SWRO Permeate

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I am an operator and have SWRO permeate produced in Equatorial Pacific waters, that isn't getting remineralised/reconditioned.

I am aware of RO permeate not fit for consumption generally, but I have also worked with RO's that generally do remineralise/recondition.

Severe corrosion is occurring in and on the steel storage tanks that are lined internally, but it is difficult to determine whether this is just the seawater salty environment as this island is very small or the RO permeate.

1 out of the 3 SWRO's I operate has a Calcite filter but increases pH to approx 9.10, so acid gets dosed to control pH.

Can anyone give me an easy description of the Langelier Saturation Index and your professional opinion on whether the calcite filter is required, obviously without a feed water analysis? I believe it is. Thanks

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14 Answers

  1.             We are presenting worldwide a new process which relies on room temperature evaporation which is the only physical phenomenon separating pure water out of any saline water WITHOUT THE SUPPLY OF ANY EXTERNAL ENERGY. The energy needed is obtained by cooling the bulk of water (naturalseawaterdesalination.com = manufacturer of desalination equipment).

                Replacing the RO membrane is more expensive than our complete potable water generator. As our process operates at room temperature and low pressure then most of the components are relying on plastic materials avoiding corrosion completely, and the dilemma of reverse osmosis.

     

  3. The Langelier and Ryznar index's are both well accepted as useful primary prediction tools for likelihood of corrosion and scale formation but that is all they can be as a simple equation can't cover every possible water environment.  For example, the equations don't incorporate biological corrosion factors, are based on pure iron and no ones builds a water tank or pipe out of pure iron! and they don't incorporate ORP.

    In addition, there can be different causes of severe corrosion in a water plant, especially an SWRO, a significant number of which the water chemistry is not the cause.  With corrosion in and out of tanks there is high likelihood you need the services of both a competent corrosion consultant and a water treatment specialist process engineer.

    I happen to know such people if your company is serious about addressing the adverse corrosion effects.

    stewart@slswaterfilters.com.au

    1 Comment

  4. LSI, Believe it or not, is one of the toughest things to really truly understand. It is much more complex than most people in the water treatment industry lend credit to it.  If you truly want to understand it and all that is encompassed in evaluating your specific situation, then might I suggest using a program that will model damn near everything you need.  I really like “French Creek”.  - Sean

    1 Comment

  5. LSI, Believe it or not, is one of the toughest things to really truly understand. It is much more complex than most people in the water treatment industry lend credit to it.  If you truly want to understand it and all that is encompassed in evaluating your specific situation, then might I suggest using a program that will model damn near everything you need.  I really like “French Creek”.  - Sean

  6. Without going into mechanism and LSI do follow as under before or inside the tank:

    1. Allow pH at around 9 don't reduce.

    2. Retain calcite filter. if it is not increasing Ca add CaCl2 to increase Ca  to 10 ppm.

    3. Inject Sodium Hexameta Phosphate ( SHMP ) to maintain 3 to 5 ppm as PO4 in the permeate

    This will control corrosion in the tank and transporting line.

    1 Comment

    1. Hi Guru Das Saha.

      Thanks for your help.

      The potable water tanks that are corroding are the storage for each of four sites i deliver potable water to. These tanks also have a Chlorine dosing system incorporated. I cannot allow the water to be stored at a pH of 9 due to the effect on Chlorine.

      1 Comment reply

      1. The pH of 9 of SWRO is not because of high alkalinity or other alkaline ions it is high because it is RO water and ionic strength is less. Acid dosing to reduce pH may make the water more corrosive if the control is proper which is likely. Chlorine will be equally effective if dosed into this water at pH 9, as pH will come down to some extent during chlorination. Try implementing my suggestions as it is which can be subsequently standardised when required.

  7. WOW----WAIT. Everybody is shooting all they know about LSI and remin, but I don´t know if they read about the issue: Corroded tanks, but is it INSIDE only? why then you mention the saline environment? How about outside? is it OK? Is the lining looking good? Actually I am not clear from the writing if the tank holds seawater. I must assume not, AND that it is fit for the location.

    Also you have not said HOW MUCH remin (ppm) is provided.  You are using ONE calcite filter? what about the others? working? not needed? ACID: You dose how much? what about the alkalinity destruction?

    Suggestion: Try to get some CO2 if you can. Possibly you cannot, so, my gut feeling is that the ration acid/calcite is not good. Reduce calcite and check pH avoiding acid if possible. I disagree that you should shoot as high as 9. At 8.3-8.5 it could be your solution. Let us talk disinfection afterwards

    1 Comment

    1. Hi Miguel.

      Thanks for your help.

      We have had a representative from the tank company, but obviously they would lay blame on another entity or product as to the cause. The tanks in question indeed hold potable water, not sea water. The 160mm outlets on each tank has severe corrosion on the outside, and corrosion also is evident on almost the entire circumference on the bottom 33% of the tank. The liner had no tears as such, but for an unexplained reason has been compromised. How, we dont know. I have 2 calcite filters working on 1 RO unit , and this unit produces 15m3/hr, another produces 24m3/hr and another produces 36m3/hr, into concrete storage tanks 300,000 ltrs each, the flow rates from consumption is undetermined as of yet. With this unit running, the permeate in the 300,000 ltr storage tanks sit around high 8 to 9.10 pH. When not in use, the permeate sits around 7.9 pH. I add acid to control pH to 7-7.2

      Alkalinity isnt measured due to reasons unknown to me but i am making my best efforts to have this entire situation rectified, in the best interests of the public.

