LSI is a formula that gives ...
Published by Pat Stanford, General Manager
LSI is a formula that gives you the probability of calcium carbonate precipitating in a given water stream It is based on Calcium, TDS, alkalinity, pH and temperature. A positive result means that the water will tend to precipitate calcium carbonate. a negative results means the water will tend to dissolve calcium carbonate or be acidic. Normally in distribution systems you want to adjust the RO permeate enough to have a slightly positive LSI. LSI is very good for RO permeate as it is really only applicable up to about 5000 mg/l TDS, so it is not good for seawater RO feed calculations.
If your calcite is taking the pH to 9.1, I don't think it is pure calcite. Calcite (Calcium Carbonate) should have an equilibrium pH of about 8.3. If the pH is going higher, then there is probably some calcium oxide or sodium hydroxide in the mix to take the pH higher. Talk to you supplier about the analysis of what they are selling you. If you can't get a true calcite, you could bypass part of the permeate around the calcite bed and then blend them back together to get the pH you are looking for.
Yes, RO Permeate needs to be adjusted before going into the distribution system. Straight off the membranes it is probably at a pH of 5.5-6.5. If you put this pH in steel or copper pipes they will dissolved.
Here is California, alkaline water is very popular, with entire stores specializing in selling bottles of alkaline water. 9 is very high and should be brought down, but I doubt you will have much in the way of health risks at a pH of 8-8.3.
If you get the pH up to 9, you are probably putting in two much calcium and have a potential to precipitate calcium carbonate on the downstream piping.
2 Comments
Hi Pat.
I appreciate your knowledge and help.
I will look into the manufacturer of the calcite and get an appropriate indication of what it contains.
From a new operators point of view, who has only been operating full time in the industry for 4 years, i am controlling pH for disinfection. In your opinion is this correct ?. The 4 work sites i deliver to have back up chlorine dosing, however the island public do not have any form of backup/monitoring/vermin proofing and the infrastructure has seen better days. Hence this is my reason why i am controlling pH, corrosion in a steel tank is a distant second. I was instructed to have my 2x 300,000 ltr concrete storage tanks maintained at a chlorine ppm of 0.30ppm with the resultant pH hovering around high 8-9.10 was the highest i saw. I didnt like this so i immediately began to control pH with the view to keep it around 7-7.5.
No alkalinity is tested for. I believe there is much more testing i can do to confirm the water quality, but for reasons unknown to me i have been instructed not too.
Published by Chris Riley, Water/Waste water treatment operator/Plumber at Broadspectrum
SIR,LSI means tendency calcium carbonate scaling.LSI is -0.2 means /-0.2 below the point of calcium carbonate saturation.
In your case please concentrate on Prechlorination and pretreatment with antiscalant.
Published by MOHAMMED NAZEER, Lead role in GE Power water
1 Comment
Hi Mohammed.
Thankyou for your help.
We have antiscalant, pre membranes, but we dont prechlorinate. We currently have no means to remove chlorine before the membranes.
My pH control is purely from a disinfection and public protection point of view.
Published by Chris Riley, Water/Waste water treatment operator/Plumber at Broadspectrum