Factors affecting the performance of reverse osmosis and nanofiltration membranes

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2023-09-08

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Factors affecting the performance of reverse osmosis and nanofiltration membranes

1. Recovery rate

The percentage of water supply in a membrane system that is converted into permeate or permeate. The design of the membrane system is based on the preset inlet water quality, and the concentrated water valve set on the concentrated water pipeline can adjust and set a reasonable recovery rate. Enhanced pre-treatment configurations can greatly improve recovery rates.

2. Desalination rate

The percentage of total soluble substance concentration removed from the system inlet through reverse osmosis membrane, or the percentage of specific components such as divalent ions or organic matter removed through nanofiltration membrane.

3. Flow

Flow rate refers to the inflow rate of water into the membrane element, usually expressed in cubic meters per hour (m ³/ h) Or expressed in gallons per minute (gpm). Concentrated water flow refers to the "inflow" flow rate of the non permeable part of the membrane leaving the membrane element system. This portion of concentrated water contains soluble components brought in from the raw water source, often expressed in cubic meters per hour (m ³/ h) Or expressed in gallons per minute (gpm).

4. Flux

The flow rate of liquid through a unit membrane area, usually expressed in liters per square meter per hour (l/㎡ h) or gallons per square foot per day (gfd).

5. The impact of stress

The inlet pressure affects the water production flux and desalination rate of RO and NF membranes. Permeation refers to the flow of water molecules from the dilute solution side through the membrane to the concentrated solution side. Reverse osmosis and nanofiltration technologies apply operating pressure on the inlet water flow side to overcome natural osmotic pressure. When an operating pressure higher than the osmotic pressure is applied to the concentrated solution side, the natural flow direction of water molecules will be reversed, and some of the influent (concentrated solution) will pass through the membrane to become purified water on the dilute solution side.

Due to the impossibility of absolute interception of soluble salts in the influent by RO and NF membranes, there is always a certain amount of permeability. With the increase of pressure, this increase in permeability is quickly overcome because the rate of membrane permeability through water is faster than the rate of salt transfer. However, there is an upper limit to the removal rate of salt by increasing the inlet pressure. If the pressure exceeds a certain value, the desalination rate will no longer increase, and some salts will also couple with water molecules to penetrate the membrane.

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