Efficacité de l'assainissement des eaux usées sur le bassin de la rivière Chaudière (Québec, Canada)

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Le Québec a consacré des efforts techniques et financiers substantiels à l'assainissement des eaux usées municipales et à l'entreposage des déjections animales afin de satisfaire les demandes des citoyens en matière de restauration des usages des cours d'eau. L'assainissement de l'eau a, dans l'ensemble, par ses choix technologiques et administratifs, engendré des investissements publics dépassant 7,2 milliards de dollars et plus de 400 millions de dollars annuellement au chapitre de l'exploitation. Ces choix ont-ils permis d'atteindre un niveau de qualité de l'eau correspondant à un optimum social? À l'aide d'une étude de cas portant sur le bassin versant de la rivière Chaudière (Québec, Canada), cet article met en évidence les facteurs qui ont nuit à l'efficacité des politiques de contrôle de la pollution de l'eau au Québec. Sur ce bassin, 125 M$ ont été consacrés entre 1981 et 1992 à l'érection d'usines d'épuration utilisant différents types de traitement, 8,6 M$ ont été alloués pour la construction de structures d'entreposage de fumiers et le service de la dette pour l'assainissement des eaux usées municipales atteindrait près de 527 M$ selon une hypothèse de financement de 25 ans. La performance des usines d'épuration a permis de réduire significativement les apports au cours d'eau, notamment en DBO5 et en phosphore. Enfin, cette performance et le coût total de l'assainissement municipal sur le bassin de la Rivière Chaudière permettent d'évaluer, sur la base d'une relation coût-efficacité, qu'il y aurait un niveau optimal de qualité de l'eau pouvant résulté de l'établissement d'infrastructures d'assainissement des eaux usées municipales. Ainsi, dans l'optique d'une prise en charge sociale du problème collectif de la pollution de l'eau sur la base du bassin versant, il serait approprié que les gestionnaires et les usagers-contribuables de la ressource-eau, prennent en compte, non pas uniquement les objectifs de rejets, mais également les coûts et les performances de l'ensemble des usines d'épuration sur le bassin afin de retirer le maximum de charges polluantes là où les équipements sont les plus performants.

Considerable technical and financial effort has been invested by Québec in the cleaning up of municipal wastewaters and storage of animal manure to meet demands by citizens to restore the province's rivers to their former state. Water pollution control has required technological and management choices that have resulted overall in public investments in excess of $7.2 billion, with over $400 million going to operation costs annually. Have these choices enabled Québec to attain a water quality level consistent with a social optimum?Based on a case study taken from the Chaudière river watershed, Québec, Canada, this article posits two conditions for achieving a social optimum and underscores the factors that have offset the efficiency of water pollution control policies in Québec. According to the data collected on this watershed, between 1981 and 1992, $125 M was invested in the construction of sewage water treatment plants using various treatment methods, while $8.6 M went towards manure storage facilities. On the whole, $527 M is expected to be spent over 25 years to service the debt for municipal wastewater treatment within the watershed.While inputs of pollutants, especially BOD5 and phosphorus, have dropped significantly with the construction of the wastewater treatment plants, levels of residual pollution in the watershed remain high. It is suspected that total residual loads of phosphorus from municipal and agricultural sources are still well above the loads eliminated through wastewater treatment. If they are to achieve an efficient watershed-based approach to water management, decision-makers are faced with two conditions: the first addresses intersectoral efficiency in controlling pollution in a watershed and the second involves minimizing intrasectoral costs of pollution control. The condition explains the administrative and technical choices made as well as the importance of the political market in allocating resources to water pollution control among the socioeconomic sectors responsible for water quality deterioration. The condition explains how to minimize the costs in a specific socioeconomic sector among the available water treatment solutions. Using performance data from wastewater treatment plants and the total cost of wastewater treatment in the Chaudière river watershed, it can be assumed, based on a cost efficiency ratio, that an optimal level of water quality should occur as a result of the establishment of municipal wastewater treatment infrastructures. However, it would appear from the results obtained that Québec's water treatment program has deviated from a social optimum, i.e., restoration costs have not been shared equitably among users/polluters within the watershed, and measures to ensure maximum removal of pollution at minimum cost have not been secured. The play of political forces is central to the allocation of resources among pollution sources. Without a proper hard core concept, a water pollution control policy will not be able to elaborate the best solutions oriented towards attaining a social optimum. In the context of the high residual pollution loads within the watershed, there remains the issue of what water quality level is desirable at what cost, particularly with respect to the community's contribution to date and the efficiency of the response strategies that have been implemented. Now that wastewater treatment infrastructures have been set up, and a watershed-based approach to water management becomes effective, water resource managers and users/taxpayers should turn their attention away from discharge objectives only to focus also on the costs and performance of the watershed's treatment plants as a whole, so that removal of pollutant loads at high-performance facilities may be maximized.