HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 52(2) pp.29–34 (2024) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2024-16 EVALUATION OF EXPERIENCES OVER THE EIGHT-YEAR-LONG OPERATION OF THE ULTRAFILTER SETUP CONSTRUCTED FOR THE PROTECTION OF DRINKING WATER IN MISKOLC LILLA HÁRS1*, GÁBOR RÁCZ1, GÁBOR LAKNER1, JÓZSEF LAKNER2 AND NÁNDOR NEMESTÓTHY3 1 Hidrofilt Ltd., Magyar u. 191, Nagykanizsa, 8800, HUNGARY 2 Óbuda University Alba Regia Technical Faculty, Budai u. 45, Székesfehérvár, 8000, HUNGARY 3 Research Group on Bioengineering, Membrane Technology and Energetics, University of Pannonia, Egyetem u. 10, Veszprém, 8200, HUNGARY As a result of careful, persistent work over several years, since the successful technical handover in September 2015, the new water treatment plant is one of the most powerful ultrafiltration drinking water purifiers in Central Europe and continuously ensures high-quality drinking water for the population. Not only was the construction of the water treatment plant essential for the safe supply of drinking water to the city, it is also capable of solving regional water supply problems in the event of a disaster, as the last eight years of its operation have demonstrated. Our present study is intended to summarize the most important operating experiences since the technical handover in September 2015. Keywords: ultrafiltration, drinking water, karst water 1. Introduction The waterworks on average meets more than half of the water demand required by the city of Miskolc and has been continuously operating since 1913. Over the second half of the last century, anthropogenic pollution has become significant. In the event of water pollution caused by raw sewage above the acceptable limit (in the absence of water purification technology), the operator often had to stop producing water. The main cause of the emerging problems is the increase in turbidity and microbiological contamination of karst springs on the surface of the land after periods of rainfall. Since the technology of ultrafiltration provides a suitable and safe solution to the problems that arise, in 2013, after the conclusion of the construction tender, the winning MI-DU-HI Consortium (Duna Aszfalt Ltd., Hidrofilt Ltd.) was able to start constructing a new water purification plant, which was successfully handed over in September 2015. Karst aquifers are an important supply of freshwater for about 25% of the world’s population. The preservation of chemically safe karst water should be of paramount importance to protect human health. Because of the peculiar geographical and hydrogeological circumstances of karst regions, this water is vulnerable to pollution from human activities and the effects of climate change, e.g. steadily decreasing average annual Received: 18 June 2024; Revised: 27 June 2024; Accepted: 28 June 2024 *Correspondence: hars.l@hidrofilt.hu precipitation and global warming [1]-[5]. Water infiltrating the aquifer may originate from precipitation (rain and/or snow) in the karst region itself (diffuse infiltration; autogenic recharge) or accumulate in adjacent regions before flowing into the karst as sinking streams (concentrated, point infiltration; allogenic recharge). Underground, it flows fast through large openings, for instance, caves or conduits (turbulent flow). Alternatively, it can also move slowly through small and narrow openings in low-permeability zones (laminar flow) [6]-[7]. Filtration processes through membranes are increasingly used as alternatives for treating freshwater and wastewater in the anticipation of more stringent quality standards [8]-[11]. However, meeting the quality standards of drinking water using conventional water treatment processes has become increasingly challenging because of the discharge of various organic chemicals and heavy metals into water bodies. The problem is induced by rapid industrialization, urbanization