Deltafact Biocides: no clear picture as yet of impact on water quality

For water managers there is still only limited knowledge available about biocides and their impact on water quality, even though these substances can still unexpectedly turn up in Dutch surface water. The recently published Deltafact Biocides of the Water Quality Knowledge Impulse (KIWK) programme presents the current knowledge and the knowledge gaps.

Few people know what biocides actually are and how extensively they are used. Only a quarter of the biocidal products permitted in Europe are monitored in surface water in the Netherlands. As in the case of the recently published Deltafacts Microplastics and Consumer Products, the purpose of the Deltafact Biocides (all three publications in Dutch) is to inform water managers about these substance groups. The Deltafacts are products of the Chain Explorer project of the KIWK, which is a four-year research programme being conducted by Wageningen Environmental Research, RIVM, Deltares and KWR, at the request of the national government, the provinces and the water managers. The webinar Emerging Substances is being organised on 3 June for water managers and other water professionals, for the purpose of sharing knowledge from the Chain Explorer project.

Biologically active substances

Biocides are biologically active substances intended to control, attract or repel harmful living organisms. The literal meaning of ‘biocide’ is ‘life-killing’. The Deltafact Biocides covers the four main biocidal product groups: disinfectants, preservatives, pest control agents and ‘other biocidal products’, which include antifouling products used to control the growth of organisms on vessels. The need to monitor the emission of plant protection products into water is obvious. However, in the case of biocides this is often not so self-evident, says Ivo Roessink, researcher at WUR: ‘The use of biocides is more diffuse than that of plant protection products, which are applied for example to control aphids or other pests. Because of the existence of an exposure route into the water, it’s logical for you, as a water manager, to keep an eye on these products. We are increasingly coming to understand that biocides can also take unexpected backdoors to end up in our environment. It is therefore a matter of raising awareness.’

Analytical method and monitoring

The degree to which biocides are kept track of in Dutch water was investigated by KWR’s data scientist Tessa Pronk. ‘Of the 254 biocides permitted in Europe, only 55 are being monitored. In other words, only a fraction of those that could occur in surface water.’ One of the reasons for this low level of monitoring is explained by Joke Wezenbeek, a researcher at RIVM. ‘In many instances, there are as yet no standardised analytical methods for biocides. Nor do we have a clear picture of their emission routes into water. How, for instance, would biocides used as preservatives in a can of paint end up in the water?’ An additional factor is that biocides often have a double application, adds Pronk. ‘The question is therefore whether they are monitored because they are a biocide. In the end, it appears that only half a dozen products are used solely for biocidal purposes.’

Most important emission route via WWTPs

It is therefore very illuminating to see, in the Deltafact Biocides, the emission routes of the 22 product types listed in the European Biocidal Products Directive. For instance, it shows that disinfectants and preservatives can reach surface water primarily via wastewater treatment plants (WWTPs). Roessink: ‘It’s not that people flush paint down the toilet. But they do for instance paint many garden fences. And when it rains, paint particles are washed away, and then reach the WWTP through the sewer system, and then ultimately end up in surface water.’


Apart from an initial exploration of biocides of importance for the water cycle and their presence in the Dutch water system, the Deltafact Biocides also addresses governance. ‘Like plant protection products, biocides are substances that are very well regulated,’ says Wezenbeek. ‘You can only use these substances if they have been approved, and if the biocidal products containing these substances are permitted. You must therefore demonstrate that the product is safe for humans and the environment. Nevertheless, our exploratory work indicates that some biocides can be present in water at levels above the threshold values. A good example is DEET, a substance found in insect repellents.’

Follow-up research

For Roessink, one of the most important outcomes of the Deltafacts Biocides is the presence in water of biocides that one would not expect, given what they are used for – DEET or wood preservatives, for instance. ‘We have in this way started by mapping the landscape of these substances with reference to the water cycle.’ The research will be pursued further to provide water managers with even more information. Based on a list of all European biocides and their associated substance properties, a follow-up study will determine which of them might end up in the water, and what the possible toxic effects would be. ‘For water managers there is no need to monitor all biocides,’ Wezenbeek points out. ‘For instance ozone and alcohol won’t have any harmful effect on water quality, because they degrade rapidly. You therefore won’t come across them in the water.’