1. Introduction
Ductile iron is an alloy of iron and carbon in which, during solidification of the casting—owing to the addition of small amounts of magnesium or cerium—graphite nodules are produced. This form of graphite improves the mechanical properties of the material when compared to grey cast iron. Ductile iron pipes display higher plasticity, which makes them survive local strain without cracking and produces only local pipe deformations. These pipes are also highly resistant to lateral stress and high internal pressure. For the above reasons, for the past few decades they have been used for building water supply networks, especially in areas with shock risk, that is underneath motorways and streets with heavy traffic, in urbanised areas, in large urban and industrial agglomerations and in areas with complex geology. Ductile iron, just like most metal goods in contact with water, undergoes chemical and biological corrosion [
1,
2]. To prevent corrosion, different types of protective coatings are applied to the internal and external surfaces of the pipes. The most commonly used internal linings are made of cement mortar. In the case of the water intended for consumption, linings made of Portland cement or alumina cement are usually employed for contact with water characterized by greater corrosivity (lower pH value, low alkalinity and high CO
2 content). External pipe surfaces are covered with zinc or zinc and aluminium coatings or with black varnish or epoxy plastic [
3,
4,
5].
The primary ingredient of the cement is Portland clinker, its content—depending on cement type—ranging from 95% (additive-free Portland cement) to 15% (blast-furnace cement). Portland clinker is the product of heating (sintering) the mixture of limestone and aluminosilicate materials at the temperature of about 1450 °C. Primary elements contained in the cement include silicon, calcium, aluminium, iron and sulphur, possibly complemented with potassium, titanium, magnesium, manganese and phosphorus. During fuel combustion in rotary furnaces, some amount of heavy metals contained in the fuels are accumulated in the clinker. In recent years, different types of waste materials have been combusted in these furnaces, such as used motor oils or car tyres, which may have significantly higher heavy metal content than natural raw materials.
Elements contained in the primary components of the cement coating released into water in most cases do not affect its health claims. They do, however, have some impact on its organoleptic properties such as taste, odour or turbidity. Leaching of mechanical impurities, such as sand, may lead to corrosion of the fittings and any other components of water supply networks and systems [
6,
7,
8]. Examination of damage to new cement coatings caused by water with varied corrosive properties conducted by Douglas and Merill [
9] showed that low-alkalinity aggressive water may significantly raise pH (from 7 to 12 after one week of testing), alkalinity and calcium content in water contacting cement mortar coatings. Application of a bitumen coating covering the cement layer considerably reduces the solubility of cement components [
9]. Vik and Hedberg [
10] listed the following water quality parameters that are recommended to be met in order to prevent dissolution of cement-based materials, including asbestos cement, cement pipes and cement coatings: pH > 7, alkalinity > 15 mg/L in the form of CaCO
3, calcium >10 mg/L, sulphates < 200 mg/L, aggressive CO
2 < 5 mg/L.
Trace elements such as chromium, lead, zinc, nickel, arsenic, cadmium, vanadium or copper [
11] are released from the cement lining into the water flowing through the pipeline. A study examining the leaching of trace elements from the cement, conducted by Achternbosch et al. [
12] also showed that the use of waste materials for clinker firing increases element cement content to a minor extent. Reported relationships were not clear-cut however, as they were influenced by many factors, including method used and solubility testing. The study by van der Sloot [
13] presented leaching of metals from cement mortar as a function of pH and duration of the period in which cement was in contact with water. Based on the revealed relationships, it was observed that the smallest amounts of metals leached into the water when it was neutral. Guo et al. [
14] observed increased amounts of chromium, arsenic, barium and cadmium leached from the cement coating, recording their concentrations above the limit values for drinking water. Analyses conducted by Moudilu et al. revealed a correlation between leaching of heavy metals from the Portland cement and temperature, as well as the period of the experiment, using a new, highly sensitive method that allowed to determine very low concentrations of these elements [
15]. Use of cement mortar coatings may lead to penetration of aluminium into the water, with the aluminium content depending on the type of cement. After two months of experiments on steel pipes with cement mortar lining, aluminium content was found to increase from 0.005 mg/dm
3 to 0.690 mg/dm
3. This element affects human health and it is particularly dangerous for patients undergoing kidney dialysis [
16]. Due to the fact that the leaching of aluminium from Portland cement occurs in low-alkalinity water, Berend et al. [
16] recommend that internal coatings made in situ from regular Portland cement should not be used in water with an alkalinity lower than 55 mg/L as CaCO
3 and that factory-made internal cement coatings should not be used in water with alkalinity lower than 25 mg/L as CaCO
3. It is also recommended that the use of high alumina cement lining should be limited in pipelines used for distributing drinking water, due to much higher aluminium content leached from this type of cement into the water, when compared to Portland cement [
3,
6].
The discussed topic is not new. However, previous studies focused on the laboratory, mainly static, leaching tests. This paper presents new studies conducted under real dynamic conditions, based on a new 90 m experimental set. The hydraulic conditions in this set reflected the average conditions in the entire network, determined on the basis of field measurements and numerical calculations (using a numerical model devised in EPANET 2.0 software, US EPA, Washington D.C., USA, 2000).
In this paper, the composition of cement lining was connected with the changes in the quality of water transmitted via lined pipeline. The occurrence of changes in the water quality was indicated both in short (hourly) and long-term (daily).
