Characteristics of pipe-scale in the pipes of an urban drinking water distribution system in eastern China

Abstract

Pipes that carry drinking water have gradually aged. Events occurring with increasing frequency, such as substandard water quality in residential taps, red water, and black water, reveal the deterioration of the chemical stability of a drinking water distribution system (DWDS). Pipes in the DWDS serving City S, located in eastern China, were sampled to analyze the concentration and distribution of pollutants in pipe-scale of pipes of different materials, ages and diameters, and the factors (such as materials, age, and diameter) influencing the accumulation of pollutants were also investigated. The quantity of pipe-scale in the most commonly used gray cast iron pipe and ductile cast iron pipe (DN150) was 151.5-195.0 g·m-1 and 7.1-29.4 g·m-1, respectively. The concentration of heavy metals in pipe-scale was positively correlated with the quantity of pipe-scale (R2=0.874); the sequence of concentration of metals was Fe>Al >Mn, Zn >Pb, Cu>Cr, Cd. Galvanized steel pipe, with the highest degree of corrosion, had the highest concentration of heavy metals in pipe-scale. The morphology and composition of pipe-scale were substantially influenced by pipe material and age. For example, in the oldest galvanized steel pipe-scale, there was not only a large number of iron compounds but also some zinc composite oxides. In addition to hydrocarbons produced by microbial metabolism, there were microalgae metabolites and exogenous contaminants such as polycyclic aromatic hydrocarbons (PAHs). The concentrations of microbial metabolites increased with increasing service time.