and population growth, which gradually deteriorate surface water quality, affect drinking water security and increase the treatment costs of drinking water [12]. In developing countries, drinking water production is potentially a very large market for ultrafiltration (UF) membranes. Given that one of the most critical problems is the lack of drinking water, people in these regions are supplied with surface water which contains a significant number of https://doi.org/10.33927/hjic-2024-16 mailto:hars.l@hidrofilt.hu HÁRS, RÁCZ, G. LAKNER, J. LAKNER AND NEMESTÓTHY Hungarian Journal of Industry and Chemistry 30 microorganisms that can cause several diseases. Globally, the application of UF technology is increasing with regard to drinking water treatment, with an estimated market value projected to reach $2.14 billion in 2023 [13]. Filtration processes in the treatment of surface water are the fastest developing application of membrane technology [14]-[17]. UF uses low-pressure membranes with pore sizes of between 0.001 and 0.1 µm [18] as well as pore diameters from 1,000 to 100,000 Da [19]-[20]. UF membranes are physical barriers which are able to efficiently remove suspended particles, bacteria, colloids, algae, parasites and viruses as well as prevent turbidity for purification and disinfection purposes. UF technology has many advantages such as the superior quality of treated water, its compact system, easy operation and maintenance, lack of chemicals as well as the minimal production of sludge [11],[12],[14],[21]. Although UF is a promising technology for water and wastewater treatment, it has many significant disadvantages, e.g. membrane fouling and the insufficient removal of soluble contaminants [20]. Membrane fouling refers to the accumulation of particulates and colloids, dissolved organic and inorganic matter as well as microorganisms on the membrane surface and within the membrane pores, resulting in the reduction in its permeability. Consequently, membranes must be cleaned periodically to reduce membrane fouling [19],[22]-[25]. The objectives of this study were to evaluate UF performance in terms of permeability and the quality of treated water as well as present the most important operating experiences. 2. Brief introduction to the water treatment system By modernizing the existing waterworks, an ultrafiltration capacity of 1,500 m3/h was realized with two 500 m3 outdoor filtered water storage tanks. The role of ultrafiltration technology during water treatment is to reduce the content of suspended matter and microbiological pollutants in the water, i.e. to remove colloids, bacteria, viruses and protozoa as well as macromolecules with a molecular weight greater than 100 kDa. During the operation, the dissolved salts and water molecules flow over the surface of the UF membrane before the purified water, after being disinfected, is sent to the surface tanks then into the network. The technical parameters of the equipment are shown in Table 1. 2.1. Feedwater The feedwater for the UF membrane is karst water from caves. The raw water is first sent to the pre-filter unit to ensure its safe. Here, impurities larger than 300 µm are filtered out. 2.2. Ultrafiltration process During water production by ultrafiltration, the well pumps provide the pressure and flow rate necessary for the treatment process during which the resulting filtrate is sent to the clean water storage tank with a nominal flow rate of 198 m3/h per unit. In order to optimize the degree of stress on the membrane, all processes involving liquid flow (backwashing, rinsing, water production, etc.) are carried out alternately from below and above. The UF modules are cleaned immediately based on a timed program or when the TMP (Trans Membrane Pressure) of the filtrate increases (during backwashing). The backwashed water is supplied per device at a flow rate of 600 m3/h as well