Ask the Membrane Expert

RO membrane cleaning should be performed with high and low pH cleaning chemicals. The high pH cleaning should always be performed first to remove biological or organic foulants, both of which contain carboxylic functional groups. These weak acids gain an anionic charge at high pH, allowing them to disperse. High pH cleaning chemicals chelate the calcium that bridges foulants and biofilms to each other and to the membrane surface. Commodities such as NaOH are only mildly effective because of their inability to remove the calcium bridging. The use of citric acid is recommended prior to high pH cleaning by some...

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 to result from imperfections in the membranes. It can also be exasperated by mechanical or chemical damage which can result in a loss of salt rejection. In a paper titled “Removal of biological and non-biological viral surrogates by spiral-wound reverse osmosis membrane elements with intact and compromised integrity”, the authors found a correlation between rejection of viruses and rejection of sodium chloride, where an increase in...

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, avoid or minimize any sulfuric acid dosing because it will add to the dissolved SO42-  ions in your feed water. For calcium sulfate saturations > 300%, a specialty CaSO4 antiscalant should be used. The AWC A-104 can control calcium sulfate at saturations as high as 600%.

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 (iron, aluminum) Silicate scales Calcium hardness alone cannot be used to determine scaling tendency, as it must combine with an anion to form a scale. In order to calculate the antiscalant dosage, a comprehensive water analysis must be performed that contains the following parameters: Cations: Ca, Mg, Ba, Sr, Fe, Al, Mn, Na Anions: Alkalinity, SO4, F, PO4, SiO2 Other Required Information: Feed water pH (measured upon collection at...

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 will be available as assimilable carbon sources for the survivors.   Many components of existing TOC will also be broken down to AOC (Assimilable organic carbon). These will also become a source of carbon for increases in biological populations and a resulting biomass. This is especially a problem with seawater which has very high organics content.   Furthermore, dead bacteria are suspended solids and are just...

Using citric acid is very expensive. If the water source contains oxygen, has been exposed to oxidizers, or ferric based coagulants have been used, the iron will be in the ferric state and can typically be controlled by dosing sulfuric acid to reduce the pH to ~6 and dosing antiscalant. If the water does not contain any dissolved oxygen, most of the iron will be in the Ferrous state.  Ferrous ions are extremely soluble and easily controlled by most antiscalants without acid. However, in many cases, some soluble oxygen will be present. It only takes 0.1 ppm dissolved oxygen to...

Sodium metabisulfite (SMBS) is not a disinfectant. When used in high dosages, it is an oxygen scavenger and many biofilm forming bacteria are anaerobic, making oxygen scavenging useless. SMBS does however prevent fungal growth in membranes when used as a storage solution.

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 limestone, dolomite, phosphorite, metal hydroxides) are acid soluble and can be removed by low pH cleaning. Suspended solids that are made of crystalline aluminosilicates such as silts and clays are insoluble in water.  The use of an effective high pH cleaning chemical with dispersants can help remove silts and clays as long as they are small enough to pass through the feed channels.  However, if...

Q: Can hydrogen peroxide be used effectively in place of sodium bisulfite for dechlorination of seawater RO feedwater? We are concerned about dissolved oxygen levels in the concentrate that is disposed back into the sea… Sodium sulfite  and sodium metabisulfite both work very well for dechlorination at a stoichiometric ratio of 1:1: $${SO{_3}{^2}{^-} + HOCl \rightarrow SO_4{^2}{^-} + Cl^- + H^+}$$$${HSO{_3}{^-} + HOCl \rightarrow SO{_4}{^2}{^-} + Cl^- + 2H^+}$$ This works out to about 1.5 ppm sodium bisulfite — to 1 ppm hypochlorite (active) by mass, and typically a slight excess is used to increase the rate of reaction. If...

There are two major symptoms that we typically associate with membrane damage: 1. Increase in permeate conductivity coupled without a loss in productivity, or in extreme cases, with an increased productivity– this could indicate delamination or chemical damage.  Membrane probing can determine whether this increase permeate conductivity is simply due to leakage at the permeate tube interconnectors (damaged O-ring). 2. Increase in differential pressure (ΔP) in both stages without a loss in productivity – this typically indicates membrane “ballooning” or “pouching” that occurs during system shutdown as a result of excessive permeate backpressure.   This causes the membrane to stretch out...