Water Quality Control Of Desalinated Water Distribution By Cast Iron Pipe Network

With the sustainable development of cities, the utilization of desalinated water is increasingly prevalent in the future. However, when the desalinated water enter into the municipal distribution pipe network, it often leads to the phenomenon of "yellow water" and "red water" due to the unique characteristics of desalinated water: low alkalinity and hardness; and it often causes severely corrosion of the cement lining during the distribution in new pipeline network. What’s more, it would causes seriously exceeded of some water quality indexes, it has a detrimental influence upon the water security of local residents. Therefore, the safe distribution of the water is greatly significant to maintain the drinking water security. The subject considers the representative gray cast iron pipe as the studying objects, and controlling water quality stability as the research objectives. In addition, we also carry out the in-depth and systematical study the effect of the different water quality on the iron release in the pipe network.At first, scanning electron microscopy, X-ray energy dispersive spectrometer and X-ray diffraction were used to analyze the exterior feature, microstructure, chemical elements composition and compound constitute of the corrosion scale from gray cast iron pipe and welded steel pipe in drinking water distribution systems. The outer of the corrosion scale was compact, while the inner was porous. Iron was the primary chemical element of the corrosion scale, and the composition of the scale was various iron compounds. The surface of welded steel pipe corrosion scale are composed ofα-FeOOH andγ-FeOOH, while the“shell-like”layer are composed of Fe3O4, the porous layer are composed ofα-FeOOH and Fe₃O₄. The characteristics of the corrosion scale were lying on the characteristics of iron pipe materials, water quality in distribution systems, water treatment process and hydraulic conditions.Secondly, we also analysis the effect of the water quality to the total iron concentration by dynamic simulation experiment of the pipe network and changing the mixing ratio of the tap water and the desalinated water and pH value and other water quality indexes. The result shows that there is a positive correlation between the release of iron and the soaking time, water temperature, and a negative correlation between the release of iron and the pH value, alkalinity and hardness. We also carry out the continuous stay and water sources switch testing of dynamic pipe network at the same time, the result indicates that the methodology of mixing the top water and the desalinated water in the old pipe and adjusting the water quality is desirable; the rapid switching water source has little influence upon the total iron concentration and turbidity of the water sample in the pipe network, so it does not appear the fluctuation and deterioration of water quality.Meanwhile, this paper also establishes the empirical statistic model and kinetic model of the iron release in the dynamic pipe network. The prediction accuracy of the whole regression and stepwise regression in empirical statistic model is 86.9% and 89.1% respectively. We also establish the kinetic model of iron release base on the possible iron release reaction. From the overall prediction of the iron release kinetic model, the average relative error of the predicted value is about 15.74%, which can basically meet the forecast requirements of iron release in the water network.Finally, we also carry out the simulation experiment of the desalinated water distributing in the new cast iron pipe network. The result shows that in order to maintain the safe distribution of the desalinated water in the pipe network, we should maintain the pH value at around 8.0 is reasonable, and silicate dissolution rate gradually decline with the increasing of the total alkalinity and hardness, so we should rise appropriately the total alkalinity and hardness; and appropriately control the concentration of the free chlorine, and it is more appropriate that maintain it at around 1.0 mg/L.