The investigation of parameters affecting the treatment of synthetic bilge water by continuous electrooxidation/flotation process

This study addresses the pressing issue of bilge water pollution from ships, a highly oily and hazardous wastewater source. The research employs the electrooxidation/flotation process, known for its effectiveness in organic matter removal. Key parameters, such as initial pH, current density and flow rate, were investigated for their impact on the removal of chemical oxygen demand (COD) and oil‐grease (OG) from bilge water. Initial pH showed minimal effects on COD and OG removal, while current density significantly enhanced removal efficiency by influencing anodic electrochemical reactions. Conversely, higher flow rates reduced residence time and lowered removal efficiency. Optimal conditions, with a current density of 10 mA/cm2, pH 7.5 and a flow rate of 20 ml/min, achieved impressive results, removing approximately 80% of COD and 99% of OG from bilge water. These findings highlight the potential of this method for effective bilge water purification.


Highlights
• Bilge water treatment is very important in terms of water pollution.
• A continuous flow electrochemical reactor was used for bilge water treatment.
• With the EO-EF treatment process, 99% oil-grease and 80% COD removal were obtained.
• The results provided a new approach for the treatment of industrial wastewater containing oil.serious problem, as it greatly damages the aquatic environment.
Among the pollutants of large water bodies, bilge water emulsions have been a cause for concern (Jiyoung et al., 2019).Bilge water contains all contaminants including a mixture of fresh and seawater, as well as diesel fuels, oily liquids produced from the engine room, grease, suspended solids, heavy metals, and surfactants produced on board.Due to the increase in international trade, sea transportation is used intensively.This situation causes more bilge water to be discharged into the sea and resulting in higher environmental pollution levels.Heavy metals as well as oily pollutants are extremely dangerous as they accumulate in marine organisms (Gryta, 2020).
Although the direct discharge of bilge water without treatment to the sea is forbidden, sometimes, bilge water is discharged to the sea from the ship's hull with the help of a discharge line.The discharged bilge water is rapidly diluted in seawater.Due to the rapid dilution of the discharged bilge water, toxicity from the mixture can be expected to be a problem only around the discharge line (Magnusson et al., 2018).
Due to the complexity of the bilge water, it is not possible to fully purify with a single treatment method.Physical, chemical, and biological treatment methods are used for bilge water treatment.Gravity (Caplan et al., 2000), flotation (Hanafy & Nabih, 2007), and filtration (Ahmad et al., 2018;Tomaszewska et al., 2005) methods were used as physical separation methods.Coagulation (Fard et al., 2021;Hamidi et al., 2021), Fenton oxidation (Öz & Çetin, 2021), membrane distillation (Gryta, 2020), and photocatalysis (Cazoir et al., 2012) methods were used as chemical treatment methods.Anaerobic treatment (Emadian et al., 2015), biofilms (Vyrides et al., 2018), and aerobic treatment (Nisenbaum et al., 2020) methods were used as biological treatment methods.Another treatment method is the electrochemical treatment method.The most used electrochemical treatment method is the electrocoagulation process.Some of the studies performed with electrochemical treatment methods are shown in Table 1.Some of these examined studies were carried out in batch mode, and the rest were in continuous mode.Except for one study where aluminium and iron electrodes were used simultaneously, an aluminium electrode was used in all electrocoagulation studies.Pt/Ir and oxidized titanium were used as anode electrodes in the electrooxidation studies carried out in batch mode.Considering the bilge water volume, it is thought that it is very difficult to convert batch studies into real practice.Besides, the biggest disadvantage of the electrocoagulation process is the electrochemical dissolution of the anode electrode.Electrode dissolution leads to depletion of the anode electrode at certain time intervals depending on the applied current density.This causes the operating costs of the process to increase.However, the treatment efficiency is quite high in most of the studies.
Electro-oxidation (EO) is another treatment method that can be used to remove and treat bilge water.This new method has high removal efficiency for almost all kinds of organic matter.For this reason, it is a treatment method that is frequently used in the treatment of organic pollutants.In EO, contaminants are absorbed by the anode surface and subsequently oxidized directly or indirectly (Fil et al., 2014).
Moreover, recent work by Hajalifard et al. (2023) highlights the integration of advanced oxidation processes (AOPs) with electrocoagulation (EC) methods for water and waste water treatment, particularly emphasizing the removal of emerging contaminants and showed potentials for reducing energy consumption and sludge production.
The mechanism and detailed explanation of the electrooxidation process are given in the previous works of the author (Fil et al., 2014;Kul et al., 2015).Although the number of studies on different wastewaters is very large, there has not been enough work on the removal of bilge water by EO in continuous mode.
This study aims to investigate the treatability of bilge water by electrooxidation/flotation method in continuous mode.The electrooxidation/flotation process has been preferred because it has higher organic matter removal efficiency and does not form waste sludge.
Furthermore, this study aims to pave the way for more extensive research into electrooxidation/flotation method as a sustainable solution for bilge water treatment.Exploring its applicability and efficiency in real-world marine settings can lead to practical and cost-effective solutions for addressing this pressing environmental issue.

