Electrochemical treatment of industrial cooling tower blowdown water using magnesium-rod electrode

Water Resources and Industry 23 (2020) 100121

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Electrochemical treatment of industrial cooling tower blowdown water using magnesium-rod electrode Hussein I. Abdel-Shafy a, Madiha A. Shoeib b, Mohamed A. El-Khateeb a, Ahmed O. Youssef c, Omar M. Hafez d, * a

Water Research & Pollution Control Department, National Research Centre, P.O. Box 12622, Doki, Cairo, Egypt Surface Coating Department, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 12422, Helwan, Cairo, Egypt c Department of Chemistry, Faculty of Science, Ain-Shams University, P.O. Box 11566, Cairo, Egypt d Chemical Laboratories Section, Helwan Fertilizer Company (HFC), P.O. Box 1081, El-Tabbin, Cairo, Egypt b

A R T I C L E I N F O

A B S T R A C T

Keywords: Cooling tower blowdown water Electrocoagulation Magnesium-rod electrode Scale ions removal Operating cost Sludge characterization

Cooling tower blowdown water (CTBW) was treated with simple electrocoagulation (EC) using magnesium-rod electrode. This study examined the effects of the treatment parameters (current density, electrolysis time, electrode distance, initial pH and stirring speed) on the EC ability to remove hardness ions (Ca2þ, Mg2þ) and dissolved silica from CTBW. Under the optimized con­ dition, magnesium-rod electrode removed 51.80% and 93.70% respectively for total hardness and silica; with an operating cost of 0.88 US$/m3 treated CTBW. EC sludge has been characterized by SVI, SEM-EDX, XRD, and FTIR exploring the ability of sludge to settle, surface morphology, elemental composition, crystalline type, and functional groups. It can be concluded that EC using magnesium-rod electrode can be successfully applied for the treatment of CTBW to facilitate its reuse.

1. Introduction Cooling towers operate on the basis of evaporative condensation and sensible heat exchange. The residual heat of the vaporization is emitted by mixing two types of fluids with different temperatures, resulting in a decrease in the temperature of the hot fluid. The system of evaporative cooling water loses its pure vapor and tends to raise the concentration of dissolved solids in the remaining water. If this cycle continues, the solubility of many solids will finally be reduced. The solids will then be placed in the shape of shale on the warm surface of the condenser pipes. The formation of scales in cooling water systems reduces the heat transfer efficiency by insulating the metal surface from the cooling water, thereby preventing the transfer of heat. When the formation of the scale reaches a critical point, the hydraulic hindrance impact reduces the flow of water dramatically [1]. Depending on the minerals in the water, the most commonly found cooling salts are calcium carbonate, calcium phosphate, iron oxides, magnesium silicate and silica. Factors that affect the scale leaning on the cooling tower include water quality, pH, temperature, and water flow rate [2]. The control of scale deposits in cooling water systems is necessary to maintain maximum heat transfer efficiency. The formation of scale in the cooling tower can be controlled by increasing CTBW to limit cycles of concentration [3]. The volume of blowdown water required for optimal cooling tower operation could be reduced by removing or eliminating the scale-forming ions from cooling tower * Corresponding author. E-mail addresses: [email protected] (H.I. Abdel-Shafy), [email protected] (M.A. Shoeib), [email protected] (M.A. ElKhateeb), [email protected] (A.O. Youssef), [email protected] (O.M. Hafez). https://doi.org/10.1016/j.wri.2019.100121 Received 4 April 2019; Received in revised form 29 October 2019; Accepted 6 December 2019 Available online 12 December 2019 2212-3717/© 2019 Published by Elsevier B.V. This is an open access article (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Nomenclature and units: Symbol EC CTBW V A s min T.H Ca.H Mg.H rpm SVI SEM EDX XRD FT-IR OC Cenergy Celectrode

