How the calculation works
- Feed flow = product water flow ÷ recovery. At 75 % recovery, 10 m³/h of product water needs 13.3 m³/h of feed.
- Concentrate flow = feed flow − product water flow.
- Concentration factor = 1 ÷ (1 − recovery). At 75 % recovery the salts in the concentrate are about four times as concentrated as in the feed.
- Product water salinity ≈ feed salinity × (1 − rejection) × mean concentration factor on the feed side.
- Energy cost = specific energy × electricity price, per m³ of product water.
Typical values
| Water | Salinity (TDS) | Recovery | Specific energy |
|---|---|---|---|
| Tap or well water | 200–1,000 mg/L | 75–85 % | 0.4–0.8 kWh/m³ |
| Brackish water | 1,000–10,000 mg/L | 65–85 % | 0.5–1.5 kWh/m³ |
| Seawater | 35,000–45,000 mg/L | 35–50 % | 2.5–4.0 kWh/m³ with energy recovery |
Limits of this calculator
The recovery you can reach is limited by scaling: calcium carbonate, calcium sulphate, silica and barium sulphate become more concentrated by the concentration factor. The real permeate quality depends on the membrane type, the temperature and the membrane age. A membrane projection based on your water analysis is part of every proposal for our industrial reverse osmosis systems and seawater reverse osmosis plants.
Frequently asked questions
What is recovery in reverse osmosis?
Recovery is the share of the feed water that leaves the plant as product water (permeate). The rest leaves as concentrate and carries the rejected salts.
How much water does a reverse osmosis plant waste?
At 75 % recovery, one quarter of the feed leaves as concentrate. Seawater plants run at 35–50 % recovery, so 50–65 % of the seawater intake returns to the sea as brine.
How much energy does reverse osmosis need per m³?
Brackish water plants typically need 0.5–1.5 kWh per m³ of product water. Seawater plants need 2.5–4.0 kWh per m³ with an energy recovery device.