Electric field assisted membrane separation for oily wastewater with a novel and cost-effective electrocoagulation and electroflotation enhanced membrane module (ECEFMM)

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Chemical Engineering & Processing: Process Intensi fi cation

journal homepage:www.elsevier.com/locate/cep

Electric fi eld assisted membrane separation for oily wastewater with a novel and cost-e ff ective electrocoagulation and electro fl otation enhanced

membrane module (ECEFMM)

Ankita Mazumder

^a

, Zinnia Chowdhury

^a

, Dwaipayan Sen

^b,

*, Chiranjib Bhattacharjee

^a

aChemical Engineering Department, Jadavpur University, Kolkata 700032, India

bDepartment of Chemical Engineering, Heritage Institute of Technology, Kolkata 700107, India

A R T I C L E I N F O

Keywords:

Electrocoagulation and electroflotation enhanced membrane module (ECEFMM) PES membrane

PSf membrane Oily wastewater Permeateflux

A B S T R A C T

The present study discusses the simultaneous application of electricfield during membrane separation of oil from oily wastewater on eliminating the fouling propensity over the membrane. A novel hybrid electro- coagulation and electroflotation enhanced membrane module (ECEFMM) was proposed here to expect the benefits of electrocoagulation and electroflotation with the membrane separation. The synergistic effect of ap- plied voltage and membrane operation facilitates demulsification for emulsified oily wastewater along with substantial turbulence creation over the membrane surface through hydrogen bubbling. Such turbulence reduces the deposition over the membrane surface that eventually restricts the permeateflux decrease by 43–72 %.

Voltage was applied both continuously and periodically, along with a gradual voltage increment in each mode.

Further with both the mode of applied voltage, polyethersulfone (PES) and polysulfone (PSf) membrane were employed with each of the modes. It was observed that the permeation of the PES membrane is not substantially affected by the mode of the applied voltage. On the contrary, with relatively low hydrophilic PSf membrane, the permeation is highly dependent on the applied voltage and its mode of application. The minimumflux decline for PSf membrane can be achieved with periodical voltage application at 10 V with substantial oil rejection of 94–96 %.

1. Introduction

Industrial growth along with urbanization has substantially in- creased the consumption of oil that eventually raises the different forms of oil pollution due to either lack of conscience among the people or negligence by them. Oil, being an acrimonious pollutant, imposes po- tential adverse impacts on the environment. Especially, the pollution of water with oil becomes intricate to treat, when oil is being mixed with water to form the emulsion. Oil-water emulsions discharged from sev- eral industrial works in automotive shops, machining, metal processing industry, offshore oil explorations, refineries, oil-drilling, oil transpor- tation, and oil distribution have been the major sources of pollution and eventually became an unavoidable environmental threat [1–5]. Thus, the major concern is the discharge of oil into the environment in their different forms like free (oil and grease), dispersed and emulsions.

Moreover, the stability of emulsified form is largely depending on the

particle size of dispersed oil droplets in water and is a big challenge to treat for the separation of oil from water.

So far, different technologies have already been studied for the treatment of such emulsified oily wastewater. Among them the mostly used technologies are coagulation [6], flotation/flocculation [7], salting-out process [8], membrane separation [9], biological treatment [10,11] and oil adsorption [12]. However, all these technologies even become limited in treating the highly stable emulsion to effectively separate oil from oil-water biphasic mixture. Dissolved airflotation (DAF) is another mostly used technology for the oily wastewater ef- fluent treatment that allows the increased rate of oil-water separation by destabilizing oil-water followed by separation of oil. The efficiency of DAF units depends on the buoyancy force created by the purged air from the bottom of the tank and the added chemicals to facilitate coagulation/flocculation. However, the addition of chemicals not only makes the expensive technology but also non-ecofriendly for further

https://doi.org/10.1016/j.cep.2020.107918

Received 2 August 2019; Received in revised form 14 March 2020; Accepted 31 March 2020

Abbreviations:ECEFMM, Electrocoagulation and electroflotation enhanced membrane module; PES, Polyethersulfone; PSf, Polysulfone; DAF, Dissolved airflotation;

SDS, Sodium dodecyl sulfate; SS, Stainless steel; TPH, Total petroleum hydrocarbons; TMP, Trans-membrane pressure; DC, Direct current; TFMM, Turbineflow membrane module; RFMM, Radialflow membrane module; CI, Confidence interval; SCE, Saturated calomel electrode; ANOVA, Analysis of variance

⁎Corresponding author.

E-mail addresses:dwaipayanju@gmail.com,dwaipayan.sen@heritageit.edu(D. Sen).

Chemical Engineering & Processing: Process Intensification 151 (2020) 107918

Available online 23 April 2020

0255-2701/ © 2020 Elsevier B.V. All rights reserved.

T

processing of the chemicals used. Among the aforesaid technologies, membrane technology has evolved as an effective separation tool be- cause of its high oil removal capacity, low-energy consumption and compact design compared to robust space-consuming traditional pro- cesses. However, the efficiency in terms of running cost might be compromised due to fouling of the membrane that reduces its throughput and longevity. Studies have shown that even with the high water permeability, slow diffusing oil molecules enhances oil con- centration adjacent to the membrane surface by around ten folds [13].

Such increased concentration of oil near the membrane surface pro- motes oil adsorption at the surface and within the pores as well. Though periodical hydraulic cleaning can reduce fouling to an extent, irrever- sible fouling caused by oil adsorption and pore plugging cannot be completely eliminated without further chemical cleaning. However, chemical cleaning of a membrane may be limited, as repeated cleaning involves a high operational cost of the membrane process and the membrane replacement cost at the same time. Therefore, often prior to membrane separation, a primary and secondary treatment process usually employed to reduce membrane fouling followed byflux decline.

In case with the oily wastewater, though dissolved airfloatation is the conventional one for the separation of oil and dirt from water asflocs, it might not absolutely guarantee the complete demulsification, which ultimately fails to reduce oil contaminants loads from the water.

