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Water Research 252 (2024) 121248
Available online 1 February 2024
0043-1354/© 2024 Elsevier Ltd. All rights reserved.
Advancements in combined electrocoagulation processes for sustainable
wastewater treatment: A comprehensive review of mechanisms,
performance, and emerging applications
Aatif Ali Shah
a
,
c
,
*
, Sunil Walia
a
, Hossein Kazemian
a
,
b
,
c
,
*
a
Materials Technology & Environmental Research (MATTER) lab, University of Northern British Columbia, Prince George, BC, Canada
b
Northern Analytical Lab Services (Northern BC’s Environmental and Climate Solutions Innovation Hub), University of Northern British Columbia, Prince George, BC,
Canada
c
Environment Science Program, Faculty of Environment, University of Northern British Columbia, Prince George, BC V2N4Z9, Canada
ARTICLE INFO
Keywords:
Combined electrochemical processes
Water treatment
Wastewater treatment
Electrocoagulation
Advanced oxidation processes (AOP)
ABSTRACT
This review explores the potential and challenges of combining electrochemical, especially electrocoagulation
(EC) process, with various - wastewater treatment methods such as membranes, chemical treatments, biological
methods, and oxidation processes to enhance pollutant removal and reduce costs. It emphasizes the advantages
of using electrochemical processes as a pretreatment step, including increased volume and improved quality of
permeate water, mitigation of membrane fouling, and lower environmental impact. Pilot-scale studies are dis-
cussed to validate the effectiveness of combined EC processes, particularly for industrial wastewater. Factors
such as electrode materials, coating materials, and the integration of a third process are discussed as potential
avenues for improving the environmental sustainability and cost-effectiveness of the combined EC processes.
This review also discusses factors for improvement and explores the EC process combined with Advanced
Oxidation Processes (AOP).
The conclusion highlights the need for combined EC processes, which include reducing electrode consumption,
evaluating energy efciency, and conducting pilot-scale investigations under continuous ow conditions.
Furthermore, it emphasizes future research on electrode materials and technology commercialization. Overall,
this review underscores the importance of combined EC processes in meeting the demand for clean water re-
sources and emphasizes the need for further optimization and implementation in industrial applications.
1. Introduction
Water is indispensable for sustaining life and supporting various
activities in both domestic and industrial settings. However, global
water consumption has steadily increased by 1 % annually since 1980,
primarily driven by the escalating demands of the residential, agricul-
tural, and industrial sectors (Almukdad et al., 2021). Unfortunately, this
growing water usage has led to a worldwide water crisis, exacerbated by
the rapid expansion of polluting industries such as pharmaceuticals,
distilleries, textiles, fertilizers, tanneries, and mining (Malik et al.,
2020). Moreover, the rise in human population, urbanization, and
improved living standards have intensied water pollution, posing sig-
nicant risks to public health and the environment (Kumar et al., 2023;
Li et al., 2022). The scarcity of freshwater resources and pollution in
freshwater reservoirs has emerged as a critical global challenge,
potentially resulting in social and health risks, reduced agricultural
yields, compromised industrial production, droughts, and even res
(Jadhav et al., 2023). In light of these pressing concerns, minimizing
water wastage and optimizing water usage has become imperative to
ensure the sustainability and security of domestic, industrial, and agri-
cultural water supply, especially in climate change (Santos et al., 2023).
Consequently, wastewater reclamation, recycling, and reuse have been
proposed as viable alternatives to alleviate water scarcity and mitigate
the impending water crisis (Rashid et al., 2021; Semaha et al., 2023).
When considering wastewater treatment, it is crucial to recognize
that the pollutants present in wastewater vary in composition and
concentration depending on their origin. Therefore, carefully consid-
ering and selecting appropriate treatment methods and steps are
* Corresponding authors at: Materials Technology & Environmental Research (MATTER) lab, University of Northern British Columbia, Prince George, BC, Canada.
E-mail addresses: aatifali.shah@unbc.ca (A.A. Shah), Hossein.kazemian@unbc.ca (H. Kazemian).
