Reverse Osmosis Water Treatment Technology: Principles, Applications, Membrane Fouling Prevention,

Reverse Osmosis Water Treatment Technology: Principles, Applications, Membrane Fouling Prevention, Membrane Technology Comparison, and Equipment Analysis

Reverse osmosis is one of the commonly used water treatment technologies.
The principle involves applying pressure higher than the solution's osmotic pressure to prevent other substances from passing through a semipermeable membrane, thereby separating these substances from water. This effectively removes dissolved salts, colloids, microorganisms, pyrogens, organic matter, and other contaminants from water.

In other words, the desalination principle of reverse osmosis involves applying pressure greater than the natural osmotic pressure to saline water (such as raw water), reversing the direction of osmosis. This forces water molecules from the raw water to the other side of the membrane, producing clean water and achieving the goal of removing salts from the water.

Reverse osmosis is a membrane separation technology driven by pressure differences, representing a permeation process opposite to natural osmosis. It stands as one of the most advanced membrane separation techniques available today.

Reverse osmosis finds applications not only in boiler feedwater preparation but is also gaining increasing attention in wastewater treatment.

Compared to technologies like microfiltration, ultrafiltration, and nanofiltration, reverse osmosis offers higher salt rejection rates and recovery rates. It effectively reduces the volume of concentrated wastewater discharged, thereby lowering energy consumption in subsequent evaporation and solidification processes.

However, reverse osmosis technology imposes stringent requirements on feedwater quality. Typically, the pH range must be between 4 and 10, temperature below 40°C, Silt Density Index (SDI) less than 5, free chlorine below 0.1 mg/L, turbidity under 1 NTU, and iron content below 0.1 mg/L.

I. Causes of Reverse Osmosis Membrane Fouling
Similar to other membrane treatment technologies, reverse osmosis inevitably experiences membrane fouling during operation. The primary causes include:

1. Scaling Fouling
Reverse osmosis involves concentrating feedwater. As feedwater is concentrated, insoluble salts exceed their solubility product and precipitate, forming scale deposits on the membrane surface. Concentration polarization further exacerbates this phenomenon.

2. Microbial Contamination
Reverse osmosis feedwater may contain various microorganisms, including bacteria, algae, fungi, viruses, and other higher organisms.

3. Organic Contamination
Reverse osmosis feedwater may contain a wide variety of organic compounds. During system operation, these organic substances adsorb onto the membrane surface, causing membrane flux loss. In severe cases, irreversible flux loss may occur.

4. Colloidal Contamination
Colloidal contamination primarily results from feedwater concentration and pH changes. Contaminants may be organic or inorganic monomers or complexes.

II. Preventing Reverse Osmosis Membrane Contamination

1. Adjusting Operating Parameters to Protect Membranes
Key parameters include pH, temperature, and operating pressure.
2. To prevent inorganic salt scaling on the membrane surface, add acid or scale inhibitors.
3. Prevent colloidal contamination through media filtration, coagulation-flocculation processes, cartridge filtration, or integrated treatment methods.
4. Chlorination is the most effective method for preventing microbial contamination. However, to avoid membrane oxidation, consider dechlorination of the reverse osmosis feed water.

III. Comparison of Microfiltration, Ultrafiltration, Nanofiltration, and Reverse Osmosis

Membrane Process

Microfiltration (MF):
Driving Force: Pressure difference (0.01–0.2 MPa)

Separation Mechanism: Sieving

Permeate: Water, solvent-dissolved substances, gases

Retained: Suspended solids, particles, fibers, and bacteria (0.01–10 μm)


Ultrafiltration (UF):
Driving force: Pressure difference (0.01–0.5 MPa)

Separation mechanism: Sieving

Permeate: Water, solvent, ions, and small molecules (MW < 1000)

Retentate: Bioproducts, colloids, and macromolecules (MW 1000–30000)



Nanofiltration (NF)

Driving force: Pressure difference (0.5–2.5 MPa)

Separation mechanism: Sieving, dissolution/diffusion

Permeate: Water, solvents, monovalent salts | Solutes (MW < 200)

Retentate: Solutes, divalent salts, sugars, dyes (MW 200–1000)


Reverse Osmosis (RO)

Driving Force: Pressure difference (0.5–2.5 MPa)

Separation Mechanism: Sieving, dissolution/diffusion

Permeate: Water and solvent

Retentate: All suspended solids, solutes, and salts


Electrodialysis (ED)

Driving force: Potential difference

Separation mechanism: Ion exchange

Permeate: Electrolyte ions

Retentate: Non-dissociated and macromolecular substances

IV. Common Equipment in Membrane Separation Technology

01 Small-scale reverse osmosis system flow

02 Pretreatment equipment: Quartz sand filter and its working principle

03 Quartz sand filter + activated carbon filter + nanofiltration equipment

04 Precision Filter + Two-Stage High-Pressure Pump + Reverse Osmosis Equipment