IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE MOJCA POBERŽNIK, LUDVIK TRAUNER, ALBRECHT LEIS and ALEKSANDRA LOBNIK About the authors Mojca Poberžnik University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia E-mail: mojca.poberznik@uni-mb.si Ludvik Trauner University of Maribor, Faculty of Civil Engineering, Smetanova 17, 2000 Maribor, Slovenia E-mail: trauner@uni-mb.si Albrecht Leis Joanneum Research Institute, Institute of Water Resources Management, Hydrogeology and Geothermics Elisabethstrasse 16, A-8010 Graz, Austria E-mail: albrecht.leis@joanneum.ac.at Corresponding author Aleksandra Lobnik University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia E-mail: aleksandra.lobnik@uni-mb.si Abstract The presented research work is focused on the development of an innovative way of economically improving the quality of underground water by the application of carbon dioxide. The basic pilot plant was developed with the idea of applying carbon dioxide in order to maintain the carbonate equilibria in water, pumped from under the ground. The optimal content of this natural component in underground water, used in urban areas for different purposes (e.g. drinking water, heating systems etc), simultaneously prevents corrosion and precipitated mineral deposits. The adaptation of an existing pilot plant started with operational testing and preliminary measurements. An appropriate sensor system for the online monitoring of the tap water's heating process measuring parameters such as the temperature, the concentration of dissolved carbon dioxide, the conductivity and the pH value was built in. Different amounts of carbon dioxide were applied during the two experimental stages. The composition of the water was regularly analysed using automatic titration and ionic chromatography. The gas-dosing control in the first phase was volumetric. Due to an observed inaccuracy in the gas-dosing system, the volumetric dosing control was replaced with a system for the mass control of the introduced gas. Closing the system was considered to be suitable, as the water composition remained almost constant during the entire experiment. Improvements to the gas-dosing control and the closing of the system were carried out in the final phase of the pilot plant's adaptation. Further measurements were made in different seasons to test the seasonal influences on underground water's composition and the efficiency of the carbon dioxide supply. Keywords underground water, drinking water quality, pilot plant, carbonate equilibria, carbon dioxide, precipitation, scale 1 INTRODUCTION Underground water, percolating through different geological structures, dissolves certain minerals. The behaviour of this underground water is most interesting in areas that consist of carbonate rocks, e.g. limestones and dolomites. The main minerals in these rocks are calcium and magnesium carbonates, which dissolve easily in underground water and give the water its hardness. The quality of the water depends significantly on its hardness. In urban areas underground water is widely used as a source of water for various purposes, e. g. as drinking water, sanitary water and communal water [1]. Municipal companies that manage distribution systems for underground water and wastewater are constantly confronted by the necessity to invest large amounts of money in maintenance programmes with regard to water installations and canalizations in order to prevent the ACTA GeOTeCHNICA SLOVENICA, 2008/2 21. M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE consequences of limestone deposits and corrosion [2]. Healthy water for human consumption should not be corrosive in waterworks with either form limestone deposits. Therefore, the water has to be kept in a thermodynamic balance with the mineral phases of calcite or aragonite in order to fulfil this condition [3]. The natural water content depends on a combination of the geochemical and biological processes, in addition to the water's pH and alkalinity, which are two of the most important parameters affecting the composition of natural water [4]. Conventional drinking-water preparation as well as cooling and boiling feed-water preparation is based on softening and demineralization. Softening units