Abstract

This paper investigates the impact of pipe material on the quality of water in distribution systems over various time intervals. A comparative study was performed between two commonly used pipe types for water transmission: ductile iron pipes with internal cement lining and polyethylene pipes. The physical and chemical properties of the water were analyzed. The two pipe types under investigation—ductile iron pipes with cement lining and polyethylene pipes—were introduced, and their characteristics reviewed. Water samples were collected at different time points as it passed through these pipes to evaluate qualitative parameters. Factors such as concentrations of organic and mineral chemicals, microbial and bacterial content, pH, dissolved oxygen, and other key indicators were measured. The results were analyzed to accurately assess the influence of pipe type and material on water quality over time. Statistical differences and their possible causes are discussed in this paper. This study provides valuable information for pipeline designers and decision-makers in hydrology to select the most suitable pipe for water transmission systems and can help enhance water quality and the efficiency of water resources.

Keywords: Water quality, ductile iron pipes, cement lining, potable water hygiene, polyethylene pipes.

1- Introduction

Today, pipelines and water supply systems serve not only for water distribution; providing continuous, sufficient, and safe drinking water is essential for human health, human rights, welfare, and sustainability (1). More broadly, safeguarding the health of drinking water consumers is crucial as a matter of civil rights. A significant portion of the global disease burden results from inadequate drinking water services (2).

Over the past decades, water pipes have been made from steel, cast iron (including gray and ductile cast iron), and even copper. However, due to their low cost and availability, plastics such as polyethylene (PE) and cross-linked polyethylene (PEX) have largely replaced these materials. Many recent reports indicate that plastic pipes can release significant amounts of various chemicals into the water, which may worsen the taste and odor of drinking water and cause a range of adverse health effects (3). Nevertheless, information on the potential health impacts of using plastic drinking water pipes remains limited (4). The harmful effects of chlorine and other disinfectants on polyethylene pipes have also been discussed in several studies (5).

Many organic additives are present during the manufacturing of plastic pipes (PE, PEX), including oxygen-containing compounds such as ethyl tert-butyl ether (ETBE), methyl tert-butyl ether (MTBE), and tert-butyl alcohol (TBA). Their degradation products include antioxidant byproducts like 2,4-di-tert-butylphenol (2,4-DTBP). Studies have shown that solvents such as benzene, toluene, ethylbenzene, and xylene (4, 6, 7) are easily released from plastic pipes (PE, PEX) after contact with water. Furthermore, these molecules are readily washed out, meaning that ETBE, MTBE, and TBA can easily alter the taste and odor of water (8).

Cast iron pipes have been used in water supply and distribution pipelines worldwide since around 1901 (1280 in the Iranian solar Hijri calendar). With technological advancements, the limitations of gray cast iron—such as brittleness and fluid velocity loss—have been addressed. The introduction of ductile cast iron and the use of internal coatings, combined with the high durability and environmentally friendly lifecycle of ductile cast iron pipes with internal cement lining, have led to their increased adoption in water distribution systems that meet global standards. Ductile cast iron pipes with various internal linings made from different types of cement are used to prevent corrosion of the inner cast iron surface. Typically, pure aluminum linings are applied in sewage systems. In some cases, slag cements are used in sewage applications, and increasing the thickness of the internal lining can further enhance resistance to the corrosive sewage environment. However, only Portland cement is used for internal linings of potable water transmission pipes. The use of high-alumina cements for drinking water is not recommended, as the amount of aluminum released into drinking water is significantly higher compared to Portland cement.

The use of high-alumina cements for drinking water applications is not recommended, as they release significantly more aluminum into the water compared to Portland cements. Portland cement clinker is produced by calcining raw materials—including limestone and aluminum silicates—at a kiln temperature of 1450°C. During this stage of cement production, elements such as silicon, calcium, aluminum, iron, sulfur, potassium, titanium, magnesium, manganese, and phosphorus are incorporated into the final cement composition. There are ongoing concerns that heavy metal pollutants, such as lead, mercury, and others, may leach from the internal cement lining into the water. Additionally, combustion products from the rotary kiln can mix with the clinker, introducing heavy metals into the cement. However, to date, there have been no claims that these elements present in cement have adverse health effects (10). Only very small amounts of elements such as chromium, lead, zinc, nickel, arsenic, cadmium, vanadium, or copper are released from the cement lining into the water flowing through the pipeline (11).

Research shows that aluminum can leach from Portland cement under alkaline conditions (12). The release of heavy metals and other harmful elements from cement mortar depends on the pH of the drinking water and the length of contact between the cement and the internal lining (13).

This topic is not new; efforts have been made to move beyond laboratory-scale testing and fixed samples by using conditions that closely resemble real pipeline systems, with circulating fluid sampled accordingly. Additionally, changes resulting from water contacting cement were examined over weeks and months, comparing Portland cement lining in one sample to water contacting plastic pipes in another. The present study does not provide comprehensive, general, or definitive information regarding the impact of pipe type and material on water quality, as the interaction between the lining or pipe’s internal surface and water always depends on several variable parameters, the most important of which is the chemical composition of the transported water (14). Therefore, even the most advanced laboratory testing methods cannot fully capture the complexities of real-world water quality variations within pipelines under actual conditions.

