The effect of pulsed voltage application on energy consumption during electrocoagulation was investigated. Three voltage profiles having the same arithmetic average with respect to time were applied to the electrodes. The specific energy consumption for these profiles were evaluated and analyzed together with oil removal efficiencies. The effects of applied voltages, electrode materials, electrode configurations, and pH on oil removal efficiency were determined. Electrocoagulation experiments were performed by using synthetic and real wastewater samples. The pulsed voltages saved energy during the electrocoagulation process. In continuous operation, energy saving was as high as 48%. Aluminum electrodes used for the treatment of emulsified oils resulted in higher oil removal efficiencies in comparison with stainless steel and iron electrodes. When the electrodes gap was less than 1 cm, higher oil removal efficiencies were obtained. The highest oil removal efficiencies were 95% and 35% for the batch and continuous operating modes, respectively.

INTRODUCTION

Emulsified oils or cutting oils are widely used in metal forming, metal cutting, and in the galvanic industry where cooling, lubrication and rust control are important in operations (Byers 1994). Emulsified oils are usually composed of a mineral oil (40–80%), a surfactant, and some additives such as biocides, anti-degrading, and anti-corrosive chemicals. Oil-water emulsions having oil droplets in free form can be separated easily by using gravitational settling and flotation. However, emulsified oils are stable in water and their separation can be problematic (Rubio et al. 2002; Sangas et al. 2013), i.e., emulsion breaking (demulsification) is needed before the oil can be successfully removed.

Emulsions can be destabilized by chemical, physical, and electrochemical methods. Chemical methods (coagulation/flocculation) (Zouboulis & Avranas 2000; Bensadok et al. 2007) are the most widely used processes in the treatment of oily wastewaters and aluminum/ferric salts are used commonly as coagulants. Physical methods include heating, centrifugation, precoat filtration, ultrafiltration, and membrane processes (Cheryan & Rajagopalan 1998; Gryt et al. 2001; Cambiella et al. 2006). Cutting oils can contain biocides to prevent their degradation and, therefore, biological processes can also be used for demulsification (Perez et al. 2007). When the effluent is highly polluted with soluble compounds and pollutants cannot be treated by other techniques, distillation (Canizares et al. 2014) can also be applied despite its high operation cost.

Electrocoagulation is an electrochemical separation process that uses a direct current between metal electrodes immersed in water. Aluminum or iron electrodes generate in situ coagulant agents that destabilize pollutants. Electrocoagulation offers some advantages over traditional coagulation: less coagulant is required, less sludge is formed, the equipment is simpler and the amount of coagulant can be controlled by adjusting current (Bernal-Martinez et al. 2013). It has been successfully employed for the removal of metals (Parga et al. 2005), dyes (Khandegar & Saroha 2013), oils (Yang 2007; Kobya et al. 2008), and organics (Chen et al. 2000) from wastewaters. Emamjomeh & Sivakumar (2009) presented a list of pollutants from wastewaters that could be treated successfully by electrocoagulation. Despite the proven efficiency of electrocoagulation to treat wastewater, high energy consumption, and anode passivation phenomenon have limited further application (Rena et al. 2011). Hence, the development of pulsed or alternating current application in electrocoagulation was explored in recent years (Vasudevan et al. 2011; Bernal-Martinez et al. 2013).

In this study, the effects of pulsed voltage applications on energy consumption during electrocoagulation were investigated by using oily wastewaters. Three voltage profiles having the same arithmetic average were applied to the electrodes under the same operating condition. The experiments were performed in batch and continuous operating modes. In addition, the effects of applied voltages, electrode materials, electrode configurations, and pH on removal efficiency were investigated.

EXPERIMENTAL

Wastewater samples

Preparation of synthetic wastewaters

Bor oil (Petrol Ofisi) is one of the commonly used oils in the metal cutting industry in Turkey. It can be used for almost all metal types and can be mixed by water up to 10% (volume). However, 2% mixing is usually applied in industry. It is a paraffin-based mineral oil and contains surfactants and other chemicals (biocides, lubricating agents, pressure additives, anti-foam agents, and corrosion inhibitors). The synthetic wastewater samples were prepared by adding bor oil to tap water (2%). The mixture was then stirred mechanically for 30 min. The initial pH, conductivity, and turbidity of synthetic wastewaters were around 6, 350 μs/cm and 1,200 NTU, respectively. The chemical oxygen demand (COD) was measured by titrimetric method and evaluated as 250 mg/L.

