A comparative social life cycle assessment of urban domestic water reuse alternatives

Purpose: The social benefits and impacts of four alternative approaches to urban domestic non-potable water reuse were compared: (1) central wastewater treatment, no urban reuse. Reclaimed water is discharged to nature; (2) central wastewater treatment and urban reuse of the wastewater treatment plant’s tertiary effluent; (3) semi-distributed greywater treatment and reuse, at cluster scale (8 residential buildings); (4) distributed greywater treatment and reuse, within each apartment building. Methods: The impacts of the four aforementioned approaches to water reuse on three relevant stakeholders were investigated: public, community, and consumer. A hierarchical structure of impact categories and sub-categories under these stakeholder groups was established. Expert judgment elicitation was used to attribute weights to the social criteria, through an analytical hierarchy process (AHP). AHP is an established multi-criteria decision analysis method, based on series of pairwise comparisons. It was also used to evaluate impact intensities for both quantitative and qualitative social indicators. All expert judgements were integrated into an overall weight vector, and a comprehensive social score was calculated for each compared scenario. Results and discussion: Public commitment to water saving was ranked as the most important factor in assessing the social impacts of urban domestic reuse, with a weight of 29.6%. Two sub-categories of the community category ranked second: urban landscape and community engagement (12.6% and 12.0%, respectively). The two distributed alternatives are advantageous over the other two approaches in terms of water saving and urban landscape. The semi-distributed alternative has a significant benefit of community engagement, which the other three lack. The business-as-usual (BAU, no reuse) scenario scored highest in the categories: public equality, consumer health concerns, and consumption habits. Final scores for the compared scenarios indicate that central reuse is somewhat more socially beneficial than no urban reuse, but the two distributed alternatives are far better. Conclusions: In social life cycle assessment (SLCA) often quantitative and qualitative criteria/indicators exist side by side and their harmonious integration is challenging. The challenge arising is twofold: quantifying qualitative indicators and homogenizing all indicator evaluations into a uniform comparable scale. Both tasks were achieved in this study using the AHP. The AHP also successfully served as a platform for participatory processes of eliciting expert judgments regarding criteria weights and may be very useful for stakeholder participation in any social assessment. Regarding the case study—distributed urban water reuse was found to be socially beneficial, both in terms of promoting public commitment to conservation of natural water resources and in advancing community engagement.