Safe and high quality drinking water is an important part of human life.Drinking water saf ety is an important research direction in the field of environmental science and engineering. Like water source protection and water plant treatment, drinking water distribution system is a key link of drinking water safety. There are various types of sedi ments in the pipe network. The presence of sediment s in the pipe network not only directly reduces the water delivery capac ity, but also affects the chemical and biological stability of the water quality, resulting in the decline of water quality at the ta p water , and even sometimes causes problems such as "red water","black water" and other events
This work mainly focused on the accumulation, speciation and risk assessment of metals found in pipe scales and loose deposits. Consensus based sediment quality guidelines (CBSQGs) CBSQGs), risk assessment code (RAC) and global contamination factor (GCF) were adopted for the potential heal th risk assessment of contaminants. The risk assessment method ology and index system of heavy metals in water supply network are esta blished. The objective of this study i s to evaluate the pollutant enrichment capacity of pipe scale and to identify speciat ion changes in heavy metals under variations in water quality. The accumulation/release risk of metals in pipe scales and loose depos its during drinking water distribution system was studied through typical cases. To provide theoretical basis and technical support for drinking water safety assurance based on waterplant pipe network coordination. The main conclusions are as follows:
(1)Through the investigation of the pipe scales and loose deposits in the pipe network, it wa s found that there were many kind s of metals enriched in the pipe network sediments. The metals with the highest content in pipe scales and loose deposits were Fe, fo llowed by Al, Mn and Ba, and some trace heavy metals. The Tessier sequential extraction method was used to study the specia tion distribution of metals in fourteen samples (six pipe scale samples, eight loose deposit samples)collected from real drinking wa ter distribution systems. Mn, Cu, Pb, Zn, Ni, Co and Ba in the samples were mainly associated with the Fe Mn oxides fractio n, which indicated Fe Mn oxides might play an important role in the accumulation and release of these metals . Fe, Al, As, Cr, V and C d mainly existed in the residual fraction, which indicated their low mobility. Morphology and surface areas analytical resu lts showed that the pipe scale samples have porous structures, and the resulting, relatively large,specific surface area (maximum wa s 52.94 m 2 /g, higher than natural adsorbents) might result in the ability of pipe scale samples to adsorb contaminants. XRD results also showed that pipe scale samples were rich in substances with heavy metal adsorption capacities, such as Fe 3 O 4
(2)The I CF evaluation found that Cu in loose deposits had a higher ICF value 6), indicating a high ecological risk. The ecologica l risks of As, V and Co in all samples were relatively low, while heavy metals such As Cd, Cr, Pb and Ni presented moderate risks. GC F evaluation results showed that GCF value of most sampling points was greater than 12, which presented a high ecological r isk. The highest GCF value reached 101.8, which was much higher than the high risk baseline value (24)assessed by GCF. The RAC evaluation results showed that the high risk of pipe scales and loose depos its was related to Cd, and the RAC value of Cd in mos t sampling sites was greater than 30, reaching the high risk level. Ba was of moderate risk in pipe scale samples and of high risk in loose deposits. CBSQGs evaluation results showed that the risk of hea vy metals in loose deposits was higher than that in p ipe scale samples, and Ba was the highest risk among all heavy metals
(3)The maximum adsorption capacity of Pb 2+2+, Cd 2+2+, Cr 6+6+, Ba 2+2+, Cu 2+ and Zn 2+ reached 18.2, 34.9, 94.5, 255.4, 699 and 668.1 μg/g, r espectively. When the pH changed from 6 to 10, the ch anges in amounts of released heavy metals were consistent under different pH conditions, and there was no significant difference. The maximum release amounts of Cu, Cr, As, Pb, Cd were 0.56, 0.51, 1.82, 0.84 and 0.72 μg/g, respectively, under pH=8. Althoug h the amounts were small, the speciation distributions of heavy metals changed, and the proportion of unstable fractions increased, which aggravated the release risk of pipe scale samples. The presence of humic acid a ccelerated the dissolution of organic m atter in the pipe scale samples,which might be combined with metals. This further proved that the pipe scales were unstable and susceptible to water quality conditions, which should be taken seriously.
(4 ) The water quality of rural users under the mode o f integrated urban and rural water supply wa s selected as the research object. The change of eight metals (Al, Fe, Mn, Zn, Cu, Ba, Cr, As) were investigated from the finished water (water treated by drinking water tre atment process before entering distribu tion process) to tap water of 10 water treatment plants in 5 cities of eastern China. The metal concentration in the tap water was mostly higher than the finished water and almost no samples remained constant with fin ished water indicating that the water q uality could deteriorate during DWDS in the rural areas. Both the HQ values and HI values for all studied elements were far below 1, indicating there were negligible non carcinogenic risks for inhabitants (adults and children) due to water intake. Carcinog enic risk values showed that As in studied areas was up to 1.94×10 4 and the value was higher than 1×10 4 .Trace amounts of As could accumulate in drinking water distribution system. There were obvious variations in t he heavy metals between finished water and tap water, and the accumulation and re release effect of metals in drinking water distribution system could not be ignored.