Real wastewater samples

Wastewaters were collected from a turning machine workshop where bor oil was used as the cutting oil. The collected wastewater samples were mainly composed of bor oil, but also contained some metal chips, lubricating oils and grease. The initial pH, conductivity and turbidity of wastewaters were around 7,280 μs/cm and 2,800 NTU, respectively. After the separation of metal chips and grease, the COD of wastewater samples were measured and were in the range of 1,100 to 1,400 mg/L. When the samples were diluted by tap water in a ratio of 1:3, their properties resembled synthetic wastewater samples.

Stability of emulsified oil droplets in wastewaters

Zeta potential measurements

Emulsions are generally stabilized by repulsive charges on the surface of droplets and by adsorbed layers that act as an interfacial barrier to prevent the close contact or coalescence of droplets (Rios et al. 1998). In practice, the droplet surface charges are usually characterized in terms of zeta potential and it is defined as the difference in potential between the surface of the tightly bound layer of ions on droplet surface and the electro-neutral region of the solution. The zeta potentials of bor oil droplets were measured by Malvern Zetasizer-4.

Creaming analysis

Creaming, in an oil-water emulsion, is the movement of oil droplets under gravity to form a concentrated layer at the surface (Binks et al. 2000). Creaming analysis is relevant for this study because of the determination of the stability and separation of bor oils droplets from water by gravity. Simple visual observation technique was used to determine the creaming behavior of bor oil droplets. 50 mL wastewaters were placed into test tubes and visually observed for three months. Almost no phase separation in the wastewaters was recorded during this time period. These results showed that a stable emulsion was formed and it was not possible to separate bor oil droplets by gravity.

Electrocoagulation experimental setup

The electrocoagulation experimental apparatus consisted of an electrokinetic cell (27.3 × 16.5 × 14 cm3), a power supply (40 V, 5 A), conductivity and pH probes (Mettler Toledo), a data logger and a peristaltic pump (Masterflex). The electrokinetic cell was made of pyrex. The tested electrodes were aluminum, iron and steel plates at 13 cm in length, 2 cm in width and 0.3 cm in thickness. A single pair of the same metal plate was used as the anode and cathode electrodes. The electrodes were submerged into water.

Water samples (15 mL) were taken at every 2 min during the electrocoagulation experiments. The samples were shaken for a short period of time to free the gas bubbles. The colloids formed during electrocoagulation were collected at the surface of samples by using magnets. The turbidities of the samples were then measured (Aqualytic AL450T-IR). In addition, COD measurements were performed every 30 min during the experiments. The standard titrimetric method was used in the determination of COD. The regression coefficients between oil content-turbidity and oil content-COD were found to be 0.998 and 0.92, respectively. All the removal efficiencies presented in this study were evaluated depending on turbidity measurements. COD measurements were only used in the power consumption evaluation, where the power consumptions were represented in terms of kW-hr/kg COD as shown in Table 1.

Table 1

Oil removal percentages and specific energy consumption (SEC)

  Batch Continuous 
Run A 
 Turbidity removal (%) 90 18 
 COD removal (%) 92 15 
 Energy consumed (kW-hr) 0.0294 0.0248 
 Specific energy (kW-hr/m319.6 13.8 
 Specific energy (kW-hr/kgCOD85.2 368.2 
Run B 
 Turbidity removal (%) 91 35 
 COD removal (%) 88 36 
 Energy consumed (kW-hr) 0.0342 0.0310 
 Specific energy (kW-hr/m322.8 17.2 
 Specific energy (kW-hr/kgCOD103.6 191.1 
  Batch Continuous 
Run A 
 Turbidity removal (%) 90 18 
 COD removal (%) 92 15 
 Energy consumed (kW-hr) 0.0294 0.0248 
 Specific energy (kW-hr/m319.6 13.8 
 Specific energy (kW-hr/kgCOD85.2 368.2 
Run B 
 Turbidity removal (%) 91 35 
 COD removal (%) 88 36 
 Energy consumed (kW-hr) 0.0342 0.0310 
 Specific energy (kW-hr/m322.8 17.2 
 Specific energy (kW-hr/kgCOD103.6 191.1 

