Pipes in Drinking Water Distribution System: Comparison
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Please note this is a comparison between Version 2 by Lindsay Dong and Version 1 by Magdalena Bosomoiu.

The inner walls of the drinking water distribution system (DWDS) are expected to be clean to ensure a safe quality of drinking water. Complex physical, chemical, and biological processes take place when water comes into contact with the pipe surface. 

  • tap water
  • polymeric pipe
  • leaching
  • corrosion

1. Introduction

Water is essential for life development throughout the world, all plants and animals need water. Its quality (expressed by chemical, physical and biological characteristics) influences the population’s health. Drinking water quality varies from place to place and is affected by the water source, the treatment that water undergoes, and the pipe distribution system.
Water in contact with the pipe material can cause component leaching, pipe corrosion over time, or promote microorganism growth on the inner pipe surface. The first two mechanisms lead to chemical contamination of water whilst microorganism multiplication deteriorates the biological stability of water (development of pathogens, deterioration of taste, odor, and color). Water quality at the tap and the problems that arise are specific to each water distribution system.
Among the first materials used for the water distribution systems (concrete and lead), lead was extensively employed. Lead exposure has severe consequences (affecting mental and physical development), with symptoms that do not appear until dangerous amounts have accumulated in the human body [8][1]. Adverse health effects have also been observed in the prenatal stage [9][2]. The lead limit in drinking water has been decreased over time to 10 μg/L in the European Union in compliance with the value recommended by the WHO, but the aim is to reduce the limit to 5 μg/L in the next 15 years [10,11][3][4]. Because there is no natural source of lead in drinking water, the only way to achieve the new limit is by replacing the lead pipes. The US EPA proposed the complete elimination of lead (concentration of 0 μg/L) from drinking water [12][5]. Because of that, during the last decades, it was decided to replace lead plumbing with other materials such as iron, galvanized steel, PVC, and copper [13,14,15,16,17][6][7][8][9][10].

2. Materials Used to Manufacture Drinking Water Distribution Pipes

2.1. Lead

The main lead source in a lead pipe is the pipe itself. Besides lead-manufactured pipes, another source of lead in non-lead pipe systems are brass fixtures and lead-solder connections [41,42,43,44,45][11][12][13][14][15]. These fittings represent discrete sources of lead that may give rise to significant lead concentrations in drinking water. There is no safe lead concentration in drinking water. Lead concentration can vary within a home, at the same tap over time, or between homes. Currently, new lead systems are no longer installed but many older DWDS still contain lead pipes. In recent years, lead pipes have been gradually replaced worldwide, especially in the cases where Pb concentrations are high. Concentrations higher than 10 μg/L were consistently measured in old pipe systems in Taiwan, which represents a health risk for the population [46][16]. However, care must be taken when replacing parts of the lead pipe system. In Canada, replacement of about 80% of lead pipe with copper pipe caused sustained lead release (sometimes worse than a full lead system) up to 12 weeks upon substitution [47][17]. The same behavior has been reported in the USA [48][18]. This was attributed to galvanic corrosion between aged lead pipes and new copper pipes [47,49][17][19]. The factors that control the lead release from corrosion scale are [47,50,51,52,53][17][20][21][22][23]:
(a)
 Water chemistry parameters:
(i)
pH, alkalinity
(ii)
The content of dissolved inorganic carbon
(iii)
Presence of disinfectants: chlorine, chloramine, dissolved oxygen
(iv)
Presence of corrosion inhibitors (orthophosphate, orthophosphate/polyphosphate mixture, silica)
(v)
Oxidation Reduction Potential
(vi)
Presence of organic matter
(b)
 Flowing regime: alternate flow/stagnant regime; pipes flushing.
Lead scale is a complex structure that accumulates many different metallic compounds, often in different layers. Because of that, lead release into the flowing water is more complex than the corresponding solubility mechanism. One of the factors affecting the lead leaching from the corrosion scale is the pH. Lead solubility from the scale increases with decreasing pH, allowing the detachment of lead particles from the scale into drinking water [54,55][24][25]. The lead corrosion scale can contain other heavy metals, such as V, Sb, Cu, Mn, and Cr [56,57,58,59,60][26][27][28][29][30]. In total, 2.8% of water samples analyzed in a DWDS in USA exceeded the maximum allowable concentration for Pb, As, and Cu [60][30]. Characterization methods showed that hydrocerussite phase (Pb3(CO3)2(OH)2) is the major lead crystalline corrosion compound in the scale [51,56][21][26]. Cerussite (PbCO3) was also present in some cases. In the inner scale layers, PbO2 and Pb3O4 were present [56][26]. Lead dioxide acts as a protective layer. The formation of PbO2 was reported to be beneficial in terms of pipe protection against future corrosion [61][31]. Metallic lead reacts with water only in the presence of oxidizing compounds to form lead hydroxide, Pb(OH)2 [62][32].  In soft water, traces of lead form colloidal lead compounds that are difficult to remove [63][33]. It has been experimentally observed that higher lead levels in water are often accompanied by high iron concentrations [64][34]. Lead with iron and natural organic matter present in the environment forms colloidal particles that are easily detached and transported in the DWDS. This was confirmed by size exclusion chromatography with UV and multi-element (ICP-MS) detection. Lead is mobilized via adsorption to iron colloidal particles. The change of disinfectant from chlorine to chloramine caused an increase in lead corrosion. This is because the predominantly tetravalent lead (PbO2) scale is destabilized in the presence of chloramine [65][35]. Analysis of tubercle scales collected from pipelines in Columbus, Ohio, found they contained microorganisms, such as sulfate reducers, nitrate reducers, nitrite oxidizers, ammonia oxidizers, and sulfur oxidizers [66][36].