In this context, prior to membrane separation, electrocoagulation can be used as an effective pretreatment technology providing better demulsification in a shorter duration of time without using any addi- tional chemical coagulant. Bensadok et al. [14] applied electro- coagulation to de-emulsify cutting oil emulsions and separate oil from water. According to them, sole electrocoagulation treatment was not sufficient to bring down the pollutant load below the permissible limit but can be effectively used as pretreatment step followed by a mem- brane treatment for the effective oil separation. In the subsequent year, Li et al. [15] adopted a similar approach, where the oily wastewater was treated using combined electrocoagulation and ultrafiltration, manifesting the deduction of total fouling ratio, defined as the per- centage reduction in water flux compared to initial waterflux, from 78.9 % to 33.3 %. In a recent study, Changmai et al. [1] reported that with electrocoagulation as a pretreatment step, around 70 % removal of oil and grease from wastewater reduce oil load on the membrane sur- face and eventually reducing the fouling tendency. Apart from the several advantages with electrocoagulation as a pretreatment step, high capital investment and more importantly, the requirement of the large installation area for both primary and tertiary unit operations was the major limitation. Moreover, when electrocoagulation and membrane operation are carried out separately followed by one other, the effect of

the applied electricfield for controlling membrane fouling cannot be properly utilized. It has been reported that applying an electricfield across the membrane will reduce membrane fouling and increase permeateflux [16,17]. The charged contaminants can get repelled from the membrane surface depending on the surface charge of the mem- brane because electrostatic repulsive forces, which alleviates the fouling under the electricfield [18,19]. Moreover, because of theflocs polarization and aggregation under the influence of the applied electric field, the polarized cake layer is formed over the membrane, which was found to be crucial to control the permeateflux decline. The porosity of thisflocs’cake layer largely depends on theflocs’characteristics such as size, irregularity and its stability in the presence of any shear stress [20–22]. One of the most common techniques in reducing the fouling of the membrane is increasing the shear rate or turbulence over the membrane surface, which ultimately reduces the fouling after sweeping away the adsorbed oil from the surface. Thus, with simultaneous ap- plication of both the electrocoagulation and membrane operation, the hydrogen gas generated at the cathode creates an intensified turbulence on the membrane surface that reduces the cake layer over the surface.

Henceforth, such electroflotation because of hydrogen evolution will be an additional benefit in alleviating the membrane fouling.

With the objective of increasing sustainability, process industries are recently attracted towards the approach of process intensification for lower space and energy consumption, better performance, reduced capital cost, minimal waste generation leading to decreased environ- mental hazards [23,24]. In case of oily wastewater treatment, Krystynik et al. [25] have carried out a process intensification study through the integration of two different treatment technologies, namely, electro- coagulation and photo-oxidation in a single system. The study revealed that the integrated system has removed almost 100 % contaminants from wastewater which was not possible by any of the mentioned in- dividual treatment methods. However, the process becomes less eco- nomic because of inclusion of photo-oxidation process. A similar ap- proach was tried by Huang et al. [26] in 2018, where the unification of cyclonic separation with airflotation separation was done in a single floatation column for treatment of oily sewage which efficiently re- moved 97 % of the total oil from water. Industrial wastewater treatment fraternities are also equally interested in the development and im- plementation of the process intensified treatment technologies for ef- ficient handling of wastewater [27]. Recently, Akvola Technologies has entrenched an integratedflotation-filtration system for processing oily wastewater with the intention of reducing energy consumption [28–30]. Similarly, CDM Smith has combined multiple commercially available technologies such as microfiltration, ion exchange, UV dis- infection and reverse osmosis for the efficient separation of produced Nomenclature

N_H2 Nucleation rate for hydrogen bubble

z Number of electrons required to produce 1 mol of gas Fconstant Faraday’s constant (Cmol⁻¹)

F Force imparted by hydrogen bubble on the oil droplet CD Drag coefficient

v Velocity of the hydrogen bubble (m.s⁻¹) Re Reynolds number

I Current (Amp)

A Electrode surface area (m²) d Corrosion rate (mil per year) Icorr Corrosion current density

EW Equivalent weight of aluminum electrode L Length of the electrode (m)

Vw Volume of the wastewater treated (m³) N Number of the treatment cycles in a day

CR Replacement cost of aluminum electrode (INR.m⁻³)

EAl The aluminum electrode cost per kg (INR.kg⁻¹) V Applied voltage of the cell (V)

t Operational time (h)

EE Local electrical unit charges (INR.kWh⁻¹) CE Electrical cost for running the cell (INR.m⁻³) Doil Oil droplet diameter (m)

Greek letters

ρAl Density of the aluminum electrode (kg.m⁻³) θ Contact angle (o)

φ Angle of the cone created after contact (o)

σxy Interfacial surface tension between x and y component in the system

ρL Density of the liquid ρG Density of the hydrogen gas μ Viscosity of the liquid

water. Almost 85 % freshwater recovery was achieved through this technology [30,31].

Considering the aspects of process intensification, the present study is thus focusing on the synergistic effect of electrocoagulation and electroflotation along with membrane operation in a single integrated set-up called ECEFMM. The integration of electrochemical cell and membrane module in a single hybrid setup will be a significant process intensification approach as this will substantially reduce the capital investment cost by obsoleting an additional pretreatment (electro- coagulation) setup and thereby lowering the installation area require- ment. Moreover, the voltage application within the electrolytic cell restricts the permeateflux drop during oily wastewater treatment be- cause of the turbulence created due to the evolution of hydrogen gas at the cathode. This eventually results in less membrane fouling in the hybrid technology unlikely to any other conventional membrane se- paration. Because of the low fouling propensity, membrane aging will be restricted for a prolonged time period, demanding less frequent membrane replacement. Thus, the development of novel ECEFMM module for handling oily wastewater treatment will be a process in- tensified technology aided with the benefits of lower space require- ment, reduced operational step, less energy intensive and lower op- erational expenses. As an important consideration, the effect of the applied voltage, and its mode of application on the permeation was enumerated with membranes like PES and PSf. Further, the study was extended to determine the corrosion rate of the anode along with an understanding of its effect on the membrane module’s operating cost.

2. Materials

2.1. Chemicals and reagents

Detergent was procured from a local grocery shop and spent oil was collected from local car service stations. Ultrapure deionised water was collected from the hybrid Arium RO unit and the Arium 611DI ultra- pure water system (Sartorius, Göttingen, Germany). Sodium dodecyl sulfate (SDS) (CAS No. 151-21-3), sodium hypochlorite (NaOCl) (CAS No. 7681-52-9), sodium hydroxide (NaOH) (CAS No. 1310-73-2) and ethanol (CAS No. 64-17-5) were purchased from Merck (Mumbai, India).

