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Nr. 99, 06.07.2019  voriger  Übersicht  weiter

Water protection with grassland

Substances such as nitrogen or phosphorus are nutrients when they are absorbed by plants, but pollutants when they are released into groundwater. Under scientific observation, grasslands are being used as natural filters at a biogas plant near Strande.

Hernan Abad Ortega, Thomas Zakel, Joachim Schrautzer, Kirsten Rücker, Peter Janson (from left to right). Credit: Geist

Silage and animal waste are used in a considerable number of biogas plants that have been built in Schleswig-Holstein since the energy transition. These materials are stored in the immediate vicinity of the eco-reactors, and this is not without risks, no matter how carefully they are managed. Especially during heavy rainfall, surface water accumulates, which is heavily contaminated and much too rich in nutrients to be released into nature.

This is precisely the problem that the biogas plant at Gut Eckhof near Strande faces. The power plant is owned by five agricultural enterprises, which together cultivate 1,200 hectares of land and maintain a common machine park. Collectively, these farms had problems maintaining a crop rotation system that also makes economic sense.

"It was becoming increasingly difficult," says Count Nicolaus zu Reventlow, Managing Director of the biogas plant, which went into operation in 2011. The idea of cultivating maize in addition to wheat and rapeseed in order to generate energy was a logical next step. This is now being done on about 250 hectares, or about 20 per cent of the total area of the farms, resulting in an equilibrium that is reasonable for both the soil and the business side of things.

Things are also looking good with regard to the wastewater problem. The basic idea is simple: transporting the wastewater to fields or meadows is time and money consuming - and is not without risk for the environment because of soil compaction and the possible seepage of nutrients. Distributing the water to fields of grass via a sprinkler system so that the plants absorb it completely, on the other hand, is an elegant solution in every respect as it even produces a side benefit in the form of fuel for the wood-chip heating system at the neighbouring farm.

But does this system really reliably protect the groundwater from nutrient discharge? This is of particular interest to officials at the State Agency for Agriculture, Environment and Rural Areas (LLUR). The authority commissioned the Institute for Ecosystem Research at Kiel University with an accompanying study. A major aim was to find out whether the system could be used as a model. After all, there are about 600 biogas plants in the federal state, explains Peter Janson, head of the Technical Water Protection Department at LLUR. He adds that the disposal of surface water does not run smoothly everywhere.

Under the direction of Professor Joachim Schrautzer, Dr Kirsten Rücker and Thomas Zakel from Kiel University are now investigating the Strande model for the second year. The surface water is first discharged into a lagoon. As soon as it arrives in this catchment basin, the scientific staff begins its work. "We are interested in the complete material cycles," explains Kirsten Rücker. What is particularly important is whether nitrogen or phosphorus are present in the fields and, if so, at what depths, and what the chemical oxygen situation is, from which conclusions can be drawn about the quality of the water.

The measurement technology was completely installed in the summer of 2018. The results were "very good", says Dr Rücker. At least 90 and in some cases up to 98 per cent of the nutrients and organic impurities were absorbed by the plants. However, at a depth of 80 centimetres, i.e. where it becomes relevant for a possible groundwater contamination, nothing was found that would go beyond the naturally occurring nutrients. Since the young grasses grow better roots in the soil over time, Professor Schrautzer expects future results to be even more positive. However, it is likely to be some time before universally valid statements can be made.

The summer of 2017 was far too wet and the following year far too dry, making at least one normal summer necessary to complete the picture. Considering the importance of the teaching aspect, Professor Schrautzer would also welcome an extension of the accompanying study beyond the planned period, at least until 2020. Numerous Bachelor's and Master's theses have already been written about Strande's biogas. The project also offers students the opportunity to gain valuable practical experience.

Martin Geist
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