Iron stability in drinking water distribution systems in a city of China

ISSN 1001-0742Journal of Entironmentud Sciences Vol. 18. No. 1, pp. 40- -46, 2006CN11-2629/XArticle ID: 1001-0742(2006)01 0040-07CLC number: X131.2 Dncunent code.AIron stability in drinking water distribution systems in a city ofChinaNIU Zhang-bin!", WANG Yang', ZHANG Xiao-jian', HE Wen jie2, HAN Hong-da, YIN Pei-jun2(1. Depar(ment of Environmenl Science and Technology, Tsinghua University; Beiing 100084, China. F.-mail: nzb03@xnails.tsinghua.cdu.cn; 2.Tianjin Water Works Limited Company, Tianjin 300000 China)Abstract: A field study on the estimation and analysis of iron stability in drinking water distribution system was carried out in a city ofChina. The stability of iron ion was estimated by pC-pH figure. It was found that iron ion was unstable, with日high Fe (OH),precipitation tendency and obvious incrcase in lurbidity. The outer layer of the corrosion scale was compact, while the inner core wasporous. The main composition of the scale was iron, and the possiblc compound constitutes of the outer scale were ax-FcOOH,y-FcOOH, a-Fe2O, y-Fc2O, FeCl, while the inncr were Fe;O4, FeCl, FeCO, According to the charateristics of the corosion scalc, itwas thought that the main reason for iron instbility was iron release fom corosion scale. Many factors such as pipe materials,dissolved oxygen and chlorine residual afect iron release. Cienerally, higher iron release occurred with lower dissolved oxygen orchlorine residual concentration, while lower iron rclease occurred with higher dissolved oxygen or chlorine residual concentration.The reason was considered that the passivated out layer of scale of fric oxide was broken down by reductive reaction in a conditionof low oxidants concentration, which would result more rapid corrosion of the pipe and red water phenomenon.Keywords: chlorine residual; corrosion scale; dissolved oxygcn; drinking water distribution syslems; iron release; iron stabilityyears, correspondingly, the USEPA estimate of 77.2Introductionbillion USD(McNeill and Edwards, 2001). ComplaintsAlthough many water facilities have adoptedof "red water” at the tap is becoming the mostpretreatment and advanced trcatment technologics totroublesome problem for water utilties all over theimprove drinking water quality, iron, turbidity andworld (Xu and He, 2002; Benjanun, 1996). Thereforebacteria in tap water still cannot rcach thepeople must know the mechanism of iron stability to .requirements, indicating that water in distributionsolve problems in water distribution systems.systcms was biological instability and chemicallron stability is one problem of chemical stabilityinstability (Yuan and Wang, 2003; Wu and Liu, 1998;in distribution systems, including iron pipe corrosion,Xu and He, 2002). Maintaining water quality andcorrosion scale formation and iron rclcase phenomena.keeping water stability in drinking water distributionIt is very complicated because of the involutedsystems is a challenge and is viewed as a new frontierphysical, chemical and biological reactions andby thc water industry. Consequently, many researchesprocesscs and reticular relationship among theon the water quality problems in drinking watercorrosion, scale formation and iron rclcase.distribution systems werc carried out.Iron intability not only obstructs the flow ofThe majority of distribution systems pipes werewater, but also degrades the water quality. Manycomposed of iron material: cast iron (51.67%) andproblems causcd by iron instability are as follows: (1)galvanized stcel (23.85%) in China (Xu et al., 2004),it can form corrosion scalc, increasing head loss andsimilarly, cast iron(38%), ductile iron(22%) and steeldecrcasing water capacity (Larson and Sollo, 1967);(5%) in the USA (Benjanmin, 1996). Moreover, a(2) it can cause iron release from the corrosion scalesurvey in the year of 1997 of the 100 largest waterand severely causc“red water" problem(Sarin, 2001);utilties was found that the most common distribution(3) it can react with dissolved oxygen and chlorinesystem problem is the corrosion of iron material piperesidual in drinking water distribution systems (Sarin(Edwards, 2004). Edwards considered that “con-el al, 2004; Fratueret al., 1999); (4) it