Lake therapy

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Lake therapy encompasses all measures to improve the ecological condition of lakes .

Sub-areas are

The water maintenance uses lake therapy measures. Within the scope of the preservation of the possible uses, it must observe the aquatic ecologically oriented development in accordance with (§39 (1) Water Management Act ). The times of use and the use of equipment must be coordinated with the requirements of nature conservation law .

Lake remediation

  1. Systems for wastewater treatment, especially sewage treatment plants , but also rainwater clarifiers , etc.
  2. Wastewater collectors and circular sewer systems: Limitation of nutrient imports through permanent or seasonal separation and isolation from the above-ground inflow (" circular sewer system ") and subsequent central wastewater treatment .
  3. Soil filters, reed polders, sedimentation basins: Reduction of the material pollution of the lake by passage through upstream filter systems such as aquatic plants - (macrophyte) rich herbaceous basins or stretches, retention soil filters (see DWA - M 178) / artificial wetlands or reed bed structures (with the effect of a Herbal sewage treatment plant ); or the mechanical sedimentation of particulate bound phosphates and solids in upstream settling tanks .
  4. Surface decoupling and decentralized rainwater management: material and hydraulic relief of the lake by avoiding, reducing or removing runoff-forming surface sealing or by using central or decentralized rainwater management procedures (cf. DWA-M 138 and DWA-M 153).
  5. Adapted agriculture: Reduction of nutrients (in particular phosphate) entry of adjoining agricultural land by minimizing and clarify the fertilizer and manure use , extensive grassland and unused buffer strips .
  6. Bank rehabilitation: Support or improvement of self-cleaning by creating near-natural flat banks with typical vegetation zoning and small-scale interlocking of water and land areas. Reduction of erosive nutrient inputs through permanent green plant cover, hedge and fringe plantings and bank structures parallel to the slope. These measures also improve the habitat function .
  7. improved surface ventilation: keeping fresh air corridors free or clearing closed bank vegetation in order to improve the aeolian oxygen input into the epilimnion (at the same time reduced foliage input).
  8. User and visitor control: Reduction of nutrient and pollutant inputs as well as development of near-natural bank structures and undisturbed lake areas through spatial and temporal control and allocation of leisure uses (e.g. temporary bathing or fishing protection zones). Creation of a needs-based infrastructure for recreational waters (mooring and entry zones; supply / disposal options, sanitary facilities). Reduction of user pressure through environmental education measures (information and awareness-raising) and user agreements (cf.).
  9. Handling chemicals that are hazardous to water: Accident ( average ) prevention (emergency planning, equipment infrastructure, training of emergency personnel and operational services).
  10. Securing and remediation of contaminated sites .

Lake restoration

Nutrient reduction in open water

The open water ( pelagic ) is the water area away from the bank.

  1. Nährstoffinaktivierung by chemical precipitation : reduction of the phosphorus content in the water body and reduction of the phosphate solution back from sediments (P-inactivation) by precipitation by means flokkulierender Al - Fe - or Ca - salts .
  2. External phosphorus elimination: Reduction of the phosphate concentration by removing water, which is cleaned in external P-elimination plants by physico-chemical processes (precipitation, flocculation , flotation , adsorption , filtration ) and returned in a less or not polluted form.
  3. Dilution and flushing: Dilution or removal of nutrients and phytoplankton mass through high water exchange (chemostat principle). At the same time, the water is renewed and, if necessary, destratified .
  4. Weed removal (removal of plants from the water with a mowing boat): The weed removal serves u. a. the targeted extraction of the nutrients bound in the biomass and has a preventive (prophylactic) effect against oxygen deficiency situations that could arise from the decomposition of the dead biomass. The cuttings must be removed from the water body. In larger or deeper lakes, the aquatic plants (macrophytes) are harvested by floating equipment with underwater cutting units and integrated collecting or conveying devices, in smaller lakes it can be done by land-based excavators, rakes or chains. For very small or narrow bodies of water and shallow water, manual weeding with a weed rake is an alternative.
  5. Population regulation of waterfowl : Reduction of nutrient input and excessive bank grazing (with nutrient input through the bird's droppings) by preventing waterfowl feeding (dismantling of known feeding places, raising visitor awareness ; possible administrative fines ), limiting nesting opportunities, deterring or hunting.
  6. Change in lake morphology : punctiform or channel-shaped bottom depressions as small-scale sedimentation traps, which also reduce the risk of extensive phosphorus redissolution or to intensify self-cleaning (bank extensions and flattening; generally also to improve the habitat function , see bank rehabilitation ).

Prevention of nutrient redissolution from lake sediments

The sedimentation of a year in a lake is called lake sediment ( warve ) .

