1. Introduction
Public health is closely related to the safety of drinking water [
1]. The quality of drinking water usually deteriorates when it is delivered through drinking water distribution systems (DWDSs) owing to pipe material release, biofilm detachment, and loose-deposit resuspension [
2], [
3]. Additionally, sediments and microorganisms in DWDSs are responsible for discoloration, metal ion release, and bacterial level increases [
4], [
5], [
6]. Water companies clean pipelines to remove sediments and microorganisms in DWDSs for a continuous, safe, and reliable water supply [
7].
Ice pigging has been introduced and applied in DWDSs as an environmentally friendly pipeline cleaning technique [
8]. In practical applications, saline ice slurry used as a viable semisolid pig is pumped into water pipes via a hydrant/air valve or a 2 in (1 in = 2.54 cm) fitting using high-speed water to push the ice pig through the pipe. Subsequently, the ice slurry is pushed out of the system via a downstream hydrant/fitting or an upstream air valve. The mechanism and procedure of ice pigging described in the materials and methods sections are shown in
Fig. 1(a). The ice pig contains liquid water, microscale ice particles (with diameters of < 1 mm), and an anti-freezing depressant (NaCl) is commonly used in ice pigging for DWDSs) [
9].
Ice pigging offers high cleaning efficiency because the ice slurry presents a “solid” piston-like profile when it flows through a straight pipe. The ice pigs provide shear stress that is 2-4 orders of magnitude higher than that of water at the same speed [
10]. Ice pigs are no-sticking, namely, they eventually melt away when negotiating typical DWDS equipment, including bends, tees, contractions, expansions, valves, and pumps [
11], [
12]. Moreover, ice pigs can be directly discharged into a waste tank or drainage pipelines through hydrants and air valves without requiring a unique catcher [
10]. Photographs of effluent samples obtained from downstream hydrants during the whole ice pigging project are shown in
Fig. 1(b). The original fluid sample was initially clear then turned progressively darker, finally became clear again. A similar change could also be observed in Movie S1 in Appendix A.
Nonetheless, controversy about the application of ice pigging exists. Current studies have indicated that massive quantities of sediment could be removed from DWDSs via ice pigging [
13], [
14], [
15]. However, knowledge of whether ice pigs will strip passivated oxide surfaces from metal-based pipes is scarce. Ervin et al. [
15] provided photos of the cross-section of a pipe before and after ice pigging, reporting no apparent visual disturbance to the pipe surface. Visual observations cannot reveal underlying mechanisms. Additionally, the influence of ice pigging on the bacterial community in bulk water, which is closely related to drinking water safety, has rarely been reported. Furthermore, lost ice pigs can travel through networks to enter customer faucets owing to improper valving control and incomplete slurry discharge, increasing the risk of localized water quality degradation and customer complaints [
16]. There are few reports on the practicalities of ice pigging, including the control of hydraulic conditions and pipe network valves, selection of ice fractions, and prevention of the leakage of ice pigs in pipes or user faucets. Therefore, an in-depth understanding of ice pigging application in DWDSs is highly desired to design effective cleaning strategies and simultaneously resolve concerns.
The objectives of this study were as follows: ① evaluate the discharge degree of ice pigs and provide suggestions to optimize ice pigging plans; ② analyze whether passivated oxide surfaces are disrupted by ice pigs; ③ compare the bacterial community structure in bulk water before and after ice pigging in different pipe networks; and ④ ascertain the correlation between pipe service age, temperature, concentrations of chloride and total iron, and the 15 most abundant taxa in bulk water to inform ice pigging operations.
2. Materials and methods
2.1. Pipe network description
The eight networks in this study, located in southeastern China, were received water supply from the same water treatment plant. The water treatment plant used coagulation, sedimentation, filtration, ozonation, biologically activated carbon, and chlorination. The features of the eight networks, including different pipe materials, pipe service ages, pipe diameters, lengths, bends, and branches, are listed in Table S1 in Appendix A.
