Reliability analysis of urban rainwater harvesting: a case study of Dhaka city

Abstract

Rainwater harvesting is a decentralized practice that provides both water supply and runoff reduction benefits that are often difficult to assess. In a city like Dhaka where the water table is falling rapidly & concrete surfaces and landfill dumps are taking the place of water bodies due to increased population and water demand as well, non-conventional water resources such as rainwater harvesting must be introduced to partially offset the increasing water demand. According to the upcoming Bangladesh National Building Code, every building proposed for constructing on plots having extent of 300 square meter or above shall have the facilities for conserving and harvesting rainwater though a very few studies have been carried out to check the efficiency of rainwater harvesting in Dhaka city. The research presented in this paper is undertaken in the light of current knowledge gaps to access the efficiency of a RWH system in Dhaka to provide guidance to water authorities to enhance the acceptance of a RWH system. The objectives of this research is to analyze the reliability of a rainwater harvesting system, to utilize programming and visualization to assess the efficacy of a rainwater harvesting system and to assess the economic savings if rainwater harvesting system is integrated with the supply water system. This paper investigates the applicability and reliability of rainwater harvesting (RWH) systems to meet the daily water demand in the multistoried residential buildings in combination with the conventional water supply systems. The major difference with the other studies carried out in this field is that the model presented in this research uses the existing underground water reservoir as rainwater harvesting tank. In order to calculate the reliability, 20 years rainfall data starting from 1988 to 2007 was considered and all the calculations were performed for three different climatic conditions (i.e. wet, average and dry years). Years corresponding to maximum and minimum annual rainfall were considered wet year (2007) and dry year (1992) respectively. The average year (1990) was considered the year having annual rainfall close to the average annual rainfall over 20 years’ period. A water balance model was developed considering the Daily rainfall runoff, Underground reservoir capacity and Daily water demand. Cumulative water stored in the water balance model is the sum of rainfall runoff and the stored water of the previous day less the daily water demand. When volume of stored water exceeded the tank capacity, the excess iii amount of water is considered as spilled water. When water demand exceeded the volume of stored water then the additional water was taken from the town water supply. Based on the water balance model a rainwater harvesting analysis software has been developed. The software has been developed using C# (C sharp) language in Microsoft Visual Studio. It calculates reliability, percent of water saved, overflow ratio and at the same time benefit-cost ratios based on the given input. Different authors and organizations estimated the daily water demand for Dhaka city differently. . For this study a range of 120-180 lpcd has been selected as most of the recent estimated values lies within this range. The roof of the building is the the rainwater catchment area. This study is concerned with the rainwater harvesting in the multistoried residential buildings of Dhaka city having a plot size 3 to 5 katha i.e. 200-334 m2. The catchment area was considered as 60- 70% of the plot size i.e. for a plot size of 200 m2 the catchment area is 140 m2 and for a plot size of 334 m2 the catchment area is 200 m2. The reliability relationships with the varying tank sizes showed that both the time base and volumetric reliability increases up to a tank volume of 30 m3 for the wet year and beyond that reliability does not increase with the increasing tank volume. But in case of average and dry conditions tank size have no significant impact on reliability. For all the cases volumetric reliability was found 5-8% more than the time based reliability. Both time based reliability and volumetric reliability decreased with the increasing water demand. The demand being high and the tank sizes being large no spilled water was found for any of the cases. So the overflow ratio relationships with the varying water demand and tank size remained zero. Economic savings showed that around BDT 4000 on average can be saved each year if rainwater harvesting system is integrated with the conventional water supply system. The present analysis indicates that about 20-30% reliability can be achieved if sufficient rainfall is available throughout the year. Approximately 250-550 kL of rainwater can be harvested each year from a catchment area of 200 m2.The study also revealed that the current underground tank sizes of the residential buildings of the city are sufficient to prevent the overflow of the harvested rainwater. Though the reliability and the economic savings found is not much greater, it will have a significant impact if rainwater harvesting system is practised at a larger scale as it will reduce a certain portion of the pressure from the conventional water supply and it will also play a significant role in alleviating the water clogging problem.