LWW recently got a question from one of our Reverse Osmosis and Softening (ROS) systems operators in the Yucatan. They asked the following:
The ROS system was installed in July 2012. The system operator was concerned about the membranes because the TDS was measuring ~50 ppm even after a softener regeneration. What does LWW recommend?
My short answer to the question was that the RO has probably lost some capacity in the last 8 months of operation. It doesn’t sound like it’s time to wash the membrane, but it is time to take notice and start or continue to take data on system operation. I said that they should plan on washing the membrane after a year of operation. This would be the washing process done by a trained person such as Carlos McGregor with acid and base solutions, overnight soaking, etc.
On the RO system, the regeneration of the resin in the softener protects the RO from Calcium and Magnesium contamination in the water. Calcium and Magnesium are the elements that show up as hardness in the water. These contaminates can foul a membrane very quickly and you can’t wash them out. For that reason, you need to measure the hardness of the water as well as the TDS after the regeneration. With a TDS of 50 ppm you could estimate the hardness at 20 – 30 ppm. I would say that 50 ppm of TDS in the product water is OK as long as you can’t taste any salt in the water.
As an experiment, try measuring the TDS in the waste water as well as the TDS going into the RO. You may be able to run a rough balance of TDS (in) = TDS (out). You could also calculate the %TDS removed and going down the drain in the waste stream. As long as the RO is removing 90% plus of the TDS, then the membrane is doing its job. (This assumes that you are keeping the waste flow about equal to product flow.)
The best way to determine when it is time to wash the membrane is to track the gallons per minute (gpm) output of the RO. If when you start out, the RO product water flow is 3.0 gpm, that is your baseline. The manufacturer says that when the product water flow drops by 10% to 2.7 gpm, then it is time to wash the membrane. The problem is that the drop in flow is very slow, so unless you are writing down data every week or more, the operator will lose track of how the system is performing.
Joanie Lukins added to this advice by reminding ROS system operators that it is recommended that water softeners should get an extra regeneration about every month. This extra regeneration uses more salt, but ensures that the resin is returned to full softening capacity extending the life of the membranes.
If there are other comments on this posting, please submit them.
Thanks, Ralph Young, Technical Director, Living Waters for the World