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.

INTRODUCTION

The pipeline is one of most important parts of an urban drinking water distribution system (DWDS). Although many treatment plants keep improving water treatment processes to guarantee water quality, water at customers' taps is usually worse than that at the treatment plant owing to contamination during network transmission. In addition to serious pollution effects, such as ‘red water’ or ‘colored water,’ drinking water may contain high levels of turbidity, excessive amounts of iron and manganese, excessive bacteria, and other contaminants (Cerrato et al. 2006; Yang et al. 2012). In many cases, these are caused by the pipe-scale in the DWDS.

Drinking water from a treatment plant enters a DWDS with a physical load (particles), a chemical load (nutrients, heavy metal salts and organic matter) (Schock et al. 2008; Peng et al. 2010), and a microbial load (Liu et al. 2013), which gradually accelerate the formation of pipe-scale. Drinking water is usually delivered from the treatment plant to the client through tens or even hundreds of kilometres of pipeline. A series of complex reactions occur during this process, such as nitrification, corrosion (Regan et al. 2003; Renner 2008), and biofilm formation (Schwartz et al. 2003). In addition, trace heavy metal ions and organic matter in bulk water interact with pipe-scale and then gradually accumulate in the pipe, constantly changing the composition of the pipe-scale (Yu et al. 2010; Gonzalez et al. 2013) Thus, pollutants in pipe-scale are complicated and diverse, comprising a variety of metal oxides, salts and other inorganic matter, disinfection byproducts, microbial metabolites and other organic matter, as well as pipe biofilm.

Pipe-scale is widely present in all drinking water supply networks and may cause serious harm (Boulay & Edwards 2001; Edwards & Sprague 2001; Lytle et al. 2005; Calle et al. 2007; Copeland et al. 2007; Renner 2008; Kim & Herrera 2010; Xie & Giammar 2011; Benson et al. 2012; Świetlik et al. 2012; Gonzalez et al. 2013). A greater degree of corrosion can cause network damage and a reduction in the pipes' service life. When disturbed by events such as water quality changes, pipe breaks, and fire-fighting, pipe-scale can release metallic contaminants, reducing the quality of drinking water. Many metallic contaminants, including arsenic, lead, cadmium, mercury and uranium-based metals, are toxic to humans and other organisms (Domingo 1994; Valko et al. 2005; Beech et al. 2014). Any of these factors can cause water quality to fail to meet standards during water distribution.

Corrosion of pipes and growth of biofilm usually vary widely with differences in pipe material and age and bulk water properties, and the composition of pipe-scale also varies (Lee et al. 1980; Stanimirova et al. 2008; Ray et al. 2010; Pinto et al. 2012). In particular, the pipe material affects the composition of pipe-scale. Many studies have revealed that pipe material may dominate the composition of metallic compounds in pipe-scale. For example, the typical composition of iron corrosion products in cast iron pipes includes the oxy-hydroxides goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), magnetite (Fe3O4), hematite (Fe2O3), ferrous hydroxide (Fe(OH)2), ferric hydroxide (Fe(OH)3), and siderite (FeCO3) (Sarin et al. 2004, Wang et al. 2012). In addition, quartz (SiO2), calcite (CaCO3) and other non-iron mineral components have frequently been detected (Peng et al. 2010). The most commonly detected copper compounds are cuprite (Cu2O), tenorite (CuO), and malachite (CuCO3·Cu(OH)2) (Merkel & Pehkonen 2006). These compounds store labile copper until it is released by a change in conditions (Calle et al. 2007). In PVC pipe, Cerrato found that manganese compounds accumulated substantially more than in cast iron pipes (Cerrato et al. 2006). Most of the lead compounds present in DWDSs are from a variety of sources, including lead pipes, tin-coated pipes, galvanized steel pipes, lead-soldered joints and fittings, brass materials, and cement-mortar lining. The corrosion products that normally form inside pipes are crosstie (PbCO3), hydrocerussite (Pb3(CO3)2(OH)2), plumbonacrite (Pb10(CO3)6(OH)6O), litharge (PbO), and plattnerite (PbO2) (Lee et al. 1989; Kim & Herrera 2010). Galvanized steel pipe will usually accumulate zinc compounds (Gonzalez et al. 2013).

