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Treatment of real printing wastewater using electrocoagulation process with titanium and zinc electrodes
Journal of Water Process Engineering 34 (2020) 101137
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Treatment of real printing wastewater using electrocoagulation process with titanium and zinc electrodes
T
Safwat M. Safwat Sanitary & Environmental Engineering Division, Faculty of Engineering, Cairo University, PO Box 12613, Giza, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: COD Current density Electrochemistry SEM Wastewater
Electrocoagulation (EC) has numerous benefits over traditional coagulation. It is noteworthy that most researchers have studied EC using either iron electrodes or aluminum electrodes. The key objective of this study is to investigate the performance of the EC process with zinc and titanium electrodes for treating real printing wastewater through several experimental conditions. Experiments were done through batch systems. The effects of the operating variables, inclusive of varying current densities (CDs), spacing between electrodes, electrolysis time, and varying electrode materials (titanium and zinc) on the efficiency of the removal of different parameters were studied. Oil and grease, total dissolved solids (TDS), and chemical oxygen demand (COD) were among these parameters. The highest removal efficiency for COD after 90 min using a zinc electrode was approximately 50 % attained at 20 mA/cm2. For the titanium electrode, after 90 min, the highest removal efficiency for COD was 46 % attained at 15 mA/cm2. For the removal efficiency of TDS, the zinc electrode performance was better than that of the titanium electrode for every CD. After 90 min, the optimal TDS removal efficiencies were 9 % and 19 % for titanium and zinc electrodes, respectively. The titanium electrode offers the maximum efficiency of removal for oil and grease. The highest efficiencies of removal were attained at a separation distance of 4 cm. At a CD of 15 mA/cm2, the best COD elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 24 %, 47 %, and 30 %, respectively. At a CD of 15 mA/cm2, the best COD removal efficiencies using the zinc electrode at the separation distances of 6, 4, and 2 cm were 27 %, 41 %, and 38 %, respectively. In both the zinc and titanium electrodes, following the experiments, corrosion occurred in the anodes, which is evidence that the process of treatment took place. The EC performance was observed to be much better than that of conventional coagulation during the treatment of printing wastewater. Zinc and titanium electrodes demonstrated a good capability of removing pollutants when used in the EC process.
1. Introduction Wastewater produced by printing poses an environmental risk because of its components [1]. Printing wastewater contains non-biodegradable compounds, high concentrations of chemical oxygen demand (COD), trace amounts of toxic metals, intense colors, adhesives, and pigments [2]. The concentrations of pollutants present in printing wastewater vary with the techniques used to generate ink [3]. Printing wastewater must be treated before discharging to water streams to prevent serious effects on the environment and on human health and to make the water aesthetically appealing [2,3]. The concentration of COD should be reduced to discharge the effluent to domestic sewage networks. In Egypt, the regulations state that the effluent COD concentration from industries to sewage networks should not exceed 1100 mg/L. Biodegradation and chemical coagulation lie in the category of conventional processes applied in the
treatment of printing wastewater [4,5]. Chemical coagulation generates large quantities of sludge. Sludge treatment is quite expensive, and if it is not handled well, it may lead to secondary pollution [1]. In biodegradation treatment, inhibition of microbial functions may occur due to toxic pollutants within the printing wastewater [1,6,7]. Accordingly, it is important that the treatment of printing wastewater is conducted through efficient and innovative treatment technology. Efficient wastewater treatment technologies include electrochemical wastewater treatment technologies, for example, electro-oxidation, electroflotation, and electrocoagulation (EC) [8,9]. These technologies are such that they are environmentally friendly and generate minimal sludge quantities, have a small footprint, and do not need chemical additives [10,11]. In wastewater treatment plants, EC may function as a conventional coagulation substitute [12]. During the EC process, an applied current makes sacrificial anodes dissolve, generating active coagulant compounds [12].
E-mail address: [email protected]. https://doi.org/10.1016/j.jwpe.2020.101137 Received 21 September 2019; Received in revised form 2 January 2020; Accepted 5 January 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 34 (2020) 101137
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wastewater, they applied EC, and the findings indicated that the process efficiently removed a high percentage of polycyclic aromatic hydrocarbons (around 86 %) [31]. Furthermore, the EC process was capable of treating fluoride, with an initial concentration of 6 mg/L by lowering its concentration to less than 0.5 mg/L [32]. A group of scholars applied EC for the treatment of baker’s yeast generation in wastewater. The results indicated that the optimal efficiencies of removal for the COD, turbidity, and all the organic carbon through the optimal functional conditions were 69 %, 56 %, and 52 % for the iron electrode, and 71 %, 90 %, and 53 % for the aluminum electrode, respectively [33]. Additionally, EC was used for the treatment of real printing wastewater with copper electrodes. The findings showed that the maximum efficiency of removal of COD was 67 % [34]. The EC can be used as a pre-treatment phase to minimize the levels of contaminants of heightened strength wastewater, like wastewater produced from printing works, before subsequent treatment phases [10]. It is noteworthy that majority researchers have studied EC using either iron electrodes or aluminum electrodes as the sacrificial anode. Because other electrode forms, such as zinc and titanium electrodes, indicated efficient results if applied in EC, it is important to study these forms with actual wastewater to determine their performance compared to the majority of widely applied electrodes (iron or aluminum) [35–38]. Zinc electrodes were able to remove arsenite by up to 99.89 % [39], urea by up to 66 % [38], lead by up to 99.9 % [18], and total phenolic content by up to 84.2 % [40]. Titanium electrodes were able to remove arsenic by up to 58 % [41] and humic acids by up to 65 % [42]. To the best of the author’s knowledge, EC with zinc and titanium electrodes has not been explored yet in printing wastewater treatment. Therefore, applying the EC process using zinc and titanium electrodes can be considered a possible advanced technique for treating printing wastewater. These two electrodes showed promising results in the treatment of other types of wastewater; therefore, they are used in this study to assess their feasibility. The key objective of this research is to study the performance of the EC process using zinc and titanium electrodes. Experiments were performed in batch systems for the treatment of real printing wastewater under various experimental circumstances. The EC process configuration used in this investigation is the simplest form. The effects of the operating variables, inclusive of varying current densities (CDs), spacing between electrodes, electrolysis time, and varying electrode materials (titanium and zinc) on the efficiency of removal of different parameters were studied. These parameters were COD, oil and grease, and total dissolved solids (TDS). In this work, the consumption of energy for the EC cell was computed as well.
