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The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process
DES-12989; No of Pages 9 Desalination xxx (2016) xxx–xxx
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The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process Barbara Tomaszewska a,b,⁎, Mariola Rajca c, Ewa Kmiecik a, Michał Bodzek c,d, Wiesław Bujakowski b, Katarzyna Wątor a, Magdalena Tyszer a a
AGH University of Science and Technology, Mickiewicza 30 Av., 30-059 Krakow, Poland Mineral and Energy Economy Research Institute, Polish Academy of Sciences, Wybickiego 7, 31-261 Krakow, Poland Silesian University of Technology, Institute of Water and Wastewater Engineering, Konarskiego 18, 44-100 Gliwice, Poland d Institute of Environmental Engineering, Polish Academy of Sciences, M. Curie-Skłodowskiej 34, 41-819 Zabrze, Poland b c
H I G H L I G H T • • • •
Research areas investigated include problems during the pre-treatment of geothermal water, Study involved nanofiltration with a commercial NF-270 (Dow Filmtec) membrane, Research was carried out on different temp. variations 30 °C and 17 °C and the use of raw water and water with antiscalant, Qualitative and quantitative identification of the deposits precipitated on a NF-270 membrane has been presented.
a r t i c l e
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Article history: Received 23 February 2016 Received in revised form 22 June 2016 Accepted 7 July 2016 Available online xxxx Keywords: Geothermal water Water treatment Membrane Nanofiltration Scaling
a b s t r a c t The use of the reverse osmosis process in water treatment often requires the careful selection of methods of pretreatment. This especially applies to mineralised water, ferruginous water, water with a high carbonate hardness and also water with a tendency to precipitate sulphate and silica deposits. Increased feed water temperature may be a further factor which could encourage scaling of the membrane. To decrease the number of divalent ions, the nanofiltration process was tested with the use of a commercial NF-270 membrane (Dow Filmtec). Filtration was performed under a transmembrane pressure of about 10 bar with cross-flow filtration. Tests were carried out on two highly mineralised geothermal waters mineralisation (TDS 2.2–2.3 g/L), more than 600 mgCaCO3/L and a high silica concentration. Before laboratory tests, the tendency to precipitate mineral deposits was determined by means of geochemical modelling. In addition, appropriate doses of antiscalant were selected to avoid the precipitation of deposits on the NF membrane. The research was carried out on different variations of: 1) two temperatures: 30 °C and 17 °C and 2) the use of raw water and water with the addition of antiscalant (Hydrex). During tests the changes of membrane effectiveness with time were observed. Qualitative and quantitative identification of the precipitate deposits on the NF-270 membrane was made for each of the variants of the water pretreatment process. © 2016 Published by Elsevier B.V.
1. Introduction The efficient and sustainable management of natural resources, primarily energy resources, and increasing the share of renewable sources of energy are both goals of European Union Member States in the era of
⁎ Corresponding author at: AGH University of Science and Technology, Mickiewicza 30 Av., 30-059 Krakow, Poland. E-mail addresses: [email protected], [email protected] (B. Tomaszewska), [email protected] (M. Rajca), [email protected] (E. Kmiecik), [email protected] (M. Bodzek), [email protected] (W. Bujakowski), [email protected] (K. Wątor), [email protected] (M. Tyszer).
the fight against global warming. These objectives, especially economic ones, should undoubtedly be harmonised with environmental protection. The use of geothermal energy, which offers significant resources that are widely available on a global scale, also plays an important role in this regard. The fact that this resource is practically limitless and renewable, together with its independence of changing climate and weather conditions, present clear advantages which militate in favour of its increased utilisation. Irrespective of the obvious climate protection benefits, the direct and indirect development of these resources in the generation of electricity, heating, agriculture, horticulture, balneotherapy, recreation, etc. should also respect other elements of the natural environment. This primarily relates to finding a suitable
http://dx.doi.org/10.1016/j.desal.2016.07.007 0011-9164/© 2016 Published by Elsevier B.V.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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way of dealing with chilled geothermal waters that have already been spent and constitute waste. The desalination of geothermal waters used for energy purposes is increasingly seen as a good method of securing high-quality water for various areas of the economy. In countries with warm climates it is mainly used for the irrigation of agricultural crops [1–7]. Given the increasing deficit of freshwater worldwide, the possibilities for desalinating and treating geothermal waters for drinking and household purposes should be considered [8–13]. The use of the reverse osmosis process in water treatment often requires careful methods of selection and preliminary removal of undesirable components. This applies especially to mineralised water, ferruginous water, water with high carbonate hardness and also water with a tendency to precipitate sulphate and silica deposits. An additional factor which could encourage scaling of the membrane might be increased feed water temperature. The nanofiltration (NF) process has been analysed in order to decrease the number of divalent ions. NF is mostly used for the softening of and the removal of organic compounds from surface and brackish water. NF membranes are characterised by a high retention of bivalent ions, while the retention of monovalent ions is limited. The aim of the study was to evaluate the use of NF as a pre-treatment method for the softening of geothermal water. The research was carried out using 1) water at two alternative temperatures, 30 °C and 17 °C and 2) raw water and water with the addition of antiscalant. 2. Materials and methods
Table 2 Results of Bańska PGP-3 geothermal water using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 Al+3 Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2274 648.6 477.3 46.21 195.1 39.37 6.090 440.3 797.1 313.9 74.90 0.018 0.210 0.0061
2274 660.2 475.1 46.25 200.9 38.60 6.02 465.8 785.8 281.2 74.62 0.018 0.220 0.0061
882.0 94.00 249.5 23.49 29.41 5.00 0.84 395.6 3.00 156.9 61.16 0.005 0.010 0.0061
1063.3 143.90 295.10 27.06 42.50 9.21 1.32 469.6 6.92 199.5 71.63 0.007 0.010 0.0061
61.2 85.5 47.7 49.2 84.9 87.1 86.2 10.2 99.6 50.0 18.34 72.22 95.24 0
53.3 78.2 37.9 41.5 78.8 76.2 78.1 0.00 99.1 29.1 4.01 61.11 95.45 0
Total hardness in mg CaCO3/L.
the process with a recycling of the permeate into the reservoir. The NF270 membrane is the most suitable for antiscaling pretreatment because of the high rejection rate of scale forming ions as well as the high permeate flux [14,15]. The active area of the membrane was 155 cm2. The membrane has a negative load surface in a wide range of pH [16]. The new membrane was conditioned by filtration of deionised water to stabilise the permeate flux.
2.1. Geothermal waters 2.3. Methodology of physico-chemical analysis Tests were carried out using two geothermal waters with high mineralisation (TDS 2.2–2.3 g/L), more than 600 mgCaCO3/L and a high silica concentration. The physical and chemical composition of the water tested is shown in Tables 1 and 2. According to the Szczukariew-Priklonski classification, the waters fall into the hydrogeochemical type SO4-Cl-Na-Ca. The feed water had a temperature of 1) 30 °C and 2) 17 °C. 2.2. Testing equipment A schematic diagram of the system used for the nanofiltration process is shown in Fig. 1. Tests were conducted using the SEPA CF-HP type high-pressure membrane module from the American company, Osmonics Inc. This used a “cross-flow” system utilising an NF270 Dow Filmtec membrane at a specified pressure of 10 bar and completing
Water pH and temperature were measured using the electrometric method immediately after sampling water from the system. Inorganic components were determined in an accredited laboratory of the Department of Hydrogeology and Engineering Geology of the AGH University of Science and Technology in Kraków (PCA certificate No AB 1050) using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Chloride ion content and water alkalinity were determined by titration in accordance with accredited testing procedures. 2.4. Antiscalant dosing and methodology for predicting scaling The Hydrex Antiscalant Dosing Calculator (Veolia Water Solution & Technologies) was used to choose the recommended product and calculate the dosage applied in feed water. The saturation index (SI) of the
Table 1 Results of Bańska PGP-1 geothermal water using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 +3 Al Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2228 636.1 467.2 45.2 190.4 39.2 5.95 435.2 784.0 294.3 74.06 0.038 0.16 0.0061
2342 666.8 492.8 47.41 200.8 40.29 6.240 482.4 812.5 279.6 76.93 0.089 0.32 0.0061
891.5 97.10 251.0 24.50 29.77 5.540 0.870 403.3 5.32 145.5 61.67 0.005 0.010 0.0061
994.6 123.5 277.9 25.75 37.85 7.07 1.12 449.2 3.00 167.6 69.4 0.005 0.01 0.0061
59.9 84.7 46.3 45.8 84.4 85.9 85.4 7.33 99.7 50.6 16.73 86.84 93.75 0
57.5 81.5 43.6 45.7 81.2 82.5 82.1 6.89 99.6 40.1 9.79 94.38 96.87 0
Total hardness in mg CaCO3/L.