Organic pollutants not only affect the taste of drinking water but also accumulate in the pipe-scale, threatening water quality. Worse, in many cases they become nutrients for pipeline biofilms. Because of the limitations of trace analysis technology and methods of organic intermediates, there have been very few studies of organic pollutants in pipe-scale. Only a few preliminary reports on the types of organic pollutants have been published. These reports categorized organic pollutants into four sources: source water (Matilainen et al. 2010), pipe materials (Skjevrak et al. 2003; Durand & Dietrich 2005; Whelton & Nguyen 2013), metabolites of pipeline organisms (Skjevrak et al. 2004; Höckelmann & Jüttner 2005; Skjevrak et al. 2005; Beale et al. 2013) and disinfection by-products.

In contrast to developed countries, urban DWDSs in China did not come into wide service until the 1980s. At present in China, cast iron pipes account for 68% of these systems. Gray cast iron pipes, which are weakly resistant to corrosion, account for 51%; in some cities the figure is as high as 90%. Additionally, 85% of newly built pipelines still use metal pipes.

It was usually difficult to carry out systematic research of Chinese urban DWDSs, so much of the pipe-scale was collected from retired pipes, burst pipes, and simulation pipes (Yang et al. 2012). With the help of several social sectors of City S, located in eastern China, pipes in the DWDS serving the city were sampled to analyze the concentration and distribution of pollutants in the pipe-scale of pipes of different materials, ages and diameters, and the factors influencing the accumulation of pollutants were also investigated.

MATERIALS AND METHODS

Sampling sites

At one time, the source water quality of the DWDS of City S was the highest in China. However, with increased development of the DWDS, the pipeline became complex and diverse (Figure 1). The water source was a natural river before 2002, when it switched to a reservoir. The flow velocity was low throughout the year. All the target pipes were serving the area without bends in the pipeline to ensure consistent hydraulic conditions. Additional information on the pipelines and water quality is given in Tables 1 and 2.
Table 1

Parameters of pipes

Sample IDPipe materialPipe age (years)Pipe diameter (mm)Flow velocity (m·s−1)DateQuantity of pipe-scale (g·m−1)
GSP1 Galvanized steel pipe 12 DN80 <0.1 2013.09.29 16.0 
GSP2  17 DN80 <0.1 2014.03.11 262.9 
GSP3  23 DN80 <0.1 2013.04.03 47.8 
DCIP1 Ductile cast iron pipe DN150 <0.1 2013.10.15 28.0 
DCIP2  DN150 <0.1 2014.03.13 7.1 
DCIP3  DN150 <0.1 2014.03.18 8.0 
DCIP4  11 DN150 <0.1 2013.09.25 29.4 
GCIP1 Gray cast iron pipe 11 DN150 <0.1 2013.10.22 170.9 
GCIP2  17 DN150 <0.1 2013.10.29 195.0 
GCIP3  23 DN150 <0.1 2013.04.19 151.5 
PVCP PVC pipe 13 DN100 <0.1 2013.09.17 0.9 
SSP Stainless steel pipe 10 DN80 <0.1 2013.10.10 2.8 
Sample IDPipe materialPipe age (years)Pipe diameter (mm)Flow velocity (m·s−1)DateQuantity of pipe-scale (g·m−1)
GSP1 Galvanized steel pipe 12 DN80 <0.1 2013.09.29 16.0 
GSP2  17 DN80 <0.1 2014.03.11 262.9 
GSP3  23 DN80 <0.1 2013.04.03 47.8 
DCIP1 Ductile cast iron pipe DN150 <0.1 2013.10.15 28.0 
DCIP2  DN150 <0.1 2014.03.13 7.1 
DCIP3  DN150 <0.1 2014.03.18 8.0 
DCIP4  11 DN150 <0.1 2013.09.25 29.4 
GCIP1 Gray cast iron pipe 11 DN150 <0.1 2013.10.22 170.9 
GCIP2  17 DN150 <0.1 2013.10.29 195.0 
GCIP3  23 DN150 <0.1 2013.04.19 151.5 
PVCP PVC pipe 13 DN100 <0.1 2013.09.17 0.9 
SSP Stainless steel pipe 10 DN80 <0.1 2013.10.10 2.8 
Table 2

