Accumulation And Release Of Contaminants In Pipe Scale Of Drinking Water Distribution System

Urban drinking water safety is one of the most main problems of the people’s livelihood pointed out in the "12th Five Year Development Plan of Science and Technology". With the increase of service life, drinking water pipes aged gradually. Increasing events, such as substandard water quality in residents’taps, red water, black water and so on, revealed the deterioration of the chemical stability of drinking water distribution system (DWDS). To explore the formation of pipe scale, reduce secondary pollution and guarantee drinking water safety, pipes in DWDS serving in City S, Zhejiang Province were sampled, and the accumulation and release of pollutants in pipe scale were studied. On one hand, digging pipes to analyses the concentration and distribution of pollutants in pipe scale of pipes with different materials, age or diameter were determined, and the factors influencing accumulation of pollutants were investigated as well. On the other hand, stagnant pipe reactor were assembled, then the accumulation and release of pollutants experiment were carried to simulate the influence of stagnant on water quality of DWDS, and to investigate the ability of accumulation of pollutants in pipe scale. In addition, biofilms culture test was carried in the overflow pipe reactor system, to explore the biological stability of DWDS brought by nutrient. The main results of this study are as follows:(1) The investigation of 12 sampling points showed that, the quantity, morphology and composition of pipe scale were influenced by pipe material and pipe age. Pipe material was the most important factor, for example, the quantity of pipe scale in the most common used grey cast iron pipe and ductile cast iron pipe (DN150) was 151.5 g·m-1-1950.0 g·m-1 and 7.1 g·m-1-29.4g·m-1, respectively; The concentration of disinfection by-products was very low in pipe scale; The concentration of heavy metals in pipe scale was positively related with the quantity of pipe scale (R2=0.874), and the concentration sequence of metals was Fe>Mn>Al>Zn>Pb>Cu>Cr>Cd; Besides some hydrocarbons produced by microbial metabolism, there were also some microalgae metabolites and exogenous contaminants, such as PAHs.(2) The investigation of two DN600 grey cast iron pipes in main pipeline showed that, the distribution of pollutants had significant radial differences; The pipe scale of the lower part had the maximum amount and average particle size; The spatial distribution of heavy metals was significantly different. The concentrations of manganese, lead, copper and chromium were higher in the upper pipe scale, which indicated that those metals may come from pipe material. For example, the concentration of manganese was up to 11.33 mg·g-1 in the upper part, but 6.79 mg·g-1 in the lower part. The higher concentration of aluminum was detected in the lower part, which was up to 19.79 mg·g-1 in the lower part, but only 9.12 mg-g"1 in the upper part. It was closely related with chemical precipitation; Types of organic pollutants varied in different spatial parts likewise.(3) The simulation experiment about release of pollutants in stagnant pipe reactors showed that, the release process of pipe scale pollutants was quite different in pipes of different material, and related with the concentration of pollutants in pipe scale. The more pollutans in pipe scale, the more and faster it would release. Generally, the release quantity of disinfection by-products and heavy metals would not exceed drinking water standard limit, resulting in insignificant health risk. But the luminescent bacteria toxicity test results showed that there was potential biological toxicity risk in stagnant water.(4) Experiment of accumulation and release of pollutants showed that, the accumulation of pollutants in cast iron pipes increased with the increasing concentration of pollutants in water, and the adsorption and release quantity was obviously different for different pollutants. The accumulation and release rate was faster for disinfection by-products. Pipe scale would accumulate more heavy metals and keep releasing for a long time. The release quantity of heavy metals (Pb, Mn and Zn) were only 2.0%-63.8% of the accumulation quantity, indicating the strong accumulation ability of heavy metals in pipe scale, which may due to some reaction between the metal ions and compounds of pipe scale.(5) Biofilms culture test in overflow pipe reactor system showed that, in biofilms of cast iron pipe, Proteobacteria phylum, Actinobacteria phylum, Firmicutes phylum and Bacteroidetes phylum were the dominant bacterium population. The proportion of these bacteria will change along with the concentration of nutrient, which also resulted in the community structure diversity at phyla level. And the diversity variation trend may be influenced by the concentration of nutrient. Compared with the real pipes, when the concentration of nutrient was C:40 mg·L-1, N:2 mg· L-1,P: 1 mg·L-1, it will promote the diversity of community structure of biofilms. But at higher concentration (C:100 mg·L-1,N:5 mg·L-1 and P:2 mg·L-1), the trend was opposite.