are designed to replace of multiply charged positive ions (Ca2+, Mg2+) with sodium (Na+) ions and demineralization units are designed to remove a fraction of all the ions from water [5]. A more recently developed alternative method for preventing waterworks from developing lime scale is introduction of electromagnetic water conditioning. An electromagnetic conditioner works by creating a magnetic field around the pipework, which alters the ions' crystallization behaviour, so removing their ability to form scale [6]. Drinking water pre-preparation, based on the subsequent addition of CO2 gas as a natural component of healthy drinking water is a low-cost process for improving the quality of drinking water and, simultaneously, not altering the natural water's mineralogical composition. It represents an innovative and environmentally friendly method, as no unwanted by products or pollution arises. The aim of the presented research work was to develop a pilot plant and to test different methods for increasing the efficiency degree of the CO2 supply. To our knowledge, the presented pilot plant and such an application of carbon dioxide are original and have never been published before. To date a similar application of CO2 has been developed by the Messer Company but this one is related only to wastewater neutralization. The advantage of using carbon dioxide rather than sulphuric acid is reflected in simplifying the technological process, while at the same time reducing the costs [7]. 2 CARBONATE SPECIES IN NATURAL WATERS Sedimentary rocks, such as limestones and dolomites represent the most extended rocks of aquifers and their main minerals, calcium and magnesium carbonates dissolve easily in groundwater. Consequently, calcium is found to be the highest concentration of totally dissolved solids in average river compositions from different continents [8]. With regard to the composition of natural waters, 90% of the impurities come from just eight inorganic species i.e., Ca2+, Mg2+, Na+, K+, HCO3- , SO42- , and Cl-. Some of them are inert, e.g. Na+, K+, NO3- and Cl-, meanwhile the components of the carbonate equilibria are involved in numerous chemical reactions (1), which results in variations in their concentrations [9]. co^) t C02(aqy H20 I I (1) OH~ +H+ H++HCO- I H+ + CO3" + Ca2+ + CO3" CaC03(s) The slowest reaction in this dynamic system is the gas transfer into the aqueous phase, which is slower than other reactions that produce or consume carbon dioxide during the aqueous phase, e.g. calcite dissolution and precipitation. The formation of carbonic acid is reported to equilibrate within 20-200 s in a stirred solution, while the dissociation of bicarbonate and carbonate ions equilibrates within seconds [10]. Under all pH conditions, most of the dissolved CO2 exists as CO2 (aq) and only a small amount of the dissolved CO2 (aq) is hydrated to produce carbonic acid [9]. Various ratios of CO2 (aq)/H2CO3 (aq) have been reported e.g. 385/1 or 250/1 [11]. It is conventional to express the total dissolved carbon dioxide as H2CO3*, which represents the sum of the CO2 (aq) and the H2CO3 (aq) [9]. The dissolved carbon dioxide in an aqueous solution is distributed among three species, H2CO3* (dissolved CO2), HCO3- (bicarbonate) and CO32- (carbonate). Table 1 shows the equilibrium equations and the corresponding constants over different pH ranges [8, 9, 12 ]: 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE Table 1. The carbonate equilibria equations and the corresponding equilibrium constants over different pH ranges. pH range Equilibrium equation Equilibrium constant pH < 4.3 CO2 (g) + H2O ^ h2co3* Kco2 = IO-1.47 atm-1 4.3 > pH < 8.2 H2CO3* + H2O ^ hco3- + h3o+ K = 10-6.35 KH2CO2 = 10 pH > 8.2 hco3- + h2o ^ co32- + h3o+ K = 10-10.38 KHCO3- = 10 The reaction between calcite (CaCO3) and carbon dioxide (2), derived from biological processes, is the fundamental reaction for understanding the dissolution and precipitation of CaCO3 in nature [8]: C02 (g) + H2O(lq) + CaCO3 (5) Ca2+(aq) + 2HC0- (aq) (2) The redundancy of the dissolved CO2 causes the formation of a weak carbonic acid, a decrease of pH value and dissolution of CaCO3. The acidic pH of drinking water has harmful influence on humans' physiological environment