Water quality parameters of source water and treated water

ParameterSource waterTreated water
pH 6.7–7.1 7.1–7.5 
Turbidity (NTU) 0.94–2.77 <0.10 
TDS (mg/L) 59–77 80–94 
Alkalinity (mg CaCO3/L) 14.0–22.4 22.1–28.5 
Hardness (mg CaCO3/L) 25.1–34.4 40.1–44.6 
Total iron (mg/L) 0.024–0.056 <0.010 
Cl (mg/L) 2.8–4.2 6.2–8.0 
SO42− (mg/L) 12.6–15.5 12.3–15.6 
TOC (mg/L) 1.5–3.4 0.7–1.9 
DO (mg/L) 5.6–12.2 6.9–10.1 
ParameterSource waterTreated water
pH 6.7–7.1 7.1–7.5 
Turbidity (NTU) 0.94–2.77 <0.10 
TDS (mg/L) 59–77 80–94 
Alkalinity (mg CaCO3/L) 14.0–22.4 22.1–28.5 
Hardness (mg CaCO3/L) 25.1–34.4 40.1–44.6 
Total iron (mg/L) 0.024–0.056 <0.010 
Cl (mg/L) 2.8–4.2 6.2–8.0 
SO42− (mg/L) 12.6–15.5 12.3–15.6 
TOC (mg/L) 1.5–3.4 0.7–1.9 
DO (mg/L) 5.6–12.2 6.9–10.1 

TDS: total dissolved solids; TOC: total organic carbon; DO: dissolved oxygen.

Figure 1

The DWDS of City S.

Figure 1

The DWDS of City S.

Sampling method

Twelve pipe samples were taken after 10:00 pm to avoid the influence of sunshine and high temperature. Pipe-scale was then collected, put into vials with Milli-Q water and refrigerated at 4 °C. All turbid liquid of pipe-scale was carefully metered in the laboratory. The liquid was centrifuged at 4,500 rpm for 10 minutes by a super-centrifuge (Thermo Sorvall RC+ centrifuge). The supernatant was collected and weighed using a sensitive balance (Mettler-Toledo AL204).

Analysis method

Analysis of metals

Turbid liquid of pipe-scale was put into Petri dishes, air-dried, and then ground to pass through a 100 mesh sieve. Approximately 0.2 g of air-dried scale was weighed using the sensitive balance (Mettler-Toledo AL204). Samples were acid-digested following a method (US EPA 3050B) based on microwave digestion (CEM MARS 5). An atomic absorption spectrometer was used for analysis (Thermo MKILM6) of metals, including Fe, Mn, Zn, Pb, Cu, Cr and Cd. Al content was determined via chrome azurol S complex formation (living standards for drinking water testing methods GB/T5750-2006) and UV-VIS spectrophotometry (UNICO UV-2100).

Analysis of organic pollutants

Wet pipe-scale samples were freeze-dried. The extraction steps for the freeze-dried solid samples were as follows. (1) Approximately 1 g of solid sample was placed in a 40-mL vial, then 5 g baked sodium sulfate and 30 mL methylene chloride (DCM) were added (2). The top of the vial was cleaned and sealed with a PTFE-sealed cap, shaken to test for leaks, and then carefully loaded in a shaker and shaken overnight at 150 rpm. (3) After being centrifuged at 1,200 rpm for 30 minutes, 1 mL liquid was removed for gas chromatography–mass spectrometry (GC-MS) analysis. (4) The concentrated solution was analyzed by the Agilent GC-MS. (5) The above four steps were repeated by using the mixed solvent acetone/hexane (1:1).

Semivolatile organic compound (SVOC) composition was analyzed using GC-MS (Agilent Technologies, Wilmington, DE, USA). Chromatography separations of samples were performed on an HP-5 ms fused silica capillary column (30 m × 250 μm i.d., 0.25 μm film thickness). The oven temperature was initially held at 40 °C for 1 min, then raised 15 °C min−1 to 120 °C and held for 1 min, followed by a second heating at 10 °C min−1 to 290 °C and held for 3 min. The helium gas flow rate of the column was 1.0 mL/min, and the temperature of the injector was maintained at 250 °C with a splitless mode and 1-µL injection volume of the samples. All analyses were performed in the SCAN mode of the MS, and the unknown organic contaminant was identified using the NIST 08 mass spectral library.

