A Study On Water Quality Change In Reclaimed Water Distribution System With Different Pipes

Water shortage is one of the most serious resource problems nowadays.Reclaimed water,as a new resource to alleviate water shortage,is becoming more and more important.Reclaimed water produced by reverse osmosis process is pure,it has a strong corrosive effect to pipes because of its purity,which will affect water quality,and have an adverse effect on stability,security,and serviceability of reclaimed water quality.A comprehensive study on quality variation of reclaimed water in pipe network will help guarantee safety of reclaimed water quality.In this study,a large-scale circulation pipe system was built,where variation pattern and influencing factors of reclaimed water quality produced by reverse osmosis process distributed in cast iron pipe,stainless steel pipe,PVC pipe,PE pipe,cement mortar lining pipe were studied,mainly focused on the variation of pH,TDS,residual chlorine,dissolved oxygen,and ion concentration.And impact of pH,flow rate,alkalinity,hardness on reclaimed water quality variationwere also studied.Besides,a chlorine decay model for each pipe type,iron andmanganese precipitation model for cast iron pipe,calcium and silicon precipitation model for cement pipes were built in the research.The study showedwhen water distributed in cast iron pipe,the pipe wall was corroded seriously,and the water qualitydeteriorated rapidly,turned into red water.Besides,residual chlorine decayed rapidly,releasing a large amount of iron and manganese.For stainless steel pipe,PVC pipe,and PE pipe,the water quality was stable.But when water distributed in cement pipe,pH,TDS,and concentrations of calcium and siliconwere significantly increased.The residual chlorine decay rate of each pipe ranked as follows: Cast iron pipe>> cement pipe> stainless steel pipe?PVC pipe?PE pipe.Increasing pH canreduce the increase of TDS and control release of iron ion and manganese ion in cast iron pipe.For cement pipes,increasing pH can reduce calcium ion releasing,but had little impact on silicon ion.Increased flow rate can help increase pH,TDS,iron and manganese concentrations in cast iron pipe,but had little effect with cement pipe.Increasing alkalinity within a certain extent can effectively reduce release of metal ions in cast iron pipe,however,when increased more,it wouldadversely promote the release of metal ions.And increasing alkalinity can also decrease releasing of calcium and silicon in cement pipe.When increasing hardness,the releasing of metal ions in cast iron pipe can be controlled,but there was no obvious changein cement pipe.For stainless steel pipe water and plastic pipe water,the changes of pH,flow rate,alkalinity and hardness won't affect the water quality.In stainless steel pipe,plastic pipe and cement pipe,the residual chlorine decay pattern fitted tothe second-order model.In cast iron pipe,iron and manganese concentrations can also be well simulated by a statistical model,with taking pH,alkalinity and hardness etc.into account.In cement pipe,release pattern of calcium and siliconcomplied with the solution-diffusion model,andrelease process can be well simulated by the mathematic model based on Fick's Law.