Characteristics Of Iron Stability Caused By Water Source Switch And Iron Release Control In Drinking Water Distribution Systems

The water source switch projects such as South-to-North Water Diversion and desalinated seawater instead of traditional source water brought out a new challenge for water works in China, led to the red water problem in drinking water distribution system. This paper mainly focused on the characteristics of iron stability caused by water source switch and iron release control measures in distribution system. The effects of water quality change on iron release were analyzed, and the control technologies were found out. The mechanisms of aggressive anions on corrosion scales were discussed, and the iron stability control theory systems were developed. These were very important for guaranteeing drinking water quality stability and safety.The main research productions were listed as follows.The concentration of sulphate or chloride increased suddenly caused by water source switch had an obvious effect on iron stability, the pipe water deteriorated in 1-2 days, and the iron release had a positive correlation with the concentration of aggressive anions. It was suggested that the sulfate concentration in pipe water was controlled less than 75 mg/L and the Larson Ratio was less than 0.70. The desalinated seawater need to be mixed with the traditional source water, or to be taken post treatment.Based on the tracking monitoring of tap water and the experiments in pipe section reactors, the effects of water source ratio adjustment, elevating pH and alkalinity, adjusting oxidation reduction potential and adding phosphate inhibitors on iron release control were quantitively described. The mechanisms of control measures had been analyzed, and cost effective had been evaluated. As a result, the integrated iron release control technologies in drinking water distribution system for water source switch situations in China were proposed.According to the physical chemical characteristics of corrosion scales and the pilot scale experiments, it was firstly found that iron release could be controlled through adjusting oxidation reduction potential(ORP) by adding disinfectants. This discovery provided a new insight to maintain the iron stability in distribution system. The iron release could be restrained by elevating ORP≥300mV, while corrosion scales would be reduced when ORP<300mV and iron release increased accompanied by ionic strength elevated. Chlorine had a higher ORP than chloramine, it was concluded that using free chlorine for disinfection instead of chloramine in order to control the iron release effectively by stabilizing the corrosion scales. The high concentration of residual chlorine was necessary to obtain the high ORP. To maintain the sufficient residual chlorine in distribution system, the finished water quality should be improved by switching to good water source, updating the water treatment processes with advanced treatment process of ozone and activated carbon, flushing or spraying coating in old unlined iron pipes or adding chlorine in facilities of secondary water supply.Applying instrument analysis of materialogy and theory of crystallography, it was firstly discovered that the corrosive effects of aggressive anions on magnetite(Fe3O4) and goethite(FeOOH) which were the constituents of corrosion scales, thus the mechanisms on the disruption or dissolution of corrosion scales and red water problem caused by water source switch were interpreted. The empirical formula on corrosivity of anions had been established, and the Larson Ratio had been modified.