Characterization of pipe-scale

The crystal structure of pipe-scale was analyzed using an X-ray diffractometer (XRD) (Bruker-P4). The operation parameters for XRD were as follows: Cu Kα radiation at 40 kV and 100 mA and a 2θ range from 5 to 90 °, with a 0.02 ° step and a 0.15-s count time at each step. Energy-dispersive spectrometer (EDS) analysis was conducted using a scanning electron microscope (JEM-5610LV).

RESULTS AND DISCUSSION

Total quantity of pipe-scale

The inner surface of the gray cast iron pipe (GCIP1-GCIP3) was more seriously corroded than that of other types of pipe, with a much larger tubercule (Figure 2). Therefore, the total amount of pipe-scale was higher: up to 195.0 g·m−1 (GCIP3) Figure 3. However, in the ductile cast iron pipe (DCIP1-DCIP4), there was almost no observable rust, and the inner protection liner was less corroded. Thus, the total amount of pipe-scale was smaller (7.1–29.4 g·m−1). Not surprisingly, the likelihood of corrosion varied with the pipe material, and more pipe-scale accumulated in seriously corroded pipes. The result was similar in the other pipes.
Figure 2

Inner surfaces of various pipes before and after sampling.

Figure 2

Inner surfaces of various pipes before and after sampling.

Figure 3

Quantity of pipe-scale in pipes of different age and material.

Figure 3

Quantity of pipe-scale in pipes of different age and material.

In theory, the older the pipe was, the greater would be the probability of severe corrosion, resulting in a larger amount of pipe-scale. However, this was not the case. We found no clear correlation between content of pollutants in pipe-scale and pipe age. The differences may have resulted from the electrochemical corrosion of different pipes (Lytle et al. 2004).

Heavy metals

Table 3 shows that, regardless of the pipe material, the content of eight heavy metals in pipe-scale was in the following order: Fe > Al > Mn, Zn > Pb, Cu > Cr, Cd. The concentration of Fe was the highest, ranging from 5.159 mg·g−1 to 690.858 mg·g−1. This is because iron water supply pipes are the most common pipes in the DWDS (Gonzalez et al. 2013). In the gray cast iron pipe, which has more rust than the other types, the concentration of Fe in pipe-scale was much higher than that in the ductile cast iron pipe, which was another indication that the Fe in pipe-scale was mainly from corrosion (Lytle et al. 2005; Copeland et al. 2007). The 17-year-old DN80 galvanized steel pipe (GSP2) had by far the highest concentration of Fe in pipe-scale, reaching 690.858 mg·g−1. This pipe was characterized by a long period of use, a very high degree of corrosion, and pipe tubercule that almost blocked the pipes. In addition to the PVCPs (5.159 mg·g−1), several samples of ductile cast iron pipes had a low concentration of Fe. The common feature was the smooth lining material and a shorter period of use. The concentration of Fe in the SSP was only 23.395 mg·g−1, owing to the greater corrosion resistance of its inner surface.

