Comparison of modern drinking water network maintenance methods: Evaluation of removed deposits in the form of total suspended solids (TSS)

International Journal of Environmental Research and Public Health Article Comparison of Modern Drinking Water Network Maintenance Methods: Evaluation of Removed Deposits in the Form of Total Suspended Solids (TSS) Petra Jurek Vidláˇrová* and Silvie Heviánková   Citation: Jurek Vidláˇrová, P.; Heviánková, S. Comparison of Modern Drinking Water Network Maintenance Methods: Evaluation of Removed Deposits in the Form of Total Suspended Solids (TSS). Int. J. Environ. Res. Public Health 2021,18, 4311. https://doi.org/10.3390/ ijerph18084311 Academic Editor: Paul Tchounwou Received: 12 March 2021 Accepted: 15 April 2021 Published: 19 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Environmental Engineering, Faculty of Mining and Geology, VSB-Technical University of Ostrava, 708 00 Ostrava, Czech Republic; [email protected] *Correspondence: petra.vidlar[email protected] Abstract: Water pipe sediment removal should be implemented as an integral part of water mains maintenance in order to steadily supply consumers with drinking water of high quality. Considering the number of different water pipe sediment removal methods, the article aims to evaluate the currently used methods to remove water pipe sediment from the pipes of the drinking water distribution system. The evaluation compares the implementation requirements of each method as well as the quality and the quantity of the removed products. The tested methods were unidirectional flushing, Comprex ® , and Ice Pigging ® . The results of the comparison are expressed in terms of total suspended solids (TSS) recovery, metals mass concentration and water consumption. Since contamination can settle along the entire surface of the pipeline, it is most appropriate to recalculate the results per unit area of the pipeline. The results point at the following efficiency the Comprex ® method was the most efficient in removing TSS, Ice Pigging ® was the next and unidirectional flushing removed a negligible amount of TSS compared to the other two methods. The absolute recovery of TSS was 0.12–3.01 g · m −2 in unidirectional flushing of plastic pipes, 1.58–8.54 g · m −2 in unidirectional flushing of metal pipes, 4.36–47.53 g · m −2 in Ice Pigging ® , and 5.19–69.23 g · m −2 in Comprex ® . The composition of the sediment was strongly influenced by particle origin: Pipe material affected the crystalline phase of the sediment and the water source and the age of the pipe affected the amorphous phase of the sediment. Therefore, it was found that evaluation of efficiency based on the amount of TSS removed is only suitable for sites that meet the same conditions as pipe material, water source and ideally the pipe age. It has further been found that the Comprex ® method can be advantageously used in real conditions to clean pipes with insufficient hydraulic conditions (such as with a high level of incrustation), as the cleaning has low water flow velocity requirements. Keywords: drinking water distribution system (DWDS); unidirectional flushing; air scouring; Ice Pigging®; Comprex®; total suspended solids (TSS) 1. Introduction Supplying consumers with high-quality drinking water is a challenging and complex process. It places high demands on water producers to ensure proper protection of water sources, its treatment, accumulation, and distribution to final consumers [ 1 ]. The total length of the drinking water network in Europe is 4,225,527 km [ 2 ]. This infrastructure requires maintenance and investments to provide clean and wholesome water for all. Therefore, some of the basic preconditions to ensure the supply of high-quality drinking water to all consumers are the proper operation of water infrastructure, its construction, renewal and maintenance, in particular [2]. The renewal of the water supply network and the development of water supply network monitoring significantly reduce water losses in the water supply network. In certain EU member states, the average daily demands (ADD) per capita are decreasing (e.g., in the Czech Republic ADD has practically halved since 1989 to date; from 1.25 million m3·y−1 Int. J. Environ. Res. Public Health 2021,18, 4311. https://doi.org/10.3390/ijerph18084311 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021,18, 4311 2 of 16 to 585,000 m 3 .y −1 ) [ 3 ]. This has a positive effect especially in terms of saving water resources as the Earth is currently struggling with a decrease in the yield of underground resources [ 4 ] or flow in watercourses [ 5 ] in many localities. However, the water savings may have a negative