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Source: Water Technology

Questions about reverse osmosis

August 2016

Certification Action Line features questions and answers typical of those appearing in Water Quality Association certification examinations. Some answers may not satisfy everyone or every condition.

Q. When osmotic shock occurs, it does not directly affect the membrane. (True or False)
A. False. Osmotic shock involves sudden shrinkage or swelling of the membrane.
Q. Reverse osmosis (RO) requires greater feed pressure than ultrafiltration. (True or False)
A. True. There are line pressure RO systems, but it is usually true.
Q. In laboratory use, it is not particularly unusual to find the same water treated by ultrafiltration, RO and deionization (DI). (True or False)
A. True. Such as pharmaceutical water treatment.
Q. The advantage of presoftening an RO feedwater containing hardness is the reduction of membrane fouling. ( True or False)
A. True. Presoftening will help reduce buildup of calcium carbonate and calcium sulfate on the membrane.
Q. In RO, the membrane is synthetically produced to have specific properties. (True or False)
A. True. Semipermeable membranes are manufactured from organic substances: cellulose acetate, cellulose triacetate or aromatic polyamide resins.
Q. The oxygen content of water has an adverse effect on the RO process. (True or False)
A. False. Oxygen is a dissolved gas and does not in and of itself affect the RO process.
Q. The rejection percentage is the percent of water going to waste in an RO unit. (True or False)
A. False. Percent rejection = TDS in feed – TDS in permeate x 100 TDS in feed.
Q. Since RO does not require regeneration like DI, it is the best method for producing the highest-quality water. (True or False)
A. False. RO is a physical separation of impurities. DI is more a chemical reaction (ion exchange), which produces a higher-quality product water.
Q. RO rejects all mineral ions on the same percentage basis. (True or False)
A. False. RO is selective; different minerals, different percent.
Q. Higher feedwater temperature improves the refection rate (percent rejection). (True or False)
A. False. Temperature affects the (permeate flux) product flow rate but does not significantly affect the permeate quality.