Overview
The ultra-low pressure reverse osmosis (RO) membrane is a high-performance separation membrane element based on reverse osmosis technology, specifically designed to reduce operating pressure and enhance energy efficiency. With an operating pressure as low as 0.3-0.6 MPa (traditional RO membranes require 1.0-1.5 MPa), it ensures high salt rejection rates (≥99%) while significantly reducing energy consumption and equipment investment costs. This membrane element is widely used in municipal water supply, industrial pure water production, seawater desalination pretreatment, food and beverage concentration, and other fields. It is particularly suitable for desalination of low-mineralization water sources (such as groundwater and surface water), representing a key development direction in green water treatment technology.
Working Principle
I. Fundamentals of Membrane Separation
Ultra-low pressure RO membranes are semi-permeable composite membranes, typically composed of a polyamide composite layer (desalination layer), a porous support layer, and a protective layer. Their core principle involves using pressure lower than that of traditional RO membranes to drive water molecules through the membrane via sieve effects and dissolution-diffusion mechanisms, while intercept (trapping) dissolved salts (e.g., Na⁺, Cl⁻, Ca²⁺, Mg²⁺), organic matter, colloids, bacteria, and other impurities, thus achieving separation of water and contaminants.
II. Low-Pressure High-Efficiency Desalination Mechanism
- Ultra-Thin Desalination Layer Technology: The desalination layer thickness is reduced to 0.1-0.2 μm through nanoscale coating processes, significantly lowering resistance to water molecule passage while maintaining high selectivity.
- Optimized Membrane Structure: High-flux membrane materials and wide-channel spacer designs reduce concentration polarization, allowing high water flux (15-25 L/(m²·h)) at lower pressures.
- Charge Repulsion Effect: The negatively charged surface of the desalination layer enhances interception of anions (e.g., SO₄²⁻, HCO₃⁻) via electrostatic repulsion, while pore-size sieving retains cations and macromolecules.
III. System Operation Process
After pretreatment (e.g., filtration, sterilization, pH adjustment), raw water is driven by an ultra-low pressure pump into the membrane module. Water molecules pass through the membrane to form product water (desalted water), while und透过的 (unpassed) concentrated water (brine) with high impurity content is either recycled or discharged. Systems often use multi-stage series designs to further improve desalination rates or water production.
Performance Characteristics
I. Ultra-Low Pressure and High Energy Efficiency
- Low Operating Pressure: Pressure is reduced by over 50% compared to traditional RO membranes, with energy consumption only 30%-50% of traditional membranes for the same water production. Suitable for long-distance water transmission or high-lift scenarios.
- Significant Energy Savings: Reduces pump power requirements and pipeline pressure loads, lowering equipment investment costs, especially for large-scale municipal water supply or industrial circulating water systems.
II. High Desalination Rate and Anti-Fouling Properties
- Desalination Rate ≥99%: For water sources with total dissolved solids (TDS) ≤1000 ppm, product water conductivity can be reduced to below 10 μS/cm, meeting industrial pure water and drinking water standards.
- Anti-Fouling Design: Membrane surfaces are modified for hydrophilicity or charge regulation to reduce adhesion of organic matter and microorganisms, extending chemical cleaning cycles (by over 30% compared to traditional membranes).
III. Wide Applicability and Stability
- Diverse Water Source Adaptability: Can treat low-mineralization water sources such as groundwater, surface water, and reclaimed water, especially suitable for low-hardness water in southern China or secondary treatment of RO brine.
- Chemical Resistance: Withstands pH 2-11 environments, adapting to long-term use of pretreatment chemicals (e.g., scale inhibitors, bactericides).
IV. Long Lifespan and Low Maintenance
- Extended Service Life: Under normal conditions, membrane elements last 5-7 years, reducing replacement frequency and operational costs.
- Long Cleaning Intervals: Combined with optimized pretreatment, chemical cleaning intervals can reach 6-12 months, minimizing downtime and chemical consumption.
Structure and Types
I. Typical Structure
- Spiral Wound Membrane Elements: The most common form, rolled from membrane sheets, flow channel spacers, and a central collection pipe. Sizes often include 8-inch (200 mm) or 4-inch (100 mm) diameters, with lengths of 36-60 inches (914-1524 mm).
- Composite Membrane Materials:
- Desalination Layer: Aromatic polyamide, determining desalination performance;
- Support Layer: Polypropylene or polyester nonwoven fabric, providing mechanical strength;
- Protective Layer: Polyvinyl alcohol or cellulose membrane, preventing physical damage.
II. Main Types
- Municipal Type: Focuses on high flux and anti-colloidal fouling, suitable for advanced treatment in waterworks.
- Industrial Type: Enhanced oxidation resistance (e.g., residual chlorine ≤0.1 ppm), used for electronic-grade pure water or food processing water.
- Seawater Desalination Pretreatment Type: Withstands brackish water (TDS ≤3000 ppm), providing cost-effective pre-desalination for seawater RO.
Application Cases
I. Municipal Drinking Water Advanced Treatment
- Scenario: Surface water or groundwater is treated with ultra-low pressure RO membranes to remove heavy metals (e.g., lead, cadmium), nitrates, and pesticide residues, ensuring product water meets *National Drinking Water Health Standards* (GB5749).
- Case: A southern waterworks replaced traditional sand filtration + activated carbon processes with ultra-low pressure RO membranes, achieving product water turbidity <0.1 NTU at only 0.4 MPa pressure, saving 40% energy annually.
II. Industrial Pure Water Production
- Scenario: Electronics and pharmaceutical factories use ultra-low pressure RO membranes to produce primary pure water as pretreatment for EDI systems, reducing subsequent ion exchange loads.
- Case: A semiconductor factory adopted an ultra-low pressure RO + EDI process, achieving product water resistivity ≥15 MΩ·cm at 0.5 MPa pressure, saving approximately ¥2 million/year in electricity costs compared to traditional processes.
III. Food and Beverage Concentration
- Scenario: Concentration of liquid foods like fruit juices and whey, where RO membranes retain sugars and proteins while preserving flavor compounds, replacing traditional thermal concentration.
- Case: A dairy enterprise used ultra-low pressure RO membranes to concentrate whey with >95% retention, reducing energy consumption by 70% compared to evaporation and avoiding heat-sensitive component degradation.
IV. Brackish Water Desalination
- Scenario: In brackish water areas (e.g., arid regions in Northwest China) with TDS ≤2000 ppm, ultra-low pressure RO membranes enable low-cost desalination for domestic water or irrigation.
- Case: A rural drinking water project treated groundwater with TDS=1500 ppm using ultra-low pressure RO membranes, producing water with TDS <100 ppm at only 1.2 kWh/ton, lower than 2.5 kWh/ton for traditional RO.
Conclusion
Ultra-low pressure reverse osmosis membranes have broken through the energy consumption bottleneck of traditional RO technology through material innovation and structural optimization, becoming a benchmark for "energy-saving and carbon-reducing" water treatment. With growing global demand for green technologies, their applications in municipal, industrial, and environmental fields will continue to expand. In the future, performance is expected to further improve via nanomaterial modification (e.g., graphene composite membranes), driving the water treatment industry toward higher efficiency and lower carbon emissions.