impact on the water quality due to its longer residence time in the pipeline [ 6 ], [ 7 ], where the resulting water quality is altered in sensory and microbiological parameters (i.e., heterotrophic plate count) [8,9]. All EU countries place high demands on the quality of supplied drinking water, and water management companies have begun to value consumers as valuable customers [ 10 ], [ 11 ]. Therefore, efforts have been increasing to reduce the occurrence of adverse phenomena, such as turbid events in the water supply network, which the consumer feels the most. On the other hand, the distribution system can never be free from all particles or microorganisms, being a complex set of chemical and biological reactants [ 12 ]. However, the presence of sediments and microorganisms can be reduced significantly by a correctly selected and frequent maintenance method for their removal [13]. The commonly-used methods for distribution pipe cleaning [ 14 ] may be summarized as pigging, Ice Pigging ® , air scouring (AS), and water flushing. Recently, Neutral Output Discharge Elimination System (NO-DES) method has been introduced, which functions as aboveground loops of distribution system between two hydrants using hoses and the water is circulated within the temporary loop at scouring velocities through the water main and filters using pump mounted on the truck [ 15 ]. The method most authors focus on is the so-called unidirectional flushing [ 12 , 16 – 22 ]. While the highest possible flow rates of water in the pipeline are achieved, accumulated sediment or parts of biofilm are removed from the pipe surface [ 19 , 21 ]. The principle of this method is summarized by [ 23 ] based on the knowledge of many authors. Recommended flow velocities are in the range of v = 1.0–1.6 m·s−1 [ 12 , 24 ]. These velocities are sufficient for the removal of loose and cohesive deposits [19] and partly for tuberculated pipes [24]. The water pipe can be clogged with fine sediment at the bottom of the pipe or by chemical deposits on the pipe walls, which can fill almost the entire pipe diameter [ 25 ]. These sediments may be soft, easily removable to solid requiring aggressive cleaning methods [ 7 , 18 ]. Therefore, a higher velocity and higher shear stress are necessary to clean the inner surface of the pipe and its adherent material [ 12 , 24 ]. However, this is often problematic when using the unidirectional flushing method in real operation conditions, which is confirmed by the results obtained. Higher shear stress can be achieved by air scouring and Ice Pigging ® , each of these methods uses a different cleaning principle. During air scouring compressed air is blown into the pipe, together with a liquid, which increases the shear stress at the pipe walls and scours and flushes out the sediment from the pipe [ 12 ]. Air scouring has been modernized to a software-driven impulse flushing which is marketed under trade names. During Ice Pigging ® the friction at the pipe walls is 2–4 orders of magnitude higher than at the same speed of a water stream during flushing [ 26 ], because the friction is increased using two-phase ice slurry in form of a plug moving down the pipe and dislodging built-up material [ 27 ]. Mostly a mixture of 5% food salt and potable water is used in the process [28,29]. Information about the amount and origin of deposits is useful to prevent their formation [ 19 ], therefore authors evaluate the efficiency of removal and composition of the deposits. The majority of studies focus on unidirectional flushing [ 12 , 19 , 22 , 30 ]. However, it has been found that limited knowledge exists on the comparison of the modern methods, such as Ice Pigging ® and air scouring, even in terms of operational requirements or in terms of efficiency. The existing studies only compared unidirectional flushing or pigging [ 12 , 18 ] or examined their efficiency in laboratory conditions [31]. The aim of the research reported here is to provide novel information regarding the efficiency of Ice Pigging ® and air scouring based on loose and adherent deposits removal and total water consumption. The aim is to compare two commercial methods called Ice Pigging ® and software-driven impulse flushing Comprex ® , and to contrast the results with these obtained by unidirectional flushing. This new knowledge on suitable Int. J. Environ. Res. Public Health 2021,18, 4311 3 of 16 operation conditions obtained by testing methods in practice can optimize the performance of the distribution system cleaning. Air scouring is not involved in general evaluation because only one application of AS was performed, and enough data was not obtained. Air scouring results are in the main text mentioned only for comparison with the effectivity of software-driven process Comprex®. 