Fig. 1. Diagram of the system for carrying out nanofiltration in “cross-flow” mode (Explanation: 1-heat exchanger; 2-raw water inflow; 3-rotameter; 4-membrane cell; 5permeate outflow; 6-pump; 7-raw water tank).
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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solution was also calculated using the Phreeqc Interactive 3.3.3-10424 program (PHREEQCI) using the Wateq4f minerals database [17]. 2.5. Methodology of membrane scaling analysis The mineral composition of reaction products was determined via the powder X-ray diffraction (XRD) method using a Philips X'pert APD diffractometer (with the PW 3020 goniometer), Cu lamp, and graphite monochromator. The analysis was performed within the angle range of 5–60 2θ. The Philips X'Pert and the ClayLab ver. 1.0 software packages were used to process diffraction data. The identification of mineral phases was based on the PCPDFWIN ver. 1.30 database formalised by the JCPDS-ICDD. The morphology and chemical composition of the main mineral components of the materials examined in the micro area domain were determined using a scanning electron microscope (SEM). The equipment used was an FEI Qanta 250 FEG scanning microscope additionally equipped with a chemical composition analysis system based on energy dispersion scattering – the EDS EDAX. 3. Results and discussion Owing to the fact that the desalination of geothermal waters in industrial installations is carried out at a temperature of ca. 30 °C [10], the first tests of the NF process using water from two wells (Bańska PGP-1 and Bańska PGP-3) were conducted at this feed temperature. Subsequently, tests were carried out for water at a temperature of 17 °C, since ultimately the preferred solution would involve better utilisation of the heating potential of geothermal waters, e.g. for heating soil when growing crops under film covers or on sills in special greenhouses, or for culturing other organisms, e.g. algae [18]. In this case, the temperature of chilled waters fed to the desalination plant would be correspondingly lower. Test results for waters from both geothermal wells and for two different feed temperatures are shown in Tables 1 and 2. The results of the test conducted using the nanofiltration process with NF-270 membranes and geothermal water from the two wells examined (Tables 1 and 2) demonstrated a more efficient rejection of undesirable components (by about 10%) for the lower temperature (17 °C) feed. TDS was decreased by 59.9–61.2% at a temperature of 17 °C and by 53.3–57.5% at 30 °C. The desired reduction in the total hardness of the water was obtained: 84.7–85.5% at 17 °C and 78.2–81.5% at 30 °C. The best rejection ratio was obtained for sulphate ions: 99.6–99.7% at 17 °C and 99.1–99.6% at 30 °C. For metasilicic acid, the figures were 16.73–18.34% and 4.01–9.79% respectively. Given the high content of sulphate ions in the waters tested, the effect achieved would be significant in terms of preventing the scaling of membranes at the proper water treatment stage using the reverse osmosis (RO) process. No significant decrease in permeate flux was observed during laboratory studies. The nanofiltration process proceeded in a stable manner as shown in Fig. 2. Due to the lower viscosity of water, significantly higher permeate flux was achieved at a water temperature of 30 °C. At a water temperature of 17 °C, the performance of the membrane was lower by approx. 20%. The resulting stability in performance at a laboratory scale during a test lasting approx. 5.5 h is consistent with the results obtained in tests on a long-term, semi-industrial scale [11]. Geothermal water with TDS of 2.15 to 2.7 g/L was treated for a period of eight months. During a pilot study of the desalination of geothermal water on a semiindustrial scale using DOW FILMTEC BW30HR–440i membranes, no decrease in the permeate flux was observed. At RO-1 an average permeability of 5.25 ∙ 10− 6 m3/m2·s was achieved at recovery levels of ca. 75–78% and at RO-2 at a transmembrane pressure of 10 bar, the recovery level was ca. 75% and the average permeability of the membranes was measured at 7.9 ∙ 10− 6 m3/m2·s [11]. However, the main factor that had a positive effect on the desalination process was the
Fig. 2. Changes of permeate flux during nanofiltration of the Banska PGP-1 and Banska PGP-3 geothermal waters using the NF-270 membrane (transmembrane pressure 10 bar, temperature 30 °C and 17 °C). Explanation: PGP1/30 - Banska PGP-1/water temperature 30 °C; PGP3/30 - Banska PGP-3/water temperature 30 °C; PGP1/17 Banska PGP-1/water temperature 17 °C; PGP3/17 - Banska PGP-3/water temperature 17 °C.
hydrochloric acid treatment of the water fed to the first reverse osmosis stage, which decarbonised the water and also reduced the saturation index with respect to sulphate and silicate mineral forms [11]. To determine how applicable the solution analysed is in industrial installations, one must thoroughly determine its process efficiency and reliability. A reduction in the efficiency of a membrane process is usually related to membrane scaling arising as a result of the solution (feed) being supersaturated with specific mineral forms, as a result of which deposits accumulate on the membrane surface, blocking it and reducing process efficiency. These phenomena may be particularly intense in waters with a high level of total hardness and elevated silica content. The scaling forecast prepared using geochemical modelling and taking into account the physicochemical properties of the geothermal waters (feeds) studied and the pressure used in the process in question (10 bar) yielded the following results: 1) for waters at a temperature of 30 °C: a) the oversaturation of the geothermal water solutions tested and a tendency of the following minerals to precipitate: sulphates, especially barite (BaSO4), silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (Fe2O3), magnetite (Fe3O4) and aluminium hydroxide – gibbsite (Al(OH)3) as well as various aluminium silicate forms, e.g. kaolinite (Al2Si2O5(OH)4), muscovite (KAl3Si3O10(OH)2) and others; b) a state of equilibrium with respect to carbonate forms – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2) and calcium sulphate (gypsum – CaSO4); c) undersaturation with the anhydrite form of calcium sulphate (CaSO4); 2) for waters at a temperature of 17 °C: a) the oversaturation of the geothermal water solutions tested and a tendency of the following minerals to precipitate: silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (γFe2O3), magnetite (Fe3O4), various aluminium silicate forms and aluminium hydroxide – gibbsite (Al(OH)3); b) a state of equilibrium with respect to barium sulphate (barite) (BaSO4); c) undersaturation with carbonate – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2) and sulphate forms (anhydrite and gypsum – CaSO4). The calculations performed using the Hydrex antiscalant Dosing Calculator (Veolia Water Solution & Technologies) demonstrated that among the HYDREX (Veolia) product family, the Hydrex 4104 (H4104) antiscalant would be the most appropriate to protect the NF270 membrane against scaling. Therefore, the next stage involved tests conducted at feed temperatures of 30 °C and 17 °C using the
