Rainwater harvesting systems reduce detergent use

Abstract

Purpose: Due to population growth, urban water demand is expected to increase significantly, as well as the environmental and economic costs required to supply it. Rainwater harvesting (RWH) systems can play a key role in helping cities meet part of their water demand as an alternative to conventional water abstraction and treatment. This paper presents an environmental and economic analysis of RWH systems providing households with water for laundry purposes in a life cycle thinking perspective. Methods: Eight urban RWH system scenarios are defined with varying population density and storage tank layout for existing buildings. Storage tank volume required is calculated using Plugrisost software, based on Barcelona rainfall and catchment area, as well as water demand for laundry, since laundry is a fairly constant demand of non-potable water. Life cycle assessment (LCA) and life cycle costing (LCC) methodologies are applied for this study. Environmental impacts are determined using the ReCiPe 2008 (hierarchical, midpoint) and the cumulative energy demand methods. Net present value (NPV), internal rate of return (IRR), and payback (PB) time were used in LCC. Savings from laundry additives due to the difference in water hardness was, for the first time, included in a RWH study. Results and discussion: LCA results indicate that the best scenario consists of a 24-household building, with the tank spread on the roof providing up to 96% lower impacts than the rest of scenarios considered. These results are mainly due to the absence of pumping energy consumption and greater rainwater collection per cubic meter of built tank capacity. Furthermore, avoided environmental impacts from the reduction in detergent use are more than 20 times greater than the impacts generated by the RWH system. LCC indicates that RWH system in clusters of buildings or home apartments offer up to 16 times higher profits (higher NPV, higher IRR, and lower PB periods) than individual installations. Conclusions: LCA and LCC present better results for high-density scenarios. Overall, avoided environmental and economic impacts from detergent reduction clearly surpass environmental impacts (in all categories except terrestrial acidification) and economic cost of the RWH system in most cases (except two scenarios). Another important finding is that 80% of the savings are achieved by minimizing detergent and fabric softener by using soft rainwater; and the remaining 20% comes from replacing the use of tap water.