Sand filtration has been a widely used treatment process in the drinking water sector for numerous decades. Yet we still have a limited understanding of the way sand filters actually operate. The NWO Sand Filtration research programme being executed by Vitens together with drinking water company Dunea, in which ten PhD students are collaborating, is designed to change this.
A proven technique, sand filtration is the oldest and most widely used purification process at Vitens,” explains process technologist Frank Schoonenberg. “A sand filter removes iron, manganese and ammonium dissolved in the water. Contrary to popular belief, a sand filter is not a simple sieve but a kind of reactor in which physical, chemical and biological processes take place that also influence one another. However, we are neither really sure how this influence is brought to bear, nor how the various processes actually take place. Despite the long-term application of this form of filtration, sand filters are still largely a black box, and it remains more or less a mystery why one sand filter works well while the other does not.”
“Despite the long-term application of this form of filtration, sand filters are still largely a black box”
“With a view to improving our understanding of the way sand filters work, we embarked on the Sand Filtration research programme together with drinking water company Dunea, thanks to financial support from the Netherlands Organisation for Scientific Research (NWO). Launched in 2020, this programme comprises five research projects, in which no fewer than five Dutch universities and three research institutes participate. Two of these projects are focused on rapid sand filtration, and are therefore highly relevant to Vitens. The two are being carried out by Delft University of Technology, Utrecht University and Radboud University Nijmegen. Each project is executed by two PhD students from different departments. This approach was adopted because it allows both different insights to be combined and cross-fertilisation to take place. In performing their research, the PhD students make use of filter material and water supplied by Vitens and Dunea.”
“The studies are all pretty fundamental and focus on various aspects of sand filtration. For example, the researchers from Utrecht and Nijmegen endeavour to gain an insight into the individual microbiological and geochemical processes that take place in sand filters. To this end, they combine modern DNA analyses with geochemical measurements. The DNA analyses enable them to make an inventory of all the bacteria that are present and active in the sand filters. The geochemical measurements – based on microscopic photographs taken by the researchers, among other things – map out highly precisely where in a sand filter the iron and manganese are adsorbed. Our own laboratory also contributes to the research projects. For example, our colleagues measure the amount of iron-oxidising bacteria in the sand filters using a qPCR method.”
“Delft’s research projects are executed by PhD students from its biotechnology and water management departments. Among other things, they are investigating which adjustments to the process conditions might lead to improved operation of sand filters. They are also investigating the possibility of applying more efficient process solutions. For example, should it transpire that various processes have a detrimental effect on one another or operate more favourably individually, it may prove beneficial to ‘separate’ them. And we have done just that in the pilot project being run at our Hammerflier production site. There, we first remove the iron using a sand filter under relatively low-oxygen conditions. We then increase the oxygen content by means of aeration, to remove manganese and ammonium from the water in another filter.”
“A better understanding of the way processes work should enable us to both further optimise and design them more efficiently”
Frank is enthusiastic about the research programme: “This kind of in-depth long-term research is highly suitable in gaining a better understanding of purification processes that we have been using for quite some time, yet still do not fully comprehend. Of course, it is not simply a matter of comprehension. After all, a better understanding of the way processes work should enable us to both further optimise and design them more efficiently”
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