In the experimental study, first, batch and continuous experiments were performed using synthetic wastewaters. The experiments were then repeated at the same operating conditions using real wastewaters. The comparison of operating modes was performed for the same wastewater samples. In the batch operating mode, 1.5 L of wastewater was placed in the electrochemical cell and was circulated from cathode to anode by a flow rate of 20 mL/min. The same amount of water in the electrochemical cell and the same flow rate were used in the continuous operating mode. All the experiments except pH investigations were performed without any pH adjustments. Only the electrocoagulation experimental results and evaluated power consumptions for the treatment of synthetic wastewaters were presented in this study.

Evaluations of energy consumption

The electrical energy consumed in electrocoagulation can be evaluated in terms of the energy used per unit volume of the treated wastewater, i.e., specific energy consumption (SEC, kW-hr/m3) 
formula
1
where U, I, t, and represent the applied voltage difference to the electrodes (V), the current passing through the electrodes (A), the electrocoagulation time (s) and the volume of the treated wastewater (m3), respectively. The electrical energy consumption can also be represented as a function of 1 kg COD removal from wastewater (Choua et al. 2009; Santos et al. 2013). When the applied voltage is constant 
formula
2
where CODt represents the measured COD of wastewater at time ‘t’. This analysis is highly dependent on the specific type of constituents that contribute to the COD of the water. Therefore, the results from this study would only apply to wastewaters that have similar constituents.

For the electrocoagulation experiments performed in this study, the energy consumption (kW-hr), SEC and SECCOD were evaluated at 3,000 s. This time was selected because the removal percentages in most of the experiments attained a constant value around 2,500 s and stayed constant after this time. Since the currents were measured online every 5 seconds, the integral term in the equations was calculated by the evaluation of the area under the curve presented in the current–time graph. The area was evaluated numerically using trapezoidal rule (Chapra & Canale 1998).

RESULTS AND DISCUSSION

Effects of operating parameters on oil removal efficiencies

Electrode materials and their configurations

Aluminum, iron, and stainless steel were tested as electrode materials in this study. The applied voltage was 40 V and the distance between the electrodes was 1 cm. The experiments were performed in the batch operating mode. Among the studied electrodes, the lowest oil removal efficiency (69%) was observed at the stainless steel electrodes. The basic mechanism for the oil removal at the stainless steel electrodes was electroflotation. Water electrolysis reactions were taking place at the electrodes. Therefore, the produced hydrogen and oxygen gas bubbles could promote the floatation of flocculated oil droplets out of the wastewater. In the case of iron or aluminum electrodes, metal ions were released from the anode as a result of oxidation reactions. The generated Al3+ or Fe3+ ions sustained further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides. The oil droplets were then adsorbed onto the produced hydroxides. In addition, the oil droplets were transported to wastewater surface by the hydrogen bubbles formed at the cathode. Even though the oil removal efficiency by using iron electrodes was high (96%) the color of wastewater turned black after the application of voltage for 50 min. This color change was due to the formation of iron(II) sulfides since bor oil has sulfur components in its chemical formulation. The attained removal efficiency was 95% for the aluminum electrodes after 50 min and the treated wastewater color was clear. The electrode materials were also used for electrocoagulation in real wastewater samples and produced similar results. The best electrode material was also found to be aluminum depending on removal efficiency. However, it required a longer operating time to reach 95% removal efficiency (120 min).

