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Saving fens with waste water

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​​Intact fens are usually very old and several meters thick. They provide habitats for rare animal and plant species and store enormous amounts of water and carbon in their peat layer. However the overwhelming majority of fens in Germany have dried up and can no longer fulfil these functions. Scientists have now tested the rewetting of a lowland fen in the Uckermark region using an unusual resource: purified waste water from sewage treatment plants.

All around grassland and fields, as far as the eye can see, with some small villages and forests in between: from a bird's-eye view, in the middle of the landscape, a rectangular area covered with reeds becomes visible. Ditches and two small ponds border the area which covers around ten hectares. It is the remainder of a once intact lowland fen in Brandenburg. At some point, drainage ditches were dug around it in order to lower the groundwater level and cultivate the surrounding areas. The water was drained out of the lowland fen. It has been drying out for decades. In the 1990s, the area attracted the attention of researchers and a test area was established – also to test the subsequent use of such dehydrated fens. 

In 2011, the federally funded "ELaN" project was launched, through which researchers from the Leibniz Centre for Agricultural Landscape Research (ZALF) saw an opportunity to revive the fen with an unusual idea: Purified waste water from a nearby sewage treatment plant was to be used to return the necessary moisture to the habitat and stabilize the hydrological balance.

 

Such a waste of waste water

"Water from sewage treatment plants is usually channelled into ditches or rivers in order to get it out of the landscape as quickly as possible. This is to prevent it from seeping into the soil and contaminating the groundwater", says Dr. Sebastian Maassen from ZALF. Because when it comes from the sewage treatment plant, it still contains numerous residual pollutants and nutrients, which now flow through the rivers into the seas. Instead, this water could be used more intelligently. "You could reach two goals at once: irrigate the dehydrated fens and at the same time clean the waste water more thoroughly." This also applies to the waste water from the nearby sewage treatment plant in Passow. Analyses have identified at least 67 micro-pollutants – primarily residues from medicines or personal care products. "The trace elements tell us a lot about the social structure of the region", says Maassen. In Berlin, for example, there are more residues of birth control pills, whereas here in the Uckermark there are more painkillers and sweeteners. The researchers also found corrosive agents or x-ray contrast media in the waste water in Passow. None of these substances should be allowed to find their way into groundwater. Now it was time to find out whether the fen prevented this from happening. 

The lowland fen in the Uckermark was an ideal experimental area. "The dried out fen may no longer be intact, but it still contains a lot of organic matter," explains Maassen. This creates favorable conditions for the decomposition of a great many residues. Another factor is that the water can remain there for a long time before it reaches the surrounding ditches and the groundwater. In addition, many micro-organisms live in the fen which also remove substances from the waste water. For the pilot project, Maassen and his team diluted the waste water from the Passow sewage treatment plant with ten times the amount of surface water in order to control the rewetting of the fen over a three-year period under strict conditions. The constituent substances in the waste water, soil and groundwater of the experimental area were constantly monitored and analysed. 

The results speak for themselves: "In the waste water from our sewage treatment plant, we were able to detect numerous undesirable substances that had subsequently disappeared in the groundwater of our test site, i.e. after the 'cleansing' by the fen", said Maassen. The fen apparently "swallowed and digested" the substances. It is not yet possible to describe exactly which processes are responsible for this. Micro-organisms, plants or sunlight – the influences of the individual decomposition processes are manifold. However, some residues are also bound into the peat or fen soil. So-called toxicity tests with plants, bacteria, annelids and other organisms gave cause for optimism: They did not reveal any negative effects of the treated waste water on soil life.

 

On dry land

From an ecological viewpoint, there seems to be no reason not to rewet fens using purified waste water from sewage treatment plants. Further research is required however. But since 2015, the pilot site has been left high and dry again, because of a shortage of financing for the pumps. There is hope, however: "The Michael Succow Foundation would like to test the cultivation of renewable raw materials", says Maassen. For if intact fenland was once considered economically unviable due to the high groundwater levels, ideas for sustainable use certainly exist today. Reeds can be grown here for example, which can be used as biomass for energy production. Water buffaloes find excellent conditions on the sites, as do black alder trees, which provide valuable wood. 

"The topic will become increasingly important in the future", of this Maassen is certain. Because in many areas throughout Germany – especially in the northeast – water is already a scarce resource. Predominantly sandy soils allow it to seep away quickly and land management has so far paid little attention to the storage of water resources. Climate change will further intensify the problem. The establishment of closed water cycles could help to preserve natural water reservoirs and valuable habitats such as the fen in the Uckermark.

 

Text: Heike Kampe

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