dirty water requires more pressure than warm, clean water. If pressure is raised by restricting the discharge rate below manufacturer’s recommendations, the membrane can foul and fail prematurely. Adding an influent-pressure boosting pump is a standard technique for safely raising the rate of purified water production.
Naturally, since the water is being forced through extremely tiny pores, there is a significant loss of flow and pressure when the purified water is separated by the membrane, so the permeate water usually needs to be stored and re-pressurized before use. Storage can be either pressurized or atmospheric. Atmospheric storage tanks have a greater potential for microbiological contamination, so appropriate precautions should be taken to keep the water clean and safe. Most good RO systems will also include post-storage filtration to further polish the water and remove potential tastes and odors from storage. The final polishing filter is usually a cheap and effective granular activated carbon cartridge that has a long working lifespan and is economical to replace.
Many well-meaning environmental activists denigrate RO systems as wasteful, since water is used to clean the membrane(s) and then discharged to drain. Because of this misinformation, some plumbers and dealers refuse to install them, for fear of contributing further to the rampant wastefulness that is endemic in the US. I don’t like the negative description of drain concentrate water as wastewater, since it really is not. It is far better to discharge water through an efficient RO, than frequently replacing membranes with their associated production footprint. Saying that an RO purifier wastes water is akin to saying that an apple tree dropping its unpicked over ripe fruit is wasteful. The over-ripe fruit returns nutrients to the earth and feeds the tree, which then grows more fruit. Discharge water from an RO is not lost forever and will return through the household drainage system to a municipal plant or back to the earth in an off-grid application. We can’t be blind to the opportunity cost of the purified water though, since it has to be pumped, stored, treated and distributed before it enters the RO appliance. Since the discharge from a reverse osmosis processor is sanitary potable water (this is not considered wastewater, as it is never in contact with soils, dirt or biological contaminants; it is merely concentrated clean water), the opportunity cost of the RO discharge can be recovered through the use of some simple techniques:
RO discharge can be routed to the household greywater processor where it can be stored, pressurized and reused for toilet flushing and other code-approved greywater applications.
If the local climate permits, the RO discharge can be routed directly to the garden with the use of an appropriate air gap. Many dealers will simply pipe the RO discharge into the home’s existing rainwater harvesting system with excellent results.
A lot of dealers that I coach are particularly innovative in the things that they add to their RO installations:
Alkalinity and ORP changing filters
Exotic media cartridges can be added to purified water systems that raise alkalinity and develop a negative ORP in the water. While you may or may not agree with the suggested health-benefits of these products, your client might want it and you should be able to help them.
Flavor concentrate-adding cartridges are available in certain markets that can add a variety of fruit flavors to purified water.
When routing RO water to icemakers, hot water dispensers and beverage makers, one runs the risk of exposing the metallic surfaces to water that could be corrosive. A simple solution is to introduce alkalinity with a food-grade neutralizer cartridge.
Homeowners will often want to use their centralized RO processor for more than just drinking water and ice. A common application is to manifold and separate the drinking fixtures from an aquarium filler that is polished with a mixed-bed deionizer cartridge.
Many homes include water features, and waterfall-style humidifiers. These are inevitably problematic when fed with hard water and even soft water. Reverse osmosis water is becoming the water of choice for these fixtures.
Don’t shy away from recommending reverse osmosis purification systems for your clients. It is still the most economical purification technology out there. In this new millennium, cities have less resources and employees available to clean the water that they provide, and we are continually being informed about a multitude of synthetic and organic emerging contaminants that are in aquifers and rivers. These emerging contaminants are usually most economically removed by a properly designed RO system. Your clients deserve the best water at the very best price. Your dealership also deserves the improved revenue and customer service opportunities from visiting clients to sanitize and service their purification system(s).
Images courtesy of The Innovative Water Project
About the author
Greg S. Reyneke is Managing Partner at Red Fox Advisors, a multidisciplinary research, development and consulting company with a strong emphasis on water, air, microbiology and energy projects. He also serves as an advisor to the ProFlow Dealer Network, a Pentair Platinum Partner and is a member of the WC&P Technical Review Committee.