Experimental Study On The Effect And Mechanisms Of Water Quality Variations On Iron Release In Water Supply Network

Iron pipes are still prevalent in water supply network,they have varying degreesof corrosion and scaling problems. Corrosion and scaling will reduce the service lifeof the pipe network, increase maintenance costs, increase the pipe resistance therebyincrease energy consumption.However, deterioration of water quality is the moreserious problem, it will cause users’ complaints and affect the users’ healthy.Especially when the water quality in pipe network changed, it will destroy thebalance between the pipe scale and the water, resulting in the release of iron frompipe scales, causing total iron,turbidity, chromaticity and other water qualityindicators exceeded. This research selected galvanized steel pipe from water supplynetwork,studied the effect and mechanisms of quality changes on release of iron fromscales of iron pipes, with hope of providing some guidance and advice when waterpurification process changed or source water switched,in order to control the releaseof iron,avoid the phenomenon of a large area of yellow water.Firstly, author analyzed the physical and chemical properties of iron pipescale,results have shown that the same kind of pipes with similar scale structure:thetop surface layer,the "shell-like" layer,the porous core.The Fe and O are the majorelements in scales, Fe₃O₄and α-FeOOH are the main chemical composition.Moreover, under laboratory conditions, changed the Cl^-, SO₄^2-, free/combinedresidual chlorine concentration, alkalinity and pH artificially to study the effect onrelease of iron, the results are as follows:（1）There is positive correlation between Cl-/SO₄^2-concentration and iron releasefrom pipe scales.However,SO₄^2-is less effective than Cl-on iron release even in thesame Larson ratio. Larson ratio can only reflect qualitative effect of Cl-and SO₄^2-oniron release. So,it can not use Larson ratio to judge the amount of iron released inpractice.（2） Free residual chlorine can cause iron release through oxidate Fe₃O₄, thehigher the free residual chlorine concentration, the higher the total iron content inwater.The total iron content in water are less affected by combined residual chlorine.As free chlorine decay rapidly, the disinfection at the end of the pipe network isineffective,resulting in a large number of iron bacteria and other microorganismsbreeding. Compared the free and combined residual chlorine comprehensively, combined chlorine is more conducive to control the release of iron in the pipenetwork.（3）Added the remineralization process（lime and CO₂coupled） after thedisinfection process can improve the pH and alkalinity, so that can effectively controlrelease of iron, but it may cause the turbidity, chromaticity exceeded. After thealkalinity improved,there could form protective film CaCO₃or generate FeCO₃thenconverted into Fe₃O₄to mitigate the iron release.In the end,the production experiment of remineralization process that control therelease of iron have been done in S waterworks of Shenzhen.The result showed thatadded the remineralization process before flocculation tank, compared with the targetalkalinity,the actual alkalinity of treated water has15-25%loss. Remineralizationprocess added in waterworks can improve the chemical stability of treated water,socan inhibit iron release to some extent.There is little change of water chemicalstability in the pipe network. So,improve the chemical stability of water in the pipenetwork should improve the chemical stability of the treated water.