Table 3

Concentrations of heavy metals in pipe-scale per gram

Sample IDPipe age (years)Pipe diameterFe (mg·g−1)Zn (mg·g−1)Mn (mg·g−1)Pb (mg·g−1)Cu (mg·g−1)Cr (mg·g−1)Cd (mg·g−1)Al (mg·g−1)
GSP1 12 DN80 46.908 23.284 4.012 2.599 0.184 0.024 0.406 10.656 
GSP2 17 DN80 690.858 2.613 4.355 0.524 0.097 0.354 <0.001 1.308 
GSP3 23 DN80 109.347 0.561 1.993 0.073 0.027 0.027 <0.001 1.885 
DCIP1 DN150 22.564 1.235 19.518 0.851 0.109 0.008 0.002 3.263 
DCIP2 DN150 13.243 0.917 0.266 0.100 0.020 0.020 0.007 0.385 
DCIP3 DN150 20.881 0.229 0.041 0.065 0.065 0.076 0.006 0.564 
DCIP4 11 DN150 47.208 3.669 25.025 1.157 0.385 0.034 0.002 15.825 
GCIP1 11 DN150 225.277 0.675 11.497 0.706 0.104 0.011 0.002 11.390 
GCIP2 17 DN150 258.897 2.947 32.036 1.285 0.358 0.060 0.010 19.249 
GCIP3 23 DN150 21.444 1.194 2.945 0.053 0.006 0.094 0.002 0.153 
PVCP 13 DN100 5.159 1.873 0.671 0.141 0.035 0.035 0.035 0.106 
SSP 10 DN80 23.395 6.561 2.935 0.168 0.203 0.097 0.009 1.874 
Sample IDPipe age (years)Pipe diameterFe (mg·g−1)Zn (mg·g−1)Mn (mg·g−1)Pb (mg·g−1)Cu (mg·g−1)Cr (mg·g−1)Cd (mg·g−1)Al (mg·g−1)
GSP1 12 DN80 46.908 23.284 4.012 2.599 0.184 0.024 0.406 10.656 
GSP2 17 DN80 690.858 2.613 4.355 0.524 0.097 0.354 <0.001 1.308 
GSP3 23 DN80 109.347 0.561 1.993 0.073 0.027 0.027 <0.001 1.885 
DCIP1 DN150 22.564 1.235 19.518 0.851 0.109 0.008 0.002 3.263 
DCIP2 DN150 13.243 0.917 0.266 0.100 0.020 0.020 0.007 0.385 
DCIP3 DN150 20.881 0.229 0.041 0.065 0.065 0.076 0.006 0.564 
DCIP4 11 DN150 47.208 3.669 25.025 1.157 0.385 0.034 0.002 15.825 
GCIP1 11 DN150 225.277 0.675 11.497 0.706 0.104 0.011 0.002 11.390 
GCIP2 17 DN150 258.897 2.947 32.036 1.285 0.358 0.060 0.010 19.249 
GCIP3 23 DN150 21.444 1.194 2.945 0.053 0.006 0.094 0.002 0.153 
PVCP 13 DN100 5.159 1.873 0.671 0.141 0.035 0.035 0.035 0.106 
SSP 10 DN80 23.395 6.561 2.935 0.168 0.203 0.097 0.009 1.874 

Because it is a common element in iron pipes (Lytle et al. 2004; Cerrato et al. 2006), the concentration of Mn was also relatively high. In pipes of different materials, the concentration of Mn in pipe-scale varied greatly. In the cast iron pipe-scale, the concentration of Mn was as much as 32.036 mg·g−1 (GCIP2), compared with only 0.671 mg·g−1 in the PVC pipe-scale (PVCP). The concentration of Mn was not as high as in a previous analysis of PVC pipes (Cerrato et al. 2006), indicating a trace amount of Mn in the source water of City S.

Flocculation of aluminum compounds in treated water would result in relatively high concentrations of Al in pipe-scale (Ona-Nguema et al. 2002, Huck & Gagnon 2004), especially in cast iron pipes. In the 17-year-old DN150 gray cast iron pipe (GCIP2), the Al concentration was up to 19.249 mg· g−1.

In China, lead pipes and copper pipes have seldom been used; therefore, the concentrations of Pb and Cu in pipe-scale were very low. The concentration of Cr was less than 0.1 mg·g−1in all samples except for GSP2.

Overall, the pipes with a high degree of corrosion and a long service time had more pipe-scale, and the concentrations of Fe, Al, Zn, Mn, Pb, and Cu in the pipe-scale were higher (R2 = 0.874). This indicates that metal pollutants in pipe-scale are a reaction product of precipitation accumulation and corrosion of pipes.

Crystal structure of pipe-scale

Hydrotalcite-like compounds containing zinc aluminum and sodium aluminum, as well as aluminum phosphate (AlPO4) and goethite (α-FeOOH), were observed in galvanized steel pipe-scale (Figure 4). The EDS data also showed the presence of Zn, which was consistent with the results shown in Table 3. As service time increases, a pipe's inner surface will form iron oxide (Sarin et al. 2004, Wang et al. 2012) and the inner liner may fall off. A pipe may then tend to form many iron oxides and metal composite oxides.
Figure 4

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from galvanized pipes.

Figure 4

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from galvanized pipes.

Pipe-scale from gray cast iron pipe that had not been long in service was composed mainly of quartz (SiO2), goethite (α-FeOOH) and magnetite (Fe3O4) and other iron-containing metal compounds (Figure 5). Samples from older pipe had composite oxides containing calcium, iron, aluminum and magnesium, magnesium hydroxide and other composite silicon metal compounds.
Figure 5

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from ductile cast iron pipes.

Figure 5

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from ductile cast iron pipes.