2.2. Mechanism and procedure for ice pigging
An ice slurry was produced from a 5% NaCl solution. Two ice slurry generators were used together to prepare ice slurry for the eight ice pigging projects. One generator with a rated power of 31.36 kW could produce 20 t ice slurry in 24 h. The ice slurry was preserved in a 316L stainless steel tank with a volume of 10 m3 and stirred with two groups of blenders. Another generator with a rated power of 19.61 kW could produce 10 t ice slurry in 24 h. The ice slurry was preserved in a 316L stainless steel tank with a volume of 6 m3 and stirred with one group of blenders. The refrigerant used in both of the ice slurry generators was monochlorodifluoromethane (CHClF2).
For ice fraction measurement, a coffee press (i.e., a mesh plunger) was employed to separate two phases of an ice slurry sample, and then a relative ice volume fraction was calculated by the volume ratio of solid (ice particles) and the sum of solid and liquid (ice slurry). This technique allowed for manual measurements and provided a convenient method for field analysis in the ice pigging process.
During the ice pigging process, as shown in
Fig. 1(a), valves A and C were first closed to isolate the pipe section to be cleaned. Subsequently, hydrant valves B and D were opened, and valve D was connected to a wastewater tanker. Thereafter, the ice slurry (occupying up to a fourth of the pipe volume) with a high ice fraction (at approximately 60%) was pumped into the pipe section through the upstream hydrant valve B. Afterward, valve A was opened to push the ice slurry downstream using the mains water velocity. As the ice slurry reached the downstream hydrant valve D, the ice pigs and associated deposits being removed were discharged into the wastewater tanker. The water quality of the effluent was monitored online (refer to details in Section 2.5). Once the turbidity was lower than one nephelometric turbidity unit (NTU), valves B and D were closed. Finally, valve C was opened to return the cleaned pipe section to service [
16].
2.3. Time and date of ice pigging projects conducted in the eight networks
Networks with diameters less than and more than 400 mm were cleaned during the daytime and in the evening, respectively. The 12NCI400 network was cleaned using ice pigs twice, first in February 2021 and then in November 2021. The differences between the two trials are provided in Section S1 in Appendix A. The other seven networks were intervened for the first time and cleaned by ice pigs in November and December 2021, respectively.
2.4. Water sampling method
Before pumping ice slurry into pipes, the hydrants at the end of each network were opened in the eight ice pigging projects. Five replicates of 1 L pre-bulk water samples were obtained from the flexible pipe connected with the selected hydrant after opening the hydrant for 15 min. During the ice pigging process, five replicates of 1 L flushed samples were collected when the measured turbidity reached values above 100 NTU. Another five replicates of 1 L post-bulk water samples were collected after the measured turbidity fell below 1 NTU. Out of the five replicates of the pre- and post-bulk water samples, three were used to analyze microbial community structure, and two were used to detect concentrations of chloride, sulfate, and trace elements. The five replicates of flushed samples were filtered to obtain sediment samples, which were air-dried and ground to a 100-mesh sieve size. They were analyzed by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD). The physicochemical and microbiological parameters of the bulk water sample were measured in triplicate. After collection, the samples were immediately transported to the laboratory and stored at 4 °C until use. Sample extraction and physicochemical analysis of the bulk water sample were completed within 24 h after the sample was collected.
