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A few weeks ago, we pickled all RO membranes in our trains with sodium metabisulfite for long term storage. Our membrane manufacturer recommended that we change the solution once the pH drops below 3. We measured a decrease in pH for the first two weeks, but then on the third week, the pH increased. Can you explain why the pH increased instead of decreasing?

When you mix sodium metabisulfite in water, you form a sodium bisulfite solution: $${Na_2S_2O_5 + H_2O \Leftrightarrow 2NaHSO_3}$$$${NaHSO_3 \Leftrightarrow Na^+ + HSO{_3}{^-}}$$ Bisulfite is in equilibrium with sulfite and sulfurous acid (...

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We recently installed new RO membranes in our system and observed that the permeate water tested positive for coliform bacteria. Is it possible in your experience that RO membranes could be damaged during installation and cause contamination of the water distribution system by bacteria and pathogens?

It’s unlikely that the RO membranes were damaged inadvertently during installation. While it’s possible for there to be manufacturing imperfections that allow pathogens such as viruses and bacteria to pass through, it’s not common. In...

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What is the procedure for cleaning RO membranes? What CIP chemicals should be used?

RO membrane cleaning should be performed with high and low pH CIP chemicals. The high pH cleaning should always be performed first to penetrate and disperse biological or organic foulants. Specialty high pH cleaning chemicals more effective...

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Does reverse osmosis desalination of sea water remove viruses from the product water?

Yes, viruses are substantially removed by reverse osmosis membranes. However, studies performed have shown varying viral removal efficacy, varying from 2 log removal (99% rejection) to 5.9 log removal (99.99987% rejection). This is believed...

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What is the maximum Calcium Sulfate (CaSO4) level in brine side that A-102 plus can deal with?

A-102 Plus can handle up to 300% calcium sulfate saturation on the brine side. CaSO4 saturation will vary according to concentrations of dissolved calcium and sulfates, temperature and salinity. If you already have high sulfate levels, avoi...

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How do I determine anti-scalant dosage for a 2300 gpd R/O unit, when CaCO3 is 255 gpg?

When calculating antiscalant dosage, inhibition of all scales must be considered. Potentials scales that can form are: Calcium carbonate Calcium phosphate Calcium fluoride Calcium sulfate Barium sulfate Strontium sulfate metal hydroxides (i...

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What is the disadvantage of raw water disinfection by chlorine in RO desalination?

Chlorine will irreversibly damage a polyamide RO membrane, and as such, a chlorine residual cannot be allowed to contact the membrane. This means that once you dechlorinate, surviving bacteria will once again flourish, and dead bacteria wil...

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Can Sulfuric acid or HCl be used with the addition of Citric acid for the control of iron and other scales in a reverse osmosis plant?

First and foremost, citric acid is a highly assimilable organic acid – this means that it would act as a carbon source for growth of microorganisms.  In the few cases we’ve come across where citric acid was being dosed continuously int...

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Is sodium metabisulfite an effective disinfectant for my RO system?

Sodium metabisulfite (SMBS) is not a disinfectant. SMBS dissolved in water converts to sodium bisulfite (SBS).  It is effective as a preservative because it is an oxygen scavenger and therefore prevents growth of aerobic microorganisms.  ...

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How can I clean a membrane that has been clogged with suspended solids and sludge?

This is a very common problem and there is no single solution.  Some suspended solids consist of natural organic matter (NOM) or dead bacteria, and can be removed by performing a high pH cleaning.  Other suspended solids (such as limest...

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