Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes

Accepted Manuscript Title: Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes Author: E. Demirbas M. Kobya PII: DOI: Reference:

S0957-5820(16)30239-7 http://dx.doi.org/doi:10.1016/j.psep.2016.10.013 PSEP 898

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

21-4-2016 14-10-2016 17-10-2016

Please cite this article as: Demirbas, E., Kobya, M., Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2016.10.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes E. Demirbas1,*, M. Kobya2 1

Department of Chemistry, Gebze Technical University, 41400, Gebze, Turkey

2

Department of Environmental Engineering, Gebze Technical University, 41400, Gebze, Turkey

* Corresponding author. E-mail address: [email protected], Tel: +90 (262) 6053010, Fax: +90 (262) 6053005.

Abstract The present study dealt with treatment performances and operating cost of real metalworking fluid wastewater (MWFW) using chemical coagulation (CC) and electrocoagulation (EC) processes. The optimum coagulant dosage for COD and TOC removal efficiencies from the MWFW was determined with varying concentrations of alum, aluminium chloride, ferric sulphate, and ferric chloride. COD and TOC removal efficiencies in the coagulant dosage range of 50-1000 mg/L were obtained as 65-97% and 49-81% for alum, 48-96% and 38-80% for aluminium chloride, 43-92% and 36-76% for ferric sulphate, and 55-93% and 41-77% for ferric chloride, respectively. The aluminiumbased coagulants at a pH of 7.5 showed better performances than ferric-based coagulants in terms of COD and TOC removals. COD and TOC removal efficiencies from the MWFW in the EC process were 93% and 80% for Al electrode at a pH of 5, 80 A/m2 and 25 min, and 93% and 82% for Fe electrode at a pH of 7, 80 A/m2 and 25 min, respectively. Operating costs at the optimum conditions were calculated to be 1.190 US $/m3 for Al and 1.813 US $/m3 for Fe electrodes, respectively. Alum in chemical coagulation process provided a lower operating cost (0.12 US $/m3) as compared with the EC process using

Al electrode (1.19 US $/m3). The results demonstrated that both investigated treatment processes were effective to treat wastewater from a metalworking plant. Keywords: Metalworking fluid wastewater, Coagulation, Electrocoagulation, Operating cost 1. Introduction In metalworking processes, cutting fluid is widely used for lubrication, cooling, surface cleaning, and corrosion prevention during machining of metallic pieces in metalworking operations (Cheng et al., 2005). Water-based lubricants and cutting oils have replaced some petroleum-based products in the metalworking industry, especially in steel cold rolling operations as a result of their higher performance. The metalworking fluids (MWFs) consisting of oil-in-water emulsions contain mainly emulsified oil and surfactants which allow emulsion formation and stabilization when mixed with water (Byers, 2006; Benito et al., 2002). These cutting fluids usually comprise an emulsion of 2-5% oil in water. These emulsified fluids act as lubricants and coolants in metalworking operations in order to reduce friction between the metal and mechanical equipment and to avoid metallic piece oxidation and contact welding of metal parts. MWFs lose its lubricant and coolant properties after a certain period of time because of their thermal degradations and pollution by the suspended matter. Therefore, it is necessary to carry out their replacement periodically and to remove the worn effluents. Wastewater discharged from industrial washers used to rinse the produced parts may have an emulsified oil concentration of 300-7000 mg/L and free floating oil content of 30000 mg/L (Mysore et al., 2005). Worldwide, over 2000000 m3 is used annually although the wastewater volume could be ten times higher due to the dilution of the MWFs prior to use. The average disposal cost of spent MWFs is 28-56 US $/m3, however this cost will be significantly higher for smaller businesses (56-113 US $/m3) (Macadam et al., 2012). The metalworking plant effluents contain also a high percentage of oil, chemical oxygen demand (COD), and total organic carbon (TOC) (Greeley and Rajagopalan, 2004). Moreover, oil and grease in the metalworking fluid

wastewater (MWFW) and foul the sewer system generate an unpleasant odour. They constitute a potent threat for the groundwater because of their great capacity of penetration in the ground (Sokovic and Mijanovic, 2001). Treatment processes like distillation (Canizares et al., 2004), dissolved air flotation (Bensadok et al., 2007), adsorption (Yuan et al., 2011; Mysore et al., 2005), aerobic and anaerobic oxidation (Cheng et al., 2006, Perez et al., 2006, Christopher and Thompson, 2005), chemical coagulation (Rios et al., 1998), hydrothermal oxidation (SanchezOneto et al., 2007), photo-Fenton oxidation (MacAdam et al., 2012), ultrasonic oxidation (Seo et al., 2007), and membrane processes (Milic et al., 2013; Hesampour et al., 2008) were applied to treat the MWFWs. These treatment methods have some disadvantages such as longer time requirements and regeneration problems, concentration polarisation and membrane fouling, long operating time and process control, toxicity of biocides and low biodegradability and operating cost (Yu et al., 2016; Jamalay et al., 2015). Coagulation and electrocoagulation processes play a more prominent role in the treatment of oily wastewaters due to its several advantages including very efficient, easy operating and automated process, low capital and operating cost (Asselin et al., 2008), and high filterability of the treated water (Al-Shannag et al., 2013). However, there are a few studies involved with treatments of real MWFW by chemical coagulation (CC) (Rios et al., 1998) and electrocoagulation (EC) (Kobya et al., 2008; Bensadok et al., 2008; Carmona et al., 2006), and synthetic solutions containing metalworking fluids (Bergmann et al., 2003). The CC process involves addition of a solution containing hydrolysing metal salts (Fe3+ or Al3+) while the EC requires in situ generation of coagulants by electrolytic oxidation of an appropriate anode material (iron or aluminium). The EC process was successfully used to treat various wastewaters effectively such as Baker's yeast wastewater, metal plating wastewater, paper industries wastewater, municipal wastewater (Al-Shannag et al., 2014; Al-Shannag et al., 2015; Shannag et al., 2012; Al-Shannag et al., 2013), olive mill wastewater (Lafi et al., 2010), HF

wastewater remediation (Aoudj et al., 2013), oil-in-water emulsions (Canizares et al., 2008) and oily wastewaters (Sekman et al., 2011; Asselin et al., 2008; Calvo et al., 2003). The use of electrodes with large surface area is required and performance improvement has been achieved in the EC cells with either monopolar electrodes or bipolar electrodes. Monopolar electrodes need an external electrical contact to the power supply and their two faces are active with the same polarity. Monopolar electrodes in serial connections for each pair of sacrificial electrodes are internally connected with each other. A higher potential difference is required for a given current since the cell voltages sum up. In the case of the bipolar reactor, the sacrifice electrodes are placed between the two electrodes in parallel without any electrical connection. Only two monopolar electrodes are connected to the power source with no interconnection between the sacrifice electrodes. When the current passes through the two parallel electrodes, the neutral sides of the plate acquire an opposite charge. The external electrodes are monopolar and the internal ones are bipolar (Kobya et.al., 2011; Zeboudji et al., 2013; Lemlikchi et al., 2012). The anodic material gets dissolved into the solution with a simultaneous formation of hydroxyl ions and hydrogen gas evolution at the cathode during the EC. Reactions involved during the EC with Al and Fe electrodes are: Anode reactions: Al  Al3  3e