2.5. Water quality monitoring
Every ice pigging project involved the use of a flexible pipe, which connected a hydrant at the downstream end of each pipe network to a wastewater tank. This setup enabled the control of ice slurry discharge, accompanied by visual displays of parameters, such as logging of flow, turbidity, temperature, and suspended solids concentration. The flow rate during the eight ice pigging projects is provided in Table S2 in Appendix A. The turbidity of water collected from the eight networks before and after ice pigging was lower than 1 NTU. The temperature of ice slurries discharged from the downstream fire hydrants during ice pigging was measured. The temperature of ice slurries was in the range of −6.87 to −3.22 °C when the bulk water temperature ranged from 9.22 to 16.15 °C in the eight ice pigging projects. The pH value was measured using a portable multiparameter meter (HQ40d, HACH, USA). The pH values of bulk water collected from eight networks before and after ice pigging ranged from 7.35 to 7.45. Discrete water samples were collected before and after ice pigging from hydrants for physicochemical and microbiological laboratory analysis. The chloride and sulfate concentrations were analyzed using an ion chromatography system (ICS2000, Dionex, USA). According to the standard method for drinking water analysis in China (GB/T 5750.6-2006), trace elements, including Fe, Cd, Ti, Ag, Be, Cu, Se, Pb, Ni, Sb, Mn, Mo, Zn, As, B, and Ba, were analyzed via inductively coupled plasma-mass spectrometry (ICP-MS; NexIon 300Q, Perkin Elmer, USA) after the bulk water samples were digested using 1% (v/v) nitric acid. The microstructures and elemental compositions of sediments carried by the ice pigs from the cleaned pipes were analyzed via SEM coupled with EDS. The crystalline phases of the sediments were determined via XRD (Ultima IV, JEOL, Japan). XRD was performed using Cu Kα radiation at 40 kV and 100 mA, with 2θ ranging from 5° to 80°. The physicochemical analysis of water from the eight networks before ice pigging is provided in Table S3 in Appendix A. In the long-term monitoring process, turbidity and the residual chlorine concentration were measured using a turbidimeter (2100Q, HACH) and free chlorine kit (DR300, HACH), respectively.
2.6. Microbial community structural analysis
Water samples (1 L) were filtered through 0.22 µm nitrocellulose membrane filters (Kovmiye, China). The filters were preserved in the dark at −80 °C for subsequent DNA extraction. The subsequent analyses were performed by Shanghai Personal Biotechnology Co., Ltd., China, with polymerase chain reaction (PCR) amplification (V3V4 region) of the 16S ribosomal ribonucleic acid (rRNA) using the universal primer sets 338F (5′-ACTCCTACGGGAGGCAGCA-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). The biological information was analyzed via the online platform Personalbio Genescloud
†, based on the Illumina Sequencing Platform [
17]. The PCR programs were as follows: 95 °C for 5 min, 40 cycles of 95 °C for 15 s, and 60 °C for 30 s. The results are presented as the mean value ± standard error. Correlations between bacteria (genus level) and physicochemical parameters were calculated using the Correlation Plot application in Origin Pro 2021 software (version 9.8.0.200, Origin Lab Inc., USA).
3. Results and discussion
3.1. Chloride and total iron concentrations in bulk water before and after ice pigging
The ice slurry carrying various substances will not be fully discharged from the outlet if the hydraulic conditions and valves in the network are not carefully controlled, causing localized water quality degradation. The change in chloride concentration before and after ice pigging was used to indicate whether the ice slurries were fully discharged because the ice pigs contained 3%-5% NaCl. Moreover, elevated chloride concentrations in bulk water adversely affect iron release and cause water contamination [
18], [
19]. As shown in
Fig. 2(a), chloride concentrations distinctly increased after ice pigging only in two groups of samples, namely, 11NCI200-150/PE110 and 11NCI50/PE110, which increased by (9.15 ± 0.13) and (25.84 ± 0.04) mg·L
−1, respectively. In these two projects, the ice pigs traveled through networks of cast iron and polyethylene (PE) pipe sections with small diameters, which probably had greater risks of retaining residual ice slurry. This result was attributed to the different rheological behavior, flow pattern, and melting of the ice slurry when it flowed through different conducting pipe materials [
10], [
20], [
21]. Thus, there were difficulties associated with the smooth discharge of the ice slurry over a short distance in a short period of time. In practical operation, ice pigging projects are recommended to be separately performed in pipes with two different materials with small diameters if conditions permit. When the actual on-site conditions are limited, carefully controlling network valves and increasing flushing water are recommended to avoid retention of residual saline pigs.