  1. Sediment conditioning by nitrate : Prevention of phosphate release from lake sediments by nitrate-induced (Ca (NO 3 ) 2 ) increase in the redox potential at the water-sediment contact zone ( nitrate breathing ). At the same time, the nitrate, as an anaerobic electron acceptor, promotes the mineralization of organic substances with a reducing effect (biotechnical desludging). Sediment conditioning using nitrate in combination with phosphate precipitation (see above) is known as the " RIPLOX process ".
  2. Sediment conditioning through liming: Reduction of the sludge volume by adding CaCO 3 , CaO or Ca (OH) 2 to temporarily drained ("summered") pond sediment. By increasing the pH, the microbial cellulose breakdown of deciduous sediments is promoted (biotechnical desludging).
  3. Desludging with sediment removal: the mechanical removal of the uppermost, nutrient-rich sediment layer serves to improve the water quality and / or maintain an appropriate water depth and usability. The desludging can also serve to shape the water morphology (change in lake morphology, see above). The machines used are bulldozers, tracked or floating dredgers, and caterpillars or excavators if the bottom is stable. The gentlest method is targeted suction with suction dredgers (deepening, see below).
  4. Desludging without sediment removal: Promotion of aerobic sludge degradation in situ by aerator devices or bio-activation by " effective microorganisms " (biotechnical desludging). The sediment removal can be omitted or postponed.
  5. Sediment aeration: Oxygen enrichment of the hypolimnion and the contact zone to the sediment by compressed air or gaseous oxygen, which reduces oxidizing Fe 2+ phosphate release from the sediment. At the same time, anoxic processes on the river bed are prevented.
  6. Sediment cover : Reduction of the substance diffusion from the sediment into the water body through artificial barriers (plastic sheeting, sea ​​chalk , fine particulate clays, zeolites ), which also prevents excessive macrophyte colonization of shallow lakes.

Reduction of oxygen deficits

  1. Aeration of the epilimnion : An immediate measure to combat acute oxygen deficiency in the surface water by blowing in air using fountains, compressor-operated aerator pipes, compact injector aerators, rotary aerators, turbines and much more. With continuous operation, aerobic material conversions stabilize and increase in the water body, which contributes to sludge removal without sedimentation.
  2. Aeration Hypolimnion: In deep water aeration , in stably stratified (stratified) lakes, oxygen-poor deep water is conveyed to the surface by means of mixed air lifters, from where it is returned to the hypolimnion enriched with oxygen . In this way, the hypolimnion and the contact zone with the sediment are ventilated in order to prevent persistent O 2 deficits. This reduces the risk of phosphate mobilization and prevents anoxic processes on the water floor.
  3. Forced circulation / artificial destratification: The entry of compressed air at the bottom of stratified, polytrophic lakes prevents stratification in spring (“forced circulation”) and thus lowers the risk of eutrophication spurts . At the same time, the oxygen balance of the water is generally improved.
  4. Deep water discharge: Discharge of nutrient- or pollutant-containing, oxygen-deficient deep water according to the siphon principle or through a gradient pressure pipe (" Olszewski pipe "), which prevents possible pollution of the epilimnion.
  5. Supply of calcium peroxide as powder or granules. 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.

Control of excessive phytomass (macrophytes, algae)

  1. Weed removal: In lake therapy, the mechanical removal of emersed and submerged macrophytes is used to avoid nocturnal or seasonal (seasonal) oxygen deficits , accelerated sediment formation or against odor nuisance (cf. nutrient withdrawal , see above). However, their importance in water maintenance for the maintenance or restoration of water uses (e.g. water management , boat traffic, sport, fishing, ...) is dominant .
  2. Water level regulation: aquatic plants and algae are combated by prolonged subsidence (drying out) of the lake (possibly in combination with desludging or sediment coverings, see above); Macrophyte locations can alternatively be "starved" by raising the water level (or bottom depressions , see above) (deteriorated light climate).
  3. Biomanipulation : Mass populations of submerged macrophytes and algae can be directly reduced by stocking them with herbivorous fish species (e.g. rudd , roach ). The selective limitation of the coarse fish community (e.g. through fishing , stocking with predatory fish ) can reduce the feeding pressure v. a. lower to planktivorous daphnia , which indirectly decimates phytoplankton (food web control).
  4. Shading / clouding: In the littoral of lakes, shady trees on the south and east banks can limit the phytomass of macrophytes and algae . In the pelagic area , the artificial clouding of the water body with clays or coloring chemicals ("Aquashade ") can contain plankton algae, macrophytes and cyanobacteria ("blue algae") that require light .
  5. Copper sulphate : Fighting algae by applying algicidal CuSO 4 x 5 H 2 O (due to the heavy metal accumulation in sediment and / or food webs , this method is unacceptable in the field).
  6. further processes: algae or seston repellent ; cyanophage viruses, growth inhibitors ("phytotranquilizers"); s. a. Dilution, forced circulation and sediment cover.

