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Views: 6421 Author: Site Editor Publish Time: 2026-05-23 Origin: Site
As a core separation component in the water treatment industry, reverse osmosis (RO) membrane packaging has evolved from a single wet-membrane format to a dual-format system of dry and wet membranes. This evolution is closely tied to iterations in membrane material technology, upgrades in logistics requirements, and the diversification of application scenarios.
Commercial exploration of reverse osmosis technology began in the 1950s, and industrial mass production of membrane elements started in the 1960s. Against the backdrop of growing water scarcity, seawater desalination and industrial wastewater treatment became urgent industry demands. The large-scale production and stable preservation of RO membranes—core components for water separation—emerged as key industry breakthroughs. During this period, DuPont (USA) pioneered the industrial mass production of cellulose acetate (CA) RO membranes, becoming the world’s first company to commercialize RO membranes. This laid the foundation for the initial form of wet-membrane packaging and ushered in an era where RO membranes could only be stored in a wet state.
As the first generation of commercial RO membranes, cellulose acetate membranes are made of cellulose acetate polymer. They feature moderate hydrophilicity and separation performance, effectively separating water molecules from dissolved salts and impurities. However, they have a fatal structural flaw: the membrane layer is formed by interwoven polymer chains. Once exposed to air and completely dried, the polymer chains undergo irreversible shrinkage and embrittlement, leading to permanent blockage and rupture of membrane pores—rendering the membrane element completely unusable with no possibility of recovery. This inherent property dictated that early cellulose acetate RO membranes had to remain wet at all times to preserve structural integrity and filtration performance. Thus, wet-membrane packaging was the only available factory packaging method, with no viable dry-membrane technical route.
To address the storage and transportation of cellulose acetate membranes, DuPont developed a dedicated membrane preservation solution. Centered on sodium bisulfite and blended with small amounts of preservatives and buffers, this solution kept the membrane layer wet, isolated it from air, inhibited microbial growth on the membrane surface, and prevented oxidative hydrolysis of the membrane layer. The specific packaging process was as follows: after rolling, membrane elements were rinsed with pure water to remove residual impurities and processing agents; they were then fully immersed in the dedicated preservation solution for at least 24 hours to ensure full penetration into the membrane layer and complete coverage of the desalination layer. Next, vacuum sealing technology was used to package the membrane elements with an appropriate volume of preservation solution in thickened aluminum foil vacuum bags, expelling air to prevent membrane oxidation. Finally, the sealed elements were placed in specialized corrugated boxes lined with shock-absorbing cushioning materials to avoid collision damage during transit. This packaging method became the prototype of wet-membrane packaging and set a benchmark for subsequent wet-membrane process optimization.
After DuPont, Dow Chemical, Hydranautics, and other companies entered the RO membrane sector, all adopting the wet-membrane packaging technology for cellulose acetate membranes. Leveraging their R&D strengths, they further optimized preservation solution formulations and vacuum packaging processes to reduce risks such as preservation solution leakage and membrane element damage during transportation. In this era, all global RO membranes were wet membranes. The core purpose of packaging was to protect membranes from drying damage and ensure qualified filtration performance upon delivery, catering to the then-dominant industrial engineering scenarios with no need for long-distance transportation.
Early wet-membrane packaging had prominent shortcomings that paved the way for dry-membrane R&D:
Short shelf life: Sodium bisulfite in the preservation solution oxidizes and fails over time, resulting in a typical shelf life of only about 1 year. After expiration, the solution can no longer effectively inhibit microbial growth, leading to mold, degradation, and performance degradation of membrane elements.
Strict storage and transportation conditions: The preservation solution in wet membranes freezes at low temperatures, expanding and rupturing membrane elements and vacuum bags; at high temperatures, the solution deteriorates and volatilizes, also damaging membranes.
Limited long-distance transportation: With the rise of global trade, RO membranes needed to be shipped worldwide via sea transport (1–2 months per voyage). The high temperature and humidity of marine environments easily caused preservation solution deterioration and bacterial growth, drastically reducing product pass rates and causing substantial economic losses for manufacturers.
In the 1980s, RO membrane technology witnessed a revolutionary breakthrough: FilmTec (later acquired by Dow Chemical) successfully developed thin film composite (TFC) polyamide RO membranes. Compared with traditional cellulose acetate membranes, TFC membranes offer superior salt rejection, fouling resistance, and service life, quickly replacing cellulose acetate membranes as the industry mainstream. This breakthrough also provided the core technical prerequisite for dry-membrane packaging, completely breaking the restriction that "membrane elements must remain wet at all times".