and causes corrosive damage to pipelines (Figure 1). The deficit of dissolved CO2 (degassing of water) causes the precipitation of non-soluble crystalline structures (e.g. CaCO3) and results in the obturation of mechanical parts (filters, valves), flow hindrance due to limestone precipitates, and the loss of energy (Figure 2). Rocks formed from sediments cover 93% of the Slovenia's land area. Among them limestone (25%) and dolomite (10%) are the most extensive. In Slovenia, over 95% of the water used for public supplies is drawn from groundwater, while, for example, in the United States the quantity is only 40% [3, 14]. The Drava basin, located in the Northeast of Slovenia, represents a source of potable water for the wide Maribor area. A geological map in Figure 3 shows the ground composition of the aquifer along the River Drava in the wider area of the City of Maribor. The Figure 2. Noxious consequences of limestone deposits. 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE Figure 3. Geological map of the underground water area in Maribor. 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE underground water levels (UWL) around the pumping station at Vrbanski plateau are presented in Figure 4. The underground water level variations over the period 2004 to 2008, considering water flow and the quantity of pumped water are shown in Figure 5. The presented international research work that discusses the optimization of underground water quality by introducing carbon dioxide was performed within the K-Net Centre of Water Resources Management (WRM). Figure 4. Underground water levels (UWL) in the area of Vrbanski plateau, the pumping station for Maribor waterworks. UWL variations (P1, P2, P3, average value) 252 251 g 250 ® co -C\IC\IC\IC\ICOC\IC\IC\IC\lT-eOr«-T-T-T-T-C\l CSI CSI CSI CSI i- i- 1- time (days) Figure 5. The underground water-level variations over the past four-year period (2004 to 2008) considering the water flow and the quantity of pumped water. 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE Table 2. The average values of significant carbonate equilibria parameters regarding drinking water over a ten-year (1995 -2005) period in the Maribor area. Winter Spring Summer Autumn Ca2+ (mg • L-1) 76.17 81.28 54.35 73.81 HCO3- (mg • L-1) 265 273 248 251 pH 7.52 7.53 7.47 7.57 Conductivity (|S • cm-1) 516 511.25 510.62 511.16 The aquifer is classified as a carbonate-silicate type with inter-grained porosity and belongs to the second most extended of the Slovenian aquifers' types (26.2%) [15]. Analytical data, including the averages of the past ten years characterize the tap water in the Maribor region as moderately hard water (12-18 °n) [16]. Comparing the ten-year averages of water composition over different seasons indicates some typical patterns in the most significant parameters of carbonate equilibria (Table 2) [16]. The total dissolved solids concentration (TDS), including the concentration of Ca2+, Mg2+ and CO32- is increased in the spring and winter seasons due to more intensive rock weathering and washing out. Meanwhile, the pH value does not vary significantly over the year. 3 EXPERIMENTAL The experiments were performed on a pilot plant constructed for the observation of precipitation in pipe-lines and heat exchanger. The basic pilot plant consisted of a closed primary pipeline with adjustable temperatures of the heating fluid and an open secondary pipeline, supplied by tap water, a heat exchanger, and an ultrasonic stirrer. An operational test was performed to find out the initial time for the stabilization of the system, and to indicate the scale precipitation in the heat exchanger under the following conditions: 65°C, 2 bars and 2 L/min of water flow, known tap-water composition, applied ultrasound (US, 27 kHz, 4W), and without introducing additional carbon dioxide. A preliminary set of measurements was carried out on the basic pilot plant, without the sensor system. The samples were taken periodically every hour at five points of the process. The concentrations of free carbonic acid, calcium ions and hydrogen carbonate ions were determined titrimetrically. The temperature and the pH value were measured, while the pressure and water flow were kept constant during