Moreover, EC has numerous benefits over traditional coagulation, for instance, shorter retention time, higher efficiency, simple operation, and inhibition of secondary pollution resulting from added chemicals [13,14]. Coagulation uses chemical coagulants, such as polyelectrolytes or metal salts, while EC produces coagulants in situ via the sacrificial anode material with electrolytic oxidation that leads to a much reduced production of sludge [10]. The mechanism of removal and the characteristics of flocs in EC is different when compared to conventional chemical coagulation. In EC, flotation can occur due to the generation of hydrogen gas. For equivalent coagulation-flocculation conditions, EC can generate flocs over a wider range of pH values at a fast rate. The structure of the flocs in EC are relatively fragile and porous, subjected to compaction and restructuring. EC can decrease the need for pH adjustment at low initial pH values. Since flocs generated from EC are small, more compacted, and less subjected to shear stress compared to those produced from chemical coagulation, they are more suitable in processes that operated in environment with high shear conditions [15]. During EC, a direct current (DC) is applied to a reactor with cathode and anode electrodes submerged in a conductive solution [14]. Three major processes occur in the process of EC: (i) production of bubbles of gas at the cathode, (ii) suspension and sedimentation of the generated flocs, and (iii) oxidation of the anode [16]. During the current application to the system, reduction reactions take place at the cathode, while oxidation reactions occur on the sacrificial anode generating cations [13]. In aqueous media, those cations generate metal hydroxides [13]. Floating solids are destabilized effectively through the species of metal hydroxide. At the cathode, hydrogen gas generates continuously. With the assistance of floating and scouring, they are to form flocculated particles and remove pollutants [14]. The eradication mechanism may be through charge neutralization, sweep coagulation, or adsorption [13,17]. During the process of EC, the electrochemical reactions with metal, such as zinc or titanium, acting as electrodes are outlined as follows [12]: For zinc (Zn) [18]: At the anode (coagulation): Zn → Zn2+ +2e− At the cathode (flotation): 2H2O + 2e−→ H2 +2OH− For titanium (Ti) [19]: At the anode (coagulation): Ti → Ti4+ +4e− At the cathode (flotation): 4H2O + 4e−→ 2H2 +4OH− Several wastewater types, such as domestic wastewater [20], mining waters [21], pulp and paper mill wastewater [22], water containing pharmaceuticals [23], olive oil mill wastewater [24], swine wastewater [25], and others, have been treated successfully through EC. Past research has indicated that EC is an efficient process for wastewater treatment in industrial cases, specifically those of elevated strength and poisonous materials [26,27]. One researcher showed that toxic metal ions existing in wastewater produced from metal plating were treated through EC (more than 97 % were removed) [28]. In research studying paper mill effluent treatment through EC, the efficiencies of removal through an electrode of aluminum were 75 % of the COD and 70 % of the biochemical oxygen demand after around 7.5 min. On the other hand. the efficiencies of removal were 80 %, 93 %, and 55 %, respectively, when an iron electrode was used [29]. In wastewater treatment of a poultry slaughterhouse through EC, the COD removal using aluminum electrodes was 93 %, whereas the optimal oil and grease removal achieved using iron electrodes was 98 % [30]. Moreover, treatment of wastewater produced from an olive oil mill was investigated through EC. In a hydraulic retention time of 30 min, the removal percentages of COD were 42 % and 52 % with an iron anode and aluminum anode, respectively [24]. In the investigation of the removal of polycyclic aromatic hydrocarbons in paper-making
2. Materials and methods 2.1. Characteristics of printing wastewater In this research, the wastewater was obtained in Egypt from the printing industry. Table 1 shows the characteristics of the wastewater. 2.2. Coagulation experiments Fig. 1 illustrates the EC system. The system was operated for 90 min Table 1 Wastewater analysis.
2
Parameter
Value
pH Chemical oxygen demand (COD) Oil and grease Total suspended solids (TSS) Total dissolved solids (TDS) Conductivity
6.8 6950 mg/L 405 mg/L 75 mg/L 6520 mg/L 5.62 mS/cm
Journal of Water Process Engineering 34 (2020) 101137
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Fig. 1. Electrocoagulation (EC) system.
under a batch mode and a temperature of 25 °C ± 1.5 °C. The EC system comprises an electrochemical unit that has a 0.5-liter magnetic stirrer. The system consisted of two electrodes externally linked to a DC source (Model no: LABPS300SSM, Velleman, Belgium). The anode electrode was made of either zinc (Zn) or titanium (Ti), while stainless steel was used as the cathode electrode. The electrodes were brought from a local company (Almazen, Egypt). The percentages of zinc and titanium by weight in the electrodes were 77 % and 81 %, respectively. The immersed surface area per electrode was 24 cm2. The electrode separation was 4 cm, though some runs have been conducted with 2 and 6 cm separations. The distance between electrodes has been chosen based on the diameter of the beaker used. These gap distances were chosen to represent approximately a constant increment of 2 cm. The stirring speed was maintained at a low rate (100 rpm) to prevent floc shearing [43]. Moreover, the hydraulic retention time was within the range of 5–90 min. Four different CDs: 5, 10, 15 and 20 mA/cm2, were investigated. These values were chosen based on literature [38,44]. To determine the current intensity and voltage in the EC process, an ammeter and voltmeter were used. Samples were periodically withdrawn, then filtered. Conventional chemical coagulation trials were performed using the jar test. Zinc sulfate and titanium tetrachloride were used as coagulants to simulate zinc and titanium electrodes. Conventional coagulation experiments consisted of slow mixing for 20 min at 30 rpm and flash mixing for 1.5 min at 100 rpm, then a settling period of 20 min, followed by gathering the samples [45].
Fig. 2. Efficiencies of the removal of COD over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
3. Results and discussion 3.1. Impact of CD on EC During the process of EC, the CD, which determines the rate of generation of hydrogen bubbles and floc growth, was used to enhance the electrode dissolution to create the species of coagulant [40]. The removal efficiency of pollutants is not usually in direct proportional with the value of current density. The effect of a CD of between 5 and 20 mA/cm2 was investigated for zinc and titanium electrodes. Figs. 2 and 3 represent the efficiencies of the removal of the COD and TDS, respectively. As observed, the COD removal rate for each CD grew rapidly within the first 10 min. After this, the rate of elimination efficiency for the COD was reduced because of the phenomenon of desorption [27]. Additionally, oxidation reactions, which enhanced the corrosion phenomena, may result in the creation of layers of stable oxide on the surface of the anode electrodes. These layers lead to passivation effects, lowering the EC cell efficiency [48]. This phenomena is mentioned in previous studies [48–50]. The highest removal efficiencies for COD after 10 min were 28 % and 36 % for zinc and titanium electrodes, respectively. The highest removal efficiency for COD after 90 min using a zinc electrode was approximately 50 % and was attained at 20 mA/cm2. For the titanium electrode, after 90 min, the highest efficiency of removal for COD was 47 % and was attained at 15 mA/ cm2. Zinc species have a higher molecular weight than titanium species, which leads to the production of heavier coagulant complexes that can precipitate more effectively [51]. The values of COD elimination efficiencies for CDs of 5 mA/cm2 and 10 mA/cm2 were quite similar for zinc electrodes, while the COD removal efficiencies for CDs of 5 mA/ cm2 and 20 mA/cm2 were quite similar for the titanium electrodes. When there is no significant difference between two different current densities regarding the removal efficiency, it is feasible to use the lowest current density to achieve this value of removal efficiency. For the removal efficiency of TDS, the zinc electrode performance
2.3. Analysis Effluent and influent samples were gathered. Samples were taken using a sterile syringe, then filtered before performing further analysis. The total suspended solids (TSS) of effluent was measured before filtration to determine the number of flocs formed. Moreover, oil and grease, TSS, and COD were determined with respect to the standard methods [46]. During COD analysis, mercury sulfate was used to mask the interference that may occur due to the high TDS concentration [47]. A multimeter was used to measure the TDS and pH. The formula below was used to calculate the pollutant removal efficiency (%) following the treatment: Efficiency of removal (%) = (Xo − Xe)/Xo ×100, where Xe and Xo represent the effluent and influent pollutant concentrations, respectively. The anode electrode morphologies were studied via scanning electron microscopy (SEM). All the experiments were conducted in two replicates, and the reported results were in terms of the measurement average. 3
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Fig. 4. Values of pH over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode. Fig. 3. Efficiencies of the removal of TDS over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
hydroxides. In the EC process, an increase in the pH value enhances the creation of these metal hydroxides [53]. Titanium and zinc hydroxides hold the pollutants/colloids through sweep coagulation because they precipitate, resulting in an increased COD reduction [14]. Figs. 5 and 6 represent the oil and grease and effluent TSS while using titanium and zinc electrodes at varying CDs. The titanium electrode offers the maximum efficiency of removal for oil and grease. For oil and grease, at a CD of 15 mA/cm2, intensive fine hydrogen bubbles formed, which follow the oil droplets and suspend them in the solution surface in a sweeping action known as sweep flocculation [54]. The zinc electrode produced more total flocs compared to the titanium electrode at every CD, yielding more TSS in the zinc electrode effluent. Because of the increased removal of pollutants taking place at 15 mA/ cm2, the proceeding investigations for both zinc and titanium electrodes were conducted at this CD value.