2.2. Electrocoagulation and electroflotation enhanced membrane module (ECEFMM)

Fig. 1(a) and (b) shows the schematic representation of the ECEFMM and the entire process respectively. The membrane module (C) was fitted with an electrical circuit (D and E) (Fig. 1b), where aluminum (Al) anode and stainless steel (SS) cathode, separated by 0.02 m electrode distance, were used in the cell. The module was made up of SS316 with a volumetric capacity of 0.005 m³(5 L) and was procured from Concept International Ltd, Kolkata, India. To supply the feed from a jacketed feed tank (A) (capacity: 0.02 m³or 20 L) a magnetic pump (B) (manufactured by DEV Pumps Pvt. Ltd., Gujarat, 0.12 HP; max- imum head: 3 m) wasfitted to the set-up. The temperature of the feed solution was maintained at a temperature of around 25–35 °C during all the experiments. The pressure over the membrane was provided by an air compressor (G). PES and PSf membranes of 5 kDa molecular weight cut-off(MWCO) (0.078 m effective diameter of the membrane) were procured from Sterlitech Corporation, USA. The quantification of total petroleum hydrocarbons (TPH) was done according to standard ASTM D3173-75 methods.

3. Methods

3.1. Preparation of feedstock

Two different types of feedstock solutions (emulsified oily

wastewater and a mixture of emulsion along with free oil) were pre- pared of 0.001 m³(1 L) volume each. Emulsified wastewater was pre- pared by mixing spent oil with water in the ratio of 1:200 aided by a 1

% w/v SDS acting as an emulsifier. While the synthetic mixture of free oil and emulsion was formulated after mixing free spent oil and emulsion with water in the ratio of 1:1.

3.2. Membrane compaction

Both the membranes werefitted to the membrane module for water compaction at a trans-membrane pressure (TMP) (ΔP) of 0.5 MPa, which is greater than that of the experimental TMP (ΔP= 0.3 MPa) to prevent any continuous deformation within the membrane structure during experiments. The compaction of the membrane was carried out for around 2 h beyond which the permeateflux becomes stable attri- buting to complete compaction of the membrane. It was seen that the membrane hydraulic resistance for PES and PSf membrane was 3.7 × 10¹³m⁻¹and 6.5 × 10¹³m⁻¹respectively.

3.3. Application of electricfield along with simultaneous membrane separation

The feed solutions viz. emulsified oily wastewater and the mixture (emulsion along with free oil) were separately introduced to the ECEFMM. To study the effect of electrolysis on the performance of membrane operation, the sole membrane operation without electrolytic cell was carried out with the feedstock as a control. During electrolysis, Al³⁺ion came out into the solution and Al(OH)3precipitated out as the

Fig. 1.(a) Schematic diagram for the working principle of ECEFMM; (b) Schematic processflow diagram for the operation of ECEFMM (A: jacketed feed tank; B: magnetic pump; C: membrane module; D: electrical circuit and E:

electrical circuit; F: feed line; G: air compressor; R: retentate; P: permeate; V:

voltmeter).

coagulant (Eqs.1and 2). Simultaneously hydrogen was generated on SS electrode (Eq. 3), which moved upward through the solution and at the same time, part of it was being dissolved during its upward movement.

Such a movement of the hydrogen gas bubble through the solution creates turbulence within the solution and subsequently, on the mem- brane surface. Therefore, in order to study the effect of such hydrogen bubbling on the membrane surface, two different modes of electricfield application were investigated. In one mode, the cell was continuously supplied with direct current (DC) along with the membrane separation.

In the other mode, the supply was connected to the cell periodically for 30 min along with the membrane separation with a subsequent cease in the electrical supply for 30 min. Both the modes were carried out for 3 h. 5, 10 and 15 V DC voltages were applied to the cell in both the modes of application. All the experiments were carried out in triplicate to assess the reproducibility of the result.

Reactions at the anode

→ ⁺aq + e⁻

Al(s) Al (³ ) 3 (1)

→ + ⁺+ ⁻

2H O₂ O (g) 4H₂ 4e (2)

Reactions at the cathode + ⁻→ + ⁻

2H O 2e₂ H (g) 2OH (aq)₂ (3)

The permeateflux is shown as the normalization of permeateflux (NJ) as mentioned in the Eq.(4),

= NJ J

J

t

0 (4)

where NJ, Jt and J0 represented the normalized permeate flux, the permeateflux at any time‘t’and the initial permeateflux at time‘0′, respectively. For the determination of normalized steady state flux (NJS), Jtwas considered as the steady state permeateflux in Eq.(4) during oil-water separation with membrane.

3.4. Membrane cleaning protocol

Initially, the membrane was washed with 1 (N) NaOH and 2 % (w/

v) NaOCl solution that recovered around 85–90 % of the virgin mem- brane water flux. Subsequently, the entire cell was filled up with a second washing solution comprising of 12 mM SDS, 1 N NaOH and 2 % (w/v) NaOCl so that the membrane remains dipped in it. The primary washing was continued for an average duration of 1 h until the desired flux recovery was achieved, while the secondary wash was carried out for 30 min approximately. The membrane was stored in 10 % (v/v) ethanol solution until further usage of it to avoid the formation of any biofilm on the membrane surface.

3.5. Corrosion analysis

Potentiodynamic polarization test was carried out to evaluate the passivity of the aluminum anode with a three-electrode cell using po- tentiostat (Make: M/S Concept International). Saturated calomel elec- trode (SCE) was used as the reference electrode within this three electrode test cell. The voltage was varied from−5 V to +5 V with a scanning rate of 1 mV s⁻¹.