as at a pressure of 2.5 bars. The typical operating parameters of the equipment are shown in Tables 2 and 3. Table 1: The most important technical parameters of the ultrafiltration equipment Characteristics of the UF equipment Values Quantity of modules 8 units; 40 modules/unit, i.e. a total of 320 modules Total membrane surface 19 200 m2 Module type Inge dizzer XL 0.9 MB 60 T-Rack Pore size 0.02 µm Membrane material PESM Membrane surface/module 60 m2 Table 2: Main operating parameters (during normal operation) Characteristics of the UF equipment Values Gross flux, l/m2/h 82.5 Net flux, l/m2/h 78.1 Yield, % 96.5 Filtering time, min 80 Backwash duration, s 50 Forward flush duration, s 0 Acid CEB frequency, h 168 Base CED frequency, h 168 NaOCl CED frequency, h 168 Table 3: Main forms of chemical cleaning Base Acid NaClO solution Backwashing, s 50 50 50 Chemical injection, s 60 60 60 Soaking, min 15 15 0 Chemical leaching, s 60 60 60 EVALUATION OF THE EIGHT-YEAR-LONG OPERATION OF THE ULTRAFILTER SETUP 52(2) pp. 29–34 (2024) 31 2.3. Data Querying/Service The variable parameters (temperature, flow rate, pressure, turbidity) were measured in real time by a data transmission device. Each variable was queried with millisecond precision. When multiple values were measured within a millisecond, the highest value was recorded. These values were then converted into daily datasets for each variable. Only values greater or equal to 10 were considered during this conversion process which were averaged to determine the daily values. The permeability values were determined individually for the 8 UF units on a daily basis. For easier handling, they were converted into average annual permeability values. The operational values over the past 8 years were analyzed. 3. Results and analysis 3.1. Measurements Between 2016 and 2023, the following parameters were determined on a daily basis (i = 1, 2, ..., 365 denotes the day number): turbidity (zi) from which the daily load (wi) was calculated, daily precipitation (qi), average daily permeability (pi) over 8 measurements, average daily water production and total annual water production. The summarized annual results are presented in Table 4. It should be noted that between 2016 and 2018, no significant maintenance was performed. In 2019, the system was optimized and has been regularly maintained ever since. Linear regression analysis using Excel was employed to determine the correlation between the measured values and their corresponding functions: 𝑦 = 𝛼 ∙ 𝑥 + 𝛽 (1). In addition to determining the parameters, the regression coefficient 𝑟2 was calculated. 𝑟2=1 indicates a perfect correlation, while 𝑟2=0 suggests no correlation. The actual values typically fell between 0 and 1 with values closer to 1 indicating a stronger correlation. Deciding when data are considered to be correlated is a matter of definition (typically a significant correlation is when 𝑟2>0.8). 3.2. Changes in average annual permeability The average annual permeability was calculated using the following relation: �̅� = 𝑝0 + 1 365 ∑ 𝑝𝑖 365 𝑖=1 (2), where i refers to the day number of the year for the nth year �̅� = 𝑝𝑛̅̅ ̅. When the sample sizes are small, the correlation depends not only on 𝑟2 but also on the degrees of freedom (f), which is determined by the sample size (n), e.g. when n=2, 𝑟2=1. This relationship can be expressed as follows: 𝑡𝑓 ≡ √𝑓 𝑟 √1−𝑟2 (3). In the case of linear regression, 𝑓 = 𝑛 − 2, following a Student's t-distribution. From this t-Table, the critical value t95 corresponding to the given probability level and degrees of freedom was determined. If tf > t95, then the function can be approximated by a straight line when the confidence level is equal to 95%. The change in the average annual permeability between 2016 and 2023 is shown in Figure 1. For the correlation line fitted over the entire range, 𝑟2=0.1902, indicating no significant