Contents lists available at ScienceDirect
Water Research
journal homepage: www.elsevier.com/locate/watres
https://doi.org/10.1016/j.watres.2024.121248
Received 2 August 2023; Received in revised form 25 January 2024; Accepted 31 January 2024
Water Research 252 (2024) 121248
2
necessary to ensure the treated wastewater is safe for discharge into
aquatic systems or soil (AlJaberi et al., 2022; Nawarkar and Salkar,
2019). Over the past two decades, several water treatment methods have
been employed to treat wastewater for reuse. These methods encompass
chemical precipitation, membrane-based treatments (Ge et al., 2023),
coagulation and occulation (Ho et al., 2020), ltration (Thao et al.,
2023), adsorption (Momina and Ahmad, 2023), ion exchange (Tong
et al., 2022), and biological treatment methods (An et al., 2023; Xu
et al., 2023). However, biological processes often require precise control
over operational conditions, longer treatment durations, and more
enormous physical footprints and can generate undesired by-products
(Aziz et al., 2019; Donkadokula et al., 2020; Hasan and Muhammad,
2020). On the other hand, chemical processes involve substantial
chemical inputs, resulting in increased overall costs and complicating
downstream procedures, which may lead to potential secondary
contamination (Aguilar-Asc´
on et al., 2023; Das et al., 2022). Addition-
ally, membrane ltration and adsorption, unless complemented by
comprehensive pre-treatment processes, gradually lose efciency over
time due to pore blockage by pollutants and reduced ux (the ow rate
of treated efuent per unit surface area) (Priya and Jeyanthi, 2019;
Tahreen et al., 2020).
In recent years, electrochemical technologies have gained signicant
attention due to their ability to leverage multiple parameters for
achieving efcient performance. These innovative methods have proven
highly effective in combating environmental contamination and
removing persistent organic pollutants from wastewater (Mohora et al.,
2018), (AlJaberi et al., 2022). Various electrochemical technologies
have been applied in wastewater treatment, including electrootation
(Nunes et al., 2022), electro-Fenton(EF) (Xu et al., 2019), electrodialysis
(Wang et al., 2022), electrocoagulation (Aiyd Jasim and AlJaberi,
2023), ultraviolet treatment (Yang et al., 2023), ozonation (Cui et al.,
2023), and electrooxidation (Chen et al., 2023), rely on the utilization of
electrical current for their operations.
Among the electrochemical processes, EC has emerged as a widely
used and promising technology for cost-effective and efcient treatment
of water and wastewater, while minimizing sludge production. It offers
exibility, ease of use, and the ability to handle various contaminants
(Das et al., 2021). EC represents a promising alternative to chemical
coagulation for treating heavily contaminated wastewater due to its
rapid removal rate and absence of added chemicals. Unlike chemical
coagulation, EC eliminates the risk of generating secondary contami-
nants as no chemical coagulant is used. Furthermore, the generation of
hydrogen gas bubbles during the EC process facilitates the otation of
contaminants to the water surface (Mohammed and Aljaberi, 2018). The
EC process has proven effective in eliminating various contaminants
from water and wastewater, encompassing total suspended solids, total
organic carbon (TOC), oil emulsions, heavy metals, chemical oxygen
demand, color, and turbidity (Al-Raad and Hanaah, 2021). Extensive
research has been conducted on EC process treatment for various types
of wastewater, including industrial, tannery, domestic, grey wastewater,
hospital, palm oil mill efuent, textile, and municipal wastewater,
demonstrating its efcacy in removing small colloidal particles and
suspended solids (Al-Shati et al., 2023; Ansari et al., 2022; De Maman
et al., 2022; Ensano et al., 2019; Koyuncu and Arıman, 2020; Nasrullah
et al., 2022; Syam Babu et al., 2020; Zhang et al., 2023).
However, it is essential to note that while the EC process effectively
removes heavy metals, suspended solids, and chemical oxygen demand
(COD)(“ISO 6107:2021(en), Water quality — Vocabulary,” n.d.) from
wastewater under suitable treatment conditions, it cannot eliminate
refractory organics and microorganisms. Furthermore, the generation of
sludge during the EC process necessitates additional treatment and poses
environmental challenges that must be addressed (Genawi et al., 2020;
Olvera-Vargas et al., 2019; Pandey and Thakur, 2020). It is crucial to
emphasize that internationally, the regulations governing limits for
parameters such as COD, TOC, and BOD vary from one country and
region to another, imposing distinct restrictions on different water
bodies. In specic EU Member States, the prescribed limits for BOD in
treated wastewater quality stand at 123 mg/L. The EU directives outline
guidelines for urban wastewater treatment, setting a COD discharge
limit of 125 mg/L for all types of wastewater treatment plants (WWTPs).
In Germany, COD discharge limits vary between 75 mg/L and 150 mg/L
based on the scale of WWTPs. Conversely, in the United States, there are
no universally applied COD discharge limits; instead, they differ among
states (California Region 8 - Santa Ana COD 5–90 mg/L) (Preisner et al.,
2020; “Urban wastewater treatment,” n.d.; US EPA, 2014; Wu et al.,
2022; Zhou et al., 2018).