was better than that of the titanium electrode for every CD. After 90 min, the optimal TDS removal efficiencies were 9 % and 19 % for titanium and zinc electrodes, respectively. The total ions produced as Ti2+ or Zn2+ are based on the CD used and measure the quantity of the resultant coagulant. Therefore, the metal hydroxide formation rate grew with the increase in the metal ion quantities, which dissolved into the solution. This improved the efficiencies of the removal for both the TDS and COD. Moreover, pollutant adsorption took place on the surface of the hydroxides, oxyhydroxides, and metal oxides [5,52]. The decrease in pollutants can also be attributed to the mechanism of destabilization, which entails three key steps: the compression of the double layer, then the neutralization of the charge, and lastly, floc creation [52]. Furthermore, the increase in CD resulted in an increase in the hydrogen bubble production and a reduction in their size. This leads to the elimination of the pollutant through the flotation phenomenon. In the titanium electrode case, they failed to achieve the optimal elimination efficiency of COD at the highest CD applied. This could be because elevated levels of CDs heighten the turbulence of the system. An increase in turbulence could influence the process of coagulation because the particles will not have adequate time to reduce and agglomerate the pollutants. Fig. 4 represents the change in pH values over a given period. In all CD values, the pH increased over time. This could be because of the reactions taking place at the cathode. In the EC system, molecules of water gain electrons and disintegrate into hydroxyl ions and bubbles of hydrogen, leading to a rise in the pH value [40]. For the zinc electrode, the increase in pH value was higher compared to that of the titanium electrode for every CD. This implies that, for the zinc electrode, the dissociation level of the water molecules was higher compared to that of the titanium electrode. For both forms of anodes, higher pH values were attained at higher CDs (15 and 20 mA/cm2). This is due to the high CDs. The cathode corrosion occurs because of the intensive hydroxide anion production [43]. Titanium and zinc disintegrate to generate divalent ions (Ti2+ ions and Zn2+ ions), creating metal
3.2. Effect of separation distance between the two electrodes The separation distance between different electrodes influences the
Fig. 5. Effluent TSS at different CDs using EC on printing wastewater at a separation distance of 4 cm. 4
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Fig. 6. Effluent oil and grease at different CDs using EC on printing wastewater at a separation distance of 4 cm.
Fig. 8. Efficiencies of the removal of TDS over time at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2: a) Ti electrode, b) Zn electrode.
disturbance or (ii) uniform distribution of flocs inside the reactor [34]. Moreover, the short separation distance between the electrodes resulted in an increased electrostatic effect, which inhibits the collision of the particles. Many electrochemically produced gas bubbles lead to turbulence, while the large separation distance substantially lowered the floc formation [48,55,56]. The pollutant removal behavior was nearly similar for all separation distances. The pollutant removal rate was increased in the first 10 min, then reduced until the operation of the system finished. The best COD elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 24 %, 47 %, and 30 %, respectively. The best COD removal efficiencies using the zinc electrode at the separation distances of 6, 4, and 2 cm were 27 %, 41 %, and 38 %, respectively. The best TDS elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 6 %, 5 %, and 6 %, respectively. The best TDS elimination efficiencies using zinc electrodes at the separation distances of 6, 4, and 2 cm were 5 %, 19 %, and 9 %, respectively. The oil and grease effluent values for the zinc electrode case were almost equivalent to those for the titanium electrode, as indicated in Fig. 9. The TSS values in effluent were heightened for the zinc electrode in comparison to the values attained from the titanium electrode. This is because the total flocs resulting from the zinc case were higher compared to those produced in the titanium case. This shows that the EC system containing the zinc electrode works better than the EC system using the titanium electrode. Table 2 shows a summary of the results. The zinc and titanium electrode morphologies were studied prior to and at the end of the EC at a CD of 15 mA/cm2. Fig. 10 represents the electrode images of SEM prior to and at the end of the treatment process. In both the zinc and titanium electrodes, corrosion occurred on the anodes following the EC treatment process, which is evidence that the process of treatment took place. The titanium electrode surface had cracks, whereas the zinc electrode surface was rough with dents. The
Fig. 7. Efficiencies of the removal of COD over time at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2: a) Ti electrode, b) Zn electrode.
ohmic potential and consumption of energy of the EC cell [11]. The effect of separation between the two electrodes was studied at a CD of 15 mA/cm2. Figs. 7 and 8 represent the elimination efficiencies for TDS and COD, accordingly, for varying separation distance values of 6 cm, 4 cm, and 2 cm. The highest efficiencies of removal were attained at a gap of 4 cm. A short distance between electrodes can decrease the electrical energy for ion motion because the travel path will decrease, and in turn, the motion resistance will also decrease. However, the situation for this study was different. When the gap increased to 6 cm or reduced to 2 cm, it led to a decrease in the pollutant elimination efficiencies in comparison to those with a gap of 4 cm. This occurrence could be attributed to the system configuration. The reactor had a circular cross section and a 4 cm gap between electrodes, which was almost equidistant to the gap between the two parallel electrodes and between every electrode in the cell and edge. This equidistance may result in two outcomes: i) reduction of the
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1.9 kW h/m3 for the titanium electrode and zinc electrode, respectively. The values lie within the range cited in the energy consumption literature for EC, ranging from 58 to 0.002 kW h/m3 [58]. These results confirm that zinc anodes demand more energy than titanium anodes. The electrode consumption (MC) can be calculated using the following equation:
MC (g/ m3) =
where M represents the molar mass of the electrode (g/mol), z represents the number of electrons in the oxidation-reduction reaction, and F represents the Faraday constant (96,500 c/mol). Assuming an electrolyte time of 10 min and a CD of 15 mA/cm2, the values of electrode consumption were 53.6 g/m3 and 146.3 g/m3 for the titanium electrode and zinc electrode, respectively. These results confirm that the wastewater consumes more mass of zinc compared to titanium.
Fig. 9. Effluent TSS and oil and grease at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2.
production of numerous dents and cracks in the anodes is associated with the metal consumption at active electrode sites because of the oxygen production at the surface [57]. The titanium electrode corrosion was even, while the zinc electrode experienced pitting corrosion. Even corrosion was much better compared to pitting corrosion because it is easier to determine.