(1)

Fe  Fe2  2e

(2)

Fe2  Fe3  e

(3)

Cathode reactions: 3H 2 O  3e   3OH   3/2H 2(g) (for Al electrode) 2H 2 O  2e   2OH   H 2(g) (for Fe electrode)

(4) (5)

Dissolved metal ions form various charged hydroxylated species. These charged species adsorbed on Al(OH)3(s) and Fe(OH)3(s) act as coagulants, and facilitate the removal of pollutants

in wastewater. Also, the rates of ferrous ion oxidation by dissolved oxygen increase with pH during the EC process using Fe electrodes and approximately 90% conversion may be achieved in a few minutes at a pH of 7 4Fe 2  O 2  2H 2 O  8OH   4Fe(OH) 3(s)

(6)

In both cases, the chemistry of the hydrolysed metal salts is complex. In addition to these species, formation of monomeric and polymeric (oxy)hydroxo-metallic species, especially under high metal concentrations, was also reported. Various charged mono and polyhydroxylated species of iron and aluminium are formed due to the interaction of metal ions (Me: Fe3+ and Al3+) with water in the bulk depending on the solution pH according to the following reactions: y xMe 3  yH 2 O  Me x (OH) 3x  yH  y

(7)

All these hydrolysis metal species can interact with different types of pollutants, achieving their removal from wastewaters. These interaction processes are strongly related to the metal speciation and they can be summarized into three main types: (i) the metallic ionic monomeric species can neutralize charge of the pollutants by adsorption on their surfaces (or binding to their ionized groups), (ii) the metallic ionic polymeric species can bind to several particles (or molecules) of pollutant at a time, and (iii) the pollutants can be enmeshed into growing metallic hydroxide precipitates, or can be adsorbed onto their surfaces. Similarly, when aluminium chloride, aluminium sulphate, ferric sulphate and ferric chloride are used as chemical coagulating agents, the products of hydrolysis of the salts are the charged chemical coagulants (Kobya et al., 2011; Kobya et al., 2010). The objective of this work was to study the treatment of the MWFW by CC and EC. Effects of operating conditions for CC (pH, coagulant type and coagulant dosage) and EC (electrode type, initial pH, current density and reaction time) on the COD and TOC removal efficiencies

were investigated. Operating costs at the optimum operating conditions were also calculated for both treatment processes.

2. Material and methods 2.1. Metalworking fluid wastewater The real MWFW was obtained from a heavy metal manufacturing company producing automotive engine, transmission and stamping plants in Gebze, Turkey. The MWFW in the company was semi synthetic fluids (Generax 85) used as a coolant and lubricant in large scale for continuous metal working processes. The wastewater was analysed according to the Standard Methods after the sample collection (APHA, 1998). Characterizations of the MWFW were a pH of 6.6  0.4, conductivity of 6.1  0.4 mS/cm, COD of 17312  2812 mg/L, TOC of 3155  505 mg/L, turbidity of 15350  2100 NTU and total suspended solid of 150  25 mg/L. 2.2. Experimental set-up and operation (i) Chemical coagulation experiments: Chemical coagulation is carried out by adding salts (coagulants) namely, ferric sulphate (Fe2(SO4)3.7H2O), ferric chloride (FeCl3.6H2O), aluminium sulphate or alum (Al2(SO4)3.18H2O) and aluminium chloride (AlCl3.6H2O). Their concentrations are based on their hydrated forms. Analytical grade coagulants from Merck (Germany) were used in this study. During the coagulation experiments, six different coagulant dosages (50, 100, 250, 500, 750 and 1000 mg/L) and six different pH values (4, 5, 6, 7, 8, and 9) were considered for all coagulants to determine the optimum coagulant dosage and pH values. The CC experiments were carried out in a standard jar test experimental set-up at room temperature. A jar-test apparatus with six beakers of 1 L volume was used. The CC process consisted of three subsequent stages: initial rapid mixing stage at 160 rpm for 5 min, followed by a slow mixing stage for 20 min at 30 rpm and the final settling step lasting for another 1 h. Then, a sample of supernatant water was taken for COD and TOC analyses.

(ii) EC experiments: The experimental setup was reported in the earlier study (Kobya et al., 2011). The reactor made from Plexiglas with dimensions of 100 mm  100 mm  110 mm (1.1 L) was equipped with four plate electrodes at monopolar parallel connection mode. Aluminium (99.53% purity) and cast iron (99.50% purity) plates as sacrificial electrodes were used as two anodes and two cathodes with dimensions of 45 mm  53 mm  3 mm. The active both Al electrode and Fe electrode areas were 143 cm2 and an electrode gap was set to 10 mm. The electrodes were connected to a digital dc power supply (Agilent 6675A model) operated at galvanostatic mode. All experimental runs were maintained at 300 rpm and room temperature. The pH of wastewater was adjusted to the required value with 0.1 M NaOH or 0.1 M H2SO4 (Merck). Conductivity of the solution also affects the efficiency, the cell voltage, and the electrical energy needed. The addition of an electrolyte, Na2SO4 to the existing solution can potentially lead to the transfer of current and improve the overall wastewater conductivity (Merzouk et al 2010; Daneshvar et al. 2006). In each run, 0.8 L of the MWFW was placed into the EC reactor. The current density was adjusted to a desired value and the EC process was started. The solutions were stirred at constant speed until completion of the experiment. The samples from the EC reactor at certain time intervals were collected and then filtered by a Whatman membrane filter (pore diameter of 0.45 µm).

