Water Knowledge: Introduction to Reverse Osmosis Membrane Indicators-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Water Knowledge: Introduction to Reverse Osmosis Membrane Indicators

Published Time:

2021-09-28

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1. Reverse osmosis membrane desalination rate and salt passage rate

Salt passage rate = Product water concentration / Influent concentration × 100%

Desalination rate =(1–Product water salinity / Influent salinity) × 100%

Salt passage rate = 100% – Desalination rate

The desalination rate of the membrane element is determined when it is manufactured. The high and low 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 reverse osmosis for different substances is mainly determined by the structure and molecular weight of the substance. The desalination rate for high-valence ions and complex monovalent ions can exceed 99%, and the desalination rate for monovalent ions such as sodium ions, potassium ions, and chloride ions is slightly lower, but it also exceeds 98%; the removal rate of organic matter with a molecular weight greater than 100 can also reach 98%, but the removal rate of organic matter with a molecular weight less than 100 is lower.

2. Reverse osmosis membrane water production

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

Permeation rate ——It is also an important indicator of the water production of reverse osmosis membrane elements. It refers to the flow rate of permeate per unit membrane area, usually expressed in gallons per square foot per day (GFD). Too high a permeation rate will cause the water flow velocity perpendicular to the membrane surface to increase, exacerbating membrane fouling.

3. Reverse osmosis membrane recovery rate

Recovery rate ——Refers to the percentage of feed water converted into product water or permeate in the membrane system. It is determined according to the pretreatment feed water quality and water requirements. The recovery rate of the membrane system is determined during design,

Recovery rate = (Product water flow rate / Feed water flow rate) × 100%

For large RO systems, the recovery rate is usually limited by the tendency of sparingly soluble salts to scale, that is, by the maximum concentration of concentrated water; for small RO systems, the recovery rate is usually below 30%~50%. At lower recovery rates, the RO unit can operate safely without controlling the solubility of sparingly soluble salts. Cost is usually not a priority for small RO systems.