2. Methodology 2.1. Selected Network Characteristics To address these objectives, full-scale tests were carried out in test zones in the water distribution system of a 300,000-citizen city, which consists of 1064.8 km of pipes with the capacity of 1.875 L · s −1 . Almost 16.5 km of the distribution system was monitored during the cleaning processes applying Ice Pigging ® , Comprex ® and unidirectional flushing. Ice Pigging ® was applied to 75% of investigated pipes length, unidirectional flushing to 13% of investigated pipes length, and Comprex ® to 12% of investigated pipes length. The average length of cleaned section was for (a) Ice Pigging ® l IP = 1749.1 m; (b) Comprex ® lCO = 363.3 m; (c) unidirectional flushing lUNI = 197.3 m. According to Macek and Škripko [ 16 ], it is possible to carry out unidirectional flushing up to pipe nominal diameter DN = 300 mm. Minimum recommended speed [ 12 , 24 ] for this nominal diameter is equal to the instantaneous flow of Q = 63.6 L · s −1 . In the tested operating conditions, this flow is often limited by hydrodynamic pressure and hydrant capacity, therefore it was possible to flush effectively only sections up to DN = 150 mm . Comprex ® method is capable of cleaning pipes of nominal diameters up to DN = 1200 mm [ 32 ]. Candy et al. [ 33 ] state that Ice Pigging ® can be applied to clean the diameter up to DN = 750 mm. Sections up to DN = 450 mm have been tested under real operating conditions [ 33 ]. Based on this information, pipes suitable for cleaning were selected as follows (a) pipes from DN = 80 mm to DN = 150 mm in unidirectional flushing; (b) pipes from DN = 100 mm to DN = 250 mm in Ice Pigging ® ; (c) pipes from DN = 80 mm to DN = 200 mm in Comprex ® method. Tested localities were named by code (Figure 1), which characterizes the selected method and basic properties of the cleaned section, such as pipe diameter (DN) and pipe material. The frequency of the experiments was influenced by the cost of the methods being evaluated. No cleaning interventions have been reported since the pipe was commissioned. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 3 of 16 Pigging® and software-driven impulse flushing Comprex®, and to contrast the results with these obtained by unidirectional flushing. This new knowledge on suitable operation conditions obtained by testing methods in practice can optimize the performance of the distribution system cleaning. Air scouring is not involved in general evaluation because only one application of AS was performed, and enough data was not obtained. Air scouring results are in the main text mentioned only for comparison with the effectivity of softwaredriven process Comprex®. 2. Methodology 2.1. Selected Network Characteristics To address these objectives, full-scale tests were carried out in test zones in the water distribution system of a 300,000-citizen city, which consists of 1064.8 km of pipes with the capacity of 1.875 L·s−1. Almost 16.5 km of the distribution system was monitored during the cleaning processes applying Ice Pigging®, Comprex® and unidirectional flushing. Ice Pigging® was applied to 75% of investigated pipes length, unidirectional flushing to 13% of investigated pipes length, and Comprex® to 12% of investigated pipes length. The average length of cleaned section was for (a) Ice Pigging® lIP = 1749.1 m; (b) Comprex® lCO = 363.3 m; (c) unidirectional flushing lUNI = 197.3 m. According to Macek and Škripko [16], it is possible to carry out unidirectional flushing up to pipe nominal diameter DN = 300 mm. Minimum recommended speed [12,24] for this nominal diameter is equal to the instantaneous flow of Q = 63.6 L·s−1. In the tested operating conditions, this flow is often limited by hydrodynamic pressure and hydrant capacity, therefore it was possible to flush effectively only sections up to DN = 150 mm. Comprex® method is capable of cleaning pipes of nominal diameters up to DN = 1200 mm [32]. Candy et al. [33] state that Ice Pigging® can be applied to clean the diameter up to DN = 750 mm. Sections up to DN = 450 mm have been tested under real operating conditions [33]. Based on this information, pipes suitable for cleaning were selected as follows (a) pipes from DN = 80 mm to DN = 150 mm in unidirectional flushing; (b) pipes from DN = 100 mm to DN = 250 mm in Ice Pigging®; (c) pipes from DN = 80 mm to DN = 200 mm in Comprex® method. Tested localities were named by code (Figure 1), which characterizes the selected method and basic properties of the cleaned section, such as pipe diameter (DN) and pipe material. The frequency of the experiments