2.2. Cement

Cement is frequently used for the manufacturing of pipes or as a protective layer on the inner surface of an iron or steel pipe. Cement is used as a coating to prevent the corrosion of pipe metallic surfaces because of the high pH of the cement lining. In this way, the isolation of metallic pipe from water is achieved and an alkaline environment near the pipe walls is created, which prevents corrosion. The protective layer can be applied during the manufacturing process of the pipes (prefabricated coated pipes) or during the renovation of corroded water pipes (manually coated pipes). Concrete pipes are cement-based pipes made by mixing cement (limestone), water, sand, and additives, which prevents the concrete pipe from cracking. Concrete pipes are among the oldest pipe systems to be employed for water transportation [70][37]. The types of cement allowed to be used in manufacturing the pipes for drinking pipes are Ordinary Portland Cement (OPC), High Alumina Cement (HA cement), Blast Furnace Slag Cement (BFS cement), Fly Ash Cement (FA cement), and Sulfate-Resistant Portland Cement (SR cement) [71][38]. Cement-manufactured pipes were usually used in combination with asbestos, which provides tensile strength and makes the pipes resistant to thermal and chemical breakdown. However, in the late 1980s, concerns were raised in New York regarding the use asbestos–cement pipes (AC) because the asbestos fibers were detaching from the inner pipe surface and migrating to drinking water [72,73][39][40]. The analysis of a 56-year-old AC pipe evidenced surface corrosion because of calcium leaching from the wall [74][41]. Seasonal variation of temperature was found to impact the calcium dissolution rate, which, in turn, impacts the release of asbestos fibers: Lower water temperature increased the calcium leaching, which accelerates the detachment of asbestos fibers. Cement is composed mainly of CaO, Al2O3, SiO2, Fe2O3, and gypsum in various ratios [76,77,78][42][43][44]. In addition to the characteristics of running water, the durability of asbestos–cement pipes is also directly related to the free lime content, Ca(OH)2 [79][45]. Concrete and cement pipes are mainly affected by the leaching process. Contaminants are released from the cement layer immediately after the installation of new pipes or after the rehabilitation of old pipes. The degree of water contamination is strongly related to the composition of the cement and is more accentuated at the beginning of the pipeline operation [71,80,81,82][38][46][47][48]. The leaching process means the dissolution of some compounds of the pipe walls. The dissolution takes place in the pores of the wall and assumes several steps:
(i)
 Internal diffusion of liquid water through the porous wall;
(ii)
 Dissolution of the compounds;
(iii)
 Diffusion of the dissolved compounds through the pores at the surface of the pipe;
(iv)
 External diffusion of the dissolved compounds at the wall pipe into the main water stream. The permeability of the material controls how much water diffuses through the pores of the material. The leaching rate depends on whether the pipe has just started to be operated, the permeability of the pipe material, the softness of drinking water, and the solubility of the leached compound. Soft water is water relatively free of dissolved ions and has a higher capacity for dissolution compared with more saturated water in minerals.
Cement is a source of toxic elements that can be of natural origin or introduced into the cement composition during the manufacturing process. The cement industry has modified its production methodology to include the valorization of several types of waste materials (meat and bone meal, waste tires, solidified sewage sludge) as an alternative fuel in cement kilns [83,84,85,86][49][50][51][52]. Waste tires are the most common sources of antimony, cadmium, chromium, cobalt, lead, titanium, and zinc [83][49]. Solidified sewage sludge is a source of cadmium, chromium, zinc, vanadium, and cobalt in varying concentrations [84][50]. Comparison between CEM I types (OPC) made by two different manufacturers showed similar behavior. For both types of cement, the leaching of aluminum and calcium was very intensive in the initial contact with drinking water and decreased over time. Aluminum leaching was still present after a 64-day investigation period, while calcium was leached only up to 4 days after the first contact with drinking water. Leaching of chromium was similar for both types of cement CEM I and disappeared after 7 days. Lead leaching was not detectable after the fourth day for both types of cement. The main disadvantage when using CEM I type cement was exceeding the aluminum maximum concentration in drinking water for several days after the pipe was put in use [82][48].  Another parameter that affects the leaching of dangerous compounds from cement coating is the dose of disinfectant present in drinking water. The disinfectant is added to ensure a reduced concentration of microorganisms in water transported from the treatment plant to the consumer.  Adding sodium hypochlorite to water does not increase the water pH at contact with cement lining. However, when comparing results obtained from tests on water with or without disinfectant, the presence of disinfectant does considerably increase the leaching of different compounds (such as calcium, aluminum, chromium, and lead). The lowest tested sodium hypochlorite dose was the most aggressive, causing the highest rate for calcium, aluminum, and chromium leaching.  To improve the resistance of cement pipes, PVC (polyvinyl chloride) tubes can be used as the inner layer [95][53]. There are, however, some inconveniences when using cement-based pipes with embedded polymer pipe. The parameters that were studied were pipe diameter, curing age, temperature, and resistance to chloride ions. These factors change with increasing the pipe diameter. 