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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H4104 antiscalant. Tables 3 and 4 show the physicochemical properties of the feed dosed with H4104 and the rejection rate of the ions analysed at temperatures of 17 °C and 30 °C. As concerns the rejection ratio of undesirable ingredients, slightly better results were obtained for the feed dosed with antiscalant compared to the results for the feed without antiscalant (Tables 3, 4). Unfortunately, a decrease was observed in process flux which indicated that blockage of the membrane was occurring (Fig. 3). The downward trend in performance of the membrane was maintained for most of the total filtration time (about 12,000 s), for subsequent measurement seconds it showed a significant stabilisation of the flux, in particular for the Banska PGP-3 geothermal water at 30 °C. This may prove that the blocking of membrane filtration for water with antiscalant takes place until it reaches a certain level, and that then there is a stabilisation of the conditions under which transport through the membrane takes place. The geochemical modelling of the degree of saturation of the feed dosed with antiscalant for both the geothermal wells and water temperatures (17 °C and 30 °C) tested demonstrated: a) the continued oversaturation of the tested solutions with respect to the following minerals: silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (γ Fe2O3), magnetite (Fe3O4) and aluminium hydroxide – gibbsite (Al(OH)3) as well as various forms of aluminium silicate, e.g. kaolinite (Al2Si2O5(OH)4), muscovite (KAl3Si3O10(OH)2) and others. However, an increase in solution saturation was observed with respect to phosphate minerals, e.g. apatite, hydroxyapatite (Ca5(PO4)3OH) and others, which had a tendency to precipitate. The solutions were in a state of equilibrium with respect to sulphate forms – barite (BaSO4), anhydrite and gypsum (CaSO4) and strong undersaturation with respect to carbonate forms – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2). In consequence, the geochemical modelling shows that, apart from geothermal waters (feeds), waters with the antiscalant have oversaturation with respect to phosphate minerals, e.g. apatite, hydroxyapatite and strong undersaturation with respect to carbonate forms – aragonite, calcite and dolomite. Photomicrographs of membranes after conducting tests with geothermal waters (without the antiscalant) are shown in Figs. 4 and 5, and those for feeds dosed with the antiscalant are shown in Fig. 6. As a result of the nanofiltration of water from the Bańska PGP-1 well (without the antiscalant) at a feed temperature of 30 °C, the following deposits precipitated on the membrane: sulphate minerals with a small share of chlorides (the result of halite precipitation from the water remaining on the membrane). These deposits have an irregular character (Fig. 4a-1). The mineral components mainly include gypsum
Table 4 Results of Bańska PGP-3 geothermal water with Hydrex 4104 using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
Raw water mg/L 17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 Al+3 Fe+2 PO−3 4
1977 537.2 401.9 38.60 162.2 32.23 4.99 433.9 664.9 284.5 62.99 0.008 0.15 86.48
2041 568.0 427.5 40.65 171.6 34.01 5.31 417.3 711.4 263.2 66.91 0.023 0.06 88.37
764.8 66.50 208.9 18.45 19.05 4.61 0.64 384.1 7.67 71.90 53.87 0.005 0.010 0.36
675.5 47.20 189.5 17.18 13.42 3.35 0.43 347.1 4.39 32.70 52.67 0.005 0.020 0.224
61.3 87.6 48.0 52.2 88.3 85.7 87.2 11.5 98.9 74.7 14.47 37.5 93.33 99.58
66.9 91.7 55.7 57.7 92.2 90.2 91.9 16.8 99.4 87.6 21.28 78.26 66.66 99.74
a
Permeate mg/L
R [%]
Total hardness in mg CaCO3/L.
and halite as well as iron oxides, which form a layer of sorts on the membrane – this is well illustrated by Fig. 4a-2. This layer consists of very fine grains and it is difficult to identify the presence of individual crystals. In some micro-areas, the presence of aluminosilicate phases is clearly visible on the surface of gypsum and halite deposits, which is especially visible in Fig. 4b. Deposits on the NF-270 membrane after the nanofiltration of water from the Bańska PGP-3 well exhibit similar characteristics (Fig. 4c). The membrane surface in Fig. 4c is covered with clusters of very fine-grained iron sulphates (Fig. 4c-3). In individual micro-areas, mineral clusters with a size of up to 50 μm can be found whose chemical composition is a mixture of phosphates, sulphates, and sodium and potassium chlorides (Fig. 4c-1,2). The deposits crystallised on the membrane during the nanofiltration of water at 17 °C mainly consist of fine-grained minerals from the sulphate group (Fig. 5a). The chemical spectra of this layer show that it is mostly sulphur and oxygen with trace amounts of sodium and silicon (Fig. 5a, b-3). This layer almost completely covers the surface of the membrane. Far larger clusters of mineral grains with a size of up to 5 μm are only found in a few places on the membrane. Their chemical composition is clearly different from the background in quantitative terms. Apart from the dominant sulphurs, there is also calcium that forms gypsum/anhydrite minerals (Fig. 5b-1,2, c-1,2). There are only sporadic micro-areas where, at the expense of lower sulphur content, clusters appear whose chemical composition is dominated by silicon
Table 3 Results of Bańska PGP-1 geothermal water with Hydrex 4104 using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 +3 Al Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2084 574.4 434.4 40.08 171.3 35.72 5.46 435.2 706.0 305.7 67.87 0.023 0.08 83.02
2185 579.8 448.9 40.98 172.5 36.29 5.52 472.2 733.4 181.5 70.05 0.071 0.38 79.24
844.1 77.60 229.7 20.83 22.98 4.931 0.74 410.5 29.76 63.80 60.26 0.005 0.010 0.0986
837.2 70.30 231.9 21.19 20.38 4.718 0.66 417.3 4.590 81.70 61.78 0.005 0.010 0.0061
59.5 86.5 47.1 48.1 86.6 86.2 86.4 5.67 95.8 79.1 11.21 78.26 87.5 99.88
61.7 87.9 48.4 48.3 88.2 87.0 88.0 11.6 99.4 54.9 11.81 92.96 97.37 99.99
Total hardness in mg CaCO3/L.
Fig. 3. Changes of permeate flux during nanofiltration of the Banska PGP1 and Banska PGP3 geothermal waters with Hydrex 4104 (A), using the NF-270 membrane (transmembrane pressure 10 bar, temperature 30 and 17 °C). Explanation: PGP1/30/A Banska PGP-1/water temperature 30 °C/water with H4104 antiscalant; PGP3/30 Banska PGP-3/water temperature 30 °C/water with H4104 antiscalant; PGP1/17 Banska PGP-1/water temperature 17 °C/water with H4104 antiscalant; PGP3/17 Banska PGP-3/water temperature 17 °C/water with H4104 antiscalant.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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Fig. 4. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water at 30 °C).
(Fig. 5c-3) and with a clearly elevated aluminium content. These features are probably quartz and feldspar grains in trace amounts. The results of scaling forecasts performed using geochemical modelling methods have been reflected in the deposits precipitated on the
surface of the membrane. No carbonate deposits were found on membranes, both at the higher and lower temperatures. At the same time, numerous ferruginous deposits clearly indicate that iron removal must be conducted before feeding the water to the NF membrane.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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Fig. 5. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water at 17 °C).
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
B. Tomaszewska et al. / Desalination xxx (2016) xxx–xxx
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Fig. 6. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water with Hydrex 4104).
HYDREX 4104 (H4104) used in the second stage of testing is a liquid formulation based upon phosphonates and dispersants which are extremely effective in preventing scale formation on membranes (Veolia product card). It is a non-toxic liquid suitable for automatic dosing and effective against sulphate, silica and carbonate scales. However, tests
on membranes after the water desalination process using the H4104 antiscalant at both temperatures (30 °C and 17 °C) demonstrated that the surface of the membranes was covered almost exclusively with fine-grained apatite/hydroxyapatite compounds (Fig. 6a-c). These phosphates cover essentially the entire surface of the membranes.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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Only in some micro-areas, can grain clusters be observed where calcium sulphates (Fig. 6b-c) or iron oxides are present alongside the dominant calcium phosphates. As a result of dosing the feed with antiscalant, the phosphate ion content in the feed increased from 0.0006 mg/L to greater than 80 mg/L. On the other hand, the physicochemical properties of the geothermal waters examined resulted in a very intensive precipitation of phosphate minerals and a reduction in permeate flux over time. A number of research studies suggest that the use of nanofiltration to pre-treat water is a solution to be favoured [14,15,19–22]. The advantages of NF membrane pretreatment, and the overall system design, fall into two broad categories, including an overall reduction in treatment cost while maintaining the integrity of water supplies. The benefits can be summarised as follows [22–26]: • prevention of SWRO membrane fouling by the removal of turbidity and bacteria, • elimination of scaling in the SWRO membrane by the removal of scale forming hardness ions, • reduction of the pressure required to operate SWRO plants by reducing seawater feed TDS by 30–60%, depending on the type of NF membrane and operating conditions.
