| Water quality problems related to deterioration in drinking water distribution pipes have been reported and are as old as the existence of drinking water distribution system themselves. These problems are mainly related to the loss of disinfection residual and release of iron corrosion products. Modeling efforts to help manage distribution water quality problems have focused mainly on maintenance of disinfection residuals with no model for the prediction of release of corrosion products. Many complications have been cited as hindering iron release modeling. These include the many potential sources of iron, the existence of various release mechanisms that are not fully understood, the role of pipe material and pipe age, and the effect of water chemistry. This document presents a detailed development of a new model describing iron concentration in drinking water distribution systems based on pilot-scale studies. Mathematical and mass balance derivations of a zero-order iron concentration model supporting the empirical data are presented in this document. The iron concentration model obtained is a function of a iron flux (km), pipe material, pipe diameter and hydraulic retention time. The flux term is a function of water quality and Reynolds Number. For Reynolds Numbers less than 2000, flux has a constant value characteristic of the pipe material and water quality: 1.99 mg/m2.d (0.19 mg/ft 2.d) for galvanized iron and 4.16 mg/m2.d (0.39 mg/ft 2.d) for unlined cast iron. The flux varies linearly with Reynolds Number under turbulent flow conditions.; The flux model developed by Mutoti et al, 2003, for predicting iron concentration in drinking water distribution systems was analyzed and verified using fourteen pilot distribution systems handling water with varied chemistry. Iron concentration in the water exiting the distribution system is proportional to flux and hydraulic retention time, and is inversely proportional to pipe diameter. Distribution system (pilot or full-scale) iron concentration modeling was carried out using a summation method for different pipe reaches. Predicted iron concentration values closely matched measured increases in iron concentration values. The largest differences between measured and predicted iron concentrations were observed in pilot distribution systems receiving finished waters associated with extreme values of one or more of the following: high chloride, high sulfate, and/or low alkalinity. (Abstract shortened by UMI.)... |
