Reverse Osmosis Concentrate Volume Calculator
What the Reverse Osmosis Concentrate Volume Calculator does
The Reverse Osmosis Concentrate Volume Calculator estimates how many gallons of concentrate a reverse osmosis system produces over a specified operating period. Enter the feed flow rate, operating time, and water recovery percentage to calculate the volume of water that remains after permeate is produced.
In a reverse osmosis system, feed water enters the membrane and separates into two streams. Permeate is the treated water that passes through the membrane. Concentrate, also called reject water or brine, is the remaining stream that carries away much of the material rejected by the membrane.
This calculator uses a simple volume balance: feed volume equals permeate volume plus concentrate volume. It assumes the feed flow rate and recovery percentage stay constant throughout the operating period, and that all feed water that does not become permeate leaves as concentrate. The default inputs are illustrative, so replace them with figures that reflect your system when available.
How to use the Reverse Osmosis Concentrate Volume Calculator
To estimate concentrate volume, enter these three values:
- Feed flow rate (gallons per hour): Enter the total rate at which water flows into the reverse osmosis system. Use the feed rate, rather than the rate of treated water coming out of the system.
- Operating time (hours): Enter the number of hours the system operates at the stated flow and recovery rates. For example, use 2 for two hours or 0.5 for 30 minutes.
- Water recovery (%): Enter the percentage of feed water that becomes permeate. A recovery of 50% means that half of the feed volume becomes permeate under the calculator’s assumptions.
The result is shown as Concentrate Volume in gallons, rounded to two decimal places. Make sure the flow rate is in gallons per hour and the operating period is in hours so the units match.
If you know the feed and permeate flow rates instead of the recovery percentage, you can calculate recovery before using the tool. Divide the permeate flow rate by the feed flow rate and multiply by 100. Both flow rates must use the same time unit, and the feed flow must be greater than zero.
How the Reverse Osmosis Concentrate Volume Calculator formula works
The calculator first finds the total volume of feed water processed during the operating period. It then applies the share of feed water that does not become permeate:
Concentrate volume = Feed flow rate × Operating time × (1 − Water recovery percentage ÷ 100)
For example, a recovery of 50% corresponds to a recovery fraction of 0.50. The remaining fraction is 1 − 0.50, or 0.50. Under the calculator’s assumptions, that remaining half of the feed volume leaves as concentrate.
Worked example using the default inputs
Assume a system receives 10 gallons per hour of feed water, operates for 24 hours, and has a constant 50% water recovery. These are illustrative assumptions, not measurements for a particular system.
- Calculate feed volume: 10 gallons per hour × 24 hours = 240 gallons.
- Convert recovery to a fraction: 50 ÷ 100 = 0.50.
- Find the concentrate fraction: 1 − 0.50 = 0.50.
- Calculate concentrate volume: 240 gallons × 0.50 = 120.00 gallons.
In this example, the other 120 gallons are permeate. Together, the 120 gallons of permeate and 120 gallons of concentrate equal the 240 gallons of feed water.
As another illustrative example, a feed flow of 100 gallons per hour over two hours produces 200 gallons of feed volume. At 75% recovery, the remaining 25% is concentrate, giving an estimated 50.00 gallons.
Use cases for the Reverse Osmosis Concentrate Volume Calculator
A concentrate estimate can help you understand where the feed water goes and plan around the system’s expected operating schedule. Possible uses include:
- Comparing recovery scenarios: Keep the feed flow and operating time the same, then change the assumed recovery percentage to see how it affects concentrate volume.
- Planning for a run: Estimate concentrate produced during a known number of operating hours when flow and recovery are reasonably steady.
- Reviewing water use: Compare the estimated concentrate with the feed volume to understand the proportion that does not become treated water.
- Checking a measured result: Compare the estimate with actual collected or metered volumes to identify differences that may deserve further investigation.
- Estimating collection needs: Use the result as an initial planning figure if concentrate is collected, while allowing for actual operating conditions and other water flows.
The tool can be useful for discussing system performance, but an estimate alone does not determine whether a particular operating setting is suitable. Follow the system manufacturer’s instructions and any applicable requirements for concentrate handling or disposal.
Other factors to consider when calculating Concentrate Volume
Flow and recovery may change. The formula assumes both remain constant for the entire period entered. Actual operation can vary, so a single estimate may be less representative over a long period with changing conditions. If you have separate measurements for periods with different steady rates, estimate each period separately and add the results.
Operating time is not necessarily elapsed time. Use the hours when the system is actually operating at the stated feed rate. If it cycles on and off during a day, entering the full day as operating time could overstate the feed and concentrate volumes.
Flushing and other losses are excluded. Startup flushing, leaks, and other water losses are not accounted for. If those flows occur, the calculator’s concentrate result may differ from the total water sent to a drain or collection system.
Recovery is a percentage of feed water. It is not the percentage of contaminants removed by the membrane. Using a contaminant rejection percentage in place of water recovery would produce the wrong volume estimate.
Volume is not the same as water quality. The calculator estimates gallons of concentrate; it does not calculate the concentration of dissolved substances, assess water quality, or determine whether the concentrate is appropriate for a particular use or disposal method.
Boundary values are mathematical cases. At 0% recovery, the formula assigns all feed volume to concentrate. At 100% recovery, it returns zero concentrate. These results follow the formula and should not be taken as evidence that a real system can operate at either extreme.
Frequently asked questions
What is reverse osmosis concentrate?
Concentrate is the stream of water that does not pass through the reverse osmosis membrane as permeate. It carries away much of the material rejected by the membrane and may also be called reject water or brine.
How does a higher recovery rate affect concentrate volume?
For the same feed flow and operating time, a higher recovery percentage means a larger share of feed water becomes permeate. The formula therefore estimates a smaller concentrate volume. Actual system settings and performance depend on the equipment and operating conditions.
Can I calculate concentrate volume from daily feed volume?
The calculator uses a feed flow rate in gallons per hour and an operating time in hours. If you know a daily feed volume, determine the corresponding flow rate during the actual operating hours, provided the flow is steady enough for the estimate. For example, 240 gallons processed over 24 operating hours corresponds to an assumed 10 gallons per hour.
Is concentrate volume the same as total drain water?
Not always. The estimate covers the feed water remaining after permeate production under the stated assumptions. Startup flushing, leaks, or other flows may affect the total water measured at a drain.
Why might my measured concentrate differ from the estimate?
Feed flow and recovery may vary over time, operating hours may differ from the value entered, and the system may have flushing or other losses that the calculator excludes. Use measured flow rates and actual operating time where possible, and treat the result as an estimate.