3.4. Chemical coagulation performances Zinc sulfate and titanium tetrachloride were used because previous studies mentioned the using of metal salts as coagulants. For Iron and aluminum electrodes, usually ferric sulfate, ferric chloride and aluminum sulfate are used for comparison purposes. As a result, zinc sulfate was used for zinc electrode, and titanium tetrachloride was used for titanium electrodes [45,59–61]. The application of zinc sulfate and titanium tetrachloride was at varying dosages (from 10 g/L to 50 g/L) to study the performance difference between EC and conventional coagulation. Fig. 12 represents the COD removal efficiencies through chemical coagulants. The COD removal efficiency values grew with the increase in coagulant doses. Compared to zinc sulfate, titanium tetrachloride generates a better outcome. The optimum COD removal efficiencies were 25 % and 17 % for titanium tetrachloride and zinc sulfate, respectively. These values are very low compared to those achieved using EC, but the applied coagulant quantity is very high. These findings assert that the performance of EC is much better compared to that of chemical coagulation during the treatment of printing wastewater.
3.3. Consumption of electrical energy and electrodes In the process of optimization, the main components, such as energy cost, should be considered. Fig. 11 shows the consumption of energy needed for treating printing wastewater versus the hydraulic retention time at varying CDs. The calculation of the consumption of electrical energy (EEC) was determined by applying the equation below:
EEC (kWh/ m3) =
I *t *M z *F *V
UIt V
where I represents the intensity of the current (A), t represents the EC treatment time (h), U represents the average cell voltage (V), and V represents the volume of the wastewater (m3). The outcome indicates that an increase in the consumption of energy corresponds with the CD. The consumption of energy for the zinc electrode was much higher than that for the titanium electrode. The highest values of consumption of energy were 15.9 kW h/m3 and 17.1 kW h/m3 for the titanium electrode and zinc electrode, respectively. After 10 min, the efficiency of removal of COD with a CD of 15 mA/cm2 was greater than 70 % of the efficiency of its removal achieved after 90 min, as shown above. Thus, 10 min can be assumed to be the optimum case. The best values of consumption of energy at a CD of 15 mA/cm2 were 1.7 kW h/m3 and
4. Conclusion This study investigates the treatment of printing wastewater through the EC process. It investigated the use of two varying electrodes (titanium and zinc). Zinc and titanium electrodes were able to reduce the COD concentrations of printing wastewater. The findings indicated that the removal efficiencies of pollutants when applying the zinc electrode were much improved compared to those identified when
Table 2 A summary of the experimental results. Electrode type
Experimental conditions
Removal efficiency of COD
Removal efficiency of TDS
Removal efficiency of oil & grease
Zinc electrode
CD =15 mA/cm2 Spacing =2 cm Reaction time =90 CD =15 mA/cm2 Spacing =4 cm Reaction time =90 CD =15 mA/cm2 Spacing =6 cm Reaction time =90 CD =15 mA/cm2 Spacing =2 cm Reaction time =90 CD =15 mA/cm2 Spacing =4 cm Reaction time =90 CD =15 mA/cm2 Spacing =6 cm Reaction time =90
38 %
9%
13 %
41 %
19 %
14 %
27 %
5%
9%
30 %
6%
16 %
47 %
5%
19 %
24 %
6%
12 %
Zinc electrode
Zinc electrode
Titanium electrode
Titanium electrode
Titanium electrode
min
min
min
min
min
min
6
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Fig. 10. Scanning electron microscopy images of electrodes: a) Ti electrode prior to treatment, b) Ti electrode at the end of treatment, c) Zn electrode prior to treatment, and d) Zn electrode at the end of treatment.
Fig. 12. Efficiencies of the removal of COD from printing wastewater using chemical coagulation with different coagulant doses.
applying the titanium electrode. For the zinc electrode, the highest efficiency of removal of COD was almost 50 %, attained at a CD of 20 mA/cm2. For the titanium electrode, the highest removal efficiency of COD was 47 %, attained at a CD of 15 mA/cm2. For all CD values, the pH values rose over a given period. After 90 min, the highest TDS removal efficiencies were 9 % and 19 % for the titanium and zinc electrodes, respectively. The optimal efficiencies of removal were attained at a separation distance of 4 cm. The titanium and zinc electrode morphologies were reviewed prior to and at the end of the EC process. For both titanium and zinc electrodes, as anodes, corrosion occurred after the EC process, which was evidence of metal dissociation. At the titanium electrode, there was even corrosion, whereas the zinc electrode showed pitting corrosion. The outcome indicated that the
Fig. 11. EEC over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
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consumption of energy rose with the CD. The results showed that the wastewater consumes more mass of zinc compared to titanium during the EC process. The EC performance was better than that of conventional coagulation during the treatment of printing wastewater. The results indicate that titanium and zinc electrodes can be used to implement and scale up the process in the future.
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Declaration of Competing Interest The author declares that they have no conflict of interest. Acknowledgments The author wishes to thank the specialists at the central laboratory of Egyptian mineral resources authority for their help with the analyses. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors References [1] C.H. Tung, S.Y. Shen, J.H. Chang, Y.M. Hsu, Y.C. Lai, Treatment of real printing wastewater with an electrocatalytic process, Sep. Purif. Technol. 117 (2013) 131–136, https://doi.org/10.1016/j.seppur.2013.07.028. [2] K.P. Papadopoulos, R. Argyriou, C.N. Economou, N. Charalampous, S. Dailianis, T.I. Tatoulis, A.G. Tekerlekopoulou, D.V. Vayenas, Treatment of printing ink wastewater using electrocoagulation, J. Environ. Manage. 237 (2019) 442–448, https://doi.org/10.1016/j.jenvman.2019.02.080. [3] L. Khannous, A. Elleuch, I. Fendri, N. Jebahi, H. Khlaf, N. Gharsallah, Treatment of printing wastewater by a combined process of coagulation and biosorption for a possible reuse in agriculture, Desalin. Water Treat. 57 (2016) 5723–5729, https:// doi.org/10.1080/19443994.2015.1005688. [4] S. Ahmed, E. Rozaik, H. Abdel-Halim, Performance of single-chamber microbial fuel cells using different carbohydrate-rich wastewaters and different inocula, Polish J. Environ. Stud. 25 (2016) 503–510, https://doi.org/10.15244/pjoes/61115. [5] S. Safwat, M. Matta, Adsorption of urea onto granular activated alumina: a comparative study with granular activated carbon, J. Dispers. Sci. Technol. 39 (2018) 1699–1709, https://doi.org/10.1080/01932691.2018.1461644. [6] S.M. Safwat, Performance of moving bed biofilm reactor using effective microorganisms, J. Clean. Prod. 185 (2018) 723–731. [7] S. Ahmed, E. Rozaik, H. Abdelhalim, Effect of configurations, bacterial adhesion, and anode surface area on performance of microbial fuel cells used for treatment of synthetic wastewater, Water Air Soil Pollut. 226 (2015) 300, https://doi.org/10. 