The requirement for more stringent control of water quality from reverse osmosis water treatment facilities is being seen more and more in municipal potable water systems. In addition to the traditional goal of salt removal, today’s standards call for a stable finished water that does not cause adverse distribution system reactions due to the changed water quality created by the addition of a reverse osmosis plant. Water quality standards are set for calcium hardness, alkalinity and pH to achieve a water stable with regard to calcium carbonate precipitation (i.e. Langelier Index = 0). A water stable with regard to calcium carbonate will minimize corrosion of cement lined water mains and may allow a protective coating of calcium carbonate on metal pipe surfaces. A combination of limits on pH and minimum alkalinity concentrations are established to provide appropriate pH buffering to guard against undesirable pH changes in the distribution system. Minimum calcium limits can be used in cases where irrigation could be limited by the elevated sodium concentration of the reverse osmosis permeate. The Soil Adsorption Ratio (SAR) is directly related to the sodium concentration and inversely related to the square root of the calcium concentration. Post treatment of reverse osmosis permeate to produce a finished water quality stable with respect to calcium carbonate formation (i.e. Langelier Index = 0); a pH and alkalinity properly adjusted; and appropriate concentrations of calcium and alkalinity necessary to irrigation water quality goals with respect to these parameters as well as sodium and chloride concentration control requirements. Both the calcite treatment system and the calcium hydroxide treatment system are capable of meeting these post treatment goals.
This presentation is available to AMTA Members only.
- Glenn Dunkelberger, PE, D.E.E.
- Reiss Engineering, Inc.
- AMTA Annual Meeting, San Diego, CA
- San Diego Biennial
- Reverse Osmosis, LSI, Post Treatment