correlation between permeability and time in years. In 2019, the system was optimized, resulting in an increase in permeability. If the operational periods are divided into two intervals, namely 2016–2018 and Table 4: Annual data regarding the measured parameters between 2016 and 2023 Year Annual precipitation (q) (mm) Average turbidity of raw water (z) (NTU) Average permeability (p) (L/m2/h/bar) Average load (w) (kg/day) Average water production (m3/day) Annual water production (m3/year) 2016 874 1.35 304 22.28 16 482 5 557 067 2017 773 1.42 239 26.70 18 795 6 860 189 2018 608 2.52 191 49.24 19 527 7 107 900 2019 687 0.79 267 17.27 21 963 7 533 300 2020 703 1.18 327 22.22 18 898 6 916 680 2021 663 1.35 327 28.03 20 838 7 606 020 2022 588 1.66 304 42.87 25 880 9 446 240 2023 699 2.71 291 54.16 20 023 5 286 010 HÁRS, RÁCZ, G. LAKNER, J. LAKNER AND NEMESTÓTHY Hungarian Journal of Industry and Chemistry 32 2020–2023 representing periods during which maintenance was not (before period) and was carried out (after period), respectively, and separate lines are fitted to the data series for each interval, we obtain the followings: 𝑝𝑏𝑒𝑓𝑜𝑟𝑒̅̅ ̅̅ ̅̅ ̅̅ ̅ ≡ 310 − 57.0 ∙ 𝑡 (4), 𝑝𝑎𝑓𝑡𝑒𝑟̅̅ ̅̅ ̅̅ ̅̅ ≡ 330 − 13.1 ∙ (𝑡 − 4) (5), where p represents the permeability and t denotes the number of years since 2016. In this case, segment-wise correlations show strong results: • before maintenance, 𝑟2=0.9916 and according to Equation 3, 𝑡𝑓=10.8 > t95=6.31, • after maintenance, 𝑟2=0.9683 and 𝑡𝑓=7.81> t95=2.92. Therefore, both relationships can be considered linear at a confidence level of 95%, as shown in Figure 1. In both cases, permeability decreases, however, the rate of decrease before renovation is more than four times that after renovation. Due to regular maintenance and backwashing following renovation, permeability increased, resulting in a slower rate of decrease compared to the period during which maintenance was not carried out. The annual decreases are consistent with the similar annual results obtained, although the ratio is slightly higher in the latter but not significantly. From the two periods (poorly maintained and well maintained), one year was selected for each to examine the daily permeability. Permeability was measured simultaneously through the 8 units and the averages of these measurements are shown in Figures 2 and 3. In 2017, the decrease in average daily permeability was 32%. In 2023, the decrease in average daily permeability was 6%. 3.3. Optimization in 2019 Up until 2019, the UF units only underwent warranty repairs. In this year, permeability decreased, prompting maintenance to be carried out. The trends in permeability in 2019 are shown in Figure 4. Maintenance revealed that the system was improperly optimized. Changes to the control program, CEB (chemical enhanced backwash) frequency and the chemical settings were required. The CIP (cleaning in place) of the 8 UF units was completed. During the integrity test, 20 damaged UF modules were replaced. The period during which the optimization took place is marked in Figure 4. Figure 1: The change in average annual permeability between 2016 and 2023 Figure 2: Average daily permeability as a function of time in the year 2017 (The dots represent the measured values and the dashed line denotes the fitted regression line.) Figure 3: Average daily permeability as a function of time in the year 2023 (The dots represent the measured values and the dashed line denotes the fitted regression line.) Figure 4: Average daily permeability as a function of time in the year 2019 with the optimization period marked EVALUATION OF THE EIGHT-YEAR-LONG OPERATION OF THE ULTRAFILTER SETUP 52(2) pp. 29–34 (2024) 33 4. Conclusions It can be concluded that if the system had not been optimized in 2019, the permeability would have dropped to almost zero by 2021, indicating that the maintenance of systems during the warranty period