2.3. Chemical Analyses and operating cost COD and TOC analyses were conducted by the procedures described in the Standard Methods (APHA, 1998). The COD concentration was measured by UV-vis spectrophotometer (PerkinElmer Lambda 35). The TOC levels were determined through combustion of the samples at 680 ◦C using a non-dispersive IR source (Tekmar Dohrmann Apollo 9000). The wastewater turbidity was measured using a Mettler Toledo 8300 model turbidimeter. pH was

measured using a pH meter (Mettler Toledo 2050e model), and the conductivity was determined with conductivity meter (Mettler Toledo 7100e model). In this preliminary economic investigation, the operating cost of the treated MWFW (OC, US $/m3) can be calculated by considering four parameters as major cost items (Kobya et al., 2016; Asselin et al., 2008): amount of energy, electrode material consumptions, chemicals consumed and landfill cost of sludge in the CC and EC processes. Cenergy and Celectrode consumptions (only for EC process) are calculated with the following equations (Kobya et al., 2016): Cenergy 

U  i  tEC

(8)

v

where U is cell voltage (V), i is applied current (A), tEC is the operating time (h) and v is the volume (m3) of wastewater. it C

electrode



EC

M

w

z F v

(9) where Celectrode is electrode consumption (kg/m3), Mw is molecular mass of aluminium (26.98 g/mol) and iron (55.85 g/mol), z is number of electron transferred (zAl = 3 and zFe = 2) and F is Faraday’s constant (96487 C/mol). According to the Turkish market in February 2016, prices for electrical energy were 0.13 US $/kWh, and prices for Al and Fe electrode materials were 1.78 and 0.93 US $/kg, respectively. Prices of chemicals used for adjustment of a desired pH were 1.01 US $/kg for NaOH, 0.40 US $/kg for H2SO4. Prices of coagulants were 0.14 US $/kg for alum, 0.31 US $/kg for AlCl3, 0.15 US $/kg for Fe2(SO4)3, and 0.22 US $/kg for FeCl3. Landfill cost of generated sludge was 0.10 US $/kg. In this case, operating costs for EC and CC process were calculated with the following equations: OC = a Cenergy + b Celectrode + c Cchemical + d Csludge

(10)

OC = a Cenergy + c Cchemical + d Csludge

(11)

3. Results and discussion 3.1. The effect of pH and coagulant dosage on treatment performance in CC The effect of coagulant dosage (50-1000 mg/L) on the removal efficiencies of COD and TOC from the MWFW was evaluated to determine optimum coagulant dosage (Fig. 1). COD and TOC removal efficiencies were increased to about 500 mg/L coagulant concentration for all four coagulants. COD and TOC removal efficiencies at 50-1000 mg/L of coagulant dosage range were 65-97% and 49-81% for alum (4.05-81.08 mgAl3+/L), 48-96% and 38-80% for AlCl3 (5.60-112 mgAl3+/L), 43-92% and 36-76% for ferric sulphate (10.65-213 mgFe3+/L), and 55-93% and 41-77% for ferric chloride (10.4-207 mgFe3+/L), respectively. However, addition of coagulant dose >500 mg/L (40.5 and 56 mgAl3+/L dosages for alum and aluminium chloride; 103.5 and 106.5 mgFe3+/L dosages for ferric chloride and ferric sulphate) did not improve COD and TOC removal efficiencies considerably. COD and TOC removal efficiencies over 96% (reduced from 17312 mg/L to 693 mg/L) and 80% (reduced from 3155 mg/L to 631 mg/L) from the MWFW were obtained with aluminium-based coagulants (alum and AlCl3) in comparison with ferric-based coagulants (FeCl3 and Fe2(SO4)3), respectively. pH is a factor that affects performance of the coagulation process because of its effect on polymeric species upon dissolution of the coagulants in water. When the pH is too low, protons outcompete the metal hydrolysis products for organic ligands and removal is poor because some of the organic acids do not precipitate. At high pH, the OH- ion competes with organic compounds for metal adsorption sites and metal hydroxides precipitate by co-precipitation (Stephenson and Duff, 1996). Effect of initial pH (pHi) on COD and TOC removal efficiencies at 500 mg/L coagulant dosage in the MWFW by CC was studied in the pHi range of 4.5-8.5. Fig. 2 showed that COD

removal efficiencies at a range of pH 4.5-6.5 were increased from 43% to 97% for alum and from 42% to 96% for AlCl3 whereas COD removals at pH 7.0-8.5 were decreased from 87% to 54% for alum and from 90% to 58% for AlCl3, respectively. The maximum removal efficiencies for alum and AlCl3 at a pH of 6.5 were 97% and 96% for COD, 81% and 80% for TOC, respectively. Similar results were obtained for ferric-based coagulants at a range of pH 4.5-7.5 (31-91% of COD and 25-76% of TOC removals for Fe2(SO4)3, 37-93% of COD and 33-77% of TOC removals for FeCl3). The maximum COD and TOC removal efficiencies for ferricbased coagulants were obtained at a pH of 7.5 and thereafter, COD and TOC removal efficiencies were decreased. According to these results, aluminium-based coagulants were found to be more efficient than ferric-based coagulants in terms of COD and TOC removals from the MWFW. The removals of COD and TOC were mainly attributed by the formation of precipitates from the combination of the soluble organics and the coagulant. Maximum efficiencies at a pH of 6.5 and 7.5 are due to precipitation of the metal ion in hydroxide form. Strong ion association occurs in both acidic and basic media. At neutral pH, the hydrogen bonds between neighbouring hydroxyls and between water adsorbed onto the surface and surface hydroxyls are disrupted by electrolyte adsorption, resulting in an increase in the percentages of COD and TOC removals (Mishra et al., 2002). Moreover, Amounts of sludge dried in an oven at 105°C from coagulant dosages of 50, 100, 250, 500, 750, 1000 mg/L in CC experiments were 0.021, 0.048, 0.130, 0.220, 0.330, 0.430 g/L for alum, 0.025, 0.054, 0.067, 0.280, 0.380, 0.490 g/L for AlCl3, 0.023, 0.045, 0.058, 0.260, 0.340, 0.470 g/L for Fe2(SO4)3, and 0.022, 0.044, 0.055, 0.240, 0.340, 0.450 g/L for FeCl3, respectively. Removal efficiencies of 97 and 96% for COD, 81 and 80% for TOC were achieved for alum and AlCl3 at 500 mg/L and a pH of 6.5. Similar results for Fe2(SO4)3 and FeCl3 at 500 mg/L and a pH of 7.5 were also obtained (91 and 93% for COD and 76 and 77% for TOC removals). Operating cost included coagulant, chemicals and sludge disposal at the optimum