The core structure of TFC polyamide membranes is a three-layer system: support layer + transition layer + ultra-thin desalination layer. The ultra-thin desalination layer is made of polyamide polymer and features a unique reversible drying characteristic—unlike the irreversible "dry-and-destroy" trait of cellulose acetate membranes, TFC membranes do not embrittle or rupture when completely dehydrated. The polymer chains temporarily shrink and membrane pores narrow, but when rehydrated and fully immersed, the chains expand again, membrane pores revert to their original state, and filtration performance (salt rejection, flux) is almost fully restored to factory specifications with no permanent damage. This critical characteristic laid a solid technical foundation for dry-membrane packaging R&D and enabled dry storage of membrane elements.
The development of dry-membrane technology was essentially a solution to the pain points of wet membranes in long-distance transportation and long-term storage. With rapid trade growth, demand for RO membrane transportation surged. Long-distance sea transport involved long cycles and harsh environments, and the shortcomings of wet membranes—short shelf life, sensitivity to freezing and high temperatures—severely constrained industry development. Sensing this market demand, FilmTec developed a dry-membrane production process based on the reversible drying property of TFC membranes, achieving industrial mass production of dry membranes in the 1990s. This officially launched the era of parallel dry and wet membrane packaging for RO membranes.
Subsequently, Dow Chemical, Hydranautics, Toray, and other manufacturers followed suit, optimizing dry-membrane drying processes and launching their own dry-membrane products to enrich the dual-packaging system. The advent of dry membranes not only solved the pain points of wet membranes in long-distance transportation and long-term storage but also expanded RO membrane application scenarios, enabling adaptation to cold regions and long-term inventory storage—use cases inaccessible to wet membranes—and driving the continued growth of the RO membrane industry.
Since then, RO membrane packaging has formed a clear technological divide:
Traditional cellulose acetate membranes: Due to irreversible drying characteristics, they can only use wet-membrane packaging. With the popularization of TFC membranes, cellulose acetate membranes have gradually exited the mainstream market, with only limited applications in special scenarios such as low-concentration wastewater treatment.
TFC polyamide membranes: Thanks to reversible drying characteristics, they can flexibly adopt wet or dry packaging according to market demand. The same membrane sheet and production line can produce both wet and dry membranes solely through different post-treatment processes. The two formats share identical core materials and filtration performance, differing only in storage, transportation, and usage methods.
The core differences between dry and wet membranes stem from distinct post-treatment processes of TFC membranes after factory production. These process differences directly determine their packaging forms, storage conditions, usage requirements, and performance. Based on current mainstream industry products (from leading manufacturers such as Hydranautics, Dow Chemical, and Toray), this section provides a detailed comparison of the two formats across four dimensions: production process, packaging form, core advantages, and shortcomings, clarifying their technical features and applicable scenarios.
The production process of wet membranes 延续 s the wet-storage logic of early cellulose acetate membranes, optimized for TFC membrane properties, with the core goal of keeping the membrane layer wet to ensure immediate stable performance upon installation. Wet membranes are not the industry mainstream today, with concentrated application scenarios. Only a few leading manufacturers continue production, focusing primarily on high-end industrial engineering scenarios.
The specific production flow is as follows:
After rolling, membrane elements are rinsed with pure water to remove residual impurities and processing agents.
Elements are fully immersed in an optimized dedicated preservation solution (still centered on sodium bisulfite, blended with preservatives and buffers; optimized shelf life extended to 1.5 years) for at least 24 hours to ensure full penetration into the membrane layer and coverage of the desalination layer.
Vacuum sealing technology is used to package elements with an appropriate volume of preservation solution in thickened aluminum foil vacuum bags, expelling air to prevent membrane oxidation and microbial growth.
Sealed elements are placed in specialized corrugated boxes (4, 8, or 12 pieces per box for 4040, 8040, and other specifications) lined with shock-absorbing cushioning materials to avoid collision damage during transportation.
Packaging characteristics (taking Hydranautics wet membranes as an example):
Outer packaging: Specialized corrugated boxes printed with product model, specification, production date, shelf life, storage conditions, and other information.
Inner packaging: Thickened aluminum foil vacuum bags containing transparent preservation solution; liquid flow is perceptible when gently shaken.