Electrocoagulation Cooling tower blowdown water Volt Ampere Second Minutes Total Hardness Calcium Hardness Magnesium Hardness Rotation per minutes Sludge volume index Scanning electron microscope Energy-dispersive X-ray spectrometer X-ray diffractometer Fourier transform infrared spectrophotometer Operating cost Energy consumption Electrode consumption

water and then recycling the clean water to the cooling tower circuit. Make-up water treatment is also a way of reducing the scale formation in the cooling system. Growing prices for freshwater, frequent dryness and water shortages have motivated industries to try and reduce water use. Recycling the clean water can gain substantial pecuniary profits because of the large volumes of water required in cooling towers [4]. There are several treatment technologies to reduce scaling in cooling water systems, such as reverse osmosis, nanofiltration, ion exchange, and chemical coagulation methods [5–8]. This technology is not suitable for treating cooling water with high levels of scale-forming ions due to the effect of fouling on the membranes [6]. Membrane and ion exchange processes are not enough to treat the blowdown water of the cooling tower [9,10]. Many scientists used several electrochemical techniques to draw attention to efficiency and eco-friendly processing. The most important reaction is the electron, a “clean reagent,” which explains the reduced waste output associated with EC methods [11]. One of the main advantages of EC is that no chemicals are used, consequently there is no chance of secondary pollution, making EC a green technology that can be used effectively for CTBW treatment [12]. In the present study, magnesium is used as anode and the main chemical reactions occurring in EC cells are as follows [13]: At the cathode: 2H2O þ 2e → H2 At the anode: Mg → Mg 2þ

In the solution: Mg

2þ

(aq)

(g)

(1)

þ 2OH

(2)

þ 2e

(3)

þ

þ 2H2O → Mg (OH) 2↓ þ2H

2þ

The OH formed at the cathode increases the pH of the CTBW and thereby induces the precipitation of Mg ions as corresponding hydroxides and co-precipitation with magnesium hydroxides [14]. Mg2þand OH¡ions produced in EC cells will immediately undergo additional spontaneous hydrolysis responses to form multiple monomeric and polymeric species which will eventually turn into Mg (OH)3(s) based on complicated precipitation kinetics [15,16]. Several studies in EC technology applications were published in the literature [17–20], including the removal of organic com­ pounds [21], metal ions [22,23], colloidal abrasive particles [24], phosphate [25] and viruses [26]. Limited researches were reported on the treatment of potentially scale forming ions, namely Ca2þ, Mg2þ ions and silica from the CTBW. Hafez et al. [27] researched the efficacy of EC in the removal of hardness ions and silica from CTBW using Al, Fe and Zn electrodes. Zhi et al. [28] studied the application of surface response methodology to optimize the removal of silica from CTBW by EC. Gelover et al. [29] studied the electro-generation of aluminum as a coagulant for the removal of silica in water for cooling towers. Schulz et al. [30] investigated the performance of EC by using Fe and Al electrodes to remove scale forming ions from CTBW and reverse osmosis reject water. In addition, Liao et al. [31] established the effect of anti-scaling compounds on the efficiency of EC using Al and Fe electrodes for the treatment of CTBW. They reported that anti-scaling compounds used as coagulation aids were increasing hardness ions removal. Using magnesium as electrode material was already studied in EC [32–35]. However, magnesium electrode application in the EC reactor was never earlier researched to remove scale ions from CTBW. Previous authors ’ research [27] showed that EC use of Al-electrode was effective in removing scale ions from CTBW. The primary disadvantage of Al-electrode, however, is the remaining amount of aluminum as the USEPA rules suggest that the maximum water ¡

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Table 1 Physical and chemical characteristics of CTBW collected from Helwan Fertilizer Company, El-Tabbin, Cairo, Egypt. (from period of 3 months). Parameters pH Turbidity TDS Total-Alkalinity Total-Hardness Ca-Hardness Mg-Hardness Silicates Chloride Zn Total-phosphate Fe Sulphate

Unit

Min. value

Max. value

Actual value

NTU (mg/L) as CaCO3 (mg/L) as CaCO3 (mg/L) as CaCO3 (mg/L) as CaCO3 (mg/L) as SiO2 (mg/L) (mg/L) (mg/L) as PO4 (mg/L) (mg/L) as SO4 (mg/L)

7.80 6.50 1100 100 625 365 260 24 151 1.0 6.0 0.09 680

8.60 9.50 1500 130 720 420 300 29 173 1.30 7.30 0.11 740

8.20 7.93 1297 114 672 392 280 27 162 1.20 6.61 0.1 711

Fig. 1. Schematic diagram of EC reactor.