conditions. Unit costs of alum, AlCl3, Fe2(SO4)3, FeCl3, NaOH, H2SO4 (96-98%), and sludge storage and disposal were 0.139 US $/kg, 0.314 US $/kg, 0.149 US $/kg, 0.215 US $/kg, 0.65 US $/kg, 0.11 US $/kg and 0.10 US $/kg. Total costs of alum, AlCl3, Fe2(SO4)3 and FeCl3 at the optimum conditions were calculated as 0.1163, 0.2098, 0.1203, and 0.1563 US $/m3. The solubility boundary denotes the thermodynamic equilibrium that exists between the dominant aluminium species at a given pH and solid aluminium hydroxide. The minimum solubility of Al(OH)3 (0.03 mg/L) occurs at a pH of 6.3, and solubility increases as the solution becomes more acidic or alkaline. When aluminium or iron-based coagulants were added to wastewater, a chemical reaction occurred and Al(OH)3 or Fe(OH)3 was formed. Al(OH)3 or Fe(OH)3 is immiscible in water, and causes oil droplets to become flocs and increases average diameter of oil droplets. Aluminium and iron hydroxides in water dissociate into Al+3, Fe3+, and OH−. Oil droplets with negative charge accumulated near aluminium and ferric ions and large flocs were formed in wastewater. On the other hand, at a higher pH, the coagulant particles were less positively charged; therefore, their attraction to the anionic oil droplets decreased (Cerqueira and Costa Marques, 2012). Oil removal from steel manufacturing (oil concentration of 430 mg/L) and petroleum refining wastewaters (oil concentration of 1900 mg/L) by coagulation was obtained as 99% at operating conditions (30 mg Al /L as PAC, 10 mg/L casein and a pH of 7), and 96.7% (20 mg/L Al as PAC, 100 mg/L casein and a pH of 8.2), respectively (Suzuki and Maruyama, 2005). Bottle oil washing wastewater (oil content of 3300 mg/L) by coagulation was achieved to 93% for COD, 99% for oil, and 99% for colour removal efficiencies by the addition of 180 mg/L alum, 1 g/L preastol at a pH of 6.5 (Elleuch et al., 2014). Oil-grease from automobile service station wastewater containing oil-grease concentration of 300 mg/L by alum coagulation was obtained as 100% at 200 mg/L alum and a pH of 7.8 (Mazumder and Mukherjee, 2011). Ngamlerdpokin et al., (2011) found that the chemical recovery by addition of H2SO4 at a pH of

1.0-2.5 and coagulation treatment by alum was effective to treat biodiesel wastewater (COD of 271000-341712 mg/L, oil-grease of 210-421 mg/L). Greater than 98.3% and 99.2% of COD from oil-grease were removed using alum at 2 g/L whilst 98.2% and 98.6% of COD were achieved slightly lower by PAC coagulation (1 g/L). The CC process in this work provided the removal efficiencies of 97% for COD and 81% for TOC and OC of 0.12 US $/m3. Most of the works in the literature showed some variations in the removal efficiencies due to the nature of wastewaters and experimental conditions, and no operating costs were reported except for this study.

3.2. The effects of initial pH, current density and operating time in EC Initial pH is a factor influencing the treatment performance of the EC process (Carmona et al., 2006). The effect of pHi in the wastewater on the removals of COD and TOC was first explored within the pH range of 3-8 at a current density of 80 A/m2 and an operating time of 25 min. Fig. 3 shows the removal efficiencies of COD and TOC as a function of the initial pH. As pHi increased from 3 to 5 for Al electrode, COD and TOC removals reached 65-94% and 45-83%, and it then decreased at a higher pH than 5 (66% of COD and 57% of TOC removals at a pH of 8). 60-90% of COD and 30-80% of TOC removals at pH of 3-7 were obtained with Fe electrode in the EC. Maximum COD and TOC removal efficiencies were observed at a pH of 5 for Al electrode and a pH of 7 for Fe electrode. It was interesting to note that COD and TOC removals decreased when pH was greater than 5 for Al electrode and 7 for Fe electrode. The pH after an operating time of 25 min increased from 3 to 4.7 and from 8 to 8.7 for Al electrode and from 3 to 6.4 and from 8 to 9.5 for Fe electrode, respectively. When initial pH values were 5 for Al electrode and 7 for Fe electrode, the final pHs increased to 6.5 for Al electrode and 8.6 for Fe electrode. The EC reactor equipped with iron (or aluminium) electrodes was able to produce significant quantities of hydroxyl ions at the cathode

and in fact increased the pH of the effluent. During the EC process, ferrous and aluminium ions produced by anodic dissolution reacted vigorously with the emulsified and colloidal dispersed oils, leading to phase inversion and breaking of the emulsion. Stabilized drops of emulsified and colloidal dispersed oils were then readily adsorbed and adhered to the surface of the metallic hydroxides such as Al(OH)3 and Fe(OH)3 (Asselin et al., 2008; Kobya et al., 2015). Likewise, hydrogen gas bubbles (H2) evolved at the cathode electrodes floated hydroxide flocs together with the pollutants adsorbed on the surface of hydroxides. The two processes (neutralization of charged particles and electro-flotation) gave a synergistic effect and contributed to efficient removal of pollutants from the MWFW. Applied current and electrolysis time are important parameters influencing the treatment efficiency and operating cost of the EC because it not only determines the coagulant dosage rate but also the bubble production rate, size and the flocs growth (Canizares et al., 2008). The removal efficiencies using Al electrodes in the EC reactor at current density of 20-100 A/m2, 25 min and a pH of 5 increased from 64 to 95% for COD and from 54 to 85% for TOC. The similar results were obtained for Fe electrodes in the EC reactor. The removal efficiencies at current density of 20-100 A/m2, 25 min and a pH of 7 increased from 56 to 93% for COD and from 45 to 82% for TOC, respectively. According to the Faraday’s law, the efficiency of ion production on the anode and cathode increased as the current density increased. Therefore, there was an increase in flocs production in the solution and hence, an improvement in the efficiency of COD and TOC removals. As the current density increased, the anodic dissolution was favoured so that the metallic sludge residues increased and organic pollutants were effectively removed from the wastewater. The optimum current density for both electrodes was chosen as 80 A/m2 since the highest removals of COD (94% for Al and 90% for Fe) and TOC (83% for Al and 80% for Fe) were observed at this value.

One of the most important parameters that considerably affect the application of any method of wastewater treatment is the cost. Energy and electrode consumptions for current densities of 20, 40, 60, 80, and 100 A/m2 were determined as 0.53 kWh/m3 and 0.111 kg/m3, 1.54 kWh/m3 and 0.128 kg/m3, 3.27 kWh/m3 and 0.156 kg/m3, 6.06 kWh/m3 and 0.208 kg/m3, 7.80 kWh/m3 and 0.4519 kg/m3 for Al electrode and 1.04 kWh/m3 and 0.057 kg/m3, 2.51 kWh/m3 and 0.198 kg/m3, 5.92 kWh/m3 and 0.341 kg/m3, 9.38 kWh/m3 and 0.601 kg/m3, 14.03 kWh/m3 and 0.770 kg/m3 for Fe electrode at 25 min, respectively. Amount of sludge and OC at current densities of 20, 40, 60, 80, and 100 A/m2 were calculated as 0.032 kg/m3 and 0.295 US $/m3, 0.054 kg/m3 and 0.458 US $/m3, 0.074 kg/m3 and 0.736 US $/m3, 0.082 kg/m3 and 1.190 US $/m3, 0.097 kg/m3 and 1.852 US $/m3 for Al electrode and 0.036 kg/m3 and 0.216 US $/m3, 0.058 kg/m3 and 0.541 US $/m3, 0.081 kg/m3 and 1.118 US $/m3, 0.106 kg/m3 and 1.813 US $/m3, 0.131 kg/m3 and 2.577 US $/m3 for Fe electrode, respectively. Figure 5 shows removal efficiencies of COD and TOC for Fe and Al electrodes at different operating times (0-30 min), 80 A/m2 and optimum pH values. The removal efficiencies of COD and TOC increased with the increase in the operating time which reached about 51-94% and 40-84% for Al electrode and 40-92% and 36-81% for Fe electrode and then COD and TOC removals for both electrodes almost remained constant at an operating time of 25 min. The amount of sludge, energy and electrode consumptions varied with increase in the operating time from 5 to 30 min were 0.018-0.095 kg/m3, 1.366-7.339 kWh/m3 and 0.061-0.314 kg/m3 for Al electrode and 0.016-0.143 kg/m3, 1.199-8.621 kWh/m3 and 0.051-0.634 kg/m3 for Fe electrode, respectively. Operating costs for Al and Fe electrodes were also calculated as 0.313-1.547 US $/m3 and 0.230-1.750 US $/m3 at an operating time of 0-30 min.

