The main products

Reverse osmosis (RO)

Ⅰ.Summarize

The membrane that selectively permits the passage of substances is known as a semi-permeable membrane. Generally, a membrane that allows only solvent to pass through while blocking solute is considered an ideal semi-permeable membrane. When equal volumes of dilute and concentrated solutions are placed on opposite sides of a container, separated by a semi-permeable membrane, the solvent in the dilute solution will naturally flow through the membrane towards the concentrated solution side. This results in the liquid level on the concentrated solution side rising above the level on the dilute solution side, creating a pressure difference that reaches an osmotic equilibrium state. This pressure difference is known as osmotic pressure. The magnitude of osmotic pressure depends on the type, concentration, and temperature of the concentrated solution, and is unrelated to the properties of the semi-permeable membrane.

When equal volumes of a dilute solution (e.g., fresh water) and a concentrated solution (e.g., salt water) are placed on either side of a semi-permeable membrane, the solvent in the dilute solution will spontaneously flow through the membrane towards the concentrated solution side. This phenomenon is called osmosis. When osmosis reaches equilibrium, the liquid level on the concentrated solution side will be higher than that on the dilute solution side, forming a pressure difference, which is the osmotic pressure. The size of osmotic pressure depends on the inherent properties of the solution, specifically related to the type, concentration, and temperature of the concentrated solution, and is independent of the properties of the semi-permeable membrane.

If a pressure greater than the osmotic pressure is applied to the side of the concentrated solution, the flow direction of the solvent will reverse from its original osmotic direction, beginning to flow from the concentrated solution towards the dilute solution side. This process is called reverse osmosis. Reverse osmosis is a reverse migration process of osmosis, a separation method driven by pressure that utilizes the selective retention of a semi-permeable membrane to separate solutes and solvents in a solution. It has been widely applied in the purification and concentration of various liquids, with the most common example being in water treatment processes, where reverse osmosis technology is used to remove inorganic ions, bacteria, viruses, organics, colloids, and other impurities from raw water to obtain high-quality pure water.

Ⅱ.Main indicators

1.Desalination Rate and Salt Passage Rate

Desalination Rate — The percentage of soluble impurity concentration removed from the feed water of the system through the reverse osmosis (RO) membrane.

Salt Passage Rate — The percentage of soluble impurities in the feed water that pass through the membrane.

Desalination Rate = (1–Salt Content of Product Water / Salt Content of Feed Water) × 100%

Salt Passage Rate = 100% – Desalination Rate

The desalination rate of a membrane element is determined during its manufacturing process. The level of desalination 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, but the lower the water production volume. The desalination rate of RO for different substances is mainly determined by the structure and molecular weight of the substances. The desalination rate for high-valent ions and complex monovalent ions can exceed 99%, while for monovalent ions such as sodium ions, potassium ions, and chloride ions, the desalination rate is slightly lower but still exceeds 98%. For organics with a molecular weight greater than 100, the removal rate can also reach 98%, but for organics with a molecular weight less than 100, the removal rate is lower.

2.Water Production Volume (Water Flux)

Water Production Volume (Water Flux) — Refers to the capacity of the RO system, i.e., the amount of water that passes through the membrane per unit time, usually expressed in tons per hour or gallons per day.

Osmotic Flow Rate — Also an important indicator of water production volume for RO membrane elements. It refers to the flow rate of permeate on a unit membrane area, usually expressed in gallons per square foot per day (GFD). Excessively high osmotic flow rates will increase the water flow velocity perpendicular to the membrane surface, exacerbating membrane fouling.

3.Recovery Rate

Recovery Rate — Refers to the percentage of feed water in the membrane system that is converted into product water or permeate. The recovery rate of the membrane system is determined during design and is based on the preset feed water quality.

Recovery Rate = (Product Water Flow Rate / Feed Water Flow Rate) × 100%

III. Influencing Factors

1.Impact of Feed Water Pressure on Reverse Osmosis Membranes

The feed water pressure itself does not affect the salt passage. However, an increase in feed water pressure raises the net pressure driving reverse osmosis, leading to an increase in water production volume. Meanwhile, the salt passage remains almost unchanged. The increased water production volume dilutes the salt that passes through the membrane, reducing the salt passage rate and increasing the desalination rate. When the feed water pressure exceeds a certain value, excessively high recovery rates due to the pressure intensify concentration polarization, which in turn increases the salt passage, offsetting the increased water production volume and preventing a further increase in the desalination rate.

2.Impact of Feed Water Temperature on Reverse Osmosis Membranes

The conductivity of water produced by reverse osmosis membranes is highly sensitive to changes in feed water temperature. As the water temperature increases, the flux also increases linearly. For every 1℃ increase in feed water temperature, the water production volume increases by 2.5% to 3.0% (with 25℃ as the benchmark).

3.Impact of Feed Water pH on Reverse Osmosis Membranes

The feed water pH has little effect on water production volume but has a significant impact on the desalination rate. The desalination rate peaks when the pH is between 7.5 and 8.5.

4.Impact of Feed Water Salt Concentration on Reverse Osmosis Membranes

Osmotic pressure is a function of the concentration of salts or organics in water. Higher feed water salt content results in a larger concentration difference, increasing the salt passage rate and thus decreasing the desalination rate.

IV. Applications

In food, beverage, and pure water production processes.

In the preparation of water for the pharmaceutical, electronics, and other industries using reverse osmosis principles.

In the concentration, separation, purification, and water preparation processes in chemical engineering.

In desalination and softening of boiler feed water.

In the desalination of seawater and brackish water.

In water and wastewater treatment for industries such as papermaking, electroplating, and textile printing and dyeing.

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