Combining the EC process with other treatment techniques is crucial
to make the EC process suitable for sustainable water desalination,
wastewater treatment, and water reuse. The choice of treatment tech-
nique for combination should be carefully considered to enhance
removal efciency and reduce energy consumption. In response to this
need, researchers have developed combined EC processes that integrate
it with other treatment methods, such as membranes-based, chemical-
based, biological-based, and electrochemical-based combined processes,
to improve overall performance (Asfaha et al., 2021; Genethliou et al.,
2023; Igwegbe et al., 2021; Xu et al., 2022). Electrochemical AOPs have
gained recognition among these combined processes as effective and
viable methods for eliminating contaminants from various wastewaters.
These oxidation-based treatment techniques generate reactive OH•
radicals and include processes such as ozonation, direct UV, UV/H
2
O
2
,
UV/H
2
O
2
/O
3
, Fenton, and photo-Fenton for wastewater treatment
(Mirza et al., 2020; Tanveer et al., 2022; Yasar et al., 2019). AOP
treatment of industrial efuents containing chemicals can result in the
generation of CO
2
, water, and inorganic compounds, as well as the
creation of biodegradable intermediates through the partial degradation
of biodegradable organic contaminants (Asaithambi et al., 2022a).
Notably, extensive research on combined EC systems, their ad-
vancements, and their water and wastewater treatment applications are
lacking. This review paper aims to provide an in-depth overview of
combined techniques used for wastewater treatment, address the asso-
ciated challenges, and provide future recommendations. Specically,
the paper focuses on the reaction mechanism involved in the combined
EC/AOPs process, which utilizes highly reactive hydroxyl radicals to
degrade organic pollutants effectively. Additionally, the paper summa-
rizes the experimental outcomes from combined EC/AOPs processes
reported between 2018 and 2023. The authors envision that this
comprehensive review article will enhance readers’ critical compre-
hension of combined electrocoagulation processes, encompassing their
inherent constraints and prospective remedies, aligning with the stan-
dards of scholarly literature.
2. EC process
The EC process has gained popularity as a reliable method for
treating various types of wastewaters. EC combines the advantages of
coagulation, otation, and electrooxidation, providing an alternative to
chemical coagulation (Oliveira et al., 2021). The technology operates by
applying a strong electric eld to induce a redox reaction in water
particles. During EC, coagulating agents (M
n+
) are released through the
oxidation of anode material, which neutralizes particulate matter and
forms metal hydroxide complexes.
Concurrently, the electrochemical reduction or oxidation of impu-
rities in water generates hydrogen or oxygen bubbles within, promoting
the electro otation of coagulated particles. (Ezechi et al., 2020; Patel
et al., 2022). The layout of an EC reactor is illustrated in Fig. 1.
The efciency of the EC process is inuenced by various operational
parameters, including the inter-electrode distance, solution pH, type of
electrodes, applied current density, and electrode conguration. For
instance, the pH and concentration of the coagulating metal ions in the
solution can affect the formation of ocs(Liu et al., 2021; Zhao et al.,
2021). Recent advancements indicate that these factors signicantly
contribute to the effectiveness of EC processes (Islam, 2019;
A.A. Shah et al.
Water Research 252 (2024) 121248
3
L´
opez-Guzm´
an et al., 2021). Fig. 2 illustrates the key parameters that
impact the efciency of the EC process. The efciency is determined by
the gas bubbles’ generation and size and the electrodes’ passivation.
(Sandoval et al., 2021; Villalobos-Lara et al., 2020). Additionally, the
efciency of the EC process is affected by mass transfer and mixing
conditions(He et al., 2024; Naje et al., 2020; Villalobos-Lara et al.,
2020). The physical design of the reactor and the current density are two
primary factors that inuence these conditions. The reactor’s cell ge-
ometry, electrode arrangement, and hydrodynamics are physical design
parameters that affect mass transfer and mixing. Furthermore, the
generation of bubbles through the current density also impacts mass
transfer and mixing. Therefore, to achieve optimal removal efciency,
the EC process necessitates an optimal physical reactor design and
careful control of operating parameters (L´
opez-Guzm´
an et al., 2021).
Detailed information on these factors can be found in other sources, as
they have been extensively discussed and evaluated with various types
of wastewater (Bajpai et al., 2022; Das et al., 2022; Zaied et al., 2020).
2.1. EC wastewater treatment mechanism
The EC process commonly utilizes electric current in the electrolytic
solution to destabilize pollutants present in the form of dissolved or
suspended particles (Bas¸aran Dindas¸ et al., 2020). The mechanism of EC
involves seven steps: (i) the production of metal cations at the anodes by
Fig. 1. Schematic diagram of the EC process (Das et al., 2022).
Fig. 2. Illustrates the various factors that impact the efciency of the EC process.
A.A. Shah et al.
