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One of the factors that indicate the performance of a reverse osmosis system is its recovery rate.
While it’s generally expected that reverse osmosis will waste a lot of water for each filtered portion, there are some reasonable bounds for that.
If you don’t know how to calculate your recovery rate, read on.
You can calculate your RO system recovery rate using our free calculator below or one of these two formulas:
You can approach this measurement from two perspectives: you can either use the flow rate of your purified water for reference in the calculation, or the flow rate of the wastewater line. In either case, you’ll also need to know the flow rate of your input stream.
Measuring flow rates is easier than you might think. Simply take a large container, let it fill for exactly one minute, and check how much water has accumulated. This will give you your flow rate in gallons per minute.
Take this measurement for your input water stream before it enters the RO membrane, and then either for the filtered water stream or the wastewater stream. Then, use one of the following formulas, depending on which one you’ve chosen to go with:
If you are using wastewater:
Recovery rate in % = (Input flow rate – Wastewater flow rate) / Input flow rate X 100
For example, if your input flow rate is 0.25 gallons per minute, and your wastewater flow rate is 0.2 gallons per minute, your recovery rate is:
(0.25 gpm – 0.2 gpm) / 0.25 gpm x 100 = ~20%
If you are using filtered water:
Recovery rate in % = Filtered water flow rate / Input flow rate x 100
If your input flow rate is 0.25 gpm and your filtered water flow rate is 0.05 gpm, your recovery rate is:
0.05 / 0.25 X 100 = ~20%
Our calculator is based on the above formulas. It’s no different than doing the calculations yourself, although the simplified interface makes the process much more straightforward.
You first enter your input flow rate as well as your permeate or reject water flow rate. The calculator will provide your recovery rate instantly.
Recovery rate is one of the factors that tell you how well your reverse osmosis system is doing as a whole, along with other numbers like the TDS rejection rate.
Monitoring the recovery rate and keeping track of changes in its value can help you identify problems with your system before they’ve developed into anything more serious. Plus, it’s good to maintain a relatively high recovery rate in general – it means less water is going to waste.
If you want to improve the recovery rate of your reverse osmosis system, there are several things to look at: feed water pressure, refiltering wastewater, and of course, proper maintenance of your reverse osmosis system.
If possible, try to increase the pressure on the feed line. This can be done with a pressure pump or by adjusting an existing pressure valve, if you already have one in place.
And of course, always make sure to keep your reverse osmosis system in good condition. Most importantly, replace the membrane regularly. Also make sure that pre and post-filters are working properly, and if not, replace them.
Cleaning your RO system and sanitizing occasionally can also help improve recovery rates.
Another less commonly used approach to improving RO recovery rates is to run the wastewater through an additional membrane stage in order to recycle it. This greatly improves recovery rates, however, it puts a huge burden on the second membrane, since it has to process water which already is much more contaminated.
RO reject water recycling is most often found in whole house and commercial setups.
If you have any questions about how to calculate RO recovery rates please don’t hesitate to leave a comment below!
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