Langlier Saturation Index: A Forgotten Gem

I am frequently amazed by how many water treatment professionals have never heard, much less used, the LSI to determine the water characteristics of their systems. Essentially, the calculation indicates whether or not the water will tend to form scale or be corrosive.

Scale can be problematic for most systems; it reduces flow through pipes. It can dramatically reduce the efficiency in heat exchangers; 1/16” scale reduces heat transfer by about 10%. But it’s not all bad; a nice layer of scale protects the public from the lead pipes still in service.

Corrosion on the other hand is generally a bad thing. It increases the lead and copper concentration in the drinking water. We all know about Flint. Corrosion reduces the life of the equipment the water contacts.

In the 1930s a professor, Wilfred Langlier, came up with a formula to determine if the water would cause deposits inside pipes or corrode them. He did this for a large municipal water treatment plant, which is interesting because very few of the plants I work with use the calculation. It is common, however, in cooling tower water treatment. To do the LSI calculation you need to know 4 things: the pH of the water, the TDS of the water in mg/l, the calcium hardness of the water reported as CaCO3, the m-alkalinity, and the water temperature in Celsius.

The LSI formula is:

LSI=pH-pHs

pHs= (9.3+A+B)-(C+D) where:

A=(log(TDS)-1)/10

B=(-13.12log(Temp+273)+34.55

C=log(CaH)-0.4

D=log(mAlk)

The pHs is the pH at which the water is balanced and will neither scale nor corrode. If the number is greater than 0, the water will tend to form scale. The higher the number, the more scaling the water becomes. Conversely, numbers lower than 0 indicate the water will be corrosive. The lower the number the more corrosive the water will be.

Back in the old days we had to do this calculation by hand. I even have a slide rule for these calculations. Today you can go on any one of a number of websites and simply plug in your numbers and the result is calculated for you. These same websites will have various interpretations of what the numbers mean.

Basically I like my municipal plants to have an LSI value of 0-0.5. This water is slightly scaling and I’ve never had a lead or copper issue where this was the case. If you can run in this range most of the time and occasionally dip in the 0-(-0.5) range, you can remove some of the scale deposit.

Below is the LSI for treated Flint River water during the crisis:

The EPA has a test to determine the where or not the water will tend to be corrosive or scaling called the ‘marble test’. I have compared the LSI calculation with the marble test results and found them to track quite well.

A few years ago I was contacted by a large engineering firm. They were working on a groundwater remediation project to remove VOCs from a leaking underground gasoline storage tank. They had a very nicely designed treatment system with various aerators and filters. The problem was that after a few hours of operation the system scaled up and required several days of maintenance to remove the scale. I asked for a water analysis and they had never had one done! The appropriate tests were ordered and when the results were available I ran the LSI calculation. The water was severely scaling! The use of a scale inhibitor, injected at the start of the system, kept things clean and operating well.

The LSI is a useful tool. Try it out!