July 2001
Volume 43 Number 7
 

Hollow-Fiber Technology Advances: Microbiological Disinfection Capabilities of Low-Pressure MF & UF Membranes
by Herman Beckers, Ludo Diels and Luc Stoops   Pages: 

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Summary: Extensive microbiological tests have shown that the combination of an activated carbon prefilter followed by a micro- or ultrafiltration membrane cartridge is an effective barrier against bacteria and (in the case of ultrafiltration) also against viruses.

Point-of-use (POU) water treatment systems are used on a large scale to remove taste, odor or potentially harmful contaminants such as pesticides or disinfection by-products. There is a growing public concern in obtaining also microbiological safe drinking water at POU.

Cysts and oocysts (as Cryptosporidium and Giardia) are the largest waterborne microorganisms -- 3-6 micrometers or microns (um) -- 10-6 meters. They can be highly infectious. The smaller bacteria -- Salmonella, E. coli, Legionella, etc. -- are single celled microorganisms and can cause serious diseases. P. diminuta, for instance, has a diameter of 0.3 um. Most other bacteria are slightly larger. Viruses, such as Rotavirus and Poliovirus, are a group of infectious agents ranging from 10 nanometers (nm) -- 10-9 meters -- to 25 nm in diameter (see Figure 1).

The most important processes to reduce the amount of microorganisms in drinking water are:
* Chemical disinfection,
* Disinfection by ultraviolet (UV) light,
* Distillation, and
* Membrane filtration.

Chemical disinfection

Oxidizing chemicals such as chlorine gas, hypochlorites, chloramine, iodine, peroxides and ozone can be used to disinfect drinking water. For POU installations, these techniques are very unpractical because of complexity and/or limited lifetime. With some of these products (especially with chlorine gas and hypochlorites) there's also a growing concern about health effects of disinfection by-product (DBP) formation such as trihalomethanes (THMs), which have been associated with cancer and miscarriages.

Disinfection by UV

While UV light may kill microbes at high doses, recent research has proven it effective at inactivating even cysts and oocysts at lower doses based on an attack on the microbial DNA structure. Adequate water contact with the lamp surface is required. An advantage of UV is that there's no formation of DBPs. Disadvantages are that electricity is needed and fouling, improper flow rates or a lack of effective pretreatment can block UV transmission into the water causing incomplete disinfection.

Distillation

Distillation is probably the oldest disinfection technology for water. It's effective against a broad range of contaminants -- particularly if a carbon filter is used to prevent carryover of volatile organic chemicals (VOCs); however, it's a costly process and can require large amounts of energy and water.

Membrane filtration

Depending on the size and kind of contaminants to be removed, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) can be used. MF is mostly defined as filtering in the range of 0.1 um to 10 um. UF membranes typically have pore sizes in the range from 1 nm (0.001 um) to 100 nm (0.1 um). RO membranes have a dense, non-porous structure in contrary to UF and MF membranes. Nanofiltration (NF), not covered in this article, is situated between RO and UF.

Reverse osmosis

RO systems are the most widespread membrane technology for POU applications. Practically all particles, bacteria and organics greater than 300 Daltons in molecular weight (for comparison, water has a molecular weight of 18 Daltons) are removed, although o-ring leaks can occur and cause downstream contamination. Nearly all-contaminating ions including nitrates are removed, resulting in almost mineral-free water. This very pure water can be corrosive in some circumstances and also most authorities recommend not drinking very pure water. Other disadvantages are: 1) Most POU RO systems have only a very limited capacity, which affects flow rate (typically for residential applications: 20 gallons/day with a storage tank of 2 gallons). A storage tank overcomes this partly, but the standing water in it can be a source of bacterial (re)growth; >and 2)a high level of waste water production per unit of purified water.

Ultrafiltration

UF membranes with a small pore size (for instance, 10 nm) are not only very effective against bacteria but also remove viruses. For drinking water production, mostly capillary membranes (hollow fibers) are used (see Figure 2). These are very thin tubes with an outer diameter of about 1 millimeter. A large number of such hollow fibers are placed in a tube. On one side, the open end of the fibers are closed with a special glue; on the other side, the open space between the fibers and the inner tube wall is filled with glue. The incoming water to be filtered is forced then to flow radially from the outside of the hollow fiber membranes to the inside, through the very small pores of the membrane wall. This way of operation is called "dead-end mode" and results in 100 percent use of the feed water for filtration. Cleaning of the membranes is done by periodic back washing. In combination with an activated carbon prefilter to remove bad taste or odor, DBPs and pesticides, UF is a competitive technology for RO installations in many applications. When removal of high concentrations of salts is required, RO is preferred.