Water Research 252 (2024) 121248
4
the application of electric current, (ii) hydrolysis at the cathode gener-
ating hydroxyl ions, (iii) reaction between metal cations and hydroxyl
ions to form metal hydroxides, (iv) oxidation of toxic contaminants into
intermediate products, (v) charge-neutralization of contaminants
through reaction with metal hydroxides, (vi) adsorption of
charge-neutralized contaminants onto metal hydroxides, leading to their
removal via sweep coagulation, and (vii) the formation of H
2
gas at the
cathode, which lifts the ocs to the solution surface through sweep
otation (An et al., 2017; Mao et al., 2023). Gas evolution from water
electrolysis can promote the otation of some coagulated pollutants to
the surface, and the agglomerated species can absorb other species. The
applied potential between the electrodes enhances the adsorption pro-
cess, facilitating separation and removal processes, along with the ma-
jority of suspended species (Nidheesh et al., 2021a; Pedersen et al.,
2019; ﻥﺍﺉﻝﻱﺱﺭﻱ et al., 2020).
The EC process typically involves chemical equations (Eqs. (1)–(4))
that occur when an electric current passes through electrodes, usually
made of aluminum(Al) and/or iron(Fe) (AlJaberi, 2018; Al Jaberi et al.,
2020). The general electrode reactions are as follows (Islam, 2019).
Oxidation at anode
M
(s)
→ M
n+
(aq)
+ne
−
(1)
2H
2
O → O
2
+4H
+
+4e
−
(2)
Reduction at cathode
2H
2
O +2e
−
→ 2OH
−
(aq)
+H
2
(g) (3)
Mn+ +nOH−⇔ M(OH)
n
(4)
Recent research has focused on the application of EC for wastewater
treatment. For instance, a study by Omwene et al. (2018) investigated
the removal of phosphorus from domestic wastewater using combined
Al and Fe plate anodes in an EC reactor. Under optimal conditions, the
study reported a removal efciency of 99.99 % for phosphorus from
domestic wastewater with an initial concentration of 52.1 mg/L PO
4
-P
(Omwene et al., 2018). The time required to achieve over 99.99 %
phosphate removal efciency was increased from 6 to 80 min for initial
concentrations ranging from 5.0 to 52.1 mg/L, indicating that phos-
phorus removal at a constant current density linearly depended on the
initial concentration of the pollutant.
Similarly, Sharma et al. (2021) investigated the effectiveness of EC in
treating domestic wastewater using Al and Fe metal plate electrodes in
various combinations. By employing a combination of Al (cathode) and
Fe (anode) with a mono-polar electrode connection in parallel, the EC
process achieved high removal efciencies for COD (91.8 %), Biological
oxygen demand (BOD)(“ISO 6107:2021(en), Water quality — Vocabu-
lary,” n.d.) (94.3 %), and turbidity (96.5 %) after 30 min of treatment.
The process exhibited an optimum current density of 1.25 mA cm
−2
and
low energy consumption of 0.017 kWh (Sharma et al., 2021). On the
other hand, Safwat et al. (2020) investigated the performance of the EC
process using zinc and titanium (Ti) electrodes for treating real printing
wastewater under various experimental conditions (Safwat, 2020). The
results showed that at a current density of 20 mA/cm
2
, the zinc electrode
achieved the highest COD removal efciency of ~ 50 % after 90 min. In
comparison, the Ti electrode achieved 46 % removal at a current density
of 15 mA/cm
2
. Furthermore, the zinc electrode demonstrated enhanced
total dissolved solids (TDS) removal efciency compared to the Ti
electrode across all tested current densities (5 mA/cm
2
to 20 mA/cm
2
).
The EC process also exhibits signicant promise for removing oil from
diverse high-load wastewater streams, including biodiesel wastewater,
car wash efuents, restaurant efuents, and oileld-produced water.
(Kadier et al., 2022). Despite the widespread use of the EC process for
water and wastewater treatment, certain limitations are associated with
it. First and foremost is the quick depletion of sacricial anodes, which
necessitates their periodic replacement. Furthermore, electrode passiv-
ation can reduce the overall process efciency. Therefore, it is
recommended to combine the EC process with other water treatment
techniques to overcome these limitations and enhance the overall per-
formance of the treatment system.
3. Combined EC processes
Previous studies have demonstrated that EC can effectively remove
heavy metals and suspended solids (Bajpai et al., 2022; Islam, 2019).
