1. Desalination rate and salt permeability of reverse osmosis membrane

Salt transmittance = produced water concentration / influent concentration × 100%

Desalination rate = (1 – salt content of produced water / salt content of influent) × 100%

Salt penetration rate = 100% – desalination rate

The desalination rate of the membrane element has been determined when it is manufactured and formed. The desalination rate depends on the density of the ultra-thin desalination layer on the surface of the membrane element. The denser the desalination layer, the higher the desalination rate and the lower the water yield.

The desalination rate of different substances by reverse osmosis is mainly determined by the structure and molecular weight of substances. The desalination rate of high valence ions and complex monovalent ions can exceed 99%, and the desalination rate of monovalent ions such as sodium ions, potassium ions and chloride ions is slightly lower, but also more than 98%; The removal rate of organic matter with molecular weight greater than 100 can also be more than 98%, but the removal rate of organic matter with molecular weight less than 100 is low.

2. Reverse osmosis membrane water production

Water production - refers to the water production capacity of the reverse osmosis system, that is, the water through the membrane per unit time, which is usually expressed in tons / hour or gallons / day.

Permeate flow rate is also an important indicator of the water yield of reverse osmosis membrane elements. Refers to the flow rate of permeate per unit membrane area, usually expressed in gallons per square foot per day (GFD). Too high infiltration flow rate will accelerate the water flow perpendicular to the membrane surface and aggravate membrane pollution.

3. Reverse osmosis membrane recovery

Recovery rate refers to the percentage of feed water converted into produced water or permeate in the membrane system. It is determined according to the pretreatment inlet water quality and water requirements. The recovery rate of the membrane system has been determined at the time of design,

Recovery rate = (produced water flow / inlet water flow) × 100%

For large RO systems, the recovery rate is usually limited by the scaling tendency of insoluble salts, that is, by the maximum concentration of concentrated water; For small RO systems, the recovery rate is usually less than 30% ~ 50%. At low recovery rate, RO unit can operate safely without controlling the solubility of insoluble salt. Cost is usually not a priority for small RO systems.