1007/s11270-015-2567-3. [8] S. Ahmadzadeh, M. Dolatabadi, Removal of acetaminophen from hospital wastewater using electro-Fenton process, Environ. Earth Sci. 77 (2018) 1–11, https://doi. org/10.1007/s12665-017-7203-7. [9] S. Ahmadzadeh, M. Dolatabadi, Modeling and kinetics study of electrochemical peroxidation process for mineralization of bisphenol A; a new paradigm for groundwater treatment, J. Mol. Liq. 254 (2018) 76–82, https://doi.org/10.1016/j. molliq.2018.01.080. [10] D.T. Moussa, M.H. El-Naas, M. Nasser, M.J. Al-Marri, A comprehensive review of electrocoagulation for water treatment: potentials and challenges, J. Environ. Manage. 186 (2017) 24–41, https://doi.org/10.1016/j.jenvman.2016.10.032. [11] S. Ahmadzadeh, A. Asadipour, M. Pournamdari, B. Behnam, H.R. Rahimi, M. Dolatabadi, Removal of ciprofloxacin from hospital wastewater using electrocoagulation technique by aluminum electrode: optimization and modelling through response surface methodology, Process Saf. Environ. Prot. 109 (2017) 538–547, https://doi.org/10.1016/j.psep.2017.04.026. [12] C.T. Wang, W.L. Chou, Y.M. Kuo, Removal of COD from laundry wastewater by electrocoagulation/electroflotation, J. Hazard. Mater. 164 (2009) 81–86, https:// doi.org/10.1016/j.jhazmat.2008.07.122. [13] S. Barışçı, O. Turkay, Domestic greywater treatment by electrocoagulation using hybrid electrode combinations, J. Water Process Eng. 10 (2016) 56–66, https://doi. org/10.1016/j.jwpe.2016.01.015. [14] D.D. Nguyen, H.H. Ngo, W. Guo, T.T. Nguyen, S.W. Chang, A. Jang, Y.S. Yoon, Can electrocoagulation process be an appropriate technology for phosphorus removal from municipal wastewater? Sci. Total Environ. 563 (2016) 549–556, https://doi. org/10.1016/j.scitotenv.2016.04.045. [15] T. Harif, M. Khai, A. Adin, Electrocoagulation versus chemical coagulation: Coagulation/flocculation mechanisms and resulting floc characteristics, Water Res. 46 (2012) 3177–3188, https://doi.org/10.1016/j.watres.2012.03.034. [16] M.M. Emamjomeh, M. Sivakumar, Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes, J. Environ. Manage. 90 (2009) 1663–1679, https://doi.org/10.1016/j.jenvman.2008.12.011. [17] Z. Al-Qodah, M. Al-Shannag, Heavy metal ions removal from wastewater using electrocoagulation processes: a comprehensive review, Sep. Sci. Technol. 6395 (2017) 1–28, https://doi.org/10.1080/01496395.2017.1373677. [18] F. Hussin, F. Abnisa, G. Issabayeva, M.K. Aroua, Removal of lead by solar-photovoltaic electrocoagulation using novel perforated zinc electrode, J. Clean. Prod. 147 (2017) 206–216, https://doi.org/10.1016/j.jclepro.2017.01.096.
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Treatment of real printing wastewater using electrocoagulation process with titanium and zinc electrodes
T
Safwat M. Safwat Sanitary & Environmental Engineering Division, Faculty of Engineering, Cairo University, PO Box 12613, Giza, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: COD Current density Electrochemistry SEM Wastewater
Electrocoagulation (EC) has numerous benefits over traditional coagulation. It is noteworthy that most researchers have studied EC using either iron electrodes or aluminum electrodes. The key objective of this study is to investigate the performance of the EC process with zinc and titanium electrodes for treating real printing wastewater through several experimental conditions. Experiments were done through batch systems. The effects of the operating variables, inclusive of varying current densities (CDs), spacing between electrodes, electrolysis time, and varying electrode materials (titanium and zinc) on the efficiency of the removal of different parameters were studied. Oil and grease, total dissolved solids (TDS), and chemical oxygen demand (COD) were among these parameters. The highest removal efficiency for COD after 90 min using a zinc electrode was approximately 50 % attained at 20 mA/cm2. For the titanium electrode, after 90 min, the highest removal efficiency for COD was 46 % attained at 15 mA/cm2. For the removal efficiency of TDS, the zinc electrode performance was better than that of the titanium electrode for every CD. After 90 min, the optimal TDS removal efficiencies were 9 % and 19 % for titanium and zinc electrodes, respectively. The titanium electrode offers the maximum efficiency of removal for oil and grease. The highest efficiencies of removal were attained at a separation distance of 4 cm. At a CD of 15 mA/cm2, the best COD elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 24 %, 47 %, and 30 %, respectively. At a CD of 15 mA/cm2, the best COD removal efficiencies using the zinc electrode at the separation distances of 6, 4, and 2 cm were 27 %, 41 %, and 38 %, respectively. In both the zinc and titanium electrodes, following the experiments, corrosion occurred in the anodes, which is evidence that the process of treatment took place. The EC performance was observed to be much better than that of conventional coagulation during the treatment of printing wastewater. Zinc and titanium electrodes demonstrated a good capability of removing pollutants when used in the EC process.
1. Introduction Wastewater produced by printing poses an environmental risk because of its components [1]. Printing wastewater contains non-biodegradable compounds, high concentrations of chemical oxygen demand (COD), trace amounts of toxic metals, intense colors, adhesives, and pigments [2]. The concentrations of pollutants present in printing wastewater vary with the techniques used to generate ink [3]. Printing wastewater must be treated before discharging to water streams to prevent serious effects on the environment and on human health and to make the water aesthetically appealing [2,3]. The concentration of COD should be reduced to discharge the effluent to domestic sewage networks. In Egypt, the regulations state that the effluent COD concentration from industries to sewage networks should not exceed 1100 mg/L. Biodegradation and chemical coagulation lie in the category of conventional processes applied in the
treatment of printing wastewater [4,5]. Chemical coagulation generates large quantities of sludge. Sludge treatment is quite expensive, and if it is not handled well, it may lead to secondary pollution [1]. In biodegradation treatment, inhibition of microbial functions may occur due to toxic pollutants within the printing wastewater [1,6,7]. Accordingly, it is important that the treatment of printing wastewater is conducted through efficient and innovative treatment technology. Efficient wastewater treatment technologies include electrochemical wastewater treatment technologies, for example, electro-oxidation, electroflotation, and electrocoagulation (EC) [8,9]. These technologies are such that they are environmentally friendly and generate minimal sludge quantities, have a small footprint, and do not need chemical additives [10,11]. In wastewater treatment plants, EC may function as a conventional coagulation substitute [12]. During the EC process, an applied current makes sacrificial anodes dissolve, generating active coagulant compounds [12].
E-mail address: [email protected]. https://doi.org/10.1016/j.jwpe.2020.101137 Received 21 September 2019; Received in revised form 2 January 2020; Accepted 5 January 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.
