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Scaling of RO & NF membranes may occur when sparingly soluble salts are concentrated within the element beyond their solubility limit.  

 

For example, if an RO plant is operated at 50% recovery, the concentration of salts in the concentrate stream will be almost double the concentration in the original feed stream, and as the recovery of the plant is increased so does the risk of scale precipitation.

 

For further information regarding antiscalant dosing projections Click here

 

RO and NF plant feed waters are different in ionic composition and as a result different types of scale may deposit on the membrane surface. Once scale is deposited on the membrane surface the RO Plant performance will drop  and the membranes should be cleaned to remove the scale. Some types of scale species are detailed below:

 

Calcium Carbonate scale is the most common form of scale as it precipitates quickly once beyond its solubility limit, also most natural waters are almost saturated with respect to calcium carbonate.  It appears as a powdery substance  which clogs the membrane. Generally it can be easily cleaned with a strong acid.

 

Calcium sulphate precipitates onto membranes when treating waters that have high concentrations of both calcium and sulphate.  It has a crystalline structure with sharp features which cause damage to the membrane surface and also makes it difficult to remove

 

Silica is naturally present in most feed waters in the range of 1–100 mg/L. The prevailing forms of silica are meta silicic weak acids, it is mostly in the undissociated form at or below a neutral pH. Supersaturated silicic acid can further polymerize to form insoluble colloidal silica or silica gel, which can cause membrane scaling. 

 

Barium sulphate is the most insoluble of all alkaline-earth sulfates. When present in water, it may lead to massive precipitation, possibly acting as a catalyst for calcium sulfate and strontium sulfate scaling.  It forms as white crystals with sharp features. Once present this is very difficult to remove. 

 

Strontium Sulphate (celestine) forms in a crystalline strucure  or as a needle like structure and occurs in a similar way to calcium sulphate.  Due to the structures in which it forms it can cause membrane surface damage and is also very difficult to remove

 

Magneisum Hydroxide (brucite) - is very rare and preciptates at high pH in membrane systems above 9.5.  It can be found in second pass brackish water boron rejection plants where higher pH levels are experienced due to the use of sodium hydroxide to aid the boron rejection. 

 

Calcium phosphate fouling was not common until reverse osmosis technology was applied to municipal wastewater. Due to water shortages, municipal waste water recycle or reuse has become one a major application. Along with this new application, preventive actions for calcium phosphate scaling are needed. 

 

 

 

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