Many cities are moving towards increased use of recycled water to meet water demand due to freshwater scarcity, population growth, urbanisation and climate change. Water recycling requires substantial energy. Water utilities are facing serious challenges providing cost-effective and reliable water services under rising energy cost. Energy is further linked with global climate change through carbon intensive Greenhouse Gases (GHGs) emissions. However, few studies have attempted to understand the energy use of water recycling systems and how energy intensity of those systems varies with scale and technology. In this paper, we undertook a comprehensive and systematic literature and data review to understand the energy intensity of water recycling systems. We used four "cases": (1) Centralised Potable (2) Centralised Non-Potable, (3) Decentralised Potable and (4) Decentralised Non-Potable systems to structure our work. Our analysis demonstrates how energy intensity of water recycling systems decreases with increasing size for a wide range of scale and for different treatment technologies. The treatment energy intensity for centralised systems having capacity less than 5 MLD varies from 0.48 to 2.0 kWh/kL for non-potable and 0.75 to 2.0 kWh/k for potable; for capacities between 5 and 200 MLD varies from 0.2 to 0.9 kWh/kL for potable and from 0.25 to 0.75 kWh/ kL for non-potable; and for any capacity greater than 200 MLD, the treatment energy intensity is less than 0.8 kWh/kL for potable and 0.55 kWh/kL for non-potable systems. But current centralised water recycling systems have a treatment energy intensity from 0.65 to 1.4 kWh/kL for Potable for capacity from 21 to 378 MLD and from 0.6 to 1.0 kWh/kL for non-potable systems for 6 to 350 MLD. In the case of decentralised systems, smaller systems consume higher energy than centralised systems but larger decentralised Systems (mid-size) have lower energy intensity. Though the treatment energy intensity of a centralised system is low, the reuse of treated water for non-potable water requires a dual pipe system which involves a good amount of pumping energy due to the long distance between the treatment plant and the users. Pumping energy, in this case, can vary from 0.19 to 1.43 kWh/kL. The selected treatment technology and train have also influence on the energy use. The present trend of water recycling is to produce high-quality recycled water for all non-potable reuse using Advanced Water Treatment but all non-potable water uses do not necessarily require such high quality water. Little attention has been given to introducing 'fit for purpose' water reuse using appropriate technologies and larger decentralised water recycling systems that have the potential to reduce energy intensity for cost-effective urban water services.
Cities are increasingly depending on energy-intensive water sources, such as distant rivers and the ocean, to meet their water demand. However, such expensive sources could be avoided using alternative local sources of water such as wastewater, rainwater, and stormwater. Many cities do not have robust accounts of those localized water resources, as estimating those resources requires comprehensive accounting in complex urban water systems. In this article, we investigate whether an urban metabolism evaluation framework built on the urban water mass balance can help analyze these resources, especially in a rapidly growing developing city. We first refined the water mass balance equation developed by Kenway and his colleagues in 2011 for a developing country context with the inclusion of some significant components such as system loss. Then, we applied the refined equation for the first time to Bangalore city in India, a developing country, for the year 2013-2014 as a real case example, which is a rare water mass balance analysis of its kind. The refined equation helped analyze Bangalore's urban water system. The total available wastewater, stormwater, and rainwater were 656 gigaliters (GL). The gap between water demand and supply could be met if 54% of this recycled potential were harnessed. Wastewater had enough potential (362 GL) to replace the whole centralized water supply from the Cauvery. A scenario analysis showed that the gap between water demand and supply in 2021 can be met if 60% of total recycled potential is utilized. This approach can be used to help other cities identify the potential of alternative water sources and support integrated water planning and monitoring water metabolic performance. Keywords:alternative water resources system boundary water performance indicator water reuse urban water accounting urban water planning Supporting information is linked to this article on the JIE website
Cities are increasingly depending on energy intensive water sources such as distant rivers and the ocean to meet their water demand. However, such expensive sources could be avoided using alternative local sources of water such as wastewater, rainwater and stormwater.Many cities do not have robust accounts of those localized water resources, as estimating those resources requires comprehensive accounting in complex urban water systems. In this article, we investigated whether an urban metabolism framework built on the Urban Water Mass Balance can help analyze these resources, especially in a rapidly growing developing city. We first refined the water mass balance equation developed by Kenway et al. (2011) for a developing country context with the inclusion of some significant components such as system loss. Then we applied it to Bangalore city for the year 2013-2014 which is a rare mass balance analysis in a developing country. The refined equation helped analyze Bangalore urban water system. The total available wastewater, stormwater and rainwater were 656 gigaliters. The gap between water demand and supply could be met if 54% of this recycled potential were harnessed.Wastewater had enough potential (362 gigaliters) to replace the whole centralized water supply from the river Cauvery. A scenario analysis showed that the gap between water demand and supply in 2021 can be met if 60% of total recycled potential is utilized. This approach can be used to other cities to identify the potential of alternative water sources and help integrated water planning and monitoring water metabolic performances.
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