2.3. Copper

Comparison with steel [102][54], copper and copper alloys are corrosion scale-forming materials [103,104,105][55][56][57]. This means that the corrosion by-products accumulate on the interior of the pipe. It was found that pure water does not contribute significantly to the corrosion of copper pipes [106][58]. Generally, there are three types of scale in pipes:
(a)
 Compounds that crystallize directly onto tube surfaces from the flowing water (this is the case of calcium and magnesium carbonates that distribute uniformly on the inner surface of the pipe);
(b)
 Scale consisting of inorganic materials precipitated elsewhere and transported by the flowing water;
(c) 
Scale formed by the corrosion products (characteristic for unlined iron cast pipes, copper pipes); the corrosion scale is a hard mineral consisting of densely distributed corrosion tubercles.
At the beginning of their operation, the copper pipes are covered by a protective layer of cuprous oxide. The difference in the behavior of stainless steel and copper and copper alloy materials resides in the fact that the copper corrosion is much faster, and the concentration of copper ions in the water near the pipe wall rises, so the precipitation and crystallization of corrosion products take place. Stainless steel also forms a passivation film, but the corrosion of this film is so slow that the corrosion products do not accumulate at the pipe wall. Scale build-up is a process that takes months or even years.  The main factors that were found to affect the copper corrosion and later to cause the leaching of the corrosion products in drinking water are:
-
pH [106,109][58][59];
-
temperature [110][60];
-
total organic carbon [110][60];
-
dissolved inorganic carbon (carbonates) [106,107][58][61];
-
chloride ions or other disinfectant substances [110,111][60][62];
-
corrosion inhibitors: sulfate, polyphosfate, and orthophosfate [106,109,112,113][58][59][63][64];
-
existence of microorganisms in biofilm [114,115,116,117][65][66][67][68];
The components of the scale formed by the corrosion products were products of Cu(I): cuprite (Cu2O) and copper (I) hydroxide (CuOH) and products of Cu(II): CuO, copper (II) hydroxide (Cu(OH)2), and malachite (Cu2CO3(OH)2) [106][58].

