Lake rehabilitation as a sub-area of lake therapy includes measures in the catchment area that serve to improve the condition of lakes and other still waters . Measures within lakes serve to restore the lake . They are used, for example, to prevent lakes from tipping over .
Lakes to be rehabilitated are usually eutrophic or even hypertrophic , i. that is, they contain too many nutrients, especially too much phosphate . As a result, too much biomass forms in the trophogenic layer, and oxygen is consumed when it is broken down in the tropholytic layer . This can lead to the lake becoming anaerobic (oxygen-free) at depth . As a result, aerobic organisms die, i.e. everyone who needs oxygen. The anaerobic degradation products, which are usually toxic, such as B. hydrogen sulfide and ammonia . Digested sludge forms at the bottom of the lake. Furthermore, phosphate accumulates there without being able to form an insoluble compound with trivalent iron. As a result, its fertilizer power is retained even after full circulation - the lake is considered to have overturned .
Possible remedial measures
Reducing the entry of nutrients
First of all, you can reduce the nutrient input through the inflows, for example by building sewage treatment plants or at least septic tanks or ring pipes. If the input comes from diffuse sources, more far-reaching measures must be taken. In Switzerland, during the rehabilitation of the three Mittelland lakes ( Baldeggersee , Sempachersee , Hallwilersee ), the pastures that stretch right up to the shore were transferred to fallow land or very extensive pastureland and the number of livestock units was noticeably reduced. In the case of smaller lakes, more like ponds surrounded by deciduous trees, trees have to be felled in order to reduce the entry of leaves.
The entry of nutrients from bird droppings can be reduced if a feeding ban for waterfowl is decreed (and monitored), as the feeding (and spreading of feed) attracts birds, prevents them from moving on or improves their chances of survival in the cold season and thus disturbs the natural balance . Excessive fish stocking and feeding the fish can also lead to nutrient inputs.
Sometimes there are fore lakes in nature. It is a small basin that receives the incoming water and should stay there for several days. Solids are deposited there that do not pollute the lake. This concept is used in the lake renovation and artificial fore lakes are constructed. Such a lake consists of an area that is separated from the main lake by foil and where water only enters the main lake from the upper layer. Since material is deposited, the Vorsee has to be dredged regularly. The dredged material often has to be disposed of as hazardous waste. One example of such a renovation is the airport lake in Berlin .
To prevent the lake from tipping over, it is advisable to add oxygen to the water. This can be done in a number of ways.
Air is supplied to the surface layer. This can be done by one or more fountains, or by an artificial waterfall. This measure is often used in the area of urban ponds, if there are also aesthetic requirements; From a water-ecological point of view it has many disadvantages.
- Sometimes the aesthetic considerations prevail and the fountain or waterfall is turned off at night. However, the need for oxygen is highest at night, because then the algae cannot photosynthesize and use oxygen. The lowest oxygen concentration in the water is measured at sunrise.
- When the water hits it, small algae and unicellular organisms are smashed. They die and the biomass also "fertilizes" the lake.
- The constant movement causes the water to circulate, which can prevent a thermocline from forming. This means that no nutrients can be fixed at the bottom of the lake. Everything is constantly being whirled around
Oxygen or air is pumped into the lake using hoses and introduced there through nozzles. When air is introduced, the oxygen is dissolved in the water, but the remaining air components are not completely dissolved. The rising gas bubbles lead to a mixing of the water and can result in a disruption of the thermocline (with constant and uninterrupted ventilation it is even quite safe). New systems aim to supply the water with the necessary air at staggered intervals so that, in the best case scenario, the thermocline can be fully preserved and the oxygen content in the entire water increases significantly. Ideally, however, pure oxygen ventilation is preferable. Only pure oxygen is atomized. You can calculate how much can be brought in so that it is completely used up by the time the thermocline is reached. Then the stratification is not disturbed. In both cases the lake receives oxygen in the depths, with the latter only oxygen aeration in the depths. The method of ventilating deep down to the thermocline with pure oxygen is much more expensive than ventilation with oxygen obtained from the ambient air.
Ventilation of the lake bed
Oxygen can be added to the bottom sludge by adding calcium peroxide as powder or granulate. Calcium peroxide deposited on the pond floor and then stored in the pond sludge slowly decomposes, splitting off oxygen, which is then available to sludge-degrading microorganisms. At the same time, the phosphates released during the sludge degradation process are bound as calcium phosphate (see also phosphate elimination ).
If the lake is shallow and / or a lot of mud has settled on the bottom of the lake, it can make sense to dredge the lake. This reduces the nutrient content on the one hand, if z. B. decomposing plant remains are removed; on the other hand, more oxygen can dissolve in the colder, deeper water layer. Dammed ponds are often completely drained for dredging.
If a lake has a surface drainage, an Olszewski pipe can also be installed. It is a withdrawal of the nutrient-rich deep water without additional energy expenditure. The natural flow is blocked. In its place a long tube will be laid down to the deepest point of the lake. According to the principle of communicating pipes, the deep water now rises through the pipe to the drainage point and leaves the lake instead of surface water.
The effectiveness of this procedure depends on the amount of drainage. It is based on the fact that during the stagnation phases there is a notable shift in the phosphorus content from the epilimnion to the deeper layers due to the sedimentation of biomass and detritus .
- Leaflet DWA M-606, German Association for Water Management, Sewage and Waste eV, ISBN 978-3-939057-61-1 , 2006
- Table of contents - Leaflet DWA M-606, German Association for Water Management, Wastewater and Waste eV , ISBN 978-3-939057-61-1 , 2006, download, accessed on April 27, 2017