As shown in
Fig. 2(b), the total iron in bulk water collected before and after ice pigging was determined. A sharp increase in total iron was found after ice pigging (131.98 μg·L
−1) only in 11NCI200-150/PE110, possibly because the ice slurry traveled through seven bends and two branches in ductile iron pipelines with diameters of lower than 200 mm and was discharged from three fire hydrants simultaneously. Four of the eight networks cleaned by ice pigs were located in communities where a large number of bends and branches existed (11NCI200-150/PE110, 6PE110, 11NCI150/PE110, and 12NCI150). However, only the total iron concentration in 11NCI200-150/PE110 after ice pigging increased sharply. The ice pigs were pushed through several bends and branches in all four networks but were discharged from more than one outlet at once only in 11NCI200-150/PE110. Generally, many materials are attached to the inner surface of bends and branches in pipes with small diameters. The shear stress offered by the ice slurry would probably cause iron release when the slurry travels through bends and branches [
22]. Additionally, operators usually have difficulties in precisely locating ice pigs and receiving them from several outlets simultaneously in practical projects owing to unknown and complex phenomena occurring inside the pipes [
23]. The collection of full saline slugs is critical if ice pigs travel through more than one branch. If the ice pig is discharged from several outlets at the same time, as shown in Fig. S1(a) in Appendix A, dirty slugs may be received at consumer taps, leading to complaints. It is recommended to discharge the ice slurry from one outlet in one step (cleaning branches one by one) if it travels through bends and branches, as shown in Fig. S1(b) in Appendix A. Optimal path planning is the key to success in ice pigging projects. As illustrated in Table S4 in Appendix A, the total iron concentration remained stable in the two-year monitoring period in treated and tap water after ice pigging. The temporary increase in the total iron level is not likely to deteriorate drinking water quality during long-term service.
3.2. Distribution and cluster assessment of trace elements in bulk water before and after ice pigging
The water samples used for cluster analysis before ice pigging were collected from selected hydrants approximately 15 min after the hydrants were opened. During the sampling process before ice pigging, certain trace elements experienced interference from the bulk water, subsequently being released back into the bulk water. The water samples utilized for clustering after ice pigging were obtained from the selected hydrants until the effluent turbidity dropped below 1 NTU. During sampling after ice pigging, some trace elements were disturbed by the ice pigs and were released back into the bulk water. Cluster analysis was applied to identify which trace elements would be released back into the bulk water collectively during ice pigging projects.
The trace element clustering analysis in eight pipe networks before and after ice pigging is shown in
Figs. 2(c) and
(d). Fe was divided into a single branch for clustering among the trace elements before ice pigging. B and Ba were clustered into one group in a parallel branch to Fe. Other trace elements were clustered into one category under branches parallel to B and Ba. After ice pigging, B and Ba were clustered into one group in a branch parallel to As. The category containing B, Ba, and As was under a branch parallel to Fe. Other trace elements were clustered into one group in a branch parallel to B, Ba, and As. The simple clustering relationship suggested that the sediments in the eight networks absorbed few heavy metals [
24].
The concentrations of B, Ba, and As in both treated water and tap water monitored over a period of two years (starting from a year and a half before the eight ice pigging projects) are presented in Tables S5-S7 in Appendix A. These concentrations remained below 200.0, 10.0, and 0.4 μg·L
−1, respectively, which were much lower than the regulated limits in China (500, 700, and 10 μg·L
−1, respectively). However, during ice pigging, the B, Ba, and As present in scales, loose deposits, and biofilms may be released under the interference of bulk water and the ice slurry [
25]. Consequently, the concentrations of B, Ba, and As in the bulk water before and after ice pigging within the eight networks ((138.36 ± 75.97), (66.23 ± 23.82), and (40.40 ± 35.92) μg·L
−1, respectively) exceeded the concentrations observed during long-term monitoring. Except for Fe, B, Ba, and As, the concentrations of trace elements in bulk water remained below 10 μg·L
−1. Since the concentrations of B, Ba, and As were closer, these elements displayed a higher degree of correlation compared to the others. These findings further confirm that ice pigging can be advantageous in removing trace elements that have accumulated in scales, sediments, and biofilms over an extended period of time. Additionally, long-term monitoring after ice pigging revealed that the concentrations of these elements remained at trace levels in the bulk water, demonstrating that ice pigging does not have a long-term detrimental impact on water quality.
