Reverse Osmosis Sizing and Capacity

Should a large tank not be an option, the alternative is multiple skids, some of which will sit idle. The key is to rotate skids on and off so that no one skid experiences most of the down time start-ups. It is also recommended that an off-line or shut-down flush be employed, and that the motor be equipped with a soft start. For the example above, two 350-gpm skids would be required. Both skids would be on line during the 500-gpm draw. A product tank with enough capacity to handle the 200-gpm overage would be required. 

If demand is continuously variable, the best option is still designing for the average flow rate, as described above. However, in some cases variation in demand may be such that it still may become necessary to cycle the feed pumps on and off with level in the product tank. 

Off-Site Membrane Cleaning On-Site Membrane Cleaning (CIP)  

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The main methods used in desalination units are reverse osmosis and multi-effect distillation. The energy consumption of a desalination plant using the technology of reverse osmosis depends on its size, the contents of sea water and the desired quality of the produced water and usually ranges between 5-7 kWh/m3 when sea water is used. When well water in coastal regions is used for desalination the energy consumption is less than half of the above. The nominal capacity of small-size desalination units for rural housings is of the order of 400 W. 

Hollow fibers and spiral wound modules have been constructed for this application. A module 18 inches in diameter and 5 feet long can produce up to 1,500 GPD. Plants now exist in the Florida Keys, the Bahamas, and the Cayman Islands ranging in size from 10,000 to 60,000 GPD. They have the highest capacity per unit volume for evaporation plants now in existence. It is claimed that these plants are more cost effective than multiple-effect evaporators, but in the author s view, it is doubtful they can compete with reverse osmosis. 

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