aluminum values of 0.05–0.2 mg/L due to cathodic dissolution [36]. There is no such disadvantage in the event of magnesium electrodes as the USEPA rules recommend maximum water magnesium values of 30 mg/L. The aim of this work was to study optimization of the EC technique with magnesium-rod electrode in the removal of scale-forming ions from the cooling tower recirculation water once it’s discharged. In particular, the goal of this study was to reuse of treated CTBW as makeup water to a cooling tower. 2. Material and methods 2.1. CTBW source and characteristics CTBW was collected from the effluent of an urea fertilizer plant (Helwan Fertilizer Company), Egypt. Samples were collected at the pump in clean 1 L plastic bottles that had been carefully rinsed with the respective samples prior to collection and labeled. The plant produces approximately 40 m3 of CTBW per hour. Usually, CTBW is discarded into the wastewater drain. The physical and chemical properties of CTBW are shown in Table 1. 2.2. Experimental set-up and procedure As shown in (Fig. 1), a batch monopolar electrochemical Plexiglas container of 1 L capacity with the dimensions of (L � W � h/11 � 8.0 � 8.0 cm). Magnesium-rod was used as the anode and stainless steel-rod of the same dimensions as the cathode. A pair of parallel electrodes with an active surface area of 65 cm2 operated in the monopolar mode was vertically positioned in the reactor and paired with (Dr. Meter 30V/5A) power supply. Magnesium electrodes were polished mechanically with sandpaper before each round and 3

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Fig. 2. Effect of current density on the efficiency of EC using magnesium rod-electrode (pH:8, electrode distance:1 cm and t:60 min).

soaked in a solution of 6% chromic acid and 5% HNO3, for 45 s and then washed with distilled water [37]. Such electrodes were mounted in the EC apparatus and the anode-cathode inter-electrode distance of 1.0 cm was kept. The inner container volume was nearly 0.60 L. CTBW was agitated using (INTLLAB Magnetic Stirrer MS-500) at a rotational speed of 300 rpm. The samples were filtered through 4 μm filter paper after settling time of 30 min. The filtrates were examined later. 2.3. Analytical methods The filtered CTBW was analyzed to determine the scale ions concentration. The analysis was performed using the APHA (2012) standard methods [38]. The cations and anions were determined using UNICO SQ2800 UV/VIS Spectrophotometer and Dionex ICS-1000 ion chromatograph. The pH was measured by using Fisherbrand™ FE150 pH meter and the conductivity was measured by Horiba Laqua DS70 conductivity meter. 2.4. Characterization of the sludge A scanning electron microscope (Hitachi S–3600 N) was used to evaluate the surface morphology of the produced sludge and the Oxford INCA 350 EDX device was combined with the microscope to conduct an energy dispersive X-ray spectroscopy that provides an elemental analysis. Using X-ray diffractometer (Thermo ARL X-Ray Diffractometer, USA), XLENS PD6 software was used to investigate the phase and crystal structure of this produced sludge. Fourier Transform Infrared Spectrophotometer (IRPrestige-21, Shimadzu) confirms the presence of different functional groups of the produced sludge using IRsolution software. The capacity to settle sludge was assessed using the sludge volume index (SVI). The following equation was used to determine the SVI (mL/g): (4)

SVI (mL/g) ¼ SV / MLSS x 1000

Where SV is the settled sludge volume in the transparent Imhoff cone graduated in 1 L after 30 min of settlement, and MLSS is a sustained solid mixed liquor. Sludge volume index (SVI) is a significant parameter measuring sludge settlement capability that can guide the design and control of settling tanks [39]. 2.5. Operating cost Estimation of the operating cost (OC) of EC treatment, using magnesium-rod electrode, can be calculated by considering two parameters as major cost items. They are electrode material and electrical energy costs [40]. OC as US$/m3of the treated CTBW was calculated with the following equation. OC ¼ α � C

energy

þβ�C

(5)

electrode 3

3

Where Cenergy (kWh/m ) is the energy consumption and Celectrode (kg/m ) is the electrode consumption. Unit prices α and β are given from the Egypt market in August 2019 are the electrical energy price (0.04 US$/kWh) and magnesium-rod electrode price (1.0 US$/kg). The energy and electrode consumptions levels are calculated by means of the following equations [41]: (6)

C energy (kWh/ m3) ¼ U � і � t EC / ν

Where U is the applied voltage, і is the current (A), t EC is the operating time (s or h) and ν is volume of the CTBW in the EC reactor (m3). 4

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Fig. 3. The effect of current density on (a) electrode consumption (b) energy consumption and (c) operating cost using magnesium rod-electrode (current density: 14.29 mA/cm2, pH:8, electrode distance:1 cm and t:60 min).