3.3. Microscopic structure and chemical composition of sediments carried by the ice pigs from the eight networks
Photographs of sediments carried from the eight networks by the ice pigs were obtained in Fig. S2 in Appendix A. The microstructures of sediments carried by the ice slurry from the eight networks could be categorized into two types, as shown in Figs. S3-S11 in Appendix A. The first type comprised smooth lumps of minerals interwoven with flakes or spherical crystals, inferred to be cement materials and sand particles with metallic elements, such as Mg, Al, and Ca (Figs. S11(a-i)-(a-iv)). The second type exhibited amorphous shapes with samples collected from 6PE110 (Figs. S11(b-i)-(b-iv)), which were similar to micrographs of polycarbonate film surfaces in contact with deionized water for 250 d [
26]. It was inferred that the irregularly shaped materials were microplastics stripped from plastic pipes by ice pigs. However, the samples collected from 11NCI200-150/PE110, 11NCI150/PE110, and 15NCI400/PVC425 network did not have microplastics because the deposited layers in the plastic pipes took time to generate and become stable. In practical engineering, it is critical to prevent destruction to the inner wall of relatively new plastic pipes when utilizing ice pigs.
The EDS analysis of sediments collected from the eight networks is shown in Figs. S12-S19 and Table S8 in Appendix A. Sediments collected from the eight networks contained relatively high contents of Si, O, C, and Mg, which are considered the main elements in soil and cement [
24]. The high Al content in some samples could be due to the chemical coagulants used in water treatment plants. Various trace metals, such as Pt, Mn, and Ba, were found in the sediments driven out of pipes by ice slurries [
27]. Sediments and heavy metals formed through the deposition process were likely to be released back into bulk water due to variations in hydraulic regimes in pipelines, such as water demand peaks, bursts, or firefight events [
28]. The bulk water would probably turn yellow or black after the release occurred [
29]. Dynamic and complex sediments within pipes provide shelters for bulk water contaminants [
30]. Therefore, the systematic utilization of ice pigging for sediment control can protect drinking water safety.
The crystal structures and contents of the sediments carried by the ice pigs from the eight networks are shown in
Fig. 3 and Table S9 in Appendix A, respectively. Quartz (silicon in 11NCI200-150/PE110) and muscovite were detected in the eight samples. Quartz might originate from sand used for filtration in water treatment plants or particles entering water distribution systems with the original water, followed by deposition on pipe walls [
31]. Muscovite, a common building material, may be the cement mortar lining the inner pipe wall or loose deposits from pipelines. It was reported that typical components of pipe scale were oxyhydroxides goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), magnetite (Fe
3O
4), hematite (Fe
2O
3), ferrous hydroxide (Fe(OH)
2), ferric hydroxide (Fe(OH)
3), and siderite (FeCO
3) [
32]. The abovementioned materials were not detected in the sediments removed by the ice pigs from pipes, indicating a low possibility that the ice slurry would strip away passivated oxide surfaces from metal-based pipes and expose fresh metal to corrosion effects [
32].
