LSI (Langlier Saturation Index): - LSI is a process of reporting the scaling or corrosive potential of RO membrane feed water. LSI is important to RO feed water in determining whether a water is corrosive (has a negative LSI) or will tend to scale calcium carbonate (has a positive LSI). The LSI value is calculated by subtracting the calculated pH of saturation of calcium carbonate from the actual feed pH.
The solubility of calcium carbonate decreases with the increase of temperature, higher pH, higher calcium concentration, and higher alkalinity levels. The LSI value can be lowered down to the desired value by reducing pH by the injection of an acid (typically sulfuric or hydrochloric) into the RO feed water.
A polymer-based antiscalant can also be injected to inhibit the precipitation of calcium carbonate. Chemtex also provides a LSI calculator upon request to evaluate the LSI value of the RO feed water so that a better antiscalant can be chosen for use in the feed water.
The indications for the LSI are based on the following values:
| LSI | Indication |
| LSI<0 | Corrosive in nature - water is under saturated with respect to calcium carbonate. Under saturated water has a tendency to remove existing calcium carbonate protective coatings in pipelines and equipment. |
| LSI=0 | Water is considered to be neutral. Neither scale-forming nor corrosive in nature. |
| LSI>0 | Scale forming in nature - water is super saturated with respect to calcium carbonate (CaCO3) and scale forming may occur. |
SDI (Silt Density Index): - Silt Density Index (SDI) is an empirical test developed for measuring the rate of fouling of nano filtration (NF) or RO membranes. No direct correlation exists between turbidity of a water stream and it’s SDI. This is found from experiments that water with a very low turbidity (< 1 NTU) may have a high SDI value.
Calculations: - SDI = P30 /Tt = 100*(1 - Ti/Tf) / Tt
Where, SDI= Silt Density Index
P30 = % plug gage at 30 psi feed pressure **
Tt = Total test time in minutes (usually 15 minutes, but may be less if 75% plug gage** occurs in less than 15 minutes).
Ti = Initial time in seconds required to obtain sample.
Tf = Time required to obtain sample after 15 minutes (or less).
Note: - - Time to collect 500 ml should be approximately 5 times greater than the time to collect 100 ml. If 500 ml collection time is much greater than 5X, SDI should be calculated using 100 ml collection times.
- For accurate SDI measurements, P30 should not exceed 70%. If P30 exceeds this value, re-run the test and obtain Tf at a shorter time (T).
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