By Harry Futselaar, Henk Schonewille, and Walter van der Meer

Summary: Capillary nanofiltration (NF) enables raw water to be treated in a single step to produce high-quality permeate. The permeate can be used as industrial process water. Trial studies show that a stable operation is achievable using capillary NF without pre-treatment (direct NF).


For many domestic and industrial applications, large amounts of water are only used once for cleaning purposes, after which these streams are discharged into the sewage system. Generally, the sources for this washing water is valuable potable water or locally pumped groundwater. These sources, however, are becoming scarce due to increases in the price of potable water or environmental legislation penalizing groundwater intake to prevent soil dehydration or aquifer depletion. The possibilities for new technologies are open to enter the water treatment market, enabling utilization of new, lower-quality sources such as untreated surface water and effluent from wastewater treatment plants.

Membrane technology is a relatively new treatment method for producing process water from contaminated sources. The membrane separates the contaminated input flow into a concentrated waste stream and a clean product flow (permeate). The properties of the semi-permeable membrane determine the quality of the permeate. For example, a nanofiltration (NF) membrane removes bacteria and viruses as well as organic matter, heavy metals and, to some degree, pesticides. The permeate is thus of high quality and can be used as process water for industry (producing ultrapure water, boiler supply water, water for the process industry, etc.). Membranes in current NF installations are usually designed as spiral-wound elements and placed in pressure vessels. Due to the propensity of spiral-wound membranes to foul, pre-treatment is necessary before the water can be processed with any of these elements. In practice, spiral-wound membrane filtration is part of a series of treatment steps such as flocculation/sedimentation with rapid sand-filtration and NF, and (double) rapid sand-filtration with NF, making membrane filtration not always economically viable to produce process water out of lower quality sources. This article introduces a new product—capillary fiber NF.

Nanofiltration capillaries
To improve the economical attractiveness for process water production  out of the aforementioned sources, low-pressure NF capillary fiber membranes have recently been developed. These NF membranes may bridge the gap between the bulky, robust, tubular NF membranes and the compact, fouling-sensitive, spiral-wound NF membranes.

The background of this new product is based on use of capillary ultrafiltration (UF) as pre-treatment before spiral-wound NF. Since the 1990s, UF has been used successfully as a new pre-treatment step for spiral-wound membranes, creating an effective barrier to suspended matter.1,2 A capillary fiber NF membrane combines the favorable properties of capillary UF membranes in terms of ease of cleaning with favorable properties of the NF membrane such as removal of bacteria, viruses, pesticides and heavy metals.3

Direct nanofiltration

An additional improvement to this new membrane is the way of operation—semi-dead-end NF. As such, the incoming water to be filtered is forced 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 already applied successfully for UF1,2 and has been adapted slightly to NF (and reverse osmosis). Figure 1 shows the basic idea behind semi-dead-end NF. During the production run, the concentrate valve is closed and all the feed supplied to the system is withdrawn as permeate. To stabilize the flux and rejections at an acceptable level, a small crossflow velocity is applied over the membrane. When the rejection drops too much, the concentrate valve is opened and the system is flushed by means of air-enhanced forward flushing. This latter procedure makes very effective membrane cleaning possible. It involves flushing (cross flush) with water, to which a shot of air is added for five to 10 seconds. This creates a high level of turbulence so the membrane can be effectively cleaned hydraulically. This relatively new technique is already used in various (capillary) UF installations.4 Subsequently, the concentrate valve is closed and the production run starts again. As seen in Table 1, the average cleaning frequency is about two to four times per hour.

Figure 2 shows a picture of an 8-inch NF module with 20 square meters (m2) of membrane area. This new product enables treatment of various water sources in a single step to produce high-quality process water, which is relatively free from bacteria, viruses, chemical oxygen demand (COD), heavy metals, color, and partially softened but not desalinated. The new NSF International-certified product enables the treatment of various water sources in a single step to produce high-quality process water indicated as “direct capillary NF.”

Experimental results
At one pumping station in Spannenburg, The Netherlands, direct UF was used on surface water from the Prinses Margriet Canal. During the summer, a pilot installation with a capillary NF module was installed that ran in parallel with the UF (see Figure 3). The aim was to investigate drinking water production from “area-specific” surface water.

The NF and UF were compared with each other during the research. The results showed with this use of NF, more stable operation is achieved as well as a higher quality (flux approximately 27 liters per square meter per hour, L/m2/h, at a transmembrane pressure of 2-2.5 bar). It’s common to specify the operating pressure at which a certain amount of product is produced (L/h) per square meter of membrane area that has been installed (m2). Moreover, weekly chemical cleaning is sufficient. Table 1 shows a summary of the main operation parameters. Figure 4 shows results of the change in flux for direct NF.

Conclusion
This trial study demonstrated clearly that direct capillary fiber NF results in long-term stable flux behavior with a minimum of chemical cleanings. The flushing has proved to be a very efficient, powerful way of operation to stabilize the flux and reduce the frequencies of chemical cleanings. Capillary NF enables raw water to be treated in a single step to produce high-quality permeate. The permeate can be used as process water for the industry. Examples are water for the production of ultrapure water needs and boiler water, and water for the process, chemical, food and dairy industries.

Acknowledgments
The author would like to thank Program Economy, Ecology and Technology of the Ministry of Economic Affairs (EET) and The Netherlands Agency for Energy (NOVEM) for their financial support.

References

  1. Rosberg, R., “Ultrafiltration (new technology), a viable cost-saving pretreatment for reverse osmosis and nanofiltration – A new approach to reduce costs,” Desalination 110, 107-114, 1997.
  2. van Hoof, S.C.J.M. , A. Hashim, and A.J. Kordes, “The effect of ultrafiltration as pretreatment to reverse osmosis in wastewater reuse and seawater desalination applications,” Desalination 124, 231-242, 1999.
  3. Frank, M., et al., “Capillary hollow fiber nano-filtration membranes,” Separation and Purification Technology, 22-23, 499-506, 2001.
  4. van der Meer, W.G.J., et al., “‘Luchtspoeling’ bij ultrafiltratie,” H2O 4, 20-22 (in Dutch), 1999.
  5. Duin, O., et al., “Direct nanofiltration or ultrafiltration of WWTP effluent,” Desalination 132, 65-72, 2000.

About the authors
Harry Futselaar is the technology development manager and Henk Schonewille is the sales manager at NORIT Membrane Technology, of The Netherlands. They can be reached at +31 74 242 2009, +31-74-250.0509 (fax), email: info@noritmt.nl, or website: www.norit.info. Walter van der Meer is the manager of process technology at Vitens Water, of The Netherlands. He can be reached at +31 58 294 5242 or email: walter.vandermeer@vitens.nl

 

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