2.4. Iron

The iron pipes used for DWDS are cast iron pipe, galvanized iron pipe, unlined cast iron pipe, and lined ductile iron pipe. Corrosion of metallic iron means its dissolution from the metallic pipe as Fe2+ simultaneously with the reducing of oxidizing agents present in water: O2, H+, SO42−, NO2−, NO3−, CO2; ferrous iron can be further oxidized to ferric iron. This ends up creating cracking and pitting iron corrosion. The primary parameters that affect the iron pipe stability are related to the flowing water characteristics: alkalinity, pH, presence of chloride and sulfate ions [120,121,122,123,124,125,126][69][70][71][72][73][74][75]. Orthophosphates added to the flowing water act as corrosion inhibitors by forming insoluble phosphates that create a coating protective layer [127][76]. The solid corrosion products that build up thick corrosion scale are responsible for pipe blockage, leakage, and red water formation [128,129][77][78]. Corrosion scales formed on the surface of pipes consist mainly of lepidocrocite (γ-FeOOH), magnetite (Fe3O4), goethite (α-FeOOH), hematite (Fe2O3), and ferrihydrite [130,131,132][79][80][81]. All these minerals have strong affinities to adsorb heavy metals (lead > vanadium > chromium > copper > arsenic > zinc > cadmium > nickel > uranium) [130][79].  The metal accumulation in corrosion scale represents a threat to the population’s health. A maximum accumulation rate of 3.94 mg vanadium/g of scale and 3.90 mg arsenic/g of scale, respectively, was found by He et al. (2021) on scales from cast iron pipe (20 years operation time) [132][81]. The accumulation rate depends on various factors, such as pH and temperature. The scale is mainly composed of magnetite and goethite. Chloride and sulfate anions diminish arsenic accumulation, therefore, increasing their content will reduce arsenic accumulation on corrosion scales [132][81]. Another toxic heavy metal is chromium. The accumulation and release of chromium (III) and (VI) from iron corrosion scales was investigated [139][82]. The outer layer of the scale accumulates less chromium and releases more. During the Cr6+ accumulation process, part of the Cr6+ is reduced to Cr3+ by the existing Fe2+, which is a beneficial process since the toxicity of Cr3+ is reduced. Comparing with the stainless steel scale, which contains chromium from the corrosion products, the iron scale accumulates chromium from the water flowing in the pipe or from the inner linings of the pipe. The chromium content in scales of stainless-steel pipes is higher than the chromium content of iron corrosion scales [102][54]. Manganese also can accumulate in iron pipe scales. Although not as dangerous as vanadium, chromium, and arsenic, its concentration in drinking water is monitored and limited to 0.05 mg/L [1][83]. The manganese release is enhanced in stagnant water. The factors influencing the manganese release are pH, alkalinity, sulfate ion concentrations, and the presence of disinfectants [140][84].  Over time, the corrosion scale formed on iron pipes develops a multi-layer structure consisting of the corroded floor, which is the pipe’s inner wall, porous layer, stable shell layer, and top layer (Figure 5). The shell layer gives resistance to further corrosion process by blocking the access of oxidizing compounds, being mainly composed of compact α-goetite and magnetite: α FeOOH (more than 50%), followed by calcium carbonate (about 20%) and Fe3O4 (below 10%) [142][85].  The water characteristics’ and disinfectants’ influence on iron corrosion in a ductile iron pipe were studied in accelerated laboratory tests using increased concentrations of disinfectant [144][86]. Sodium hypochlorite, like liquid chlorine, enhances iron corrosion by oxidizing ferrous to ferric ions. The effect of NaClO on the presence of calcium ions is different from the chlorine effect, NaClO is responsible for calcium carbonate accumulation on the inner pipe wall in waters with high alkalinity and hardness. NaClO not only contributes to pipe corrosion but also intensifies the CaCO3 deposition over time.  The formation of siderite (FeCO3) was also found to play an important role in iron corrosion [122,150][71][87]. Scales formed on different pipes (PVC; lined ductile iron, LDI; unlined cast iron, UCI; galvanized steel, GS) after 1 year of use, with water having different characteristics, were analyzed [122][71]. The order of total iron release decreases in the order: unlined cast iron > galvanized steel ≥ lined ductile iron > PVC. The main corrosion by-products on unlined cast iron pipe were: FeCO3, α-FeOOH, β-FeOOH, γ-Fe2O3, Fe3O4, while on galvanized steel, FeCO3 was replaced by significant amounts of zinc oxide. Lined ductile iron and PVC had low deposits of iron containing corrosion by-products.  The scale deposited on iron-based pipes provides a favorable environment for microinvertebrates (Asellus aquaticus) to develop and live in large numbers. On the contrary, on plastic pipes, there were only isolated specimens of Asellus aquaticus [152][88]. The effect of sulfate ions was studied for old cast iron distribution pipes [125][74]. Red water occurred for the pipes usually fed with groundwater, while no coloration was noticeable for pipes supplied with surface water. The difference arises from the scale composition: (i) thin and less stable for the groundwater-supplied pipes containing mainly higher proportion β-FeOOH, FeCO3, and green rust; (ii) thick and more stable for surface water-supplied pipes, having a high content of stable Fe3O4. The water sulfate content also influences the bacterial communities living on the scale surface.

