Wetenschap - 12 april 2001

Bacteria do the dirty work

Bacteria do the dirty work

Making waste water clean enough for irrigation is a 24 hour a day job

It's a smelly business, but someone has to do it: improving the treatment of waste water. In the laboratory of the sub-department of Environmental Technology in the Bornsesteeg researchers from countries including Egypt, Palestine and Brazil are doing just that. "We want to make the waste water so clean that you can use it for irrigation."

The millions of tiny organisms in the lab never stop working. Day and night the bacteria eat away at the most obnoxious stuff, swimming around in glass chambers that the researchers made for them. One of these researchers, Nedal Mahmoud, a fourth year PhD student, gives a tour through the lab. Like most of his colleagues he has his own special reactor, a glass cylinder holding the bacteria that purify the waste water coming from the sewer for the village of Bennekom. And like the bacteria, Mahmoud never seems to stop working. "It is a 24 hour a day job, also in the weekends. We even dream about our reactors."


As we walk through the maze of tubes, cylinders and machines, the Palestinian explains why bacteria are so useful. "They convert the waste into by-products like carbon dioxide and you don't need to keep on adding bacteria. Once they are in the reactor they keep on multiplying and doing their work." Another advantage is that the reactor does not require an external energy source, he explains. "This means you can place it virtually anywhere. The reactor runs on the energy it produces itself. The bacteria produce methane gas which heats the system."

These anaerobic reactors - they do not use any oxygen - have already been built in many places around the world, but Mahmoud and his colleagues are improving them, making them smaller and more efficient. "I built a relatively small reactor, and have demonstrated that it works. I cannot make the water clean enough for drinking, but it can be used for irrigating farmland." Mahmoud hopes to put his ideas into practice in his home country of Palestine. "In the Middle East a lot of polluted water is just discharges into rivers and lakes. We can change this situation by using anaerobic reactors."

Clear water

Eager to show that the anaerobic reactors are working, Mahmoud points to one of them, about one metre high, filled with dark, dirty sludge at the bottom. The water flows upward and gets much cleaner. At the top the water is clear. Mahmoud remains modest: "Without the thirty years of knowledge already accumulated in this field, including the heritage of the recently retired professor Gatze Lettinga, we wouldn't have got so far. We are at the top of the pyramid."

Hugo Bouter