Standard Practice for Calculation and Adjustment of the Langelier Saturation Index for Reverse Osmosis (Withdrawn 2019)

In the design and operation of reverse osmosis installations, it is important to predict the calcium carbonate scaling properties of the concentrate stream. Because of the increase in total dissolved solids in the concentrate stream and the difference in passages for calcium ion, bicarbonate ion, and free CO₂, the calcium carbonate scaling properties of the concentrate stream will generally be quite different from those of the feed solution. This practice permits the calculation of the Langelier Saturation Index for the concentrate stream from the feed water analyses and the reverse osmosis operating parameters.

A positive Langelier Saturation Index indicates the tendency to form a calcium carbonate scale, which can be damaging to reverse osmosis performance. This practice gives various procedures for the adjustment of the Langelier saturation index.

The tendency to form CaCo₃ scale can be suppressed by the addition of antiscalents or crystal modifiers. Suppliers of antisealents and crystal modifiers can provide information on the scale inhibition peformance of these types of chemical. Their use may be appropriate for reducing scale formation in RO systems. The RO system supplier should be consulted prior to the use of antisealents and crystal modifiers to ensure they will not have a negative impact on the RO system.