Spatio-temporal assessment of chlorine residuals in the water distribution system of Dhaka city

Abstract

The majority of the people of Dhaka City in Bangladesh are completely depending on supplying water from Production Tube wells (PTWs) under DWASA for household potable water use. Proper understanding, characterization and prediction of water quality behavior in the drinking water distribution system are critical to ensure to meet regulatory requirements and customer-oriented expectations. Water distribution systems are frequently designed to ensure hydraulic reliability, which includes adequate water quantity and pressure for domestic and industrial demand. Chlorination is usually done in large networks to ensure acceptable water quality throughout the network and water quality modeling needs to be done to predict residual chlorine concentrations in critical nodes. Although a hydraulic model is being applied by DWASA to ensure system reliability of its water distribution network, but the water quality modeling is not done before decommissioning or during operation.

To control the quality and adapt it to the need, the water quality model should be run and predict the impact of the quality of the network. Although a chlorination system is adopted in a particular District Metered Area (DMA) of Dhaka city, it is controlled by a limited number of potential chlorine injection points which are typically the PTWs of such DMAs. The present study assesses the effectiveness of the baseline chlorination system adopted in a particular DMA of Dhaka city by performing residual chlorine measurements at selected locations within the network by predicting the concentration of residual chlorine over space and time under various operating conditions using EPANET. The residual chlorine model was calibrated and validated using time-patterns of residual chlorine concentration data collected at several control points within the DMA and the sensitivity of the residual chlorine model were done over a variety of conditions. The bulk residual chlorine decay rate (kb) was found to be -0.2975h-1 through laboratory experiments. The wall residual chlorine decay rate constant (kw) has been found to be -0.0065 ft/s through calibration. Also, the residual chlorine modeling using EPANET is carried out to simulate the residual chlorine in the water distribution network of the DMA as well as to determine the most optimum locations of chlorine injection points. It has been found that more than 50% pipes do not get any chlorine under the present chlorination scheme. Inclusion of additional chlorine injection points has increased the chlorine coverage area of the network. For example, the additional three, four, five and six injection points increased the chlorine coverage over 51%, 63%, 75% and 86% of the pipes. It needs to be noted that 100% chlorine coverage was not obtained over 24 hours, even with all six injection points and therefore, additional protection measures to ensure water quality needs to be provided in certain sections of the network.

Water quality simulation for assessing the effectiveness of the current chlorination system for different chlorine injection patterns in a specific DMA in Dhaka City, an automated continuous chlorine monitoring system may be useful in better calibration and validation of the chlorine model. EPANET is a tool that may be used for controlling the concentration of residual chlorine and assessing the effectiveness of the existing chlorination system in the different DMAs under DWASA.