RO – Hardness and suspended solids removal

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Pre-treatment steps are customised depending on the water chemistry and site-specific design criteria. The preferred hardness removal method for most industrial applications consists of a conventional lime-soda ash precipitation softening system followed by filtration and weak acid cation (WAC) ion exchange. Lime-soda ash softening in a conventional solids contact clarifier, or in a dense solids process, is an economical method of removing the bulk of the hardness (calcium and magnesium) and other scale forming cations such as iron, barium and strontium. The effluent from the clarifier is filtered with dual media gravity or pressure filters for reduction of suspended solids.

Secondary softening is accomplished by WAC ion exchange, which when operated in the hydrogen cycle will efficiently remove hardness associated with alkalinity.

Hydrogen ions released in the cation exchange react with the carbonate alkalinity to form carbonic acid for subsequent removal in the degasifier. The WAC ion exchanger has a very low leakage characteristic that is important to avoid potential scaling of the RO membranes.

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