| MATERIALS AND METHOD
Bilge water prepared synthetically was used during the study.Synthetic bilge water composition and characterization have been prepared in full compliance with literature reports and samples from various ships.The preparation procedure of bilge water is the same as that used in the study by Körbahti and Artut (2013).Bilge water consists of a mixture of organic and inorganic substances as given in Table 2.In the above table, distilled water was used for the preparation of seawater substitute.
In the electrooxidation treatment studies, a reactor made of plexiglass with a volume of 1100 ml was used.The Bilge water volume in the electrochemical reactor is 800 ml.Inside the reactor, the electrodes two anodes and two cathodes with 5 mm distance were positioned horizontally, parallel to each other.Platinum-coated titanium mesh electrodes were used as the anode electrode, and pure mesh titanium electrodes were used as the cathode electrodes.The direct current required for the experiment was provided by using a programmable power supply (Chroma Programmable DC Power Supply Model 62024P-40-120).The prepared synthetic water flowed into the reactor with a peristaltic pump (Cole Parmer/Masterflex 7550-60 Computerized Drive).The schematic view of the experimental setup is given in Figure 1.The following equations were used for the calculation of the experimental data: • Calculation of treatment efficiencies: where C o is the initial pollutant concentration (mg/L) and C t is the concentration of the pollutant remaining in the wastewater at time t (mg/L).
• Calculation of energy consumption: where I is the applied current intensity (A), V is the potential difference in the system (volt), t is the reaction time (minute), and ϑ is the total wastewater volume (m 3 ).

| RESULTS AND DISCUSSION
The treatability of bilge water by electrooxidation/electroflotation process is investigated using platinum-coated titanium electrodes.

| The effect of initial pH
Wastewater pH in wastewater is an important parameter for the electrooxidation/electroflotation process, as it affects both the bubble formation, size, and the oxidation reaction that takes place on the anode surface.These effects have been studied in detail in the literature.In studies using different electrodes, it has been revealed that for the production of H 2 , the neutral condition is favourable regarding the bubble size and the optimal purification condition, but a general rule cannot be defined for the optimal pH for the O 2 bubble size (Alam et al., 2017;Jiménez et al., 2010).In this study, the current density of 5 mA/cm 2 and the flow rate of 20 ml/min were kept constant.
The pH, temperature changes, and energy consumption values that emerged at the end of the experiments are shown in Figure 2.
At the end of the 20-min hydraulic residence time, the pH and temperature values increased for all the initial pH values studied.The reason for the increase in the pH value at the end of the reaction period can be explained by Equation (3).This reaction can be expressed as the cathode reaction.
The range when aluminium electrodes are used by the electrocoagulation process (Ulucan & Kurt, 2015).Although pH is an important parameter for the anodic dissolving of aluminium ions to form flock in the electrocoagulation process, it does not show a great effect on the efficiency of COD removal from the bilge water.In another study, it was established, and the initial pH value is not important in domestic wastewater treatment with the electrochemical processes (Özyonar & Korkmaz, 2022).In the electrooxidation study, in which the treatment of paracetamol wastewater was carried out, a similar trend was determined for the initial pH value obtained in this study (Periyasamy & Muthuchamy, 2018).

| Effect of applied current density
Current density is one of the important design parameters in an electrooxidation/flotation process.In this study, 2.5, 5, and 10 mA/cm 2 were applied as current intensities.During the study, the pH of the bilge water was 7.5, and the flow rate was kept constant at 20 ml/ min.The effect of the change in current density on the effluent pH, temperature values, and energy consumption values is shown in Figure 4 below.
As it can be seen in Figure 4, when the current density was increased, the pH value of the effluent also increased due to the formation of H 2 gas.In addition, the increase in the current density under the constant electrical conductivity value caused an increase in the electrical resistance.Moreover, the increase in electrical resistance causes a part of the total energy given to the system to be converted into heat energy causing the effluent water temperature increases.In electrochemical processes, the increase in current density under a constant electrical conductivity value causes an increase in the potential difference value in the system.Considering the energy consumption value given in Equation ( 2), the potential difference value also increases as a result of the increase in current density.This situation caused the energy consumption values to reach much larger values.
The energy consumption value for 2.5 mA/cm 2 current density was 2.2 kW-h/m 3 .Increasing the current density to 10 mA/cm 2 caused the energy consumption to increase to 15 kW-h/m 3 .Demonstrating that when the current density increased four times, the energy con- The effect current density on effluent pH, temperature, and energy consumption values noted that the increase in current density causes an increase in the formation of oxidizing radicals.Therefore, higher pollutant removal efficiency was obtained at higher current density (Jacobo et al., 2015).
At 2.5, 5, and 10 mA/cm 2 current densities, the COD and OG removal efficiency were 67%, 72%, 80%, and 92%, 95%, and 99%, respectively.The use of high current densities, which generate more ÁOH radicals, will help increase both direct and indirect electrochemical oxidation of organic compounds.The results obtained from the above outlined experiment verify this fact.There are many studies in the literature supporting the result of the current study that the efficiency of pollutant removal increases with the increase of current density (Aggadi et al., 2021;Qiao et al., 2021;Sanni et al., 2022).