However, with the operation of the NF water softening process, there is also the potential for scaling with inorganic substances to occur on the surface of the NF membrane due to the supersaturation of some scalant ions in the NF retentate [19]. Relatively little research has focused on this topic [27], and almost all the designers followed the manufacturer's recommended recovery specification which was usually limited to below 15% for a single NF membrane element [19]. A large number of publications are devoted to the use of different antiscalants to prevent the formation of deposits in membrane processes. Bonnelye et al. [27] reported a general consideration concerning the comparison of different pretreatment processes in RO desalination operations. In detail, they indicate that the pretreatment process must be adapted to the quality of the water to be treated. Our research results confirm this fact. Selection of the appropriate antiscalant is also a very important and responsible task. Van der Hoek et al. [28] indicate that the use of an antiscalant (flocon 100) in combination with H2SO4 was successful in controlling scaling, but this operational mode resulted in severe biofouling as the antiscalant acted as a nutrient for microbial regrowth in the membrane elements. Farchat et al. [29] showed, on the basis of an industrial installation in Djerba, that the use of phosphonate-based antiscalant can contribute to the relatively high levels of P observed. In the Djerba plant, since organic phosphonates are much more resistant to hydrolysis or converted to orthophosphate (PO3− 4 ), it is possible that the P on the membrane resulted from the reaction of the inhibitor with the Ca present in the feed water. The same result was obtained, i.e. that the chemical composition of antiscalant is not adapted to a quality of feed water that presents a moderate salinity with organic matter and iron. Farchat et al. [29] also reported, after Vrouwenvelder et al. [30], that dosage with antiscalant could increase both the phosphate and substrate concentrations of water. All the studies present the conclusion that the selection of antiscalants appears critical in respect to the development of fouling. Our research results are in agreement with recent research by Farchat et al. [29] and Vrouwenvelder et al. [30]. 4. Conclusions In the studies discussed, the nanofiltration process was selected as a method of water pre-treatment that can reduce the divalent ion content of waters before the proper treatment process using reverse osmosis. Thus nanofiltration was considered as one of the solutions which can be used for “water softening”. The tests conducted for water with
significant hardness and metasilicic acid content demonstrated a high efficiency in rejecting undesirable ions. The total hardness of water was reduced by 84.7–85.5% at a temperature of 17 °C and by 78.2–81.5% at 30 °C. A high rejection ratio was achieved for sulphate ions (99.6–99.7% at 17 °C and 99.1–99.6% at 30 °C), and for metasilicic acid it amounted to 16.73–18.34% and 4.01%–9.79% respectively. Even better rejection ratios of undesirable ions were achieved by dosing the feed with an antiscalant. However, a decrease in permeate flux over time was found when using antiscaling protection in the form of the H4104 antiscalant. This phenomenon was caused by the membrane being overgrown with secondary deposits. Microscopic examination of the deposits formed on the membranes yielded results fully consistent with the forecast made using geochemical modelling methods. In the cases examined, the use of an antiscalant based on phosphoric acid resulted in the formation of additional deposits, primarily in the form of calcium phosphate (apatite/hydroxyapatite). Geothermal waters have specific physicochemical properties and their increased temperature accelerates the thermodynamic reactions leading to the precipitation of mineral deposits. The scaling forecast is of considerable research and predictive importance here. Owing to the complexity of the processes occurring on the membrane surface, it is difficult to evaluate them in purely mathematical terms. This is confirmed by the results of tests conducted which demonstrate that the antiscalant selected (in theory the best one), whose purpose was to reduce sulphate, silicate and carbonate scaling, contributed to the precipitation of other deposits (phosphates). Acknowledgement This work was financed by the Polish National Centre for Research and Development, grant No 245079 (2014-2017). References [1] N. Kabay, I. Yilmaz, S. Yamac, S. Samatya, M. Yuksel, U. Yuksel, M. Arda, M. Saglam, T. Iwanaga, K. Hirowatari, Removal and recovery of boron from geothermal wastewater by selective ion exchange resins — I. Laboratory tests, React. Funct. Polym. 60 (2004) 163–170. [2] N. Kabay, I. Yilmaz, S. Yamac, M. Yuksel, U. Yuksel, N. Yildirim, O. Aydogdu, T. Iwanaga, K. Hirowatari, Removal and recovery of boron from geothermal wastewater by selective ion-exchange resins — II. Field tests, Desalination 167 (2004) 427–438. [3] Ş. Şimşek, N. Yıldırım, A. Gülgör, Developmental and environmental effects of the Kızıldere geothermal power project, Turkey. Geothermics 34 (2005) 239–256. [4] N. Kabay, I. Yilmaz-Ipek, I. Soroko, M. Makowski, O. Kirmizisakal, S. Yag, M. Bryjak, M. Yuksel, Removal of boron from Balcova geothermal water by ion exchangemicrofiltration hybrid process, Desalination 241 (2009) 167–173. [5] Ş.G. Öner, N. Kabay, E. Güler, M. Kitiş, M. Yüksel, A comparative study for the removal of boron and silica from geothermal water by cross-flow flat sheet reverse osmosis method, Desalination 283 (2011) 10–15. [6] H. Koseoglu, B.I. Harman, N.N.O. Yigit, E. Guler, N. Kabay, M. Kitis, The effects of operating conditions on boron removal from geothermal waters by membrane processes, Desalination 258 (2010) 72–78. [7] N. Kabay, P. Köseoğlu, E. Yavuz, U. Yüksel, M. Yüksel, An innovative integrated system for boron removal from geothermal water using RO process and ion exchangeultrafiltration hybrid method, Desalination 316 (2013) 1–7. [8] D.L. Gallup, Treatment of geothermal waters for production of industrial, agricultural or drinking water, Geothermics 36 (2007) 473–483. [9] V.G. Gude, Geothermal source potential for water desalination — current status and future perspective, Renew. Sust. Energ. Rev. 57 (2016) 1038–1065. [10] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UFRO process. Part I: boron. Removal in pilot-scale tests, Desalination 319 (2013) 99–106. [11] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UFRO process. Part II: membrane scaling after pilot-scale tests, Desalination 319 (2013) 107–114. [12] B. Tomaszewska, M. Bodzek, The removal of radionuclides during desalination of geothermal waters containing boron using the BWRO system, Desalination 309 (2013) 284–290. [13] B. Tomaszewska, L. Pająk, M. Bodzek, Application of a hybrid UF-RO process to geothermal water desalination. Concentrate disposal and costs analysis, Arch. Environ. Prot. 40 (3) (2014) 137–151. [14] L. Llenas, X. Martinez-Llado, A. Yaroschchuk, M. Rovira, J. de Pablo, Nanofiltration as pretreatment for scale prevention in seawater reverse osmosis desalination, Desalin. Water Treat. 36 (2011) 310–318.
