Lake for bathing and recreation
Kleiner Russweiher

Problem: Sludge formation, silting

Project highlights

SchlixX Plus worked to full effect, including in dystrophic marsh water
28,000 m3 of organic sludge degraded, sediment reduced by an average of 22 cm
Organic sludge converted into sand with minimal organic content (< 1%) 

The starting point

When an old favourite cleans up well

The Kleiner Russweiher, located southeast of Bayreuth in Upper Franconia, is a lake of considerable size, measuring 26 ha in total. Part of a beautiful landscape, it forms the end of a chain of ponds and invites you to wander along its natural banks, swim, fish or even go boating. As well as appealing to the local population as a relaxing spot to visit, the lake also attracts large numbers of tourists every year, who are served by three camp sites and two hotels right on the lake shore. The absolute highlight of the Kleiner Russweiher is located on the southern bank: Here, visitors can enjoy the natural outdoor swimming pool, which is the largest lake-front mud bath spa in Bavaria.

A problem that you encounter nose first

Over the years, the perfect idyll of the Kleiner Russweiher had lost its shine: Visitors to the pool had to wade further and further through foul-smelling sludge before they could swim. The increase in silt was also making fishing difficult. And a swimmer who crossed the lake every day reported shallow spots where he was even able to sit down.  

It became clear to the authorities of the city of Eschenbach, who are responsible for the lake, that a noticeable amount of sludge was forming in the Kleiner Russweiher – and something needed to be done. However, after initial research, it quickly became apparent that the conventional solution for this problem was simply too expensive: Dredging and disposing of around 100,000 m3 of organic sludge would have cost more than the city treasury could afford. And where could so much sludge have been stored before disposal?

The solution

A vital tip for the decision-makers

Thanks to numerous press reports on the conditions at the Kleiner Russweiher, the wider public were now aware of the problem – fortunately. When an Eschenbach resident happened to see a TV report by the Bavarian broadcasting service about how SchlixX Plus had been used to successfully treat the Quellitzsee, he immediately contacted the local authorities:  

Wouldn't this solution also be ideal for the local lake? The Eschenbach authorities acted quickly and invited OASE to a meeting with the people who run the lake-front spa, tenants and city representatives, who were then informed about the procedure. Some attendees were initially sceptical about whether "some kind of powder" could solve the problem, so it was decided that a test run would be carried out after the bathing season ended in September 2016. A prerequisite for the approval of the responsible authorities was neutral monitoring of the test so that results and developments could be understood later.

A difficult start

A boat designed especially for this large area and equipped with GPS equipment spread exactly 8 t of SchlixX Plus over a specified area. However, at the time, none of the parties involved knew that exactly one week later, the Grosser Russweiher, a lake to the north, was to be drained into the Kleiner Russweiher. The effect was substantial: The flushed water swirled up the SchlixX Plus that had been applied – and also transported even more organic sediment into the lake in large quantities. As a result, the first treatment and also the data collected as part of the monitoring process came to nothing.

The successes

A long pause that paid off

In April 2019, test no. 2 was given the go ahead. This time everything went smoothly – with very good results: After six months, the sludge layer had reduced by an average of 22 cm over an area of 16.9 ha. This meant that over this period, 7.5 t of SchlixX Plus had degraded a staggering 28,000 m3 of organic material. And: When analysed, the water showed the same excellent values as before in all parameters. 

A beach with new hidden treasures

In 2020, the city of Eschenbach commissioned OASE to continue their treatment of the lake – this time also in the area of the lake-front spa, where around 9 t of SchlixX Plus were applied over 18 ha. The same method as before was used to check the effect: The distance from the surface of the water to the ground was measured and this was used to calculate the amount of sediment degraded. However, without checking the composition of the sediment. The result: At around 12,300 m3, a considerable amount of organic material was degraded in the bathing area. However, the clients had hoped for more based on the first treatment.  

Nevertheless, SchlixX Plus had actually done an excellent job. This became apparent a short while later when the lake was drained: Sand banks emerged all along the shore area of the outdoor swimming pool, where sludge had been previously. And the sand had only a minimal organic content of less than 1%. This pleased visitors to the pool, but also the authorities: In the event of dredging, the disposal costs for inorganic sand are only a fraction of those for organic sludge.

Why is sand better than sludge? 

On the one hand, sand is more pleasant for bathers to walk on than silt. However, there are other advantages, especially for water maintenance: Unlike organic sludge, it is not mandatory to dispose of sand at a landfill site, so it can easily be removed. And this has a massive impact on disposal costs: Compared to disposing of organic material, which costs about €100/t, the disposal costs for 1 m3 of sand are roughly €3.50 to €4. (To compare the units of measurement: The mass of 1 m3 of water is just under 1 t). So if you want to create more volume in a body of water by dredging the sediment, the resulting costs for disposing of sand are considerably lower.

An exemplary success

Most importantly, however, the people of the region and the numerous visitors are once again able to enjoy a beautiful piece of nature: Fishing enthusiasts are pleased with the number, size and taste of the fish. And in 2020, the lake was also certified for excellent bathing water quality according to strict EU criteria.

Facts and figures

Project nameKleiner Russweiher
Type of waterFreshwater
Type and sizeLake for bathing and recreation, 27 ha
ProblemSludge formation/silting
Product appliedSchlixX Plus
Period and type of application2019: 7.5 t of SchlixX Plus were added to 16.9 ha of the total area
Result of the application28,000 m3 of organic sludge degraded, sediment reduced by an average of 22 cm
Continued application2020: Application repeated, plus treatment of the outdoor swimming area (approx. 18 ha) with a further 9 t of SchlixX Plus
ResultSediment reduced by 7.5 cm; instead of sludge: compact, sandy substrate with <1% organic content

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