However, it cannot eliminate refractory organics and reduce COD and
microorganisms. COD and BOD are two crucial water quality parameters
used to evaluate the impact of discharged wastewater on natural water
bodies. High COD concentrations indicate a large amount of oxidizable
organic material in the solution, which can deplete dissolved oxygen
levels threaten aquatic life. The acceptable limits for COD and BOD in
surface water are typically 250 mg/L and 30 mg/L, respectively (Dhadge
et al., 2018). Reducing BOD and CODs to these limits solely through the
EC process is challenging. These contaminants may persist in treated
wastewater, leading to environmental issues such as fouling down-
stream ltration membranes Additionally, chlorides in wastewater are a
signicant environmental pollutant that can have adverse effects on
human health, including hypertension, dehydration, and gastrointes-
tinal symptoms. Moreover, in industrial settings, chlorides cause pipe
blockage, demineralization, corrosion, and scaling, as reported by
Deepti et al. (2020) (Deepti et al., 2020). The water taste changes
drastically for chloride concentration in drinking water at >250 mg/L
(Kumar and Puri, 2012). Hence, it is imperative to adequately process
wastewater efuents from diverse origins prior to their reuse, discharge
into natural water bodies, and eventual use for human consumption.
The limitations of the individual EC process in treating real waste-
water efuents are well recognized due to the complexity of such ef-
uents. Consequently, there has been increasing interest in
combinational and/or hybrid techniques, including membrane pro-
cesses (Changmai et al., 2022), chemical (Kalia et al., 2023), biological
methods (Le et al., 2021), and electrochemical methods such as AOPs
(Villase˜
nor-Basulto et al., 2022). These techniques offer rapid reaction
rates, high removal efciencies, and low operational costs (¨
Ozyonar and
Korkmaz, 2022; Yazici Guvenc et al., 2022a). Researchers have made
signicant efforts to address these challenges, developing hybrid/-
combined EC processes that combine other effective wastewater treat-
ment techniques, as illustrated in Fig. 3.
3.1. Combined EC-Membrane-based processes
Despite using different conventional treatment methods for waste-
water, such as textile efuent, these approaches have proven insufcient
in effectively eliminating resistant compounds present in the wastewater
(Yabalak et al., 2021). Textile industry wastewater contains dyes, salts,
surfactants, acids, bases, and oxidants, posing challenges for conven-
tional treatment methods. Biological and physicochemical approaches
have been historically employed but need help to remove
non-biodegradable dyes and salts despite achieving high COD removal
rates. Physicochemical methods face limitations regarding low con-
ductivity and soluble COD removal, costly chemicals, and signicant
sludge production (G¨
onder et al., 2020). Membrane processes, including
ultraltration (UF) and nanoltration (NF), have shown promise in
treating textile efuent. NF and reverse osmosis (RO) membrane pro-
cesses demonstrate exceptional performance in removing COD and
color, making them suitable for reusing textile wastewater (´
Curi´
c et al.,
2021). However, membrane treatment processes have limitations when
applied to textile efuents. Membrane fouling can lead to low water ux
and signicant ux decline, hampering the potential of membrane
processes. It has become evident that a single treatment method alone
cannot meet stringent discharge standards and achieve desired ef-
ciency levels. Therefore, implementing combined treatment technolo-
gies is necessary to ensure high-quality treated wastewater and facilitate
wastewater reuse (Anis et al., 2019; Cinperi et al., 2019).
A.A. Shah et al.
Water Research 252 (2024) 121248
5
Due to its numerous advantages, the EC process has gained attention
as a combined treatment method in wastewater treatment research
(Bili´
nska et al., 2020; GilPavas and Correa-Sanchez, 2020). These ad-
vantages include eliminating the need for chemical additions, reducing
salt content in treated wastewater, minimizing alkalinity consumption,
generating minimal sludge, enabling simple operation, suitability for
portable plants, environmental friendliness, and the ability to treat large
volumes of wastewater with diverse pollutants. The EC process effec-
tively removes inorganic colloidal and organic matter, major causes of
fouling in membrane processes. It is widely exploited as a pre-treatment
method for various water and wastewater sources (G¨
onder et al., 2020).
While numerous studies have been conducted on the combined EC
process and its applications in wastewater treatment, only a few re-
searchers have explored the integration of EC with membrane processes
(Haz et al., 2020, 2019; Mazumder et al., 2020; Sardari et al., 2019,
2018b, 2018a; Xu et al., 2022, 2021; Zhang et al., 2019).
Eda et al. conducted a study on treating biologically treated textile
efuent using the combined EC–nanoltration–reverse osmosis
(EC–NF–RO) system (Günes¸ and G¨
onder, 2021). They evaluated the
efciency of the process and compared the treatment performances and
membrane fouling behaviors of NF and the combined EC–NF systems.
The results demonstrated remarkable removal efciencies achieved by
the combined EC–NF–RO process. They observed removal efciencies of
over 93 % for COD, 99 % for conductivity, 97 % for chloride, and 91 %
for TDS. The high-quality water obtained through the combined EC +
NF 270 process, followed by reverse osmosis (RO), was deemed suitable
for all textile nishing processes.