2.5. Steel

Stainless-steel pipes are extremely resistant to corrosion. However, under long-term operation, they become susceptible to pitting corrosion, which forms thin layers of corrosion scales. It has been reported that stainless-steel corrosion scales contain a large amount of chromium compounds. The scale formation is initiated by pitting corrosion on the pipe surface, followed by the homogeneous deposition of iron and chromium corrosion compounds [80][46]. The insoluble corrosion products are α-FeOOH, α-Fe2O3, γ-FeOOH, γ-Fe2O3, Fe3O4, FeCO3, Cr2O3, CrOOH, and possibly FeSO4.  Physico-chemical characterization of pipe scales can be made by SEM, XRF, XRD, and XPS [156,157][89][90]. The presence of disinfectant products enhances the corrosion of stainless-steel pipe [102,158][54][91]. Steel pipe corrosion in the presence of chloride ions is a multi-step process that depends, besides the characteristics of water, also on the presence or absence of a coating protective layer. For uncoated steel pipes, three steps were evidenced during chloride-induced corrosion: (a) diffusion of chloride ions through the mortar/cement layer; (b) pitting corrosion by which cavities are produced in the steel layer; (c) transition from pitting corrosion to uniform corrosion as more and more chloride ions arrive at the surface of the steel layer.

2.6. Polymer-Based Pipes

Polymeric pipes used in DWDS include PE (polyethylene), PEX (cross-linked polyethylene), LDPE (low-density polyethylene), HD-PE (high-density polyethylene), PVC (polyvinyl chloride), PVC-U (unplasticized polyvinyl chloride), and Hi-PVC (high-impact polyvinyl chloride) pipes. Polymeric pipes have some advantages compared to other pipes: They are lighter, which is important if they are used in buildings, and they do not form corrosion scale, etc. Among analyzed pipes (PVC-U, galvanized steel pipe, copper pipe, and cast-iron pipe), PVC-U pipes have the minimum value in terms of resource and energy consumption in residential buildings [159][92]. By using polymer-based pipes in DWDSs, the problems related to compounds containing heavy metals are reduced significantly. However, polymer-based pipes have a reduced mechanical strength and favor the formation of organoleptic compounds [160,161][93][94] and biofilm on the inner pipe surface [162][95].  It was shown experimentally over 16 weeks of investigation that the migration of organic compounds was elevated within the first weeks of use, followed by a lower and constant level [163][96]. The organic compounds that migrate from the pipe surface (polyethylene and cross-linked polyethylene) can be further transformed and degraded in bulk water [164][97]. These compounds can be degraded biotically or abiotically, but the degradation rate is low. However, this study did not include the influence of pre-existent biofilm on the pipes’ inner surface. It is expected that the presence of the biofilm would change the degradation rate.  PVC pipes release different traces of additives used in the manufacturing process (e.g., organotins or lead-containing additives). It was recommended that PVC should be rinsed before operation to eliminate the organotin traces [169][98]. It was found that lead additives leach from unplasticized PVC pipes under the action of UV radiation (a concentration of 0.8 mg/L was detected after 14 days of exposure) [170][99]. The major migration component from HDPE pipes was 2,4-di-tert-butyl-phenol (2,4-DTBP), which is a degradation product of polymer additive (antioxidants). To evaluate the odor level, TON (threshold odor number) was used. Among tested pipes (HDPE, PEX, and PVC), PVC pipe showed no significant odor [172][100]. When comparing newly installed PE and HDPE pipes, HDPE pipe released smaller quantities of antioxidants and their degradation products in the drinking water distribution system [173][101]. Retention of lead from drinking water on biofilm grown for three months was evaluated for PEX, HDPE, and copper pipes. The lead accumulation experiments were performed for five days.  The effect of two common disinfectants (chlorine dioxide, ClO2, and sodium hypochlorite, NaOCl) on polymer-based pipes was studied in accelerated aging experiments. The evaluated pipes were HDPE, LDPE, PVC–U, and Hi-PVC pipes. The pipes have a lower resistance to oxidation in the presence of ClO2 than in the presence of NaOCl.