There were more iron-containing metal compounds in gray cast iron pipe-scale than in ductile iron pipe-scale. The XRD patterns and EDS patterns (Figure 6) of ductile iron pipe-scale showed that differences in the components of the four samples were small, perhaps because of little difference in service life, an undamaged lining, and almost no rust. Only calcite (CaCO3) and quartz (SiO2) were observed in ductile cast iron pipe-scale.
Figure 6

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from gray cast iron pipes.

Figure 6

X-ray diffraction patterns and energy-dispersive spectroscopy of pipe-scale from gray cast iron pipes.

In general, pipe material and age impacted the crystal structure of pipe-scale. Because of the undamaged lining and the similar pipe age, the differences in the composition of ductile cast iron pipe-scale were not significant. However, in galvanized pipes and gray cast iron pipes, the older the pipe was, the more complex was the composition of the pipe-scale. We conclude that, after a pipe has been in service for several years, the composition of its pipe-scale begins to change. With increasing service time, this difference becomes more pronounced.

Species of organic pollutants

Table 4 lists 40 types of organic pollutants in pipe-scale that were identified in our samples based on the best match with the MS library. A large number of organic pollutants that could not be identified are not included. In addition to some common types of hydrocarbons, such as octadecane, hexadecane, heptadecane and n-eicosane, which are formed by microbial metabolism, there were also some microalgae metabolites, such as oleamide (Bertin et al. 2012) and octadecenamide (Dembitsky et al. 2000). In addition, suspected exogenous polycyclic aromatic hydrocarbons (PAHs) (Peng et al. 2010), such as anthracene, phenanthrene, pyrene, and benzopyrene, were detected.