3.3. Comparison of treatment results for MWFs

A number of studies carried out for treating synthetic and waste MWFs are presented in Table 1. These studies were performed with simulated wastewater consisting of commercial MWFs and exhausted wastewaters from different metalworking processes. Hydrothermal oxidation was used to treat synthetic (COD of 160000 mg/L) and waste MWFs (COD of 1706 mg/L) (Sanchez-Oneto et al., 2007; Portela et al., 2001). The removal efficiency of over 97% was obtained at 500oC and 8.7-27 s. Ozone and aerobic hybrid processes (Jagadevan et al., 2013) managed to reduce COD from 3100 mg/L to 868 (72%) whereas aerobic and zero-valent nano Fe electron beam irradiation hybrid process (Thill et al., 2016) gave removal efficiency of 92.8% (COD reduced from 117180 to 8436 mg/L). Painmanakul et al. (2013) used combined advanced oxidation processes (AOP) such as ultrasonic-Fenton (P1), coagulation-ultrafiltration (UF)-Fenton (P2) and coagulation-UF (P3) to treat synthetic MWF having COD of 3051 mg/L and amongst the methods, P2 at 30 min gave a removal efficiency of 98.2%. UV-TiO2 (P1), UV/Fe2+-H2O2 (P2), bio-oxidation-UV/Fe2+-H2O2 (P3) and bio-oxidation-UV/TiO2 (P4) in AOPs were used to treat the MWFW with COD of 1050 mg/L and P3 gave highest removal efficiency of 92% and cost of 173 US $/m3 (MacAdam et al., 2012). Fenton and biological oxidation processes (COD of 11500 mg/L) achieved removal efficiency of 92.2% at steady state time of 20 days (Jagadevan et al., 2011). Other AOPs such as ultrasonication-Fenton (Seo et al., 2007), ultrafiltration-electrooxidation (Burke et al., 2004) and ultrafiltration-O3 (Chang et al., 2001) were tried to remove COD from MWFs and ultrasonication-Fenton process removed 98% of COD (reduced from 12358 to 248 mg/L). Table 2 shows treatment of MWFs by physicochemical process. Filtration-coagulation-UFpeat bed filtration (oil content of 22425 mg/L), packed bed reactor using peat (oil content of 890 mg/L), vacuum evaporation (COD of 22480 mg/L), distillation (COD of 66150 mg/L), destabilization-settling-UF-vacuum

evaporation

hybrid

(COD

of

67000

mg/L),

demulsification-reverse osmosis (COD of 156759 mg/L), ultrafiltration-microfiltration (COD

of 38000 mg/L), coagulation-nanofiltration (NF)-UF (TOC of 44209 mg/L), UF (COD of 10255925 mg/L) and microfiltration (COD of 12000-73800 mg/L) processes gave rise to removal efficiencies of 99.9%, 40%, 99.5%, 91.6%, 99.8%, 99.9%, 99.9%, 91.3%, 43.7% and 90.3%, respectively (Benito et al., 2002, Viraraghavan and Mathavan, 1990, Gutierrez et al., 2007, Canizares et al., 2004, Gutierrez et al., 2011, Zhang et al., 2008, Janknecht et al., 2004, Hilal et al., 2004, Muric et al., 2014, Schoeman and Novhe, 2007). Different biological (aerobic, anaerobic and combined processes) treatment processes were used to reduce the influent COD concentration (Table 3). Amongst the reactors giving in Table 3, semi-batch thermophilic aerobic bioreactor could remove 97.3% of COD (reduced from 230000 to 6210 mg/L) better than the rest, but the value of COD effluent was still high (Cheng et al., 2006). UF/reverse osmosis (RO), wet air oxidation and biological treatment were also used for the removal of COD from MWFs (Gutierrez et al., 2011; Zhang et al., 2008). However, wet air oxidation required high installation and operating costs as well as serious corrosion problems (Cheng et al., 2006). Likewise, many problems related to the long-term efficacy of the UF/RO membranes and issues related to the proper sizing of channels (lumens) within membrane modules remained. As different kinds of MWFs contained feed concentrations of COD in the range of 500000-1100000 mg/L for emulsions, 20000-55000 mg/L for semisynthetic and 25000-35000 mg/L for synthetic were treated with chemical or UF (COD reduced to 2000-9000 mg/L for emulsions, 3000-6000 mg/L for semi-synthetic and 2000-3000 mg/L for synthetic) or combined technologies (300-900 mg/L for emulsions, 300-1100 mg/L for semi-synthetic and 250-900 mg/L for synthetic). The operating costs of these fluids after the treatments remained in the range of 250-450 US $/m3 (Burke et al., 2004). Moreover, a few studies were reported on the application of EC for the treatment of MWFs (Table 4). The EC was performed to treat oily wastewater containing such as cutting oil-water emulsions, oil suspension used for machining and drilling operations, petroleum refinery in

great success. As pointed out by Sangal et al. (2013), the EC process using Al electrodes for treating of synthetic metal cutting solutions lowered the effluent COD concentrations from 9600 mg/L to 96 mg/L (removal efficiency of 99% in 3 h). Tir et al. (2004) achieved a removal efficiency of 90% in 2 min at 25 A/m2 for synthetic MFW solution containing COD of 62300 mg/L. Kobya et al. (2008) applied the EC process using Fe and Al electrodes at 60 A/m2 and 25 min for treatment of MWFs containing COD of 17312 mg/L and reached a removal efficiency of 92% for Fe and 93% for Al electrodes. Sludge production from optimized treatment of metal cutting wastewater by a batch EC reactor using response surface methodology was reported to be approximately 2% (Kobya et al., 2011). As seen in Table 1 and Table 4, there was limited work in the literature related to removal of real waste MWF by AOP and electrocoagulation techniques. Operational costs provided only with these studies in Tables 1 and 4. This study achieved better rate of removal efficiency with lowest operational cost as compared to the other works (Tables 1 and 4). The novel finding of this study was to treat real wastewater and provide comparisons based on the operational cost to the literature. The EC process showed more promising results for treatment of MWFs since it was regarded as a flexible, compact and fairly less expensive technology (Table 1-4).

