Study On Corrosion Characteristics Of Drinking Water Distribution Systems Based On Occluded Water

In the past five centuries,cast iron pipes have played an irreplaceable role in drinking water distribution systems.However,due to the aging of the pipe and corrosion,it will not only cause energy loss,but also cause secondary pollution of the drinking water in the pipe during the process of transportation and distribution.With the improvement of residents'quality of life,people's requirements for drinking water are getting higher and higher.Therefore,it is urgent to regulate the drinking water quality and control the corrosion of cast iron pipes for the water industry.In the drinking water distribution system,in addition to the presence of bulk water,there is also the presence of occluded water in the pipe scales of the corroded cast iron pipe.Because of its low pH,high concentration of anion and metal ion,and anaerobic characteristics,occluded water plays a crucial role in the corrosion process of aging pipes.From the view of occluded water,the corrosion process of cast iron pipe in occluded water and bulk water was studied.It was found that the pipe scales of bulk water and occluded water were transformed from unstable pipe scales to ?-Fe₂O₃.In the occluded water,Green Rust was produced,which was extremely unstable.The behavior of Fe,Mn,Cu and Cr in occluded water and the sources of Fe and Mn in occluded water were studied using a simulated occluded cell device.The results showed that Fe and Mn were enriched in the occluded water,while Cu and Cr in the occluded water gradually decreased with time.Fe and Mn in the occluded water can migrate to the bulk water,causing contamination of the bulk water.The Fe and Mn in the occluded water mainly came from the corrosion of the metal pipe.The pipe scales had a certain hindrance to the outward migration of Fe in the occluded water,but the migration inhibition of Mn was less.The presence of occluded water can contaminate drinking water,so research on the effect of water quality parameters on the migration of occluded water metal ions was essential.In this paper,response surface analysis was used to study the influence of water quality parameters in the bulk water on the change of metal ion concentration in the bulk water and the occluded water.It was found that the change of Fe and Mn concentration in the bulk water and the change of Mn concentration in the occluded water would increase with the increase of chloride ion and sulfate concentration,which was not significantly affected by pH.Fe concentration in occluded water was negatively correlated with pH as well as positively correlated with chloride ion and sulfate concentration.This paper studies the relationship between different water quality judgment index and metal ion concentration in bulk water and occluded water through correlation analysis.The study found that only larson-skold index?LR?could best reflect the changes of Fe and Mn concentrations in the bulk water and occluded water.In order to evaluate the corrosion and scaling potential of drinking water to pipe,this paper analyzed the correlation between Fe concentration and Langelier saturation index?LSI?,Ryznar stability index?RSI?,Larson-Skold index?LR?and aggressiveness index?AI?using drinking water samples from 59 regions.It was also confirmed that there was a significant positive correlation between Fe concentration and LR.Therefore,it is more suitable to use LR to evaluate the corrosivity of drinking water in China.The health risk model was used to evaluate the risk of metal ions in drinking water to human health.The risk value of carcinogenic heavy metal elements was expressed as Cr>Cd>Sr>Fe>Mn,and Cr metal accounted for 97.1% of the total risk,so Cr should be controlled first.