is crucial. Every system is different, moreover, their optimization and regular professional supervision are very important to help prevent membranes from failing as well as ensure the long-term operational functioning of the system without clogging and flux reduction. In order to correlate the available data, it was important to separate it. Further ongoing analyses of the available data need to be performed to clarify the relationships. Acknowledgements The authors are grateful for the experimental support provided by Ferenc Horváth (Chief engineer at local waterworks). REFERENCES [1] Shi, J.; Jiang, G.; Sun, Z.; Liu, F.; Wang, Q.: The migration and transformation processes of dissolved organic matter in rainwater- drip water- phreatic water of a typical karst spring catchment, in South China, J. Hydrol. (Amst), 2023, 625, 130077, DOI: 10.1016/j.jhydrol.2023.130077 [2] Lang, Y.-C.; Liu, C.-Q.; Zhao, Z.-Q.; Li, S.-L.; Han, G.-L.: Geochemistry of surface and ground water in Guiyang, China: Water/rock interaction and pollution in a karst hydrological system, Appl. Geochem., 2006, 21(6), 887–903, DOI: 10.1016/j.apgeochem.2006.03.005 [3] Selak, A.; Reberski, J.L.; Klobučar, G.: Assessing the persistence, mobility and toxicity of emerging organic contaminants in Croatian karst springs used for drinking water supply, Sci. Total Environ., 2023, 903, 166240, DOI: 10.1016/j.scitotenv.2023.166240 [4] Cusano, D.; Lepore, D.; Allocca, V.; De Vita, P.: Control of soil mantle thickness and land cover types on groundwater recharge of karst aquifers in Mediterranean areas, J. Hydrol. (Amst), 2024, 630, 130770, DOI: 10.1016/j.jhydrol.2024.130770 [5] Puigserver, D.; Giménez, J.; Gràcia, F.; Granell, À.; Carmona, J.M.; Torrandell, A.; Fornós, J.J.: Effects of global and climate change on the freshwater- seawater interface movement in a Mediterranean karst aquifer of Mallorca Island, Sci. Total Environ., 2024, 912, 169246, DOI: 10.1016/j.scitotenv.2023.169246 [6] Kogovšek, B.; Jemcov, I.; Petrič, M.: Advanced application of time series analysis in complex karst aquifers: A case study of the Unica springs (SW Slovenia), J. Hydrol. (Amst), 2023, 626, 130147, DOI: 10.1016/j.jhydrol.2023.130147 [7] Li, J.; Yuan, D.; Liu, J.; Ma, M.; Li, Y.: Evaluating the effects of water exchange between surface rivers and karst aquifers on surface flood simulations at different watershed scales, J. Hydrol. (Amst), 2023, 623, 129851, DOI 10.1016/j.jhydrol.2023.129851 [8] Alonso, E.; Santos, A.; Solis, G.J.; Riesco, P.: On the feasibility of urban wastewater tertiary treatment by membranes: a comparative assessment, Desalination, 2001, 141(1), 39–51, DOI: 10.1016/S0011-9164(01)00387-3 [9] Ang, W.L.; Mohammad, A.W.; Hilal, N.; Leo, C.P.: A review on the applicability of integrated/hybrid membrane processes in water treatment and desalination plants, Desalination, 2015, 363, 2–18, DOI: 10.1016/j.desal.2014.03.008 [10] Kaya, R.; Yuksekdag, A.; Korkut, S.; Turken, T.; Pasaoglu, M.E.; Ersahin, M.E.; Ozgun, H.; Koyuncu, I.: Impact of membrane configuration on the performance and cost of a pilot-scale UF process treating surface water, Sep. Purif. Technol., 2023, 304, 122414, DOI: 10.1016/j.seppur.2022.122414 [11] Belafi-Bako, K.; Toth, G.; Nemestothy, N. Application of polymer membranes in downstream processes, Phys. Sci. Rev., 2020, 5(7), 20180070, DOI: 10.1515/psr-2018-0070 [12] Lee, Y.