Nanofilters Convert River Water into Drinking Water—French City Finds a New Source for Its Citizens
The world’s first and largest nanofiltration plant for treating river water is located north of Paris. The plant is fully automated and the CIP (clean-in-place) loop for cleaning the membranes runs without virtually any manual intervention. Trouble-free CIP operation is ensured by, among other things, 30 plastic-lined magnetic drive pumps for dosing and conveying the chemicals.
Water supplies to the French public are mainly in the hands of private companies. A large share of the market is served by Vivendi, which was founded in 1853. In northern Paris, the Syndicate des Eaux d’lle de France (SEDIF) supplies 39 communities (800,000 people) from a plant that employs a very unique method of treating water from a river source. Located in Méry-sur-Oise, the treatment plant needed to increase production capacity. To do this, SEDIF decided to adopt a technology that had never been used before to treat river water—nanofiltration.
Nanofiltration is a membrane liquid separation technology that’s positioned between reverse osmosis (RO) and ultrafiltration. While RO can remove the smallest of solute molecules—in the range of 0.0001 micron (mm) in diameter and smaller—nanofiltration removes molecules in the 0.001 mm range.
A form of RO
Nanofiltration is essentially a lower-pressure version of RO where the purity of product water isn’t as critical as pharmaceutical-grade water, or the level of dissolved solids to be removed is less than what’s typically encountered in brackish water or seawater. As such, nanofiltration is especially suited to treatment of well water, or water from many surface supplies like rivers or lakes.
Nanofiltration is used where the high salt rejection of RO isn’t necessary. Yet, nanofiltration is still capable of removing hardness elements such as calcium or magnesium. Like RO, nanofiltration is also capable of removing bacteria and viruses as well as organic-related color without generating undesirable chlorinated hydrocarbons and trihalomethanes (THMs). Nanofiltration is also used to remove pesticides and other organic contaminants from surface and goundwaters to help ensure the safety of public drinking water supplies.
Sometimes referred to as “membrane softening,” nanofiltration is an attractive alternative to lime softening or sodium chloride zeolite softening technologies. Plus, since nanofiltration operates on lower pressure than RO, energy costs are lower than for a comparable RO treatment system.
The location for this unusual plant was chosen because the capacity of the old plant in Méry-sur-Oise had to be increased. Moreover, more effective treatment technology was required. Providing more incentive was the increasingly poor quality of the untreated water from the Oise River. The pollution of the water by organic constituents as well as pesticides from agriculture had been rising continuously for years. In 1998, the old treatment plant even had to be temporarily shut down owing to the excessively high total organic compounds (TOC) content. It was decided that a membrane process was an ideal solution for such problematic water treatment.
All known life forms are based on the separation of substances with biological membranes. It’s assumed that the membrane processes, which occur in nature, consume little energy and are extremely efficient. Researchers are trying to understand the processes in nature and convert them into industrial processes with synthetic membranes.
Industrial membranes are filigree structures, which largely consist of a multi-layer polymer film with a total thickness of a human hair (approximately 100 mm); the actual active layer only accounts for one-hundredth of this thickness.
There’s enormous future potential for membrane processes. Their application ranges from the recovery of gasoline vapors through the treatment of acids and alkalis to the removal of alcohol from beer. In addition, the fuel cell is largely based on membrane processes. Certain branches, such as the food industry, the pharmaceutical industry and biotechnology, would no longer be feasible without membrane processes.
In principle, all pressure-driven membrane filtration processes (micro-filtration, ultrafiltration, nanofiltration and RO) involve the water being pressed through a membrane by a transmembrane pressure difference. The membrane then ideally retains all the undesirable water constituents. Which process is used depends on the type and size of the substances to be separated. Nanofiltration membranes generally carry an electric charge so special separation problems can be solved, i.e., the separation of molecules of the same size but with different electric charges.
Vivendi’s decision to go forward with nanofiltration proved to be correct, as project manager Arnaud Douveneau noted, “Using nanofiltration, our plant in Méry-sur-Oise already satisfies the stringent EU (European Union) demands placed on the quality of drinking water, and that with a much lower volume of chemicals than in conventional plants.” Equally good results could only be achieved with RO but nanofiltration consumes much less energy and is therefore considerably lower-priced. Nevertheless, nanofiltration isn’t a cheap technology; however, thanks to the use of specially developed membranes for water not containing salt, the throughput at Méry-sur-Oise is substantially higher than with conventional membranes. Moreover, the operators can run the plant at a lower pressure, and both aspects cut running costs, Douveneau stresses.
Cleaning the membranes
A major factor for the economic success of nanofiltration in water treatment is the continuous monitoring and cleaning of the membranes. At the Méry-sur-Oise plant, this takes place in a fully automatic CIP process. Each membrane is equipped with pressure, flow and conductivity sensors. The condition of the membrane surfaces is therefore monitored around the clock. The control takes each of the eight membrane lines out of production every eight weeks, and initiates the cleaning process that involves acids, bases and detergents.
Dosing out these cleaning chemicals are leak-free, plastic-lined magnetic drive pumps from ITT Richter. The company specializes in pumps, valves as well as measuring and control equipment for corrosive and pure media. Although the initial investment costs of magnetic drive pumps are slightly higher than for conventional mechanical seal pumps, the high operational reliability of these leak-free pumps was more important to the operators. Furthermore, magnetic drive pumps are more economical in the medium term as there are no mechanical seals and liquid-sealed systems to be serviced and the cost of monitoring is minimal. Douveneau notes, “Only a few staff work in the fully automated water treatment sector. The reliability of the pumps must therefore be ensured in the long term.” Pumps for conveying corrosive, toxic or otherwise critical media are subject to particularly stringent standards with regard to operational reliability, sealing against the atmosphere, and service life especially in the environment of drinking water treatment. The high demands were also an important reason for the operator to install ITT Richter seal-less pumps and, in particular, magnetic drive pumps.
With the technology installed at Méry-sur-Oise, Vivendi satisfies the requirements of SEDIF with the security of supplies and drinking water quality. Douveneau says, “We are demonstrating with this plant what the state of the art is today in drinking water technology. Roughly 800,000 people in the Ile de France region obtain particularly high-quality drinking water, which is also demineralized.” Naturally, such a quality has a price. The plant cost roughly 1 billion Francs (US$150 million), but the advantages speak for themselves—people receive drinking water without any taste of chlorine and soft water that no longer causes any scaling problems in home plumbing.
About the author
Fathi Chatila is the editor of Arab Water World, where this article was first published in the January/February 2002. It’s being run here with the publisher’s approval. Please contact: email@example.com