Low pressure ultrafiltration needs fine tuning
Dipl.-Ing. Werner Ruppricht, and Dr. Ulrich Meyer-Blumenroth of NADIR Filtration (Germany), with Japan’s Tadaaki Miyano, DAICEN and Dr. Shuji Nakatsuka of DAICEL, outline the benefits of double asymmetric pore membranes.
The importance of low pressure Ultrafiltration is increasing in the treatment of potable and process water extracted from surface water. But for optimal use, components must be critically attuned.
River water, however, contains a seasonal fluctuating amount of turbidity plus bacteria and therefore must undergo treatment before its use in industrial plants.
Conventional processing of surface water is subject to a combination of chemical and physical treatments. Sand filtration, flocculation, precipitation and sedimentation reduce the turbidity of surface water. For complete disinfection of surface water additional treatments are needed (e.g. ozonization).
The disadvantage of conventional processing is that seasonal fluctuations of the composition of the river water and its temperature affect the quality of the clear water produced. These fluctuations can only be partially compensated by adapting the dosing of the added chemicals – with added high operational costs.
Ultrafiltration functions purely as a physical separation, and delivers independently of the raw water quality a clear and disinfected permeate. It has the following advantages:
- High permeate quality (clear, free of germs and no turbidity)
- Long term stable operation of downstream RO or ion exchange due to low Silt Density Index (SDI)
- Single step processing, simple system design, easy to automate
- Modular system building allows easy extension with membrane sections in case of growing water needs
- No salinization by precipitating ingredients
- Easy processing- the concentrate can be returned to the river
- Lower operational costs.
For cost-effective membrane filtration a high level of specific permeate flux has to be maintainted long term. Continuously shortening the membrane cleaning intervals is not acceptable alone.
The membrane polymer, its hollow fibre structure, together with the construction and mode of operation of the module must be attuned optimally for guaranteeing an ideal functioning of the ultrafiltration system.
Hydrophilic / low fouling?
The more hydrophilic the membrane material is, the easier the water to be filtered spreads on the membrane and passes its pores. Due to their low electrical surface charge biological deposits are substantially more weakly bound to hydrophilic membranes than to hydrophobic ones.
Hydrophilicity cannot be expressed by one parameter alone. It is a series of data which gives out information, e.g. whether a membrane material delivers a long lasting stability of the permeate flux on surface water.
Strikingly, the hollow fibres of cellulose acetate (CTA) and Polyacrylenitrile (PAN) build the smallest contact angle with a water droplet. The smaller the contact angle, the lower the pressure needed to wet the membrane surface and to pass the pore structure.
Bovine Serum Albumin (BSA) is an unpolar molecule, which is differently strong adsorbed by various membrane materials. With BSA, it is possible to simulate how strong binding forces between the membrane surface and bacteria produced slimes can be. The CTA-membrane shows by far the lowest value because the membrane hardly allows bacteria to attach to its surface. Although PAN and PES are available, CTA because of its high hydrophilicity, was selected for the MOLPURE®FW50 system.
Conventional production of hollow fibres results in a simple asymmetric pore structure. The most effective separation layer is at the inner diameter of the hollow fibre. The pore diameter continuously increases as it travels through the membrane wall from in- to outside.
The hollow fibre manufacturer Daicen of Japan produces hollow fibres with a double asymmetric pore structure. This characteristic gives the membrane a very high mechanical stability. The MOLPURE® FW50 membrane has a highly porous structure with a very dense separating layer at the inner surface of the hollow fibre. Its nominal molecular weight cut off is 150 kDa, resembling an average pore diameter of 0,08 um.
Compared with the size of germs in surface water (i.e. Giardia and Cryptosporidium) this means that the effective separation layer of the membrane forms an obstacle. Blocked germs cannot attach to the smooth, hydrophilic membrane surface and can easily be removed by back flushing. Germs cannot grow into the pores or cause ‘pore clogging’ in the MOLPURE® FW50.
All hollow fibres in the module must be subject to the same processing conditions/pressures. They are potted in 4 bundles of approximately 5000 fibres each. Between the bundles there are gaps of 2cms to reduce flow resistance in the whole membrane module during back flush. The modules are mounted, and the fibres also hang, vertically in the module housing, so complete air removal is more easily achieved than from horizontal modules.
Though the mechanical stability of the hollow fibre allows a maximum trans-membrane pressure of 2 bar, Daicen/Nadir recommend not to exceed a trans-membrane pressure of 0.6 bar. Higher trans-membrane pressures bring higher permeation for short periods only. In the long term, heavier fouling and shorter cleaning intervals must be accounted for.
In the case of relatively ‘dirty rivers’ the turbidity in the UF-feed can go over 100 NTU. At a permeate yield of some 90%, there was a strong growth in the amount of turbidity in the concentrate. Usually eight MOLPURE®FW50 modules are installed per module block, each with a permeate capacity of approx. 35-45 m³/h.
In Japan’s rivers Ibo and Sagami, hollow fibres of CTA, PES and PAN have been tested in parallel. In both the CTA-hollow fibres gave best results. Over 80 days a stable flux of 80-100 l/m²h was reached while the trans-membrane pressure did not go over 0,6 bar. In Europe, results of trials with CTA-hollow fibres in surface waters on the Rhine, Main, Weser and Aller, validate the Japanese laboratory trials.