2- Materials and Methods

2.1. Pipes

In this laboratory study, two 40-centimeter pipes—one made of ductile cast iron and the other of polyethylene, both with a DN/OD 110 diameter—were exposed to circulating water. The water exiting the system served as the sample source for water quality assessment tests.

Before initiating circulation and water transfer, both 40-cm pipe samples (ductile cast iron and polyethylene) were exposed to pure water—with the chemical composition listed in Table 1—for twenty-four hours, following the EN-14120 standard. Additionally, 4-mm thick glass plates were used to seal both ends of the pipe samples. These plates, cut to a 12-centimeter diameter, were heated to approximately 450°C for two hours to remove potential contaminants.

2.2. Reference Water

The water sample used for testing, referred to as the reference water, was prepared by reverse osmosis (RO) treatment to remove potential contaminants before transfer through the pipes.

The pure drinking water sample, obtained from a three-stage reverse osmosis process, was designated as the reference sample for chemical composition analysis. It was stored in separate glass containers for each pipe type—polyethylene and sulfate-resistant Portland cement-lined ductile cast iron.

2.3. Water Circulation System

The water circulation system used in this study consists of three main components: First, the pipes under investigation, as described in section 2-1. Second, the glass reservoirs for storing water. The third component is the water pump, which circulates water from the reservoir through the pipes and then returns it to the reservoir, maintaining continuous circulation for the required duration. The connection between the reservoir and the pipes was made using plastic tubes with a 10 mm diameter, which were washed with pure drinking water 24 hours prior to the experiment. In this setup, water was pumped from the reservoir into the sample pipe at a constant flow rate of 1 liter per minute by the pump, and then returned to the reservoir via a channel installed above the pipe, completing the circulation process. The various components of this system are illustrated in Figure 1.

Figure 1- a) Schematic illustration of the water circulation system- b) Actual image of the water circulation system for both pipe samples- c) Image of the pump in the glass reservoir

2.4. Test Water

After 7 and 30 days of water circulation in these systems, to evaluate the water quality, the test water was drained into glass bottles that had previously been heated at 450 degrees Celsius for 2 hours, and then transferred to the ViroMed laboratory.

Once again, water was filled into the reservoirs of both sets of 40-centimeter pipes, and circulation began at a constant flow rate of 1 liter per minute using a pump. This condition was maintained for one month. Afterward, a water sample was collected in a glass bottle for discharge testing. All procedures, including heating the glass containers, were also performed for this test sample.

3- Results and Discussion

3.1. Physical and Chemical Properties

As shown in Table 1, the physical and chemical properties of the tested water samples are presented for each water circulation system. Furthermore, these results reflect measurements taken after one week and after one month.

Table 1 - Physical and Chemical Properties of Test Results

Both tested water samples had a desirable appearance and color. For turbidity, both samples met standard permissible limits; however, the turbidity of water from polyethylene pipes was about 20% higher than that from ductile cast iron pipes lined with sulfate-resistant Portland cement. The total hardness (TH) of both samples was within acceptable limits, but as shown in Figure 2, the water from polyethylene pipes had higher total hardness than that from ductile cast iron pipes. Conductivity was also higher in the polyethylene pipe samples compared to the ductile cast iron samples with cement lining. The pH of the water from polyethylene pipes increased during the first week and then decreased, with these changes being significant, while the pH in ductile cast iron pipes with cement lining remained stable. The water sample from polyethylene pipes had an odor exceeding the standard limit. The odor threshold at 25°C is 5 TUN units, which rose to the maximum allowable limit of 7 after one month. The comparative chart is shown in Figure 3.

Alkalinity and total dissolved solids in the tested water samples of both systems increased compared to the reference water but were still within acceptable standard limits. Alkalinity and total dissolved solids were higher in the water samples from ductile cast iron pipes than in those from polyethylene pipes. As Shelton AK ET Al. (2007) demonstrated in their research (15), the impact of polyethylene pipes on the chemical quality and odor of water is very significant; this is also confirmed by the results of this study based on Table 1 and Figure 2. The water quality status of polyethylene pipes does not comply with Iranian water quality standards and the World Health Organization due to the aforementioned parameters.

3.2. Mineral Chemicals

The mineral chemicals measured in the tested water samples are presented in Table 2. These results were obtained using Atomic Fluorescence Spectroscopy (AFS).

