Semipermeablemembrane

Membrane manufacturer’s today construct thin-film composite (TFC) RO membranes consisting of three layers: the extremely thin polyamide top barrier, the polysulfone support layer and polyester layer. The semi-permeable polyamide barrier prevents molecules with molecular weights greater than 100 from passing through. Water molecules easily diffuse through the polyamide layer, providing pure water on the opposite side.

The 5:3 array system above should not be confused with a two-pass system, which is the combination of two RO systems where the permeate water from the first system becomes the feed for the second system. In this case, the water is “passing” through two RO systems. Two-pass systems are used in applications that need extremely high quality permeate water.

Cylindrical RO pressure vessels house the elements, and their construction material relies on the operating pressure, vibrations, temperatures, feedwater and chemicals entering the system. They also require some sort of corrosion resistance due to the high salt content of water streams and chemical cleaning. Vessels fabricated from filament-wound glass reinforced plastic (FRP) have been used for systems from 100 psi up to 1200 psi operation. These FRP vessels are inherently non-corrosive and have a smooth and precise inside surface for optimum membrane loading and sealing. Stainless steel vessels are typically only used these days for steam sterilized systems like found in dairy applications.

Reverse osmosis

Figure 3 shows the external components of elements. The outer wrap may be tape, fiberglass, or a polypropylene mesh (also known as fullfit) and keeps the element together in the spiral-wound shape. The anti-telescoping devices (ATDs) stabilize the components of the element and prevent shifting of the internal mechanical components under pressure, also known as telescoping. The brine seal wraps around the feed side ATD and guarantees the feedwater flows into the element’s internal components.

Designing an RO system requires much customization and should not be built the same exact way every time. These processes require different pre-treatment, construction materials, membranes, chemicals and more.

Figure 5 shows one membrane leaf and water flow interactions within and around that leaf. There may be up to 30 leaves in a single element (depending on the manufacturer). Each membrane leaf consists of a permeate carrier in between two active membrane surfaces. Once the water permeates through the membrane surface, the permeate carrier collects the clean water and directs it toward the permeate water tube. The leaves are glued together on the feed and concentrate side as well as the outer edge but are not glued where the permeate carrier attaches to the permeate water tube.

Reverse OsmosisSystem

Reverse osmosis (RO) systems can seem intimidating. However, this technology has been around for decades and is used globally in all types of industries. Understanding the basics of how RO membranes work, and the mechanisms of the RO process are the first steps to system optimization.

The concentrated wastewater stream must be taken into consideration since it contains the salts from the feed that have been retained by the membranes at much higher concentrations than the feed water. For some RO systems that are small-scale or residential, the concentrate can be discharged straight to the local drain or sewage. Depending on the environmental regulations and quality of the concentrate water, it can be discharged to surface waters. Zero liquid discharge (ZLD) systems minimize the amount of liquid waste by evaporating the concentrate and recovering the leftover salts as waste. However, some concentrates have dried salts that are a valuable resource and will be useful in some applications. It is also important to consider the length of the concentrate waste line because the salts may precipitate from the water and form deposits, or scale, on the piping, effectively blocking the exit flow of the system. It is always important to follow local regulations and environmental guidelines when disposing of waste streams.

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Figure 4 demonstrates the purpose of the element’s feed spacers which provide a space for water to flow as well as promote turbulence, helping the water flow much faster. Since RO membrane elements utilize crossflow filtration, the fast-moving water pushes off foulant and salts forming on the membrane surfaces.

Salt and water transport inreverse osmosismembranes: Beyond the solution-diffusion model

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UFmembrane

After the clean water is collected, it is suitable for various applications such as: drinking water, production of food and beverages, pharmaceutical development and experiments, semiconductor manufacturing and many more. However, most of these applications require further treatment before the water is ready for use.

Elements are comprised of several components: outer wrap casing, anti-telescoping devices, brine seal, membrane leaves, feed spacers, glue seals, permeate carriers and the permeate water tube.

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The number of elements that go into a vessel depends on how much water is needed in the permeate stream, the average membrane flux of the system, required active membrane surface and the type of elements. You can use multiple hydraulic programs that run calculations for the number of elements per vessel, number of vessels for stages, and number of stages needed for the whole system.  Single stage systems are typically used with low recovery systems (e.g., seawater desalination) while multi-stage systems are used to acquire a higher recovery. Two examples of system arrays are shown in the figure below. Each vessel in one stage is set up in parallel while the separate stages are set up in series. The latter stages require fewer vessels as the volume remaining on the feed/brine side reduces as permeate is produced. In the example below, since the 1st stage has five vessels and the 2nd stage has 3 vessels, the system is a 5:3 array.

The most common type of RO membranes are brackish, seawater and nanofiltration (NF). To determine the best membrane for your system, consider your feedwater source, desired recovery, water quality, and energy requirements.

Reverse osmosiswater

TFC membranes are packed into a spiral-wound configuration, allowing for several advantages compared to other configs: lower replacement costs, less space needed, simpler plumbing systems and more design freedom. Typical diameters of these spiral-wound elements for commercial use consist of 4-inch and 8-inch elements. These sizes were standardized early in the industry because of their efficient energy use, productive surface area, reliable performance, and existing infrastructure.

For maintenance and cleaning, treatment solutions such as antiscalant and clean-in-place (CIP) chemicals are used. If a CIP cannot be run on the RO system, Avista offers an off-site cleaning and restoration service (OSCAR) to clean the membranes in your system.

Ultraviolet (UV) disinfection kills or deactivates any remaining bacteria, viruses, or other microorganisms, providing an extra layer of protection for industries that require high microbiological purity.

Electrodeionization (DI) uses electricity, ion exchange membranes and resin that removes ionized molecules from water, resulting in extremely pure water used in laboratory and scientific applications.

Ion exchange (IX) resins remove trace amounts of unwanted dissolved minerals, like silica from water used in boiler loops.

Balancing the permeate flow rate between stages is important for a system to have a good average flux. Typically, the permeate flow rate will be lower toward the tail elements (elements located toward the end of each vessel) compared to the lead elements. This is due to the concentration of salts being higher toward the tail elements, which increases the osmotic pressure and makes it harder for those elements to produce permeate water. It may be necessary to boost the feed pressure between stages to ensure a balance flux across all membranes.