The considerable development of agricultural and industrial activity, has led to the emergence of new classes of organic pollutants say "persistent" which are resistant to the processes of conventional treatment of the waters. The photocatalysis represents of our days, emerging solution to the problems of pollution of groundwater environments, because that can degrade organic matter in basic products and less toxic. This technique associated with a biological method "bio-monitoring" allows to

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... rify the waters effectively, simple and less costly. The use of photocatalytic process in the presence of photocatalyst SnO2 and Lemna minor under UV light has revealed a decrease in the absorbance by report to the initial state and that the rate of absorption varies from 7% to 33% for wastewater and 7% to 34% for the waters of the dam Dahmouni. The treatment of the waters by photocatalysis associated with the lenses of water could be a reality in the countries with strong sunlight to here to a dozen years. This technique is interesting both from the point of view of technology that from the environmental point of view. In effect, this technique may be extended to large scale for addressing releases industrial or domestic or for the purification of contaminated waters.Two water resourses including the dam Dahmounie and wastewater treatment plant in the city of Tiaret are in in the focus of this study.