Conclusions COD and TOC removal efficiencies by chemical coagulation from the MWFW at optimum conditions (500 mg/L of coagulant dosage, a pH of 6.5 for aluminium-based coagulants, a pH of 7.5 for ferric-based coagulants) were determined as 97% and 81% for alum, 96% and 80% for aluminium chloride, 91% and 76% for ferric sulphate, 93% and 77% for ferric chloride, respectively. The CC results showed a high removal of COD and TOC by the aluminium-based coagulants removing a greater proportion than the ferric-based ones. Considering the treatment efficiency (residuals of 520 mg/L for COD and 504 mg/L for TOC) and landfill disposal of

treated sludge (0.22 kg/m3) together for the treatment of the wastewater using chemical coagulants, the minimum OC of 0.12 US $/m3 was obtained by alum. Maximum COD and TOC removal efficiencies by the EC process were observed at a pH of 5 for Al electrode and a pH of 7 for Fe electrode. The optimum current density for both electrodes was 80 A/m2 since the highest removals of COD (94% for Al and 90% for Fe) and TOC (83% for Al and 80% for Fe) were observed at this value. Operating costs at 80 A/m2 were calculated as 1.190 US $/m3 for Al electrode and 1.813 US $/m3 for Fe electrode. Continual research using this technique can not only improve its efficiency, but new modelling techniques can also be used to predict many factors and develop equations that will predict the effectiveness of treatment.

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100

COD removal efficiency (%)

90 80 70 60

Al2(SO4)3 AlCl3

50

Fe2(SO4)3 FeCl3

40 0

100 200 300 400 500 600 700 800 900 1000 1100 Coagulant dosage (mg/L)

(a)

85 80 TOC removal efficiency (%)

75 70 65 60 55 50

Al2(SO4)3

45

AlCl3

40

Fe2(SO4)3

35

FeCl3

30 0

100 200 300 400 500 600 700 800 900 1000 1100 Coagulant dosage (mg/L)

(b) Fig. 1. Variations in (a) COD and (b) TOC removals at different coagulant dosages from the MWFW (50-1000 mg/L, ferric-based coagulants at a pH of 7.5, aluminium-based coagulants at a pH of 6.5).

100

COD removal efficiency (%)

90 80 70 60 Al2(SO4)3

50

AlCl3 Fe2(SO4)3

40 30 4,0

FeCl3 4,5

5,0

5,5

6,0

6,5 pH

(a)

7,0

7,5

8,0

8,5

9,0

90

TOC removal efficiency (%)

80 70 60 50 Al2(SO4)3

40

AlCl3 Fe2(SO4)3

30 20 4,0

FeCl3 4,5

5,0

5,5

6,0

6,5

7,0

7,5

8,0

8,5

9,0

pH

(b) Fig. 2. Variations in (a) COD and (b) TOC removals at different pHs from the MWFW (coagulant dosage of 500 mg/L).

100

Al electrode Fe electrode

COD removal efficiency (%)

80

60

40

20

0 3

4

5

6 Initial pH

(a)

7

8

100 Al electrode Fe electrode TOC removal efficiency (%)

80

60

40

20

0 3

4

5

6

7

8

Initial pH

(b) Fig. 3. The effect of initial pH on (a) COD and (b) TOC removals from the MWFW (pH: 3-8, a current density of 80 A/m2 and an operating time of 25 min).

100

Al electrode Fe electrode

COD removal efficiency (%)

80

60

40

20

0 20

40

60

80 2

Current density (A/m )

(a)

100

100 Al electrode Fe electrode

TOC removal efficiency (%)

80

60

40

20

0 20

40

60

80

100

2

Current density (A/m )

(b) Fig. 4. The effect of current density on (a) COD and (b) TOC removals from the MWFW (25 min, a pH of 5 for Al and a pH of 7 for Fe).

100 Al electrode Fe electrode

COD removal efficiency (%)

80

60

40

20

0 5

10

15

20

EC time (min)

(a)

25

30

100

Al electrode Fe electrode

TOC removal efficiency (%)

80

60

40

20

0 5

10

15

20

25

30

EC time (min)

(b) Fig. 5. The effect of operating time on (a) COD and (b) TOC removals from the MWFW (a current density of 80 A/m2, a pH of 5 for Al and a pH of 7 for Fe).

Table 1. Comparison of results for the treatment of MWFs by AOPs and AOPs-combined process. Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

References

Hydrothermal oxidation

Synthetic MWFs solution: Servol and Biocut oils, pH = 8.6 

COD and TOC = 97%

Sanchez-

0.2, Conductivity = 3.5  0.9 mS/cm, COD = 160000  30000

Oneto et al.,

mg/L.

2007

Optimum conditions: T = 500oC, Pressure = 25 MPa, Reaction time = 8.7 s (Servol) and 27 s (Biocut). Hydrothermal oxidation

MWFs wastewater: COD = 1633-2882 mg/L, TOC = 447-724 COD = 38.7–97.4%

Portela et al.,

mg/L.

2001

TOC = 30.7-98%

Optimum conditions: COD = 1706 mg/L, TOC = 447 mg/L, T = 500oC, Amount of H2O2 (w/v) = 1 mL/L MWFs, Catalyst = COD =97.4% (optimum) 4  10-3 M KOH, Reaction time = 9.7 s.

TOC = 98% (optimum)

Ozone (O3) + Aerobic hybrid process MWFs wastewater: pH = 7.32, COD = 3100 mg/L, TOC = 675 COD = 72% mg/L.

COD = 26.9% (only O3)

Optimum conditions: pH = 9, Cozone = 2500 mg/L, T = 20oC.

COD = 44.9% (only Aerobic)

Jagadevan et al., 2013

AOPs combined processes

Synthetic MWFs solution: Castrol Clean-cut, pH = 7.4, COD = COD = 91.3 % (P1)

Painmanakul

P1: Ultrasonic(US)-Fenton

3051  120 mg/L, Turbidity = 1356  56 NTU.

et al., 2013

P2: Coagulation/UF-Fenton

Optimum conditions: For P1: [Fe2+] = 500 mg/L, [H2O2] = 14 COD = 80% (P3)

P3: Coagulation/UF

g/L, [Fe2+/H2O2] = 1:28, pH = 1.7, time = 1 h, US power = 400 W and 28 kHz; For P2: [Fe2+] = 500 mg/L, [H2O2] = 5 g/L, [Fe2+/H2O2] = 1:10, pH = 1.7, time = 30 min.

COD = 98.2% (P2)

Table 1. Comparison of results for the treatment of MWFs by AOPs and AOPs-combined process (continued). Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

References

AOP and combined process

MWFs wastewater: COD = 1050 mg/L.

COD = 82%, OC = 85 $/m3 (for P1)

MacAdam et al.,

P1: UV/TiO2

Optimum conditions: For P1: Retention time = 20 min, COD = 85%, OC = 107 $/m3 (for P2).

