Pitfalls In Specifying Reverse Osmosis Equipment In Industrial Applications

Applications

This discussion is the first of several which address pitfalls in selecting or purchasing reverse osmosis machines.

Raw Water Temperature

The density of water changes with temperature; its maximum density is reached in the form of ice at 32° F and its density decreases almost linearly as its temperature increases. Since reverse osmosis involves the passage of water molecules through a semi-permeable membrane, the "thicker" or more dense the water, the slower the passage.

In reverse osmosis applications, most feedwater temperature fluctuates based on ambient temperature, often ranging from 40° F to 70° F if sourced from lakes, rivers or streams. Well water in much of the world maintains stable year-around temperatures of ~ 55° F. In arid or tropical areas, the temperature of well and surface waters will be significantly higher.

Confusion and potential misapplication of RO equipment ensues because many years ago, manufacturers of RO membranes selected 77° F as the standard or "normalized" temperature at which to rate the flow capacity of their membranes. However, very few real world applications use a feedwater which is consistently at 77° F. For example, technical specifications from most RO manufacturers list a 100 gallons per minute (gpm) RO with small print at the bottom of the page indicating this is at 77° F. However, if the raw water fluctuates as described above, the "100 gpm" RO output will be approximately 52 gpm at 40° F and 88 gpm at 70° F. Obviously if the system is selected based on output at 77° F, there will be a deficit, equal to 50% during some months of the year. While your vendor should point this out, not all may be aware, or they may fear that sizing the machine correctly for the lowest possible feedwater temperature will make them uncompetitive since it requires a larger machine with more membranes.

In the above example, the machine must be sized for the lowest temperature, requiring one rated for ~190 gpm at 77° F in order to produce 100 gpm at the low temperature of 40° F. When the feedwater reaches 70° F, this machine will be producing about 165 gpm. This presents the following critical design considerations:

1. The RO product water storage tank should be designed with enough capacity to avoid turning the RO on and off due to high water more frequently than ~ 30 mins when operating at the peak feedwater temperature.
2. Piping design and sewer capacity must be capable of handling a reject (wastewater) stream ranging from 33 gpm to 55 gpm.
3. Pretreatment equipment (e.g., softener, depth filter, iron filter, carbon filter) must be capable of treating the range of feedwater demanded by the RO, ranging from 133 gpm to 220 gpm.
4. Incoming raw water supply must be capable of supporting a flow of 133 to 220 gpm at the minimum dynamic pressure required (typically 35 psi).
5. Integration of a variable frequency drive RO pump in the initial design can help modulate the flow since pump output is adjustable.
   

Temperature is one of several very important considerations involving RO equipment and ancillary equipment design. The following chart provides temperature correction factors for a wide range of feedwater temperatures.

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