4. Results and discussion

4.1. Effect of continuous mode applied voltage on normalized permeateflux from ECEFMM

Fig. 2shows the effect of electrical voltage on normalized steady state permeateflux in comparison to normalized steady state permeate flux without any application of the voltage. Without the application of voltage, thefigure reveals that with the variation in the feedstock there are no significant differences in the steady stateflux from ECEFMM

fitted with either PES or PSf membrane. At no voltage applied condi- tion, it is seen that the normalized permeateflux with PES membrane is more compared to PSf membrane. However, with a particular mem- branefitted to the module, theflux is the lowest at no voltage applied condition, compared to when voltage is applied to the cell. On the application of voltage, the emulsion breaks and separates into oil and water. Such separation reduces the volume of the contaminants that eventually reduces the intensity of the fouling. FromFig. 3(a) through (c) it can be seen that with the PES membranefitted to the module, no significant difference is seen with the flux for either emulsion or emulsion along with the free oil irrespective of the intensity of applied voltage. Moreover, with the PES membrane the normalized permeate flux is very close to one, manifesting no substantialflux decline when there is a continuous mode of applied voltage (Fig. 3a through c). On the contrary, with the PSf membrane, there is a substantial decrease in theflux unlike PES membrane even with the continuous application of voltage. Further, with the PSf membrane, beyond 5 V, during the continuous mode of applied voltage the permeateflux decrease sub- stantiates with the emulsion feedstock.

Here, theflux decline characteristic in case with a membrane pro- cess mainly attributes to two different properties of the membrane– one is the hydrophilicity of the membrane and another is the streaming potential (membrane surface charge). Owing to the higher hydro- philicity of PES membrane compared to PSf, an increased affinity for water permeation through the PES membrane is observed, which thereby results in elevated permeate flux with PES than PSf. At no voltage applied condition, the surface charge on the emulsion droplet was found as -74.7 mV and with the mixture (emulsion along with free oil) it was−18 mV (Fig. 4a). The streaming potential for PES mem- brane was measured−80 mV. With both emulsion and mixture feed- stock, the less negatively charged o/w droplet with respect to higher negative surface charge of the PES membrane manifests higher poten- tial difference (p.d.) between the membrane and the droplet. Such higher p.d. results an electrically induced motion towards the mem- brane. Under pressure, on the surface of the membrane, there will be a compression over the double layer that attributes to the disruption of the emulsion followed by freeing of water from its emulsified form. This eventually increases the water permeation rate through the PES mem- brane because of its hydrophilic property.

On the contrary, with PSf membrane, the streaming potential−20 mV is comparable to the surface charge of emulsion droplets mixed with free oil, while much less compared to only emulsion droplets. With thefirst one, there is a strong repulsion between the membrane and the droplet which restricts the approach of the droplet towards the

Fig. 2.Effect of applied voltage on the normalized steady state permeateflux (NJs) from ECEFMM (⬛: Js for emulsion feedstock with PES membrane; : Js

for emulsion along with free oil feedstock with PES membrane; : J_s for emulsion feedstock with PSf membrane; : J_sfor emulsion along with free oil feedstock with PSf membrane).

membrane manifesting less compression of the droplet. On the con- trary, with the later (emulsion) one, the p.d. acts outward from the membrane and hence, no compression over the double layer takes place. However, because of pressure and hydrophobicity of the mem- brane, only charge neutral oil droplets approach towards the membrane forming a secondary layer on the surface.Fig. 4(a) shows that even after application of the voltage the difference in the surface charge between emulsion and mixture system is not significant.Fig. 4(b) shows the p.d.

between the surface charge of the droplets and the membrane surface at different voltage application. Thefigure shows that the maximum dif- ference in p.d. between two different feeds for a specific membrane is observed, when there is no application of voltage. However, the change in p.d. for different feeds for a specific membrane is insignificant after

applying certain voltage within the cell. Further, from thefigure it is quite evident that with applied voltage for both the membranes irre- spective of the feed type at steady state, the droplets feel attraction towards the membrane, when the average p.d. is 7 mV for PSf mem- brane and 67 mV for PES membrane. Hence, it is evident that theflux change is not depending solely on the p.d., but also on the membrane property and the diameter of the droplets.

The size of the oil droplets at 10 V is more or less same for both emulsion and mixture. However, at 15 V the emulsion droplet size is almost 1.5 folds higher than that of the mixture. With the increase in the applied voltage more evolution of hydrogen (Fig. 5) at cathode makes a compression on the double layer around the droplet in the emulsion, which eventually intensifies the coalescence of the droplets Fig. 3.Variation of normalized permeateflux (NJ) with time at different modes of applied voltage with PES membrane ((a): 5 V; (b): 10 V; (c): 15 V) and PSf membrane ((d): 5 V; (e): 10 V; (f): 15 V). -▴- and shows the permeateflux variation in continuous mode with emulsion and the mixture (emulsion along with free oil) feedstock respectively. ( and :first cycle of the applied voltage for emulsion and emulsion along with free oil feedstock respectively; and : second cycle of the applied voltage for emulsion and emulsion along with free oil feedstock respectively; and : third cycle of the applied voltage for emulsion and emulsion along with free oil feedstock respectively).

towards a bigger size. Hence, at the high applied voltage of 15 V, droplet size variation with the feedstock type is much prominently noticed unlikely to 5 V and 10 V. However, in case with the mixture (emulsion along with the oil) because of the low volume fraction of the emulsified form was not affected much with such compression and hence, not much growth in the sizes could be seen (Fig. 4a). Because of similar surface charge and droplet size range at 5 V, the steady state permeateflux does not have too much variation in either of the two forms of the feedstock (Fig. 2). At 10 V, due to the presence of small sized droplets, the partial penetration of the droplets into the

membrane pores could be possible with the hydrophobic PSf membrane that manifest the reduction in the steady state permeateflux. However, the creation of turbulence due to hydrogen evolution will counteract the fouling of the membrane that also contributes to an increase in the flux. Further, due to the low volume fraction of the emulsion in the mixture manifests more permeation of water with the mixture than with the emulsion and thereby results in the less permeateflux decline with the mixture (Fig. 3e).Fig. 3(d) and (e) show that the steady condition on theflux is observed at around 70 min. for both 5 and 10 V applied voltage respectively. On the contrary, with 15 V (Fig. 3f) such a steady condition is seen at around 50 min. attributing to more rigorous fouling. After applying 15 V voltage, not much difference can be seen in surface charge compared to 10 V. However, the increase in average droplet size by 8 fold for emulsion and 3 fold for mixture can be seen compared to particle size at 10 V (Fig. 4a), which eventually increases the fouling propensity over the membrane. As discussed previously, the increased size oil droplets at 15 V were carried towards the membrane surface by the upward force exerted by hydrogen gas, show higher propensity of getting deposited on the membrane surface and blocking the pores. This eventually aids early attainment of the steady condition along with increasedflux decline than at 10 V.Fig. 4(c) shows the re- duction in the surface charge of the membrane because of the oil de- position over the surface attributing to membrane fouling.

