Thursday, July 9, 2015

RO Membrane Considerations for Purified Water

Certain considerations should be kept in mind to achieve high-purity water from a reverse osmosis system. A properly designed and operated RO system is able to produce water with a resistivity greater than 1 megohm and there are several factors that can affect the final product.

Factors that Affect Membrane Rejection

Controlled experiments show that membrane rejection fluctuates depending on the feed’s pH values and total dissolved solids, along with crossflow rates and element recovery levels. Minor feedwater constituents like ammonia and alkalinity are also seen having a great part in achieving high purity permeate, and these elements are seen as important ones that ensure the success of producing high-purity water.

How These Affect Membranes

Feeds having high pH levels contribute to the reduction of the membranes’ rejection, and acid addition that can lower its pH level can help in correcting this condition. Since a high pH level also contain a high concentration of hydroxide ion, membranes cannot reject this.

Feedwater with minimal total dissolved solids also reduces the membrane’s capability to reject ions, and this reduction in ion rejection must be considered when calculating the final permeate quality of a two pass system. The feedwater chemistry, meanwhile, can greatly affect permeate conductivity. Particularly feeds containing dissolved gases pass directly through these membranes, making it necessary for certain changes in the feedwaters’ pH levels to be done. These considerations should be remembered when designing an effective RO system.

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