The electric field strength between plate electrodes is proportional to the applied voltage divided by the distance between electrodes. As the electrodes gap increases the electric field strength decreases and, therefore, the electro-migration velocity of ions decreases (Khandegar & Saroha 2013). This provides more time for aluminum hydroxides to agglomerate to form flocs. However, if the distance between electrodes is too low, then aluminum hydroxides degrade by collisions as a result of high electric field strength. In this study, the distance between two aluminum electrodes was varied from 1 to 4 cm by keeping the applied voltage constant at 40 V. When the electrodes gap was 3 or 4 cm there was almost no oil removal for 60 min. On the other hand, when the electrodes gap was decreased to 1 cm, 95% oil removal efficiency was observed in 50 min.

Applied voltages

The electrocoagulation experiments were performed by applying 10, 20, 30, and 40 V potential difference to the aluminum electrodes. The corresponding oil removal efficiencies after 50 min were 50%, 85%, 90%, and 95%, respectively. With an increase in the applied voltage, the aluminum dissolution rate increased according to Faraday law, i.e., the amount of aluminum hydroxide flocs increased. Therefore, an increase in removal efficiency was expected (Lai & Lin 2004; Kobya et al. 2008). Even though increasing the applied voltage can promote reaction rates at the electrodes, a large current would result in a significant decrease in current efficiency. Most of the electrical energy can be wasted in heating water. Therefore, the critical voltage can be defined as the voltage that can supply the required amount of Al3+ ions to wastewater. It can be evaluated experimentally and higher voltages than the critical voltage do not result in any improvements in the quality of treated water. In this study, the critical voltage was found to be 40 V.

Initial pH

The best oil removal efficiencies were observed at the initial pH of 6. When the initial pH was equal to 8, no oil removal was observed for almost 60 min. The zeta potentials of oil droplets at pH 6 were measured as −80 mV. In addition, the zeta potentials of oil droplets increased in magnitude as pH increased. These results showed that bor oil emulsions became more stable in alkaline conditions. Therefore, the oil removal efficiency at pH 8 was much lower than the one at pH 6. Although the highest removal efficiency was expected to be at pH 4 (measured as the isoelectric point of oil droplets), microscopic analysis showed that oil droplet diameters decreased with increasing acidity. Therefore, flocculation and coagulation decreased in more acidic conditions.

Effects of pulsed voltages on oil removal efficiencies and energy consumption

To analyze the effect of pulsed voltage applications on energy consumption, three experiments were performed by keeping the average of the applied voltage constant:

  • Run A: 30 V was applied to the electrodes for 90 min.

  • Run B: 20, 30, and 40 V were applied to the electrodes each having a duration of 30 min.

  • Run C: 20, 30, and 40 V were applied to the electrodes randomly for a duration of 10 min.

The applied voltage profiles are presented with respect to time in Figure 1. The arithmetic average of these three voltage profiles was equal to 30 V for a time period of 90 min. The current measurements of Run A, Run B, and Run C are shown in Figure 2 for the batch and continuous operating modes. The current response times with respect to voltage variations were very quick for both operating modes. In addition, the currents for the batch operating modes were slightly higher than for the continuous mode, which was theoretically expected. The conductivity of the studied wastewaters was less than for tap water. As the oil droplets were separated from the wastewater its conductivity increased. The oil removal efficiencies in the batch operating mode were higher than those obtained for the continuous operating mode. The oil removal efficiencies for both operating modes are shown in Figure 3. When the results of the batch operating mode were analyzed, it could be concluded that the steady-state was reached after 3,000 s for the three voltage profiles studied. In addition, the steady-state removal efficiency for three voltage profiles was equal to 92%. In the case of continuous operation, however, the removal efficiencies were very low for all applied voltage profiles and showed more fluctuations. The approximate steady-state times for Run A and Run B could be stated as around 2,500 s. Even though it has not been shown in the graph, the corresponding steady-state time for Run C was around 6,700 s. The attained steady-state removal efficiencies were 18%, 35%, and 45% for Run A, Run B, and Run C, respectively.

Figure 1

Applied voltage profiles each having an arithmetic mean of 30 V.

Figure 1

Applied voltage profiles each having an arithmetic mean of 30 V.

Figure 2

The measured currents during electrocoagulation experiments for batch and continuous operating modes.

Figure 2

The measured currents during electrocoagulation experiments for batch and continuous operating modes.

Figure 3

The turbidity removal percentages during electrocoagulation experiments for batch and continuous operating modes.

