Deep water aeration

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With deep water aeration or hypolimnic aeration , the oxygen demand in the deep water is covered by the air supply from the atmosphere without destroying the natural stratification of the lake. The deep water remains aerobic , the redissolution of phosphates is significantly reduced and the mineralization of the sediments is improved. Long-term scientific findings show that it is possible to keep the deep water aerobic all year round by means of technical ventilation measures and thereby restore the balance of the lakes as effectively as possible.

Problem of eutrophication

During summer stagnation , eutrophic , dimictic lakes develop an oxygen deficit in the deep zones. As a result of high nutrient inputs , the trophy of the lakes increases. The consequences are increased phosphorus concentrations, increased algae growth and a correspondingly increased oxygen consumption in the deep zones. As a result, the oxygen stored here can be completely consumed in extreme cases and the deep water becomes anaerobic . Under anaerobic conditions, new bacterial populations appear, which continue the decomposition process in the sediment. Digested sludge is produced in an anaerobic environment while ammonium , iron and manganese as well as toxic hydrogen sulfide accumulate in the water body. The hypolimnion is now not only hostile to life, but also rising phosphate redissolutions from the sediments into the deep water occur in this milieu, which increasingly pollute the water system after the next autumn full circulation through increased phosphorus concentrations. Especially with regard to the drinking water production in reservoirs and the applicable limit values ​​of the Drinking Water Ordinance , this deterioration in the water status is a serious problem. Deep water aeration can prevent this process.

Technical possibilities of deep water aeration

Deep water aeration systems (TIBEAN or TWBA for short) consist of one or more riser pipes in which the deep water is aerated and rises, a degassing head in which the aerated water is freed of gases and one or more downpipes in which the aerated, degassed water is returned to the hypolimnion. In addition, nutrient adsorption and / or nutrient precipitation stages can be implemented in the degassing head. TIBEAN can be designed as both floating and submerged systems.

TIBEAN: structure, function and system components

technology

Atmospheric air is sucked in by means of highly efficient water jet air ejectors and injected into the riser pipe in fine bubbles. There, the mixture of air bubbles and water moves upwards and draws deep water through the suction device. Thus, the deep water drainage takes place in a targeted manner from the water lamella near the ground. At the end of the riser pipe, the mixture flows into the degassing head. There the oxygen-enriched water is separated from the residual gas. The residual gas escapes into the atmosphere. The oxygen-enriched water flows down through the downpipe and laminar in a horizontal direction via the inflow device into the hypolimnion. The optimal configuration can be determined by means of flow and mass transport calculations, which are carried out as part of the design.

Plant components

  1. Float
  2. Riser pipe (telescope)
  3. Degassing head
  4. Static mixer
  5. Suction opening
  6. Cover grille
  7. Downpipe
  8. Oxygen entry
  9. Submersible motor pump with ejector
  10. Blow tanks

material

TIBEAN can basically be made of polyethylene (PE), polypropylene (PP), stainless steel or an Al / Mn alloy. Due to the best properties, however, the systems are almost exclusively made of thermoplastics (PE, PP) and are therefore UV, weather and frost resistant.

Possible uses

TIBEAN are highly variable and cover a very wide range of applications with an oxygen input of 1.5 to 60 kg / h, an operating depth of 5 to 50 m and a volume throughput of 600 to 7,500 m 3 / h. The target formulations for water therapy can vary depending on the priority. The possible uses of deep water aeration systems are accordingly diverse:

  • Preservation of the deep zone as an aerobic habitat for fish and other higher organisms.
  • Reduction of the nutrient supply in the water.
  • Prevention of digested sludge formation, increased ammonium production and the formation of toxic hydrogen sulfide.
  • Reducing the cost of drinking water production.
  • Targeted treatment of the deep water with precipitants.

Drinking water production in dams

Particularly with regard to the production of drinking water in reservoirs, deep water aeration enables a significant reduction in costs and a simplified technical treatment of the deep water. Since the deep water below the thermocline is usually withdrawn to produce drinking water in dams, the improved water quality of the hypolimnion has a direct influence on drinking water production. With regard to the applicable limit values ​​of the Drinking Water Ordinance, the following relationships arise:

pH and corrosion

The limit value for the pH value of drinking water is 6.5 to 9.5. pH values ​​that deviate from the neutral range (pH 6.5 to 7.5) generally pose a problem, as they give an indication of the corrosion behavior of the water. Slightly acidic water (pH 4 to 6.5) attacks mostly galvanized iron pipes, but also copper and asbestos-cement pipes. This is referred to as acid corrosion . Practical experience has shown that the use of unprotected, galvanized steel pipelines is only possible with a neutral pH value of around 7.3. The erosion of the pure zinc layer is promoted by lower pH values. Natural cold water usually shows a slightly alkaline reaction due to the dissolved salts and gases. This is done by setting equilibrium concentrations of the dissolved carbon dioxide in the form of hydrogen carbonate ions and carbonate ions. Higher alkaline pH values ​​(pH 9 to 14) lead to so-called oxygen corrosion in the presence of oxygen as an oxidizing agent . To avoid the acid or oxygen corrosion described, buffer solutions are added to the raw water during the drinking water production . Thanks to the pH stabilizing effect of deep water aeration, the use of these buffer solutions can be reduced, thus lowering operating costs.