C

electrode

(7)

(Kg/m3) ¼ і � t EC � Mw / Z � F � ν

Where Mw is the relative molar mass of the magnesium electrode (24,305g/mole), Z is the number of electrons in the oxidation/ reduction reaction (z ¼ 3), and F is the Faraday constant (1 F ¼ 9.65 � 104 C/mole). 3. Results and discussion 3.1. Effect of current density One of the most important operating parameters in EC is current density, which has an integral effect on process efficiency [42]. Fig. 2 showed that by raising the current density from 1.43 to 14.29 mA/cm2.The removal efficiency of the total hardness improved from 20.0 to 51.80%. Meanwhile, the removal efficiency of silica increased from 48.0 to 93.70%. Increasing current density results in an increase in the amount of Mg2þ ions promoting the process of electrochemical precipitation [16,43]. Above optimum current density (14.29 mA/cm2), the efficiency of hardness ions and silica removal was decreased. This may be due to passivation of the electrode; this result is in full agreement with other investigations [44]. An economic evaluation of the electrode consumption and energy consumption at current density ranging from 1.43 to 18.57 mA/ cm2, at a constant electrolysis time (60 min). Fig. 3 shows the impact of current density on electrode consumption, energy consumption and total operating costs. As shown in the Fig. 3 (a–c), increased current density has resulted in an increase in electrode consumption, energy consumption and total costs. The electrode consumption was calculated as 0.19 to 1.34 kg/m3, energy consumption values were 0.18 to 3.05 kWh/m3 and total operating costs were 0.09 to 1.14 US$/m3. No significant increase in the removal efficiency of the magnesium-rod electrode at higher current density has been achieved. Therefore, the operating costs were determined to be 0.88 US $/m3 (Celectrode, exp ¼ 0.82 kg/m3 and Cenergy ¼ 1.81 kWh/m3) at the optimum conditions (current density of 14.29 mA/cm2; elec­ trolysis time 60 min; 300 rpm stirring speed and electrode distance of 1.0 cm). 5

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Fig. 4. Effect of electrolysis time on the efficiency of EC using magnesium rod-electrode (pH:8, electrode distance: 1 cm and current density: 14.29 mA/cm2).

Fig. 5. Effect of electrolysis time on (a) electrode consumption (b) energy consumption and (c) operating cost using magnesium rod-electrode (current density: 14.29 mA/cm2, pH:8, electrode distance:1 cm and t:60 min).

The contribution of this study, both in terms of efficiency and cost, can be further confirmed by comparing the cost of treatment in the previous study [45]. The operating costs of the current work are lower than the operating costs of the process reported by Sharma et al. [46] for the removal of hardness ions from wastewater using Al electrode, with an operating cost of 1.01 US$/m3. Villegas-Mendoza et al. [47] reported that operating costs on the removal of dissolved silica from CTBW using EC with Al electrodes were approximately 0.53 US$/m3. Studied by Zhi et al. [28] stated that silica removal from cooling water by EC using Al electrode is achieved by the following: 83.243% of silica removal efficiency, 1.466 kWh/m3 of energy consumption, 0.0400 kg/m3of electrode consumption and 0.245 kg/m3 of sludge production.

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Fig. 6. Effect of electrode distance (cm) on the efficiency of EC using magnesium rod-electrode (pH: 8, current density: 14.29 mA/cm2 and t: 60 min).

Fig. 7. Effect of pH on the efficiency of EC using magnesium rod-electrode (current density: 14.29 mA/cm2, electrode distance: 1 cm and t: 60 min).

