Journal of Food, Agriculture & Environment, Vol.7 (2), April 2009 815 Comparative studies of the different mechanical oxygenation systems used in the restoration of lakes and reservoirs Mustapha Gafsi 1 , Ahmed Kettab 2 , Saadia Benmamar 2 and Salim Benziada 2 1 Laboratoire de Recherches de Génie Civil (LRGC), Equipe de Recherches de Ressources Hydriques, Université Ammar Telidji de Laghouat, Algeria. 2 Laboratoire de Recherches en Sciences de l’Eau (LRS-EAU/ENP), Ecole Nationale Polytechnique, El Harrach, Avenue Hassen Badi, Algiers, Algeria. e-mail: benmamar@yahoo.fr, benziada@yahoo.fr Received 2 October 2008, accepted 25 March 2009. Abstract The techniques used for the restoration of lakes or to prevent eutrophication are numerous (chemical, biologic, mechanical, etc). Due to their excessive costs and the relatively insignificant outcomes of some of these techniques, the process of artificial aeration is one of the most promising methods. Four strategic techniques for the control of nutrients are selected for this study: artificial destratification by the bubble plume, partial (or total) lift hypolimnetic aerator, bubble plume oxygenation and Speece cone oxygenation. Each of these methods has both advantages and disadvantages. Technical and economic analyses established by different researchers reveal that hypolimnetic oxygenation is most favorable for nutrient control. In hypolimnetic aeration systems, the bubble plume appears to be most economical and perhaps most simple among the systems proposed for Standley Lake (Colorado, USA), even as other researches selected the Speece cone aeration system in other applications. Based on existing hypolimnetic aeration research, this study provides a synthesis of a number of issues related to the aeration in lakes and reservoirs, including the advantages and disadvantages of these aeration systems. This study also concentrates on the economic and technical aspects associated with these aeration systems. We found that the use of oxygen limits the nitrogen saturation and in contrast with using air. We demonstrate that the most efficient hypolimnetic aeration system is the bubble plume diffuser, although accidental destratification may occur. The destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period. Key words: Aeration, eutrophication, destratification, hypolimnetic aeration, temperature, dissolved oxygen, pollution, thermal stratification, restoration techniques, lake. www.world-food.net Journal of Food, Agriculture & Environment Vol.7 (2) : 815-822. 2009 WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI-00980 Helsinki, Finland e-mail: info@world-food.net Introduction The problems of water quality are related to a decrease in the dissolved oxygen (DO) content, particularly in the lower layers 1 , which may deteriorate significantly if the DO consumed in biochemical processes is not renewed by surface aeration or photosynthesis 2 . The concentration of DO is one measure of the water quality 1 . The amount of DO in water is an indication of the level of microbiological activity, the amount of decaying organic matter present and level of reaeration. In addition, DO is probably the most significant factor relating to the sustainability of fish habitat 3 . The artificial mixing of stratified lakes with aerators prevents thermal stratification from becoming established and increases the DO throughout the water column 1, 4 . It also prevents the surface of rivers or lakes from freezing over 5, 7 , creates barriers against saltwater intrusion into rivers and lakes 5, 8 and retards ice formation in harbors and inland waterways 9 . The artificial mixing helps to produce surface currents to protect harbor areas against high amplitude waves 9, 10 to avoid oil slicks from spreading after oil tanker accidents 5,11 and to bring about an almost complete compensation of the oxygen deficit resulting from metabolic activity 12 . The artificial aeration of oxygen-depleted lake waters is one of many methods used for remediation 13 . It is undoubtedly the most frequently used technique 13, 14 due to its relatively low cost and ease of deployment 13 . There are two main types of artificial aeration of lakes: destratification and hypolimnetic aeration 15 . In the first case, the entire lake is mixed, usually by the release of compressed air from a perforated air line laid along the bottom of the lake. In the second case, the objective is to maintain thermal stratification, while oxygenating the hypolimnion 16 . These restoration techniques can be used separately or in combination 17 . In the case of separately used systems 17 , an artificial mixing of the water column during the cold season and an input of oxygen into the hypolimnion during summer in order to preserve the stratification are assured. Combined systems can be switched between mixing mode using coarse air bubbles and hypolimnetic oxygenation or aeration mode using fine oxygen or air bubbles. Every diffuser is operated using air or oxygen during the summer and air during the winter 18 . Each one of these methods has its advantages and disadvantages.