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Biofilters and mini-wetlands can reduce nitrogen leaching

New research results indicate that constructed wetlands with biofilters can be effective measures to reduce nitrogen emissions from field drains to the aquatic environment. The Ministry of Environment and Food of Denmark is supporting further research in the area.

2017.03.31 | Janne Hansen

The aquatic environment can be protected from nitrogen leaching with the aid of measures such as biofilters. Photo: Janne Hansen

Several measures can be used to reduce nitrogen losses from cultivated fields, for example catch crops. Depending on factors such as soil type, management and climate, nitrogen losses via drainage can also pose a problem. In some situations, measures reducing nitrogen content in drain water need to be considered.

This is where constructed wetlands with biofilters enter the scene. Results from trials with pilot systems show that biofilters can reduce the annual loss of nitrogen to the aquatic environment from drains by 45-60 percent depending on the type of filter. This sounds promising but there are still many questions that need to be answered. The Ministry of Environment and Food of Denmark is therefore funding further research and testing with 31 million DKK.

Test of current and new systems

The aim of the four-year project, which is a collaboration between the Department of Agroecology and the Department of Bioscience at Aarhus University, is to establish and strengthen the knowledge base for evaluation of the nitrogen effect and the cost effectiveness for constructed wetlands with biofilters. 

Part of the project will deal with investigating the long-term effects of the already established constructed wetlands with biofilters. The aim of this is to acquire extensive research-based knowledge about constructed wetlands, including the effect on nitrogen and phosphorous as a function of the primary controlling variables, including hydraulic conditions. The current pilot systems will also be tested for possible changes in efficiency, operation and maintenance and for ways in which to take variations in the Danish landscape into consideration.

Another task in the project will be to establish three new full-scale systems at different locations in Denmark, based on experiences from the current pilot systems. One of the aims is to optimise the systems and their operation and to find out if knowledge about these systems can be generalised for other locations. 

- The establishment of these systems, and possibly a further parallel rollout of systems on a larger scale, will enable us to generate knowledge that, together with maps and other data previously generated by Aarhus University, will provide us with a qualified base for identification of potential new locations and for the dimensioning of constructed wetlands with biofilters. Further, we will obtain valuable data to calculate the costs related to removing one kg of nitrogen under various geographical conditions, explains the project’s coordinator Senior Scientist Finn Plauborg from Department of Agroecology.

The project will result in detailed operational instructions that can be adapted according to precipitation, conditions in the upper soil layer, including soil texture and especially clay content, geological conditions and drainage conditions.   

Using nature’s own processes  

The principle behind the drain filters is that drain water is led through a biofilter consisting of organic material, such as willow chips. In the filter, under anaerobic conditions, bacteria metabolise the nitrate in the drain water into free atmospheric nitrogen. The process is the same as the one that takes place in natural wetlands, the difference being that it is on a much smaller scale in biofilters.

One of the challenges is to find the optimal conditions that can make the process run efficiently. The retention time and temperature are both important factors in this regard. If the drain water runs through the filter too quickly, the nitrogen turnover will not be efficient. On the other hand, there may be unintended negative effects if the retention time is too long.

The unintended negative side effects include the formation of undesired gases. Under conditions where the nitrate turnover exceeds 90 percent, there is a risk that hydrogen sulphide gas will be formed. An unpleasant effect as this gas smells like rotten eggs. Greenhouse gases like methane and nitrous oxide may also be formed, and the latter is a highly potent greenhouse gas, 300 times worse than CO2. It is therefore crucial that the biofilters are constructed in order to optimize nutrient turnover and reduce the unintended negative effects.

- It is important that authorities, agriculture and other players have well-substantiated and thoroughly tested facts that provide a solid basis for choosing the most efficient solutions, says Finn Plauborg.

- We must find feasible, efficient and cheap solutions that can be used in the landscape to remove nitrogen. We are obligated to do so, as society will be spending millions of kroner on these new measures. The solutions concern not only a single field, a single brook, a single wetland or a single forest. They concern the entire agro-ecological landscape.

International meeting about drainage measures

Denmark is not the only country facing challenges regarding nutrients in the aquatic environment. Other countries are also working with measures to reduce emission of nutrients to the aquatic environment.

In order to gain an overview of the international research being carried out in this area, DCA – Danish Centre for Food and Agriculture has organised an international meeting that will be held on April 20, 2017, where experts from countries such as the Netherlands, Czech Republic, Sweden, Germany and USA will give a status of the work being done with regard to various measures.

The target group for the meeting includes experts and advisors from agencies, trade organisations and advisory services.

Please note that the number of available seats are limited; early registration is therefore recommended. Please register by April 12 at the latest.

Participation is free, but AU will charge a "no show fee" if you are registered, but do not attend.

You can see the programme and register here

Prior to the public meeting, the foreign researchers will meet with their Danish colleagues to exchange experiences regarding establishment and operation of various measures. 

For more information please contact

Senior Scientist Finn Plauborg,
Department of Agroecology,
Email: finn.plauborg@agro.au.dk
Telephone: +45 8715 7714
Mobile: +45 2218 1809



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