The Impact Of Water Quality Variation On Water Supply Pipeline Corrosion

Generally, the balance between water and pipe walls will be destroyed when thewater quality gets changed, before a new chemical equilibrium forming in the watersupply system. The process will affect pipe corrosion and iron releasing in the originalpipe network, which may presumably be the cause of yellow-water phenomenon inpipelines and further to be a hidden danger of the water quality security. Thisdissertation aims to detect how the qualitative change of water in water supply networkwill have effects on pipe corrosion. This research includes the following five aspects:1) This study probed into the physicochemical characteristic of scale in inner pipewalls and investigated into the mechanism of scale forming from pipeline corrosion,coming to a conclusion that the main chemical components of pipe scale wereFeCO3(siderite), FeOOH(ferric hydroxide)、FeO(OH)(goethite) and Fe3O4(magnetite).2) It researched into impact factors of pipe corrosion and iron releasing of pipenetwork in simulation system, including chloride ion concentration, sulfate ionconcentration, total alkalinity, pH, and residual chlorine. According to the analysis ofexperimental results, corrosion in pipeline and iron releasing were positive correlatedwith chloride ion concentration and sulfate ion concentration, negative correlated withtotal alkalinity and pH, and not obvious related with residual chlorine.3) From the perspective that the modification of single water quality index resultedin the alteration of chemical stability index, estimation of pipeline corrosion throughchemical stability index is analyzed. The verdict was that only the Larson Ratio (LR)could evaluate the tendency of iron releasing exactly in every single factor experiment,which proved that LR was a reliable criterion in judging the iron releasing in pipe network.4) Based on the laboratory simulation of practical water source switch process, itresearched into the variation regularity of water quality and its chemical stability in pipenetwork when transforming the water source, together with pipe corrosion caused by thechange. Experimental results indicated that:①before and after switching, ironconcentration and turbidity increases while dissolved oxygen and residual chlorine havea significant attenuation;②iron release was positive correlated with chloride ionconcentration and sulfate ion concentration, and not noticeable correlated with totalalkalinity; and③the result came from LR was in accordance with real situation, whilethe conclusion drawn from Ryznar stability index (IR) was opposite. Trough correlationanalysis, correlation coefficients between water quality index and iron release are asfollows from highest to lowest: chloride ion concentration, conductivity, sulfate ionconcentration, total dissolved solids, total hardness, and total alkalinity; amongchemical stability index, only Larson Ratio is highly relevant with iron release.5) It researched into techniques of controlling pipe corrosion, including methods ofadding phosphate, adding sodium bicarbonate for adjusting alkalinity, and addingsodium hydroxide for regulating pH. All of those three approaches were effective indominating pipe corrosion.