In another study, Hawli et al. investigated the treatment and recla-
mation of Produced Water using a novel combined system combining EC
and forward osmosis (FO) (Al Hawli et al., 2019). The authors presented
the schematic diagram of the proposed combined system. The results
showed signicantly higher removal efciencies achieved by the com-
bined system than by standalone processes. Specically, the combined
system achieved removal efciencies of 99 % for total suspended solids
(TSS), 98 % turbidity, and 16 % for conductivity. The improved water
quality obtained from the combined system highlights the practicality
and potential of this approach for water treatment.
Tavanger et al. conducted a study on the treatment of real textile
wastewater using individual EC and NF processes, as well as their
combination(EC-NF) (Tavangar et al., 2019). Various electrode mate-
rials, including AlAl), Fe, and Ti, were utilized for EC pretreatment. The
results showed that EC with an Al electrode outperformed the other
electrodes, achieving ~ 64 % and 94 % removal of COD and color,
respectively. The NF membrane exhibited excellent color removal (over
87%) and extremely low rejections of inorganic salts (less than 4 %). The
hybridization of EC-NF proved benecial by leveraging the strengths of
each process and mitigating their individual limitations. The imple-
mentation of combined EC-NF processes enables the treatment and reuse
of wastewater with high productivity, efcient dyes/salts fractionation,
and low membrane fouling. However, the practicality of the
EC-membrane combined process is constrained by several factors. These
include the complexity and cost of implementation, higher energy re-
quirements compared to standalone methods, and the maintenance and
operational challenges associated with integrating EC and membrane
technologies (Tavangar et al., 2019; Thamaraiselvan and Arnusch,
2021).
3.2. Combined EC-chemical-based processes
While this review centers on the period between 2018 and 2023,
there appears to be limited literature available on combined
electrochemical-chemical processes. Chemical coagulation (CC) is a
widely used water treatment process that relies on the addition of
chemicals to induce the destabilization and precipitation of dissolved
and suspended substances (Hendaoui et al., 2021; Shamaei et al., 2018).
However, treated wastewater from this process often retains a notable
concentration of residual chemicals, necessitating an additional process
to effectively address this challenge. The integration of the benets of
both EC and CC techniques through a combined EC-CC approach has the
potential to create a more energy- and cost-effective method for pur-
ifying water (Abujazar et al., 2022).
For instance, in 2023, Juan et al. investigated the synergistic impact
of EC-chemical precipitation treatment (EC-CP) on actual wastewater
from the denim industry (Zaldivar-Díaz et al., 2023). The combined
EC-CP treatment demonstrated synergistic effects, resulting in removal
efciencies of 91 % for COD, 93 % for color, and 94 % for turbidity. The
energetic costs associated with EC and EC-CP treatments were reported
as 1.82 and 0.25 US$/m
3
, respectively. Furthermore, the primary factors
inuencing both EC and EC-CP processes were identied as the current
density and hydraulic retention time (HRT), with the hydraulic reten-
tion time ranking as the next signicant factor. In the combined EC-CP
process, CaCl
2
was utilized as a multifunctional support serving as
both the electrolyte and precipitating agent.
Adsorption is an alternative chemical process for treating
Fig. 3. Schematic diagram of combined EC processes for wastewater treatment.
A.A. Shah et al.
Water Research 252 (2024) 121248
6
wastewater, offering a straightforward and effective approach to remove
a wide range of pollutants (Dolatabadi et al., 2022; Jalil et al., 2019; Sia
et al., 2020). However, operating costs can increase due to the need for
adsorbent regeneration and disposal. Wet air oxidation is a highly
suitable method for treating toxic wastewaters with high organic loads
(Crini et al., 2019). Nevertheless, its applicability is limited due to the
demanding requirements of high pressure and temperature during
operation. In this context, Bulca et al. (2021) conducted a study
comparing the effectiveness of adsorption and catalytic wet air oxida-
tion as sequential treatments following EC for enhancing the quality of
wastewater generated by the textile industry (Bulca et al., 2021). The
ndings demonstrated that the EC/adsorption process resulted in higher
wastewater quality compared to the sequential EC and catalytic wet air
oxidation treatment. Both processes proved effective in removing TOC,
color, turbidity, and TSS, meeting the standards for irrigation water.
In 2022, Akarsu et al. conducted a study on the combined EC and
activated sludge processes to investigate the removal of COD, oil-grease,
and color from wastewater generated by vegetable oil production
(Akarsu et al., 2022). Various operating parameters were examined,
including current density, pH levels, retention times, and the use of
different electrode types (Al-Al and Fe-Fe). The highest removal ef-
ciency was achieved with Al-Al electrodes in the EC process, specically
at a current density of 300 A/m
2
, pH 2, and a retention time of 180 min.