3.2. Effect of electrolysis time The effect of electrolysis time on the removal of scale ions within 15–90 min was evaluated in this study at constant current density (14.29 mA/cm2). Fig. 4 showed that hardness ions and silica removal efficiency depended greatly on the amount of metal ion produced by the magnesium-rod anodes. The levels of Mg2þ ions improved with increased electrolysis time [15]. As shown in Fig. 4, a rise in the electrolysis time from 15 to 60 min resulted in an increased efficiency of removal of total hardness from 19.44% to 51.80% and silica from 77.0% to 93.70%. However, above the optimum time, there was no significant removal of both hardness ions, and silica was achieved. This may be because dissolved metal ions and their hydroxides have been found at the saturation stage of floc formation [46]. Also, it would be possible to evidence passivation phenomena on the electrodes due to Mg (OH) 2 films formation. Fig. 5 shows the effect of electrolysis time on consumption of electrodes, energy consumption and total operating costs. These results showed that the anode material consumption increased linearly by increasing the electrolysis time and, subsequently, the operating costs increased. At a constant current density of 14.29 mA/cm2, the electrolysis time increased from 15 to 60 min, increasing the experimental electrode consumption from 0.38 to 0.82 kg/m3. According to Faraday’s law, the theoretical electrode consumption in the reaction from 15 to 60 min was 0.21 to 0.49 kg/m3. In fact, the experimental consumption of electrodes could have exceeded the theoretical value. It is evident that the electrode consumption contributes significantly, in addition to electrochemical and chemical reactions, to the dissolution of magnesium electrodes to the production of Mg3þ ion coagulants [36]. Moreover, the corrosion and oxidation of the electrode surface could explain this phenomenon [48].

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Fig. 8. Effect of stirring speed (rpm) on the efficiency of EC using magnesium rod-electrode (pH: 8, current density: 14.29 mA/cm2, electrode distance: 1 cm and t: 60 min). Table 2 Characterization of CTBW before and after treatment by EC. Parameters

Unit

Actual value before EC

Actual value after EC

Removal efficiency (%)

pH Turbidity TDS Total-Alkalinity Total-Hardness Ca-Hardness Mg-Hardness Silicates Chloride Zn Total-phosphate Fe Sulphate

NTU mg/L as CaCO3 (mg/L) as CaCO3 (mg/L) as CaCO3 (mg/L) as CaCO3 (mg/L) as SiO2 (mg/L) mg/L mg/L as PO4 (mg/L) mg/L as SO4 (mg/L)

8.2 7.93 1297 114 672 392 280 27 162 1.2 6.61 0.1 711

8.5 1.02 961 69.6 324 244 111 1.7 114 <0.01 0.13 <0.01 516

– 87.14 25.91 38.95 51.78 37.69 60.31 93.66 29.63 – 98.03 – 27.43

3.3. Effect of electrode distance At current density of 14.29 mA/cm2, the electrodes are set at an electrode distance range of 0.5 to 3.0 cm and the results are given in Fig. 6. The shorter distance between the electrodes (0.5 cm) resulted in a reduction in the flow of the solution from each other; thereby decreasing the removal efficiency. A further increase in the distance between the electrodes up to 1 cm improves the efficiency of scale ions removal. Further increasing the distance between the electrodes, the efficiency of the scaling ions was reduced. This is mainly due to weaker interaction between the generated flocks and the scale molecules [49]. As a result, the optimum electrode distance chosen was 1 cm. 3.4. Effect of initial pH A significant factor that impacts the efficiency of EC is the initial pH value. Only at the beginning of each test was pH evaluated. The results (Fig. 7) revealed that there was a clear trend of rising hardness ions removal efficiency by increasing initial pH from 4.0 to 10.0. As the initial pH increased from 4.0 to 10.0, total hardness ions removal efficiency increased from 38.80% to 56.63%. However, the removal efficiency of silica exposed different patterns. By increasing the pH from 4.0 to 8.0, silica removal decreased slightly from 95.70% to 93.70%. Further increase in the pH from 8.0 to 10.0 decreased the removal rates from 93.70% to 83.81%. At lower pH, magnesium hydrolysis products are in the form of Mg2þ as soluble metal and therefore the ability to adsorb any pollutants was at least. At neutral and higher pH, the Mg2þand OH were generated by magnesium-rod anode to form many mono­ meric and polymeric metal hydroxide species [41]. The hydroxides of these metals are complex and converted in part into insoluble hydroxides made up of amorphous metals, with plentiful groups of hydroxyl surfaces for adsorbing hardness ions and silica. With increasing the pH to 10.0, the rate of hardness removal increased due to the formation of Mg (OH)2. The big surfaces of these magnesium hydroxides are suitable for quick adsorption of hardness ions [40]. By adding NaOH or NH4OH as alkaline reagents to an aqueous solution containing calcium ions and bicarbonate ions, OH ions were formed during dissociation. Hydroxide ions react with HCO3 ions to form carbonate ions, followed by calcium carbonate precipitation as a solid phase [50]. At pH 10.0, the removal ef­ ficiency of the silica decreased due to the precipitation reaction at the recorded alkaline condition [40]. 8

