Towards sustainable cities through an environmental, economic and eco-efficiency analysis of urban sanitation and drainage systems /

Resumo

The growth of cities worldwide is associated with an increasing demand for sanitation and drainage infrastructure in the context of the water cycle. Combined with the effects of climate change, the situation of these systems in urban environments is critical. Part of the existing sewer networks require an imminent renovation, others must be constructed in developing areas, whereas stormwater runoff becomes a threat in terms of flooding because of the soil imperviousness. In this context, we must determine the best practices aimed at reducing these issues from an innovative environmental and economic viewpoint and at the same time adapt cities to climate change. In response to this demand, this dissertation assesses the eco-efficiency of urban sanitation and drainage systems to determine the best alternatives in different urban contexts. To this end, the industrial ecology framework is used by applying specific methods such as life cycle assessment (LCA), life cycle costing (LCC) and eco-efficiency. This interdisciplinary research requires additional methods, such as statistical studies or field experimental analyses. The life cycle of sewers was widely analyzed and it was observed that pipe materials are not the only factors that determine the environmental impacts of a sewer constructive solution. In some cases, the trench might contribute to 80% of the environmental impacts of the construction phase, which is a relevant issue to consider in decision-making. Through a structural parametric study, we found the equivalent constructive solutions that generate the lowest environmental impact. Reducing the use of concrete or reusing the excavated soil might entail environmental improvements. On the other hand, the operation stage is challenging in the context of urban planning. The coastal city of Calafell (Spain, Mediterranean climate) was compared with the city of Betanzos (Spain, Atlantic climate). The location of Calafell’s wastewater treatment plant at a higher elevation than the city resulted in Calafell consuming more pumping energy (0.47 kWh/m3) than Betanzos (0.11 kWh/m3), where wastewater flows gravitationally. Additionally, gas emissions were found in the sewer through sampling campaigns. The largest emissions were mainly detected during the summer due to high temperature, and in turbulent areas of the sewer. Furthermore, through an eco-efficiency assessment, the environmental and economic results of a sewer’s life cycle were compared. Regardless of climate and urban form, results show that the operation stage generates the largest environmental impacts (up to 74%), whereas the installation (i.e., the trench) mostly contributes to the economic costs (70-75%). The infrastructure centralization versus decentralization debate was studied in an insular context (Minorca, Spain) with a tourist-related issues. It seems that a centralized scenario that connects the settlement to an existing treatment plant with a large treatment capacity is environmentally beneficial due to economies of scale. This scenario entails a 12% impact reduction with respect to partial decentralization through septic tanks, or 36% reduction with respect to treating seasonal wastewater at a constructed wetland. In general, results depend on the duration of the seasonal period. In the field of flood prevention, this thesis provides a new vision, as these are the first studies that integrate the avoided impacts of damage prevention into the environmental and economic effects of investing in preventive measures. These analyses are of interest in the framework of urban planning. Based on two different climates and systems, ephemeral streams in the Maresme region (Catalonia, Spain) and a green system implemented in Brazil were assessed. From a methodological perspective, flooding analyses provide some ideas in the field of LCA methods and discuss how to deal with the consequences of flooding from an integrated viewpoint.