4.2. Effect of periodically applied voltage for three cycles on normalized permeationflux

When the voltage was applied with 30 min. interval for three per- iodical cycles, as seen fromFig. 3(a) through (f), the maximumflux decline is observed for the emulsion feedstock at 5 V with both PES and PSf membrane. In case with periodic voltage application at 5 V, the rate Fig. 4.(a) Effect of applied voltage on zeta potential (−mV) (◼: Emulsion feedstock; : Emulsion along with free oil feedstock) and average particle size (μm) ( :Emulsion feedstock; : Emulsion along with free oil feedstock); (b) Variation of the potential difference (p.d.) between the surface charge of the droplets and the membrane surface at different voltage application; (c) Variation of the hydrophilicity (in terms of contact angle) and streaming potential of the membranes with the filtration time.

Fig. 5.Effect of mode of applied voltage on the hydrogen bubble nucleation rate (mol. m⁻².s⁻¹) during the electrochemical reaction.

of hydrogen evolution at the cathode is very much less because of the low nucleation rate (Eq. 5) as shown in Fig. 5. Thus, the turbulence created over the membrane surface and within the bulk volume by evolved hydrogen is not substantial [32,33]. This eventually did not aid the coalescence of oil droplets. Henceforth, the average oil droplet size does not increase significantly and remain small enough (in the range of 10 μm) as in the no voltage condition (Fig. 4a). Such small size oil droplets may result in clogging of the pores due to higher residence time on the membrane due to failing in sweep away the secondary oil layer over the membrane surface. On the contrary, with continuous mode as the nucleation rate is 32 % higher than that of periodic mode for applied voltage, theflux becomes much higher with the emulsion feedstock in case with the continuous mode. However, with the feed- stock comprising of emulsion along with free oil, theflux drop at 5 V is significantly less than that of emulsion. As the volume fraction of emulsified oil droplets in the feedstock comprising of emulsion and free oil was low, even less hydrogen evolution at the cathode can restrict the membrane fouling by smaller droplets. No significant flux decline is found at 10 V and 15 V for both emulsion and emulsion along with free oil feedstock. For both PES and PSf membrane the observation is same as seen fromFig. 3(a) through (f). However, the prime observation that can be a bit ambiguous is theflux at 10 V for both PES and PSf mem- brane (Fig. 3b and e). With PSf membrane theflux with emulsion feedstock was higher than that of the emulsion with free oil feedstock at 10 V (Fig. 3e) unlike flux in continuous mode. As discussed in the section, due to enhanced positivityflux gets higher at 10 V. Now in the periodic application of voltage, the sudden impulse of hydrogen gen- eration might create step increase in the turbulence and also more co- hesive forces to have the coalescence of the droplets. With the emul- sion, such coalescence is much higher because of the higher concentration of the emulsified form. Hence, destabilization of the emulsion reduces the fouling and increases the permeateflux. On the contrary, due to the low volume fraction of the emulsified form, the compressive forces are acting with a long range, which reduces the possibility for coalescence. This enhances the possibility for the fouling of the membrane with smaller sized droplets that was forced to the surface by step increase in the nucleation rate. While, with the PES membrane at 10 V, theflux is suddenly decreased in the second cycle (Fig. 3b). In case with the PES membrane, the membrane has a strong affinity for the comparatively more positive droplets. This may even- tually create a secondary membrane layer over the surface, which is not being eliminated by the turbulence occurred due to hydrogen evolu- tion. However, in the third cycle, such layers will be washed offbecause continuous turbulence by hydrogen bubbling for the preceding cycles.

Further, at 15 V, rigorous hydrogen evolution at different cycles creates sudden turbulences over the membrane surface that eventually restricts the fouling of the both PES and PSf membrane.

= Nucleation rate for hydrogen bubble(N ) I

zAF_{cons t}

H

tan

2 (5)

where z is the number of electrons required to produce 1 mol of gas (z

= 2 for hydrogen), Fconstantis Faraday’s constant (96485.3 Cmol⁻¹), I is the current (Amp), A is the electrode surface area (m²). Here, the nu- cleation rate is directly proportional to the current density. As in case with the intermittent application of voltage, the variation in the current density is not significant (within the 5 % confidence interval), the nu- cleation rate for hydrogen bubbles is considered as the average value of the nucleation rate obtained separately for each individual cycle.

4.3. Hydrodynamic modelling of hydrogen bubble impact on the fouling layer

During the evolution of the hydrogen bubbles from electrode, after detachment the bubbles rise through a contaminated feed and collide with the membrane surface fouled with oil layer. Once, the bubble gets collided with the oil surface, the fouled layer becomes damaged be- cause of the inertia of the bubble. Here initially, in absence of any application of the voltage the oil creates a secondary layer responsible for membrane fouling. However, with continuous application of the voltage, the evolved hydrogen bubble penetrates through the film formed if any. Such penetration creates a discrete oil layerfilm over the surface, which aids the contact between oil and hydrogen gas bubble with any contact angle (θ) varying from 0 to^π2(Fig. 6). While,φin- dicates the angle of cone created after contact. Now ifφis significantly less (as seen bubble diameter is much larger than the oil droplet dia- meter), the contact plane length is equivalent to the diameter of the oil drop. Therefore, the force because of the surface action between oil drop and bubble can be said asσoil-gas.Doil. During the collision with the fouled layer, the surface tension between the oil and the membrane surface was reduced followed by the detachment of oil from the surface.

