Reverse Osmosis Processes


Basic principle

After pre-treatment, the supplied water (also called feed), which is to be treated, first runs through a booster pump and is pumped by a downstream pump RO membrane pumped. The RO membrane is a partially permeable membrane that is only permeable to the solvent water. The RO membrane is the heart of every reverse osmosis system. Depending on whether the one- or two-stage reverse osmosis process is used, there are one or two RO membranes in the system.

From time to time backwash of the membrane is necessary to avoid clogging - even with the best pre-treatment. The difference in pressure before and after the RO membrane shows when this point in time has been reached. These should therefore be precisely recorded by the operator. Permeate should always be used for backwashing.

(Source: Herbert Bendlin, Martin Eßmann, Klaus Feuerhelm: Structure of a reverse osmosis system: ''Practice book ultrapure water 2nd revised edition'', 2011, p. 68-70)


One-step process

In the single-stage process, the water to be treated only passes through the reverse osmosis unit once. The RO membrane divides the water into two partial flows: the desired product (permeate) and the Concentrate, which is enriched with the foreign substances of the input water. Part of the concentrate is fed back into the raw water side, so that part of the water is in permanent circulation. The other part is discarded and fed into the waste water.

(Source: Herbert Bendlin, Martin Eßmann, Klaus Feuerhelm: Structure of a reverse osmosis system: ''Practice book ultrapure water 2nd revised edition'', 2011, p. 70-71)


Two-step process

In the two-stage process, the water to be treated passes through two connected in series RO membranes. In front of each of the two RO membranes there is a pressure booster pump that works in the range of approx. 12 - 15 bar. p>

This type of two-stage process is used more often than the second type, where there is only a single pump in front of the first membrane. This pump works with a pressure of approx. 30 bar, because after the first membrane there has to be enough pressure left for the second membrane. Because of the higher pressure, the first membrane has to withstand a much higher pressure in this case. Therefore, this procedure is technically more complex and is rarely used.

(Source: Herbert Bendlin, Martin Eßmann, Klaus Feuerhelm: Structure of a reverse osmosis system: ''Practice book ultrapure water 2nd revised edition'', 2011, p. 71)


One-step process with electro-deionization (EDI)

The degree of purity of the permeate from a single-stage RO system is often not sufficient. Instead of a second RO level, an EDI is therefore often used. The EDI delivers a high yield of product water, which can easily be fed back in before the RO membrane. The product water achieves a very high quality or very low conductivity of < 0.1 µS/cm. Only a small amount of electrode rinsing water is produced, which is discarded.

(Source: Herbert Bendlin, Martin Eßmann, Klaus Feuerhelm: Structure of a reverse osmosis system: ''Practice book ultrapure water 2nd revised edition'', 2011, p. 74)



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