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Membrane Selection: choosing the right membrane

Membrane Technology offers reliable enhancement of conventional treatment systems. Constructed from modern triacetate, polyamide, poly urea, polysulphone and other materials, the membrane type has distinct properties in terms of surface charge, solution flux throughput, robustness and fouling potential.

Basic membranes can be modified to enhance their suitability for specific effluent characteristics. ESMIL has expertise in choosing the most appropriate membrane for your application by selecting from a wide range of state-of-the-art products. All membrane module designs, by definition, permit treatment process modularisation, one of the attractive advantages of membrane processes.

There are four principal configurations currently employed in membrane processes, each having various practical benefits and limitations. The configurations are based on either a planar or cylindrical geometry and comprise:

• Spirally wound membrane
• Tubular membrane
• Plate & frame / Flat sheet
• Hollow fibre membrane

Spirally wound membranes

Spirally wound membranes have the advantage of simple and robust construction whilst providing a reasonable membrane area per unit volume. They are widely used in reverse osmosis, nanofiltration, ultrafiltration and microfiltration applications.

The membrane element comprises two membranes sandwiched together, forming a "bag" sealed on three sides. The open side is attached to a central perforated tube which collects the permeate. The membrane element is rolled up to produce a spiral which forms a cylindrical shape, one end of which is open to the feed water. The fluid path length then equates to the length of the cylinder, and spacers are used to maintain separation of the membrane leaves both in the in the permeate channels and in the retentate, where the spacers also serve the important task of turbulence promotion. A number of membrane elements, up to seven for very large plant, can make up a single module by linking the retentate streams in series and placing the module in an appropriate pressure vessel.

There are numerous variations of construction available in this format, including:

• Feed and permeate spacer material and size.
• Membrane polymer.
• Glue type.
• Central tube size and material.
• Temperature and pH ratings.

It is critically important to assess each individual application very carefully to ensure that the optimum membrane and construction is selected. ESMIL have the experience and capability to do just that.

Tubular modules

Tubular modules provide the greatest turbulence promotion and the best access for the effluent to the membrane surface, although a modest surface area-to-volume ratio results in the highest cost per unit area of all cylindrical membrane geometries. This configuration is best suited for highly fouling feed streams or for those when a large amount of suspended material is present.

Polymeric tubes are available covering all separations (MF through to RO), and can be either supported to allow greater operating pressures, or unsupported (generally limited to MF and UF operating pressures).

Ceramic tubes tend to have a low packing density due to their monolith construction. These tubes comprise cylindrical holes in a solid ceramic support. Ceramic membranes have some major advantages in applications that require complete sterility, such as food or pharmaceutical, as they can be steam sterilized and also cleaned with highly aggressive chemicals. Ceramic membranes are expensive and tend to be used only in high value applications.

Plate & frame / Flat sheet

Plate-and-frame modules comprise rectangular or circular flat sheet membranes with separators and/or support plates. The modules can be pressure sealed, or be designed as cassettes.

Cassettes allow membrane elements to be inserted and removed individually without disassembling the module. They are however limited to low pressure operation, and hence to microfiltration or ultrafiltration processes.

Pressure-sealed modules can be operated at higher pressure (in the RO range). There may be hundreds of elements in a single stack, producing a very long retentate flow path and hence allowing high recovery of retained substances.

Hollow fibre membranes

Hollow fibre membranes exhibit the highest packing densities and hence the lowest area cost of all the geometries available.

Modules vary in manufacture, but in essence, all require at least one end of a bundle of fibres to be potted with the open end of the tube exposed to allow permeate to be removed. Hollow fibre modules usually operate out-to-in, where the feed is on the outside of the fibres and the permeate is sucked through the membrane to the inside of the fibre, such that the fouling layer is formed on the outer membrane surface. Alternatively, capillary fibre modules, comprising larger filaments, are operated in-to-out with the membrane formed on the internal surface.

Hollow fibre membranes can be cleaned by "back-flushing", whereby the direction of permeate flow is reversed, allowing permeate to clean the build up of solids from the membrane surface. Unfouling by back-flushing can be enhanced by air sparging, by using chemical cleaners, or by using a combination of the two.

Hollow fibre membranes are now widely used in Membrane Bio-Reactors (MBR) wastewater treatment systems, which utilise membranes along with traditional biological plant. This results in many advantages over traditional treatment, such as small plant footprint, and vastly superior treatment performance and reliability.