Study On Chemical Stability And Control Measures Of The Desalinated Seawater In Drinking Water Transmission And Distribution System

Northern China is short of freshwater resources,and desalinated seawater has important strategic significance as an untraditional source water to alleviate the water problem.The desalinated seawater with low pH,low alkalinity,low hardness and high concentration of chloride could make unstability of water quality in drinking water distribution system,and corrode pipes and deteriorate water quality.This is one of the technological problems need to be solved when desalinated seawater enters into drinking water distribution system.This paper mainly focused on the characteristics and control measures of chemical stability of desalinated seawater in different types of water transmission and distribution pipes.Based on pipe section reactors,the effects of three new lining materials pipes on water quality of different desalinated seawater were studied,and the water quality problems and control methods were raised.Comparison of the experiment results of the three pipe materials was a reference for new pipe choose in desalinated seawater distribution project.The effects of elevating pH and alkalinity on iron release in old unlined iron pipes was analyzed,water quality regulation method for desalinated seawater to control iron release was proposed basing on the correlativity between evaluation indicators for water quality and representative indicators for iron release.These were important reference and technical support for safe delivery of desalinated seawater.The main reseach conclusions were listed as follows:The desalinated seawater was serious corrosion to cement-mortar lined ductile iron pipe,and the total alkalinity and total hardness elevated greatly,so the desalinated seawater should be remineralized to pH>7.8,total alkalinity and total hardness above55 mg/L,and Langelier Saturation Index?LSI??0.Some chemical indexes of the experimental water in the reactors were high in earlier few days of the experiment,and total aluminum always exceeded 0.2 mg/L substantially,and the concentration of total iron fluctuated around 0.3 mg/L,and the concentration of total organic carbon?TOC?was safe.The new cement-mortar lined pipe need to be rinsed for about 5 days to stabilize the water before the practical application to desalinated seawater.The pH value could be used to mesure whether the water quality was stable.The pH value of the experimental water in epoxy lined ductile iron pipes exceeded the limit of 8.5 for 5 days.The concentration of TOC and UV₂₅₄ value rised substantially,and more organics were detected in pipe with NaClO disinfectant than that in pipe without disinfectant.The detected indicators of the experimental water in polyurethane lined ductile iron pipes were all under limits.Though the concentration of TOC from the two types of organic lining material pipes was under the limit of 5mg/L,the concentration did not decrease during the experiment.So the organics from the lining materials deserves concern.It is suggested that the two types of pipes need to be rinsed for about 1-2 days before the practical application.Only considering the effects of lining materials on water quality change of desalinated seawater,the order of priority for pipes was:polyurethane lined pipe>epoxy lined pipe>cement-mortar lined pipe.The size of the pipes used in the experiment is different from that of pipes in practice,and the chemical indexes can be calculated by the ratio between pipe diameters to supply reference for practical water transmission and distribution works.Two groups of old unlined iron pipes were used to study the water quality stability of desalinated seawater.The scale of iron pipe B was more developed and more porous than that of pipe A,so the red water problem of pipe B was worse than that of pipe A.Elevating pH and alkilinity both could restrained iron release:for pipe A,the desalinated seawater should be regulated to pH value above 7.8,total alkalinity above100mg/L,and the Larson Ratio Index?LRI??1.3 to control iron release;the control parameters for pipe B was pH value above 7.8,total alkalinity above 120 mg/L,and LRI?1.Therefore,the criterion of LRI<1 could be met to control the risk of iron release of desalinated seawater in old iron pipes.