Improved design and installation of an economic rooftop rainwater harvesting system

Abstract

Water resources are depleting at an alarming rate due to its increasing demand. Absolute water scarcity is being experienced in many parts of the country. In such a situation the only solution is to harvest water during rains and preserve it for future use. Rainwater harvesting is a method of capturing rainwater where it falls and taking all precautions to store it unpolluted. We selected an existing rooftop rain water harvesting system in the KCAET campus and modified its design to make it more economical and efficient. Various polyfilms were compared for their mechanical properties and cost, and finally UVA Clear LDPE of 200 micron thickness was selected as the lining material. A portable shed roof for the storage tank was constructed using GI pipes and UVA Clear LDPE. The existing screen filter was replaced with an upward flow, coir and activated carbon type filter. The density of the filter media was selected by analysing the quality of water and filtration rate at various proportions of the coir fiber and charcoal. The filter media having a density of 83.65 kg/m 3 with a filtration rate of 3.83 m 3 /min/m 2 and efficiency of 90.2 % was adopted. An automated first flush diversion system was also installed to prevent the entry of first rainwater after a dry season to the filter, so that the excess dirt load is reduced. An underground recharge tank has been constructed to recharge the ground water using the overflow, without losing the land area. The results of physical and chemical analysis of water from various sources and that from the poly lined well showed that, the water collected from the latter was of superior quality compared to other conventional sources. A computer program was developed in Visual basic 6 which gives various outputs for the design and operation of the rain water harvesting system. . From the economic analysis of the project, we found that, for a unit cost of Rs.0.10 per litre of water, Net present worth, Benefit Cost ratio and internal rate of returns were Rs.1,07,484.,four and 86% respectively, which indicates the profitability of the project. It is concluded that designed RRWH system can be strongly recommended for households facing the problems of water scarcity and quality deterioration.