Life cycle cost assessment of a rain water harvesting system for toilet flushing

Rain water harvesting (RWH) has gained popularity as a way of supplementing water supplies for various purposes, including drinking, sanitation and irrigation. This paper presents a methodology of life cycle cost assessment (LCCA) of a unit RWH system (hereafter RWH system) for toilet flushing in an industrial site. The life cycle cost and net present value benefits (NPVB) were estimated for the RWH system and compared with those of a conventional system. For the current system design, the analysis of the life cycle cost of the RWH system indicates negative NPVB for all plausible service lives up to 55 years, mainly because of the initial infrastructure investment costs, operation and maintenance (O&M) costs, and pumping costs for the system. However, sensitivity analysis concluded that an alternative design with no pump, low O&M costs (5%) and 1% tank refill volume may be economically viable given 7 years of service life. The sensitivity analysis also revealed that higher hypothetical water prices ($5/m³) may lead to positive NPVB after only 5 years of service. Full cost pricing for rainwater harvesting is important for the promotion of sustainable practices and life cycle based system design is critical to make RWH systems economically attractive.