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The sustainability definition in the context of water infrastructure has been extended as ‘infrastructure designed and managed to fully contribute to the objectives of society, now and in the future, while maintaining their ecological, environmental and hydrological integrity’ (Marques et al., 2015). The triangular framework of sustainability based on economic growth and efficiency, social justice and environmental protection is widely used for managing natural resources (Sahely et al., 2005). Furthermore, the sustainability of any product or system has been commonly associated with the Triple Bottom Line (TBL) approach, including social, environmental and economic dimensions (Marques et al., 2015), which is inadequate in addressing sustainability issues. Hence, subsequent studies have also considered technical and functional aspects such as durability, reliability, performance and flexibility, evaluation of policy initiatives and governing institutions (Kalbar et al., 2016; Rathnayaka et al., 2016). Sustainability is, thus, a critical perspective to be considered by planners and decision-makers while creating UWI. Achieving sustainability is one of the main drivers for decentralized UWI and it is gained by virtue of various factors, which are described in detail in Table 1.

Table 1

Sustainability aspects of decentralized UWI

Driver	Description
Economic benefits	Improved pressure in water supply networks Energy savings due to the phased and modular development of infrastructure Easy operation of sewerage systems owing to less depth of excavation Decentralized systems prove economical over the long operational life of the UWI Create opportunity to adopt nature-based solutions for wastewater treatment
Social benefits	Equitable water distribution can be achieved in the tail-end of cities Utilization of local labour and women can generate employment Delegating O&M authority to community may imbibe a sense of ownership that sustains STPs Modular designs of decentralized treatment units are relatively easy to install Decentralized systems strengthen local government and achieve equity of resources
Market for recycled water	The availability of treated water close to the end-use location avoids the redistribution cost of reclaimed water (point-of-sale reuse) Decentralized systems suitable for mixed land-use pattern wherein a water exchange network can be established between residential, commercial and industrial thus reduce freshwater dependency
Resource recovery	Wastewater and sludge, recognized as resource carriers amidst the energy-intensive production of nitrogen fertilizers and the depleting reserves of phosphorus rock, can be recovered in decentralized systems Plant-based solutions for phosphorus recovery from wetlands or algal ponds are an attractive option Adoption of natural treatment systems in a decentralized manner can generate useful by-products such as fish feed, biomass, animal feed, and biodiesel
Environmental impacts	Reduced operational energy in the decentralized UWI results in reduced greenhouse gas (GHG) emissions Opportunity to create circular economy at the local scale and closing the nutrient loops resulting in better water quality Full capacity utilization happens in decentralized systems reducing unnecessary wastage of resources
Ease of governance	The installation of decentralized systems involves less complexity due to lesser number of institutions involved The tendency of delays in large infrastructure projects due to land acquisition and tendering is avoided in decentralized systems

Driver	Description
Economic benefits	Improved pressure in water supply networks Energy savings due to the phased and modular development of infrastructure Easy operation of sewerage systems owing to less depth of excavation Decentralized systems prove economical over the long operational life of the UWI Create opportunity to adopt nature-based solutions for wastewater treatment
Social benefits	Equitable water distribution can be achieved in the tail-end of cities Utilization of local labour and women can generate employment Delegating O&M authority to community may imbibe a sense of ownership that sustains STPs Modular designs of decentralized treatment units are relatively easy to install Decentralized systems strengthen local government and achieve equity of resources
Market for recycled water	The availability of treated water close to the end-use location avoids the redistribution cost of reclaimed water (point-of-sale reuse) Decentralized systems suitable for mixed land-use pattern wherein a water exchange network can be established between residential, commercial and industrial thus reduce freshwater dependency
Resource recovery	Wastewater and sludge, recognized as resource carriers amidst the energy-intensive production of nitrogen fertilizers and the depleting reserves of phosphorus rock, can be recovered in decentralized systems Plant-based solutions for phosphorus recovery from wetlands or algal ponds are an attractive option Adoption of natural treatment systems in a decentralized manner can generate useful by-products such as fish feed, biomass, animal feed, and biodiesel
Environmental impacts	Reduced operational energy in the decentralized UWI results in reduced greenhouse gas (GHG) emissions Opportunity to create circular economy at the local scale and closing the nutrient loops resulting in better water quality Full capacity utilization happens in decentralized systems reducing unnecessary wastage of resources
Ease of governance	The installation of decentralized systems involves less complexity due to lesser number of institutions involved The tendency of delays in large infrastructure projects due to land acquisition and tendering is avoided in decentralized systems

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