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Reverse Osmosis Units: Factors Affecting Output
Veterinary & Aquatic Services Department, Drs. Foster & Smith
Equipment and Water Quality
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Q. I've recently purchased a Kent Marine 35 gallon per day reverse osmosis unit and it is only producing 15 gallons per day. Is this unit defective? If it isn't, why is it not producing the rated 35 gallons per day?
A. It's not likely that you received a defective unit due to Kent Marine's superior quality control. But there are a few factors that effect the output of reverse osmosis units. These factors include; the source water pressure, temperature, and the amount of total dissolved solids within the source water.

The ideal operating pressure for a RO (reverse osmosis) unit is 65 psi (pounds per square inch). An operating pressure of 45 psi will reduce the output of a unit by 50%. There are pressure gauges available to monitor the operating pressure of the system. If the operating pressure is too low, an RO pressure boosting pump can be added to increase the operating pressure to the ideal range.

The temperature of the source water should ideally be in the 70  to 77 F range for optimal performance. A temperature of 50 F, or lower, will reduce the output of the unit by 50% or more. Either hot or a mix of hot/cold tap water should never be used for the source water due to the minerals that are added to the water from most water heaters.

Finally, the amount of total dissolved solids in the source water can reduce the amount of water produced by a RO unit. The Kent Marine units are rated to treat water with a dissolved solid amount of 200 ppm or less. If your source water contains a higher concentration, it would be wise to install a mechanical pre-filter in line prior to the unit. These filters are available at most hardware stores.

Because of the requirements of reverse osmosis units, it is wise to purchase a unit that is rated above the gallons per day that your needs require.

Reverse Osmosis Systems: Checking Water Purity in Your Aquarium 
How to Select a Reverse Osmosis Unit for Your Aquarium 
Speed Up Reverse Osmosis Water Collection for Your Aquarium 
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