L’utilisation de l’eau de pluie comme substitut à l’eau de distribution par les ménages en Région wallonne de Belgique - État des lieux et impacts actuels

The rainwater use as a substitute for public water supply for households in the walloon Region of Belgium - State of the art and current impacts

S.A. AQUAWAL, Rue Félix Wodon, 21 - 5000 Namur, Belgique

Reçu : 8 Janvier 2007
Accepté : 30 Avril 2007

Résumé

L’utilisation de l’eau de pluie comme ressource alternative à l’eau de distribution dispose actuellement d’une grande popularité en Belgique. Son utilisation est présentée comme une méthode de gestion « durable » des ressources en eau. Cependant, aucune donnée n’est actuellement disponible sur l’utilisation réelle de l’eau de pluie par les ménages. C’est précisément le but de la présente étude de fournir les données de base sur l’utilisation de l’eau de pluie en Région wallonne et d’en évaluer les impacts concrets des points de vue environnemental, économique et social. L’analyse montre que la consommation actuelle d’eau de pluie par les ménages correspond à 6 % de la demande totale en eau. Cette consommation permet de diminuer les prélèvements totaux d’eau douce de 1,4 %. L’effet tampon sur les inondations est, pour sa part, quasiment nul. En terme économique, l’utilisation de cette ressource alternative engendre une augmentation de 5,5 % de la facture totale moyenne de l’eau de distribution. En conclusion, il convient d’être prudent si une politique globale de promotion de cette ressource alternative devait être adoptée.

Abstract

Rainwater use is often presented as an ecological and economic way to prevent overexploitation of freshwater resources. However, it may induce costs as a hole for the community. In order to estimate if the use of rainwater is a true sustainable development tool, it is necessary to estimate the current use of this alternative resource of freshwater from a global point of view. The use of rainwater as an alternative for public water supply is analyzed in a quantitative way. The aim was to get an overview of the current habits about the use of such water so that the analysis of its implications becomes possible. Impacts analyzed were the effects on water withdrawals and on global flood buffering, and the impact on total average water bill. Data about rainwater tanks came from the Economic public federal service of Belgium and are related to the year 2001. Proportion of housing equipped with a rainwater tank has been worked out for each Belgian municipality so that cartography became possible. These data were then crossed with multiple data concerning drinking water quality, rainwater falls, socio-economic and demographic characteristics of households and attribute of housings. The estimation of total rainwater consumption as an alternative to the public water supply is based on two different methods using independent data : one is based on a survey and the other one on modelling of the drinking water demand function. Both estimations converge to a very similar estimation of the total use of rainwater by households. The analysis shows that rainwater tanks are spatially distributed following a southeast – northwest pattern which is the opposite of the rainwater falls pattern in Belgium. Moreover this pattern does not look like a classic urban/rural or socioeconomically based spatial opposition. The most determining factor has been found to be the history of public water supply development. The rainwater consumption by households is estimated at eleven millions cubic meters per year. This estimation is the convergent result of both statistical methods (survey and modelling). This represents about 6 % of the total water demand (drinking water + rainwater consumptions) and about 25 % of the demand of an average household owning a rainwater tank. The impacts of this consumption have been evaluated. First it shows first that the quantitative pressure on freshwater resources is reduced of 1.4 % since the increase in average total water bill is estimated to 5.5 %. The use of rainwater has the same impacts on total water withdrawals than an improvement of the public water supply network yield of about 4.5 %. The effect on flood buffering has been found to be minimalist. As a conclusion, the analysis shows that though rainwater may contribute to the global decrease of freshwater withdrawals, it does not seem to be an effective tool for reaching sustainable development in the walloon Region of Belgium.