the entire experiment. Other experiments were performed using the same pilot plant, operating under the same conditions, but with the application of different amounts of additional carbon dioxide [14]. The new sensor system and the corresponding software package were chosen on the basis of the preliminary results. During the first adaptation phase, a new sensor system (Figure 6) for online monitoring of the dissolved carbon dioxide, the pH, the temperature and the conductivity were built in and calibrated. The disadvantage of this, the only appropriate sensor system, is that it makes it possible to control the mentioned parameters only within a limited temperature range. The CO2 electrode characteristics make it possible to measure the amount of dissolved CO2 within a temperature range up to 65°C and within a concentration range from 0 to 4000 mg/L. The heat exchanger was also replaced with a new, gasketed one. Figure 6. Sensor system with electrodes. The pilot plant after the first phase of adaptation is presented in Figure 7. The first set of experiments was carried out in the summer season. It comprised 35 measurements: seven at each of five measuring points (MP), with the addition of different amounts of CO2 (1 L/min, 3 L/min and 5 L/min), and all with and without the application of 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE aH O 5,02 85,03 245,03 1235,02 1385,04 Time (min) Figure 12. Measurement results without applied US and without introducing of additional CO2 (closed system). Table 3. Water composition analyses results without applied US and without additional CO2. Na+ K+ Mg2+ Ca2+ Cl- no3 SO4- hco3 alkalinity (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/3L) (mg/L) (mg/L3) (ml HCl) MP 1 n. a. n. a. n. a. 78.95 19.87 17.24 30.28 281.56 4.61 MP 2 13.28 2.07 15.22 78.88 19.55 15.68 30.31 284.04 4.65 MP 3 14.35 2.20 15.58 80.26 21.16 19.02 32.12 288.49 4.73 MP 4 13.01 2.14 15.37 78.15 19.58 18.23 30.53 282.19 4.62 MP 5 13.01 2.08 15.45 78.58 19.74 17.34 31.06 280.68 4.60 Table 4. Water composition analyses results without applied US and with 5 L/min of additional CO2. Na+ K+ Mg2+ Ca2+ Cl- no3 SO4- hco3 alkalinity (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/3L) (mg/L) (mg/L3) (ml HCl) MP 1 13.07 2.23 15.37 78.88 20.35 17.74 31.49 282.14 4.62 MP 2 13.68 2.32 15.53 81.38 21.60 17.82 33.01 291.41 4.78 MP 3 13.24 2.23 15.15 78.24 19.91 17.55 30.68 281.18 4.61 MP 4 13.19 4.26 15.00 78.48 22.22 18.11 31.05 282.02 4.62 MP 5 13.24 2.23 15.20 78.80 21.23 19.13 31.60 279.62 4.58 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE The system was stabilized and the terminal temperature of 62°C was reached in a period of four hours because the initial water temperature was only 11.8°C due to the winter season. During the heating the CO2 concentration of decreased by 42.8 mg/L (15.4% of the initial value) and consequently the pH value increased by 0.43 units (6.3% of the initial value). The variations in Ca2+ concentration during the process were expected, but were not significant. The highest Ca2+ concentration (85.1 mg/L) was reached at 40°C, and it decreased by 1.8 mg/L (2.1% of the initial value) until the terminal temperature was reached. The variations in the dissolved species showed -■-pH -e-Ca2+ (mg/L) C02 (mg/L) 140 i -x-Conductivity (mS) ^ Added C02(mg) 317 351 494 1394 1605 Time (min) Figure 13. Measurement results without applied US and with introducing 570 mg of additional CO2 (closed system). that, also in the closed system, operating at a temperature of 62°C meant that only slight precipitation was present. In the case of introducing 570 mg of additional CO2 the initial temperature was 11.1°C (winter season). The system reached the terminal temperature (63.1°C) after 256 minutes of heating, when the introduction of additional gas started. The added gas caused a drop in the pH value, which after the introduction of 280 mg of CO2 decreased by 0.74 units (10.3% of initial value). The lowest pH value (6.06) was reached after the addition of 550 mg of gas, when the initial value was decreased by 1.11 units (15.3% of initial value). As expected, analyzing the contents of the Ca2+ ions, less significant variations were noticed. The content of Ca2+ decreased only by 0.2 