Part 2 of this two-part series describes how reverse osmosis (RO) treatment helps remove minerals in tap water and makes the permeate stream ideal for consumption or for use in cooking and cleaning. Click here to read part one of the series.
Stagnation’s effect on mineralization
A standard home RO water filter (see Figure 1) typically contains a mineralization cartridge filled with natural (mined, grinded and sieved) calcite (CaCO3) granules 3 to 6 millimeters (mm) in diameter. The typical cartridge volume is approximately 250 milliliters (mL). The cartridge is located at the outlet of the RO purifier, after the RO tank and before the drinking water faucet. Calcite dissolution in water is regulated by the pH and its equilibrium with the carbon dioxide. The dissolution practically stops at the equilibrium of pH 8.5.
CaCO3 + CO2 + H2O <– –> Ca(HCO3)2
The RO membrane permeate contains a scant amount of carbon dioxide and the dissolution rate of calcite is slow. When the RO permeate water flows through the cartridge, a small amount of calcium (Ca) is dissolved. When the RO water becomes stagnate in the cartridge overnight with zero flow, the Ca concentration in water is increased to the equilibrium (10 to 20 ppm of Ca depending on water temperature and other conditions). When the faucet is turned on, water flows though the cartridge, and the initial Ca concentration is 5 to 10 ppm in the first volume, equal to the mineralization cartridge volume. The typical flow rate varies from 0.32 to 0.65 gallons per minute (gpm). After the initial volume of water, Ca concentration drops rapidly (see Table 1).
In classical RO, after stagnation (no flow) the mineralizer contains approximately 125 mL of the Ca salt solution at nearly equilibrium concentrations. The first glass of water (approximately 200 mL) will contain approximately 10 to 15 ppm Ca ions at pH 7.5. After that, the Ca concentration is miniscule. Larger volumes of filtered water contain smaller quantities of Ca ions and a pH smaller than seven.
In water-on-water RO (see Figure 1) with the double mineralization method, the mineralization cartridge with the volume of 600 mL contains granules that are 2 to 4 mm in size of magnesium (Mg) corallite. The Mg corallite granule comprises crystals of MgCa (CO3)2 with the submicron sized inclusions of oxides of Mg and Ca in between the granules. Mg and Ca oxides are dissolved in water on contact, but because of this, the Mg and Ca oxides are slightly porous Mg corallite. The dissolution of small quantities of Mg oxides is limited by diffusion and dissolution of the main crystals. The dissolution and diffusion slow down as soon as the pH exceeds 7.5 to 7.8. In double mineralization, RO water passes slowly through the mineralization cartridge into the collection tank. Water in the tank has pH 6.8 to 7.0 and Mg/Ca mineralization of 0.88 to 1.31 grains per gallon (gpg). When a drinking water faucet is turned on, water flows rapidly through the mineralization cartridge, again increasing the total mineralization to 1.17 to 1.75 gpg.
Worldwide, people are increasingly using home RO water purifiers. One main function of the RO water purifier is to reduce water hardness so that heating water home appliances last longer and drinks are tastier. The second function is to make water safer by removing harmful substances. The first function is equally important in developed and developing countries, while the second function is especially important in developing countries. At the same time, the mineral content of the RO permeate is not highly regulated in the U.S. or the majority of the other countries.
According to the World Health Organization (WHO), the Mg drinking water mineralization level is expected to be more and more important to companies that market RO water purifiers. The manufacturers of the home water RO systems and components will follow the market demand, developing better, more precise and user friendly post-RO mineralizers. The authors believe that the goal of duplicating the bottled water manufacturing process at home will necessitate the development and use of better post-RO mineralizers.
The mineralization results with the Mg corallite mineral featured in this article are another step in that direction as a practical compromise between WHO drinking water recommendations and generally softer water requirements for heating water appliances and for cooking and baking.
Alexey Aksenov is senior research chemist of WestAqua Ou, Aquaphor Group, Narva, Estonia. He is a graduate of the St. Petersburg State Institute of Technology Electrochemistry Department. He may be reached at firstname.lastname@example.org.
Alexander Mitilineo is head of the R&D Chemistry Department of WestAqua Ou, Aquaphor Group, Narva, Estonia. He is a graduate of the St. Peterburg State University Inorganic Chemistry Department. He may be reached at email@example.com.
Alexander Polyakov is vice president, technical sales of WestAqua Ou, Aquaphor Group, Narva, Estonia. He is a graduate of the Leningrad State University Physical Chemistry Department’s Ph.D. program. He may be reached at firstname.lastname@example.org.
Joseph L. Shmidt, CEO of Aquaphor Group, was also co-founder of Membrex Inc. (now a part of Osmonics, GE Water). He earned his Ph.D. and M.S. degrees in chemical engineering from Columbia University. He may be reached at email@example.com.