| The effect of flow rate
The The results showed the COD removal efficiency for bilge water slightly decreased from 78% to 72% as the flow rate increased from 10 to 20 ml/min (Figure 7).Subsequently, the COD removal efficiency fell sharply from 72% to 53% by increasing the flow rate to The effect of flow rate on effluent pH, temperature, and energy consumption values The effects of flow rate value on COD and OG removal efficiencies under constant current density and initial pH The effects of current density value on COD and OG removal efficiencies under constant flow rate and initial pH 40 ml/min.In terms of COD removal efficiency, a flow rate of less than 20 ml/min can be considered a waste of energy, time, and material.At higher flow rates (more than 20 ml/min), the time required to remove organic matter and the possibility of oxidant formation are reduced.The sharp decrease in the removal efficiency with the increase of the flow rate was also observed in the electroflotation method of removal of industrial wastewater by Khalek et al. (2019).
Considering the terms of OG removal, it has been noted that the treatment efficiency decreases with the increase in the flow rate in the electroflotation process.This result can be explained in terms of electroflotation as follows: oily emulsions collide with gas bubbles, attach, and slowly rise to the surface to scape.Each stage of this process is time dependent; that is, a sufficient amount of time ensures the completion of the process.As such, it is important to allow sufficient retention time for the bubble-particle collision and bonding in the solid-water separation process (Mohtashami & Shang, 2019).OG removal of 97.6% was achieved at a flow rate of 10 ml/min, 95.6% at a flow rate of 20 ml/min, and 90% at a flow rate of 40 ml/min.The results obtained are proof of this situation.

| CONCLUSIONS
The performance of a continuous flow electrooxidation process equipped with platinum-coated titanium mesh electrodes was evaluated in synthetic bilge water treatment.Under the optimal conditions of current density 10 mA/cm 2 , pH 7.5, and flow rate 20 ml/min, the experiments conducted showed that the electrooxidation process is a strong contender for the treatment of bilge water.Treatment of the bilge water with these specific conditions resulted in achieving a high removal efficiency in a short time.The maximum COD and OG removal efficiency of the electrooxidation process under these conditions was approximately 80 ± 2% and 98 ± 2%, respectively.
This study demonstrates that the electrooxidation method is effective and successful in the treatment of bilge water with competitive energy consumption.In summary, the electrooxidation process, when operated under the optimal conditions mentioned, surpasses the effectiveness reported in previous literature studies.

CONFLICT OF INTEREST
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

DATA AVAILABILITY STATEMENT
Due to confidentiality and privacy restrictions, access to the data is subject to approval and may require the completion of a data access agreement.Researchers interested in accessing the data can request permission and obtain further information by contacting Alper Erdem YILMAZ (at aerdemy@atauni.edu.tr).The data will be made available to qualified researchers upon agreement with the data access requirements and in compliance with relevant ethical and legal considerations.

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E Y W O R D S bilge water, current density, electrooxidation/flotation, flow rate, pH 1 | INTRODUCTION Wastewater pollution discharged into the oceans and seas has been a