      I am concerned about the quality of the water,more so from a consumption point of view, but also to clarify our corrosion issues,  if post RO there is no remineralisation. Testing was very minimal until i arrived here

      1 Comment reply

      1. Chris, I believe you are on the right road, especially with your concern about quality. The comment of Steward Shipard should be taken seriously. I believe that as soon as you get some alkalinity reading you will have an important part of the puzzle. You have not commented if you have CO2 available. Life is easier operating mild acids such as carbonic acid. Whish you could.

  8. Hi Mohannad, please give a reference to your statement that LSI has been disproved as a useful method of predicting corrosion.  I think we generally know that it is an indicator and not an accurate predictor, but it would be good to read more about the results you refer to.  Many thanks. Ian

    3 Comments

    1. It's very simple.  No study has ever found a calcium carbonate buildup in a passive layer.  But they have found ferrous carbonate.

      LSI appears to work some of the time because you get a high LSI when your alkalinity and pH are high - both criteria that are also applicable for ferrous carbonate formation.  However, if the alkalinity is insufficient, and the LSI is high because of high calcium, you will experience corrosion.

      I've had clients that have dosed calcium chloride to increase their LSI and encountered severe red water problems.  Our corrosion coupon studies found a slightly higher rate of corrosion than the control on sidestream racks.  But when they discontinued calcium chloride dosing, turned off their decarbonator, and adjusted their pH up to ~8, their red water issues immediately ceased.

    3. American Water Works Association Research Foundation & DVGW-Technologiezentrum Wasser. Internal Corrosion of Water Distribution Systems, Second Edition. American Water Works Association, Denver, CO (1996).

  9. Chris,

    LSI has been disproved as a useful method for predicting corrosion. Calcium carbonate won't protect steel. What forms is actually ferrous carbonate. As iron corrosion occurs, ferrous ions release into water and react with carbonate alkalinity to form a protective scale.  As with other carbonate scales, higher pH will create a more scaling environment.

    You need to keep your pH above 8.5 and your carbonate alkalinity above 40 ppm. Calcium doesn't matter except for taste.

    Using a phosphate based corrosion inhibitor will result in further improvement, but is no substitute for high carbonate alkalinity and optimal pH. A phosphate inhibitor won't work below pH 7.5.

    A calcite contactor should help but may not reach the required carbonate alkalinity unless you reduce the pH ahead of it to about 6 using CO2 or sulfuric acid.  

    A finished pH of 9.1 would be optimal for corrosion control, especially for iron and lead. What's the reason for the pH reduction, is it for the biocidal activity of the chlorine? Corrosion will deplete your chlorine as released ferrous ions oxidize to the ferric state.

     

    2 Comments

    1. Hi Mohannad.

      Thanks for your help.

      Yes, my primary reason for pH control is for disinfection. We were instructed to keep our 2x 300,000 ltr concrete storage tanks at 0.30ppm with a pH of high 8-9.10. At 4 of the work sites i deliver water to, these sites have additional Chlorine dosing, however, the public of 10-15,000 people have no disinfection and the conditions of the infrastructure/vermin proofing is minimal at best. From this point of view to protect the public, i control pH. Would i be correct in doing this as corrosion in steel tanks is a distant second in regards to water quality control ??

    2. First implement what has been suggested, which is based on field experience, thereafter review the corrosion prevention procedure. . The tank external can be protected by protective coating like ( IOZ + HBE + AAPU ). LSI is more so relevant for circulating Cooling Water System.

  10. 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

    1. 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.

    2. 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.

      1 Comment reply

  11. Hi Chris,  I was also operating a SWRO plant in Namibia some years back, and we had a filter bed of limestone chips that we passed the permeate through before putting it into the distribution or storage containers.  This is not an accurate way to achieve PHs, but it certainly helped.  To determine the LSI for your water, a helpful tool has been developed by the South African Water Research Commission appropriately called STASOFT.  You can download it from their website.  I tried to upload it for you but because it is an exe. file, this website will not allow it to be uploaded.  Please try it - it is safe to download and very useful.

    1 Comment

  12. Typically RO permeate is going to be aggressive as it will want to "pick-up" molecules, especially from basic metals. We use either Polyphosphate or Calcite prior to any metals in the plumbing systems for protection from leeching of the metals. Stainless Steel is resistant to the corrosive effects. And in regards to consumption of RO or Distilled water, it has been proven to rehydrate the body's cells faster than untreated water. Remember the stomach is filled with Hydrochloric  Acid with a pH of about 1. Highly Alkaline water will neutralize the stomach acid, so the body has to produce more. It can often hamper proper digestion.

    1 Comment

  14. Chris

    RO permeate is corrosive as has a very low LSI (Langelier index) that is pH dependent. In order to reduce the corrosion potential, you must adjust the pH to get a positive LSI. Note that if the value of LSI is to high then the permeate can result in scaling in case of high content in carbonates depending of the remineralization process.

    The best solution is to install a proper remineralization system, but if not, at least balance the LSI with caustic.

    1 Comment

    1. Thanks Manuel.

      Primarily i am controlling pH for Chlorine disinfection. If i dont control the pH, it will sit around mid to high 8 pH, and with the island population not protected due to poor infrastructure/lack of vermin proofing, disinfection is my primary concern. Steel tank corrosion is a distant second.