Most sensors have been installed in bypass-flows in order to minimise the risk of contamination and to facilitate maintenance. However, this also means that the water flowing through the bypass pipeline is not returned to the process, but lost for consumption or other use. As a result, this water loss can be considerable; up to several hundred thousand cubic metres per year, especially when a large number of sensors are used in this way (the utility we looked at used between 10 and 61 sensors at their treatment plants, with an average of 28).

We investigated the availability of in-line alternatives for the most commonly used sensors (pH, dissolved oxygen, conductivity and turbidity) in order to reduce water loss during treatment. During this investigation, a number of important requirements were identified, such as:

1. The risk of contamination of the water flow needs to be minimal during placement, removal and maintenance of in-line sensors, as water quality must be ensured at all times;
2. The measurement principle of in-line sensors must be based on a proven technology for the detection and quantification of the parameters in question;
3. Performance characteristics such as accuracy, stability, linearity and measurement range must be similar to or better than the sensors currently in use;
4. In-line sensors applied must comply with the requirements for materials in contact with drinking water, as stipulated in e.g. European regulations. Luckily, a large number of technology suppliers offer retractable fittings to enable in-line measurement of a number of water quality parameters in pressurised pipes. These fittings allow for safe placement and removal without the risk of contaminating the water flow. Some companies also offer retractable options to install multiparameter probes at a single location.

However, as for applications in food and beverage production, glass electrode sensors are not ideal for in-line water measurements because of the risk of breakage. An alternative to glass electrodes is the use of an ISFET sensor (ISFET: ion-sensitive field-effect transistor). ISFET sensors were invented in 1970, and since then, have found their way into many different application areas, ranging from the medical sector to food and beverages. The use of ISFET sensors for pH-measurements is much more common in marine water than in freshwater or drinking water. Besides being virtually unbreakable, ISFET sensors have the advantage of having a short response time, and can be dry-stored. However, they are generally not as stable or accurate as glass electrodes, are sensitive to light and temperature, and have known issues with drift. But these drawbacks may soon belong to the past, as machine-learning techniques may be able to effectively correct for temporal and temperature drifts of ISFET pH-sensors.

Thus, such ISFET sensors may be able to meet three of the four criteria mentioned above. The fourth requirement, however, proved to be more difficult. In Europe, the requirements for materials in contact with drinking water are currently covered under the European Drinking Water Directive (DWD), 98/83/EC. The current Directive offers no harmonised approach to materials in contact with drinking water with each Member State being free to set its own requirements. As a result, manufacturers of such products and materials are forced to obtain certificates of approval in each individual Member State where they intend to sell their products, which is a very costly and laborious process. They will only go through this process if there is a significant market potential justifying the effort and expense, which is of course perfectly understandable. In the Netherlands, we found that none of the technology suppliers offering glass-free sensor technology for drinking water quality monitoring have the required certificate of approval for use in contact with drinking water. Therefore, drinking water utilities generally show no interest in such technologies, since they cannot be implemented without the necessary certificates. No interest means no market potential, no market potential means no incentive to certify products, and no certificate means no interest from water utilities… a chicken-and-egg situation which is very difficult to get out of.

But there is reason for optimism. The recent revision process of the DWD, published by the European Commission on 1 February, 2018, offered a good opportunity to improve regulations related to materials in contact with drinking water, at the request of many stakeholders involved. As agreed in December 2019, the newly proposed Directive aims to establish an EU-wide scheme for materials in contact with drinking water, in order to harmonise these requirements across the EU and create a truly single market for such products and materials. Although this process will take time and will not be implemented overnight, it offers tremendous opportunities for technology suppliers, and we are optimistic that it will give a major boost to the deployment of in-line sensors for drinking water quality monitoring and thus public health in the long run.