Study On Iron Release In Drinking Water Distribution Systems Under The Variations Of Flow Rate And Water Quality

At present,steel and cast iron pipes are still the most widely used in drinking water distribution systems（DWDS）.During the long-term use of iron pipes,corrosion occur and corrosion products accumulate on the pipe wall.When water quality or hydraulic conditions change suddenly,the steady state between the corrosion scales and water will be destroyed,leading to iron release and secondary pollution of drinking water.In this paper,the effects of p H,alkalinity,sulfate(SO₄^2-)and flow rate on iron release from water distribution systems were investigated,and a multivariate polynomial regression model was established.This study aims to provide scientific basis for the prevention and control of iron pollution in DWDS,the main conclusions are as follows:（1）The initial corrosion scales in steel pipes were spherical,yellow-brown and loose,while that in cast iron pipes were brown nodular and thick.After experiments,the corrosion scales were more loose under stagnant conditions.Under the flow conditions,the corrosion scales had obvious layered structures.（2）The initial corrosion scales in both pipes were composed of several elements,and the contents of iron（Fe）are highest.In steel pipes,the content of silicon（Si）was higher,whereas in cast iron pipes,the content of aluminum（Al）was higher.After experiments,under stagnant and flow conditions,the proportions of Fe in steel and cast iron pipes were increased significantly.Moreover,the proportions of calcium（Ca）and Si were increased significantly in steel pipes and cast iron pipes,respectively.（3）The initial corrosion scales of steel and cast iron pipes were mainly composed of goethite（α-Fe OOH）,fibrous iron（γ-Fe OOH）and magnetite（Fe₃O₄）.After experiments,the peak of calcite（Ca CO₃）in steel pipes were increased,and quartz（Si O₂）was produced in cast iron pipes.（4）Under stagnant conditions,the total Fe concentrations in the effluents of steel and cast iron pipes were increased during the first 10-15 days,and then gradually decreased and stabilized at a stable level.When p H=6.5,alkalinity=200 mg/L（calculated by Ca CO₃）,and SO₄^2-=250 mg/L,the total Fe concentration in steel pipes was the maximum（2.22 mg/L）.When p H=7.5,alkalinity=100 mg/L Ca CO₃,and SO₄^2-=250 mg/L,the total Fe concentration in the cast iron pipes was highest（2.19 mg/L）.Under the same conditions,the total Fe concentrations in steel pipes were higher than that in cast iron pipes.Furthermore,the established multivariate polynomial regression model of total Fe showed that the influences of the three parameters on total Fe concentrations in steel pipes followed the order:p H>SO₄^2->alkalinity,while in cast iron pipes,that followed:SO₄^2->p H>alkalinity.（5）Under flow conditions,the total Fe concentrations in steel and cast iron pipes were continuously increased,and the total Fe concentrations in steel pipes were much higher than that in cast iron pipes.When p H=7.5,alkalinity=100 mg/L Ca CO₃,SO₄^2-=250mg/L,and flow rate=0.12 m/s,the total Fe concentration in both pipes were the highest,which were 2.17 mg/L and 0.59 mg/L,respectively.In steel pipes,p H,alkalinity,SO₄^2-,and flow rate had significant effects on the total Fe concentration.The order of the influences was:alkalinity>flow rate>SO₄^2->p H.However,in cast iron pipe,the order of was:p H>alkalinity>SO₄^2->flow rate.