How scale and technology influence the energy intensity of water recycling systems-An analytical review

Abstract

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 (distributed) water recycling systems that have the potential to reduce energy intensity for cost-effective urban water services.