3.4. Bacterial community structure in bulk water before and after ice pigging
The ten most abundant phyla in water samples collected from the eight networks before and after ice pigging are shown in
Fig. 4(a). The remaining reads were placed in the “others” category. Water samples collected from the eight networks were dominated by members of the phylum Proteobacteria (45.25%-98.35%), followed by Firmicutes (1.16%-23.94%), which was consistent with previous studies [
33], [
34]. The sum of the relative abundances of Proteobacteria and Firmicutes in 15NCI600 (64.22% before ice pigging and 75.52% after ice pigging) was considerably lower than that in the other seven networks (higher than 89.28% both before and after ice pigging), probably owing to the influence of the pipe diameter. Liu et al. [
35] reported that bacterial communities in pipes with larger diameters had greater diversity. A relatively high diversity denoted a metabolically versatile community that is capable of adapting to changes in environmental conditions within the pipe [
36], [
37]. This was also validated by the larger mean Chao1 and Shannon indices before (1931.87 and 7.26) and after ice pigging (2145.67 and 6.74) in 15NCI600 than in other samples (Table S10 in Appendix A).
As shown in
Fig. 4(b), the Proteobacteria classes were detected in the following order of decreasing abundance: Gammaproteobacteria (5.42%-80.33%), Alphaproteobacteria (11.98%-76.73%), and Deltaproteobacteria (0-3.22%). Firmicutes mainly comprised Bacilli at the class level. The abundance of Bacilli was relatively high (47.08%) in 12NCI400 before ice pigging. Bacilli contain many members capable of producing endospores, favoring regrowth and multiplication in low-nutrient conditions [
38]. The 12NCI400 network before ice pigging had been cleaned using ice pigs nine months prior, which was likely to change the living environment of bacteria. Bacilli could persist in the environment changed by ice pigging, indicating that the long-term influence of ice pigging on the bacterial community of DWDSs requires further research. The absolute concentrations of the ten most abundant taxa in the water collected from the eight networks before and after ice pigging at the phylum level and class level are shown in Tables S11-S14 in Appendix A.
The heatmap of the 15 most abundant genera in water samples collected from the eight networks before and after ice pigging is shown in
Fig. 5(a). At the genus level,
Pseudomonas,
Phreatobacter,
Vulcaniibacterium,
Cupriavidus,
Bacillus,
Hyphomicrobium,
Chloroplast, and
Paenibacillus were abundant in the water samples before and after ice pigging.
As shown in
Fig. 5(b), the Chao1 richness median value of bulk water collected from the eight networks decreased from 914.57 before ice pigging to 784.22 after ice pigging. The Shannon diversity median value of bulk water collected from the eight networks decreased from 4.84 before ice pigging to 3.81 after ice pigging. The decline in Chao1 and Shannon indices indicated that ice pigging is a promising technique to control the bacterial richness and diversity of bulk water. The water samples from the other seven networks before and after ice pigging had more than half of the genera in common, as shown in
Fig. 5(c). Additionally, 95% of the most abundant taxa (> 1% relative abundance) were shared by the samples before and after ice pigging in each network, as shown in Table S15 in Appendix A. Therefore, it could be speculated that the relative abundance of the genera in bulk water changed via ice pigging was probably below 1%, and ice pigging had a low probability of substantially affecting the bacterial community of bulk water. Based on the bacterial community structure before and after cleaning, the main species (relative abundance > 5% in at least one network) in bulk water originating from biofilms were
Vulcanibacterium, Cupriavidus, Hyphomicrobium, Anoxybacillus, and
Paenibacillus (Fig. S20 in Appendix A).
3.5. Correlations between physicochemical parameters and bacterial community in bulk water before and after ice pigging
The relation between the six physicochemical parameters, namely, pipe service age, temperature, conductivity, chloride, sulfate ion, and total iron, and the 15 most abundant taxa, were measured and computed using Pearson’s correlation (
Fig. 6). Pipe service age, temperature, and chloride and total iron concentrations were the four most influential parameters. In contrast, conductivity and sulfate ion concentration did not significantly impact the most abundant taxa, and the correlation coefficients were lower than 0.30 and 0.37, respectively.