Some industrial wet membranes add an extra waterproof plastic film outside the vacuum bag for enhanced transportation protection.
Authentic wet membranes (e.g., Hydranautics) feature product serial numbers and anti-counterfeiting codes on vacuum bags; the box includes a product certificate and user manual specifying preservation solution composition, storage temperature (5–35°C), and a ban on freezing.
Common Hydranautics wet-membrane models: CPA3-LD, ESPA2-4040, LFC3-LD, all dedicated to high-end industrial engineering.
Core advantages: Ready-to-install, stable performance. The membrane layer is constantly immersed in preservation solution, keeping membrane pores open and the desalination layer structurally stable. No additional activation is required upon installation; water flow immediately achieves designed flux and salt rejection, quickly meeting project acceptance and emergency water demand. Wet membranes also offer superior long-term performance stability: without dehydration-rehydration cycles, the membrane’s microstructure remains intact, with slower flux and salt rejection attenuation over 2–3 years of use. They are suitable for scenarios demanding strict water quality and operational stability, such as pharmaceutical pure water, electronic-grade pure water, power plant boiler feedwater, and large-scale pure water plants.
Shortcomings (focused on storage and transportation):
Short shelf life: 1–1.5 years, far shorter than dry membranes.
Strict storage conditions: Requires a constant temperature of 5–35°C, avoiding direct sunlight, high-temperature exposure, and low-temperature freezing. Preservation solution freezes below 0°C, rupturing membrane elements and vacuum bags and scrapping membranes.
Slightly higher transportation costs: Preservation solution increases membrane weight; temperature control and shockproof measures further raise costs.
High risk in long-distance transportation: High temperature and humidity in long-distance sea transport easily cause preservation solution deterioration and vacuum bag leakage, compromising product quality—one of the main reasons most manufacturers have phased out wet-membrane production.
The core production process of dry membranes is complete dehydration and drying for long-term storage and transport resistance. Dry membranes are now the industry mainstream, covering household, general industrial, long-term inventory, long-distance transportation, and most other scenarios. Most manufacturers have shifted to dry-membrane production.
The production flow differs significantly from wet membranes:
After rolling, membrane elements are repeatedly rinsed with high-purity pure water to completely remove impurities, processing agents, and residual moisture inside the membrane layer.
Low-temperature dehydration and drying are performed: elements are placed in specialized drying equipment and slowly dehydrated at 30–50°C under low-humidity conditions for 48–72 hours to ensure full drying. Drying speed and humidity are strictly controlled to avoid micro-cracking of the membrane layer due to rapid drying.
Immediately after drying, elements are placed in vacuum-sealed bags and vacuum-packed to prevent moisture absorption from air.
Outer packaging varies by application: household dry membranes use colored independent paper boxes with shock-absorbing cushioning, printed with product model, specification, activation method, etc.; industrial dry membranes use bulk corrugated boxes with multiple pieces per case, marked with storage conditions and shelf life.
Packaging characteristics:
Outer packaging: Independent paper boxes (household) or corrugated boxes (industrial) based on usage.
Inner packaging: Vacuum-sealed bags with no liquid; dry and hard to the touch, no liquid flow when shaken.
Some high-end dry membranes use double-layer vacuum packaging for enhanced moisture resistance.
Authentic dry membranes feature product serial numbers and anti-counterfeiting codes on vacuum bags; the box includes a certificate and user manual specifying activation method and storage conditions.
Core advantages (focused on storage and transportation):
Long shelf life: Complete drying eliminates microbial growth conditions, with a shelf life of 2–3 years (3–5 years for some high-end products), meeting long-term inventory and storage needs.
Transport and storage resistance: No liquid components; storable at -20°C to 40°C, resistant to freezing and high temperatures. No temperature-controlled transport is needed, drastically reducing transportation costs and risks—especially suitable for long-distance sea transport.
Light weight: 10–20% lighter than wet membranes (no preservation solution), facilitating handling, storage, and transportation.
Relatively low cost: No large consumption of preservation solution; large-scale drying processes further reduce production costs, suitable for cost-sensitive household and bulk-purchase scenarios.
Shortcomings (focused on usage):
Requires activation treatment: Dried membrane pores are in a shrunken state and cannot be used directly after installation. Low-pressure (0.1–0.2 MPa) pure water flushing for 30–60 minutes is required to fully rehydrate the membrane layer and reopen pores. A small amount of residual desiccant or impurities is discharged during flushing; initial flux is low, and stable flux and salt rejection are achieved only after 1–2 hours of operation.