According to Manica et al. [34], as the bubble size ranges from 80 to 500μm, the rise of bubble is in a straight path and will have an elastic impact with the oil layer. Post impact the bubble will collapse after releasing its energy. Such energy will be sufficient enough to remove the depositedfilm over the membrane surface. In this manuscript, we have derived a model for interpreting the impact and attempted to analyze the case, when the oilfilm can be removed from the membrane surface.Fig. 6shows the mechanism through which the oil layer can be removed from the surface. As seen from thefigure, the hydrogen bubble will be in contact with the oil droplet with an angle‘θ’and will impart a force‘F’on the droplet. Fouling can only be alleviated, when Fx=Fcosθ is enough to create a force, which will be substantially higher than that of -σx. While the Fy=Fsinθmust be low enough to avoid a normal force on the layer which will aid the fouling process. The force balance can be given by Eq.(6):

= −

+ + ⎛

⎝

⎞

⎠ − ⎛

⎝ ⎞

F πR ρ ρ ρ ⎠

ρ ρ πR ρ g C π

μRv

0.68 ( )

(0.5 )

4

3 Re

G 4

L G

L G

Inertia

L D

Dragforce 3

Force

3

Buoyancy force

    

(6)

Where, CDRe = 24(1 + 0.15Re^0.687) (7)

However, the model does not include the mathematics on the

Fig. 6.Schematic representation of the hydrogen bubbling mechanism through which the foulant oil layer can be removed from the membrane surface.

property of the membrane and their interaction with the oil/emulsion during the operation of the module.Fig. 7shows the variation of force for different applied voltage at different impact angle and bubble dia- meter. From the figure, it is vividly observed that for the maximum impact force, the range of the impact angle with which the bubble is supposed to be colliding with the oil is widespread at 10 V. While it is minimum for the 15 V and moderate with the 5 V applied voltage provided the actual droplet diameter reaches that maximum value. On the contrary, the maximum impact with 600 × 10⁻⁹N is simulated for a bubble diameter of 500 μm and at an impact angle of 0°. Such a possibility of penetration of 500μm bubble into the gel layer, which can only make an impact at an angle of 0° with the oil droplet is low en- ough. The interpreted result was again validated with the experimental observation at 5 V, where the maximum bubble diameter was 250μm (simulated point shown inFig. 7as‘C1′). Such a low hydrogen bubble diameter might not create much significant tangential force to the de- posited layer even if the impact angle is low enough at 1° that can actually sweep away the oil layer over the surface. With 10 V applied voltage, the maximum bubble diameter that reached was 300μm and the simulation shows that it exterts 400 × 10⁻⁹N on the oil layer with impact angle lies 3.5° (point shown as C2 inFig. 7). There is a shift in the impact angle to 5.5°, once the voltage applied is 15 V, while the simulated impact force reduces to 300 × 10⁻⁹N, when the maximum hydrogen bubble diameter was seen 400 μm. However, with the in- crease in the impact angle at 15 V, the sinθcomponent of the impact force will get increased, which is normal to the membrane surface.

Therefore, such an enhanced force acting normal to the membrane surface will increase the possibility of foulant adherence over the membrane surface and enhance the propensity of fouling. On the con- trary, the tangential shear creating cosθcomponent of the impact force is primarily responsible for the shear enhancement. Therefore increase in the impact angle reduces the magnitude of the tagential force over the membrane and thus reduce the chances towards the alleviation of the fouling of the membrane. Experimental observation in permeate flux variation with time is also validating the observation obtained for PSf membrane from the simulation. However, the hydrodynamics of colliding bubble with the oil droplet is similar for the PES membrane.

The only difference is the additional component of force that is enacted because of the hydrophilicity of the PES membrane.

4.4. Effect of continuous and periodically applied voltage on the percentage oil rejection and permeate conductivity for both types of membrane

Fig. 8(a and b) shows the percent rejection of oil by both PES and PSf membrane once the voltage is applied for both the feedstock. As seen fromFig. 8that with PES membrane the oil rejection is lower than PSf. Moreover, the rejection with PES membrane for emulsion feedstock was less than the combined feedstock beyond 5 V. With the PES membrane the hydrophilicity of the membrane increases the affinity for water along with the rigorous hydrogen formation that compels the permeation of emulsified oil droplets through the membrane in case for emulsion (Fig. 8a). While, on the contrary, with the combined feedstock the hydrophobicity of the system is much higher due to the increased free oil percentage in the system. Therefore, the rejection of oil with the mixture is higher compared to emulsion feedstock only (Fig. 8b).

However, with the PSf membrane for both types of feedstock, the nature of the variation of the percentage rejection for oil with the ap- plied voltage is similar. As discussed in Sections4.1and4.2, at 10 V the sizes of the droplets in case of both types of the feedstock are less than that of the 15 V, which may allow the passage of the microsized oil droplets through the membrane attributing to less rejection. On the contrary, because of the larger sized droplets at 15 V and less nucleation at 5 V aid the development of the secondary layer over the membrane that restricts the passage of the oil manifesting higher rejection. The effects of mode of applied voltage and membrane type on the oil re- jection are statistically interpreted byFig. 8(c and d) for emulsion and

the mixture (emulsion along with free oil) feedstock, respectively. It is seen from the inset ofFig. 8(c) and (d) that irrespective of the feedstock nature, the relative deviation is less than 5 % (confidence interval (CI) is chosen as 5 %) for both the PES and PSf membrane at any mode of the applied voltage. While inFig. 8(c), it is observed that only at 15 V the deviation in continuity is above 5 % with emulsified feedstock for any mode of the applied voltage. Such deviation is mainly attributed to the variation in permeation characteristics depending on the hydrophilicity of the used membrane. The high hydrophilicity of PES membrane is forcing the increased permeation of emulsified oil droplets at applied 15 V compared to combined feedstock having higher free oil percentage and results in the lower oil rejection for emulsified feedstock. While for hydrophobic PSf membrane, no such effect is observed.

The effect of mode of voltage application on permeate conductivity is also in contiguous to the oil rejection. As seen fromFig. 9(a), with both the membrane the conductivity becomes reduced as the voltage increases to 15 V complying with the aforesaid incident of fouling over the membrane. This restricts the passage of the ions and reduces the conductivity of the permeate. At 15 V, the conductivity is minimum with the PES membrane, when voltage is applied periodically. With the continuous voltage application, the hydrogen generation at the cathode is much prompt because of the higher nucleation rate. Hence, a con- tinuous force is given to the ions to pass through the membrane com- pared to the periodically applied voltage that increases the permeate conductivity with both the membrane for the continuous voltage ap- plication. One of the significant observations fromFig. 9(a) is the lower permeate conductivity with the PES membrane compared to PSf membrane. The enhanced hydrophilicity of the PES membrane allows more passage of water through the membrane that reduces the con- centration of ions in the permeate and thus permeates conductance.

Thus, with the PES membrane, even though there is low turbulence because of low hydrogenation at 5 V, the higher hydrophilicity of the PES membrane allows the passage of dissolved ions. There the con- ductivity is almost near to the value of conductivity at 10 V in both the voltage application mode. However, at 15 V the passage of ions was restricted due to fouling of the membrane while the permeation of water is high with PES membrane. Such condition manifests less con- centration of ions in the permeate attributing to low conductivity.