P2: UV/Fe2+/H2O2

Aeration rate = 10 L/min, Irradiation time = 0.33 h, pH COD = 92%, OC = 173 $/m3 (for P3)

P3: Bio-oxidation + UV/Fe2+/H2O2

3 = 9, [TiO2] = 10 g/L); For P2: [Fe2+] = 0.165 g/L, COD = 90%, OC = 515 $/m (for P4)

P4: Bio-oxidation + UV/TiO2

[H2O2] = 4 g/L, pH =3, UV-dosage = 20 J/cm2,

2012

Irradiation time = 3 h); For P3: Biodegradation time = 20 day, Irradiation time = 5 h, For P4: Irradiation time = 5 h. Aerobic + Zero-valent nano Fe + MWFs wastewaters: P1 for: pH = 7.9  0.5, COD = COD = 92.8%  1.4% (P1) electron beam irradiation hybrid 117180  5680 mg/L, TOC = 22132  4574 mg/L; For COD = 85.9%  3.4% (P2) process

P2: pH = 7.5  0.1, COD = 19188  151 mg/L, TOC = 3326  133 mg/L; the pristine MWFs (P1), the exhausted MWFs (P2). Optimum conditions: Acclimation period = 21 days and airflow rate = 0.4 L/min for bio-oxidation process, Zero Fe dosage = 10 g/L and diameter = 1-100 nm (BET area = 17.63 m2/g) for zero-valent Fe process, ebeam dosage = 50.1  2.4 kGy.

Thill et al., 2016

Table 1. Comparison of results for the treatment of MWFs by AOPs and AOPs-combined process (continued). Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

References

Fenton + Biological oxidation

MWFs wastewater: pH = 7.4, COD = 11500 mg/L, COD = 64.73% (P1) and 92.2% (P2)

Jagadevan et al.,

P1: Fenton

TOC = 2100 mg/L, BOD5 = 1840 mg/L.

2011

P2: Combined process

Optimum conditions: [Fe2+] = 5.62 mM, [H2O2] =

TOC = 55.3% (P1) and 85.7% (P2)

14.92 mM, Air flow = 150 L/min, T = 28oC, Steady state time = 20 days. Ultrasonication-Fenton process

MWFs wastewater: pH = 9.2, Total-N = 690 mg/L, COD = 98% Total-P = 3.51 mg/L, COD = 12358 mg/L,

Seo et al., 2007

Total N = 75%

Optimum conditions: [H2O2] = 10%, [FeSO4] = 3 g/L, Total-P = 95% pH = 3, Ultrasonication time = 30 min Ultrafiltration + Electro-oxidation

Synthetic MWFs solution: COD = 16000 mg/L, BOD5 COD = 80%, BOD5 = 60% = 6080 mg/L, Phenol = 350 mg/L.

Burke et al., 2004

Phenol = 99.9%, OC = 12-17 $/m3

Optimum conditions: Electro-oxidation time = 7-8 h. Ultrafiltration + O3 process

MWFs wastewater: COD = 102400 mg/L, TOC = COD = 93.2% 28200 mg/L. Optimum conditions: Flux = 53 L/m2 h, O3 dosage = 0.4 g/L.

TOC = 93.5%

Chang et al., 2001

Table 2. Comparison of results for the treatment of MWFs by physicochemical process. Treatment Processes

Source of waste MWFs and optimum conditions

Filtration/coagulation+Ultrafiltration

MWFs wastewater: pH = 9-10, Oil content = 22425 Oil = 99.87%

+peat bed filtration (The modular mg/L, COD = 65000 mg/L. pilot plant)

Removal efficiency and cost

References Benito et al., 2002

COD = 90%

Operating conditions: Tubular ceramic membrane pore diameter = 50 nm, Membrane area = 1.7 m2, Trans-membrane pressure = 2 bar, Coagulant concentration = 1000 mg/L, Peat bed filter: height = 0.3 m, diameter = 1 m, filtration surface = 0.785 m2.

Packed bed reactor using peat

Synthetic MWFs solution: pH = 8.25, Coil = 890 mg/L. Oil = 21-40% (for 890 mg/L)

Viraraghavan and

Optimum conditions: Equilibrium time = 2 h, BET Oil = 87% (for 115 mg/L)

Mathavan, 1990

isotherm: qe = 70.4-75.7 mg/g peat for Coil = 890 mg/L and qe = 233.6 mg/g peat for Coil = 115 mg/L. Vacuum evaporation

Synthetic MWFs solution: COD = 22480 mg/L.

COD = >99.5%

Optimum conditions: Operating pressure = 40 kPa,

Gutierrez et al., 2007

bath temperature = 180 oC, non-ionic surfactant (Brij76) = 200 mg/L. Distillation process

MWFs wastewater: pH = 8.8, Total COD = 66150 COD = 91.6% mg/L and soluble 6200 mg/L, TOC = 17,680 mg/L. Optimum conditions: 70 mmHg, T = 150oC

OC = 57.4 $/m3

Canizares et al., 2004

Table 2. Comparison of results for the treatment of MWFs by physicochemical process (continued). Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

References

Destabilization/settling + UF +

MWFs wastewater: pH = 7.05, Conductivity = 1411 COD = 98.7 mg/L

Gutierrez et al.,

Vacuum evaporation hybrid process

μS/cm , COD = 67000 mg/L, Turbidity = 5570 NTU. NTU = 99.7%

2011

Optimum conditions: Coagulant dosage = 0.05 M CaCl2, Operating pressure = 10 kPa, Vapour temperature = 54oC, Bath temperature = 134oC, Evaporation rate = 24  102 L/h, Flat 30nm TiO2 ceramic membrane (  P = 0.1 MPa). Demulsification + Reverse osmosis

MWFs wastewater: pH = 6.4, COD = 156759 mg/L, Demulsification stage: COD = 94.9%, Zhang et al., 2008 Coil = 36,000 mg/L, Turbidity = 23018 NTU.

NTU = 97.1% and Oil = 100%

Optimum conditions: For Demulsification stage: Reverse osmosis (RO) stage: COD = Demulsifer dosage = 0.1% (w/v), Operating time = 30- 99.2%, NTU = 100% and Oil = 100% 50 min, T = 80-85 °C; For RO stage: Operating time = 20 h, Driving pressure = 3.6 MPa, T = 35-40oC, Combined process: COD = 99.96%, Average flux of permeate = 15.2-15.8 L/m2 h, Oil and NTU = 100% Recovery rate of water = 80%. Ultrafiltration and Microfiltration

Synthetic MWFs solution: Oil = 38000 mg/L. Optimum

conditions:

T=22oC,

Transmembrane pressure = 2-7 bar

1-60

Oil = 99.9% kDa,

Janknecht et al., 2004

Table 2. Comparison of results for the treatment of MWFs by physicochemical process (continued). Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

Coagulation+NF+UF processes

MWFs wastewater: Mobilcut 232, pH = 8.6, TOC = TOC = 87.1% and pH = 7.4 (P1)

P1: Coagulation

44209 mg/L.

