A life cycle perspective of water conservation and resource recovery strategies in the urban water system

Abstract

The transition towards sustainable and resource efficient urban water systems is a major challenge nowadays. To meet the high demands of urban life resources need to be efficiently used and resource recovery from generated “waste” streams should become a “new normal”. Water and wastewater can provide an alternative and environmentally viable source of resources supporting the resilience of natural systems under water stress. There are many resources that can be recovered via the water path, such as water itself, energy and components such as nutrients and metals. However, the urban water system is an assembly of complex and interconnected processes which have to be taken into account in order to create an overall sustainable value. Managing the entire water-energy-nutrient nexus for the built environment requires a full system analysis of the life cycle energy consumption, water consumption, global warming potential and freshwater eutrophication potential of the municipal water and sanitation services. This work is an assessment of the environmental impacts of various urban water systems which aim to the water conservation and resource recovery. Each alternative system was assessed under four main planning scenarios applied at a dwelling level$\colon$ greywater treatment and reuse, rainwater harvesting and use, usage of water-efficient appliances, food waste valorisation via the sewer system. The scenarios were investigated from two different perspectives (eight scenarios in total), all based on the principles of water fit-for-purpose and resource recovery. In the first perspective the four scenarios limited the interventions at a dwelling level, while in the second one additional techniques for phosphorus and nitrogen recovery were applied at a centralised wastewater and sludge management level. The comparative results revealed that the usage of water efficient devices (low-flush toilets, water efficient shower heads, waterless washing machines, waterless dishwashers) at a household level, coupled with centralised wastewater treatment with simultaneous recovery of struvite and biogas, and sludge waste drying to produce biofuels promotes a robust and resource efficient urban water system. This strategy was estimated to offset 79% of the overall embodied energy consumption, 72% of life cycle carbon footprint, 22% of life cycle freshwater eutrophication potential and 56% of embodied water consumption. A sensitivity analysis revealed that the energy requirements for water heating in showers, dishwashers and washing machines and the carbon intensity of the national electricity mix were the most important parameters for determining the comparative life cycle global warming potential results.