Fig. 9(b) is a statistical interpretation of the effect of mode of ap- plied voltage and membrane type on the permeate conductivity. It is seen from the inset ofFig. 9(b) that the relative deviation is more than 5

% (CI is chosen as 5 %) for the PES membrane with different periodi- cally applied voltage. While inFig. 9(b) it is observed that at 15 V for any mode of applied voltage irrespective of the membrane type the deviation in continuity is maximum of 9 %. Hence, it is evident that periodically applied 15 V with PES membrane is primarily controlling the permeation of the ions through the membrane. As discussed

Fig. 7.Variation of the impact force exerted by hydrogen bubbles for different applied voltage with varying impact angle and bubble diameter.

previously, the permeate conductivity is much lower at this condition compared to other condition (Fig. 9a). Moreover, at 10 V the relative deviation is also seen slightly above the 5 % CI. Such a deviation is manifested by the type of the membrane as seen fromFig. 9(a). As PSf membrane shows more hydrophobic character compared to PES

membrane, so permeation of water is restricted and shows much higher conductivity compared to PES membrane. Such permeation of water with PES membrane is much rigorous because of higher nucleation compels the passage of water. Hence, it can be realized that permeation of ions is primarily controlled by the membrane type until 10 V beyond Fig. 8.Variation of percentage oil rejection with both of the continuous and periodically applied voltage for (a) emulsion feedstock and (b) emulsion along with free oil feedstock ( : PES with continuous voltage applied;⬥: PES with periodically applied voltage; : PSf with continuous voltage applied; : PSf with periodically applied voltage); Relative deviation (%) in oil rejection percentage (Ratio between standard deviation and average multiplied by 100) for an applied voltage for all membranes with different modes of voltage application for (c) emulsion feedstock and (d) emulsion along with free oil feedstock (inset: Relative deviation (%) in oil rejection percentage for a particular membrane at a particular mode of different applied voltage (A: PES with periodically applied voltage; B: PES with continuously applied voltage; C: PSf with periodically applied voltage; D: PSf with continuously applied voltage)).

Fig. 9.(a) Variation of permeate conductivity with the applied voltage for both feedstock (◼: PES membrane with periodically applied voltage;●: PES membrane with continuously applied voltage;□: PSf membrane with periodically applied voltage;○: PSf membrane with continuously applied voltage); (b) Relative deviation (%) in conductivity for an applied voltage for all membranes with different modes of voltage application (inset: Relative deviation (%) in conductivity for a particular membrane at a particular mode of different applied voltage (A: PES with periodically applied voltage; B: PES with continuously applied voltage; C: PSf with periodically applied voltage; D: PSf with continuously applied voltage).

which the control shifts to the mode of applied voltage along with the membrane type.

4.5. Operating cost analysis for ECEFMM

The operating cost of the membrane module is primarily manifested by the electrode consumption cost and the electrical energy cost to generate an electric potential difference within the cell (electrical en- ergy spent out for pump and compressor have been neglected here). To understand the electrode consumption cost, the corrosion current density was measured from Tafel extrapolation method (Fig. 10). Cor- rosion rate (d) (mil per year) was calculated using Faraday’s equation (Eq. 8)

= × ×

d 0.13 ρI EW

Al corr

(8) Where, Icorr is the corrosion current density, EW is the equivalent weight of aluminum electrode andρAlis the density of the aluminum electrode (2700 kg.m⁻³). The operating cost assumed here is for the

electrode consumption cost and the electrical energy cost. Hence, the weight loss for the electrode can be said as ^{π(2.54×10−×d)}Lρ_Al

4

5 2

kg per year (L is the length of the electrode). If the volume of the wastewater treated was Vwm³, while the treatment cycles in a day were N, the total feed volume treated in a year is equal to 320 N V (It is assumed that a membrane module is under operation for 320 days in a year). Hence- forth, the weight loss per year per m³ of the feed is given by

× ⁻ ×

( )

π d Lρ

NV (2.54 10 )

4 320

Al w

5 2

. The replacement cost of the aluminum electrode is thus given by^CR=^{π(2.54×104− ×d)}

(

^320LρNVAlw

)

^EAl

5 2

, where EAlis the alu- minum electrode cost per kg. The unit cost of aluminum metal is as- sumed as INR 144.95 per kg as per the market value in India for the year 2018−19. Considering, the values for feed volume = 0.001 m³ and number of cycles = 4, it was seen that the replacement cost in a year is almost nil for any value of the current density (Fig. 11). Hence, the operational cost for running the proposed membrane module is majorly governed by the electrical energy because of the applied vol- tage through the DC source.

Electrical energy consumption per m³of wastewater treated is given by^VIt

Vw kWh. m⁻³, where, V is the applied voltage of the cell (in V), I is the current (A), and t is the operational time (h). Now, if the local electrical unit charge is given as EEper kWh, the electrical cost for running the cell is given by^CE=

( )

^VItVw ^EE. The expense for electrical energy consumption with the varied current density is shown inFig. 11, considering the unit price of electricity as INR 5.53/ kWh as per the tariffof CESC Kolkata. It shows that with the increase in the current density, the electrical consumption cost ranging from INR 20 to INR 218 per m³ of wastewater treated is also increasing owing to the Fig. 10.Tafel plot or polarization curve representing the Tafel lines for an

anodic and cathodic reaction in the electrochemical process.

Fig. 11.Operating cost of ECEFMM module with varied current density.

Table 1

Energy consumption for collecting one ml of permeate across unit area (m²) of the membrane at operating conditions 0.2 MPa TMP and room temperature (25 °C).

Membrane module Energy required (W) per ml of permeate collection across unit area (m²) of the membrane Reference

RFMM 92.4 [8]

TFMM 9.2 [8]

ECEFMM 8.1 Present study

Table 2

ANOVA test assessing the effect of membranes types and feedstock nature on permeateflux.

Source of Variation SS df MS p-value

Membrane types 6.39 × 10¹¹ 1 6.39 × 10¹¹ 0.000

Feedstock types 5.21 × 10⁹ 1 5.21 × 10⁹ 0.639

Error 1.76 × 10¹¹ 9 2.20 × 10¹⁰

Total 8.25 × 10¹¹ 11

SS: Sum of Squares; df: Degrees of freedom; MS: Mean Squares.

