Modern‑day production activities and daily life generate large volumes of sewage and wastewater. Direct discharge will easily lead to environmental pollution and water‑resource waste. Harmless treatment and resource recovery of various types of wastewater are vital measures to ease water shortages and implement water‑saving and eco‑friendly development.
Reverse osmosis membrane technology is an efficient physical water‑treatment process. It requires no chemical dosing, involves no chemical reactions and produces no secondary pollution. Featuring excellent purification performance, stable operation and wide adaptability, it has been widely applied in municipal sewage treatment, industrial wastewater treatment and pure‑water production, boasting remarkable engineering value and broad application prospects.
I. Reverse Osmosis Membrane and Its Working Principle
A reverse osmosis membrane is a selectively permeable semi‑permeable membrane. It allows water molecules to pass through while effectively trapping impurities such as dissolved salts, heavy‑metal ions, colloids, organic matter and microorganisms in water. This achieves efficient separation of water from contaminants for water purification.
Under natural conditions, pure water permeates through the semi‑permeable membrane toward the salt‑water side. This process is known as natural osmosis.
Based on the pressure‑driven reverse‑osmosis principle, reverse osmosis technology applies a pressure higher than the natural osmotic pressure on the wastewater or salt‑water side to break the original osmotic equilibrium. Water molecules pass directionally through the membrane, whereas salts and contaminants are retained, so as to produce high‑purity purified water.
Membrane material directly determines the RO membrane’s temperature resistance, acid‑alkali resistance, chlorine resistance, fouling‑resistance performance and service life.
At present, mainstream membrane‑making materials fall into three categories: cellulose esters (cellulose diacetate, cellulose triacetate), special polymers (polybenzimidazole, polyimide, etc.) and polyaramids. Among them, polyaramids are the most widely‑used material for industrial applications.
II. Material Selection and Properties of Mainstream Reverse‑Osmosis Membranes
The performance of membrane materials directly affects the purification efficiency, operational stability and service life of a water‑treatment system. Therefore, rational material selection is a critical preliminary step for reverse‑osmosis water‑treatment projects.
Reverse‑osmosis membranes are divided into organic polymer membranes and inorganic membranes by material. Inorganic membranes (ceramic, metallic membranes, etc.) feature outstanding mechanical properties, temperature resistance and corrosion resistance. However, their high cost limits their application only to special extreme working conditions. Organic polymer membranes are widely adopted in conventional water‑treatment fields, mainly including cellulose acetate membranes, aromatic polyamide membranes and thin‑film composite membranes.
Practical membrane selection requires comprehensive consideration of wastewater composition, water temperature, pH value, as well as membrane mechanical strength, chemical stability, fouling resistance, service life and operating costs. The specific performance characteristics of the three mainstream RO membranes are described below:
(I) Cellulose Acetate Membrane (CA Membrane)
As a typical traditional reverse‑osmosis membrane, the cellulose acetate (CA) membrane has good chlorine resistance and fouling resistance, and can achieve a salt‑rejection rate above 95% under normal operating conditions.
It also has obvious drawbacks: vulnerable to microbial degradation, prone to aging during long‑term operation with gradual decline of desalination performance, and narrow acid‑base tolerance range, which may cause membrane damage and decomposition under extreme‑pH conditions. Its optimal operating pH ranges from 5 to 6, and it is only suitable for working conditions with mild water quality and low microbial content.
(II) Polyamide Membrane
Currently the most widely used reverse‑osmosis membrane, the polyamide membrane boasts high chemical stability, good hydrolysis resistance and excellent anti‑microbial properties, with a much longer service life than cellulose acetate membranes. It is applicable to a wide operating range of pH 4‑11 and water temperature ≤ 40 °C.
Its biggest disadvantage is extremely poor chlorine resistance. It can be easily oxidized and damaged by residual chlorine and oxidants. Accordingly, a dechlorination pretreatment process must be installed in the supporting system to ensure stable operation of membrane elements.
(III) Thin‑Film Composite Membrane
Manufactured by compositing multiple polymer materials, composite membranes deliver balanced overall performance and overcome the disadvantages of single‑material membranes. They can operate stably under a low pressure of 1.6 MPa for lower energy consumption. The general pH tolerance is 2‑12, and the optimal pH for long‑term stable operation is 5‑9.