Suitability of sea restoration measures

Suitability of sea restoration measures
Lake surface (km 2 ) > 1 > 1 > 1 > 1 <1 <1 <1 <1
Max. Depth (m) > 5 > 5 <5 <5 > 5 > 5 <5 <5
theoret. Water renewal (in years) > 5 <5 > 5 <5 > 5 <5 > 5 <5
Nutrient inactivation + + + + + + + (?)
dilution + + + + + + + +
Sediment conditioning + +
Desludging + + + +
Sediment cover (?) (?) + + (?) (?)
Ventilation hypolimnion + + - - + + - -
artificial Destratification + + - - + + - -
Deep water drainage (?) + - - (?) + - -
Weeding (?) (?) (?) (?) + +
Shading + +
(+: suitable; - unsuitable;?: questionable success)

The specific causes of pollution in lakes are material, morphological and management deficits, which often occur in parallel. Only through detailed, repeated analysis of the causes and taking into account the seasonal dynamics of the lake ecosystem , the choice of ecologically and economically suitable methods can be made. The suitability of the lake therapy procedures depends on the causes of pollution, the current or intended use (“model”) and the morphometry and hydrometry of the lake (surface, maximum depth and stratification, water renewal) (according to B. Scharf et al., 1992).

Often only combinations of measures from renovation and restoration are promising. In principle, however, the principle of restoration before restoration applies to the selection of measures - also with regard to the polluter pays principle : restorative measures are only effective in the long term after extensive restoration of the catchment area and sufficient reaction time of the water body.

See also

further reading

Further information on the basics, requirements and procedures of lake therapy can be found in the DWA-M 606 and DWA-M 610 rules in addition to the DWA-M 612. Possibilities for the nature-friendly recreational use of water bodies are shown in DWA-A 618 (E). The DWA-M 616 (E) provides information on aspects of the duty to ensure traffic safety in water maintenance. Practical examples of lake therapy can be found u. a. in the "Guideline for planning measures for surface waters - Part B: Still waters".

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Laws and Regulations

  • Law on the organization of the water supply (Water Management Act - WHG) of July 31, 2009 ( Federal Law Gazette I p. 2585 ).
  • Federal Soil Protection and Contaminated Sites Ordinance (Federal Soil Protection Ordinance - BBodSchV) of July 12, 1999 ( BGBl. I p. 1554 ).

Rules and standards

  • Abwassertechnische Vereinigung eV (ATV) 2005: Worksheet ATV-A 138 Planning, construction and operation of systems for the infiltration of rainwater. Hennef.
  • Abwassertechnische Vereinigung eV (ATV) 2007: Leaflet DWA-M 153 Recommendations for dealing with rainwater. Hennef.
  • German Association for Water Management and Cultivation (DVWK) 1999: Information sheet DVWK-M 250 Natural development of lakes and their surroundings.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2005: Leaflet DWA-M 178: Recommendations for the planning, construction and operation of retention soil filters for further rainwater treatment in the mixed and separated system. Hennef.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2006: Leaflet DWA-M 606 Basics and measures of lake therapy. Hennef.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2007: Leaflet DWA-M 603 Leisure and recreation on flowing waters. Hennef.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2010: Leaflet DWA-M 610 New ways of water maintenance - maintenance and development of rivers. Hennef.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2012: Leaflet DWA-M 612 Boundary strips - Part 1: Basics and functions, information on design Hennef.
  • German Association for Water Management, Wastewater and Waste eV (DWA) 2012: Leaflet DWA-M 616 (E): Obligation to maintain safety in the expansion and maintenance of rivers - recommendations for handling Hennef.
  • German Association for Water Management, Sewage and Waste eV (DWA) 2013: Worksheet DWA-A 618 (E): Recreation and recreational use at lakes - requirements, planning, design. Hennef.

Guides and monographs

  • Federal Agency for Nature Conservation (BFN): Voluntary agreements between nature conservation and nature sports. A guide. (= bfn scripts. 106). Bonn 2004, DNB 970705026 .
  • M. Dokulil, A. Hamm, J.-G. Kohl (ed.): Ecology and protection of lakes. Vienna 2001, ISBN 3-85076-438-9 .
  • Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation [NLWKN] (Ed.): Water Framework Directive. Volume 3: Guideline for planning measures for surface waters. Part B: Still waters: Recommendations on the selection, setting of priorities and implementation of measures for the development of the great lakes in Lower Saxony. Hanover 2010, DNB 1069363081 .
  • B. Scharf, A. Hamm, C. Steinberg: Lake restoration. In: WK Besch et al: Limnology for Practice. Basics of water protection. 3. Edition. Ecomed, Landsberg / Lech 1992, ISBN 3-609-65630-1 .

Individual evidence

  1. ↑ German Association for Water Management, Sewage and Waste e. V .: Basics and measures of lake therapy
  2. ^ Leibniz Institute for Freshwater Ecology and Inland Fisheries: Workshop Lake Therapy; Experiences and new challenges. ( Memento from March 6, 2014 in the Internet Archive )
  3. ↑ Federal Agency for Nature Conservation (BFN): Voluntary Agreements Nature Conservation - Nature Sports - A Guide. (= bfn scripts. 106). Bonn 2004. (bfn.de)
  4. ↑ Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation [NLWKN] (Ed.) 2010: Guideline for planning measures for surface waters B: Still waters. download