mg/L (0.25% of initial value), when the system was heated to the terminal temperature (63.1°C). During the introduction of the additional gas, the Ca2+ concentration increased and reached a maximum value of 86.2 mg/L (103% of initial value) after 494 minutes, when 530 mg of gas was introduced. The initial Ca2+ concentration was exceeded at that point, which showed that the existing mineral deposits were dissolved. The experiment was concluded after 1605 minutes (26.75 hours), when the Ca2+ content reached 100.35% of its initial value and the pH value was in slightly acidic range (6.14 or 1.03 units lower than initial value). Taking into account the early phase of the experiment, it could be concluded that despite the closing of the system, the Table 5. Water composition results without applied US and without additional CO2 (closed system). Time of experiment (min) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L) Cl- (mg/L) no3 (mg/3L) SO4-(mg/L) alkalinity (ml HCl) hco3 (mg/L3) 5.02 12.4 2.39 17.71 84.4 18.1 16.2 27.60 5.10 311.2 85.03 12.7 2.42 18.03 85.1 18.5 16.4 28.15 5.12 312.4 245.03 12.7 2.40 18.05 83.3 18.7 16.3 28.32 5.00 305.1 1235.02 12.5 2.37 17.73 83.4 18.3 16.0 27.90 5.00 305.1 1385.04 12.5 2.38 17.76 83.2 18.4 16.1 27.98 5.02 306.3 Table 6. Water composition results without applied US and with 570 mg of additional CO2 (closed system). Time of experiment (min) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L) Cl-(mg/L) no3 (mg/3L) SO4-(mg/L) alkalinity (ml HCl) hco3 (mg/L3) 0 12.2 2.27 17.53 83.7 19.6 17.1 30.40 5.03 306.9 256 12.5 2.40 17.85 83.5 n.a. n.a. n.a. 5.02 306.3 317 12.4 2.32 17.83 85.1 10.0 9.4 16.78 5.14 313.6 351 12.4 2.33 17.84 85.3 11.0 10.3 18.04 5.18 316.1 494 12.5 2.35 18.04 86.2 11.9 10.9 19.33 4.70 286.8 1394 12.4 2.69 17.79 84.9 12.7 11.4 20.03 4.80 292.9 1605 12.3 2.32 17.58 84.0 12.9 11.7 20.62 5.07 309.4 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE precipitation process (during heating) was not intensified significantly, because the operational temperature was too low. The water composition results, presented in Tables 5 and 6, confirmed the above statements. They were made using ionic chromatography and automatic titration. The quantities of cations, such as Na+, K+, Mg2+, Ca2+ and anions, such as Cl-, NO3- , SO^- were defined using ion chromatography. The automatic titration along with the incremental method was used to define the amount of hydrogen carbonate ions (HCO3- ) and the alkalinity of the investigated water samples. On the basis of the analyses of the corresponding calcium and magnesium contents it was find out that only slight variation in their quantities occurred during the experiments. These results were relevant for the preliminary hydro-chemical modelling, which showed that a slightly negative Langelier saturation index could be expected at such a process temperature. Measurements were carried out over different seasons, in summer and winter, to study the seasonal influence on tap-water composition and to test the applied method's efficiency in various conditions. The tap-water composition used for the experiments was regularly analyzed in detail. The data in Table 7 represents the average values of the significant carbonate equilibria parameters regarding the drinking water used over the summer and winter periods. Table 7. The average values of significant carbonate equilibria parameters regarding drinking water over summer (2006) and winter (2006/07) period in the Maribor area. Winter Summer Ca2+ (mg • L-1) 84..05 79.05 Mg2+ (mg • L-1) 17.62 15.26 HCO3- (mg • L-1) 309.05 284.16 pH 6.96 7.64 5 CONCLUSION The water quality optimization by introducing carbon dioxide is a very promising method in terms of reduction the costs of technological process and energy consumption. Another advantage of the described method is that it does not change the original water's mineralogical composition as it does not deplete the water of the minerals in comparison to the conventional methods, i.e. water softening. This should be taken into account especially when the preparation of the drinking water is an issue. The recently developed gas application is environmentally friendly. It does not produce any pollution or waste. This completely