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Desalination journal homepage: www.elsevier.com/locate/desal
The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process Barbara Tomaszewska a,b,⁎, Mariola Rajca c, Ewa Kmiecik a, Michał Bodzek c,d, Wiesław Bujakowski b, Katarzyna Wątor a, Magdalena Tyszer a a
AGH University of Science and Technology, Mickiewicza 30 Av., 30-059 Krakow, Poland Mineral and Energy Economy Research Institute, Polish Academy of Sciences, Wybickiego 7, 31-261 Krakow, Poland Silesian University of Technology, Institute of Water and Wastewater Engineering, Konarskiego 18, 44-100 Gliwice, Poland d Institute of Environmental Engineering, Polish Academy of Sciences, M. Curie-Skłodowskiej 34, 41-819 Zabrze, Poland b c
H I G H L I G H T • • • •
Research areas investigated include problems during the pre-treatment of geothermal water, Study involved nanofiltration with a commercial NF-270 (Dow Filmtec) membrane, Research was carried out on different temp. variations 30 °C and 17 °C and the use of raw water and water with antiscalant, Qualitative and quantitative identification of the deposits precipitated on a NF-270 membrane has been presented.
a r t i c l e
i n f o
Article history: Received 23 February 2016 Received in revised form 22 June 2016 Accepted 7 July 2016 Available online xxxx Keywords: Geothermal water Water treatment Membrane Nanofiltration Scaling
a b s t r a c t The use of the reverse osmosis process in water treatment often requires the careful selection of methods of pretreatment. This especially applies to mineralised water, ferruginous water, water with a high carbonate hardness and also water with a tendency to precipitate sulphate and silica deposits. Increased feed water temperature may be a further factor which could encourage scaling of the membrane. To decrease the number of divalent ions, the nanofiltration process was tested with the use of a commercial NF-270 membrane (Dow Filmtec). Filtration was performed under a transmembrane pressure of about 10 bar with cross-flow filtration. Tests were carried out on two highly mineralised geothermal waters mineralisation (TDS 2.2–2.3 g/L), more than 600 mgCaCO3/L and a high silica concentration. Before laboratory tests, the tendency to precipitate mineral deposits was determined by means of geochemical modelling. In addition, appropriate doses of antiscalant were selected to avoid the precipitation of deposits on the NF membrane. The research was carried out on different variations of: 1) two temperatures: 30 °C and 17 °C and 2) the use of raw water and water with the addition of antiscalant (Hydrex). During tests the changes of membrane effectiveness with time were observed. Qualitative and quantitative identification of the precipitate deposits on the NF-270 membrane was made for each of the variants of the water pretreatment process. © 2016 Published by Elsevier B.V.
1. Introduction The efficient and sustainable management of natural resources, primarily energy resources, and increasing the share of renewable sources of energy are both goals of European Union Member States in the era of
⁎ Corresponding author at: AGH University of Science and Technology, Mickiewicza 30 Av., 30-059 Krakow, Poland. E-mail addresses: [email protected], [email protected] (B. Tomaszewska), [email protected] (M. Rajca), [email protected] (E. Kmiecik), [email protected] (M. Bodzek), [email protected] (W. Bujakowski), [email protected] (K. Wątor), [email protected] (M. Tyszer).
the fight against global warming. These objectives, especially economic ones, should undoubtedly be harmonised with environmental protection. The use of geothermal energy, which offers significant resources that are widely available on a global scale, also plays an important role in this regard. The fact that this resource is practically limitless and renewable, together with its independence of changing climate and weather conditions, present clear advantages which militate in favour of its increased utilisation. Irrespective of the obvious climate protection benefits, the direct and indirect development of these resources in the generation of electricity, heating, agriculture, horticulture, balneotherapy, recreation, etc. should also respect other elements of the natural environment. This primarily relates to finding a suitable
http://dx.doi.org/10.1016/j.desal.2016.07.007 0011-9164/© 2016 Published by Elsevier B.V.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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way of dealing with chilled geothermal waters that have already been spent and constitute waste. The desalination of geothermal waters used for energy purposes is increasingly seen as a good method of securing high-quality water for various areas of the economy. In countries with warm climates it is mainly used for the irrigation of agricultural crops [1–7]. Given the increasing deficit of freshwater worldwide, the possibilities for desalinating and treating geothermal waters for drinking and household purposes should be considered [8–13]. The use of the reverse osmosis process in water treatment often requires careful methods of selection and preliminary removal of undesirable components. This applies especially to mineralised water, ferruginous water, water with high carbonate hardness and also water with a tendency to precipitate sulphate and silica deposits. An additional factor which could encourage scaling of the membrane might be increased feed water temperature. The nanofiltration (NF) process has been analysed in order to decrease the number of divalent ions. NF is mostly used for the softening of and the removal of organic compounds from surface and brackish water. NF membranes are characterised by a high retention of bivalent ions, while the retention of monovalent ions is limited. The aim of the study was to evaluate the use of NF as a pre-treatment method for the softening of geothermal water. The research was carried out using 1) water at two alternative temperatures, 30 °C and 17 °C and 2) raw water and water with the addition of antiscalant. 2. Materials and methods
Table 2 Results of Bańska PGP-3 geothermal water using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 Al+3 Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2274 648.6 477.3 46.21 195.1 39.37 6.090 440.3 797.1 313.9 74.90 0.018 0.210 0.0061
2274 660.2 475.1 46.25 200.9 38.60 6.02 465.8 785.8 281.2 74.62 0.018 0.220 0.0061
882.0 94.00 249.5 23.49 29.41 5.00 0.84 395.6 3.00 156.9 61.16 0.005 0.010 0.0061
1063.3 143.90 295.10 27.06 42.50 9.21 1.32 469.6 6.92 199.5 71.63 0.007 0.010 0.0061
61.2 85.5 47.7 49.2 84.9 87.1 86.2 10.2 99.6 50.0 18.34 72.22 95.24 0
53.3 78.2 37.9 41.5 78.8 76.2 78.1 0.00 99.1 29.1 4.01 61.11 95.45 0
Total hardness in mg CaCO3/L.
the process with a recycling of the permeate into the reservoir. The NF270 membrane is the most suitable for antiscaling pretreatment because of the high rejection rate of scale forming ions as well as the high permeate flux [14,15]. The active area of the membrane was 155 cm2. The membrane has a negative load surface in a wide range of pH [16]. The new membrane was conditioned by filtration of deionised water to stabilise the permeate flux.
2.1. Geothermal waters 2.3. Methodology of physico-chemical analysis Tests were carried out using two geothermal waters with high mineralisation (TDS 2.2–2.3 g/L), more than 600 mgCaCO3/L and a high silica concentration. The physical and chemical composition of the water tested is shown in Tables 1 and 2. According to the Szczukariew-Priklonski classification, the waters fall into the hydrogeochemical type SO4-Cl-Na-Ca. The feed water had a temperature of 1) 30 °C and 2) 17 °C. 2.2. Testing equipment A schematic diagram of the system used for the nanofiltration process is shown in Fig. 1. Tests were conducted using the SEPA CF-HP type high-pressure membrane module from the American company, Osmonics Inc. This used a “cross-flow” system utilising an NF270 Dow Filmtec membrane at a specified pressure of 10 bar and completing
Water pH and temperature were measured using the electrometric method immediately after sampling water from the system. Inorganic components were determined in an accredited laboratory of the Department of Hydrogeology and Engineering Geology of the AGH University of Science and Technology in Kraków (PCA certificate No AB 1050) using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Chloride ion content and water alkalinity were determined by titration in accordance with accredited testing procedures. 2.4. Antiscalant dosing and methodology for predicting scaling The Hydrex Antiscalant Dosing Calculator (Veolia Water Solution & Technologies) was used to choose the recommended product and calculate the dosage applied in feed water. The saturation index (SI) of the
Table 1 Results of Bańska PGP-1 geothermal water using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 +3 Al Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2228 636.1 467.2 45.2 190.4 39.2 5.95 435.2 784.0 294.3 74.06 0.038 0.16 0.0061
2342 666.8 492.8 47.41 200.8 40.29 6.240 482.4 812.5 279.6 76.93 0.089 0.32 0.0061
891.5 97.10 251.0 24.50 29.77 5.540 0.870 403.3 5.32 145.5 61.67 0.005 0.010 0.0061
994.6 123.5 277.9 25.75 37.85 7.07 1.12 449.2 3.00 167.6 69.4 0.005 0.01 0.0061
59.9 84.7 46.3 45.8 84.4 85.9 85.4 7.33 99.7 50.6 16.73 86.84 93.75 0
57.5 81.5 43.6 45.7 81.2 82.5 82.1 6.89 99.6 40.1 9.79 94.38 96.87 0
Total hardness in mg CaCO3/L.
