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Pall/Asahi

Asahi developed membrane technology based on polyacrylonitrile (PAN) UF fibre for ultrapure water (UPW) applications in the 1980s. Together with Nitto Denko, they continue to dominate this market. During the 1990s, Asahi developed membrane products based on PAN and PVDF for UF and MF markets in general water treatment. Asahi markets their products directly in Asian markets, particularly Japan , China and Korea , and for several years has marketed exclusively through Pall for other markets, particularly North America and Europe.

Asahi's initial market offering for general water treatment was based on a UF fibre made from PAN, a development of its UPW fibre. In the early 2000s, the PAN fibre structure was improved significantly, providing a product with improved robustness. The fibre has moderate permeability, and an impressive resistance to fouling, hence operates with stable permeability and a low cleaning requirement. It is not as strong as PVDF and therefore has a more limited tolerance of air scour. It has not been as widely applied as PVDF, though it offers an attractive combination of features.

There are three main international suppliers offering outside feed format technology, and several new entrants or regional players. Most of the products are based on polyvinlydifluoride (PVDF). The largest supplier both in this group, and in the whole UF/MF market for water and wastewater, is Zenon.

The PVDF fibre has extensive references in the North American municipal water market and has an impressive track record in terms of integrity and life. The unsupported fibre is very strong, though this has been achieved at the expense of permeability. Since PVDF is more prone to fouling than other membranes, regular vigorous air scour is required to control permeability. The robustness of the fibre enables Pall/Asahi systems to use high flux, which gives higher operating pressures and chemical cleaning frequency than other membrane systems. Sometimes warm cleaning chemicals are used to improve cleaning efficiency. The aggressive design tends to make capex competitive and opex high, though with low membrane replacement cost.

Siemens-Memcor

Memcor was one of the early pioneers of the use of MF in water treatment, and developed a unique product in the 1980s using a very fine polypropylene (PP) fibre cleaned periodically by a gas backwash. PP is inexpensive and flexible, but unfortunately has low chlorine tolerance, which is a disadvantage in municipal applications. In the late 1990s, Memcor developed a PVDF fibre, and has now commercialised its technology in pressure driven and submerged formats.

The Memcor PVDF MF fibre has a fine pore size rating to ensure reasonable virus rejection and stable performance. In addition, their fibre is significantly hydrophilic, which reduces fouling and operating pressures compared to an unmodified PVDF. A typical Memcor installation tends to operate with relatively low fluxes, very low operating pressures, and infrequent chemical cleaning.

The technology is supplied in both submerged and pressure driven formats, both as standard package and customised plants.

Dow (Omexell)

The rapid development of the Chinese economy in the past few years has led to a major investment in industrial water treatment, particularly in industrial boiler systems and power plants. Membrane technology is often specified for the water supply to these plants, and several western companies have focused on this market. Within China , numerous companies have rapidly developed technology to meet this rapidly growing demand, and one of the most successful has been the membrane system company, Omexell. In 2006, Dow, one of the largest international RO companies, bought Omexell, with the intention of introducing their UF technology to the worldwide markets.

Pressure driven outside feed formats have many similarities, with a mainly common approach to fibre and module dimensions, and in process design. Despite the similarities, there are no directly interchangeable products.

Omexell's product has been designed to be similar to Asahi, in that it uses a PVDF membrane in a pressure driven outside feed format, in similar sized modules. As with the pressure driven inside feed formats reviewed in the previous article, though close in design, the modules are not directly interchangeable. In addition to the module dimensions being slightly different, the end cap design is different, and allows use of a close coupled rack design, similar to inge's new T rack plus.

Note that the pore size rating suggests that the membrane has a fine MF rating, in common with the other PVDF products, whereas the MWCO value suggests a moderately tight UF.

Polymem

Polymem is a small independent company specialising in membrane systems, and based in the south of France . It is the only company with an outside feed format to use PS fibre. It had previously been assumed that PS would not be suitable for use with air scour, but Polymem's module design overcomes the problem by using loops of fibre, which are potted at only one end. The freedom of movement appears to overcome the limitation expected from the lack of flexibility in PS fibre.

The module is relatively short and wide to accommodate the fibre loops. Use of a fine fibre diameter means that a high membrane area is achieved, though operating fluxes appear to be lower than for inside feed PES products discussed previously.

Toray

The final product in this review is from Toray. Toray is known as an international RO supplier. However, their MF product, which has been available for some years, has been mainly restricted to the Japanese water market. The concept is similar to Asahi's PDVF product, but is available in a wider module diameter, ie 216 mm compared to 165 mm (8 inch NB cf 6 inch), providing much higher membrane area per module. There does not appear to be a technical reason for the choice of narrower module diameters of most pressure driven outside feed modules, compared to the wider diameters used by inside feed modules.

Toray claim a very narrow pore size distribution for their MF membrane. The fibre diameter is slightly wider than Asahi, presumably to improve hydrodynamics in the larger module.

Conclusions

. Two submerged products are offered by Zenon and Memcor, but there are a number of significant differences between them.

. Pressure driven outside feed formats have many similarities, with a mainly common approach to fibre and module dimensions, and in process design.

. Despite the similarities, there are no directly interchangeable products.

. Most of the product offerings are based on PVDF, but there is some use of PAN, and one system that uses PS.

Contact
Graeme Pearce
E-mail: graemekpearce@btinternet.com

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