was influenced by the cost of the methods being evaluated. No cleaning interventions have been reported since the pipe was commissioned. Figure 1. Code characterizing the experiments’ numerical order and cleaning conditions-used method, pipe nominal diameter and pipe material (in case of one cleaned section is created by more types, e.g. a hyphen (-) divides particular pipe nominal diameters and a slash (/) divides used types of pipe materials). Figure 1. Code characterizing the experiments’ numerical order and cleaning conditions-used method, pipe nominal diameter and pipe material (in case of one cleaned section is created by more types, e.g. a hyphen (-) divides particular pipe nominal diameters and a slash (/) divides used types of pipe materials). 2.2. Fieldwork Procedure, Monitoring and Sampling According to Vreeburg [ 12 ], Ellison [ 18 ], Carrière et al. [ 19 ], Barbeau et al. [ 22 ], Fann et al. [ 30 ] and Miller et al. [ 32 ], the effectiveness of pipe cleaning methods is often evaluated as total suspended solids removal. For this purpose, in-situ samples (volume of 2 L each) were collected and shipped to the laboratory for analysis. In the case of unidirectional flushing and Comprex ® , sampling was carried out immediately after the Int. J. Environ. Res. Public Health 2021,18, 4311 4 of 16 start of cleaning. In the case of Ice Pigging ® , the first sample was taken after a hydraulic test (described by Fann et al. [ 32 ]) and further samples at the moment as the front of the ice began to approach, which in most cases met one of the conditions: (a) Temperature dropped below 5 ◦ C; (b) conductivity began to rise above normal levels in the area (up to K = 87.3 mS·cm−1). Qualitative properties of flushed water (turbidity, temperature) were monitored on site during the whole process by digital optic sensor: (a) METTLER TOLEDO InPro 8000 Series (880 nm wavelength, Back-scattered Light principle, Mettler - Toledo, s.r.o., Praha, Czech Republic) for Ice Pigging ® and (b) PONSEL ® (850 nm wavelength; Side-scattered Light principle, TECHNOAQUA, s.r.o., DolníBˇrežany, Czech Republic) for unidirectional flushing. In the case of Comprex ® method, turbidity was measured only by portable turbidimeter Eutech TN-100 (850 nm wavelength, Side-scattered Light principle, Fisher Scientific, spol. s r.o., Pardubice, Czech Republic), as the presence of air in the water impairs the accuracy of the continuous measurement. In all cases, the measurement of turbidity was complemented by measuring total iron by Ferrover ® method using Multiparameter Portable Colorimeter HACH DR900 (HACH LANGE s.r.o., Praha, Czech Republic). Other parameters measured in-situ are listed in Table 1below as parameters measured on-site during the whole process using on-line sensors. Table 1. Parameters of water monitored on-site by on-line sensors. Method Ice Pigging®AS & Comprex®Unidirectional Flushing Temperature x x Pressure hydrostatic/hydrodynamic x x x Turbidity x x Conductivity x Flow rate x x x 2.3. Cleaning Parameters Unidirectional Flushing was carried out at velocities ranging from 0.42 to 2.76 m·s−1 for metal pipes, and 0.73 to 1.87 m · s −1 for plastic pipes as a function of pipe diameter and available pressure. Only 4 out of 10 sections met the recommended speeds [ 12 , 24 ] for performing unidirectional flushing. Air scouring without the possibility of airflow adjustment was performed only at one location as a test before Comprex ® cleaning. The used compressor was capable to create maximum pressure of 1 MPa and air injection was controlled only by front valve closing. Air pulses were created while the final water flow velocity was v = 0.28 m · s −1 , while airflow could not be controlled. Set operating parameters of Comprex ® cleaning method as a pressure of individual pulses ranged from 1.9 to 5.3 bar (190 kPa to 530 kPa), and the pulse length from 2 to 7 s. These parameters were a function of regular hydrostatic pressure in cleaned pipe and pipe material. A total of 179 to 537 pulses were used. The volume of ice used for Ice Pigging ® is limited to 9 tons by ice manufacturing and storage capacity. This volume together with pipe diameter, material, flow rate and ambient temperature influence the total length of pipe which can be cleaned at a time [ 33 ]. The total volume of ice used for Ice Pigging ® cleaning varied 4–10 m 3 . Ice fraction varied from 75 to 85% (Mode of 80%). The cleaning parameters, including the properties of the cleaned pipes, are summarized in Table 2. In cases where groundwater predominates over surface water (U > G) a specific groundwater-surface water ratio varies according to the instantaneous water-use and it cannot be accurately