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Fig. 9. The settling characteristics of the EC sludge generated using magnesium rod-electrode after the EC.

Fig. 10. SEM images and EDX analysis of EC generated sludge using magnesium rod-electrode (pH: 8, current density: 14.29 mA/cm2 and t: 60 min).

3.5. Effect of the stirring speed Fig. 8 shows the effect of stirring speeds between 80 and 600 rpm on the effectiveness of removal for both hardness ions and silica. The results show that hardness ions and silica have increased their removability by increasing the stirring speed from 80 to 300 rpm. An additional increase in the stirring speed from 300 to 600 rpm reduces the effectiveness of scale ions removal. Therefore, the 300rpm stirring speed was selected as the optimum value. Increasing the stirring speed to the optimum value increases the removal of the scale forming ions. As a consequence of improved ion mobility at increased stirring speed, this can be ascribed to floc formation. The latter is responsible for the sedimentation of the pollutants. Nevertheless, the increase in the stirring speed beyond the optimum value induces the breakup of the formed flocs. Furthermore, collision degrades the flocks due to higher rpm leading to a less successful removal [42]. 3.6. Efficiency of the EC performance for the treatment of CTBW An experiment was carried out by applying the previously determined optimum conditions. The optimum conditions were a current density of 14.29 mA/cm2; electrolysis time of 60 min; electrode distance of 1.0 cm and a stirring speed of 300 rpm. Table 2 illustrates CTBW characterization before and after EC. The results showed a marked reduction in all concentrations of the scale forming ions. The total hardness and silica were removed by 51.80% and 93.70%, respectively. On the other hand, the con­ centrations of iron and zinc decreased from 0.1 mg/L and 1.2 mg/L to less than 0.01 mg/L. 3.7. Analysis of produced sludge from the EC Produced sludge characterization was conducted to explore the mechanism for the removal of scale forming ions through EC. The chemical analysis of the sludge was evaluated by SVI, SEM-EDX, XRD and FT-IR. The final disposal of the sludge formed at the end of the EC would be confined in a special container for further treatment. 9

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Table 3 Elemental composition of sludge based on weight (%) using magnesium-rod electrode. Electrode Type Magnesium

Element (wt. %) Mg (K)

Ca (K)

Si (K)

O (K)

35.06

3.64

0.54

60.77

Fig. 11. X-ray diffraction patterns of EC generated sludge using magnesium rod-electrode (pH: 8, current density: 14.29 mA/cm2 and t: 60 min).

Fig. 12. FTIR spectrum of EC generated sludge using magnesium rod-electrode (pH:8, current density:14.29 mA/cm2 and t:60 min).

3.7.1. Settling Fig. 9 presents the settling characteristics of magnesium-rod electrode generated sludge after the EC. The obtained results revealed that the sludge volume was 110 mL within 60 min sedimentation time. The SVI sludge volume index was 169 mL/g, indicating that the sludge produced was clear and well settled. 3.7.2. SEM-EDX characterization The generated sludge spectrum SEM and EDX are shown in an optimal operating condition (Fig. 10). SEM images showed knotty 10

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Table 4 Assignment of the IR spectra bands of functional groups present in the EC -generated sludge using magnesium-rod electrode. Band position (cm

1

)

