Treatment of Water Supplies by the Technique of Dynamic Aeration

Abstract

The problems of water quality are related to the decrease in the dissolved oxygen content, particularly in the lower layers. These lower layers may deteriorate significantly if the dissolved oxygen consummated in biochemical processes is not renewed by surface aeration or photosynthesis. Thermal stratification of lakes and reservoirs can result in substantial hypolimnetic oxygen depletion, which may have a negative impact on the cold-water fisheries, the drinking- water treatment process, and water quality downstream of hydropower reservoirs. The main purpose of this study is to show the effect of hypolimetic aeration and destratification mode to improve the concentration of oxygen dissolved and level of phosphorus in the depths of the lake. The 10.3 km2 of the water of Hallwil lake are an important tourist center for the canton of Lucerne (Switzerland).Before its restoration in the winter of the year 1985/1986, this lake eutrophication showed significant disruption of aquatic activities, and pollution damage to its various uses, and thus inhibited the development of tourism in the region. Thus, the long periods of stratifications, associated to an important amount of an organic material (principally phosphorus), were originally caused by frequent anoxic phases of the deep layers. This anoxia could be noted beyond 3 m of depth. For this, the Swiss Federal Institute for Aquatic Science and Technology (Eawag) in Switzerland, to address the eutrophication of Hallwil lake, put into service in the winter of 1985/1986, an installation of a aeration system in two alternate modes of aeration namely by a aeration system in winter by destratification mode and aeration hypolimnetic in summer (air / pure oxygen). Through the exhibition of the different results of the chemical parameters (dissolved oxygen, total phosphorus) in the two aeration modes, we were able to ensure an improvement of the lake water quality As a result, the average value of oxygen dissolved and the amount of phosphorus is found in the range of allowed values are: -Winter Season: [O2]min ≥ 8g/m3; [P2]max ≤ 0.15g/m3 - Summer Season: (30 m ≥H ≥15 m): [O2]min ≥ 5g/m3; [P2]max ≤ 0. 13g/m3 Consequently, we can conclude that both techniques aeration method has contributed radically in valuable way to improve the amount of dissolved oxygen and decrease the concentration of phosphorus in the deep layers of the lake.