Effect Of Chloride On Heavy Metal Release From Water Distribution System And Control Of Iron Release From Desalinated Seawater

The long-term use of water distribution system results in corrosion and scaling.When the water supply conditions change,the chemical balance between water body and pipe scales are broken,resulting in iron release into the water body,which causes"yellow water"phenomenon in serious cases,and becomes a hidden danger of water quality safety in water distribution systems.In addition,desalinated water has been gradually used in municipal water distribution system in recent years.Due to the characteristics of its treatment technology,the water quality has obvious corrosiveness,which makes desalinated water easy to cause excessive iron release in the transmission and distribution process of existing water water distribution system,and faces great water quality problems.Taking cast iron pipes and steel pipes commonly used in urban water distribution system as the research object,this paper explores the influence of chloride on the physical and chemical characteristics of pipe scales and heavy metal release,and also discusses the iron release problem and control method after desalinated seawater enters the existing water distribution system.1)The physical characteristics,chemical characteristics and microbial composition of the initial scale on the inner wall of cast iron pipes and steel pipes were analyzed.Pipe scale of steel pipe and cast iron pipe is composed of various iron compounds,and the main element is Fe.The binding energy of inner and outer layers of cast iron pipe is larger than that of steel pipe.Proteobacteria is the main flora in the two kinds of pipes.The average richness of Proteobacteria is 92%?steel pipe?and 52%?cast iron pipe?.2)When the concentration of chloride in tap water is increased,Fe₃O₄ does not appear in the inner layer of steel pipe scale,?-FeOOH appeared in the outer layer,and?-FeOOH in the inner layer of cast iron pipe and Fe₃O₄ does not appear in the outer layer.The binding energy of Fe in the inner and outer layers of the two kinds of pipe scales is greater than that before operation.The average richness of Proteus in steel pipe scale decreased from 91.88%to 66.77%.A small number of WS6,TM6,SBR 1093 and Nitrospirae appeared.3)Increasing the concentration of chloride in tap water results in an increase in the release of Fe,Mn,Sr,V,Co,Cr,As and Mo from steel pipe and cast iron pipe.The release of iron from steel pipes is significantly greater than that from cast iron pipe,and chloride affects steel pipes more significantly.Fe²⁺and soluble iron account for about 2%-5%of total iron concentration,indicating that iron release mainly existed in the granular form of trivalent iron.In addition,the release of Fe was positively correlated with the release of Mn,As and Co,and the release of Mn was positively correlated with the release of As,Mo and Co.4)The increase of chloride concentration in desalination water leads to the increase of iron release.Adjusting the pH,alkalinity and phosphate concentration of desalinated water can control iron release in varying degrees.The order of control effect is:increasing alkalinity>adding phosphate inhibitor>adjusting pH value.The optimum conditions of iron release control by response surface model experiment are as follows:cast iron pipe:adjusting pH value to 8.5,adjusting alkalinity to 250 mg/L and adding phosphate to 0.1 mg/L;steel pipe:adjusting pH value to 8.0,adjusting alkalinity to 250 mg/L and adding phosphate to 0.1 mg/L.