Carbon costing for mixed-use greywater recycling systems

Abstract

Urbanisation in the twenty-first century is accompanied by higher water demands per unit area of available space. The ability of water providers to meet these demands long term will require sustainable innovations in terms of water supply. Previous research by the authors has shown that urban mixed-use systems that share greywater (GW) between high-rise domestic dwellings (where GW production exceeds non-potable demands) and high-rise offices (where non-potable demands exceed GW production) could overcome these difficulties. This paper explores the carbon costs (embodied and operational) of such an urban arrangement by investigating the influence of membrane bioreactors (MBRs), constructed wetlands (CWs), building heights, floor plate areas and cross-connection distances. Five water supply scenarios are considered: scenario 1 (conventional mains treated offsite); scenarios 2a and 2b (individual GW treatment via CW/MBR); scenarios 3a and 3b (shared GW treatment via CW/MBR). Over a 15-year period it is shown that shared CW treatment had the lowest carbon dioxide emissions, saving up to 11% compared with conventional mains, whereas a shared MBR increased carbon dioxide emissions by up to 27%. Furthermore, most carbon dioxide savings occur when the ratio (height or floor area) of office building to residential building is 2:3.