expressed. Int. J. Environ. Res. Public Health 2021,18, 4311 5 of 16 Table 2. Properties of the cleaned pipes and cleaning parameters (NDA—no data available; DN—nominal diameter; p—hydrodynamic pressure; QA—flow achieved; QR—flow required; WS—water source; G—groundwater— U—underground water). Method Code DN Commissioning Material Length pQA QR WS mm Year m bar L·s−1L·s−1 Unidirectional flushing 1UNI150PVC 150 1984 PVC 299.5 0.39 18.6 17.19 G 2UNI80PVC/PE 80 2000–2007 PVC-PE 279.8 0.15 6.3 5.41 G 3UNI80PE 80 2001 PE 122.9 0.28 9.4 4.30 G + U 4UNI100PVC 100 NDA PVC 199.2 0.41 6.6 8.01 U > G 5UNI100PE 100 2015 PE 134.2 0.05 5.8 6.36 U > G 6UNI100OC 100 1964 OC 93.5 0.06 4.5 7.85 U > G 7UNI80LT 80 1977 LT 135.9 0.04 3.1 5.02 U 8UNI80LT 80 1988 LT 250.5 0.28 13.9 5.02 G + U 9UNI100LT 100 1971 LT 314.2 0.07 3.8 7.85 U > G 10UNI80LT 80 1920 LT 143.5 0.07 2.1 5.02 U > G AS 12AS150LT 150 1985 LT 198.8 NDA 5.0 - G Comprex® 11CO100PVC 100 1975 PVC 371.0 0.50 2.1 - G + U 12CO150LT 150 1985 LT 654.3 1.00 2.7 - G 13CO200LT 200 1970 LT 481.7 0.70 6.9 - U 14CO80LT 80 1977 LT 135.9 0.50 1.4 - U 15CO80LT 80 1953 LT 173.5 1.50 0.9 - G Ice Pigging® 16IP150250PVC/PE/OC 150, 250 1964–2001 PVC-PE-OC 1661.0 5.50 15.2 3.00 U 17IP250OC/LT 250 1964 OC-LT 1381.0 5.80 13.8 3.00 U 18IP150-200OC/LT 150, 200 1953–1966 OC-LT 633.0 NDA 3.00 G 19IP80150OC/LT/PE/PVC 80, 150 1953–1994 OC-LT-PE-PVC 1005.0 NDA 3.00 G 20IP150PVC/PE 150 1994–2008 PVC-PE 2052.0 3.80 14.2 3.00 G 21IP100-150PE 100, 150 2009–2012 PE 2327.0 2.80 11.5 3.00 G 22IP100-150PVC/PE 100, 150 1987–2016 PVC-PE 2638.0 0.50 8.8 3.00 U 23IP100-200PVC/PE 100, 200 1997–2011 PVC-PE 1495.0 0.60 7.5 3.00 G 24IP100-200LT/PVC 100, 200 1979 LT-PVC 687.0 0.40 14.5 3.00 G 25IP150-200LT/PVC 150, 200 1979 LT-PVC 323.0 NDA 14.5 3.00 G Int. J. Environ. Res. Public Health 2021,18, 4311 6 of 16 2.4. Determination of Total Water Consumption As the water quality was monitored during the whole cleaning process, after reaching a turbidity value below 1 NTU, the mass concentration of total iron was measured. The cleaning was completed when total iron concentration met the requirements of Directive (EU) 2020/2184 of the European Parliament and of the Council of 16 December 2020 on the quality of water intended for human consumption. To reach the prescribed limits c(Fe) = 0.200 mg·L−1 , the selected cleaning method had to be combined with conventional flushing. The total water consumption was calculated as the total volume of water consumed from the start of the cleaning process to its end. The total consumption, therefore, included the volume of water consumed for the operation of the selected method and the volume of water consumed for conventional flushing until the drinking water quality standards were reached. The evaluation did not include possible loss in the event of a failure and its removal, if this failure occurred during cleaning. 2.5. Calculation of the Total Deposits Removed as TSS The concentration of total suspended solids (TSS) in collected samples were determined using filtration through glass fiber filters according to European Standard EN 872:2005. Continuous monitoring of turbidity allowed to construct a curve of dependence of the immediate mass concentration of total suspended solids on turbidity and to calculate missing values to obtain more accurate results. The total amount of TSS in each experiment was calculated using the system for modern technical computing Wolfram Mathematica [ 34 ] as a sum of definite integrals of partial areas under a TSS concentration/consumed water volume curve. The values of the cumulative volume of water consumed for flushing were plotted on the x-axis and on the y-axis there were plotted the measured values of TSS corresponding to the consumed amount of water. A linear regression of the parts between the individual measurement points was performed and found equations were used to calculate the definite integral. The equations thus differed for each locality. 2.6. Solid Phase Composition The composition of solid phase was compared using Inductively coupled plasma optical emission spectrometry (ICP-OES) EN ISO 11885:2009, powder X-ray diffraction analysis (Bruker AXS D8 Advance 2 Θ / Θ LynxEye, 40 KV/40 mA radiation, emission line Cu-Kalpha, Bruker EAS GmbH, Hanau, Germany) and electron microscopy using Quanta 650 FEG-Field Emission Scanning Electron Microscope (FEI Czech Republic s.r.o., Brno, Czech Republic) at low-vacuum-50 Pa, 1.4 nm @ 30 kV (SED). Organic matter in the solid phase of sediment was estimated from the loss on ignition (LOI 550 ) of the mass of measured total suspended solids oxidized at 550 ± 5 ◦ C; t = 1 h according to ˇ CSN 75 7350:2008 Further on also as Volatile suspended solids (VSS). 