Figure 3

The turbidity removal percentages during electrocoagulation experiments for batch and continuous operating modes.

The residence time of wastewater in the continuous operating mode was 4,500 s, which was higher than the residence time in the batch mode (2,500 s). The residence time in the continuous operating mode was increased further to 9,000 s by decreasing the volumetric flow rate of wastewater to 10 mL/min. However, the attained oil removal efficiency (50%) was still much lower than 92%. Since the electrocoagulation process involves not only the electrode reactions, but also separation processes, such as adsorption, sedimentation/floatation, it is not possible to compare the removal efficiencies obtained in the batch and continuous operating modes based on the residence time. The same electrokinetic cell was used in both operational modes and its length was 27.3 cm. Aluminum electrodes were placed at 2 cm away from wastewater influent. The gap between electrodes was 1 cm. This was the region where alum, hydrogen and oxygen were produced. After these formed, alum and gas bubbles were transported by wastewater velocity. Therefore, flotation and sedimentation became dominant mechanisms for oil removal in 80% of the electrokinetic cell volume. In the batch operating mode, wastewater was recirculated at the same volumetric flow rate as for the continuous mode. This was creating mixing and could increase the removal efficiency in flotation. In addition, because of recirculation, the contact time of wastewater with electrodes was higher in the batch operating mode. Oil removal efficiencies in the continuous operating mode were expected to increase by increasing contact time using another electrode arrangement.

It can be seen in Figure 3 that the oil removal efficiency for Run C was almost zero up to 5,000 s. By the application of potential difference to the electrodes, Al3+ released from the anode accumulated on the surface of the electrode. An aluminum oxide layer then formed on the anode causing decreases in the aluminum ion dissolution rate, which is known as electrode passivation (Fouad 2013). Since the applied voltage was the main parameter controlling the reaction rate at the anode, it is predicted that the variation of applied voltages in 10 min promoted the formation of oxide layer. Therefore, aluminum ions are not released into the wastewater and oil removal cannot be observed.

The energy consumption, SEC and SECCOD were evaluated at 3,000 s and presented in Table 1. The energy consumption for Run C was not included in Table 1 since there was no oil removal up to 3,000 s (Figure 3). According to the results presented in Table 1, there was no advantage of using the pulsed voltages in the batch operating mode. Conversely, the removal efficiencies for Run B were almost twice that of the removal efficiencies for Run A in the continuous operating mode. In addition, the SECCOD for Run A and Run B were evaluated as 368.2 kW-hr/kg COD and 191.1 kW-hr/kg COD, respectively. The saving in energy consumption by applying the pulsed voltages was 48%. The pulse durations for Run B were 30 min. It is proposed that as the pulsed voltage durations were increased from 10 to 30 min the formation of oxide layers at the aluminum electrodes could be eliminated and more aluminum hydroxides could be formed resulting in higher oil removal efficiencies.

CONCLUSIONS

Based on the results of the electrocoagulation experiments performed by using 2% bor oil, the following conclusions are drawn:

  • 96% oil removal efficiency was obtained by using iron electrodes but the wastewater color turned to black as a result of the chemical reactions between iron and sulfur.

  • The use of aluminum electrodes resulted in 95% oil removal efficiency and the treated wastewater color was clear.

  • The oil removal efficiency increased as the applied voltage was increased but this increase was not linear.

  • The critical applied voltage was found to be 40 V in this study because the oil removal efficiency showed no improvements for applied voltages greater than 40 V.

  • The optimum distance between aluminum electrodes was found to be 1 cm since the highest oil removal efficiencies were obtained at this distance.

  • Lower removal efficiencies were obtained in the continuous operating mode at the same operating conditions in comparison to the batch operating mode.

  • The use of pulsed voltages in the electrocoagulation process could cause 48% saving in energy consumption in the continuous operating mode. Pulsed durations of 30 min or longer were suggested for preventing electrode passivation.

ACKNOWLEDGEMENTS

The financial support of TUBITAK [111Y233] and Bulent Ecevit University [BAP: 2012-17-17-01] is gratefully appreciated.

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