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wastewater, they applied EC, and the findings indicated that the process efficiently removed a high percentage of polycyclic aromatic hydrocarbons (around 86 %) [31]. Furthermore, the EC process was capable of treating fluoride, with an initial concentration of 6 mg/L by lowering its concentration to less than 0.5 mg/L [32]. A group of scholars applied EC for the treatment of baker’s yeast generation in wastewater. The results indicated that the optimal efficiencies of removal for the COD, turbidity, and all the organic carbon through the optimal functional conditions were 69 %, 56 %, and 52 % for the iron electrode, and 71 %, 90 %, and 53 % for the aluminum electrode, respectively [33]. Additionally, EC was used for the treatment of real printing wastewater with copper electrodes. The findings showed that the maximum efficiency of removal of COD was 67 % [34]. The EC can be used as a pre-treatment phase to minimize the levels of contaminants of heightened strength wastewater, like wastewater produced from printing works, before subsequent treatment phases [10]. It is noteworthy that majority researchers have studied EC using either iron electrodes or aluminum electrodes as the sacrificial anode. Because other electrode forms, such as zinc and titanium electrodes, indicated efficient results if applied in EC, it is important to study these forms with actual wastewater to determine their performance compared to the majority of widely applied electrodes (iron or aluminum) [35–38]. Zinc electrodes were able to remove arsenite by up to 99.89 % [39], urea by up to 66 % [38], lead by up to 99.9 % [18], and total phenolic content by up to 84.2 % [40]. Titanium electrodes were able to remove arsenic by up to 58 % [41] and humic acids by up to 65 % [42]. To the best of the author’s knowledge, EC with zinc and titanium electrodes has not been explored yet in printing wastewater treatment. Therefore, applying the EC process using zinc and titanium electrodes can be considered a possible advanced technique for treating printing wastewater. These two electrodes showed promising results in the treatment of other types of wastewater; therefore, they are used in this study to assess their feasibility. The key objective of this research is to study the performance of the EC process using zinc and titanium electrodes. Experiments were performed in batch systems for the treatment of real printing wastewater under various experimental circumstances. The EC process configuration used in this investigation is the simplest form. The effects of the operating variables, inclusive of varying current densities (CDs), spacing between electrodes, electrolysis time, and varying electrode materials (titanium and zinc) on the efficiency of removal of different parameters were studied. These parameters were COD, oil and grease, and total dissolved solids (TDS). In this work, the consumption of energy for the EC cell was computed as well.
Moreover, EC has numerous benefits over traditional coagulation, for instance, shorter retention time, higher efficiency, simple operation, and inhibition of secondary pollution resulting from added chemicals [13,14]. Coagulation uses chemical coagulants, such as polyelectrolytes or metal salts, while EC produces coagulants in situ via the sacrificial anode material with electrolytic oxidation that leads to a much reduced production of sludge [10]. The mechanism of removal and the characteristics of flocs in EC is different when compared to conventional chemical coagulation. In EC, flotation can occur due to the generation of hydrogen gas. For equivalent coagulation-flocculation conditions, EC can generate flocs over a wider range of pH values at a fast rate. The structure of the flocs in EC are relatively fragile and porous, subjected to compaction and restructuring. EC can decrease the need for pH adjustment at low initial pH values. Since flocs generated from EC are small, more compacted, and less subjected to shear stress compared to those produced from chemical coagulation, they are more suitable in processes that operated in environment with high shear conditions [15]. During EC, a direct current (DC) is applied to a reactor with cathode and anode electrodes submerged in a conductive solution [14]. Three major processes occur in the process of EC: (i) production of bubbles of gas at the cathode, (ii) suspension and sedimentation of the generated flocs, and (iii) oxidation of the anode [16]. During the current application to the system, reduction reactions take place at the cathode, while oxidation reactions occur on the sacrificial anode generating cations [13]. In aqueous media, those cations generate metal hydroxides [13]. Floating solids are destabilized effectively through the species of metal hydroxide. At the cathode, hydrogen gas generates continuously. With the assistance of floating and scouring, they are to form flocculated particles and remove pollutants [14]. The eradication mechanism may be through charge neutralization, sweep coagulation, or adsorption [13,17]. During the process of EC, the electrochemical reactions with metal, such as zinc or titanium, acting as electrodes are outlined as follows [12]: For zinc (Zn) [18]: At the anode (coagulation): Zn → Zn2+ +2e− At the cathode (flotation): 2H2O + 2e−→ H2 +2OH− For titanium (Ti) [19]: At the anode (coagulation): Ti → Ti4+ +4e− At the cathode (flotation): 4H2O + 4e−→ 2H2 +4OH− Several wastewater types, such as domestic wastewater [20], mining waters [21], pulp and paper mill wastewater [22], water containing pharmaceuticals [23], olive oil mill wastewater [24], swine wastewater [25], and others, have been treated successfully through EC. Past research has indicated that EC is an efficient process for wastewater treatment in industrial cases, specifically those of elevated strength and poisonous materials [26,27]. One researcher showed that toxic metal ions existing in wastewater produced from metal plating were treated through EC (more than 97 % were removed) [28]. In research studying paper mill effluent treatment through EC, the efficiencies of removal through an electrode of aluminum were 75 % of the COD and 70 % of the biochemical oxygen demand after around 7.5 min. On the other hand. the efficiencies of removal were 80 %, 93 %, and 55 %, respectively, when an iron electrode was used [29]. In wastewater treatment of a poultry slaughterhouse through EC, the COD removal using aluminum electrodes was 93 %, whereas the optimal oil and grease removal achieved using iron electrodes was 98 % [30]. Moreover, treatment of wastewater produced from an olive oil mill was investigated through EC. In a hydraulic retention time of 30 min, the removal percentages of COD were 42 % and 52 % with an iron anode and aluminum anode, respectively [24]. In the investigation of the removal of polycyclic aromatic hydrocarbons in paper-making
2. Materials and methods 2.1. Characteristics of printing wastewater In this research, the wastewater was obtained in Egypt from the printing industry. Table 1 shows the characteristics of the wastewater. 2.2. Coagulation experiments Fig. 1 illustrates the EC system. The system was operated for 90 min Table 1 Wastewater analysis.
2
Parameter
Value
pH Chemical oxygen demand (COD) Oil and grease Total suspended solids (TSS) Total dissolved solids (TDS) Conductivity
6.8 6950 mg/L 405 mg/L 75 mg/L 6520 mg/L 5.62 mS/cm
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Fig. 1. Electrocoagulation (EC) system.
under a batch mode and a temperature of 25 °C ± 1.5 °C. The EC system comprises an electrochemical unit that has a 0.5-liter magnetic stirrer. The system consisted of two electrodes externally linked to a DC source (Model no: LABPS300SSM, Velleman, Belgium). The anode electrode was made of either zinc (Zn) or titanium (Ti), while stainless steel was used as the cathode electrode. The electrodes were brought from a local company (Almazen, Egypt). The percentages of zinc and titanium by weight in the electrodes were 77 % and 81 %, respectively. The immersed surface area per electrode was 24 cm2. The electrode separation was 4 cm, though some runs have been conducted with 2 and 6 cm separations. The distance between electrodes has been chosen based on the diameter of the beaker used. These gap distances were chosen to represent approximately a constant increment of 2 cm. The stirring speed was maintained at a low rate (100 rpm) to prevent floc shearing [43]. Moreover, the hydraulic retention time was within the range of 5–90 min. Four different CDs: 5, 10, 15 and 20 mA/cm2, were investigated. These values were chosen based on literature [38,44]. To determine the current intensity and voltage in the EC process, an ammeter and voltmeter were used. Samples were periodically withdrawn, then filtered. Conventional chemical coagulation trials were performed using the jar test. Zinc sulfate and titanium tetrachloride were used as coagulants to simulate zinc and titanium electrodes. Conventional coagulation experiments consisted of slow mixing for 20 min at 30 rpm and flash mixing for 1.5 min at 100 rpm, then a settling period of 20 min, followed by gathering the samples [45].