2.7. Liquid Phase Composition Color, turbidity, metals and suspended solids concentration were analyzed in all the samples according to the methods of Inductively coupled plasma optical emission spectrometry (ICP-OES) EN ISO 11885:2009, gravimetric analysis EN 872:2005, spectrophotometry EN ISO 6271:2015 and nephelometry ASTM D7726-11(2016)E1. 3. Results and Discussion Three calculation approaches were used to evaluate the amount of removed sediment in the form of TSS. The total amount of TSS from one specific section cleaning, which was determined by integration of the area under a TSS concentration/consumed water volume curve, was recalculated for a unit of length, a unit of volume and a unit of area of the cleaned pipe. All the results are summarized in Table 3and divided according to implemented cleaning method and the chosen evaluation approach. As the results obtained from unidirectional flushing showed a dependence on the pipe material and sufficient data Int. J. Environ. Res. Public Health 2021,18, 4311 7 of 16 were obtained, this method of cleaning is in Table 3divided into two sections UF of plastic pipes and UF of metal pipes. For each cleaning method, an average and a median are listed. The results are commented on in the following subsections. Table 3. Summary of total sediments recovery as TSS per unit length, unit volume and unit area and water consumption of tested methods including average and median. (UNI—Unidirectional flushing; AS—Air scouring; Recovery 1—Total TSS calculated per length of the pipe; Recovery 2—Total TSS calculated per the inner volume of the pipe; Recovery 3—Total TSS calculated per the inner area of the pipe; * estimated water consumption). Method Code Recovery 1 Recovery 2 Recovery 3 Water Consumption-Pipe Volume Ratio g·m−1g·m−3g·m−2 UNI of plastic pipes 1UNI150PVC 0.14 8.0 0.29 13.7 2UNI80PVC/PE 0.15 29.0 0.59 4.5 3UNI80PE 0.72 143.0 2.86 6.0 4UNI100PVC 0.95 120.0 3.01 2.2 5UNI100PE 0.04 5.0 0.12 1.6 Average 0.40 61.0 1.37 5.6 Median 0.15 29.0 0.59 4.5 UNI of metal pipes 6UNI100OC 2.03 258.0 6.46 3.9 7UNI80LT 0.76 151.0 3.01 9.9 8UNI80LT 2.15 427.0 8.54 4.6 9UNI100LT 0.50 63.0 1.58 1.4 10UNI80LT 0.74 148.0 2.96 4.1 Average 1.23 209.4 4.51 4.78 Median 0.76 151.0 3.01 4.10 AS 12AS150LT 1.82 103.0 3.86 6.0 Comprex® 11CO100PVC 1.63 208.0 5.19 4.0 12CO150LT 5.20 294.0 11.04 2.9 13CO200LT 10.37 330.0 16.50 4.9 14CO80LT 5.83 1160.0 23.21–50.31 13.1–25.3 * 15CO80LT 17.40 3612.0 69.23 9.2 Average 8.09 1120.8 27.70 6.9 Median 5.83 330.0 16.50 4.9 Ice Pigging® 16IP150250PVC/PE/OC 25.27 1065.0 47.53 4.9 17IP250OC/LT 14.46 295.0 18.41 4.6 18IP150200OC/LT No data available 2.5 19IP80150OC/LT/PE/PVC No data available 2.2 20IP150PVC/PE 6.07 344.0 12.89 1.8 21IP100-150PE 1.77 130.0 4.36 3.9 22IP100150PVC/PE 9.86 677.0 23.39 2.6 23IP100200PVC/PE 18.91 965.0 39.58 2.4 24IP100200LT/PVC 15.90 736.0 31.10 4.3 25IP150200LT/PVC No data available 3.1 Average 13.18 601.7 25.32 3.2 Median 14.46 677.0 23.39 2.6 Int. J. Environ. Res. Public Health 2021,18, 4311 8 of 16 3.1. Total Sediments Removed as TSS Total amount of removed impurities in the form of TSS determined by integration was recalculated per (a) unit length, (b) unit volume and (c) unit area of the cleaned pipeline. The results differ depending on the chosen evaluation method. In case of (a) the most impurities were removed as follows: Ice Pigging ® (60%), Comprex ® (36%) and unidirectional flushing (4%). In case of (b) the most impurities were removed by Comprex ® (60%), Ice Pigging ® (33%) and unidirectional flushing (7%), and in case of (c) the most impurities were removed by Comprex ® (50%), Ice Pigging ® (45%) and unidirectional flushing (5%). The percentage was calculated as the ratio of the amount of TSS removed by the selected method to the total amount of TSS removed by all methods always related to a dimension unit. See also Figure 2. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 8 of 16 3.1. Total Sediments Removed as TSS Total amount of removed impurities in the form of TSS determined by integration was recalculated per (a) unit length, (b) unit volume and (c) unit area of the cleaned pipeline. The results differ depending on the chosen evaluation method. In case of (a) the most impurities were removed as follows: Ice Pigging® (60%), Comprex® (36%) and unidirectional flushing (4%). In case of (b) the most impurities were removed by Comprex® (60%), Ice Pigging® (33%) and unidirectional flushing (7%), and in case of (c) the most impurities were removed by Comprex® (50%), Ice Pigging® (45%) and unidirectional flushing (5%). The percentage was calculated as the ratio of the amount of TSS removed by the