Ask the Membrane Expert

Soluble iron and manganese can be removed by using a Manganese Greensand Filter. However, in order to separate salt from water, either Reverse Osmosis or Evaporation have to be used.

Antiscalant dosage cannot be determined by TDS (Total Dissolved Solids).  The reason is that TDS can be made up entirely of sodium and chloride which do not have a scaling potential, or it could be made up entirely of calcium and sulfate which have a very high scaling potential. In order to determine the correct antiscalant and dosage, the ion species in the water must be identified, and their concentrations have to be calculated based on the concentration factor.  The concentration factor is variable with the type of membrane (Reverse Osmosis vs. Nanofiltration) and the % salt rejection (NF membranes...

Chlorine dioxide is a highly potent biocide that has a higher efficacy than hypochlorite in destroying protozoa, bacteria and viruses, without allowing them to build a tolerance.  It is effective over a wide pH range as both a biocide and metal precipitant and has gained popularity because it creates significantly lower concentrations of disinfection byproducts such as trihalomethanes (THM’s) and haloacetic acids (HAA’s) when compared to chlorine. However, at least one study has shown that THM formation is still significant when naturally occuring organic matter (NOM) such as humic acids are present (A.A.Stevens, “Reaction Products of Chlorine Dioxide”,1982). Chlorine dioxide...

It is not possible to have colloidal silica in your feedwater unless your silica concentration is above the solubility limit under those conditions, resulting in polymerization and hence colloidal silica formation. The difference between results from ICP-AA vs molybdate reactive silica is often mistakenly attributed to the presence of colloidal silica – but 99% of the time, the difference is due to calibration issues. Silica leaks through demineralizers because it only partially deprotonates at neutral pH and is not fully charged unless the pH is very high. That has no relevance to its existence in colloidal or silicic acid form....

H2S acts as a weak acid, much like carbonic acid: $${H_2S \Leftrightarrow HS^- + H^+}$$$${HS^- \Leftrightarrow S{^2}{^-} + H^+}$$ If your pH is above 5.5 and oxygen is allowed to contact with air, then sulfide ions will oxidize to element sulfur. We have recently come accross a case where a plant in South America made the mistake of using peroxide to oxidize the H2S and then attempting to remove the resulting elemental sulfur by ultrafiltration prior to the RO. We just performed an RO membrane autopsy and their membrane was completely coated with elemental sulfur. The fact that you have H2S implies that...

There are different types of antiscalants. Some are phosphonate based,and some are polymer based. Many are blends to provide synergistic effects of the two chemistries. Antiscalants have to be non-toxic, or they would not be approved by NSF for potable water applications. Both phosphonate and polymer based antiscalants are biodegradable over time. Ultraviolet light increases the rate of degradation as the chemical bonds are broken, and certain types of bacteria produce enzymes that cleave the bonds. Recently some “green” antiscalants have been introduced into the market, with the claim that they biodegrade at a faster rate. In reverse osmosis membrane...

What is the relationship between pH and alkalinity? In simple terms, pH is the concentration of acid protons [H+]. On the other hand, the alkalinity of a solution is its ability to neutralize acids. Alkalinity consists of ions that incorporate acid protons into their molecules so that they are not available as a free acid that can lower the pH. This is known as buffering. For example acid reacts with CO32- to make HCO3-, and converts PO42- to HPO4-  .  This makes it so that a significantly higher quantity of acid is required to lower the pH compared to a solution that does not...

What is delta P (ΔP) and what does it mean when logging RO data? The differential pressure (dP) is the pressure lost due to friction as water passes through the system. You take a reading before the pressure vessels and after the pressure vessels of each stage, and that tells you how much pressure was lost across that stage. A very high concentrate flow rate will result in a high dP because of the high friction. From a fouling perspective, scale, suspended solids, and biological growth, etc… build up in the feed channels and interfere with flow, thereby increasing friction...

In the case of high purity applications, the main criterion for clean vs. replace is salt passage. Any increase in salt passage to the demineralizers can be enough financial justification for replacement. As an example, even a slight decrease in salt rejection from 99.6% to 99.2% may not seem troublesome to someone treating brackish water for a potable application. But in a high purity application that generally employs mixed bed demineralizers to polish the RO permeate, the amount of salts that have to be removed by the demineralizer system would double, which in turn doubles the number of regeneration cycles....