MESA+ University of Twente
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Natalia Hoog

Stub resonators

transmission – line – based water sensors

Promotion date: September 12.

Promotor: Albert van den Berg

Assistant Promotor: Wouter Olthuis

Drinking water, surface water and waste water may contain toxic or otherwise undesired components at a wide range of concentration levels. In order to safeguard water quality, the early detection of pollutants and control of process conditions by continuous monitoring is mandatory. However, currently existing (bio)chemical detection methods are offline and by implication labor-intensive and thus expensive.

This conclusion drives the search to develop a new class of inline sensors for the cost-effective monitoring of water quality. The focus of this PhD study was to explore the use of antenna or transmission-line-based technology to measure water quality in terms of its composition.

In addition, several other applications of this technology have been investigated, ranging from biofilm and corrosion development to measuring the load of ion exchange columns.

Was your research theoretically oriented or instead more focused on application?

During my PhD time I studied both the theoretical and practical aspects of this technology. I like the combination of fundamental and applied science, even more so because, to my opinion, it’s the most effective way to come to a first prototype and, later on, commercialization of the research. This last aspect is of course a key parameter of all the projects within the Wetsus research program.


Based on promising results, a first production type of the coax sensor was finished in December 2012. This milestone was published in the Wetsus Science and Technology letter. The water company Vitens helped to validate the sensor. In parallel, detailed mathematical modelling of the sensor behaviour was executed at Wetsus, providing solid scientific evidence of the physics involved.

In 2013, the sensitivity of the sensor concept was greatly improved, partly a result of the successful modelling. At the same time the applicability of the concept was successfully broadened to the detection of early stages of biofilm and corrosion formation, as well as the determination of the loading of resins used in ion exchange columns.

How was the working atmosphere?

Even though I was officially employed by the University of Twente, all research has been performed at Wetsus in Leeuwarden ( The groups involved at Twente were BIOS Lab-on-a-Chip and the MESA+ Institute of Nanotechnology. In addition, EasyMeasure B.V and Smart Frequencies, two start-up companies originating from Wetsus, were very closely involved. Because of this I had a great opportunity to work with many exceptional people. Practical experience with prototypes was obtained at the Vitens drinking water production facility at Spannenburg.

During all these years it has been a wonderful experience to work together with so many different people. Even though the working environments were quite different, the atmosphere was always a warm one.

Which remarkable outcomes resulted from your PhD work?

Together with Dr. ir. M.J.J Mayer, from Easymeasure B.V. at Amersfoort, we started this project from scratch five years ago. I am very proud that within a period of just five years we were able to develop a fully automated and on-line prototype of the coaxial sensor. This prototype is actually already announced on the website of Easymeasure B.V. because the expectation is that it goes into production and will find its way into the market before the end of this year. (See It is just such a great feeling to see that the prototype performs so well not only in the lab but also under real-life conditions.


My study further resulted in six patents, five accepted publications (e.g. in Sensors & Actuators) and three submitted manuscripts.

What are your future plans?

I would love to continue working in the field of water technology and sensors. At the moment I am working at Wetsus on a feasibility study concerning a sensor based on the same concept but different of design. Goals to achieve include enhancing the sensitivity of the inline system and increase the total number of detectable components.

Looking far ahead, the holy grail probably is miniaturization which, in turn, allows for a design based on an array of separate sensors, all sensitive to specific analytes.