selected method to the total amount of TSS removed by all methods always related to a dimension unit. See also Figure 2. Figure 2. Effectiveness of methods depending on the chosen evaluation procedure per (a) unit length, (b) unit volume and (c) unit area of the cleaned pipe: The percentage was calculated as the ratio of the amount of TSS removed by the selected method to the total amount of TSS removed by three methods (IP−Ice Pigging®; UNI−Unidirectional flushing; CO−Comprex®). In the case of unit length (a) the smallest amount of removed TSS was gained during unidirectional flushing. Unidirectional flushing of plastic pipes removed 0.04–0.95 g·m−1 and unidirectional flushing of metal pipes removed 0.50–2.15 g·m−1. The total amount per unit length depended on the material of the cleaned pipe and its age, the parameters influencing hydraulic conditions. Comprex® removed 1.63–17.40 g·m−1 (an average of 8.09 g·m−1 TSS of 5 applications) and Ice Pigging® removed 1.77–25.27 g·m−1 (an average of 13.18 g·m−1 TSS of 7 applications). For comparison, Vreeburg [12], Carrière et al. [19] and Barbeau et al. [22] obtained by unidirectional flushing results of removed impurities in form of TSS from 0.1 to 0.4 g·m−1 and Miller et al. [35] using Ice Pigging® from 0.1 to 58 g·m−1. However, comparisons based on unit length are possible to make only for pipes of identical nominal diameters. Therefore, this evaluation approach is not suitable in the case of different pipe nominal diameters and may distort the results. In case of unit volume (b) amount of removed TSS per unit volume, the least impurities were removed by unidirectional flushing of plastic pipes, namely 8.0–143.0 g·m−3 (61.0 g·m−3 of TSS in average) and unidirectional flushing of metal pipes 63.0–427.0 g·m−3 (209.4 g·m−3 of TSS in average). Comprex® removed 208.0–3612.0 g·m−3 (1120.8 g·m−3 of TSS on average) and Ice Pigging® removed 130.0–1065.0 g·m−3 (601.71 g·m−3 of TSS on average). Researchers do not use this expression of results very often, and no comparable data were found. In the case of unit area (c), which is the most appropriate method of result expression as deposits can settle over the entire pipe surface [12]. The following results were Figure 2. Effectiveness of methods depending on the chosen evaluation procedure per ( a ) unit length, ( b ) unit volume and ( c ) unit area of the cleaned pipe: The percentage was calculated as the ratio of the amount of TSS removed by the selected method to the total amount of TSS removed by three methods (IP − Ice Pigging ® ; UNI − Unidirectional flushing; CO−Comprex®). In the case of unit length (a) the smallest amount of removed TSS was gained during unidirectional flushing. Unidirectional flushing of plastic pipes removed 0.04–0.95 g · m −1 and unidirectional flushing of metal pipes removed 0.50–2.15 g · m −1 . The total amount per unit length depended on the material of the cleaned pipe and its age, the parameters influencing hydraulic conditions. Comprex ® removed 1.63–17.40 g · m −1 (an average of 8.09 g · m −1 TSS of 5 applications) and Ice Pigging ® removed 1.77–25.27 g · m −1 (an average of 13.18 g · m −1 TSS of 7 applications). For comparison, Vreeburg [ 12 ], Carrière et al. [ 19 ] and Barbeau et al. [ 22 ] obtained by unidirectional flushing results of removed impurities in form of TSS from 0.1 to 0.4 g · m −1 and Miller et al. [ 35 ] using Ice Pigging ® from 0.1 to 58 g·m−1 . However, comparisons based on unit length are possible to make only for pipes of identical nominal diameters. Therefore, this evaluation approach is not suitable in the case of different pipe nominal diameters and may distort the results. In case of unit volume (b) amount of removed TSS per unit volume, the least impurities were removed by unidirectional flushing of plastic pipes, namely 8.0–143.0 g · m −3 ( 61.0 g·m−3 of TSS in average) and unidirectional flushing of metal pipes 63.0–427.0 g · m −3 (209.4 g · m −3 of TSS in average). Comprex ® removed 208.0–3612.0 g · m −3 (1120.8 g · m −3 of TSS on average) and Ice Pigging ® removed 130.0–1065.0 g · m −3 (601.71 g · m −3 of TSS on average). Researchers do not use this expression of results very often, and no comparable data were found. In the case of unit area (c), which is the most appropriate method of result expression as deposits can settle over the entire pipe surface [ 12 ]. The following results were achieved: unidirectional flushing of plastic pipes removed 0.12–3.01 g/m 2 (an average of 1.37 g · m −2 Int. J. Environ. Res. Public Health 2021,18, 4311 9 of 16 TSS of 5 applications), 1.58–8.546 g · m −2 from the metal pipes (an average of 4.51 g · m −2 TSS of 5 applications). Comprex ® removed 