The application, technical principles, and differences between primary and secondary reverse osmosis of reverse osmosis membranes
1、 Technical principle of reverse osmosis membrane
Reverse Osmosis (RO) membrane technology is based on the “sieving effect” and “dissolution diffusion theory” of semi permeable membranes, and overcomes natural permeation phenomena by applying pressure to achieve the separation of solutes and solvents. The specific principle is as follows:
Natural infiltration and reverse infiltration
Natural permeation: Water molecules diffuse from a low concentration solution (fresh water) through a semi permeable membrane to a high concentration solution (saltwater) until the osmotic pressure on both sides reaches equilibrium.
Reverse osmosis: When pressure exceeding the osmotic pressure is applied to the high concentration solution side, water molecules reverse permeate to the low concentration side, achieving the interception of impurities such as salt and organic matter.
Separation mechanism of membrane
The semi permeable membrane material is usually aromatic polyamide (PA), with nanoscale micropores (pore size of about 0.1nm) that only allow water molecules to pass through, intercepting ions, colloids, bacteria, organic matter, etc.
The separation process relies on the physical sieving and chemical selectivity of the membrane (such as charge repulsion and hydrogen bonding), with a retention rate of over 99% for soluble salts.
2、 Application Fields of Reverse Osmosis Membrane
Reverse osmosis membrane technology is widely used in the following scenarios due to its efficient desalination and purification capabilities:
Specific application scenarios
Drinking water treatment includes deep purification of municipal tap water, production of bottled water, and household water purifiers (such as RO water purifiers) to remove heavy metals, nitrates, bacteria, etc.
Industrial Water Treatment – Electronic Industry: High purity water (resistivity>18M Ω· cm) is used for semiconductor chip manufacturing, which requires the removal of trace ions and organic matter.
-Power industry: Boiler feedwater treatment to prevent scaling and corrosion.
-Chemical/Pharmaceutical: Desalination of process water to meet production standards (such as GMP certification).
Desalination of seawater is used to supply fresh water to islands, coastal cities, and ships. Reverse osmosis technology is used to convert seawater (with a salt content of approximately 35000mg/L) into drinking water (<500mg/L).
Municipal wastewater and industrial wastewater (such as printing and dyeing, electroplating wastewater) are treated and reused to reduce water resource consumption, such as in the deep treatment unit of the reclaimed water reuse system.
The food and beverage industry concentrates fruit juice, produces dairy products, purifies brewing water, removes pigments, odors, and microorganisms, and maintains product quality.
The preparation of blood dialysis water and pharmaceutical injection water (WFI) in the medical field requires strict control of endotoxin and ion content.
3、 The difference between primary and secondary reverse osmosis
The core differences between Single Pass RO and Double Pass RO are reflected in system design, effluent quality, and application scenarios. The specific comparison is as follows:
1. System composition and process
First level reverse osmosis
Process: Raw water → Pre treatment (sand filtration, carbon filtration, softening, etc.) → Security filter → Primary high-pressure pump → Reverse osmosis membrane module → Produced water (partial discharge of concentrated water).
Features: Single stage membrane filtration, usually equipped with one set of membrane elements, operating pressure is generally 1.0-1.5MPa, system recovery rate is about 75% (limited by the osmotic pressure on the concentrated water side).
Secondary reverse osmosis
Process: First stage reverse osmosis water production → intermediate water tank → second stage high-pressure pump → second stage reverse osmosis membrane module → water production (concentrated water can be returned to the first stage system or discharged).
Features: Two stage membrane filtration, the second stage membrane inlet is the first stage produced water (desalinated), with lower operating pressure (0.5-1.0MPa), usually using different membrane element combinations (such as first stage anti fouling membrane+second stage high desalination membrane).
2. Comparison of effluent quality
Indicator Level 1 Reverse Osmosis Level 2 Reverse Osmosis
Desalination rate 98% -99% (depending on raw water quality) 99.5% -99.8% (further removal of residual ions)
Conductivity 5-50 μ S/cm (after tap water treatment)<1 μ S/cm (close to pure water, can be used in high-purity scenarios)
Total dissolved solids (TDS) 10-100mg/L ＜ 5mg/L
Applicable scenarios: ordinary industrial water, municipal water supply, seawater desalination, primary treatment, electronic high-purity water, pharmaceutical water, precision chemical industry, etc
3. Technical differences and cost comparison
Energy consumption and pressure: Secondary reverse osmosis requires lower pressure due to better inlet water quality, reducing energy consumption by about 20% -30% compared to primary systems, but increasing equipment investment (with an additional set of membrane components and pumps).
Recovery rate: The secondary system can increase the overall recovery rate (up to 90%) through concentrated water reflux, reducing water resource waste and making it suitable for water scarce areas.
Anti pollution: The secondary membrane has less impurities in the incoming water, low risk of membrane fouling, longer cleaning cycle, and lower maintenance costs; The primary membrane requires more frequent chemical cleaning (such as acid washing, alkali washing).
4. Typical application scenarios
First level reverse osmosis:
Deep treatment of municipal tap water, desalination of seawater (such as island water supply), reuse of ordinary industrial circulating water, and primary purification in the food and beverage industry.
Secondary reverse osmosis:
Cleaning water for semiconductor wafers (TOC<10ppb, ion<1ppb), pure water for injection (in accordance with USP/EP pharmacopoeia standards), and production water for lithium battery electrolytes (with extremely low conductivity).
4、 Technological development trends
High desalination and anti pollution membrane: The desalination rate and anti organic pollution ability of the membrane are improved through nano coating and composite structure design (such as grafting hydrophilic groups on the surface of polyamide membrane).
Energy saving reverse osmosis: Develop low-pressure membranes (such as operating pressures below 800psi) and energy recovery devices (such as PX pressure exchangers) to reduce seawater desalination energy consumption to below 3kWh/m ³.
Integrated system: Combining reverse osmosis with ultrafiltration and EDI (electrodeionization) to achieve “ultrafiltration+RO+EDI” full membrane water treatment, simplifying the process and improving water quality stability.
By combining primary and secondary reverse osmosis, the system can be flexibly designed according to different water quality requirements, optimizing cost and efficiency while ensuring effluent standards.
Post time: Jul-07-2025