The trouble in between

One of the biggest problems with the filtration of water is bacteria and other micro-organisms clogging up the filters. Hans Vrouwenvelder studied the formation of these biofilms and came up with some completely new potential solutions.

“In 2025, almost half of all the countries in the world will have a shortage of drinking water. And don’t think clean water is just a problem for Third World countries. Also in the West, our water is increasingly polluted with pesticides and other contaminants. Techniques for purifying water are therefore extremely important”, says Hans Vrouwenvelder. For his PhD project at the department of environmental biotechnology, he studied a common problem in membrane filtration systems that transform seawater or contaminated fresh water into potable water: biofouling. Micro-organisms, like bacteria, form biofilms within the filters, resulting in problematic declines in water production or quality. Vrouwenvelder received his doctorate on October 30th.

People have been trying to come up with a universal solution for biofouling since the early 1980s, but in vain. “They tried pretreating the feed water to remove or inactivate bacteria, and thus prevent biofouling from occurring,” says Vrouwenvelder. “But that didn’t work in all cases. The focus has also been on the semi-permeable membranes that comprise the most important part of the filter systems. But adaptations of these membranes weren’t sufficient to prevent biofouling.”

In order to find new solutions for this problem, Vrouwenvelder studied some of the fundamental processes underlying biofouling in a laboratory setting. “Until now, people have mainly been conducting trial-and-error studies in the field,” he says. “But recently more people have been publishing articles on fundamental research concerning biofouling.”
The type of water filters Vrouwenvelder studied are called spiral wound reverse osmosis membrane systems, which are currently the most commonly used type worldwide. Each filter unit consists of a cylinder of tightly wound filtration membrane, which allows for as much filtration surface as possible within a limited space. In between the sheet of membrane is a sheet of gauze-like spacer material, which helps the water flow more easily between the membrane layers. A small amount of the water that is led along such a filter unit seeps through the membrane and is collected in a central collection pipe. This water is potable. The rest of the water is transported to another filter unit for further filtration.

One of the most surprising outcomes of Vrouwenvelder’s research is that the biggest problem doesn’t seem to be the filtration membrane, but rather the spacer material. Vrouwenvelder: “Both in our tests with a scale model and in tests with the filter modules used in practice, we saw that biofilm grows mainly on the spacer, creating a sort of system of blockages and water channels.” Because of this growth of biofilm on the spacer, some parts of the filtration membranes receive little water and some a lot, which can explain filtration problems. The biofilm growth also disrupts the water flow through the entire cylinder, causing pressurisation problems.

“It may seem logical that the spacer is one of the main problems,” says Vrouwenvelder, “but it was generally accepted that the membrane was the main problem, which also sounds logical. Until now, laboratory studies on biofouling used systems with only the membrane, not the spacer. We’re the first to use a scale model that has exactly the same properties as a real filter unit.” It also helped that Vrouwenvelder made use of MRI scanning techniques, enabling him to study the 3D-structure and effect of fouling development in the filters. Previously, fouled filters were usually taken apart to study them, thereby destroying such information.

A possible solution to prevent biofouling problems would be to make adjustments to the spacer geometry. Vrouwenvelder however has also made some other discoveries, mostly concerning the hydrodynamics. Like the fact that there seems to be less biofouling when the speed at which the water is allowed to run through the filters is decreased. Vrouwenvelder: “I’ve heard scientists mention that you should increase the water velocity to prevent biofouling. But it doesn’t seem to work that way. Also, if you increase the velocity in a system, you need more energy, and biofilm formation results in more pressurisation problems and a higher risk of filter damage.”
In his thesis, Vrouwenvelder describes these and other possible solutions based on his fundamental research. A great deal of interest has already been shown his work, and the International Water Agency is considering publishing his thesis as a book.  

Hans Vrouwenvelder: ‘Biofouling of spiral wound membrane systems.’ Thesis defence: 30 October 2009.

The Royal Netherlands Academy of Arts and Sciences (KNAW) has advised the Dutch minister of Education, Culture & Science, Ronald Pasterk, to withdraw the 2007 Iran Sanction Regulation. This regulation excludes students from Iran from nuclear-related fields of research and education in the Netherlands. The Academy regards the far-reaching implications of the Iran Sanction Regulation as contrary to the free and international conduct of scientific research.

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