Woodchip bioreactor as a site specific approach to reduce nitrate loads from agriculture: demonstration in practice

In the South-eastern part of the Netherlands, nitrate concentrations often exceed the limit values in surface water due to agricultural activities. Water bodies should be in good chemical status by 2027 according to the Water Framework Directive, so measures should be taken to reduce nitrate loads to the streams. In this area, many agricultural fields are equipped with tile drainage systems. A way to reduce the nitrate emissions is the installation of woodchip bioreactors which are connected to these tile drainage systems. Residence times of the drainage water should be taken into account, for negative side effects may prevail when these are too high. This includes the production of nitrous oxides, sulfide and ammonia, and leaching of heavy metals.

A woodchip bioreactor was installed and connected to 4ha arable land on sandy soil in the southern part of the Netherlands in 2023. During drainage season, the influent of the bioreactor is connected to the tile drainage system, whereas it is disconnected during low or no flow periods (mainly in summer). The effluent is connected to a small stream. Continuous nitrate sensors were installed at both the in- and effluent, as well as discharge measurements to determine the removal efficiency of the bioreactor. Several piezometers were installed inside the reactor to monitor the biogeochemical processes taking place.

An extensive sampling campaign was carried out in autumn 2023, during the drainage season 2024, autumn 2024 and during the drainage season 2025. It showed considerable removal of nitrate (between 40% and 90%), especially in the first half of the reactor. At specific moments, some leaching of sulfide, ammonia, phosphorus and iron was observed. These leaching events appear to be related to start-up of the reactor in autumn 2023 and 2024 and not specifically to flow rate. The formation of nitrous oxide was determined during operation in 2024/2025 and was negligible.
The woodchip bioreactor proved to be a good area-specific measure to reduce nitrate loads in small streams, but care has to be taken on possible side-effects of the bioreactor on the stream.