The Effect Of Water Quality Variations On Iron Release In Corroded Pipe

Owning to the phenomenon of"red water"in water distribution system(WDS) caused by water quality variations, it has aroused more and more attention, especially the coming trend of desalinated water into WDS, thus maintaining water quality in WDS is significant for water security and stability. The purpose of this paper is to control iron release,turbidity and color in WDS through adjusting water quality indexes, and steel pipe, as the most serious iron release pipe, is studied here.First of all, immersion test is conducted with the variation of water quality indexes, the mixing of tap water and desalinated water, pH, alkalinity, chlorides and sulfates are included. Iron concentration and related water quality indexes such as immersion time, pH, alkalinity, temperature and hardness are monitored. Iron release shows positive correlations with immersion time, temperature, chlorides and sulfates, and negative correlations with pH, alkalinity and hardness.Moreover, Statistical models and kinetic model are established on iron release. The prediction accuracy of all regression and stepwise regression model is 60%, Through principal component analysis(PCA), the average accuracy of PCA regression model is enhanced to 73%, especially at high iron concentration; according to the possible reaction of iron release in WDS, zero-order kinetic model is established which incorporates pH, dissolved oxygen, alkaline and immersion time, the prediction accuracy is over 80%. On the whole, the kinetic model and PCA model can be used on iron release prediction, and the prediction accuracy of kinetic model is higher than other statistical models.In addition, in order to estimate the possibility and rate of iron release in WDS more quickly and accurately, LR'index, a modified form of the Larson Ratio is developed, which adds influence factors including temperature, hardness and immersion time. LR'index has perfect practicality. According to the experimental water quality, critical value of iron release rate in steel pipe and the index of water chemistry stabilization are proposed, when LR'≤1, water meets the requirement of water chemistry stabilization, and the related iron release rate is v临界×LR' mg/(m2h). Verified by experimental data, LR ' result is consistent with the current water chemistry stabilization index, and the prediction accuracy of LR' on iron release rate is 85%.