Reliability and economic analysis of urban rainwater harvesting in a megacity in Bangladesh

•Investigated the reliability and economic benefits of the urban RWH systems.•A daily water balance modelling approach was used.•Maximum achievable reliability was found to be about 15–25%.•Current underground tanks are sufficient to prevent the overflow during monsoon.•Economically feasible under wet and average year climate conditions. This paper investigates the applicability, reliability and economic benefit of rainwater harvesting (RWH) systems to partially offset the daily water demand in the multistoried residential buildings in combination with the town water supply systems in Dhaka city. A comprehensive computer software was developed with a view to assessing the reliability and feasibility of the RWH systems in an urban setup. The software was developed using daily water balance modelling concept, which uses input data like daily rainfall, roof catchment area, runoff losses and tank volume. Three distinct climatic scenarios, i.e. wet, average and dry years were chosen by analysing historical 20-years daily rainfall data. Typical residential buildings of plot size 2.5–5.0katha (168–335m2) were considered for the study. Results indicated that about 15–25% reliability can be achieved under the wet climatic condition and for catchment sizes varying from 140m2 to 200m2, 250kL to 550kL of rainwater can be harvested each year. Several reliability curves have been presented for two roof catchment sizes (140m2 and 200m2) under three climatic scenarios and an insignificant increase in the reliability of the RWH system beyond the tank volume of 30m3 was observed. The current underground tank sizes of the residential buildings are sufficient to prevent the potential overflow during monsoon. A monetary saving of around 2000 BDT can be achieved for the catchment size of 140m2 with tank size of 40m3 under average year climate condition and the monetary saving increases with increase in catchment size.