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I G U R E 1 Schematic view of the experimental setup: (1) wastewater inlet chamber, (2) reactor, (3) peristaltic pump, (4) DC power supply, (5) anode electrodes, (6) cathode electrodes, (7) wastewater outlet chamber, and (8) pH control cell COD analyses were performed according to the closed system reflux method specified in standard methods (APHA, 2017).The pH and electrical conductivity values of the samples were measured with the WTW pH 330i pH metre.Oil and grease analyses were made by the Infracal TOG/TPH Analyzer device.The device was calibrated before the analyses were made.This method was carried out according to the standards specified in APHA (2017) standard methods for the examination of water and wastewater in section 5520.The device works in the IR range, and 10 W 40 engine oil is used as a calibration standard.
H + ions released as a result of the hydrolysis of water at the cathode electrode turn into H 2 gas.The resulting H 2 gas moves away from the aqueous medium.This causes the pH balance to deteriorate in favour of basicity.The temperature change at the end of the reaction time is not directly explained by the pH change.The pH value of the synthetically prepared bilge water is approximately 7.5.Concentrated HCL was added to synthetic bilge water to adjust the initial pH to different levels.The amount of concentrated HCL required varied depending on the target pH, with the greatest amount used to achieve the lowest pH.As the amount of HCl added to the bilge water increases, the electrical conductivity value of the solution also increases where the electrical resistance values decrease.The decrease in the electrical resistance value prevents the increase in the solution temperature.This confirms the temperature change demonstrated in Figure 2. The change in energy consumption can be explained by Equation (2).Under constant current density and reaction time, the only thing that causes the energy consumption value to change is the applied potential difference value.The change in the potential difference value under similar conditions can be explained by the change in the electrical conductivity value of the wastewater.As the initial pH value decreases, the electrical conductivity value will increase, so energy consumption values are expected to decrease.F I G U R E 2 The effect of initial pH on effluent pH, temperature, and energy consumption valuesThe effect of the initial pH value on the change of COD and oil/grease removal efficiencies under a constant current density of 5 mA/cm 2 experimented is demonstrated in Figure3above.According to the initial pH values, the COD removal efficiency was between 77% and 70%, respectively.The results show that the COD removal efficiency changes is slightly depending on the decrease in the initial pH value of the bilge water.The COD removal efficiency does not change or changes negligibly at low pH values such as 2, 3, and 4. The difference in COD removal efficiency between the lowest and natural pH values showed that the initial pH value is not a dominant system parameter for the bilge water treatment by electrooxidation.In the experiment conducted at the natural pH, the OG removal efficiency was determined to be 95.22%.The highest OG removal efficiency obtained in the experiments carried out in acidic conditions is approximately 97%.The data obtained showed that the initial pH value is not an important parameter in the ranges examined for OG removal.The difference between the OG removal efficiencies (96.9%À95.2%= 1.7%) in the experiments performed with the lowest and highest initial pH values is not large enough to show whether the investigated parameter is effective or not.It is seen that the COD removal efficiency does not change significantly in the pH 6-8.5 sumption value disproportionately increased to approximately 7.5 times.The effects of current density on COD and OG removal are shown in Figure 5. Electrochemical degradation of organic matter occurs by two mechanisms; (a) contaminants are adsorbed on the anode surface, and contaminants are destroyed by the anodic electron transfer reaction called direct anodic oxidation; and (b) electrochemically produced oxidizers oxidize in the liquid bulk called indirect oxidation.In indirect oxidation, oxidants such as chlorine, hypochlorite, hydroxyl radicals, ozone, and hydrogen peroxide are used (Un et al., 2008).It has been F I G U R E 3 The effects of initial pH value on COD and OG removal efficiencies under a constant current density flow rate of the bilge water into the electrochemical reactor determines the hydraulic residence time in the reactor.Since the flow rate determines the exposure time of the bilge water to the electrochemical reactions, it is an important parameter that affects the removal of pollutants.The effect of flow rate on COD and OG removal from bilge water was studied in a range of 10 ml/min (80 min hydraulic retention time), 20 ml/min (40 min hydraulic retention time), and 40 ml/min (20 min hydraulic retention time) at a constant current density of 5 mA/cm 2 .The raw pH value of the bilge water was used in the experiments in which the effects of flow rate were investigated.The pH and temperature changes and energy consumption values obtained at the end of the electrochemical reaction period are shown in Figure 6.Increasing flow rate caused a decrease in the effluent pH value and hydraulic residence time in the reactor.The reduced hydraulic residence time resulted in less exposure of the pollutant to electrochemical reactions.Therefore, different effluent pH values were obtained at different flow rates.The effluent water temperature values also showed a decreasing trend depending on the increasing flow rate.The decrease in the effluent water temperature value was due to the decrease in the hydraulic residence time and the change in the flow rate.These two factors cause the potential difference values to change.The main reason for the difference in energy consumption values is the change in hydraulic residence times.Considering Equation (3), one of the factors affecting energy consumption besides current intensity and potential difference values is the duration of the electrochemical reaction.As the increased flow rate caused the reaction times to decrease, it led to a decrease in the energy consumption values.The above-mentioned points are clearly shown in Figure 6.The changes in the COD and OG removal efficiencies were also investigated in the experiments where these values were obtained.The changes observed in COD and OG removal efficiencies are demonstrated in Figure 7.
Composition of synthetic bilge water and synthetic seawater