Table 4

Organic pollutants in pipe-scale

 Chemical nameCAS no.GSP1GSP2GSP3DCIP1DCIP2DCIP3DCIP4GCIP1GCIP2GCIP3SSP
1,2,3,4,5,6,7,8-Octathiocane 10544–50–0 – – – – – – √ √ √ √ – 
1,2-Benzenedicarboxylic acid bis(2-ethylhexyl)ester 84-69-5 √ √ – √ √ √ √ – √ √ – 
1,2-Benzenedicarboxylic acid, dibutyl ester 117-81-7 √ – – – – – – – – – – 
1.2:4,5-Dibenzpyrene 196-42-9 √ – – – – – √ – – – – 
1-Hexadecene 000629-73-2 – – – √ – – – – – – – 
28-Nor-17beta(H)-hopane 036728-72-0 – – – – – – – – – – √ 
2-Chlorobenzyl bromide 611-17-6 – – – – – – – – – – √ 
2-Methylanthracene 613-12-7 – √ √ – – – – – – – – 
7-Methylbenzo[b]phenanthrene 2541-69-7 – – √ – – – – – – – – 
10 9-Methylacridine 611-64-3 – – – √ – – – – – – – 
11 9-Octadecenamide 124-26-5 √ – – √ √ – √ √ – – – 
12 Anthracene 120-12-7 – – √ – – – – – – √ – 
13 Benzenemethanol 100-51-6 √ – – – – – – √ – – – 
14 Benzo(a)pyrene 50-32-8 – – √ – – – – – √ √ – 
15 Benzo[e]pyrene 192-97-2 – – – – – – – – – – √ 
16 Benzo[k]fluoranthene 207-08-9 – √ – – – – – – – √ √ 
17 Chrysene 218-01-9 – – √ – – – – – – – – 
18 Cyclopenta[cd]pyrene 27208-37-3 – – – – – – – – √ – – 
19 Docosane 629-97-0 – – – – – √ √ – – – – 
20 Ethane, 2-chloro-1,1-diethoxy 621-62-5 √ – – – – √ – – – √ – 
21 Fluoranthene 206-44-0 – – – √ – – √ √ √ √ √ 
22 Fluorene 86-73-7 – – √ – – – – – – √ – 
23 Heptadecane 629-78-7 – – – – – √ √ √ – – – 
24 Hexadecanamide 629-54-9 √ – – √ √ – √ – – – – 
25 Hexadecane 544-76-3 – – – – – – – – – – – 
26 n-Dotriacontane 544-85-4 – – – – – – – – √ – – 
27 n-Eicosane 112-95-8 – – – – – √ √ √ √ – – 
28 n-Heneicosane 629-94-7 – – – – – – √ – – – – 
29 n-Hexacosane 630-01-3 – – – – – – √ – – – – 
30 n-Nonadecane 629-92-5 – – – – – √ √ √ √ – – 
31 n-Tricosane 638-67-5 – – – – – – √ – – – – 
32 Octadecanamide 124-26-5 √ – – √ – – √ √ – – – 
33 Oleamide 301-02-0 – – – – √ – √ – √ – – 
34 Perylene 198-55-0 – – – – – – – – – – √ 
35 Phenanthrene 85-01-8 – – – – – – √ – √ √ – 
36 Pyrene 129-00-0 – – – – – – √ – √ √ √ 
37 Stigmastane 601-58-1 – – – – – – – – – – √ 
38 Styrene 100-42-5 √ √ – – – – – √ – √ – 
39 n-Tricosane 638-67-5 – – – √ √ – – – – – – 
40 Triphenylene 217-59-4 – – √ – – – – – – √ √ 
 Chemical nameCAS no.GSP1GSP2GSP3DCIP1DCIP2DCIP3DCIP4GCIP1GCIP2GCIP3SSP
1,2,3,4,5,6,7,8-Octathiocane 10544–50–0 – – – – – – √ √ √ √ – 
1,2-Benzenedicarboxylic acid bis(2-ethylhexyl)ester 84-69-5 √ √ – √ √ √ √ – √ √ – 
1,2-Benzenedicarboxylic acid, dibutyl ester 117-81-7 √ – – – – – – – – – – 
1.2:4,5-Dibenzpyrene 196-42-9 √ – – – – – √ – – – – 
1-Hexadecene 000629-73-2 – – – √ – – – – – – – 
28-Nor-17beta(H)-hopane 036728-72-0 – – – – – – – – – – √ 
2-Chlorobenzyl bromide 611-17-6 – – – – – – – – – – √ 
2-Methylanthracene 613-12-7 – √ √ – – – – – – – – 
7-Methylbenzo[b]phenanthrene 2541-69-7 – – √ – – – – – – – – 
10 9-Methylacridine 611-64-3 – – – √ – – – – – – – 
11 9-Octadecenamide 124-26-5 √ – – √ √ – √ √ – – – 
12 Anthracene 120-12-7 – – √ – – – – – – √ – 
13 Benzenemethanol 100-51-6 √ – – – – – – √ – – – 
14 Benzo(a)pyrene 50-32-8 – – √ – – – – – √ √ – 
15 Benzo[e]pyrene 192-97-2 – – – – – – – – – – √ 
16 Benzo[k]fluoranthene 207-08-9 – √ – – – – – – – √ √ 
17 Chrysene 218-01-9 – – √ – – – – – – – – 
18 Cyclopenta[cd]pyrene 27208-37-3 – – – – – – – – √ – – 
19 Docosane 629-97-0 – – – – – √ √ – – – – 
20 Ethane, 2-chloro-1,1-diethoxy 621-62-5 √ – – – – √ – – – √ – 
21 Fluoranthene 206-44-0 – – – √ – – √ √ √ √ √ 
22 Fluorene 86-73-7 – – √ – – – – – – √ – 
23 Heptadecane 629-78-7 – – – – – √ √ √ – – – 
24 Hexadecanamide 629-54-9 √ – – √ √ – √ – – – – 
25 Hexadecane 544-76-3 – – – – – – – – – – – 
26 n-Dotriacontane 544-85-4 – – – – – – – – √ – – 
27 n-Eicosane 112-95-8 – – – – – √ √ √ √ – – 
28 n-Heneicosane 629-94-7 – – – – – – √ – – – – 
29 n-Hexacosane 630-01-3 – – – – – – √ – – – – 
30 n-Nonadecane 629-92-5 – – – – – √ √ √ √ – – 
31 n-Tricosane 638-67-5 – – – – – – √ – – – – 
32 Octadecanamide 124-26-5 √ – – √ – – √ √ – – – 
33 Oleamide 301-02-0 – – – – √ – √ – √ – – 
34 Perylene 198-55-0 – – – – – – – – – – √ 
35 Phenanthrene 85-01-8 – – – – – – √ – √ √ – 
36 Pyrene 129-00-0 – – – – – – √ – √ √ √ 
37 Stigmastane 601-58-1 – – – – – – – – – – √ 
38 Styrene 100-42-5 √ √ – – – – – √ – √ – 
39 n-Tricosane 638-67-5 – – – √ √ – – – – – – 
40 Triphenylene 217-59-4 – – √ – – – – – – √ √ 