Slightly faster long-term performance attenuation: Dehydration-rehydration cycles cause slight irreversible shrinkage of the membrane microstructure, leading to faster flux and salt rejection attenuation than wet membranes in long-term use, making them unsuitable for scenarios demanding ultra-high long-term stability.
Improper activation impairs performance: Insufficient flushing time or excessive flushing pressure may damage the membrane layer, affecting filtration performance and service life.
Today, over 99% of RO membranes on the market are TFC polyamide membranes, which can theoretically be packaged as dry or wet membranes. The two formats have not formed a technological replacement relationship but complement each other based on scenario differentiation—dry membranes are the industry mainstream, while wet membranes are limited to high-end industrial engineering and other niche scenarios.
Dry membranes now cover household water purification, general industrial wastewater treatment, long-term inventory, long-distance transportation, and most other scenarios. Most manufacturers have fully shifted to dry-membrane R&D and production, with only a few leading manufacturers retaining wet-membrane production lines. Wet membranes are not mainstream, with highly concentrated applications limited to high-end industrial engineering scenarios demanding strict water quality and operational stability, and relatively low market demand.
From a manufacturer layout perspective, leading companies still producing wet membranes include Hydranautics, Dow Chemical, and Toray. Among them, Hydranautics has the highest market visibility and widest application of wet-membrane products, with models focused on high-end industrial engineering (e.g., CPA3-LD, ESPA2-4040, LFC3-LD) for pharmaceutical pure water, electronic-grade pure water, and other high-water-quality scenarios. Dow Chemical’s wet membranes mainly supply large pure water plants and power plants, with lower market visibility. Toray’s wet membranes are even scarcer, used only in a small number of high-end special projects.
The core reason most manufacturers have shifted to dry-membrane production is that dry membranes offer easier storage and transportation, longer shelf life, adaptability to diverse scenarios, and relatively lower production costs—meeting most market demands. In contrast, wet membranes have high production costs, strict storage/transportation conditions, and short shelf life, only satisfying the special needs of high-end industrial engineering with limited market demand. Thus, most manufacturers have abandoned wet-membrane production to focus on dry membranes, further consolidating the mainstream status of dry membranes.
The core logic for the long-term existence of wet membranes lies in their irreplaceable performance advantages: in scenarios such as pharmaceutical pure water, electronic-grade pure water, and power plant boiler feedwater—demanding ultra-high water quality and operational stability—the "ready-to-install, long-term stable performance" of wet membranes cannot be replaced by dry membranes. This is why leading manufacturers such as Hydranautics continue to retain wet-membrane production lines.
With continuous optimization of RO membrane technology, dry and wet membrane processes are also being upgraded:
For wet membranes: Manufacturers extend shelf life, improve high/low temperature resistance, and reduce transportation risks by optimizing preservation solution formulations.
For dry membranes: Improved drying processes shorten activation time and enhance long-term performance stability, narrowing the performance gap with wet membranes.
Regardless of technological upgrades, their core positioning remains unchanged: dry membranes as the industry mainstream for convenient storage and long-distance transportation in most scenarios; wet membranes as a niche supplement for ready-to-install, high-stability performance in high-end industrial engineering. Long-term coexistence of the two formats will remain the mainstream industry trend.
Based on the differences between dry and wet membranes and application scenarios, the following selection recommendations are provided:
High-end industrial engineering (pharmaceutical pure water, electronic-grade pure water, power plant boiler feedwater, large pure water plants, etc.): Prioritize wet membranes. For projects with strict water quality requirements, rigorous acceptance, and tight schedules, wet membranes enable ready-to-install, stable performance, reducing acceptance risks and commissioning costs.
Long-distance transportation, long-term inventory, and cold-region applications: Prioritize dry membranes. They adapt to complex transportation environments, extend shelf life, and reduce transportation and inventory losses.
Household water purification, general industrial wastewater treatment, and other cost-sensitive scenarios with no urgent acceptance needs: Prioritize dry membranes. They offer lower costs and easier storage/transportation, meeting basic usage requirements.
Bulk-purchase projects with low requirements for long-term operational stability and focus on procurement/transportation cost control: Choose dry membranes for a balance of cost-effectiveness and practicality.
