Ask the Membrane Expert

Reverse Osmosis Antiscalants can cause membrane fouling in Reverse Osmosis systems under the following conditions: Fungal growth in the antiscalant -  some antiscalants contain impurities that can provide the necessary nutrients and carbon for biogrowth.  It is not uncommon to find fungal growth in polymer-based antiscalants.  The growth in the drum can seed growth on the membrane surface.  Reputable manufacturers manufacture higher purity products and use NSF approved anti-fungal preservatives to prevent these issues. The dosing point too close to acid addition point - pH is typically adjusted using concentrated sulfuric acid.  In the absence of good mixing, the acid...

Citric acid is an organic acid that is often used for removal of calcium carbonate scale and iron hydroxide.  Citric acid is not very effective at removing phosphate salts such as calcium and iron phosphates.  It cannot be used to dissolve sulfate scales or silica, and is ineffective for biofilm removal. Although it is often recommended for organics removal, it is not very effective due to the protonation of the carboxylic acid functional groups in natural organic matter (NOM) in the low pH environment created by citric acid.  For this reason, attempting to clean at low pH when organics are present...

Once the RO membrane system is placed back online after CIP, you can get a relatively good idea by comparing post-cleaning feed pressures and differential pressures for each stage to pre-cleaning values.  The most reliable method would be to observe the trend in your normalized permeate flow chart, since that corrects for changes in temperature, conductivity and % recovery.

Always flush your membranes with RO permeate until the pH measured at the waste end is within a pH range of 5 – 9.  You can then proceed to flush with feed water until the pH of the water exiting the RO membrane system is similar to the pH of the water that is entering the RO membrane system.  Conductivity of the water exiting the RO membrane system should also be similar to the conductivity of the water entering the RO membrane system.  No foaming from the RO cleaning chemical should be detectable.  Once these three conditions are met, it...

No, if a high pH RO cleaning solution is mixed with RO brine, it would result in severe scaling.  Always consult with your hydrogeologist before sending any solution to the deep injection well.

That is not recommended.  A high pH cleaning solution is designed to dissolve membrane fouling/scale that is soluble in basic conditions.  A low pH cleaning solution is designed to dissolve membrane fouling/scale that is soluble in acidic conditions.  Neutralizing these cleaning solutions will precipitate most of the materials that were dissolved back onto the RO membrane surface, rendering the cleaning ineffective. If you do not have a neutralization tank, you should neutralize the RO cleaning solution in your CIP tank, and drain.  Then fill the tank with permeate water, circulate through your system to displace more RO cleaning solution from...

No, pH extremes can be very disruptive to bacteria and can upset the operation of the wastewater treatment plant.  The RO cleaning solution should always be neutralized prior to sending to a wastewater treatment plant.  The solution is usually considered “neutral” in most cases if it is within a pH 5 – 9 range, however, always follow local regulations and comply with your discharge permits.

Bicarbonate is in equilibrium with carbonic acid, and carbonic acid is in equilibrium with carbon dioxide. [1] $${HCO{_3}{^-} \Leftrightarrow CO{_3}{^2}{^-} + H^+}$$ [2] $${H_2CO_3 \Leftrightarrow HCO{_3}{^-} + H^+}$$ [3] $${CO{_2}{_(}{_a}{_q}{_)} + H_2O \Leftrightarrow H_2CO_3}$$ [4] $${{CO{_2}{_(}{_g}{_)} \Leftrightarrow CO{_2}{_(}{_a}{_q}{_)}}}$$ When RO permeate passes through a degasifier, the carbon dioxide gas is displaced by oxygen and nitrogen from the air that is blown through the solution.  The loss of carbon dioxide cause a shift in the equilibrium reaction to the left (see Eq. [1],[2],[3]).  The bicarbonate reacts with acid protons to form more carbon dioxide (Le Chatelier’s Principle), and in turn, carbonate proceeds to react with...

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 (just like bicarbonate is in equilibrium with carbonate and carbonic acid): [1] $${HSO{_3}{^-} \Leftrightarrow SO{_3}{^2}{^-} + H^+}$$ [2] $${H_2SO_3 \Leftrightarrow HSO{_3}{^-} + H{^+}}$$ [3] $${SO{_2}{_(}{_a}{_q}{_)} + H_2O \Leftrightarrow H_2SO_3}$$ [4] $${S{_2}{_(}{_g}{_)} \Leftrightarrow SO{_2}{_(}{_a}{_q}{_)}}$$ When H2SO3 gasses off as SO2, the pH rises due to a shift in equilibrium to the left, causing bisulfite to react with H+ to form more sulfurous acid (see Eq.[2],[3],[4]).  This is exactly like what happens when H2CO3 gasses...

It is doubtful that the membranes were damaged inadvertently during installation.  A significant amount of effort would be required to damage RO membranes to the extent that bacteria would easily pass through.  While it’s not impossible for there to be manufacturing imperfections that allow pathogens such as viruses to pass through, bacteria are quite large, so that’s also an unlikely scenario. In our experience, total coliform counts increase in the permeate almost every time membranes are replaced.  This is because bacteria are introduced to the permeate side components (such as the permeate tube in the membrane and interconnectors), which will...