Types of scaling expected on different types of membrane

The types of scaling expected on the different types of membrane must be specified to make it easier to choose the type of antiscalant that is suitable for your station, of course, along with testing the source water.

We can know whether the antiscalant is good or not simply by observing the extent of its success in preventing the deposition of salts on the membranes, taking into account the following considerations before judging the quality of the antiscalant:

Storage conditions

Its storage conditions in the station before and during use, including its temperature and dilution water…etc. The lower the temperature, the lower the ability to dissolve salts, and thus the hardness increases:

Low temperatures reduce the solubility of mineral scales (the exception here is the formation of calcium carbonate scales, which are enhanced at higher temperatures).

The dilution water must be free of hardness and preferably of high quality. Diluted antiscalants/dispersants can become biologically contaminated in the day tank depending on room temperatures and how much they are diluted. The suggested residence time for the diluted solution is 7-10 days. Usually, undiluted antiscalant/disperse solutions do not suffer from biological contamination. Failure to use the exact dosage:

The chemical metering pump used to inject the scale inhibitor/dispersant should be modified to maximize injection frequency. The suggested minimum stroking frequency is once every 5 seconds. Typical dosage rates for antiscalants/dispersants range from 2 to 5 ppm.

Poor choice of injection site

The design of the anti-scale/dispersant injection system into the RO feed stream needs to ensure it is mixed properly before it enters the RO elements. Using a stationary mixer is the most effective mixing method, but it increases cost to the system. Most systems have an injection point just before the RO feed cartridge filter and rely on the cartridge filter dwell time and RO feed pump movement to affect mixing.

Expected types of scaling

The shape and colour of scaling on the surface of the membranes play an effective role in determining the appropriate antiscalant for it, in cooperation with the suppliers responsible for chemicals and membranes. The expected types of scaling that have been classified on R.O membranes are as follows:

Calcium carbonate scaling

It is one of the most common types of scaling to which membranes are exposed, and it comes in the form of off-white powder. It is usually easy to clean with strong acid.

Silica scaling

It has a colloidal or colloidal form in which the aluminosilicates appear at the beginning of the series of membranes or in the form of precipitation at the end of the series… and it is difficult to remove.

Calcium sulphate scaling

It takes the form of plaster or gypsum and is characterized by a crystal shape with very sharp ends that can cut the surface of the membrane. This type of sulphate content is formed in the water used and is also difficult to remove.

Calcium phosphate scaling

It usually appears due to the nature of the feed water, which is either waste water or agricultural leachate, as a result of high phosphate loading.

Magnesium hydroxide (Brucite) scaling

This species is found in the second pass of brackish water plants with a high pH. To treat boron present in feed water. This is done by raising the PH above 9.5 in the presence of sodium hydroxide.

Barium sulphate (barite) scaling

The picture shows white crystals that have the shape of blades, and they are also a type that is difficult to clean.

Strontium sulphate (Celestine) scaling

It takes the form of needles or crystals and can cut through membranes and is also difficult to remove.

Symptoms resulting from bad antiscalant:

The following is observed at the station as a result of the ineffectiveness of the antiscslant:

• Low productivity
• High pressure on the membranes (pressure difference).
• High concentration of salts.

In the event that salts are deposited on the membranes as a result of calcium carbonate or magnesium carbonate, the membranes are washed with HCL at PH = 2.

Suppose the deposits are due to sulphate salts such as calcium sulphate or magnesium sulphate. Washing is done with EDTA at a concentration of 0.1% at PH=12 and a temperature of 30°C.

It should be added that antiscalant is necessary if LSI for Brackish Water or S&DSI for Sea Water is calculated. If the result is positive, do so in Reject Stream because the chance of deposition is high. Especially at the end of the membrane because it is the maximum concentration. If we avoid sedimentation there, sedimentation will not occur anywhere else because it is the worst place and most susceptible to sedimentation.

The other point is to make sure that there is no conflict between the antiscalant and the polymer, meaning that one of them is not positively charged and the other is negatively charged, so they attract each other, causing the formation of gum. Or a complex compound that causes Fouling to occur on the Membrane. Thus, we have added an additional problem to the RO System. We work to provide all the information needed by station owners, project managers, operating technicians and responsible engineers. If you need further clarification, just contact our Carewater Solutions experts.