Conventional polyamide thin‑film composite membranes inherit the merits of microbial resistance and hydrolysis resistance. Thanks to high cost‑effectiveness and broad adaptability, they are the preferred option for industrial wastewater treatment and pure‑water production. Only specially modified composite membranes are chlorine‑resistant, while ordinary composite membranes are susceptible to chlorine‑induced oxidation.
III. Applications of Reverse‑Osmosis Membrane Technology in Water Treatment
(I) Industrial and Municipal Wastewater Treatment
Wastewater treatment and resource recycling represent the core application scenario of reverse‑osmosis technology. This technology can efficiently treat complex water bodies such as power‑plant circulating water, printing‑dyeing wastewater, heavy‑metal wastewater, municipal sewage and landfill leachate. It effectively removes contaminants, recycles water resources and cuts pollutant discharge and water consumption.
During operation, impurities accumulate and turbidity rises in circulating cooling water of thermal power plants, and direct discharge may cause environmental pollution. After being treated by the ultrafiltration + polyamide composite RO dual‑membrane process, the effluent turbidity is lower than 0.02 NTU and the SDI (Silt Density Index) is below 0.7, fully meeting the reuse standard for circulating water and enabling water recycling.
Printing‑dyeing wastewater is characterized by high salinity, high chroma and high organic content, making it difficult to treat. The dual‑membrane combined process can effectively intercept pigments, salt ions and organic pollutants. After treatment, the COD of effluent can be kept below 10 mg/L with excellent purification performance, so that the treated water can reach discharge standards or be reused as a resource.
Conventional treatment of heavy‑metal wastewater mostly adopts chemical precipitation, which involves complicated procedures, high chemical consumption and risks of secondary pollution. The multi‑stage reverse‑osmosis process can efficiently retain heavy‑metal ions without large‑dose chemicals, realizing harmless and resource‑oriented wastewater treatment with simpler workflow and better environmental‑friendliness.
Municipal landfill leachate is a kind of hard‑to‑treat wastewater featuring complex composition, high pollutant concentration and unstable water quality. The advanced treatment process with polyamide RO membranes can effectively remove organic matter, ammonia‑nitrogen, salts, heavy metals and other pollutants to achieve qualified effluent discharge and eliminate leachate‑related pollution.
(II) Purified‑water and Ultra‑pure‑water Production
Drinking‑water supply, manufacturing and pharmaceutical industries impose strict requirements on water purity. Reverse osmosis is a proven core process for producing purified water and ultra‑pure water. The industry‑standard RO‑EDI (Electrodeionization) combined system can stably generate high‑purity water for various high‑precision water‑use scenarios.
The chemical industry can adopt an ultra‑low‑pressure spiral‑wound composite‑membrane RO system for process‑water production. The daily water yield of a single membrane element can reach 41.4 m³ with an impurity rejection rate of no less than 99.7 %. It efficiently removes scaling ions such as Ca²⁺ and Mg²⁺ and guarantees stable feed‑water quality for industries including caustic soda and chlor‑alkali production.
Pharmaceutical purified‑water production generally uses an ultra‑low‑pressure polyamide composite membrane with a two‑stage reverse‑osmosis system. This highly‑stable process achieves a system water recovery rate of ≥ 80 %, satisfies stringent pharmaceutical water standards, saves water resources and reduces operating costs.
Single‑stage reverse osmosis cannot meet ultra‑pure‑water specifications. The RO + EDI combined process produces ultra‑pure water with conductivity as low as 0.055 μS/cm, which serves high‑end sectors such as electronics, precision manufacturing and laboratories.
For membrane fouling during operation, regular chemical cleaning can restore membrane performance and ensure long‑term stable system operation.
IV. Conclusion
With the advantages of purely‑physical filtration, zero secondary pollution, high purification efficiency, easy operation and maintenance, and wide applicability, reverse‑osmosis water‑treatment technology has become a core technology for water purification and wastewater resource recovery. It carries great practical significance for industrial water conservation, pollution reduction and ecological protection.
With continuous R&D of membrane materials and process optimization, new‑generation reverse‑osmosis membranes will achieve performance upgrades including low energy consumption, superior fouling resistance, high‑low temperature resistance and strong acid‑alkali tolerance. Their operational stability and service life will be further improved. The technology will keep expanding into more application fields and enjoy broad development prospects for the sustainable utilization of water resources.