Fig. 2. Efficiencies of the removal of COD over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
3. Results and discussion 3.1. Impact of CD on EC During the process of EC, the CD, which determines the rate of generation of hydrogen bubbles and floc growth, was used to enhance the electrode dissolution to create the species of coagulant [40]. The removal efficiency of pollutants is not usually in direct proportional with the value of current density. The effect of a CD of between 5 and 20 mA/cm2 was investigated for zinc and titanium electrodes. Figs. 2 and 3 represent the efficiencies of the removal of the COD and TDS, respectively. As observed, the COD removal rate for each CD grew rapidly within the first 10 min. After this, the rate of elimination efficiency for the COD was reduced because of the phenomenon of desorption [27]. Additionally, oxidation reactions, which enhanced the corrosion phenomena, may result in the creation of layers of stable oxide on the surface of the anode electrodes. These layers lead to passivation effects, lowering the EC cell efficiency [48]. This phenomena is mentioned in previous studies [48–50]. The highest removal efficiencies for COD after 10 min were 28 % and 36 % for zinc and titanium electrodes, respectively. The highest removal efficiency for COD after 90 min using a zinc electrode was approximately 50 % and was attained at 20 mA/cm2. For the titanium electrode, after 90 min, the highest efficiency of removal for COD was 47 % and was attained at 15 mA/ cm2. Zinc species have a higher molecular weight than titanium species, which leads to the production of heavier coagulant complexes that can precipitate more effectively [51]. The values of COD elimination efficiencies for CDs of 5 mA/cm2 and 10 mA/cm2 were quite similar for zinc electrodes, while the COD removal efficiencies for CDs of 5 mA/ cm2 and 20 mA/cm2 were quite similar for the titanium electrodes. When there is no significant difference between two different current densities regarding the removal efficiency, it is feasible to use the lowest current density to achieve this value of removal efficiency. For the removal efficiency of TDS, the zinc electrode performance
2.3. Analysis Effluent and influent samples were gathered. Samples were taken using a sterile syringe, then filtered before performing further analysis. The total suspended solids (TSS) of effluent was measured before filtration to determine the number of flocs formed. Moreover, oil and grease, TSS, and COD were determined with respect to the standard methods [46]. During COD analysis, mercury sulfate was used to mask the interference that may occur due to the high TDS concentration [47]. A multimeter was used to measure the TDS and pH. The formula below was used to calculate the pollutant removal efficiency (%) following the treatment: Efficiency of removal (%) = (Xo − Xe)/Xo ×100, where Xe and Xo represent the effluent and influent pollutant concentrations, respectively. The anode electrode morphologies were studied via scanning electron microscopy (SEM). All the experiments were conducted in two replicates, and the reported results were in terms of the measurement average. 3
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Fig. 4. Values of pH over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode. Fig. 3. Efficiencies of the removal of TDS over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
hydroxides. In the EC process, an increase in the pH value enhances the creation of these metal hydroxides [53]. Titanium and zinc hydroxides hold the pollutants/colloids through sweep coagulation because they precipitate, resulting in an increased COD reduction [14]. Figs. 5 and 6 represent the oil and grease and effluent TSS while using titanium and zinc electrodes at varying CDs. The titanium electrode offers the maximum efficiency of removal for oil and grease. For oil and grease, at a CD of 15 mA/cm2, intensive fine hydrogen bubbles formed, which follow the oil droplets and suspend them in the solution surface in a sweeping action known as sweep flocculation [54]. The zinc electrode produced more total flocs compared to the titanium electrode at every CD, yielding more TSS in the zinc electrode effluent. Because of the increased removal of pollutants taking place at 15 mA/ cm2, the proceeding investigations for both zinc and titanium electrodes were conducted at this CD value.
was better than that of the titanium electrode for every CD. After 90 min, the optimal TDS removal efficiencies were 9 % and 19 % for titanium and zinc electrodes, respectively. The total ions produced as Ti2+ or Zn2+ are based on the CD used and measure the quantity of the resultant coagulant. Therefore, the metal hydroxide formation rate grew with the increase in the metal ion quantities, which dissolved into the solution. This improved the efficiencies of the removal for both the TDS and COD. Moreover, pollutant adsorption took place on the surface of the hydroxides, oxyhydroxides, and metal oxides [5,52]. The decrease in pollutants can also be attributed to the mechanism of destabilization, which entails three key steps: the compression of the double layer, then the neutralization of the charge, and lastly, floc creation [52]. Furthermore, the increase in CD resulted in an increase in the hydrogen bubble production and a reduction in their size. This leads to the elimination of the pollutant through the flotation phenomenon. In the titanium electrode case, they failed to achieve the optimal elimination efficiency of COD at the highest CD applied. This could be because elevated levels of CDs heighten the turbulence of the system. An increase in turbulence could influence the process of coagulation because the particles will not have adequate time to reduce and agglomerate the pollutants. Fig. 4 represents the change in pH values over a given period. In all CD values, the pH increased over time. This could be because of the reactions taking place at the cathode. In the EC system, molecules of water gain electrons and disintegrate into hydroxyl ions and bubbles of hydrogen, leading to a rise in the pH value [40]. For the zinc electrode, the increase in pH value was higher compared to that of the titanium electrode for every CD. This implies that, for the zinc electrode, the dissociation level of the water molecules was higher compared to that of the titanium electrode. For both forms of anodes, higher pH values were attained at higher CDs (15 and 20 mA/cm2). This is due to the high CDs. The cathode corrosion occurs because of the intensive hydroxide anion production [43]. Titanium and zinc disintegrate to generate divalent ions (Ti2+ ions and Zn2+ ions), creating metal
3.2. Effect of separation distance between the two electrodes The separation distance between different electrodes influences the
Fig. 5. Effluent TSS at different CDs using EC on printing wastewater at a separation distance of 4 cm. 4
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Fig. 6. Effluent oil and grease at different CDs using EC on printing wastewater at a separation distance of 4 cm.
Fig. 8. Efficiencies of the removal of TDS over time at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2: a) Ti electrode, b) Zn electrode.
disturbance or (ii) uniform distribution of flocs inside the reactor [34]. Moreover, the short separation distance between the electrodes resulted in an increased electrostatic effect, which inhibits the collision of the particles. Many electrochemically produced gas bubbles lead to turbulence, while the large separation distance substantially lowered the floc formation [48,55,56]. The pollutant removal behavior was nearly similar for all separation distances. The pollutant removal rate was increased in the first 10 min, then reduced until the operation of the system finished. The best COD elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 24 %, 47 %, and 30 %, respectively. The best COD removal efficiencies using the zinc electrode at the separation distances of 6, 4, and 2 cm were 27 %, 41 %, and 38 %, respectively. The best TDS elimination efficiencies using the titanium electrode at the separation distances of 6, 4, and 2 cm were 6 %, 5 %, and 6 %, respectively. The best TDS elimination efficiencies using zinc electrodes at the separation distances of 6, 4, and 2 cm were 5 %, 19 %, and 9 %, respectively. The oil and grease effluent values for the zinc electrode case were almost equivalent to those for the titanium electrode, as indicated in Fig. 9. The TSS values in effluent were heightened for the zinc electrode in comparison to the values attained from the titanium electrode. This is because the total flocs resulting from the zinc case were higher compared to those produced in the titanium case. This shows that the EC system containing the zinc electrode works better than the EC system using the titanium electrode. Table 2 shows a summary of the results. The zinc and titanium electrode morphologies were studied prior to and at the end of the EC at a CD of 15 mA/cm2. Fig. 10 represents the electrode images of SEM prior to and at the end of the treatment process. In both the zinc and titanium electrodes, corrosion occurred on the anodes following the EC treatment process, which is evidence that the process of treatment took place. The titanium electrode surface had cracks, whereas the zinc electrode surface was rough with dents. The
Fig. 7. Efficiencies of the removal of COD over time at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2: a) Ti electrode, b) Zn electrode.