Fig. 1. Diagram of the system for carrying out nanofiltration in “cross-flow” mode (Explanation: 1-heat exchanger; 2-raw water inflow; 3-rotameter; 4-membrane cell; 5permeate outflow; 6-pump; 7-raw water tank).
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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solution was also calculated using the Phreeqc Interactive 3.3.3-10424 program (PHREEQCI) using the Wateq4f minerals database [17]. 2.5. Methodology of membrane scaling analysis The mineral composition of reaction products was determined via the powder X-ray diffraction (XRD) method using a Philips X'pert APD diffractometer (with the PW 3020 goniometer), Cu lamp, and graphite monochromator. The analysis was performed within the angle range of 5–60 2θ. The Philips X'Pert and the ClayLab ver. 1.0 software packages were used to process diffraction data. The identification of mineral phases was based on the PCPDFWIN ver. 1.30 database formalised by the JCPDS-ICDD. The morphology and chemical composition of the main mineral components of the materials examined in the micro area domain were determined using a scanning electron microscope (SEM). The equipment used was an FEI Qanta 250 FEG scanning microscope additionally equipped with a chemical composition analysis system based on energy dispersion scattering – the EDS EDAX. 3. Results and discussion Owing to the fact that the desalination of geothermal waters in industrial installations is carried out at a temperature of ca. 30 °C [10], the first tests of the NF process using water from two wells (Bańska PGP-1 and Bańska PGP-3) were conducted at this feed temperature. Subsequently, tests were carried out for water at a temperature of 17 °C, since ultimately the preferred solution would involve better utilisation of the heating potential of geothermal waters, e.g. for heating soil when growing crops under film covers or on sills in special greenhouses, or for culturing other organisms, e.g. algae [18]. In this case, the temperature of chilled waters fed to the desalination plant would be correspondingly lower. Test results for waters from both geothermal wells and for two different feed temperatures are shown in Tables 1 and 2. The results of the test conducted using the nanofiltration process with NF-270 membranes and geothermal water from the two wells examined (Tables 1 and 2) demonstrated a more efficient rejection of undesirable components (by about 10%) for the lower temperature (17 °C) feed. TDS was decreased by 59.9–61.2% at a temperature of 17 °C and by 53.3–57.5% at 30 °C. The desired reduction in the total hardness of the water was obtained: 84.7–85.5% at 17 °C and 78.2–81.5% at 30 °C. The best rejection ratio was obtained for sulphate ions: 99.6–99.7% at 17 °C and 99.1–99.6% at 30 °C. For metasilicic acid, the figures were 16.73–18.34% and 4.01–9.79% respectively. Given the high content of sulphate ions in the waters tested, the effect achieved would be significant in terms of preventing the scaling of membranes at the proper water treatment stage using the reverse osmosis (RO) process. No significant decrease in permeate flux was observed during laboratory studies. The nanofiltration process proceeded in a stable manner as shown in Fig. 2. Due to the lower viscosity of water, significantly higher permeate flux was achieved at a water temperature of 30 °C. At a water temperature of 17 °C, the performance of the membrane was lower by approx. 20%. The resulting stability in performance at a laboratory scale during a test lasting approx. 5.5 h is consistent with the results obtained in tests on a long-term, semi-industrial scale [11]. Geothermal water with TDS of 2.15 to 2.7 g/L was treated for a period of eight months. During a pilot study of the desalination of geothermal water on a semiindustrial scale using DOW FILMTEC BW30HR–440i membranes, no decrease in the permeate flux was observed. At RO-1 an average permeability of 5.25 ∙ 10− 6 m3/m2·s was achieved at recovery levels of ca. 75–78% and at RO-2 at a transmembrane pressure of 10 bar, the recovery level was ca. 75% and the average permeability of the membranes was measured at 7.9 ∙ 10− 6 m3/m2·s [11]. However, the main factor that had a positive effect on the desalination process was the
Fig. 2. Changes of permeate flux during nanofiltration of the Banska PGP-1 and Banska PGP-3 geothermal waters using the NF-270 membrane (transmembrane pressure 10 bar, temperature 30 °C and 17 °C). Explanation: PGP1/30 - Banska PGP-1/water temperature 30 °C; PGP3/30 - Banska PGP-3/water temperature 30 °C; PGP1/17 Banska PGP-1/water temperature 17 °C; PGP3/17 - Banska PGP-3/water temperature 17 °C.
hydrochloric acid treatment of the water fed to the first reverse osmosis stage, which decarbonised the water and also reduced the saturation index with respect to sulphate and silicate mineral forms [11]. To determine how applicable the solution analysed is in industrial installations, one must thoroughly determine its process efficiency and reliability. A reduction in the efficiency of a membrane process is usually related to membrane scaling arising as a result of the solution (feed) being supersaturated with specific mineral forms, as a result of which deposits accumulate on the membrane surface, blocking it and reducing process efficiency. These phenomena may be particularly intense in waters with a high level of total hardness and elevated silica content. The scaling forecast prepared using geochemical modelling and taking into account the physicochemical properties of the geothermal waters (feeds) studied and the pressure used in the process in question (10 bar) yielded the following results: 1) for waters at a temperature of 30 °C: a) the oversaturation of the geothermal water solutions tested and a tendency of the following minerals to precipitate: sulphates, especially barite (BaSO4), silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (Fe2O3), magnetite (Fe3O4) and aluminium hydroxide – gibbsite (Al(OH)3) as well as various aluminium silicate forms, e.g. kaolinite (Al2Si2O5(OH)4), muscovite (KAl3Si3O10(OH)2) and others; b) a state of equilibrium with respect to carbonate forms – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2) and calcium sulphate (gypsum – CaSO4); c) undersaturation with the anhydrite form of calcium sulphate (CaSO4); 2) for waters at a temperature of 17 °C: a) the oversaturation of the geothermal water solutions tested and a tendency of the following minerals to precipitate: silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (γFe2O3), magnetite (Fe3O4), various aluminium silicate forms and aluminium hydroxide – gibbsite (Al(OH)3); b) a state of equilibrium with respect to barium sulphate (barite) (BaSO4); c) undersaturation with carbonate – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2) and sulphate forms (anhydrite and gypsum – CaSO4). The calculations performed using the Hydrex antiscalant Dosing Calculator (Veolia Water Solution & Technologies) demonstrated that among the HYDREX (Veolia) product family, the Hydrex 4104 (H4104) antiscalant would be the most appropriate to protect the NF270 membrane against scaling. Therefore, the next stage involved tests conducted at feed temperatures of 30 °C and 17 °C using the