can provide antrolling corrosion in drinking water distributionexcellent breeding ground for microorganismsystems is a grand challengc for the 21st century"(Tuovinen et al, 1980; LcChevallieret al, 1993); (5)(McNeill and Edwards, 2001). According to the dataespec中国煤化工ulate arsenic(Ravenprovided by AWWA, 325 billion USD was cost toet al.CNMHG1995).upgrade water distribution systems over the next 20Tl ' Uujcuvc UI UIII stuuy was to estimate theFoundation item: The National Natural Scicnce Foundation of China (No. 50578081) and the Hi-Tech Research and Developneat Program (863) ofChina(No. 2002AA601 140); *Corresponding authorNo.1Iron stability in drinking water distribution systcms in a city of China41iron stability, Lo describe the characteristics 01.2 Sampling and analysis methods for watercorrosion scale and to analyze the infiuence factors onqualityiron release in drinking water distribution systcms in aThe analysis items of water quality includc pH,city of China. The study was conducted using fieldwatcr temperature, turbidity, iron concentration,experiments. Based on the findings the mechanismdissolved oxygen concentration, chlorine residual andand control measurements of iron instability were alsoalkalinity. The analysis methods for each item arediscussed.shown in Table 1.1 Materials and methodsTable 1 Analysis items and methods for water quality1.1 Setting of sampling locationsAnalysis itermsAnalysis methods .As shown in Fig1, the sampling locations werepHB- -4 analysis insrumcntset along a orbicular main lined cast iron pipelineWater tempcrature, CCthanol thermomneler(used about 10 years)， including finished water,TurbidityHach2100 turbidityordinary sampling locations(sampling point I- -7) andanalysis instrumentdead end(sampling point 8- -9). According to the act-ual case in the city, other sampling locations withTotal iron, mg/LButhophenanthroline methodunlined cast iron and PVC pipes were also set.Dissolved oxygen. mg/L.SecnsION analysis instrumentChlorine residlual. myLDPD fitration methodAlkalinity, mgL 2Hydrochloric acid titration methodCaCO,1.3 Sampling, pretreatment and analysis methodsfor corrosion scale1.3.1 Sampling method，■The corrosion scale was obtained from cast ironWater utilitypipe and galvanized steel pipe in water distributionsystems. In course of distribution systems upgrading,Fig.1 Sampling locations in the distribution systemsTable 2 Analysis items and methods for corrosion scaleAnalysis itemsAnalysis methodsAnalysis insrumneniSample kindKYKY-2800 scuningThe outer layer bulk sampleMicrostructureSEMelectron microscopeThe inncr corc bulk samplcSX-[200 X-rayChemical compositionXRFThe powder sanplelluorescence spectroscopeBRUKFR-P4 X-rayThe ouler layer powder sampleCirsalline structureXRD .diffraction spectruscopeThe inner core powder samplePHI- 5300 X-rayThe outer layer powder sampleCompound constituteXPSphoueiectron spectroscopeThe iner core powder samplethe pipes were excavatcd and cut. The pipes were1.3.3 Analysis methodfiled with watcr and sealed in the two ends, thenThe analysis iecms, analysis methods, analysisclean up the extemal mud of the pipe. The pipcs wereinstrument and sample kind are shown in Table 2. Allput into a box flld with cotton and foam in order tothe expcriments were operated in the Analysis Centre,prevent the scale being destroyed.Tsinghua University.1.3.2 Pretreatment methodThe corTosion scale was dried under vacum2 Results and discussioncondition, and four kinds of scale were prepared. The中国煤化工，samples were the outer layer bulk sample, the inner2.1.1core bulk sample, the outer layer powder sample, theMYHC N M H Gntration in Tablc 3,inner corc powder sample. The scale was cut with athe pC-pH figure on F(II) is drawn as Fig.2(Larry etblade, and the bulk samples were prepared. The bulkul，1982). In the figurc， the bound of ironsamples were powdered with a mortar and pestle.concentration and pH of the samples were in the solid42NIU Zhang-bin et al.Vol.18Table 3 lron concentration of the samplesSanpling locations234pH7.968.148.08.057.93Tron concentration, mgL0.120.230.290.250.430.541.060.83Iron conccntration, μmol/L2.144.115.164.467.689.6418.9314.82 .7.6)H.0 t.o-s.0一f(0H)(s)3.0 t.。[Fe(OH);1]0.501.001.50[Fel*] !Iron coDcentration, mg/L12 t(Fe(OH)+-i]\F