FilterXpert: monitoring the operation of sand filters online

Water companies have traditionally used sand filters to purify water. However, the way in which these filters work is poorly understood. Accordingly, Oasen plans to participate in a joint study which will involve making online measurements of working sand filters. The study is a collaborative project involving TU Delft, the DHV consultancy, Oasen, Vitens, Thales, Waternet, and the Vallei en Eem district water board. We are looking for a kind of thermometer that will enable us to see exactly how the sand filters operate, moment by moment.

Traditional know-how
Virtually all Dutch water companies use sand filters in their water purification facilities. The filters capture dirt and remove dissolved substances such as iron and manganese. They also mediate the conversion of ammonia. While these processes are very effective, there is a lack of genuine scientific knowledge about how they actually work. Water companies operate their sand filters on the basis of traditional know-how and historical assumptions. A significant proportion of these companies have invested many years of research in attempts to optimise the control of these systems.

Patience is a virtue
The operation of sand filters is currently checked by water analysis. This method gives us an indication of when a sand filter needs to be flushed out or furnished with clean sand. The drawback with this approach is that it always takes a few days before the results of these tests are known. If the process is stable this is no problem, but we want to be able to monitor each filter’s status immediately. If the filters should experience a loss of function or if they should start to function anomalously, then we would prefer to know about this immediately. The system at the centre of this study would enable us to track developments online.

3D sand filters
As part of this study, we will be using ultrasound imaging to view the filter bed of each sand filter directly. A tomograph is used to transmit acoustic signals through the sand filter. The reflected signals are then processed by a PC to produce a three dimensional image of the filter’s contents. We aim to use this technique in conjunction with other sensors to determine whether the filter is changing and to see whether the flushing procedures have been effective. This will enable us to achieve improved process control, and may eventually cut our analysis costs. The topic of this study is closely related to the PhD research currently being conducted by our drinking water specialist, Weren de Vet. He, too, is investigating the operation of sand filters. Accordingly, his experiments and results will contribute significantly to the study in question. The study into the online monitoring of our sand filters is expected to take several years.

Marcela Coroiu is working on FilterXpert (the official name of this project), which is being funded by SenterNovem via the InnoWator programme.

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