When employing the combined EC-SBR process, impressive removal
efciencies of 99.9 % for COD and 93.0 % for color were achieved.
Furthermore, the efuent resulting from the combined process exhibited
exceptional clarity, surpassing the direct discharge standards outlined in
water pollution control regulations.
In a similar study, Myllym¨
aki et al. (2018) conducted a research
study to explore the impact of a novel approach combining activated
carbon (AC) adsorption and EC techniques on the removal of TOC from
peat solution (Myllym¨
aki et al., 2018) as shown in Fig. 4. By imple-
menting the combined method, a remarkable TOC removal efciency of
~ 95 % was achieved. The process involved an initial adsorption
treatment followed by EC using either AlAl or Fe electrodes. In contrast,
when employing either the adsorption or EC method individually, the
removal of organic substances ranged from 79 % to 89 %.
These ndings suggest that the combined method combining acti-
vated carbon adsorption with Al/Fe-EC has been demonstrated as an
effective approach for removing organic matter from aqueous solutions,
specically evident in the successful removal of TOC from peat solu-
tions. However, the combined EC-chemical wastewater treatment pro-
cess faces challenges including increased system complexity, higher
energy consumption compared to standalone chemical methods,
elevated chemical costs, potential residual chemicals in treated water,
process optimization difculties, and limited research and literature on
the topic.
3.3. Combined EC-biological-based processes
In recent years, combined treatment systems combining EC with
biological treatment processes have emerged as promising alternatives
for effectively treating heavily contaminated wastewater and producing
high-quality efuents. This section examines studies conducted between
2018 and 2023 involving EC followed by biological treatment processes.
While biological methods are effective for removing organic compounds
in industrial wastewater, they may struggle with non-biodegradable and
toxic pollutants, and high loads of colloidal matter and suspended solids.
Single conventional biological processes may not meet necessary envi-
ronmental standards, leading to a growing need for innovative, cost-
effective, and sustainable techniques for wastewater treatment (Al-Qo-
dah et al., 2020; 2018; Al-Qodah and Al-Shannag, 2019). EC has become
a promising standalone or combined treatment process alongside bio-
logical and other wastewater treatment processes (Al-Qodah et al.,
2019). However, limited research has been conducted on the waste-
water treatment using a combination of biological and EC processes
(Al-Qodah et al., 2020; Shin et al., 2020). A schematic diagram illus-
trating the combined EC-biological treatment system is presented in
Fig. 5.
A study conducted by Deveci et al. in 2019 showcased the potential
of EC and Biological Fungal Treatment (BFT) as effective technologies
for removing impurities from tannery wastewaters, particularly those
with toxic or highly organic content. The research focused on investi-
gating the treatment performance of an integrated EC and BFT process
using response surface methodology (RSM) to analyze the impact of
different operating variables on the treatment efciency of tannery
wastewater (Deveci et al., 2019). In a separate investigation, Dermouchi
et al. (2021) examined the feasibility of integrating an EC process with a
biological treatment for the biodegradation of cutting oil emulsions
(COE). The results were highly satisfactory, with an additional COD
reduction of over 98.5 % achieved, along with a favorable biomass ac-
tivity of 40 mg O
2
/Lh for the pretreated COE. Furthermore, the
non-pretreated COE solutions achieved a COD reduction of 94 % after 33
days of operation following a 52-day biomass acclimatization period.
Overall, a total COD removal of 99.8 % was attained, with a residual
value of 177 mg/L, highlighting the successful combination of a bio-
logical treatment with an EC pretreatment process.
In 2022, Al-Othman conducted a study on a modied biological-
integrated EC method for treating municipal wastewater (MWW) with
the aim of using the treated wastewater for irrigation purposes
Fig. 4. A schematic diagram of the combined EC-adsorption method (Adopted from (Myllym¨
aki et al., 2018)).
A.A. Shah et al.
Water Research 252 (2024) 121248
7
(Al-Othman et al., 2022). The study focused on the removal of various
contaminants, including turbidity, hardness, conductivity, TDS, TSS,
chloride, ammonia nitrogen (NH
3
-N), BOD, COD, and total coliform, to
ensure the suitability of the treated wastewater for irrigation. The results
demonstrated signicant removal efciencies achieved by the combined
process, including 78.8 % for turbidity, 56.8 % for hardness, 28.4 % for
conductivity, 37.4 % for TDS, 98.3 % for TSS, 27.6 % for chloride, 26.7
% for NH
3
-N, 78 % for BOD, 81 % for COD, and 99.9 % for total coliform.