3. Biofilm Formation on Inner Pipe Wall

Biofilms formed on the inner walls of water distribution pipes are responsible for several types of problems, such as the deterioration of water quality, corrosion of metallic pipe walls, intensifying the leaching processes in concrete-based pipes, proliferation of pathogens, etc. The negative action of microorganisms in biofilm can be intensified by the existence of scale on the inner pipe walls. It is known that the number and diversity of microorganisms in the film is higher than that in flowing water [178][102]. The biofilm and the scale formed on the pipe walls can exfoliate, releasing heavy metals and pathogens into drinking water [171,179,180][103][104][105]. The biofilm can contain microorganisms, such as Pseudomonas aeruginosa and Legionella pneumophila, that create health-related issues [181,182,183][106][107][108]. Moreover, the biofilm can be responsible for the deterioration of the taste and odor of drinking water [184][109]. The source of microorganisms can be the water itself or cracks in the pipe distribution system that are not remediated. The biofilm produced by microorganisms present in the flowing water adheres to the surface of the inner pipes and is formed by different constituents of extrapolymeric substances: carbohydrates, lipids, proteins, uronic acids [131][80], etc. The steps of biofilm formation include the following successional steps: (i) microorganisms from the bulk water attach to the pipe surface; (ii) microorganisms start to develop, releasing extracellular polymeric substances (EPS); (iii) modify the surroundings to exclude or enhance the development of other microorganisms [2,185][110][111]. The factors influencing the growth of the biofilm include the water source [186,187[112][113][114],188], availability of the nutrients [189][115], disinfectant concentration [185[111][116][117],190,191], water flowing regime [192[118][119],193], and the pipe material [162,185,190,194,195,196,197,198,199,200,201][95][111][116][120][121][122][123][124][125][126][127]. The main microorganisms involved in the pipe corrosion are sulfate-reducing bacteria (SRB) [205[128][129],206], nitrate-reducing bacteria (NRB) [207[130][131],208], acid-producing bacteria (APB) [209][132], and metal-oxidizing bacteria (MOB) [210,211][133][134]. A particularity of the biofilm effect on pipe corrosion is its non-uniform composition, inducing a non-equal distribution of the corrosion process on the pipe surface [210][133]. The BART test can be used to identify and count different bacteria in DWDS. It provides multiple environments in the presence of which specific bacteria are activated [212][135]. IOB isolated from different pipes scale samples was grown on a Winogradsky nutrient medium and determined by the plate-counting method [131,210][80][133]. Bacteria in biofilm are characterized using several tests: bacterial growth on R2A broth to analyse cell surface hydrophobicity; determination of zeta potential; assessment of cell motility that included swimming, swarming, twitching, and autoaggregation [205][128]. Besides bacteria, the biofilm composition can also include viruses [213][136].  Microbiologically induced corrosion (MIC) affects metallic pipes. Worldwide, there is a major concern related to the economic impact generated by maintenance of the water distribution system. Microbial-induced corrosion, together with scale accumulation, may damage the pipeline, resulting in pipe leakage or bursting. Biofilm formation can contribute to the acceleration of metallic pipe corrosion [114,115,116,117,217][65][66][67][68][137]. Pitting corrosion starts at the interface biofilm–metallic pipe because the water pH is modified by the release of metabolic degradation compounds [117][68]. It was found that in iron scales, the microorganism community has a bigger influence on Fe3O4 formation than water chemical parameters. This means that in iron pipes distributing drinking water, the microbial community influences the capture of iron from unstable corrosion products and enhances the formation of more stable and compact scale mainly consisting of Fe3O4 [218,219][138][139]. Corrosion of steel pipes used in water distribution systems in the presence of microorganisms has been extensively studied [131,220,221,222][80][140][141][142]. MIC of carbon steel enhances the uniform and localized corrosion of pipe material [109][59]. Iron-oxidizing bacteria (IOB) are responsible for the corrosion of cast iron pipes and carbon steel pipes [222][142]. Iron-oxidizing bacteria (Pseudomonas sp. strain DASEWM2) determinate a higher corrosion rate of the pipe by: (i) forming wider cracks in the corrosion layer in the presence of extrapolymeric substances; (ii) the presence of corrosion products with the non-protective effect of surface [131][80]. Comparison among four pipe materials (steel, copper, stainless steel, and polyvinyl chloride) showed that bacterial communities are more developed on steel and copper pipes [223][143]. These pipes form important corrosion scales compared with stainless-steel and PVC pipes. Stainless steel has the lowest bacterial count at the end of the operation. However, steel pipe has the highest bacterial diversity among metallic pipes. Nitrate was found to influence the bacterial communities and iron release from the corrosion scales on cast iron and stainless-steel pipes [207][130]. The increase in nitrate content promotes the growth of nitrate-reducing bacteria while decreasing the activity of iron-reducing bacteria.  Comparison among three pipe materials (ductile iron, cement-lined stainless steel, and polyethylene) used in the same conditions of flow and water characteristics showed significant differences in bacterial film growth and community [162][95]. The difference in the bacterial communities means that in the presence of disinfectant, each pipe will generate other disinfection by-products. Ductile iron had the highest disinfection by-product formation potential.