Microfiltration

The lowest range of MF (with a pore size < 0.22 um) is very effective against cysts, oocysts and bacteria. MF systems with larger pore sizes -- such as in 0.5 um carbon blocks -- don't remove smaller bacteria. Because of the larger pores compared with UF and RO, MF capillary membrane systems have higher flow rates. Apart from the larger pore size, a MF filter cartridge is identical to an UF filter cartridge. As is the case with UF, MF filter cartridges are in many cases used in combination with an activated carbon prefilter (in a "dual-stage filter system") to remove bad taste or odor, DBPs and pesticides.

Bacterial retention

Since bacteria aren't killed by membrane systems, a major concern for POU drinking water production is the long-term protection against bacterial contamination. At the Flemish Institute for Technological Research (VITO) in Belgium -- see www.vito.be -- a dual-stage filter system was investigated on its long-term retention capability. This system consists of a 9.8-inch molded carbon block filter followed by microfiltration membranes using hollow fiber technology that have a maximum pore size of 0.15 um.

At the start of the test, the dual-stage filter system was heavily contaminated with E. coli (CM2529) up to 5.9 x108 bacteria/milliliter (ml) of filter cartridge volume. This is roughly a factor of 60 higher than the concentration of coliform bacteria in sewage water treatment plants -- (about 107/ml). During a two-month period, water was regularly drawn. No significant levels of bacteria were found until day 63, when a borderline infectious level (103 bacteria/ml) was measured in the first 100 ml taken -- after about 2,800 liters (740 gallons) had passed. This immediately decreased to an insignificant level (between 0.07 and 1 bacteria/ml) as more water was drawn through the filter (the instruction sheet also recommends flushing the filter when it hasn't been in use for a prolonged period of time). This finding was consistent in the subsequent testing period and confirms that the filter is an effective barrier against extreme bacteria challenges.

More details on this and also test results of other independent test institutes can be found at the website listed in Reference 3.

Removal of viruses by UF

The U.S. Environmental Protection Agency's (USEPA) "Guide Standard and Protocol for Testing Microbiological Water Purifiers" requires a 4-log (99.99 percent) microbiological reduction for Poliovirus 1 and Rotavirus.

VITO tested a combination activated carbon prefilter followed by a UF cartridge again using hollow fiber technology. The UF membranes used have a pore size of 6 nm. Two cartridges were tested with MS-2 coliphage (25 nm), a microorganism of similar dimensions as Poliovirus 1 and Rotavirus. The test results are shown on Table 1. It can be concluded that the combination of an activated carbon prefilter followed by an UF cartridge exceeds the 4-log (99.99 percent) microbiological reduction requirement of the USEPA Guide Standard for microbiological water purifiers.

The capillary membranes used in the tests are made of a blend of polyethersulphone (PES), polyvinylpyrolidone (PVP) and zirconium oxide (ZrO2), which creates a permanently hydrophilic (water absorbing), highly porous structure, yet with a very consistent pore size. The negligible pressure difference across the membranes and the associated high flow rates make them very suitable for POU applications.

Conclusion

The combination of an activated carbon prefilter followed by a MF or UF capillary membrane cartridge offers an effective protection against bacteria and (in the case of UF) also against viruses. The UF cartridge meets the USEPA 4-log virus removal requirement established for water purifiers and is, in most cases, an alternative to other technologies such as RO.

References

1. American Water Works Association, "Water Quality and Treatment," 4th Ed., McGraw-Hill Inc., New York, 1990.

2. Scharstuhl, J.J., and J.E. Richards, "Capillary Filtration: The Evolution from Plasma Separation to a Water Sterilizing Membrane," WC&P, March 1998.

3. Prime Water Systems GmbH, http://www.primewater.com/quality.htm

About the authors

Herman Beckers is project engineer at the Flemish Institute for Technological Research (VITO) in Belgium and is working in the field of capillary membranes for POU drinking water applications. He holds a master’s degree in engineering from the Leuven University in Belgium. Beckers can be reached by email: herman.beckers@vito.be

Ludo Diels is head of the Expertise Centre "Environmental Technology" at the VITO. His lab is the microbiological analysis reference lab in Flanders, Belgium. He holds a Ph.D. degree in microbiology from the University of Antwerp (Belgium). He can be reached by email: ludo.diels@vito.be

Luc Stoops is responsible for capillary membrane production at Prime Membrane Technologies, n.v., in Belgium. He holds a bachelor's degree in chemical engineering from the Technical University of Hoboken in Belgium. He can be reached by email: luc.stoops@vito.be


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