Less nitrogen in ditches thanks to bioreactor with wood chips

In many places in the Netherlands there is still too much nitrogen present in surface water. Agriculture is an important source, mostly as a consequence of fertilisation. This leads for instance to the leaching of nitrate via drainage pipes to the ditches. An excess of minerals – like nitrogen – is not good for water quality. Among other things, it leads to algae and the excessive growth of plants and duckweed. And this, at a time when we want and need to improve water quality in the Netherlands. The fertiliser policy focusses primarily on tackling the issue at-source. As a complementary measure, nitrate could also be removed from the drainage water on land plots where nitrate leaches easily. With this in mind, in Westerbeek (Land van Cuijk municipality) a bioreactor with wood chips is being tested as a means of removing nitrate from drainage water.

Image 1. The installation of the wood chip bioreactor in Westerbeek in Brabant.

The drainage water from the agricultural plot ends up in the bioreactor. The wood chips in the bioreactor stimulate bacteria which are naturally present in the soil. These bacteria convert the nitrate into harmless nitrogen gas. ‘So that we can once again have clean water flowing in the ditch,’ says Inge van Driezum, researcher in groundwater quality and protection at KWR. 

Nitrogen has a negative impact

Fertilisers remain behind in the soil after the crops have been harvested, since the plants don’t uptake them all. Some of them leach via the drainage pipes to the ditch. ‘The wood chip bioreactor is simple and relatively inexpensive, while also being highly effective. You can have a major impact on the water quality with it, and at the same time you don’t need to greatly restrict agriculture,’ says Stefan Jansen, microbiologist at Deltares. Inge van Driezum adds: ‘We designed and installed the wood chip bioreactor together with colleagues from Deltares. In the design we took into account the intensive monitoring, which makes it possible to closely follow the processes in the bioreactor. This increases our knowledge of wood chip bioreactors and of their practical application in the Netherlands.’

‘The advantage of this approach is that you can complete the bioreactor underground and install it in places where these kinds of problems are an issue,’ according to Frank van Herpen, project leader, on behalf of the Aa and Maas Water Authority. The bioreactor is filled with wood chips from trees from the surroundings, and is expected to work for about ten years without having to be replaced. A completed bioreactor underground makes it possible to use the ground for another purpose – as buffer strips for instance.

Experiences abroad

Field tests of this kind have already been carried out with wood chip bioreactors in Denmark and the United States. In those cases the leached drainage water contains 20 to 40 percent less nitrogen. With the Westerbeek field test the research institutes are jointly investigating with the Water Authority how the bioreactor works in a typical Dutch situation. A similar test set-up has been built in Limburg, and there are plans for a field test in North Holland.

Several parties involved

The field test is a collaboration of Deltares, KWR, the National Institute for Public Health and the Environment (RIVM), and Wageningen University & Research. It is being financed by the Aa and Maas Water Authority, the Zuiderzeeland Water Authority, the Foundation for Applied Water Research (STOWA), the Ministry of Infrastructure and Water Management, the Ministry of Agriculture, Nature and Food Quality, as well as the provinces of North Brabant and Flevoland.

Press event

On Tuesday, 19 March, at 11:00, representatives of Deltares, KWR and the Aa and Maas Water Authority will present the field test on-site in Westerbeek. It will also be possible to make film recordings at the time. Interested media are requested to register in advance with Jens Verhagen, press officer of the Aa and Maas Water  ( or 06-39351217).