VARIABLE FREQUENCY DRIVE SYSTEMS ARE THE BEST CHOICE FOR RO WATER TREATMENT WHEN THE VARIABILITY OF WATER TEMPERATURE IS HIGH.

by Rick Ide, Senior Applications Engineer, AdEdge Technologies Inc.

Direct or fixed speed drives on centrifugal pumps are the most common choice for reverse osmosis (RO)systems. This is because the operation is simple and once the required flow rate and total head (i.e.,pressure) has been specified, the pump only operates on that pump curve. The direct speed drive type of design is well suited to applications where the feed water temperature is relatively constant or the variability of temperature is low, probably less than 5°C between high and low temperatures.

VFDs (Variable Frequency Drives) on centrifugal pumps should be considered when there is greater than 5°C difference between the low and high feed‐water temperatures. The use of VFDs allows you to use less energy because you operate on more than one pump curve depending on the feed pressure required. Capital cost of the VFD is higher, but operating cost is lower due to energy savings.

RO membranes require control of feed and brine flows and pressures to accommodate changes in feed water temperature, feed chemistry and membrane fouling. This important function should be handled by the integrated RO system. The optimal solution is to use a VFD to change pump‐operating speed as required for the desired feed pressure without energy-wasting throttling.

The VFD provides efficient control of the pump discharge pressure by control of the pump operating speed. The VFD also eliminates the need for a throttling valve and allows for super‐soft motion starting and easy regulation of feed water pressure.