References Hilal et al., 2004

TOC = 91.3% and pH = 7.4 (P2)

P2: Coagulation + Nanofiltration Optimum conditions: pH = 8.5, Coagulant dosage = TOC = 87.9% and pH = 8.6 (P3) (NF)

324 mg Al3+/L, Flat sheet NF membrane (BM-20D,

P3: Ultrafiltration (UF)

2000 Da, 476 kN/m2), Effective membrane area = 4.1 cm2, and flux = 6.65 0.73 L/m2 h; Polysulfonate UF membrane (100000 Da) and filtration pressure = 150 kN/m2.

Ultrafiltration process

Synthetic MWFs solution: Ultra Safe 620 oil, Oil COD = 4.02-43.68 (HFPM) concentration = 1% (COD = 1025-5925 mg/L). Optimum membrane

conditions: (HFPM),

Hollow

fibre

Multi-channel

Muric et al., 2014

COD = 0.77-42.3% (CM)

polymeric ceramic

membrane (CM), Operating time = 1 h, pH = 5-9, Inlet pressure = 3 bar Microfiltration

MWFs wastewater: pH = 7.86-9.21, Conductivity = COD = 75.5-90.3%

Schoeman

161-393 mS/cm, COD = 12000-73800 mg/L, O&G = O&G = 97.2-99.7%

Novhe, 2007

13690-19794 mg/L. Optimum

conditions:

Membralox

MF

ceramic

membrane, T = 40oC, permeate flux=78-264 L/h m2.

and

Table 3. Comparison of results for the treatment of MWFs by biological and combined processes. Treatment Processes Submerged

anaerobic

Source of waste MWFs and optimum conditions

Removal efficiency and cost

membrane Synthetic MWFs solution: pH = 8, Oil content = 0.1- TOC= 94.5% batch)

bioreactor

References Teli et al., 2015

1% w/w (Cooledge BI, Castrol, UK), TOC = 725- TOC = 97.9% (continuous) 7250 mg/L, COD = 2,830-28,300 mg/L. Operating conditions: MLSS = 4300 mg/L, MWF content = 0.1-1% w/w, T = 30oC, Operating time = 0-20 days (batch) and 20-70 days (continuous), TMP = 0-2.5 kPa.

Aerobic (P1) and electrocoagulation MWFs wastewater: COD = 14200  950 mg/L, COD = 87.3%, and BOD5 = 97.4% (P1)

Muszynski et al.,

(P2) processes

COD = 81.7%, and BOD5 = 99.3% (P2)

2007

COD = 97.3%

Cheng et al., 2006

COD = 85%

(Van der Gast et

BOD5 = 5400  620 mg/L. Optimum conditions: Al electrodes and current = 100 A.

Semi-batch bioreactor

thermophilic

aerobic MWFs wastewater: COD = 230000 mg/L. Optimum conditions: Operational time = 77.5 h, T = 50oC, pH = 6.8-7.2, removal rate = 2877 mg COD /h).

Aerobic process (using bacterial Synthetic MWFs solution: COD = 48000 mg/L. consortium)

Optimum conditions: Operating time = 400 h, Qair = 200 L/min, T = 28  1oC.

al.,2004)

Table 3. Comparison of results for the treatment of MWFs by biological and combined processes (continued). Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

References

Aerobic treatment

Synthetic MWFs solution: COD = 13,500 mg/L (P1: COD = 34  4% (P1)

Connolly et al.,

with polymer) and 9980 mg/L (P2: without COD = 30  4% (P2)

2006

polymer). Optimum conditions: Air flow = 200 L/min, T = 28oC, Hydraulic retention time = 21 days. Anaerobic sequencing batch biofilm Synthetic MWFs: COD = 500-2000 mg/L. reactor

COD = 87-80%

Optimum conditions: Total operating time = 12 days

Carvalhinha et al., 2010

(36 cycles), Fed-batch feeding time = 4 h, T = 30 °C, Organic load = 6.1 g COD/L day, Alkalinity = 200 mg/L NaHCO3, Produced CH4 = 5.2 mmol/L Up flow anaerobic fixed-film reactor

The mixed vinasses + MWFs wastewater: COD = COD = 85.8% and TOC = 58.1% (P1) 2500 kg O2/m3 and TOC = 108.6 mg/L (the cutting COD = 87% and TOC = 94.6% (P2)

P1: mixed feed composed of wine oil wastewater), COD = 10214-19780 and TOC = vinasses and cutting oil wastewater. 5163-3640 kg O2/m3 (the vinasse wastewater). P2: feed composed of wine vinasses.

Optimum conditions: T = 55oC, pH = 7-8, Organic loading rate = 16.7 (P1) and 22.3 kg COD/m3 days (P2), Hydraulic retention time = 0.15 (P1) and 0.8 days (P2).

Perez et al., 2006

Table 4. Comparison of results for the treatment of MWFs by electrocoagulation process. Treatment Processes

Source of waste MWFs and optimum conditions

Removal efficiency and cost

Electrocoagulation process

Synthetic MWFs solution: Hysol-X cutting oil, pH = Oil = >99%

(Al anodes)

6.5, Conductivity = 4.32 mS/cm, Coil = 9600 mg/L,

References Sangal et al., 2013

Turbidity = 2960 NTU. Optimum conditions: Current density = 138.8 A/m2 (i = 3.5 A), T = 20 °C, pH = 6.5, Operating time = 3 h, Cenergy = 9.88 kWh/m3. Electrocoagulation process

Synthetic MWFs solution: Tasfalout 22 B cutting oil, COD = 90%

Tir and Moulai-

(Al anodes)

pH = 8.65, Conductivity = 1.12 mS/cm, COD = NTU = 99%

Mostefa, 2008

62300 mg/L, Turbidity = 29700 NTU. Optimum conditions: Aluminium (Al) electrodes, Operating time = 2 min, pH = 5-8, Current density = 25 mA/cm2. Electrocoagulation process

MWFs wastewater: pH = 7.1, COD = 17312 mg/L, COD = 93% and TOC = 80% (For Al)

(Al and Fe anodes)

TOC = 3155 mg/L

Kobya et al., 2008

COD = 92% and TOC = 82% (For Fe)

Optimum conditions: pH = 5(Al) and 7(Fe), Current OC = 0.497 $/m3 (For Fe) density = 60 A/m2, Operating time = 25 min. wastewater:

Railroad

industry

OC = 0.768 $/m3 (For Al)

Electrocoagulation process

MWFs

oily O&G = 70%, COD = 48%

Cheryan

(Al anodes)

wastewater, COD = 1500 mg/L, TSS = 100 mg/L, OC = 1.99 $/m3

Rajagopalan,

Oil &grease (O&G) = 100 mg/L.

1998

and