Technology such as MRI helps us to understand the processes that take place in biomass when we purify waste water with micro-organisms. We can use this knowledge to extract more energy and resources (and new pollution) from our own waste water. But we also need new knowledge to tackle problems in the rapidly expanding world cities, where waste water purification is often a relatively new concept. We can and must get much more out of our waste water. This is argued by Professor Merle de Kreuk, who will give her inaugural address at TU Delft on Wednesday, 23 May.


The first sewers were built thousands of years ago, and the way we purify our waste these days is based on an activated sludge system that is over a century old. You might think we know all there is to know about water purification by now. But you couldn’t be more wrong, says Professor Merle de Kreuk, Professor of Environmental Technology in the faculty of Civil Engineering and Geosciences, for whom processes such as pellet sludge formation still hold a daily fascination: “How does the process of pellet formation start precisely, and what happens to the solid particles floating in the waste water? Using MRI, we can now visualise the structure of the pellets inside the pellet sludge right down to the millimetre, while keeping bacteria alive and with conversion processes continuing.”

More efficient and sustainable

De Kreuk hopes to find a way of extracting more resources and energy from waste water. This would not only be sustainable, but it would also improve the quality of the processing itself. “Phosphate mines have nearly been exhausted, but we still need phosphate as a fertiliser to increase food production throughout the world. Extracting it from waste water means you will have recycled a valuable resource, which also would no longer cause issues later in the processing chain.” The same applies to biogas. “If you convert as much sludge as possible into biogas, not only do you have a great source of energy, but the remaining sludge is much easier to process too. At the moment, bacteria only digest 40% of the organic substances inside the sludge. That percentage could be increased if we had a better understanding of the digestion processes.”

Compact systems

We need all the knowledge acquired here to address waste water problems elsewhere in the world, although the solutions needed there will be different.  “We can't just export a huge purification plant to emerging mega-cities in Asia or other parts of the world. We've learned from experience that things often go wrong once the foreign construction firms leave.” De Kreuk is working on projects in Delhi and Jakarta, where smaller, local systems are being developed and installed close to the source of the pollution together with local parties. “Small, compact systems in the middle of a city have much more potential. You can put the purified water, the resulting biogas and the fertilisers extracted (such as nitrogen and phosphor) to immediate local use.”


In these countries, education is just as important as technology, which is why De Kreuk makes sure that her students are closely involved in her research. Together with a company called Drainblock and students in Delhi and Jakarta, she is studying a new, simple method for purifying waste water in open sewers. “We hope that projects like this will encourage local students to choose subjects like environmental technology. Experts in these fields are desperately needed.” De Kreuk feels that project-based education is important for Dutch students too. This is why she co-founded the Water Technology Design Lab, where students can work on actual corporate projects. “The idea is for students to continue a project where previous students left off, with knowledge being passed on. We must keep innovating our education in the Netherlands too!”

Merle de Kreuk,, +31 15 27 85274
Carola Poleij, Science Communication Advisor Technical University of Delft,, +31 (0)15 27 87538, +31 (0)6 41611510