THE QUALITY PROBLEM IN WATER DISTRIBUTION SYSTEM

Introduction
In the last decades attention towards human health and the rise of life quality standards have lead to a higher interest in the quality of water flowing within pressurized flow systems, such as Water Supply Systems (WSSs) and urban Water Distribution Systems (WDSs). Indeed, it is even more necessary to guarantee high quality levels for drinkable water during the ordinary operation of the system, trying to avoid, for example, the formation of harmful elements such as disinfection by-products (DBPs). Another important issue is the intentional contamination, through the introduction of biological, chemical or radio-active contaminants, whose effects could have a strong repercussion on both public health and safety feeling. As a consequence, to adequately simulate contamination phenomena and individuate alternative intervention policies, such as optimum sites for chlorination booster stations (very important, in ordinary conditions, to decrease DBPs) and monitoring early warning stations (very important to deal with emergency conditions rising from intentional contamination), it is necessary to use appropriate analysis tools and therefore produce specific mathematical models in order to analyze space and time variability of the parameters that characterize the quality of water flowing within both WDSs and WSSs.
One of the most complete models quoted in literature for the analysis of changes of water quality characteristics along WDSs, is undeniably the one proposed by Rossman et al. [10], which allows to evaluate the variability of chlorine concentration in the water distribution systems. The model uses a Lagrangian approach and is based on the subdivision of the water volume contained in each pipe into a given number of segments (elementary cells), which have the same velocity inside the pipe as that of the flow and behave like completely stirred tanks. Because of its characteristics, last version of this model, denominated EPANET 2 [10], is considered, at present, the world standard, even though it presents some problems, hereafter described. Starting from these considerations, the aim of this paper is to propose a new mathematical problem, denominated QualSim_PFN (Quality Simulation in Pressurized Flow Networks), which is able to overcome EPANET’s lacks. As a consequence, in this paper it will first described the EPANET model, then the approach used in the QualSim_PFN model and, finally, it will be synthesized the results of a few runs carried out in order to compare their features and performances. In particular, the contaminants moving within WDSs are usually characterized by very low concentrations and small particle size.  […]