-G.; Shin, J.; Kim, S.J.; Cho, K.H.; Westerhoff, P.; Rho, H.; Chon, K.: An autopsy study of hollow fiber and multibore ultrafiltration membranes from a pilot-scale ultra high-recovery filtration system for surface water treatment, Sci. Total Environ., 2023, 866, 161311, DOI: 10.1016/j.scitotenv.2022.161311 [13] Xu, D.; Xie, Y.; Jin, X.; Ren, J.; Song, J.; Tang, X.; Zhang, Z.; Li, X.; Li, G.; Liang, H.: A comparison of typical ultrafiltration processes in drinking water treatment: Implications for fouling control and disinfection performance, Sep. Purif. Technol., 2024, 338, 126426, DOI: 10.1016/j.seppur.2024.126426 [14] Guo, X.; Zhang, Z.; Fang, L.; Su, L.: Study on ultrafiltration for surface water by a polyvinylchloride hollow fiber membrane, Desalination, 2009, 238(1-3), 183–191, DOI: 10.1016/j.desal.2007.11.064 [15] Teychene, B.; Touffet, A.; Baron, J.; Welte, B.; Joyeux, M.; Gallard, H.: Predicting of ultrafiltration performances by advanced data analysis, Water Res., 2018, 129, 365–374, DOI: 10.1016/j.watres.2017.11.023 [16] Yuasa, A.: Drinking water production by coagulation-microfiltration and adsorption- ultrafiltration, Water Sci. Technol.; 1998, 37(10), 135–146, DOI: 10.1016/S0273-1223(98)00308-4 [17] Chu, K.H.; Yoo, S.S.; Yoon, Y.; Ko, K.B.: Specific investigation of irreversible membrane fouling in excess of critical flux for irreversibility: A pilot-scale operation for water treatment, Sep. Purif. Technol., 2015, 151, 147–154, DOI: 10.1016/j.seppur.2015.07.033 [18] Zakrzewska-Trznadel, G.: Advances in membrane technologies for the treatment of liquid radioactive waste, Desalination, 2013, 321, 119–130, DOI: 10.1016/j.desal.2013.02.022 https://doi.org/10.1016/j.jhydrol.2023.130077 https://doi.org/10.1016/j.apgeochem.2006.03.005 https://doi.org/10.1016/j.scitotenv.2023.166240 https://doi.org/10.1016/j.jhydrol.2024.130770 https://doi.org/10.1016/j.scitotenv.2023.169246 https://doi.org/10.1016/j.jhydrol.2023.130147 https://doi.org/10.1016/j.jhydrol.2023.129851 http://dx.doi.org/10.1016/S0011-9164(01)00387-3 https://doi.org/10.1016/j.desal.2014.03.008 https://doi.org/10.1016/j.seppur.2022.122414 https://doi.org/10.1515/psr-2018-0070 https://doi.org/10.1016/j.scitotenv.2022.161311 https://doi.org/10.1016/j.seppur.2024.126426 https://doi.org/10.1016/j.desal.2007.11.064 https://doi.org/10.1016/j.watres.2017.11.023 https://doi.org/10.1016/S0273-1223(98)00308-4 https://doi.org/10.1016/j.seppur.2015.07.033 https://doi.org/10.1016/j.desal.2013.02.022 HÁRS, RÁCZ, G. LAKNER, J. LAKNER AND NEMESTÓTHY Hungarian Journal of Industry and Chemistry 34 [19] Gao, W.; Liang, H.; Ma, J.; Han, M.; Chen, Z.-L.; Han, Z.-S.; Li, G.-B.: Membrane fouling control in ultrafiltration technology for drinking water production: A review, Desalination, 2011, 272(1-3), 1–8, DOI: 10.1016/j.desal.2011.01.051 [20] Huang, H.; Schwab, K.; Jacangelo, J.G.: Pretreatment for low pressure membranes in water treatment: A review, Environ. Sci. Technol., 2009, 43(9), 3011–3019, DOI: 10.1021/es802473r [21] Li, P.; Yang, J.; He, Y.; He, M.; Ma, J.: Preparation of efficient and durable ultrafiltration membranes based on supramolecular assembly enhanced surface separation method, Chem. Eng. J., 2023, 475, 145842, DOI: 10.1016/j.cej.2023.145842 [22] Taniguchi, M.; Kilduff, J.E.; Belfort, G.: Modes of natural organic matter fouling during ultrafiltration, Environ. Sci. Technol., 2003, 37(8), 1676–1683, DOI: 10.1021/es020555p [23] Yoon, J.; Amy, G.; Chung, J.; Sohn, J.; Yoon, Y.: Removal of toxic ions (chromate, arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration membranes, Chemosphere, 2009, 77(2), 228–235, DOI: 10.1016/j.chemosphere.2009.07.028 [24] Utasi, A., Sebestyén, V., Rédey, Á.: Informative environment qualifying index, Hung. J. Ind. Chem., 2020, 48(2), 23–36, DOI: 10.33927/hjic-2020-24 [25] Yoo, S.S.; Chu, K.H.; Choi, I.-H.; Mang, J.S.; Ko, K.B.: Operating cost reduction of UF membrane filtration process for drinking water treatment attributed to chemical cleaning optimization, J. Environ. Manage., 2018, 206, 1126–1134, DOI: 10.1016/j.jenvman.2017.02.072 https://doi.org/10.1016/j.desal.2011.01.051 https://doi.org/10.1021/es802473r https://doi.org/10.1016/j.cej.2023.145842 https://doi.org/10.1021/es020555p https://doi.org/10.1016/j.chemosphere.2009.07.028 https://doi.org/10.33927/hjic-2020-24 https://doi.org/10.1016/j.jenvman.2017.02.072 https://doi.org/10.1016/j.jenvman.2017.02.072