Environmental life cycle, carbon footprint and comparative economic assessment of rainwater harvesting systems in schools – a South African case study

Abstract

Rainwater harvesting (RWH) provides a unique opportunity for water conservation. This research aimed to assess the performance of two types of RWH systems (gravity and pump-driven) at a local public school in replacing non-potable water for toilet flushing. The volume of harvested water, efficiency to meet demand, expenses involved and associated environmental burdens were key criteria of performance. Economic considerations included capital costs and return periods, while the environmental aspects encompassed simplified life cycle assessments (LCAs) as well as specific carbon footprints. The gravity-fed system supplied 452.5 kL/annum and covered 31.8% of the demand for flushing water for toilets for the school investigated. The pumped system provided 476.8 kL/annum representing 33.5% of the demand.  Together they would be able to supply 65.3% of the demand. The catchment area of these two systems differed and there was no overlap. As expected, the gravity-fed system outperformed the pumped system, both economically and environmentally, because no energy for pumping was needed. In terms of costs, the difference was small, and the payback periods of both systems were similar.  However, environmentally, the LCA scores for the pumped system were an order of magnitude higher for all 18 impact categories considered. Carbon footprints showed that in the construction stage both systems have similar footprints. For the operation stage, the comparison was extended, as there were higher energy requirements for the pumped system (about 4 times higher than those from the provision of municipal potable water), but in the same range or lower when compared with other alternative sources of water like groundwater abstraction, recycling of municipal water and desalination. The gravity-fed system required no energy for pumping. This study shows how trade-offs in assessing the overall performance of RWH systems can be considered, leading to better decision making.