adapted pilot plant with an advanced sensor system and an improved and more accurate gas-dosing system represents a useful technical tool for studying the behaviour of the described system operating under closed conditions with different tap-water compositions, and various seasonal influences. The used tap water from the Drava basin aquifer in the summer of 2006 and the winter of 2007 generally contained larger quantities of dissolved minerals compared to ten-year averages. The concentrations of all the carbonate equilibria relevant contents correlate to the existing average values and their variations due seasonal influence, which means that the groundwater originating from the alluvial aquifer is richer in minerals in the winter and spring seasons, compared to the summer and autumn seasons. The results were relevant for preliminary hydro-chemical modelling with a determination of saturation indexes, which confirm our presumptions and shows in most cases only a slightly negative Langelier saturation index (-0.3 < LSI < -0.2) at such an experimental temperature. The modelling will be continued and regularly updated with new data from experiments planned on the pilot plant at higher process temperature but without the online monitoring. The measured values of all the parameters correlate to the existing data for ten-years' averages (Table 2), except that the values are generally higher than the averages, which could be related to the intensity of the rock weathering or shorter response time due to less precipitation. The calcium ions concentration was 10.3 % higher in winter 2007, and 45.4% higher in summer 2006 season compared to the ten-year (1995-2005) average. The water from the Maribor area also contains 12.2% more Mg2+ ions in summer and 15.1% more Mg2+ in winter season compared to the ten-year average. ACKNOWLEDGEMENTS The presented research work is part of the project founded by the Waterpool Competence Network K-Net, Water Resources Management (WRM) and the Messer Company, Slovenia. The analytical part of the research work was also supported by an Ad Futura scholarship. 52. ACTA GEOTECHNICA SLOVENICA, 2008/2 M. POBERZNIK ET AL.: IMPROVEMENTS TO THE QUALITY OF UNDERGROUND WATER BY INTRODUCING CARBON DIOXIDE references [1] Žlender, B., Dolinar, B. (2008). Geologija. Univerza v Mariboru, Fakulteta za gradbeništvo. [2] Sander, A., Berghult, B., Elfström Broo, A., Lind Johansson, E., and Hedberg, T. (1996) Iron corrosion in drinking water distribution systems - the effect of pH, calcium and hydrogen carbonate. Corrosion science, 38, 3, 443-455. [3] Drever, J.I. (1997). The geochemistry of natural waters, Surface and Groundwater environments. Third edition; Prentice Hall, Upper Saddle River. [4] Lower, S. K (2006). Carbonate equilibria in natural waters. http://www.chem1.com/acad/pdf/c3carb.pdf [5] http://www.lenntech.com/softening.htm, (2007) [6] http://www.water-treatment.org.uk/index.html, (2007) [7] http://www.messergroup.com/de/Presse/wpresse/ 020407 Kohlensaeure/index.html; (2007) [8] Fitts, C., R. (2002). Groundwater science, Academic press, Elsevier science, San Diego, California. [9] Appelo, C. A. J., Postma, D. (2005).Geochemistry, groundwater and pollution. 2nd edition, A.A. Balkema publishers, Amsterdam, Netherland. [10] Skidmore, M., Sharp, M., and Tranter, M. (2004) Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: implications for detection of microbial CO2 signatures using S13C-DIC. Geochimica et Cosmochimica Acta 68, 21, 4309-4317. [11] Carbonate equilibria (2006). http://www.ees.nmt. edu//Hydro/courses/hyd507/Lecture Notes/13 october.pdf [12] Greiner, G. (1993). Grundlagen der industriellen Wasserbehandlung. Vulkan Verlag, Essen. [13] Sternad, L. (2007). The preparation of raw waters for use in textile industry with introduction of carbon dioxide (diploma thesis), University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia. [14] http://www.sdzv-drustvo.si/si/VD%2007 Referati/ Predavanja/15%20ARS0 vodni%20dnevi%202007 MK kratko.pdf, 2008 [15] www.mb-vodovod.si, (2008) [16] Plateis, G. (2008). Drinking water decarbonization by introducing carbon dioxide (Master thesis); University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia. 52. ACTA GEOTECHNICA SLOVENICA, 2008/2