Flux or Water Flux - Flux or water flux is typically expressed as volume per area per unit of time. Flux is used to express the rate at which water permeates a reverse osmosis membrane. Typical units of measurement are gallons per square foot per day (i.e. GFD or GSFD) or litres per square meter per hour (l/m2/hr).

The flux of a RO membrane is directly proportional to temperature and pressure. As a rule of thumb, flux decreases by about 1.5% per 1^oF.

Salt flux is the amount of TDS that has passed through a given area of membrane per unit of time. It is important to remember that Salt flux is a function of concentration gradient and not driving pressure. Therefore with increasing driving pressure, the concentration of salts in the permeate decreases due to constant salt leakage (e.g., milligrams) and increased water flux (e.g. liters). The net effect of increased drive pressure is to dilute a constant amount of salt with more pure water.

Note: 1 GFD = 1.66 l/m2/hr

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