Sustainability evaluation of water supply technologies : By using life-cycle and freshwater withdrawal impact assessment & multi-criteria decision analysis

Abstract

Sustainability evaluation of water supply systems is important to include in the decision making process when planning new technologies or resources for water supply. In Denmark the motivations may be many and different for changing technology, but since water supply is based on groundwater the main driver is the limitations of the available resource from the groundwater bodies. The environmental impact of products and systems can be evaluated by life-cycle assessment (LCA) which is a comprehensive and dominant decision support tool capable of evaluating a water system from the cradle to the grave. The first aim of this PhD thesis was to assess the environmental impacts of water supply technologies. For this LCA was used to compare the impacts of Copenhagen’s water supply technology of today with relevant cases considered for implementation in future water supply. The importance of placing the system boundaries right so the cases are comparable was emphasized due to the nature of the included cases. LCA was also found suitable to evaluate the effects of water quality parameters such as water hardness. The second aim was to evaluate the sustainability of the technologies and for this a multi-criteria decision analysis method was used to develop a decision support system and applied to the study. In this thesis a standard LCA of the drinking water supply technology of today (base case) and 4 alternative cases for water supply technologies is conducted. The standard LCA points at the case rain- & stormwater harvesting as the most environmentally friendly technology followed by the cases relying on groundwater abstraction. The least favorable case is desalination of seawater. Rain- & stormwater harvesting and desalination have markedly lower environmental impacts in the use stage compared to the base case, due to the reduced water hardness leading to e.g. a decrease in electricity consumption in households. To make relevant comparisons, it is therefore essential to include the effects of water hardness when the environmental impacts of water systems of different hardness are compared. However, a shortcoming of the standard LCA is that it does not cover the impacts of freshwater withdrawal. Therefore we further developed an existing method to evaluate the impacts of water use on a regional scale and it was applied to the local groundwater bodies from where water is abstracted for Copenhagen. Local data was extracted from the national implementation of the EU water framework directive. When incorporating the impacts of freshwater withdrawal in addition to the standard LCA the rank order is partly reversed since rain- & stormwater harvesting and desalination are significantly more preferable compared to the groundwater based cases. This shows the importance of integrating impacts of freshwater withdrawal in the environmental evaluation. A decision support system is needed which takes all identified criteria of relevance into account when choosing between several technologies for drinking water supply. During this PhD a decision support system called ASTA (acronym for: Assess the most SusTainable Alternative) was developed based on the multicriteria decision analysis methods rank ordering distribution weights and analytic hierarchy process. The ASTA decision support system incorporates the criteria of the 3 sustainability dimensions – environment, economy and society – referred to as categories in ASTA. After having assessed the 4 water supply technologies for Copenhagen with the developed system (ASTA), the results point at one preferable water supply technology. However, the results also showed that the result depends upon the weighting of the sustainability categories. This study shows that when the highest weight is assigned to environment then the case of rain- & stormwater harvesting is the most sustainable followed by desalination of seawater. When the highest weight was assigned to economy or society then the most sustainable alternative is the case of compensating actions followed by either rain- & stormwater harvesting or desalination. For all 3 sets of weighting the case new well fields has the lowest sustainability. The development of methods for combining the 3 pillars of sustainability with special attention on the environmental evaluation is presented in this thesis. It is new that LCA also covers parameters of water quality and in addition to the standard impact categories also includes freshwater withdrawal impacts on a local scale. The main contributions of the thesis are methods to include the effects of water hardness and freshwater withdrawal in addition to the environmental evaluation of the standard LCA. Finally, in the last part of the thesis (chapter 4) the environmental evaluation is combined with economy and society in a joint decision support system.