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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H4104 antiscalant. Tables 3 and 4 show the physicochemical properties of the feed dosed with H4104 and the rejection rate of the ions analysed at temperatures of 17 °C and 30 °C. As concerns the rejection ratio of undesirable ingredients, slightly better results were obtained for the feed dosed with antiscalant compared to the results for the feed without antiscalant (Tables 3, 4). Unfortunately, a decrease was observed in process flux which indicated that blockage of the membrane was occurring (Fig. 3). The downward trend in performance of the membrane was maintained for most of the total filtration time (about 12,000 s), for subsequent measurement seconds it showed a significant stabilisation of the flux, in particular for the Banska PGP-3 geothermal water at 30 °C. This may prove that the blocking of membrane filtration for water with antiscalant takes place until it reaches a certain level, and that then there is a stabilisation of the conditions under which transport through the membrane takes place. The geochemical modelling of the degree of saturation of the feed dosed with antiscalant for both the geothermal wells and water temperatures (17 °C and 30 °C) tested demonstrated: a) the continued oversaturation of the tested solutions with respect to the following minerals: silicates such as chalcedony, quartz, silica gel (SiO2) and amorphous forms of silica (SiO2(a)), iron oxides and hydroxides – goethite (FeOOH), hematite (Fe2O3), maghemite (γ Fe2O3), magnetite (Fe3O4) and aluminium hydroxide – gibbsite (Al(OH)3) as well as various forms of aluminium silicate, e.g. kaolinite (Al2Si2O5(OH)4), muscovite (KAl3Si3O10(OH)2) and others. However, an increase in solution saturation was observed with respect to phosphate minerals, e.g. apatite, hydroxyapatite (Ca5(PO4)3OH) and others, which had a tendency to precipitate. The solutions were in a state of equilibrium with respect to sulphate forms – barite (BaSO4), anhydrite and gypsum (CaSO4) and strong undersaturation with respect to carbonate forms – aragonite, calcite (CaCO3), dolomite (CaMg(CO3)2). In consequence, the geochemical modelling shows that, apart from geothermal waters (feeds), waters with the antiscalant have oversaturation with respect to phosphate minerals, e.g. apatite, hydroxyapatite and strong undersaturation with respect to carbonate forms – aragonite, calcite and dolomite. Photomicrographs of membranes after conducting tests with geothermal waters (without the antiscalant) are shown in Figs. 4 and 5, and those for feeds dosed with the antiscalant are shown in Fig. 6. As a result of the nanofiltration of water from the Bańska PGP-1 well (without the antiscalant) at a feed temperature of 30 °C, the following deposits precipitated on the membrane: sulphate minerals with a small share of chlorides (the result of halite precipitation from the water remaining on the membrane). These deposits have an irregular character (Fig. 4a-1). The mineral components mainly include gypsum
Table 4 Results of Bańska PGP-3 geothermal water with Hydrex 4104 using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
Raw water mg/L 17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 Al+3 Fe+2 PO−3 4
1977 537.2 401.9 38.60 162.2 32.23 4.99 433.9 664.9 284.5 62.99 0.008 0.15 86.48
2041 568.0 427.5 40.65 171.6 34.01 5.31 417.3 711.4 263.2 66.91 0.023 0.06 88.37
764.8 66.50 208.9 18.45 19.05 4.61 0.64 384.1 7.67 71.90 53.87 0.005 0.010 0.36
675.5 47.20 189.5 17.18 13.42 3.35 0.43 347.1 4.39 32.70 52.67 0.005 0.020 0.224
61.3 87.6 48.0 52.2 88.3 85.7 87.2 11.5 98.9 74.7 14.47 37.5 93.33 99.58
66.9 91.7 55.7 57.7 92.2 90.2 91.9 16.8 99.4 87.6 21.28 78.26 66.66 99.74
a
Permeate mg/L
R [%]
Total hardness in mg CaCO3/L.
and halite as well as iron oxides, which form a layer of sorts on the membrane – this is well illustrated by Fig. 4a-2. This layer consists of very fine grains and it is difficult to identify the presence of individual crystals. In some micro-areas, the presence of aluminosilicate phases is clearly visible on the surface of gypsum and halite deposits, which is especially visible in Fig. 4b. Deposits on the NF-270 membrane after the nanofiltration of water from the Bańska PGP-3 well exhibit similar characteristics (Fig. 4c). The membrane surface in Fig. 4c is covered with clusters of very fine-grained iron sulphates (Fig. 4c-3). In individual micro-areas, mineral clusters with a size of up to 50 μm can be found whose chemical composition is a mixture of phosphates, sulphates, and sodium and potassium chlorides (Fig. 4c-1,2). The deposits crystallised on the membrane during the nanofiltration of water at 17 °C mainly consist of fine-grained minerals from the sulphate group (Fig. 5a). The chemical spectra of this layer show that it is mostly sulphur and oxygen with trace amounts of sodium and silicon (Fig. 5a, b-3). This layer almost completely covers the surface of the membrane. Far larger clusters of mineral grains with a size of up to 5 μm are only found in a few places on the membrane. Their chemical composition is clearly different from the background in quantitative terms. Apart from the dominant sulphurs, there is also calcium that forms gypsum/anhydrite minerals (Fig. 5b-1,2, c-1,2). There are only sporadic micro-areas where, at the expense of lower sulphur content, clusters appear whose chemical composition is dominated by silicon
Table 3 Results of Bańska PGP-1 geothermal water with Hydrex 4104 using a NF-270 Dow Filmtec nanofiltration membrane (transmembrane pressure 10 bar, R-retention coefficient). Parameters
TDS a TH Na+ K+ Ca+2 Mg+2 Sr+2 Cl− SO−2 4 HCO− 3 H2SiO3 +3 Al Fe+2 PO−3 4 a
Raw water mg/L
Permeate mg/L
R [%]
17 °C
30 °C
17 °C
30 °C
17 °C
30 °C
2084 574.4 434.4 40.08 171.3 35.72 5.46 435.2 706.0 305.7 67.87 0.023 0.08 83.02
2185 579.8 448.9 40.98 172.5 36.29 5.52 472.2 733.4 181.5 70.05 0.071 0.38 79.24
844.1 77.60 229.7 20.83 22.98 4.931 0.74 410.5 29.76 63.80 60.26 0.005 0.010 0.0986
837.2 70.30 231.9 21.19 20.38 4.718 0.66 417.3 4.590 81.70 61.78 0.005 0.010 0.0061
59.5 86.5 47.1 48.1 86.6 86.2 86.4 5.67 95.8 79.1 11.21 78.26 87.5 99.88
61.7 87.9 48.4 48.3 88.2 87.0 88.0 11.6 99.4 54.9 11.81 92.96 97.37 99.99
Total hardness in mg CaCO3/L.
Fig. 3. Changes of permeate flux during nanofiltration of the Banska PGP1 and Banska PGP3 geothermal waters with Hydrex 4104 (A), using the NF-270 membrane (transmembrane pressure 10 bar, temperature 30 and 17 °C). Explanation: PGP1/30/A Banska PGP-1/water temperature 30 °C/water with H4104 antiscalant; PGP3/30 Banska PGP-3/water temperature 30 °C/water with H4104 antiscalant; PGP1/17 Banska PGP-1/water temperature 17 °C/water with H4104 antiscalant; PGP3/17 Banska PGP-3/water temperature 17 °C/water with H4104 antiscalant.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
B. Tomaszewska et al. / Desalination xxx (2016) xxx–xxx
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Fig. 4. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water at 30 °C).
(Fig. 5c-3) and with a clearly elevated aluminium content. These features are probably quartz and feldspar grains in trace amounts. The results of scaling forecasts performed using geochemical modelling methods have been reflected in the deposits precipitated on the
surface of the membrane. No carbonate deposits were found on membranes, both at the higher and lower temperatures. At the same time, numerous ferruginous deposits clearly indicate that iron removal must be conducted before feeding the water to the NF membrane.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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Fig. 5. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water at 17 °C).
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
B. Tomaszewska et al. / Desalination xxx (2016) xxx–xxx
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Fig. 6. SEM-EDS images of the NF-270 membrane surface in membrane autopsy (after treating geothermal water with Hydrex 4104).
HYDREX 4104 (H4104) used in the second stage of testing is a liquid formulation based upon phosphonates and dispersants which are extremely effective in preventing scale formation on membranes (Veolia product card). It is a non-toxic liquid suitable for automatic dosing and effective against sulphate, silica and carbonate scales. However, tests
on membranes after the water desalination process using the H4104 antiscalant at both temperatures (30 °C and 17 °C) demonstrated that the surface of the membranes was covered almost exclusively with fine-grained apatite/hydroxyapatite compounds (Fig. 6a-c). These phosphates cover essentially the entire surface of the membranes.
Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007
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Only in some micro-areas, can grain clusters be observed where calcium sulphates (Fig. 6b-c) or iron oxides are present alongside the dominant calcium phosphates. As a result of dosing the feed with antiscalant, the phosphate ion content in the feed increased from 0.0006 mg/L to greater than 80 mg/L. On the other hand, the physicochemical properties of the geothermal waters examined resulted in a very intensive precipitation of phosphate minerals and a reduction in permeate flux over time. A number of research studies suggest that the use of nanofiltration to pre-treat water is a solution to be favoured [14,15,19–22]. The advantages of NF membrane pretreatment, and the overall system design, fall into two broad categories, including an overall reduction in treatment cost while maintaining the integrity of water supplies. The benefits can be summarised as follows [22–26]: • prevention of SWRO membrane fouling by the removal of turbidity and bacteria, • elimination of scaling in the SWRO membrane by the removal of scale forming hardness ions, • reduction of the pressure required to operate SWRO plants by reducing seawater feed TDS by 30–60%, depending on the type of NF membrane and operating conditions.
However, with the operation of the NF water softening process, there is also the potential for scaling with inorganic substances to occur on the surface of the NF membrane due to the supersaturation of some scalant ions in the NF retentate [19]. Relatively little research has focused on this topic [27], and almost all the designers followed the manufacturer's recommended recovery specification which was usually limited to below 15% for a single NF membrane element [19]. A large number of publications are devoted to the use of different antiscalants to prevent the formation of deposits in membrane processes. Bonnelye et al. [27] reported a general consideration concerning the comparison of different pretreatment processes in RO desalination operations. In detail, they indicate that the pretreatment process must be adapted to the quality of the water to be treated. Our research results confirm this fact. Selection of the appropriate antiscalant is also a very important and responsible task. Van der Hoek et al. [28] indicate that the use of an antiscalant (flocon 100) in combination with H2SO4 was successful in controlling scaling, but this operational mode resulted in severe biofouling as the antiscalant acted as a nutrient for microbial regrowth in the membrane elements. Farchat et al. [29] showed, on the basis of an industrial installation in Djerba, that the use of phosphonate-based antiscalant can contribute to the relatively high levels of P observed. In the Djerba plant, since organic phosphonates are much more resistant to hydrolysis or converted to orthophosphate (PO3− 4 ), it is possible that the P on the membrane resulted from the reaction of the inhibitor with the Ca present in the feed water. The same result was obtained, i.e. that the chemical composition of antiscalant is not adapted to a quality of feed water that presents a moderate salinity with organic matter and iron. Farchat et al. [29] also reported, after Vrouwenvelder et al. [30], that dosage with antiscalant could increase both the phosphate and substrate concentrations of water. All the studies present the conclusion that the selection of antiscalants appears critical in respect to the development of fouling. Our research results are in agreement with recent research by Farchat et al. [29] and Vrouwenvelder et al. [30]. 4. Conclusions In the studies discussed, the nanofiltration process was selected as a method of water pre-treatment that can reduce the divalent ion content of waters before the proper treatment process using reverse osmosis. Thus nanofiltration was considered as one of the solutions which can be used for “water softening”. The tests conducted for water with
significant hardness and metasilicic acid content demonstrated a high efficiency in rejecting undesirable ions. The total hardness of water was reduced by 84.7–85.5% at a temperature of 17 °C and by 78.2–81.5% at 30 °C. A high rejection ratio was achieved for sulphate ions (99.6–99.7% at 17 °C and 99.1–99.6% at 30 °C), and for metasilicic acid it amounted to 16.73–18.34% and 4.01%–9.79% respectively. Even better rejection ratios of undesirable ions were achieved by dosing the feed with an antiscalant. However, a decrease in permeate flux over time was found when using antiscaling protection in the form of the H4104 antiscalant. This phenomenon was caused by the membrane being overgrown with secondary deposits. Microscopic examination of the deposits formed on the membranes yielded results fully consistent with the forecast made using geochemical modelling methods. In the cases examined, the use of an antiscalant based on phosphoric acid resulted in the formation of additional deposits, primarily in the form of calcium phosphate (apatite/hydroxyapatite). Geothermal waters have specific physicochemical properties and their increased temperature accelerates the thermodynamic reactions leading to the precipitation of mineral deposits. The scaling forecast is of considerable research and predictive importance here. Owing to the complexity of the processes occurring on the membrane surface, it is difficult to evaluate them in purely mathematical terms. This is confirmed by the results of tests conducted which demonstrate that the antiscalant selected (in theory the best one), whose purpose was to reduce sulphate, silicate and carbonate scaling, contributed to the precipitation of other deposits (phosphates). Acknowledgement This work was financed by the Polish National Centre for Research and Development, grant No 245079 (2014-2017). References [1] N. Kabay, I. Yilmaz, S. Yamac, S. Samatya, M. Yuksel, U. Yuksel, M. Arda, M. Saglam, T. Iwanaga, K. Hirowatari, Removal and recovery of boron from geothermal wastewater by selective ion exchange resins — I. Laboratory tests, React. Funct. Polym. 60 (2004) 163–170. [2] N. Kabay, I. Yilmaz, S. Yamac, M. Yuksel, U. Yuksel, N. Yildirim, O. Aydogdu, T. Iwanaga, K. Hirowatari, Removal and recovery of boron from geothermal wastewater by selective ion-exchange resins — II. Field tests, Desalination 167 (2004) 427–438. [3] Ş. Şimşek, N. Yıldırım, A. Gülgör, Developmental and environmental effects of the Kızıldere geothermal power project, Turkey. Geothermics 34 (2005) 239–256. [4] N. Kabay, I. Yilmaz-Ipek, I. Soroko, M. Makowski, O. Kirmizisakal, S. Yag, M. Bryjak, M. Yuksel, Removal of boron from Balcova geothermal water by ion exchangemicrofiltration hybrid process, Desalination 241 (2009) 167–173. [5] Ş.G. Öner, N. Kabay, E. Güler, M. Kitiş, M. Yüksel, A comparative study for the removal of boron and silica from geothermal water by cross-flow flat sheet reverse osmosis method, Desalination 283 (2011) 10–15. [6] H. Koseoglu, B.I. Harman, N.N.O. Yigit, E. Guler, N. Kabay, M. Kitis, The effects of operating conditions on boron removal from geothermal waters by membrane processes, Desalination 258 (2010) 72–78. [7] N. Kabay, P. Köseoğlu, E. Yavuz, U. Yüksel, M. Yüksel, An innovative integrated system for boron removal from geothermal water using RO process and ion exchangeultrafiltration hybrid method, Desalination 316 (2013) 1–7. [8] D.L. Gallup, Treatment of geothermal waters for production of industrial, agricultural or drinking water, Geothermics 36 (2007) 473–483. [9] V.G. Gude, Geothermal source potential for water desalination — current status and future perspective, Renew. Sust. Energ. Rev. 57 (2016) 1038–1065. [10] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UFRO process. Part I: boron. Removal in pilot-scale tests, Desalination 319 (2013) 99–106. [11] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UFRO process. Part II: membrane scaling after pilot-scale tests, Desalination 319 (2013) 107–114. [12] B. Tomaszewska, M. Bodzek, The removal of radionuclides during desalination of geothermal waters containing boron using the BWRO system, Desalination 309 (2013) 284–290. [13] B. Tomaszewska, L. Pająk, M. Bodzek, Application of a hybrid UF-RO process to geothermal water desalination. Concentrate disposal and costs analysis, Arch. Environ. Prot. 40 (3) (2014) 137–151. [14] L. Llenas, X. Martinez-Llado, A. Yaroschchuk, M. Rovira, J. de Pablo, Nanofiltration as pretreatment for scale prevention in seawater reverse osmosis desalination, Desalin. Water Treat. 36 (2011) 310–318.
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Please cite this article as: B. Tomaszewska, et al., The influence of selected factors on the effectiveness of pre-treatment of geothermal water during the nanofiltration process, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.007