Chloride concentration exhibited strong correlations with
Vulcaniibacterium,
Cupriavidus, and
Undibacterium, with correlation coefficients of 0.64, 0.58, and 0.80, respectively. The highest relative abundance of these three genera in all water samples was in the 4.53%-46.67% range. Moreover, pipe service age, temperature, and total iron concentration were highly correlated with
Paenibacillus (correlation coefficients = 0.45, −0.42, and −0.48),
Chloroplast (correlation coefficients = 0.41, −0.49, and −0.48),
Lactobacillus (correlation coefficients = 0.32, −0.48, and −0.55),
Rhodococcus (correlation coefficients = 0.41, −0.50, and −0.59),
Obscuribacterales (correlation coefficients = 0.44, −0.65, and −0.67), and
Delftia (correlation coefficients = 0.43, −0.41, and −0.50). The highest relative abundance of these six genera in all water samples was in the 2.70%-16.65% range. The relative abundance of the six genera correlated positively with pipe service age. However, they correlated negatively with temperature and total iron concentration. The correlation values followed the order of total iron concentration > temperature > pipe service age. However, the level of difficulty associated with obtaining data followed the order of total iron concentration > temperature > pipe service age. The total iron concentration measurement required samples taken from practical engineering and detected by ICP-MS, while the temperature and pipe service age could be obtained from the water supply company databases. In practical projects, obtaining one of the three parameters according to the actual conditions can help predict the relative abundance of some specific genera and guide ice pigging operations [
39].
3.6. Long-term monitoring of turbidity, residual chlorine, and total iron concentration after ice pigging
The long-term monitoring data for turbidity and residual chlorine (for four months from October 2021 to January 2022) before and after ice pigging are shown in
Fig. 7, respectively. Four-month monitoring data (from October 2020 to January 2021) of residual chlorine a year before ice pigging in bulk water collected from taps supplied by the eight networks are provided in Fig. S21 in Appendix A. The trend in turbidity (decreased significantly and remained at a stable level) demonstrated that turbidity and associated discoloration risk could be reduced after ice pigging over a long period of time. The increased residual chlorine concentration indicated that the intact cell count of bulk water in eight networks decreased after ice pigging [
40]. The residual chlorine concentration of the eight networks without ice pigging remained relatively stable from October 2020 to January 2021, which excluded the factor that chlorine decays more slowly at lower temperatures [
41]. The total iron concentration data for long-term monitoring (for two years from January 2020 to December 2022) before and after ice pigging is shown in Table S4. The total iron concentration increased from its stable low level only temporarily after ice pigging, causing no deterioration in drinking water quality.
4. Implications and conclusions
In this study, the impact of ice pigging on the passivated oxide surfaces on pipe walls and the bacterial community structure in bulk water was comprehensively determined based on field work. First, the simultaneous discharge of ice pigs from several outlets in small-diameter networks with bends and branches should be avoided to reduce the risk of ice pigs losing and iron release. Second, no disturbance to the passivated oxide surface was observed in all pipe networks. Third, ice pigging significantly decreased bacterial richness and diversity in bulk water. Finally, a satisfactory correlation was established between pipe service age, temperature, total iron, and chloride concentrations and the 15 most abundant taxa in bulk water, providing potential evidence to guide practical ice pigging operations.
This study can enable ice pigging operators to adjust individual factors, mitigating the risk of water quality degradation and subsequent customer complaints. Furthermore, the longer-term impact of ice pigging on water quality in DWDSs needs further study. The data would help water utilities select proper maintenance intervals as bulk water quality changes and maintain a balance between capital investment (e.g., water treatment improvement and pipe renewal) and operational expenditure (e.g., network maintenance frequency decisions). In conclusion, the systematic use of ice pigging for distribution system management could protect drinking water safety and public health.
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (52100015), the Zhejiang Provincial Natural Science Foundation of China (LQ22E080018), and the China Postdoctoral Science Foundation (2021M692860).
Compliance with ethics guidelines
Yujing Huang, Zhiwei Chen, Guilin He, Yu Shao, Shuang Song, Feilong Dong, and Tuqiao Zhang declare that they have no conflict of interest or financial conflicts to disclose.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2023.09.016.
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