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Reverse osmosis (RO) unit
Reverse osmosis (RO)
I. Overview A thin film that exhibits selectivity towards the substances it allows to pass through is referred to as a semipermeable membrane. Generally, a membrane that permits only the solvent to pass while retaining the solute is considered an ideal semipermeable membrane. When equal volumes of a dilute solution (e.g., freshwater) and a concentrated solution (e.g., saltwater) are placed on either side of a container, separated by a semipermeable membrane, the solvent from the dilute solution will naturally flow through the membrane towards the concentrated solution side. This results in a higher liquid level on the concentrated solution side compared to the dilute solution side, creating a pressure differential that reaches an osmotic equilibrium. This pressure differential is known as the osmotic pressure. The magnitude of the osmotic pressure depends on the inherent properties of the solution, specifically the type, concentration, and temperature of the concentrated solution, and is independent of the semipermeable membrane's properties. If a pressure greater than the osmotic pressure is applied to the concentrated solution side, the direction of solvent flow will reverse, initiating a flow from the concentrated solution to the dilute solution side. This process is termed reverse osmosis. Reverse osmosis is a reverse migratory movement of osmosis and a separation method driven by pressure that utilizes the selective retention properties of a semipermeable membrane to separate solutes from solvents in a solution. It has found widespread application in the purification and concentration of various liquids, with one of the most common applications being in water treatment processes, where reverse osmosis technology is employed to remove inorganic ions, bacteria, viruses, organic matter, colloids, and other impurities from raw water to obtain high-quality purified water. II. Key Indicators 1. Salt Rejection Rate and Salt Passage Rate （1）Salt Rejection Rate: The percentage of soluble impurities removed from the system feedwater by the reverse osmosis membrane. （2）Salt Passage Rate: The percentage of soluble impurities in the feedwater that pass through the membrane. （3）Salt Rejection Rate Formula: (1 - Salt Content in Permeate / Salt Content in Feedwater) × 100% （4）Salt Passage Rate Formula: 100% - Salt Rejection Rate The salt rejection rate of a membrane element is determined during its manufacturing process and depends on the density of the ultra-thin desalting layer on the membrane element's surface. A denser desalting layer results in a higher salt rejection rate but a lower permeate flow rate. The salt rejection rate of reverse osmosis for different substances is primarily determined by their structure and molecular weight. It can exceed 99% for multivalent ions and complex monovalent ions, slightly lower but still above 98% for monovalent ions such as sodium, potassium, and chloride, and reach 98% for organic matter with a molecular weight greater than 100, though it is lower for organic matter with a molecular weight less than 100. 2. Permeate Flow Rate (Water Flux) Refers to the production capacity of a reverse osmosis system, i.e., the volume of water passing through the membrane per unit time, typically expressed in tons per hour or gallons per day. The permeate flow rate per unit membrane area, often referred to as the flux rate and expressed in gallons per square foot per day (GFD), is also an important indicator of the membrane element's permeate production capacity. Excessively high flux rates can accelerate the water flow velocity perpendicular to the membrane surface, exacerbating membrane fouling. 3. Recovery Rate Refers to the percentage of feedwater converted into permeate or permeate solution in a membrane system. The recovery rate of a membrane system is predetermined during design based on the expected feedwater quality. Recovery Rate Formula: (Permeate Flow Rate / Feedwater Flow Rate) × 100% III. Influencing Factors 1. Effect of Feedwater Pressure on Reverse Osmosis Membrane Feedwater pressure itself does not affect the salt passage amount, but an increase in feedwater pressure elevates the net driving pressure for reverse osmosis, resulting in an increased permeate flow rate while keeping the salt passage amount nearly constant. The increased permeate flow rate dilutes the salt passing through the membrane, reducing the salt passage rate and improving the salt rejection rate. However, when the feedwater pressure exceeds a certain value, excessively high recovery rates can exacerbate concentration polarization, leading to an increase in salt passage and offsetting the increased permeate flow rate, thereby preventing further improvement in the salt rejection rate. 2. Effect of Feedwater Temperature on Reverse Osmosis Membrane The conductivity of the permeate water from a reverse osmosis membrane is highly sensitive to changes in the feedwater temperature. As the water temperature increases, the water flux also increases linearly. For every 1°C increase in the feedwater temperature, the permeate flow rate increases by 2.5% - 3.0% (based on a standard of 25°C). 3. Effect of Feedwater pH on Reverse Osmosis Membrane The feedwater pH has little effect on the permeate flow rate but a significant impact on the salt rejection rate. The highest salt rejection rate is achieved when the pH is between 7.5 and 8.5. 4. Effect of Feedwater Salt Concentration on Reverse Osmosis Membrane Osmotic pressure is a function of the salt or organic matter concentration in the water. Higher salt content in the feedwater results in a greater concentration difference, an increased salt passage rate, and consequently, a decreased salt rejection rate. |
