Characteristics And Risk Management Of Iron Release In Drinking Water Distribution Systems

Nowadays, water quality safety in drinking water distribution systems (DWDSs) is one of the focuses in water supply industry. With development of further research, iron corrosion and iron release in DWDSs have become two clear concepts with different meanings.In the paper, the physical and chemical characteristics of corrosion scales and iron release rules in DWDSs were firstly studied. Then, from the perspective of iron speciation, migration and transformation of iron in pipes were studied. Based on the analysis of iron release mechanism, iron release flux model and iron release risk evaluation system for DWDSs were established, respectively. Finally, management measures for control of iron release in DWDSs were proposed.The results indicated that the finished water historically transported was corrosive, which resulted in heavier iron corrosion and formation of developed and layered corrosion scales. In corrosion scales of both cast iron pipe and galvanized steel pipe, iron and oxygen were the main elements and their average mass percent sums were above 80% and 90%, respectively. Magnetite, goethite,lepidocrocite, akaganeite, green rust and siderite existed in corrosion scales, whose contents varied in different corrosion scales.Iron corrosion rates rose when sulfate increased from 30mg/L to 195mg/L and chloride increased from 10mg/L to 190mg/L, respectively. Iron corrosion rates dropped when pH increased from 7.05 to 8.25 and alkalinity increased from 125mg/L to 185mg/L, respectively. With dissoved oxygen increasing from 8.0mg/L to 16.0mg/L, iron corrosion rates dropped at first and then rose. When residual chlorine increased from 0 mg/L to 1.9 mg/L, iron corrosion rates decreased at first, and then increased, and at last stabilized.In the operation management of DWDSs, when flow direction changed, iron released seriously in a short time. Beyond a critical value, when flow velocity became bigger, the amount of iron release was higher. The amount of iron release rose with HRT lengthening.From July to December, water quality was monitored for more than 140 days, including total iron, pH, temperature, DO, sulfate, chloride, alkalinity and hardness. The results indicated that temperature was a main factor that affects iron release in seasonal change for DWDSs.Sulfate and chloride had a slight effect on the ferrous iron oxygenation rate, respectively. The ferrous iron oxygenation rate significantly rose with pH increasing over the range of 7.03 to 7.89. With constant pH, the difference of ferrous iron oxygenation rate was not significant as DO concentration increased from 2.49 to 8.46 mg/L. The ferrous iron oxygenation rate rose with alkalinity increasing. The ferrous iron oxidation rate rose with monochloramine concentrations increasing from 0 to 1.12 mg/L.The concentrations of iron as suspended particles are positively correlated with the increasing turbidity values in actual DWDSs. The changes of sulfate (34-389mg/L) and chloride (9-292mg/L) had little or no effect on the suspended iron particle mass fraction. The suspended iron particle mass fraction increased dramatically with pH increasing from 7.03 to 7.89. With DO concentration ranging between 2.49 and 8.46 mg/L, the suspended iron particle mass fraction increased slightly. With alkalinity increasing over the range of 118 to 247 mg/L, the suspended iron particle mass fraction increased significantly. The suspended iron particle mass fraction increased with the monochloramine concentration rising between 0 and 1.12mg/L. What’s more, when the reaction time was prolonged, the suspended iron particle mass fraction increased.When the iron release level was high, both total iron and free iron oxides in corrosion scales rose. At the same time, instable and intermediate iron compounds increased. With water temperature declining and DO, pH and alkalinity increasing gradually, iron release amount was relatively stable, both total iron and free iron oxides in corrosion scales dropped, instable and intermediate iron compounds oxidized slowly and finally magnetite and goethite were the main components of iron corrosion scales.Sulfate had no significant change in water purification process. Chloride increased slightly in both pre-chlorination and coagulation, and alkalinity decreased slightly. However, either chloride or alkalinity had little change. pH declined in coagulation and DO changed in ozone process. Larson ratio rose with the increase of coagulant dosage. During the experimental period from August to November, CCPP rose in time, but had little change in water purification process.A novel iron release flux model for DWDSs was derived from the one-dimensional diffusing equation and mass balance, respectively. The measured value and prediction value calculated with the model showed the absolute error and relative error were-0.008-0.019mg/L and -6.74～7.77%, respectively. The flux model could accurately predict the amount of iron release in pipes.Based on the uncertainty of iron release in DWDSs, under the guidance of systems engineering and operational research, fuzzy evaluation system of iron release for DWDSs was established, which could embody the risk status of iron in DWDSs, optimize iron sample location and facilitate the operation management of water supply enterprises.Analysis of the characteristics of corrosion scales, water chemical stability of Danjiangkou Reservoir and completing time of the middle route of South-to-North-Water-Diversion Project has been done, which indicats that obvious "red water" phenomenon will not occur on the premise that the water is protected in water source and water delivery canal. However, in order to ensure the safety of the people’s drinking water supply, the water supply areas and boundaries of each covering area must be determinated. The key pipes need to be determinate and water quality must be traced and monitored after water source switch.