Table 3

ANOVA test with the effect of applied voltage and its mode on steady state permeateflux for (a) PES membrane and (b) PSf membrane.

Source of Variation SS df MS p-value

(a)

Applied voltage 4.82 × 10¹⁰ 2 2.4 × 10¹⁰ 0.395

Mode of voltage application 1.57 × 10¹² 2 7.86 × 10¹¹ 0.002

Error 8.17 × 10¹⁰ 4 2.04 × 10¹⁰

Total 1.7 × 10¹² 8

Source of Variation SS df MS p-value

(b)

Applied voltage 1.1 × 10¹¹ 2 5.52 × 10¹⁰ 0.178

Mode of voltage application 9.6 × 10¹¹ 2 4.8 × 10¹¹ 0.006

Error 8.06 × 10¹⁰ 4 2.02 × 10¹⁰

Total 1.15 × 10¹² 8

SS: Sum of Squares; df: Degrees of freedom; MS: Mean Squares.

elevated energy consumption. The evaluated energy consumption cost for ECEFMM is found to be minimal and quite feasible in view of its industrial application.

Moreover, a comparative analysis on energy requirement for redu- cing fouling propensity is carried out with our previous membrane modules, where fouling was controlled by shearing force induced by two different rotating membrane modules namely turbineflow mem- brane module (TFMM) and radialflow membrane module (RFMM) [8].

It is observed fromTable 1that the energy consumed in ECEFMM for permeating ml of permeateflux per m²of membrane is found 1.14 fold and 11.4 fold less than required in case with previously studied TFMM and RFMM respectively. Additionally, this indigenous ECEFMM tech- nology was found promising for highly efficient oil water separation (94–96 % oil rejection) with around 43–72 % reduction in theflux drop compared to the previously discussed modules. On account of the less flux decline, membrane fouling will be substantially reduced and membrane longetivity will also be enhanced by restricting the mem- brane ageing for prolonged time period. Thus, less frequent membrane replacement will demand, which thereby reduces the membrane re- placement cost (maintanence expense) to a great extent. Another no- teworthy benefit in context with the process intensification is that this developed technology demands less unit operations through integrating the electrochemical process setup with the membrane module in a single hybrid ECEFMM setup. This will significantly reduce the initial capital investment expense along with the additional advantage of re- duced installation area requirement. Therefore, the above linear eco- nomic assessment manifests that the newly developed ECEFMM tech- nology can be opted as an economical and cost-feasible viable option for the treatment of oily wastewater.

4.6. Evaluation of parametric significance on permeateflux using analysis of variance (ANOVA) test

The significance of feedstock nature and membrane type on the permeateflux drop was enumerated statistically by two way ANOVA test (Table 2) with 5 % CI. It is seen fromTable 2 that the p-value (probability of making type-I error, i.e. rejecting the null hypothesis even it is true) is much less than 5 % for membrane types, while it is much higher (63.9 %) with the feedstock nature. With the variation in membrane types, permeation characteristics were substantially varied with the membrane morphology. It was discussed previously that hy- drophilicity of PES membrane in comparison to PSf membrane was significantly affecting the permeation characteristics. On the contrary, because of the presence of the emulsified system in both the feedstock the size variation is not so significant in comparison to membrane, al- though there was much difference in the surface charge between the two feedstocks. As the variation in size of the droplets in both the feedstock doesn’t have many differences with the membrane, the size based permeation is not significantly depending on the feedstock.

However, the charge difference with the membrane made a significant effect on the permeation manifested by lowp-value against the mem- brane types.

Table 3(a and b) shows the significance test analysis with the effect of voltage and its mode of application on the steady state permeateflux through PES and PSf, respectively.Table 3manifests that the mode of the applied voltage is much significant with both of the membrane in havingflux. On the contrary, the intensity of the applied voltage is less significant towards controlling the permeation characteristics. Espe- cially, with the PES membrane voltage applied is much less significant compared to PSf membrane as seen after comparing the p-values in Table 3. From the previous discussions, it is quite evident that the PES membrane is solely controlling the permeateflux due to its much ne- gative surface charge density and its high hydrophilicity. On the con- trary, relatively low hydrophilic PSf membrane requires rigorous tur- bulence through hydrogenation at the cathode in order to come across the permeation against the fouling during the run. Such instances

increase the permeation dependency on the applied voltage along with the mode of application in case with the PSf membrane.

5. Conclusion

On the application of the voltage, the stubborn biphasic emulsion breaks and disintegrates into two phases i.e. oil and water. Such se- paration reduces the volume of the contaminants that eventually re- duces the possibilities of fouling. Substantial increase in the steady state permeateflux, which was around 43–72 %, was observed on the ap- plication of the voltage, compared to theflux obtained at no voltage condition. In case with PES membrane, there was no as such decrease in the permeateflux on application of voltage, while a substantialflux decline was observed with the PSf membrane. It was observed that the flux decline with PSf membrane was much less and was better con- trolled with the periodically applied voltage than continuously applied voltage. At 10 V applied voltage, less reduction in theflux along with the highest fouling mitigation was attained for PSf membrane with both emulsion and emulsion along with free oil feedstock. It was seen that the oil rejection through membrane separation was high and permeate conductivity was low as the intensity of the applied voltage was in- creased to 15 V. Moreover, the operational energy consumption is minuscule with the ECEFMM that made it a promising and cost-effec- tive hybrid technology for oily wastewater treatment with much re- duced membrane fouling propensity.

Declaration of competing interest

The authors declare that they have no conflict of interest.

CRediT authorship contribution statement

Ankita Mazumder:Conceptualization, Data curation, Formal ana- lysis, Methodology, Software, Visualization, Writing - original draft, Writing - review & editing. Zinnia Chowdhury: Conceptualization, Formal analysis, Investigation, Methodology. Dwaipayan Sen:

Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Visualization, Writing - review & editing. Chiranjib Bhattacharjee: Conceptualization, Funding acquisition, Project ad- ministration, Resources, Supervision, Validation, Writing - review &

editing.

Acknowledgments

The authors thankfully acknowledge Department of Science &

Technology (DST), Government of India for sponsoring the project (vide sanction letter no. DST/TSG/AMT/2015/276 dated 11.06.2016).

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