ohmic potential and consumption of energy of the EC cell [11]. The effect of separation between the two electrodes was studied at a CD of 15 mA/cm2. Figs. 7 and 8 represent the elimination efficiencies for TDS and COD, accordingly, for varying separation distance values of 6 cm, 4 cm, and 2 cm. The highest efficiencies of removal were attained at a gap of 4 cm. A short distance between electrodes can decrease the electrical energy for ion motion because the travel path will decrease, and in turn, the motion resistance will also decrease. However, the situation for this study was different. When the gap increased to 6 cm or reduced to 2 cm, it led to a decrease in the pollutant elimination efficiencies in comparison to those with a gap of 4 cm. This occurrence could be attributed to the system configuration. The reactor had a circular cross section and a 4 cm gap between electrodes, which was almost equidistant to the gap between the two parallel electrodes and between every electrode in the cell and edge. This equidistance may result in two outcomes: i) reduction of the
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1.9 kW h/m3 for the titanium electrode and zinc electrode, respectively. The values lie within the range cited in the energy consumption literature for EC, ranging from 58 to 0.002 kW h/m3 [58]. These results confirm that zinc anodes demand more energy than titanium anodes. The electrode consumption (MC) can be calculated using the following equation:
MC (g/ m3) =
where M represents the molar mass of the electrode (g/mol), z represents the number of electrons in the oxidation-reduction reaction, and F represents the Faraday constant (96,500 c/mol). Assuming an electrolyte time of 10 min and a CD of 15 mA/cm2, the values of electrode consumption were 53.6 g/m3 and 146.3 g/m3 for the titanium electrode and zinc electrode, respectively. These results confirm that the wastewater consumes more mass of zinc compared to titanium.
Fig. 9. Effluent TSS and oil and grease at different separation distances using EC on printing wastewater at a CD of 15 mA/cm2.
production of numerous dents and cracks in the anodes is associated with the metal consumption at active electrode sites because of the oxygen production at the surface [57]. The titanium electrode corrosion was even, while the zinc electrode experienced pitting corrosion. Even corrosion was much better compared to pitting corrosion because it is easier to determine.
3.4. Chemical coagulation performances Zinc sulfate and titanium tetrachloride were used because previous studies mentioned the using of metal salts as coagulants. For Iron and aluminum electrodes, usually ferric sulfate, ferric chloride and aluminum sulfate are used for comparison purposes. As a result, zinc sulfate was used for zinc electrode, and titanium tetrachloride was used for titanium electrodes [45,59–61]. The application of zinc sulfate and titanium tetrachloride was at varying dosages (from 10 g/L to 50 g/L) to study the performance difference between EC and conventional coagulation. Fig. 12 represents the COD removal efficiencies through chemical coagulants. The COD removal efficiency values grew with the increase in coagulant doses. Compared to zinc sulfate, titanium tetrachloride generates a better outcome. The optimum COD removal efficiencies were 25 % and 17 % for titanium tetrachloride and zinc sulfate, respectively. These values are very low compared to those achieved using EC, but the applied coagulant quantity is very high. These findings assert that the performance of EC is much better compared to that of chemical coagulation during the treatment of printing wastewater.
3.3. Consumption of electrical energy and electrodes In the process of optimization, the main components, such as energy cost, should be considered. Fig. 11 shows the consumption of energy needed for treating printing wastewater versus the hydraulic retention time at varying CDs. The calculation of the consumption of electrical energy (EEC) was determined by applying the equation below:
EEC (kWh/ m3) =
I *t *M z *F *V
UIt V
where I represents the intensity of the current (A), t represents the EC treatment time (h), U represents the average cell voltage (V), and V represents the volume of the wastewater (m3). The outcome indicates that an increase in the consumption of energy corresponds with the CD. The consumption of energy for the zinc electrode was much higher than that for the titanium electrode. The highest values of consumption of energy were 15.9 kW h/m3 and 17.1 kW h/m3 for the titanium electrode and zinc electrode, respectively. After 10 min, the efficiency of removal of COD with a CD of 15 mA/cm2 was greater than 70 % of the efficiency of its removal achieved after 90 min, as shown above. Thus, 10 min can be assumed to be the optimum case. The best values of consumption of energy at a CD of 15 mA/cm2 were 1.7 kW h/m3 and
4. Conclusion This study investigates the treatment of printing wastewater through the EC process. It investigated the use of two varying electrodes (titanium and zinc). Zinc and titanium electrodes were able to reduce the COD concentrations of printing wastewater. The findings indicated that the removal efficiencies of pollutants when applying the zinc electrode were much improved compared to those identified when
Table 2 A summary of the experimental results. Electrode type
Experimental conditions
Removal efficiency of COD
Removal efficiency of TDS
Removal efficiency of oil & grease
Zinc electrode
CD =15 mA/cm2 Spacing =2 cm Reaction time =90 CD =15 mA/cm2 Spacing =4 cm Reaction time =90 CD =15 mA/cm2 Spacing =6 cm Reaction time =90 CD =15 mA/cm2 Spacing =2 cm Reaction time =90 CD =15 mA/cm2 Spacing =4 cm Reaction time =90 CD =15 mA/cm2 Spacing =6 cm Reaction time =90
38 %
9%
13 %
41 %
19 %
14 %
27 %
5%
9%
30 %
6%
16 %
47 %
5%
19 %
24 %
6%
12 %
Zinc electrode
Zinc electrode
Titanium electrode
Titanium electrode
Titanium electrode
min
min
min
min
min
min
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Fig. 10. Scanning electron microscopy images of electrodes: a) Ti electrode prior to treatment, b) Ti electrode at the end of treatment, c) Zn electrode prior to treatment, and d) Zn electrode at the end of treatment.
Fig. 12. Efficiencies of the removal of COD from printing wastewater using chemical coagulation with different coagulant doses.
applying the titanium electrode. For the zinc electrode, the highest efficiency of removal of COD was almost 50 %, attained at a CD of 20 mA/cm2. For the titanium electrode, the highest removal efficiency of COD was 47 %, attained at a CD of 15 mA/cm2. For all CD values, the pH values rose over a given period. After 90 min, the highest TDS removal efficiencies were 9 % and 19 % for the titanium and zinc electrodes, respectively. The optimal efficiencies of removal were attained at a separation distance of 4 cm. The titanium and zinc electrode morphologies were reviewed prior to and at the end of the EC process. For both titanium and zinc electrodes, as anodes, corrosion occurred after the EC process, which was evidence of metal dissociation. At the titanium electrode, there was even corrosion, whereas the zinc electrode showed pitting corrosion. The outcome indicated that the
Fig. 11. EEC over time at different CDs using EC on printing wastewater at a separation distance of 4 cm: a) Ti electrode, b) Zn electrode.
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consumption of energy rose with the CD. The results showed that the wastewater consumes more mass of zinc compared to titanium during the EC process. The EC performance was better than that of conventional coagulation during the treatment of printing wastewater. The results indicate that titanium and zinc electrodes can be used to implement and scale up the process in the future.
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