Membrane Anti-scale Solutions for Water Treatment R.O System
Any membrane system such as the RO system is naturally clogged by the presence of suspended substances, soluble solutes and microbial parameters in the feed water, which necessitates proper physical and chemical pre-treatment of the system.
The first possibility for the formation of scale in the raw water treatment system is when the solubility of each low soluble salt is increased. The mechanism of deposition on the membrane surface is related to a concentration gradient that occurs as water passes through the membrane continuously, leaving a large amount of soluble salts. This occurs a concentration on the surface of the separating membrane, which causes the formation of a boundary layer. An effect known as concentration polarization inside this boundary layer causes the salts to precipitate, and the suspended solids can also settle on the membrane surface and form scale particles, and scale layers into separated layers of membranes.
If the natural waters are not treated, the calcium carbonate will deposit on the membrane surface. Calcium sulfate is another common scale in natural water. Deposition usually begins in the next steps. This scale decreases the pressure and thus requires more pressure to maintain the stability of the water outlet. Antiscalants are used to reduce the effects of scale that clog the pores of reverse osmosis membranes with water-soluble salts.
Antiscalant has a great impact through the life span of reverse osmosis membranes and reduces the cost of replacing the annual consumable filters.
Comparison of Scale Inhibitors:
Most scale inhibitors have a molecular structure with functional groups including carboxylic acid (-COOH) or phosphate. Low molecular weight polyacrylate molecules (molecular mass between 100-5000) contain several carboxylic acid functional groups and are commonly used in many inhibitors. These inhibitors are among the best in preventing the formation of sulfate and carbonate, but their dispersing properties are limited.
Sodium hexa-metaphosphate (SHMP) is one of the substances that is chosen as an inhibitor because it is inexpensive in addition to its good inhibitory role. One of the disadvantages of this substance is its instability and difficult dissolution in water. In fact, if the SHMP is not stirred every three days, the hexa-metaphosphate is hydrolyzed to phosphate, which in neutral pH is combined with calcium and forms phosphate. This salt can cause clogging the membrane of the system. The use of hexa-metaphosphate as a scale inhibitor for R.O systems has not become popular due to its high dependence on proper consumption.
Organophosphonates are more stable than Sodium hexa-metaphosphate. The inhibitory and dispersive properties of this substance are similar to sodium hexa-metaphosphate. But it is stable unlike sodium hexa-metaphosphate.
Polyacrylate with heavy molecular weight (between 6000 and 25000) has the best effect on dispersion, but they are not as effective as light acrylic in inhibiting deposition.
As previously mentioned, the use of mixture inhibitors exhibits better results than single-molecule inhibitors. If used a single-molecule inhibitor, the over-injection of the inhibitor is more likely to cause the inhibitor to exit as a multivalent cation. By combining products and the use of multi-molecular inhibitors, other inhibitors can be effective in preventing the deposition of the first inhibitor. It will also require less concentration than any of the independent components of the inhibitors.
Some types of combinatory inhibitors, including light and heavy molecular weight polyacrylate, are sufficient to inhibit and disperse. In addition, other inhibitors are a combination of lighter molecular mass organophosphate acrylates that they have the properties of an inhibitor, in addition to dispersing well.
Sometimes, biological activity in the scale inhibitor tank can cause problems. Biological growth can block the feed path of the injection pump and stop the inhibitor flow. Inhibitors or suspensions usually contain some biocides. Pay attention when diluting these solutions in daily tanks, that the concentration of these biocides in dilute solution should not be less than the minimum effective amount. This amount is usually informed by the manufacturer. As the inhibitors slow down (not stop) the deposition process, the system should not remain out of service for long periods of time in the salt-saturated state. Usually, an automatic electric valve is installed in parallel with the drain current to open just before service and drain all the elements from the over-saturated salts.
In some cases, a tank is used to inject two or more chemicals at different times. In this case, careful attention should be paid not to mix the materials together so as not to damage their performance and the system.
|Introduce & Calibrate of Using Phosphonated Antiscalants|
|Dosage||Type of use||Antiscalant code|
|3-6 ppm||Membrane anti-scale for industrial use|
Use for waters with hardness less than 900ppm
Ducts with relatively freer pores
|3-10 ppm||Industrial anti-scale for hardness water|
Use for hardness water + 1000ppm to 4000ppm
Ducts that are almost capillary and have closed pores
|2-6 ppm||Industrial anti-scale|
Use for waters with hardness -1000ppm
|3-10 ppm||Specially for saline waters|
|3-8 ppm||Industrial anti-fouling|
Anti-sediment and industrial bio-pollutants
Suitable for systems with contaminated feed water