4. Microplastic (MP) Fate in DWDS

In the recent years, the presence and fate of microplastics in drinking water has received increasing attention [237,238,239][144][145][146]. There are numerous MPs sources from various environments [240,241,242,243,244][147][148][149][150][151]. Once in DWDS, the MPs can sediment and be incorporated into the scale formed on the pipe inner surface. Variation in total MPs content (polypropylene, polyethylene, polystyrene, polyethylene terephthalate, and polyvinyl chloride) along a pipeline showed a decrease in MPs concentration in water from 1570.8 n⋅L−1 at water intake to 377.0 n⋅L−1 at the end of the pipeline network [245][152]. Attempts to remove MPs in conventional water treatment plants evidenced that this is not enough to reduce MPs. Although some retention of bigger particles takes place, there is also a fragmentation process, which generates even more small particles [244][151]. Negrete Velasco et al. (2023) evidenced the persistence in drinking water of some types of plastics (such as polyethylene and polyethylene terephthalate) after the water passage through Geneve’s water treatment plant [246][153]. In aquatic environments, such as DWDS, the interaction mechanisms between microplastics and heavy metals include:
-
Electrostatic interactions and electrostatic interactions with surface complexation: Polar regions formed on corroded pipe surface interact with polar MPs surface. The polar surface of microplastics is given by different functional groups generated during MPs aging (e.g., the number of oxygen-containing groups increases with the MPs aging [253][154]). He et al. (2022) found that the pH and temperature influence the interaction between manganese ions and aged polystyrene microplastics. They proposed a mechanism based on the establishment of hydrated functional zone (an inner film, an outer film, and their enclosed space) [250][155]. The water chlorination underwent significant morphology O-functional group changes and created C-Cl bonds in MPs (polyethylene and thermoplastic polyurethane) [251][156]. This enhanced the aggregation ability of the MPs and their interaction with Cr(VI), which is a highly toxic compound.
-
Sorption or bioaccumulation is enhanced by the biofilm developed on pipe or MP surface [254,255,256][157][158][159]. The interaction between metals and MP could be intensified in the warmer season because biofilm growth is much faster. This aspect requires more study to better understand the pipe–biofilm–MPs interactions.
-
Electrostatic retention and bioaccumulation appear to be the main mechanisms of interaction between heavy metals and MP in aquatic environments [257,258,259,260,261,262][160][161][162][163][164][165]. More studies performed in set-ups that approximate the conditions from DWDS are needed to investigate how factors, such as pH, MPs size, water composition, and disinfectant type, influence the heavy metal–biofilm–MPs interaction.
It was found that the capacity of metal adsorption is different among polymers [257][160]. The adsorption capacity for cadmium ions decreases in the order PVC > PS > PP > PE.

