Let’s face it – talking about reverse osmosis (RO) membranes isn’t exactly dinner party material but if you’re even remotely involved in RO treatment, you’ve probably heard the terms “wet-packed” and “dry-packed” being thrown around. In this blog we’re going to discuss what these are, what the difference is and why it matters. So here we go.
What is the difference between Dry and Wet RO membranes?
One helpful way to think about this is to imagine a sponge. If you buy it dry, you need to soak it before you use it. If it comes already moist, it’s ready to go right away. Dry and Wet RO membranes also work in the same way – except they are designed to purify water at a molecular level, a sponge is something you left in the sink overnight.
Wet membranes arrive pre-soaked, stored in a special preservative solution and dry membranes exist in a state completely absent of any water or preservative. While both types aim to remove impurities from water, how they are stored, installed, and maintained can vary significantly.
Structural Differences
Leading manufacturers like DuPont have been at the forefront of membrane innovation, offering high-performance RO elements designed for both wet and dry applications. It was John Cadotte (future founder of FilmTec Corporation) who invented the first generation of thin film composite (TFC) membranes in the late 1970s – a technology which has been further improved since its conception and has become the benchmark for modern day RO membranes.
A typical TFC membrane consists of several carefully engineered layers: a sturdy polyester fabric base for mechanical strength and dimensional stability, a microporous polysulfone interlayer that provides a porous yet supportive transition, and an ultra-thin polyamide selective layer, which acts as the true filtration barrier. This polyamide layer is just nanometres thick but critically important, allowing water molecules to pass while rejecting dissolved salts, organics, and other impurities through a combination of size exclusion and charge repulsion mechanisms. Modern advancements have further improved membrane chemistry to enhance resistance against fouling, chlorine degradation, and mechanical stress, ensuring longer service life and greater operational reliability.
Because the polyamide layer is formed through a water-based process, the result is initially a “wet” membrane. To produce a dry version, manufacturers simply take that same membrane and put it through a carefully controlled drying step before packaging. While the fundamental structure, remains the same, the drying process removes much of the water content from the membrane’s internal pores. This causes the polyamide chains to contract and compact slightly, leading to a denser polymer structure. The Polysulfone support layer also undergoes minor shrinkage as its pore walls dry and tighten. Although the overall membrane design and functionality are preserved, the internal morphology differs between wet and dry forms, reflecting the physical effects of dehydration on the membrane’s structure.
Performance Comparison
As we previously discussed, a dry RO membrane undergoes slight structural changes during the drying process. Due to the denser polymer matrix, the initial flux and salt rejection performance is not at its peak until the membrane is properly rehydrated and conditioned during system startup – you wouldn’t wash your dishes with a dry sponge right? Once fully saturated, dry membranes perform just as well as wet membranes. The break-in period for dry membranes varies based on manufacturer guidelines. Some membranes require up to 24 hours to fully reach their optimal salt rejection levels, while others can be conditioned in under an hour.
On the other hand, wet membranes tend to have better performance right out of the box compared to dry membranes because they’re already fully saturated and individually tested by the manufacturer. A wet RO membrane will produce water at its rated flow and rejection rates almost immediately upon installation. This means when you install it, you can be confident it will achieve the expected salt rejection and throughout from day one. This is advantageous in situations where water quality is a critical requirement for a process or system downtime is critical, a wet membrane may be a better choice.
Storage and Handling
The storage and handling of wet and dry RO membranes require distinct approaches due to their differing structural properties. As previously mentioned, wet membranes are stored in sealed bags containing a preservative (usually a solution of sodium metabisulphite) to preserve their hydrated state and prevent microbial growth during storage. Consequently, they can be more difficult to store and handle. Wet membranes must be stored in stable environments to avoid drying out, or contamination. Exposure to air can cause them to dry out and become ruined, like that one houseplant you swore you’d water regularly. The shelf life of wet membranes is much shorter when compared to dry membranes due to their susceptibility to microbial contamination and physical damage. In addition, wet membranes are heavier compared to dry membranes, making them more cumbersome to handle and more expensive to transport.
Dry membranes, in contrast, are lighter and more stable during storage and transport, reducing shipping costs and simplifying manual handling. Because they lack moisture, dry membranes are less susceptible to microbial contamination and can be stored for longer periods without the same risk of degradation.
Conclusion
At the end of the day, both wet and dry RO membranes do the same job once they’re up and running. The key difference lies in how they’re prepared, stored, and installed.
Wet membranes, pre-hydrated and stored in a preservative solution, offer immediate performance but require careful storage and handling to prevent degradation. They have a shorter shelf life, are heavier, and more challenging to transport. Dry membranes, on the other hand, are lighter, easier to transport, and have a longer shelf life. However, they require rehydration before use, which can take some time for the membrane to reach optimal performance.
The choice between wet and dry membranes ultimately depends on your specific application, with wet membranes being ideal for systems requiring immediate, reliable performance, while dry membranes offering advantages in long-term storage, transportation, and cost-efficiency. Understanding these differences will help you choose the right membrane for your specific water treatment setup.
