Study on water demand and availability for purbachal new town using mathematical models

Abstract

Purbachal New Town, a large-scale township project, has been reinitiated by the government to meet the increasing demand for population influx to the capital city, Dhaka. The development of new townships like Purbachal will add more pressure on the city's overall water supply. The present study has focused on predicting the future water demand and assessing the water availability for this newly planned township using mathematical models. By 2060, the total daily water demand for Purbachal will increase to 155 MLD, 193 MLD, and 251 MLD for the low, moderate, and high growth projections. Alongside the use of surface water, the study has explored alternative supply sources like rainwater harvesting and greywater reuse and achieved savings from demand site management for meeting the incurred water demand. The Geospatial Stormwater Management Model (GeoSWMM) quantifies rainwater harvesting as a low-impact development (LID) measure. The estimated yearly stormwater runoff is found to be 24476 million liters. Of these, 14432 million liters can be harvested from the rooftop area, which can meet up to 26% of the total water demand in a low-growth scenario. The current study has constructed different water demand and supply scenarios to examine the best possible combination of supply sources to minimize the difference between water demand and delivered supply. The Water Evaluation And Planning (WEAP) model is set up for 2020 and runs for the next 40 years using the projected water demand and available supply sources for the study area. The model result shows that the average unmet demand will increase to 24 MLD (16% of the total daily demand), 63 MLD (33% of the total daily demand), and 129 MLD (52% of the total daily demand) for the three-demand projection if none other than surface water is considered as a supply source. Seven other scenarios, each having three sub-scenarios (A, B, and C), have evaluated the impact of using alternative supply sources. The result shows that 45% utilization of total rainwater (Scenario 1B), 20% savings from demand site management (scenario 2C), or 25% of total demand covered by greywater reuse (scenario 3B) are found to be optimum for low growth demand. Combining 20% savings from demand site management and 30% of total demand by greywater reuse (Scenario 6C) is the optimum scenario for the moderate growth case. Besides, the combined use of 60% of total rainwater, 20% savings from demand site management, and 30% of total demand by greywater reuse (Scenario 7C) is the optimum scenario for the high growth case. These model results will open the path to effective decision-making and good water management for the Purbachal New Town despite having scope for further improvements.