Pumps press water into a microfiltration membrane with openings measuring 5 microns in diameter. Then, the water is pushed through polymer membranes whose holes measure just under 1 nanometer each.
	The Mery-sur-Oise water treatment facility is just across the Oise River. With the quality of water resources deteriorating and tightening quality standards being imposed by the European Union, utilities have been looking for innovative ways to clean wate

Just across the Oise River from the town center stands Generale des Eaux’s 3.5 acre Mery-sur-Oise treatment facility. The company, a unit of conglomerate Vivendi Universal, is contracted by SEDIF, the water utility for the area surrounding Paris, to supply four million people with drinking water.

With the quality of water resources deteriorating and tightening quality standards being imposed by the European Union, utilities have been looking for innovative ways to clean water. Generale des Eaux developed the nanofiltration technology in its own labs in collaboration with Dow Chemical subsidiary Filmtec.

“We have been cleaning water at this plant using a biological treatment system here since the 1960s,” said Daniel Dubois, senior engineer at Generale des Eaux. “The nanofiltration began two-and-a-half years ago and now makes up 80 percent of the water that leaves the plant on a typical day.”

With little groundwater available in the Paris suburbs, 95 percent of the city's water must come from three different rivers: the Oise, the Marne and the Seine. Traditionally, river water was cleaned by injecting positively charged aluminium salts. Working much like a magnet, the salts would instantly attract negatively charged floating matter and slowly bring it to the bottom of the basin. The water would then go through a series of filters and an ozonization process. The final step, chlorine disinfection, killed whatever was left, but also gave water an unpleasant taste.

“The problem we had in this location is that the water in the Oise is particularly heavy with organic material during the wintertime and only nanofiltration could take it out,” Dubois explained. The water contains an average of 10 milligrams of organic matter per liter, more than double what you might find in the nearby Seine.

The nanofiltration process starts with the same positively charges aluminum salts to take out the biggest floating particles. The water is then ozonated to kill the algae. Then comes the anthracite and sand filters with holes of 1.2 and 0.8 square millimeters.

From then on, things gets smaller and smaller. After nitrification to remove ammonia, pumps press the water into a microfiltration membrane with openings measuring around 5 microns in diameter.

After this, the water is pushed through polymer membranes whose holes measure just under 1 nanometer each. Pumps create between 8 and 15 bars of pressure, the equivalent of standing under 80 to 150 meters of water, to drive the water through the tiny holes. As a final measure, the water is disinfected with ultraviolet rays.

The 6,000 or so residents of Auvers-sur-Oise have been drinking this nanofiltered water for almost 10 years, since the town was chosen as an experimental test site. They say the nanofiltered water tastes better because it doesn't have the taste of chlorine found in other tap water. In fact, the Cordeville restaurant in town has stopped serving bottled water altogether, figuring the tap water is now just as good.

But there are drawbacks. This treatment system is more expensive than the traditional process. All that water pumping makes for a higher energy bill - 60 percent higher.

In addition, the holes in the nanofiltration membranes are so small, that they take out useful particles along with the bad ones. “It takes out a good part of the calcium and magnesium present in the water,” said Antoine Montiel, a water expert from SAGEP, the company in charge of managing Paris’ water. He added that the plant at Mery-sur-Oise must remineralize the nanofiltered water by adding 20 percent of water cleaned by traditional means.

The advantages, on top of better tasting water, are logistical. For one, it can be managed from a distance. The plant at Mery-sur-Oise is strangely deserted. Most of the personnel works in the command center, where they control everything going on in the plant. “Nanofiltration plants can be highly automated and possible to manage from afar,” said Dominique Tricard, head of the French Ministry of Health’s Water Risk Evaluation Unit. “This means less personnel is required, making it particularly attractive for treating difficult-to-get-to underground water sources.”

Generale des Eaux isn’t the only utility harnessing the power of small tech to clean water. Competitor Ondeo, the water unit of French conglomerate Suez, has installed what it calls an ultrafiltration system, with holes of 0.1 microns in size, in one of its plants outside Paris.

Generale des Eaux has also been selling its technology to others. Most recently, Israel signed a turnkey contract for a nanofiltration plant adapted for seawater.