The available literature offers no exhaustive information on the impact of cement mortar lining of water conduits on the quality of the conveyed water. This is because the interaction between water and cement lining is determined by many factors; a significant role is played by water composition and type of lining but also properties of the contacting water. Also, state-of-the-art laboratory methods vary and quite often fail to reflect real-life conditions found in water supply networks. In light of the above, this paper presents a properly designed study conducted using the experimental setup, with the main purpose of determining the impact of cement coating covering the internal surface of a water conduit made of ductile iron on water quality in terms of changes in its physico-chemical properties.
2. Materials and Methods
The study was conducted in an experimental setup composed of the main pumping station supplying a water supply network in a selected city (
Figure 1). Water was abstracted from intakes located in underground Cretaceous formations using 8 wells, from 55 to 70 metres deep. Abstracted water met the requirements for drinking water set out in WHO [
17] and Polish National Standards [
18] and except for periodic chlorination, it requires no treatment.
The experimental setup was made of ductile iron pipes with a total length of 80 m and an internal diameter of 65 mm. The working volume of the experimental setup was 250 dm3.
The study was composed of three testing series:
Series I—long-term (multiple day) stagnation of water in the setup. Duration of the series until stabilisation of water quality in the setup. Water samples were collected once a week.
Series II—short-term (few hour) stagnation of water in the setup. Water samples were collected from the setup after 0, 1, 2, 4, 8, 16 and 32 h.
Series III—water flow through the installation at the velocity of 0.1 m/s. Water samples for the analyses were collected during the supply and drainage of the water from the setup, considering retention times of the water in the pipeline.
The experimental setup was chlorinated and washed before every series. Before each series of measurements, a water sample was collected in order to determine the initial conditions. Physico-chemical analyses of water samples determining changes in its quality were conducted in accordance with applicable analytical standard methods. The standards according to which the measurements of particular water quality parameters were conducted are presented in
Table 1. The analysis of every water quality parameter in each sample was performed in triplicate and the average of the results are presented in the tables. Trace element content in the water and in the cement coating was measured using the JY238 Ultrace Inductively Coupled Plasma—Optical Emission Spectrometer (ICP-OES) (Jobin Yvon-Horriba, Montpellier, France) [
19]. The calibration solutions were prepared using CentiPUR
® VIII multi-element calibration solution (Merck group, Darmstad, Germany, 2013). In order to conduct a more accurate quantitative determination of elements, the investigations were carried out in two phases. In the initial phase, a fast semi-quantitative analytic methodology was employed. This phase, as well as the previous results of studies on cement lining, enabled the determination of the elements that had to be subjected to further quantitative indication with a more accurate method (second phase). In that phase, determination was carried out by means of inductively coupled plasma mass spectrometer (ICP-MS, 7700x, Agilent, Polo Alto, USA). Similar to the initial phase, the quantitative determinations were performed using calibration curves and the same pattern. The scope of analytical control of water samples is presented in the tables containing results of relevant analyses.
4. Summary
The aim of this research was to examine the impact of cement-mortar lining covering the interior surfaces of ductile iron water conduits on the quality of water conveyed in these conduits. Tests were performed using an experimental setup made of commercially available pipes. The setup was supplied with underground water collected and provided, without treatment, to a water supply network in a mid-sized city. Water examined during the experiment was of variable quality, however for the entire period of the experiment it displayed good physico-chemical properties in terms of its fitness for consumption, as well as poor corrosive properties with a tendency to form protective coatings, which made it a minor hazard for cement and metallic materials.
During the experiment, attempts were made to determine changes in the physico-chemical quality of the water during long-term (multiple day) and short-term (few hours) stagnation of the water in the setup and during its flow through the setup at the velocity of 0.1 m/s (which is typical of working water supply networks). While examining the impact of cement mortar lining on water quality, changes in the quality in terms of increased indicators such as primarily turbidity, pH, alkalinity, hardness, chloride, sulphate and dry residue content were reported, as well as reduced carbon dioxide water content. Values of indices characterising water stability pointed out that water introduced into the setup gradually lost its corrosive properties and acquired an increasing ability to precipitate calcium carbonate. This provided evidence of the penetration of components of the cement lining into the water. Such changes were recorded incrementally for the first 5 weeks of water contacting the cement lining. After shorter contact (up to 32 h), water quality changes were primarily related to increased turbidity and hardness and minor pH rise. The phenomenon of interaction between water and cement-mortar lining was also observed while testing the water flow through the setup at the velocity of 0.1 m/s. In this case, the change in water quality was very small. It did confirm, however, the leaching of cement components into the water, which raised the water’s ability to precipitate calcium carbonate.
In the experiment, besides changes in the typical water quality indicators, an attempt was made to determine water penetration by metals found in the cement-mortar lining due to the applied cement manufacturing method. While testing the sample collected from the cement-mortar lining, besides the primary cement components, the following trace elements were detected: iron, magnesium, manganese, zinc, barium, chromium, lead, nickel and cobalt. Analyses conducted of samples of chlorinated and non-chlorinated water having contact with the cement lining at the initial stage of setup operation failed to confirm penetration of significant amounts of trace elements into the water (whereas leaching out of trace elements from cement was more intense in chlorinated water). Repeated analyses of samples of water stagnant in the setup for 32 h, after 8 months of setup operation, showed that the majority of trace elements, including those detected at the initial stage of operation, were below the detection limit. Among elements detected in the stagnant water, the largest increase in content with respect to the feeding water was observed for zinc, boron and gallium. Increases in sodium, iron, barium and magnesium water content after stagnation were scarce. It should be stressed that final concentrations of all elements detected in the water after its contact with the cement-mortar lining were much lower than the permitted limits for drinking water. Low concentrations and small (excluding zinc) increases in element concentrations were most probably caused by their effective immobilisation in the cement structure and by earlier leaching from the lining.