5.19–69.23 g · m −2 (an average of 25.03 g · m −2 TSS of 5 applications), and Ice Pigging ® removed 4.36–47.53 g · m −2 (an average of 25.32 g · m −2 TSS of 7 applications). The amounts of sediment removed varied only in the multiples of tens to hundreds based on the chosen method. The 100–1000 times greater efficiency has not been achieved as is stated by Miller et al. [ 35 ]. Despite the fact, that the required flow rates in the pipeline were not reached in the case of unidirectional flushing of metal pipes, a larger amount of sediment per 1 m 2 was removed from the metal pipes than from the plastic pipes. The larger amount of sediment per 1 m 2 was also removed by Comprex ® method in metal pipes. The same observation was not valid for Ice Pigging method ® , as there were often operational complications with the dosing of ice into the water supply system when cleaning metal pipes, which can have a significant effect on the cleaning process and total recovery of impurities. Next, the results can be compared with the research reported by Fann et al. [ 30 ] who present the results of Ice Pigging ® and unidirectional flushing cleaning method of asbestos cement pipes DN = 150–400 mm, Miller et al. [ 35 ] who present the results for Ice Pigging of cast iron pipes DN = 200 mm, and Barbeau et al. [ 22 ] investigating unidirectional flushing of pipes of DN = 200 mm of cement-lined ductile iron and unlined grey cast iron. The results vary considerably: 2.91–28.85 g · m −2 for asbestos cement pipes, 90.498 g · m −2 and 23.13 g · m −2 for cast iron pipes, and 0.41 for grey cast iron and 0.64 g · m −2 for cement-lined ductile iron. The results may differ due to different samples’ volumes, sampling intervals or cleaning termination points. A commonly used limit for ending the cleaning is by achieving a turbidity value ≤ 5 NTU [ 17 , 19 ]. Different pipe age, which has a direct effect on the condition of the pipeline, can have a great effect (Figure 3). The comparison above confirms the findings of Barbeau et al. [ 22 ] that deposit accumulation (and its recovery) is highly site-specific. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 10 of 16 chosen monitored parameter (turbidity X total iron) and the value [15,39], when cleaning is completed. In the case of Ice pigging consumption may also be influenced if the total water consumption includes water consumed for the necessary hydraulic test. These data are not usually listed-see [30,37]. Figure 3. Total suspended solids and water consumption (red-framed columns indicate incrusted sections, for 18IP150200OC/LT, 19IP80-150OC/LT/PE/PVC and 25IP150-200LT/PVC-data is not available). 3.3. Maximum Impurities Concentration Regardless of the chosen cleaning method, there was always a sharp increase in measured values of observed chemical parameters (iron, manganese and aluminum mass concentration, color and turbidity) when the required hydraulic conditions (or the front of the ice) were reached. All the tested methods equally show a gradual decrease in the concentration of removed impurities until the moment when all parameters met the requirements of Directive (EU) 2020/2184 and the cleaning was finished. However, the time needed to remove the largest portion of impurities differed (Figure 4). Therefore, the achieved maximum values of TSS, color, turbidity and metals’ mass concentration also differ. The highest values of measured parameters were achieved using Ice Pigging ® when removed particles are absorbed the most in front of the ice method (TSS = 924–8100 mg·L −1 ; color > 4000 mg·L −1 Pt; turbidity > 7500 ZF(t); ρ(Fe) = 65.7–627 mg·L −1 ; ρ(Mn) = 3.06–329 mg·L −1 ; ρ (Al) = 32.1–274 mg·L −1 ). While in Comprex ® (TSS = 478–1200 mg·L −1 ; color = 480– 2100 mg·L −1 Pt; turbidity = 66–2300 ZF(t); ρ(Fe) = 62.1–285 mg·L −1 ; ρ(Mn) = 1.06–67.7 mg·L −1 ; ρ(Al) = 1.15–22.9 mg·L −1 ) and unidirectional flushing (TSS = 6.4–450 mg·L −1 ; color = 190– 1600 mg·L −1 Pt; turbidity = 35–950 ZF(t); ρ (Fe) = 0.573–138 mg·L −1 ; ρ(Mn) = 0.098–91.6 mg·L −1 ; ρ(Al) = 0.181–5.02 mg·L −1 ) particles are gradually entrained by the water stream, shear stress influences the results. Since samples taken during Comprex ® had a character of a mixed sample given by a single block of water, therefore the maximum measured concentrations of individual parameters probably do not correspond to the absolute maxima that were reached during cleaning and may be underestimated. The measured results thus showed that color, turbidity or concentration indicators are not suitable for comparing the methods between one other, as three tested methods showed a different Figure 3. 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