Functional groups

3649 3069 2628 2355 1929 1688 1540 1422 1376 1225 959

O–H stretching ¼C–H stretching C–H stretching O¼C¼O stretching C¼C¼C stretching O–H bending C¼C stretching C–H bending C–H bending C–O stretching C¼C bending

particles with a small micrometer-sized particle. The flocs and sludge material appeared to be consisted of small particles, indicating of a large surface area of magnesium hydroxide colloids (i.e. favors the formation of dense flocs). The EDX spectra of sludge generated by magnesium-rod electrode showed the presence of Ca, Mg, Si, O, etc. (Table 3). This table shows that the percentage of Mg was at a higher weight compared with other elements, which may be related to the use of magnesium as an electrode. The EDX analysis showed that a significant part of the scale ions from CTBW was successfully captured in this procedure. 3.7.3. XRD and FT-IR analysis XRD spectra of the sludge produced by the magnesium-rod electrode are presented in (Fig. 11). XRD pattern clearly showed crystalline peaks that appeared at the degree of 18 and 20 and were identified as magnesium hydroxide (i.e. they are known as “Brucite” and “Magnesian” minerals). FT-IR of the generated sludge by magnesium-rod electrode showed in (Fig. 12). Table 4 shows all the allocated bands of FT-IR range of the produced sludge in relation to IR spectral libraries [51]. This FT-IR spectrum showed a powerful 3649 cm 1 adsorption band that was assigned to be free OH stretching vibration as Mg (OH)2 [52]. The adsorption band was found at 1688 cm 1 and can be allocated as H – O – H bending vibration [53]. A broad absorption band at 3445 cm 1 indicates the alteration from free protons into a proton conductive state in brucite [54]. The broad adsorption band is related to O–H vibration in the center of 3000 and 3500 cm 1. Functional groups of the sludge showed electrolyte interaction between flocks and cations. The findings showed the significant function of the sludge functional groups produced during the EC in removing colloids. 4. Conclusion � The removal of hardness ions (Ca2þ, Mg2þ) and dissolved silica from CTBW can be successfully achieved by EC using magnesiumrod electrode. � In this study, the removal of scale ions from the CTBW results showed that a maximum removal efficiency of 51.80% and 93.70% can be reached for hardness ions and silica respectively using magnesium-rod electrode with a current density of 14.29 mA/cm2; 60-min electrolysis; 300 rpm stirring speed and 1.0 cm electrode distance. � Under optimum operating conditions, the electrode consumption, energy consumption and total operating cost were found to be 0.82 kg/m3,1.81 kWh/m3 and 0.88 US$/m3 respectively. � The sludge SVI was 169 mL/g, which shows better settling properties, SEM images of sludge depicted knotty particles with a small particle size and the crystalline peaks were clearly shown in the XRD analysis of the sludge. Acknowledgments The authors would like to thank Helwan Fertilizer Company (HFC), Cairo, Egypt for making this research possible. References [1] G.B. Hill, E. Pring, P.D. Osborn, Cooling Towers: Principles and Practice, third ed., Butterworth-Heinemann, 2013. [2] J. Suitor, W. Marner, R. Ritter, The history and status of research in fouling of heat exchangers in cooling water service, Can. J. Chem. Eng. 55 (4) (1977) 374–380, https://doi.org/10.1002/cjce.5450550402. [3] D.J. Flynn, The Nalco Water Handbook, McGraw-Hill, New York, 2009. [4] H.M. Herro, R.D. Port, The Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, 1993. [5] Z. Amjad, K.D. Demadis, Mineral Scales and Deposits: Scientific and Technological Approaches, Elsevier, 2015. [6] H.I. Abdel-Shafy, S.H. Abdel-Shafy, Membrane technology for water and wastewater management and application in Egypt, Egypt. J. Chem. 60 (3) (2017) 347–360, https://doi.org/10.21608/EJCHEM.2017.3480. [7] R. Al-Sa’ed, et al., Identification and mapping of the research organizations in the field of membrane technology. Ch2, in: A. Lorenzo, A. Vega (Eds.), Membrane Technology in Water Treatment in the Mediterranean Region: PROMEMBRANE, IWA Publishing, London, 2011. [8] H. Abdel-Shafy, Chemical Treatment for Removal of Heavy Metals from Industrial Wastewater, vol. 58, 2015, pp. 1–12. No. 1.

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