Iron and manganese

The limit values ​​200 µg / l and 50 µg / l respectively apply to the concentrations of iron and manganese in drinking water. Although they are absolutely desirable as essential trace elements in drinking water, even slightly increased iron or manganese concentrations are seen as disturbing from a technical and hygienic point of view. Iron and manganese are dissolved as ions in low-oxygen water. The naturally present iron and manganese are mainly present as divalent, soluble iron (II) or manganese (II) compounds. At very high concentrations, the water becomes yellow in color. If oxygen is added to this water, it is oxidized to trivalent iron or manganese, with iron forming red-brown precipitates and manganese black. These precipitates cause discoloration and cloudiness of the water and lead to the discoloration of laundry known as rust stains. The precipitation can also lead to constriction of pipes and deposits on fittings. Iron contents above 0.3 mg / l and manganese contents above 0.5 mg / l are noticeable as an unpleasant metallic taste. The deep water aeration ensures an aerobic environment for the hypolimnion and thus ensures an oxidation and precipitation of the dissolved iron and manganese compounds before the water is processed into drinking water in the corresponding systems. In this way, further operating costs for the removal of the dissolved iron and manganese compounds can be saved. The amount and mobility of iron is also important for influencing the redox-controlled phosphorus balance. The divalent iron that diffuses from anaerobic sediment layers is oxidized at the boundary zone between anaerobic sediment and aerobic water and can therefore accumulate in the uppermost sediment layer. The greater this enrichment, the more effectively the aerobic boundary between sediment and water can act as a diffusion barrier for phosphate.

Nutrient concentrations and digested sludge formation

As already mentioned, deep water aeration also serves to a large extent to reduce nutrient concentrations. Aerobic conditions promote nitrification and the subsequent denitrification, which contributes to the nitrogen relief of the system. The chemical and microbial oxidation of reduced substances such as hydrogen sulfide and methane can intensify the degradation activity of organic matter and thus reduce the formation of digested sludge . Aerobic conditions in deep water are also an important prerequisite for reducing the redox-controlled re-dissolution of phosphorus from the sediment and / or allowing released phosphorus to re-precipitate. In this way, additional costs in drinking water production can be saved through deep water aeration, for example by dispensing with a denitrification stage or a lower need for flocculants .

Planning and design

A TIBEAN is designed in different phases. The first step should always be a morphometric measurement of the water in order to be able to assess the water subsoil and the associated requirements for the technical design and later to determine the optimal location of the system. For an exact technical design it is necessary to evaluate various series of measurements of parameters such as nutrient concentrations, temperature stratification, pH value and temporal oxygen curve in order to be able to calculate flow velocities, mass transport quantities and suspended matter distribution in the hypolymnion.

Examples of successful deep water aeration

"Schönbrunn" type deep water aeration system at the Bleilochtalsperre, 1978

Individual evidence

  1. ↑ a b c d e f g h i C. Steinberg, H. Bernhardt: Handbook of Applied Limnology. 14th result. 4/0. Verlag Hüthig Jehle Rehm, 2002, ISBN 3-609-75820-1 .
  2. ^ D. Jaeger: TIBEAN - a new hypolimnetic water aeration plant. In: Verb. Internat. Society. Limnol. 24, 1990, pp. 184-187.
  3. ↑ H. Klapper: Eutrophication and water protection. Gustav Fischer, Stuttgart / Jena 1992, ISBN 3-334-00394-9 .
  4. ↑ JL Doke, WH Funk, STJ Juul, BC Moore: Habitat availability and benthic invertebrate population changes following alum treatment and hypolimnetic oxygenation in Newman Lake , Washington. In: J. Freshwat. Ecol. 10, 1995, pp. 87-100.
  5. ↑ B. Wehrli, A. Wüest: Ten years of lake ventilation: experiences and options. EAWAG, Dübenedorf-Zürich, Switzerland, 1996, ISBN 3-906484-14-9 .
  6. ↑ a b Annex 3 of the Drinking Water Ordinance
  7. ↑ The importance of individual drinking water parameters, water association for the greater Ansfelden area, August 29, 2003, http://wasserverbandansfelden.riscompany.net/medien/download/50330502_1.pdf
  8. ↑ Water quality: special part corrosion, www.waterquality.de, know-how online, http://www.waterquality.de/trinkwasser/K.HTM
  9. ↑ The importance of individual drinking water parameters, water association for the greater Ansfelden area, August 29, 2003, http://wasserverbandansfelden.riscompany.net/medien/download/50330502_1.pdf
  10. ↑ The importance of individual drinking water parameters, water association for the greater Ansfelden area, August 29, 2003, http://wasserverbandansfelden.riscompany.net/medien/download/50330502_1.pdf
  11. ↑ DRS Lean, DJ McQueen, VR Story: Phosphate transport during hypolimnetic aeration. In: Arch. Hydrobiol. Volume 108, 1986, pp. 269-280.
  12. ↑ H. Klapper: Eutrophication and water protection. Gustav Fischer, Stuttgart / Jena 1992, ISBN 3-334-00394-9 .