- c. Calcium carbonate is perhaps the most common and most abundant scale forming from natural waters and as pH and/or temperature rises it does become less soluble in water. But, calcium compounds as well as barium, strontium, silica and silicates, are slightly soluble in most natural waters. Metals, such as ferric iron (Fe3+), manganic manganese (Mn3+ or Mn4+) and aluminum (Al3+), on the other hand, for all practical purposes are insoluble. If present in a RO feed water, for example, prefiltering of these three metals should be practiced to prevent their crystallization and precipitation (fouling) on the RO membrane. It is usually necessary to consider silicate solubilities only if the pH will be greater than eight.
- b. Salt concentrations in RO concentrate can be estimated by the following formula: Concentration factor = concentration polarization ÷ (1-recovery). Concentration polarization in this case is 1.20, i.e., 20 percent greater salt concentration at the membrane surface. Recovery is permeate flow rate ÷ feed flow rate or 10 gpd ÷ 50 gpd or 0.20. Therefore, the concentration, from 1,000 mg/l in the feedwater, of salts at the membrane surface can be estimated to be 1,000 mg/l x [1.20 ÷ (1-0.20)] = 1500 mg/l.
- a. Non-carbonate salts tend to form crystals and precipitate more slowly than do carbonate salts. When a RO system is continually displacing its water, there is often little concern about a non-carbonate salt if it is only on the edge of its solubility. It will likely be washed out of the RO membrane element before a crystal large enough to fall out of suspension can be formed.
- e. For small POU RO systems, prevention of scale formation can often be accomplished by simply operating the system at a low (less than 30 percent) permeate flow recovery to minimize the concentration of salts in the concentrate. The most certain way to prevent calcium carbonate scale is to remove water hardness in the RO feedwater with ion exchange water softening. Acid injection converts bicarbonates to carbon dioxide and also increases the solubility of most metals, including CaCO3, due to the lower pH. Scale inhibitors injected in the RO act to slow the formation of scale.
- c. If water warms in summer months, backflow rates may need to increase to maintain 50 percent bed expansion. And, as water temperature drops during winter, care should be taken to not backwash resin out through the top of the unit.
Certification Action Line – Answers – December 2013
Answers from the December 2013 edition of Certification Action Line.