Bioreactors with wood chips (also known as ”constructed mini-wetlands with biofilters”) is a so-called collective measure with the purpose of reducing the leaching of nitrogen (N) to the aquatic environment. The principle behind the drain filters is that drain water is led through a biofilter consisting of organic material, such as wood chips. In the filter, under anaerobic conditions, bacteria metabolize the nitrate in the drain water into free atmospheric nitrogen.

Testing bioreactors

Six test facilities have been established in various regions in Denmark (Fig. 1). The facilities remove nutrients from drainage water from catchment areas ranging between 16 and 120 ha. These facilities will help us achieve knowledge on rinsing of water at large variations in annual precipitation – ranging from 600 mm on average to 1100 mm. Four of the facilities consist of a basin for phosphorous precipitation and a wood chip basin for nitrogen removal (cf. the photo). The facilities in Slagelse and North Funen are major facilities each with three wood chip basins.

The new bioreactors are designed in a way that ensures that the minimum retention time of the drainage water is min. 10 hours at maximum inflow into the system. This allows for achieving an excellent nitrogen reduction impact even at low winter temperatures, when bacteria are less efficient at reducing nitrogen. Water temperatures can be so low that nitrate removal actually stops.

 Large effect at reduced space

Tests during the first year demonstrated that the facilities removed between 29 and 49 percent of total nitrogen in the drainage water. However, the Gyldenholm plant only removed 14 percent, which was due to the fact that the pumps actually performed up to 50 percent more than expected.

The results show that the bioreactors are more efficient and have a higher impact than open mini-wetlands, where the expected N reduction is 20-30 percent. At the same time, bioreactors require a much smaller area than open mini-wetlands. Preliminary results demonstrate that the area required may be reduced by as much as 75 percent.

Need for further measurings in more seasons

It is necessary to accomplish more measurings – during more seasons – in order to identify the impact of varying precipitation and different temperatures. For instance, we expect the heavy rainfall in February 2020 as well as the relatively high temperatures to have a significant impact on the N reduction.  

Concurrently with monitoring the impact, research efforts are carried out with a view to optimizing the plants and thus reduce any negative side effects, including climate gas emissions.

Project facts