Research On Water Quality Stability Of Drinking Water Pipes In A Mountainous City

Water stability in drinking water systems had great influence on tap water quality.However, the corrosion property of drinking water was one of the important factors thataffected water stability. In this paper, a350mm diameter,28year-old unlined cast ironpipe, a250mm diameter,15year-old unlined steel pipe and a new400mm diametercement-lined ductile iron pipe were collected and tested in order to research corrosioncharacteristics of water supply pipes in mountainous cities, to find the variation of ironrelease and water quality stability in the pipe reactors made from the pipes mentionedabove.According to the test results of corrosion scale from cast iron pipe and steel pipe,the structures and compositions of these corrosion scales were almost similar. Typicaltubercles were semi-uniform, and their cross-sections presented significant stratification.The outer layer loosely attached to the middle layer, with less than1mm thickness; themiddle layer called “shell-like” layer appeared dark black, about0.7mm thickness, withfirm and tight texture; the interior layer took much more volume, presenting porous andloose. Fe and O were the main elements of the tubercles, accounting for about90%. Aspot of Al, Si, Cl and Ca can also be detected in the corrosion scales. The maincomponents of tubercles in cast iron pipe were α-FeOOH, Fe₃O₄,γ-FeOOH and Fe（OH）₃and in steel pipe were α-FeOOH, Fe₃O₄,γ-FeOOH and a small amounts of CaCO₃.The variations of iron release, chemical stability and biological stability underdifferent influent flow conditions were observed in the pipe reactors. The results were asfollows:①Iron release was greatly affected by pipe material and retention time.Cement-lined ductile iron pipe can effectively prevent iron release. In both corrodedcast iron pipe reactor and corroded steel reactor, iron concentrations increased withretention time. Higher initial pH can lead to slower iron release. Compared with initialresidual chlorine concentrations at0.35mg/L and1.24mg/L，iron released slower whenat0.62mg/L and1.01mg/L. There was an apparent linear correlation between ironconcentration and turbidity, which can come to the conclusion that iron release incorroded cast iron pipe existed mainly in the form of suspended iron.②With retention time increasing, the bulk water in the pipe reactors presented less corrosive and more scaling tendency. There was a positive correlation between pHand IL, but a negative correlation between pH and IR. The bulk water with higher initialresidual chlorine concentration presented less corrosive tendency and better chemicalstability relatively.Properly increasing pH and initial residual chlorine would bebeneficial to maintain stabler pipe network water. Pipe materials had influence onchemical stability. The bulk water in the new cement-lined ductile iron pipe reactorshowed strongest corrosive, then was the corroded steel pipe reactor, and the last wasthe corroded cast iron pipe reactor.③The BDOC regularity decay of water supply system in the mountainous cityduring28d can be expressed as BDOCt=1.798×（1-10-0.077t）. Both theoretical values andmeasured values of BDOC₃/BDOC₂₈varied between38%⁴6%, which indicated thatBDOC₂₈ of the water supply system can be replaced by BDOC₃in the experimentalstudy. Rapid BDOC consumption appeared in the early reaction period. About20hourslater, BDOC showed a little fluctuation and changed slowly. Properly higher pH wouldbe beneficial to biological stability of drinking water. During the early reaction period,higher initial residual chlorine would result in rapider BDOC consumption. However, inthe late reaction, BDOC of the four experiments with different initial residual chlorineconcentrations appeared little difference. Pipe materials had great influence onbiological stability: the slowest residual chlorine decay can be observed in thecement-lined ductile iron pipe, resulring in the lowest BDOC and slowest changes, andthe water quality in corroded cast iron pipe reactor showed relatively poor biologicalstability.