Possibilities for rooftop rainwater harvesting for off-grid households

Abstract

The stress on water supply systems is increasing, among others due to pollution, increasing demands, source depletion and climate change. More sustainable solutions with respect to water supply will decrease the stress. Rainwater harvesting can be one of these sustainable solutions. Depending on the characteristics of the rainwater harvesting system it can increase water availability and limit urban flooding and erosion by water retention. Furthermore it can limit subsidence by limiting groundwater extraction or by increasing infiltration. Finally rainwater is a relatively clean and free water source. This research investigates the possibilities for domestic rainwater harvesting in an off-grid urban area in Serang (Indonesia). Current water infrastructure in this area is found insufficient. Technical, economical, legal, social and cultural aspects are tackled in this research. This is done by conducting semi-structured interviews, creating a conceptual model of a rainwater harvesting system, and by doing water quality measurements, cost calculations and literature research. While rainwater harvesting is not preferred socially, use is accepted in the case study area. Also, no relevant restrictions are present regarding rainwater use on national level. In the analysed existing rainwater harvesting tanks the water quality meet the WHO guidelines for all measured parameters, except microbial contamination. Suggested treatment steps include first flush, cloth filtration and boiling. Boiling is solely required for potable purposes and first flush water can be used for low end purposes. Furthermore, closed tanks or small fish can prevent mosquitos. Risk groups, including young children, elderly, people with weakened immune systems or skin injuries should be careful with the use rainwater. Depending on the roof size, tank size and operating scenario a certain tap flow can be expected from a rainwater harvesting system. It is found that for a tank of 2 m3 and a roof of 100 m2 an average tap flow of between 116 and 186 liter/day can be expected. This tank size has the lowest water price per cubic meter (between 0.40-1.11 euro per m3), in case a realistic roof size is connected. Payback periods were found to vary largely depending on the assumption regarding the fresh water requirement, and the operating scenario. Payback periods are around 2 and 8 years in case a fresh water need of 80 and 20 liter a family a day is taken into account. Payback periods decrease for larger fresh water needs, due to the fact that more of the expensive bottled water is replaced. In this case yearly savings based on investment increases. This research shows that rainwater harvesting is a suitable technique for the improvement of both quality and quantity of the water supply in Serang. Focus can be on the awareness, acceptance and knowledge of the population. Small changes can improve the operation of existing systems. Systems should be closed, contain overflow, tap and first flush. Harvest can increase largely by connecting the entire roof, and by considering the operating scenario carefully. Governmental institutions could help the local population with these improvements and by water quality measurements. Scientific research can focus on the remaining health risk related to rainwater use, methods for quality control of systems, constructions to bridge the large investment costs or improvements of the conceptual model used.