Traitement des eaux de pluie par photocatalyse solaire

Bien que l’usage de l’eau pluviale en France se limite essentiellement à l’arrosage et au nettoyage, ses propriétés physico-chimiques intéressantes devraient lui permettre de trouver d’autres applications. Toutefois, il est important de connaître la qualité de l’eau de pluie et les risques de contamination chimique et bactériologique afin d’adapter un traitement adéquat pour une utilisation optimale. Parmi les nombreuses techniques de traitement existantes, une technique innovante peut être utilisée : la photocatalyse solaire. L’étude proposée a pour but d’établir la qualité des eaux de pluie récupérées avant et après traitement.

Abstract

Although the uses of rainwater in France are restricted to watering and cleaning, others needs could be satisfied considering its physicochemical properties. However, it is important to know its quality and the risks of chemical and bacteriological contamination in order to adapt the appropriate treatment for an optimal use of rainwater. Among the many techniques available for water treatment, an innovative one should be mentioned, solar photocatalysis. The following study aims to evaluate the quality of rainwater recovered after streaming on roofs before and after this treatment. Physicochemical and bacteriological composition of rainwater is very changing : it depends on many parameters like the environment of the site, the weather conditions, the type of roof, and the human and animal activities. Thus, many organic compounds (pesticides, PAH’s, haloacetic acids ...) and minerals (metals, various anions and cations) can be found in rainwater, often accompanied by microorganisms. Among the activity spectrum of photocatalytic water treatment used in this study, two parameters of rainwater contamination will be pointed out : pesticide and bacteria. Indeed, heterogeneous photocatalysis is an electronic process occurring on the surface of a catalyst which, under a solar or artificial radiation generates oxidizing entities reacting with organic compounds or microorganisms. The rainwater was recovered respectively from a roof, a gutter and a tank, collected in plastic tank, located outside the Laboratory of water and environment chemistry in Poitiers (France). The water samples collected have neutral or very slightly acid pH; they contain very few minerals and heavy metals and have an average DCO content of 1,2 mg C/L. The bacterial load is also very weak considering the immediate environment of the roof which does not seem very favourable. Although rainwater can initially be contaminated by pesticides, an artificial doping (in the order of µg/L) was undertaken before each experiment of photocatalysis. With a flow of approximately 10 L/h, a total abatement of 2 log in coliforms and enterococci is noted whereas 20 to 60 % of the pesticides are eliminated (alachlore and metolachlore are more easily eliminated than atrazine and lindane). These performances are the sum of several phenomena : oxidizing species generated by the photocatalysis, photolysis, effects of temperature and adsorption. Although this solar photocatalysis process generates a rather low flow, it can be considered for the improvement of the quality of rainwater recovered after streaming on roof for household uses in countries where water resources are lacking. The improvement of this technology of photocatalysis is the subject of research programmes at the Laboratory of Water and Environment Chemistry.

Key words: disinfection / rain water – pesticide / photocatalysis- TiO₂