Alkaline Water and Excessive Alkalinity

Alkaline Water and Excessive Alkalinity. Secondary Mel: pH between 6.5 and 8.5

Alkaline water and excessive alkalinity are not necessarily the same thing. True alkaline waters are rare and occur mostly in desert areas or regions with geologic deposits of “trona,” bauxite, borax and other alkaline ores. Such waters are often undrinkable due to high TDS or salinity, regardless of the pH. Excessive alkalinity may be paired with high hardness, or it may be due to less extreme levels of the same minerals that produce alkaline waters. The notion of excessive alkalinity is relative, also. Generally, more than 250 mg/L alkalinity can be expected to cause problems with lime scale, but levels as low as 80 mg/L are preferred by some producers of “post-mix” beverages requiring carbonated water for soft drinks. That is because any alkalinity in the water will consume some of the acidity from acidulants such as citric acid in the syrup concentrate that give the necessary tartness to the drinks. In addition, alkalinity in the water fights against the carbonation process itself. Thus, commercial dealkalization technology is very important.

All remedies to excessive alkalinity involve adding acid to the water in some way. It is not advisable to add acid to the chlorine solutions being fed for disinfection and/or oxidation, as is often done with alkaline additives used to treat acid conditions, because that would produce toxic chlorine gas in the storage tank before it could be fed. Acids must be dosed separately. One way is with a simple “pot-feeder” cartridge similar to those used for many polyphosphate feeders, except that the material being dissolved and fed is a food-grade acidulant such as citric acid, tartaric acid, malic acid, etc. A portion of the incoming water is directed to the feeder, which dissolves some of the acid to form a concentrated solution which is then metered back into the main stream through an orifice that is sized to produce the proper proportional feed. The ion exchange approach is often preferable, especially if the water is also too hard or has excessive TDS. Dealkalization by ion exchange involves the use of a cationic ion exchange resin in “hydrogen form” (loaded up with H⁺ ion or “acid”): any cations such as the hardness ions (Ca⁺² and Mg⁺²), plus sodium, Na⁺, potassium, K⁺, etc. are exchanged for H⁺ ion, which then immediately neutralizes one H^- equivalent of alkalinity. This may be by direct combination with OH⁺ to make water or the H+ may combine with one or the “alkalinity ions” (carbonate or bicarbonate), driving that equilibrium one step to the left and causing one molecule of CO₂ to bubble away and be lost from the system. Either way, this treatment approach represents a “double-whammy:” not only is the excess alkalinity reduced; an equivalent amount of hardness or sodium ion is also removed, and the overall TDS is reduced as well, A degree of added control can be achieved by using a weakly-acidic cation resin (“WAC” resin) instead of the strong-acid cation resin used for water softening. (See the next section for a full discussion of “strong” and “weak” resins,) The result in this case is that such a resin will remove only the amount of hardness that is “balanced” by alkalinity. For example, if a water has 250 ppm of hardness but only 200 ppm of alkalinity, WAC resin will remove only 200 ppm of the hardness, and the H⁺ that is liberated from the resin will neutralize all of the alkalinity. If the alkalinity level is greater than the hardness level, WAC resin will remove all of the hardness but only an amount of alkalinity equal to the original hardness level. (This kind of arithmetic is permitted only when the hardness and alkalinity values are expressed “as CaCO₃.”)