The organic pollutants accumulated in different pipe-scales varied substantially. The ages of the DCIP4, GCIP1, GSP1 and SSP were similar (approximately 11 years). The organic species in the DCIP4 and the GCIP1 were mainly hydrocarbons. Several amides and lipids were also detected. DCIP4 had the most abundant organic species, whereas GCIP1 had a higher content of amide, suggesting that more algae accumulated in the early service period, when the water source was a natural river. The two samples of galvanized steel pipe-scale were significantly different. Amides and lipids dominated in galvanized steel GSP1. Galvanized steel pipes are one of the most common peripheral pipes in the DWDS. As the last part of pipes into the client, the water was always in a low flow state, which was very conducive to the formation of pipe-scale and accumulation of pollutants. The organic species in the SSP were mainly PAHs, which was completely different from those in the cast iron pipe and galvanized steel pipe. This was due mainly to the smooth inner surface and less pipe-scale, which were not conducive to the accumulation of organic pollutants. Overall, the pipe material substantially impacted the organic pollution in pipe-scale. As a cumulative pollutant sinks, the more pipe-scale forms and the more microalgae metabolites accumulate.

The ages of ductile cast iron pipe samples DCIP1, DCIP2, DCIP3 and DCIP4 were 6, 6, 9 and 11 years, respectively. Fewer types of organic pollutants were detected in DCIP2, whose age was only 6 years, and they were mainly microalgae metabolites such as octadecenamide. A wide variety of organic hydrocarbons and PAHs were detected in the older pipe DCIP4 (11 years), suggesting that the initial pipes accumulated much more microalgae from the source water and accumulated relatively little biofilm. As service time increased, conditions became more conducive to pipeline biofilm growth. Thus, in the pipe-scale from older pipes, there were fewer types of microalgae metabolites and more types of microbial metabolites. The correlation between types of organic pollutants in the gray cast iron (GCIP1, GCIP2 and GCIP3) and galvanized steel (GSP1, GSP2 and GSP3) pipe-scale and pipe age was similar to that for the ductile cast iron pipe.

In summary, the impact of pipe age on the type and content of organic pollutants in pipe-scale was greater than that of other factors. With an increase in service time, pipeline biofilms grow and exceed those of early microalgae from source water. As a result, the types of metabolites also change.

CONCLUSIONS

The following results were obtained.

  1. When pipe ages and diameters were similar, the amounts of pipe-scale and pollutants in gray cast iron pipe were greater than those in ductile cast iron pipe; the amount of pollutants in galvanized pipe-scale was greater than that in PVCP and stainless pipe-scale. The morphology and composition of the pipe-scale from different types of pipes varied appreciably.

  2. The concentration of heavy metals in pipe-scale was positively related to the quantity of pipe-scale (R2 = 0.874). The sequence of metal concentrations was Fe > Al > Mn, Zn > Pb, Cu > Cr, Cd. The pipe material and the state of the inner surface lining clearly affected the type and content of heavy metal pollutants in pipe-scale. The concentrations of heavy metals in the pipe-scale of PVC pipe and stainless steel pipe were quite low because of their material properties and better corrosion resistance. The high concentrations of heavy metals in galvanized steel pipe-scale were due to severe corrosion and other reasons. The degree of corrosion in gray cast iron pipe and ductile iron pipe varied widely, so the concentration of heavy metals was different. The differences in the composition of different pipe-scales were also substantial. There were more types of iron-containing compounds in gray cast iron pipe-scale. However, in ductile cast iron pipe-scale, quartz and calcite dominated, and there were fewer iron-containing compounds because of the intact lining and less rust. More severely corroded galvanized steel pipe-scale had more iron-containing metal composite oxides, and some zinc-containing compounds were also detected.

  3. In addition to hydrocarbons produced by microbial metabolism, there were microalgae metabolites and exogenous contaminants. The organic pollutants accumulated in different pipe-scales varied greatly, and the concentrations of microbial metabolites increased with increasing service time.

ACKNOWLEDGEMENTS

The authors thank the Key Special Program on the S&T Pollution Control and Treatment of Water Bodies (no. 2012ZX07403-003), the National Key Technology R&D Program (no. 2012BAJ25B07) and the National Natural Science Foundation of China (no. 51378455).

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