5. Mathematical Modeling

The pipes affected by the leaching process were found to be metallic pipes and cement and cement-protected pipes. Leaching depends on the material pipe and the water characteristics (disinfectant dose, pH, if it is soft water). The compounds leached from the pipe surface will accumulate in water over the distance of the pipeline. Therefore, the concentrations of different dangerous compounds will be higher at the ends of drinking water supply pipeline sections. Few studies in the literature address the modeling and prediction of compounds leaching from different pipe surfaces. This is due to the complexity related to the processes and the fact that, depending on the type of pipe, the leaching processes are time-dependent. Hence, there is need for large spatial-temporal data to develop a model that predicts the leaching of pipeline components. Moreover, when developing a tool for modeling contaminant release from the pipe distribution system, it is necessary to include a model for microbial contamination and disinfection by-product (DBP) formation because these processes interfere with the release of contaminants from the pipeline [268][166]. Chlorine-based disinfectants react with natural organic matter and form DBP. DBP include THMs (trihalomethanes), HAAs (haloacetic acids), HANs (haloacetonitriles), HNMs (halonitromethanes), HAMs (halogenated acetamides), etc. Aldehydes (ALs) and iodinated DBP can be formed when water is disinfected by ozone and iodine [269][167]. The water distribution system is a highly dynamic environment, with changes occurring throughout the whole distribution system. Great attention is given to disinfection by-products in the water treatment plant, but few studies are dedicated to what happens afterwards and how DBP transform in the water distribution system. Because PVC-manufactured pipes are more susceptible to biological film formation, the mathematical model developed for these distribution systems must include the interaction between the organic matter and the disinfection substances. Models to account for the longitudinal dispersion of pollutants in water distribution systems have been developed [270][168]. For a pipe system coated with cement, mathematical modeling of heavy metal (chromium and lead) release shortly after pipeline operation was studied by accounting for sorption [32,271][169][170]. By using this model, the chromium and lead concentrations at pipeline sections can be calculated after rehabilitation by cement lining. The developed mathematical model considers several mechanisms to describe the leaching of chromium and lead: dissolution, diffusion, advective transport, and sorption. The sorption included in the model considers the adsorption of heavy metals inside porous cement coatings. The diffusion accounts for chromium and lead transport in water phase in coating capillaries and in water phase inside the pipe. Experiments were performed in static [271][170] and in dynamic conditions [32][169]. The experiments performed under dynamic conditions used fresh water or water replaced periodically [32][169]. The water was characterized by its pH, alkalinity, and hardness. The calculated and experimental concentrations of lead and chromium were in good agreement. The calculated concentrations corresponding to a pipeline 30 km long reached values that are well below the threshold of the current regulations (2.5 mg/m3 for chromium vs. 25 mg/m3 in regulations and 2 mg/m3 for lead vs. 5 mg/m3 in regulations) [1][83].  To prevent unplanned pipe failure, the lifetime of a piping system must be approximated. Because pipes are affected by flowing water, soil, and environmental characteristics, it is difficult to predict what will be the degradation along a pipeline. In this case, estimation of probability of failure based on Monte Carlo simulations may help. This method was applied in the case of AC pipe distribution system [272][171].  Using 3D computed fluid dynamics calculations, the influence of pipeline geometry, sampling methods, and the flowing regime on the lead concentration in drinking water was studied [44][14]. The case includes a system of copper pipes connected by lead solders and brass valves. It was found that galvanic corrosion takes place when lead is in contact with copper in the presence of water. The galvanic corrosion is an electrochemical process in which one metal corrodes when it is electrically connected with a dissimilar metal in the presence of a conductive liquid. In this case, lead is the anode and corrodes, while copper is the cathode.

6. Conclusions

Metal-based pipes form scale, which contains and accumulates heavy metals (e.g., lead, vanadium, chromium, copper, arsenic, zinc, cadmium, and manganese) from the environment. The scales can detach and end up in the flowing water. Cement-based pipes release chemicals after commissioning. Calcium, aluminum, chromium, lead, and cadmium are the main ions released by cement-containing pipes. In the case of asbestos-containing cement pipes, asbestos fibers can be released into drinking water. Polymeric pipes are susceptible to releasing organoleptic substances (e.g., alkylphenols, aldehydes, ketones, and organotins generated or used in the manufacture process) within first weeks after the commissioning. The efficiency of the disinfection process depends on the pipe material. Metallic pipes (e.g., copper) may require a higher concentration of disinfectant. Regarding the corrosion process, there is a clear advantage of polymeric-based pipes versus metal-based pipes. The polymeric pipes do not develop scale deposits. They are, however, subjected to degradation by chain scission in the presence of water disinfectants that generate free radicals (e.g., ClO2). All types of pipes develop biofilm on their surface. This biofilm retains and accumulates microplastics and compounds from water. Moreover, the biofilm accelerates the proliferation of opportunistic pathogens and bacteria. When changing the flow conditions or water parameters, the biofilm can detach, causing an instant increase in some compound concentrations that may pose a health risks (especially heavy metals and microplastics). Microorganisms present in the biofilm enhance metal-based pipe corrosion (microbiologically induced corrosion), making them even more vulnerable to mechanical failure and accelerating the release of dangerous compounds in drinking water. Changing from flowing to stagnant conditions will cause an enhanced corrosion of metal-based pipes. For the polymeric pipes, there are no studies that discuss this aspect, but it is expected that the stagnant conditions will affect the microorganism community in the biofilm.

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