Resource optimization for Dhaka city water supply system using WEAP model

Abstract

With rapid urbanization, Dhaka city has been facing shortage of water supply for many years. In Dhaka city, Dhaka Water Supply and Sewerage Authority (DWASA) is responsible for municipal water supply through a piped network and can meet up to 95% of the total demand. DWASA faces difficulties in meeting this water demand with its water production system due to groundwater depletion and pollution in the peripheral rivers of Dhaka. Therefore, alternatives like rain water harvesting, wastewater reuse, and demand management practices such as water efficiency improvement are essential to run the water supply system effectively. It is also important to optimize the resources available from different sources under different scenarios. This study intended to produce a planning and management tool based on Water Evaluation and Planning model (WEAP) for resource optimization through scenario construction. The reference scenario describing the growth of demand was developed using the trend in population growth, per capita demand and system loss. This study assessed the additional resources available from recycled grey water and rainwater, and optimizes water resource allocation under different resource and demand management scenarios. Public perception on water availability from these alternative sources was also assessed in this study. Demand scenario is future consumption where low, intermediate and high supply requirement is assumed in the best, realistic and worst case demand, respectively. In case of supply scenarios, future supply were more, intermediate and less supply from the treatment plants, which assumes in high, medium and low supply cases respectively. A reference scenario has been constructed by linking low surface water supply case with best, realistic and worst case demand for determination of higher unmet demand. Twenty one scenarios have been constructed and analyzed also with the reference scenario to determine the optimum scenarios. The criteria for selecting optimum scenarios was minimization of unmet demand in the year 2030. Where more than one alternative gives the same minimum unmet demand, the scenario with the least number of alternatives is preferred since it would be the most practical option in terms of implementation. In best case demand scenario, WEAP model simulations revealed that the unmet demand was found to be zero in 10 Scenarios where more than one alternative gives the same minimum unmet demand. The preferred scenario was found to be the one which considers 30% of total demand for grey water reuse. Total amount of supply water saved from the treatment plants is 694 ML /day. In realistic case demand scenario, the unmet demand was found to be zero in two scenarios which were combination of two or three alternatives. The preferred scenario was found to be the one which considers 20% saving from demand management and 30% of total demand for grey water reuse. Total amount of supply water saved from treatment plant is 1287 ML /day. In worst case demand scenario, the unmet demand was minimum (5% of total demand) compared to other scenarios. The preferred scenario was found to be the one which considers 20% saving from demand management, 30% of total demand for grey water reuse and 60% utilization of total Rainwater. Total amount of supply water saved from the treatment plants is 2086 ML/day. A household survey was carried out in a selected area in Dhaka city mainly to assess the knowledge and attitude of household owners toward the use of rainwater, recycled grey water and demand management. According to the survey, 98% respondents would like to use recycled grey water for toilet flushing and 87 % respondents would use rainwater for other purposes. The survey indicates that about 58% respondents did not use water wisely. So, awareness building program would be needed to change attitude of people for water use. Outcome of this study indicates that DWASA would be able to meet the demand through optimization of alternative resources. Therefore some of the planned or proposed water treatment plants will not be required to meet the growing demand. However, future study on cost estimates and groundwater assessment will be required to refine the models. This optimization model for DWASA water supply system can be used in decision-making and water supply and demand management for DWASA.