Hydraulic modeling software helps determine the effects of chlorine reactivity on Beirut's water networks
BY MAHER KAHIL
The Middle East and North Africa (MENA) region suffers from a severe water shortage. The region holds 5 percent of the world's population, but possesses less than 1 percent of global water resources. Not only is the quantity of renewable water per capita the lowest in the world, but the quality of water in MENA is also inferior and often unacceptable.
In most cases, lack of funding, technology, and resources have impeded the development of water management in this region.Water authorities in Beirut, Lebanon, for example, an area that has suffered from long periods of war, have not been in a position to implement an adequate water distribution system throughout the city. For this reason, Beirut has been the subject of many water resource studies and research projects to improve the city's infrastructure and drinking water.
Such projects require innovative dataprocessing techniques and assessment methodologies that only a computer simulation model can provide. Although water quality modeling software cannot increase the natural availability of water, it has proved to be a valuable tool for developing cost-effective solutions for managing water quality, scarcity, and variability issues.
A recent water quality study was conducted as part of a master's degree thesis at Beirut Arab University, in conjunction with Khatib & Alami, a multi-disciplinary architectural and engineering consulting firm based in Beirut with offices throughout the MENA region. The study of Beirut's water networks, which used Haestad Methods' WaterCAD water distribution modeling software, had three objectives: to establish protocols for sampling distribution systems; to obtain a better understanding of factors affecting chlorine reactivity in the system's pipes; and to provide the results of these investigations to Beirut's water authorities to help plan new infrastructure improvements, develop operational maintenance strategies, and proactively manage their systems.
The area under investigation covered the Mtaileb and Rabieh district, which is located in the northern tip of Beirut. The distribution system receives its water from the Dbaye treatment plant. The 50-year-old network comprises several types of pipe materials, including cast iron and ductile iron, with diameters ranging from 150 mm to 600 mm. The network includes many illegal water connections throughout the city, and 22 percent of the population remains unconnected. Those who are connected may experience poor water quality, which can lead to waterborne diseases and other severe health risks.
Beirut's water supply network is prone to water quality deterioration caused by the following:
supply sources taken off and on line;
contamination via cross connections;
system component failures;
loss of disinfectant residual in storage facilities with long residence times;
bacterial re-growth and other microbial activities; and
increases in turbidity, dissolved lead, and pipe corrosion. Haestad Methods' WaterCAD was used to construct a hydraulic model of the Mtaileb-Rabieh water-distribution network, including the tanks and pressures throughout the system. Extended-period simulations allowed an examination of how the system behaved over time in order to understand the current operating characteristics (Figure 2). WaterCAD's scenario management and constituent analysis capabilities were used to store water quality data to model chlorine concentrations in the system and to track water sampling points free of residual chlorine. Next, a sensitivity analysis judged how calibrated parameter adjustments affected the decay rate of chlorine. These adjustments included pipe diameter variations, bulk and wall reaction rates, molecular diffusivity, and kinematic viscosity and velocity.
A completed, calibrated model of the existing system demonstrated that none of the consumers are at risk of receiving water with chlorine concentration less than the minimum allowable value, which warrants the use of the delivered water from the Dbaye water treatment plant for potable use. The greatest chlorine loss is due to pipe wall reactions, and the age and material of the pipe is the most significant factor in the decay rate of chlorine in the system. The sensitivity analysis concluded that the kinematic viscosity and molecular diffusivity did not affect the overall decay rate. The change of pipe size for diameters less than 600 mm has a great effect on the value of the overall decay rate, while for diameters greater than 600 mm, its effect is relatively small (Figure 1). The rate of chlorine decay increases with the increase of the fluid velocity between 0 and 1.8 m/sec, while its effect becomes relatively small for velocities greater than 1.8 m/sec.
The results of this study prove that modeling is an effective tool for water quality research. It is recommended that Beirut's water authorities continue to use the WaterCAD model for ongoing projects, including assessing the amount of free residual chlorine and its affect on water quality, along with detecting chlorine losses due to leakage, cross contamination, and terrorist actions.
Maher Kahil is an environmental and water resources engineer for Khatib & Alami, Beirut, Lebanon, and an assistant teacher at Beirut Arab University. He can be contacted at firstname.lastname@example.org.