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62 WINES&VINES January 2017 VIEWPOINT If not, don't despair. Even the federal gov- ernment has had its difficulties, as we shall see. High-pressure sales pitch The most important thing to understand about RO is filter porosity. This is as critical as know- ing your brake from your gas pedal. Membrane porosities are expressed in a number of peculiar ways. Unlike sterile filters measured in microns, the porosity of tangen- tial-flow membranes is typically expressed as the molecular weight (in daltons) of a mole- cule that will be rejected by the membrane half the time. For example, a 180-dalton membrane working on a 20% glucose solution (molecular weight, or MW=180) would produce a 10% glucose filtrate. Another way to rate porosity is salt rejec- tion. The U.S. Navy developed RO in the 1960s so sailors could drink seawater. The industry's main focus remains on water ap- plications. A single 8-inch x 40-inch 80-dalton membrane will produce drinkable water (300 ppm sodium chloride) from seawater (30,000 ppm sodium chloride), so the salt rejection is said to be 99%. While salt rejection can be related to dalton ratings, it isn't a very good way for you to shop for your RO membranes, be- cause it doesn't discriminate between poros- ity and craftsmanship. An 8-inch system with 16 elements in series (membrane surface area of 4,800 square feet), which costs about $250,000, can produce 50 gpm of drinkable water (enough to keep a battleship's crew hydrated and happy). Water flows readily through these mem- branes, so tiny imperfections in sealing get diluted and don't affect the permeate salt content very much. A cheap, poorly made membrane can still perform acceptably. But alcohol inhibits flow, so on wine, that same system might yield only 4 gpm of permeate. Bleed-through from tiny perforations or glu- ing defects is instantly apparent, especially in red wine. Unearthing a supplier of high-integrity membranes for our tiny niche market was a major research focus for Vinovation in the 1990s. Manufacturing costs are higher for fine wine filtration than for water applica- tions. There are only a small number of reli- able manufacturers that make dependable membranes for wine. Today, most RO manu- facturers use the membranes we championed (see below). Osmose invers was in experimental use in France in the 1980s for juice concentration, essential to remove rainwater by employing a membrane that retained all flavor constituents, while H 2 O (MW=18) passed readily. Wine applications are still forbidden in Europe out- side an experimental permit basis. This means that European equipment manufacturers have three decades of experience with RO on juice but originally almost none on wine, though some have acquired good experience in New World markets in recent years. Water, juice and dry wine behave entirely differently. When the feed stream is a 13% alcohol wine, the alcohol seems to form a gel on the surface, decreasing the flow about 15- fold. That not only means you need a lot more machine to get anything done, but its optimal design is very different. Juice membranes have similarly slow flow, but for a different reason. Each degree Brix adds about 2 bars to the juice's osmotic pressure, so a 21° Brix juice pushes back with about 40 bars. Only a high-pressure machine, preferably with a 70-80 bar upper limit, has the differential pressure to con- centrate juice. A 40-bar (600 psi) machine won't work at all. As the juice is concentrated and the Brix climbs, the flow declines accordingly. The practical limit for a 72-bar machine (1,000 psi) is about 35° Brix. In these circumstances, a machine you can occasionally push to 80 bars can be a godsend. The other problem with juice is pectin and pulp, which will clog a membrane and require cleaning with warm water and pectinolytic enzymes. (Important tip: It's critical to remove pectin before raising pH for phenolic cleaning, lest precipitated pectin permanently seal your membrane surfaces.) If pulp is present, membrane elements need wider spacers to prevent plugging. This de- creases the amount of surface area that can be included in the spiral wind by 30%. The in- creased open cross-section also increases the pump flow necessary to achieve turbulence. The nice thing about juice is that its aro- matic elements are largely tied up as glycones. That means they are much bigger than they will be after fermentation liberates them. Juice membranes can be more porous. That's why many European manufacturers install highly porous membranes, with unfortunate conse- quences for wine applications. In practice, the winery should decide whether the machine is to be used primarily for juice or wine. If both are important, pump speed adjustment should be built in. Separate banks of membranes may be an advisable purchase, in which case designs that accom- modate easy switching of multiple banks would be preferred. You don't need wider spacers if you clarify juice prior to processing to less than 0.5% solids via settling, centrifugation or flotation. Since clarifying juice is an unwelcome distrac- tion during crush, it's great to work on a small volume. If you desire a Brix bump from, say, 21° to 23° Brix, you can clarify 20% of your juice and squeeze it to 31° Brix, then recom- A 90-foot-long sheet of membrane 40 inches wide is folded back on itself into 14 leaves arranged around a permeate collection tube like the spokes of a wheel. The edges are sealed with glue, and the whole thing is twisted to create a cylinder 8 inches in diameter resembling a giant roll of paper towels. Mesh spacers between the leaves provide channels for the 40 gpm feed stream to flow turbulently across 300 square feet of membrane. Adding filters is an affordable way to get more for your buck. You should also rig for unattended operation using a properly designed fail-safe loss-prevention system that can double your throughput. Perforated collection tube Anti-telescoping device Permeate out Feedwater Feed flow across spacer Permeate carrier Covering & bypass spacer Feed-channel spacer Membranes Permeate flows spirally toward collection tube