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60 WINES&VINES January 2017 VIEWPOINT struggling. I love our industry because every- one in it "gets" the spiritual appeal of wine, often sacrificing more lucrative career options in its service. While I intend to acquaint you with some suppliers' positions I find mis- guided, I do so with respect and in the hope that better informed customers will encour- age them to shift their strategies. I also have no monopoly on the truth. In sharing my personal perspective, I hope to benefit both wineries and suppliers by illumi- nating the purchase process. My intention here is to spell out every use- ful distinction in purchasing and operating an RO machine. I include every technical nuance and trade secret I have found valuable in a quarter-century of experience with tens of thousands of processing jobs. I will share my mistakes, and I've made plenty that I hope you won't repeat. Each winery has its own unique needs and will have different applications it considers most important when selecting equipment. The winemaker must consider filter porosity, pump configuration, dead volume, operating pressure, automatic operation and resin sourcing. Besides system cost, the selection process should weigh tangential flow rates and operating pressure against limitations such as watt draw, heat generation, noise and refrigeration requirements. It is easy to buy the wrong machine. As with any emerging technology, buyers have a very poor understanding of high performance. Imag- ine trying to buy a car without knowing the difference between a Yugo and a Lamborghini. When it comes to RO, your typical winemaker understands the desired result but not the prac- tice—why you want one, but not how it works or what system suits your specific situation. RO purchasers face several hindrances to smart shopping. Just as with cars, commercial suppliers can't be expected to provide an unbi- ased comparison with their competitors. They also may, in the same way as printer manufac- turers, guard as proprietary some features of the systems they are selling in order to retain purchasers for after-market replaceables. For winemakers, a second barrier to techni- cal learning is self-doubt. The anti-manipula- tion views of the natural wine movement, in the faith that wine makes itself, have created an atmosphere in which it's challenging to joyfully embrace new technologies. For many of us, our younger selves are the most insidious of enemies. Many flinch at re- minders of our embarrassing high school chem- istry experiences. If you doubt that you may be your own worst enemy, here's a simple test. Can you define what reverse osmosis actually means? RO IS A CROSS-FLOW TECHNOLOGY W hat does tangential flow filtration mean? Many of you are likely more familiar with perpendicular or "dead-end" filtration, in which a fluid simply flows through a porous membrane that retains any solids larger than its pores. Those solids deposit on the surface until all the pores are plugged and the surface is blinded, at which time the filter is replaced. This works pretty well for traditional wine filtrations to ac- complish clarification and exclusion of yeast and bacteria. If there were some way to scrub the surface to keep the solids moving, there would be no reason to stop. In the 1950s, we began to explore filtrations at the sub-micron level and below. At tighter porosities, constant scrubbing is required. To achieve this, the feed is pumped at great speed across the mem- brane surface to create a turbulent scouring action that constantly cleans the filter. The minor flow that passes through the filter is called "permeate." Ultrafilters exclude not only particulates but also suspended col- loids such as those containing proteins, tannins and pigments. Reverse osmosis filters are about 10,000 times tighter than conven- tional sterile filters and can retain dissolved sugars, salts and aroma molecules, ideally producing a permeate that is devoid of color and flavor. These are all crossflow technologies. Cross-flow clarification, con- fusingly abbreviated to "cross flow," is the loosest type (about the size of sterile filters) and ideally passes all flavor, color and colloids into the permeate while removing particulates. Cross-flow applications are deployed in thousands of industries and have revolutionized water purification, cheese making, paint manufacture and pharmaceuticals production. The hilarious history of crossflow's introduction to the wine industry is chronicled in my other aricles The Crossflow Comix and The Crossflow Manifesto, which contains a table of the various cross- flow filter formats that include spiral-wound, tubular, ceramic, hol- low fiber, scraped disk, vibrating disk and others. For wine applications of reverse osmosis, the pressures involved, low cost and high surface area required restrict us almost entirely to spiral-wound filters. The salient feature of an RO machine is that it retains flavor, color, tannin and vinous character. Simply put, an RO device is a flavor-proof mem- brane. That's how I explain the process to a sommelier or wine lover, but if you're spending six figures, you need greater depth of understanding. The 1996 International Union of Pure and Applied Chemistry recom- mendations defined RO as a "liquid-phase pressure-driven separa- tion process in which applied transmembrane pressure causes selec- tive movement of solvent against its osmotic pressure difference." If you construct a box with two chambers separated by a mem- brane of porosity in the RO range and place 20° Brix juice on one side and water on the other, the water will migrate into the juice with great osmotic force. It takes about 600 psi of pressure (or a juice column a quarter-mile high) to prevent this flow. We can even compel the water to flow backward out of the juice if we pressurize the juice side even more. About 1,000 psi of pressure is necessary for reasonable flow. 0.001 0.01 0.1 1 10 100 Particle or molecular diameter dp [µm] Operative range of pressure driven membrane processes: The filtration spectrum. Increasing operating pressure signifies increasing hydraulic resistance of the barriers employed. Note overlap of nano and reverse osmosis. Nano begins around 1nm, or 500 daltons. Source: Karl W. Böddeker. "Liquid Separations with Membranes." 100 10 1 Pressure difference ∆p [bar] Reverse osmosis Nano- filtration Filtration Microfiltration Ultrafiltration Saline solutions Viruses Bacteria Macromolecules Yeast cells