Table 2- Toxic Inorganic Chemical Test Results

As shown in Table 2, the elements arsenic, barium, and lead were below the detection limit of the measurement method and are therefore indicated with the symbol ">" in the test results table. The magnesium content increased in both water samples tested compared to the reference water. Although this increase is not significant and remains within the acceptable and permissible range, the increase in the polyethylene pipe water sample was greater than that observed in the ductile iron pipe water sample. As mentioned in the study by X Huang et al. (2020) (16), heavy metal deposition on the surface of polyethylene pipes is inevitable, leading to pipe degradation and the release of organic carbons that can cause serious health problems. Figure 4 shows the comparative status of magnesium. Despite the presence of nitrate, the tested water in both circulation systems showed an increase compared to the reference water; however, this increase was not significant and remained within the acceptable and permissible range. The one-week tested water sample from polyethylene pipes had a lower nitrate level than the sample from ductile iron pipes, but after one month, the nitrate level in the polyethylene pipe sample increased by 2 units, making it higher than that in the ductile iron pipe water sample. Fluoride, chloride, and alkalinity also increased in the tested water samples from both systems compared to the reference water but remained within the acceptable standard range. The alkalinity of the ductile iron pipe water sample was higher than that of the polyethylene pipe sample.The alkalinity of the ductile iron pipe water sample was higher than that of the polyethylene pipe sample. The calcium hydroxide concentration in the reference water was 10 mg/L, which increased to 19.5 mg/L after one week in the polyethylene pipe sample and decreased to 17 mg/L after one month. In the ductile iron pipe water sample, calcium hydroxide increased by 2 units compared to the reference water after one week, reaching 12 mg/L. After one month of water cycling in the ductile iron pipe system, this value remained stable. This comparison is illustrated in Figure 5.

3.3. Organic Chemicals in Water

The test results for organic chemicals in the water samples are presented in Table 3. Measurements included dissolved oxygen (DO), biological oxygen demand (BOD)—the amount of oxygen required by microorganisms to consume all organic materials in the water—and several parameters for chlorinated alkanes and other organic compounds such as MTBE, tetrachloroethane, and ethylenediaminetetraacetic acid.

Table 3 -Organic Chemical Test Results in Water

The dissolved oxygen in the reference water was 3 mg/L, and this value remained unchanged in the ductile iron pipe water sample after one week, staying similar to the reference water. However, in the polyethylene water circulation system sample, this parameter decreased by 1 unit. After one month, the decrease in dissolved oxygen was much more pronounced.

Methyl tert-butyl ether (MTBE), 1,2,3-trichlorobenzene, ethylenediaminetetraacetic acid, tetrachloroethane, and carbon tetrachloride were all negligible and below the detection limit in the tested ductile iron pipe water and reference water samples, but were detected in the polyethylene pipe water sample at the levels listed in Table 3, which were, nevertheless, below permissible limits for drinking water. However, considering the odor in the sample, this value corresponds to the MTBE level (8). Total organic carbon (TOC) in water at 23°C increased to 10 mg/L in the polyethylene pipe sample after one week, but decreased to 4 mg/L after one month. In this study, the water temperature was considered to be 25°C, as Hong et al. (17) showed in their 2020 study that the release of organic carbon intensifies with increasing temperature, and the released organic carbon reacts with chlorine to produce trihalomethanes (THMs). In contrast, in the ductile iron pipe water sample, a decrease was observed in the first week, dropping to 5 mg/L, followed by a further decrease of 1 unit after one month to 4 mg/L. The TOC comparison is shown in Figure 6.

3.4. Microbial Characteristics

Microorganisms in the tested water samples were analyzed according to SMWW standards Nos. 3759, 5271, and 19265. As shown in Table 4, thermotolerant coliforms, Escherichia coli, heterotrophic microorganisms, and Clostridium perfringens spores were not detected in the 100 mL samples.

Table 4 - Microbial Test Results

4- Conclusion

Regarding the physical and visual characteristics of the tested water samples, both the appearance and color met the standards. However, turbidity, total hardness (TH), and conductivity were higher in the water sample from polyethylene pipes compared to those from ductile iron pipes with an internal cement lining. The total hardness (TH) of water from polyethylene pipes was greater than that from ductile iron pipes. Additionally, the water sample from polyethylene pipes exhibited a smell exceeding the standard limit. The odor threshold at 25°C is 3 TUN units; in this case, after one month, it reached the maximum allowable limit of 3. This is attributed to the release of chemicals from the polyethylene pipes into the water, a phenomenon not observed in the ductile iron pipe samples. Numerous studies have reported the adverse effects of polyethylene pipes on the chemical quality and odor of water. For certain mineral chemical properties, such as sulfate, chloride, fluoride, chromium, aluminum, and nitrate, the water sample from ductile iron pipes showed a very slight increase compared to the reference water, but these remained within acceptable limits according to standards. However, increases in magnesium and calcium hydroxide were observed in the water sample from polyethylene pipes. Although the release of organic carbons in the polyethylene water sample was minimal, this issue—as reported in many studies—increases the risk of pipe damage and creates conditions favorable for the formation of halomethanes, ultimately negatively impacting the quality of the flowing water.

5- Acknowledgements

Hamoun Nyzeh Company’s mission is “Water Protection,” and this article has been supported by Hamoon Naizeh Company. The authors would like to express their gratitude to Mr. Takian and Mr. Tavakoli (engineers) for their support. Additionally, the authors appreciate the valuable guidance of Ms. Flora Heshmatpour (Ph.D., Medical Engineer) and Mr. Ne’mat Hasani (Ph.D., Medical Engineer).