15 years of research

The current large-scale use of the membrane filter technology is the result of many years of research work in on-site research centres of the waterworks operators in France, Great Britain, Australia and in the USA. Important contributions to the development of the technology were provided by scientists of the University of Hong Kong and the Asian Technology and Research Network in Kuala Lumpur. The first research projects with regard to the use of filter membranes in drinking water purification started in 1985 in the framework of the European programmes EUREKA and ESPRIT. In the meantime nothing stands in the way of their use in large-scale water purification plants.

Membranes are thin, semi-permeable materials holding back many microscopically small pollutants like pesticides or pathogens by mechanical means. Another major advantage is the reduction of purification chemicals. Different terms are used depending on the pore size of the membranes: microfiltration, ultrafiltration or nanofiltration. Theses technologies are used depending on the condition of the water to be purified.

Nanofiltration for clean water

Générale des Eaux has decided to employ nanofiltration at the waterworks of Méry-sur-Oise in the north of Paris. Since the end of 1999, 200,000 cubic metres of surface water have been passing through the membranes of the nanofiltration units every day. Prior to industrial use, the nanofiltration method was tested at an experimental plant with 2,800 cubic metres.

Nanofiltration is now replacing the previously used processing methods with ozone and granulated activated carbon. The membranes, acting like a sieve, filter out not only microscopically small pollutants, but also pathogens that are resistant to disinfectants, and organic substances promoting the development of bacteria in water. The water softened by filtration is re-mineralised afterwards. As to sulphate-containing water, the French water experts have also achieved very good results with membrane filtration.

Combined technology with ultrafiltration

The challenge to purify 55,000 cubic metres from the river Seine for more than 200,000 people in the south-east of Greater Paris is being met by means of a combination of several methods: the effect of ultrafiltration membranes is combined with powdery activated carbon. After enrichment with oxygen, the water is mixed with pulverised activated carbon and pressed under high pressure into the ultrafiltration membranes. The tiny pores hold effectively back microscopically small pollutants like bacteria or viruses.

The combination of the two methods allows to directly filter out particles larger than 0.01 µm. Smaller particles are bound by the activated carbon. Only the mineral salts and the natural water balancesucceed in overcoming the barrier of the filter membranes. The size of the plant is enormous: the membranes have a filter surface of 1.2 hectares divided in 224 modules. These disinfecting properties of the membranes can reduce the amount of chlorine previously used by 90 percent.

In the meantime the method used by Lyonnaise des Eaux has come into vogue: the American city Del Rio and Lausanne in Switzerland are building waterworks with this purification technology. The daily capacities of the new waterworks in the US amount to about 100,000 cubic metres.

Research continues

After the success of the current plants the issue now is how to increase the performance of the filter systems. In addition, the engineers are developing exchangeable non-reusable membranes. Another major challenge is also to lower the cost of membrane filtration technology. The reason why today only about 10 percent of the water amounts are purified by means of membrane filtration may very well be the fact that it is more expensive. (Source: aqua press Int. 5/2000)

Dr. Alexander Tempelmayr