The energy consumption was measured at 9.9 Wh/L, and the operating
costs associated with the chosen synthesis method amounted to 0.76
$/m
3
of MWW.
The disadvantage of EC-biological wastewater treatment is the
complexity of the integrated system, which may require additional
equipment, monitoring, and control mechanisms. This complexity can
result in higher operational and maintenance costs. Additionally, the
optimization of operating parameters for both EC and biological pro-
cesses can be challenging, requiring extensive experimentation, and
ne-tuning, which limits its further application.
3.4. Combined EC-Electrochemical processes
While there are multiple methods used for water and wastewater
treatment, including biological, chemical, membrane-based processes,
and electrochemical treatments like EC, electrooxidation, and electro-
otation, EC has shown promise. Table 1 provides a comprehensive
presentation and detailed discussion of the performance parameters and
characteristics of combined EC-electrochemical processes. However, it
has limitations in effectively removing persistent organic compounds
(Islam, 2019). Consequently, there is a growing need to explore and
adopt combined technologies that integrate EC with other electro-
chemical treatments, such as AOPs, to improve treatment efciency and
effectively address the full spectrum of contaminants found in waste-
water. In recent years, electrochemical (EC) wastewater treatment has
emerged as a viable decentralized wastewater treatment option. It has
proven effective in eliminating a wide range of microbial pathogens and
organic pollutants in diverse water sources. The on-site generation of
disinfectants through EC offers environmental sustainability and
user-friendly characteristics, including low energy consumption and
ease of operation (Martínez-Huitle et al., 2018). Moreover, EC waste-
water treatment systems hold the potential to be powered by solar en-
ergy making them particularly valuable in regions with limited access to
reliable energy supplies (Hand and Cusick, 2021; Martínez-Huitle and
Panizza, 2018). Tharaa et. al has put forward the cost analysis of
pharmaceutical wastewater treatment using EC process when combined
with chemical coagulation and using solar for electrical needs. It clearly
shows that the combing solar can make it practical option and reduce the
overall cost by 20–30 % (Al-Zghoul et al., 2023).
Signicant advancements have also been made in the development
of electrochemical advanced oxidation processes (EAOPs) as highly
efcient and effective methods for wastewater treatment. These pro-
cesses harness the generation of diverse reactive species capable of
oxidatively breaking down organic pollutants across various classes.
EAOPs are particularly appealing due to their environmentally friendly
nature, as they rely on the utilization of electrons as a clean reagent.
Moreover, they offer notable advantages such as versatility, adapt-
ability, and safety, as they can be performed under ambient conditions
(Ganiyu et al., 2020, 2018). The formation of reactive species in EAOPs
is inuenced by several key factors, including the composition of the
contaminated water, the material of the electrodes, and the applied
current density or electric potential. These factors play a crucial role in
determining the type and quantity of reactive species generated.
Furthermore, studies have highlighted the signicant impact of inor-
ganic components in the contaminated water on the production of sec-
ondary oxidants and the scavenging of primary reactive species,
particularly hydroxyl radicals (
•
OH) (Ganiyu et al., 2021; Ganiyu and
Martínez-Huitle, 2019; Garcia-Rodriguez et al., 2022).
Additionally, there are different methods of generating radicals in
EAOPs such as electroreduction, electro-fenton, photo-fenton, sono-
fenton and ozonation. This categorization helps in understanding the
different mechanisms and pathways involved in the electrochemical
generation of reactive species for wastewater treatment. Overall, the
integration of EC with electrochemical processes, such as AOPs, offers
great potential for enhancing the treatment efciency and addressing
the complex contaminants present in wastewater. These combined EC-
electrochemical processes provide a comprehensive approach to
wastewater treatment, combining the advantages of both EC and
advanced oxidation processes. However, further research is needed to
optimize the operating parameters and assess the economic feasibility of
these combined systems.
3.4.1. Combined EC-electrooxidation
Electrooxidation (EOx) is an advanced oxidation process that in-
volves electrolytic reactions at the electrode surface. During this process,
anodic metal and metal hydroxide cations are formed in the aqueous
phase. Soluble or colloidal pollutants can be adsorbed onto the surface of
metal hydroxides, and organic pollutants can be removed through
Fig. 5. Schematic diagram of EC-biological wastewater treatment process.
A.A. Shah et al.
Water Research 252 (2024) 121248
8
Table 1
Combined EC processes and their performance characteristics.
Combined processes Wastewater Design and process parameters Combined process operating
parameters
Process efciency Comments Year/References
EC-
Electrooxidation
(EOx)
Container washing
wastewater
Electrodes Fe, Al, and BDD
(Cathode).
IED =2 cm. pH =11.5. Area =36
cm
2
Current =25 mA/cm
2
Initial pH =5
Operation time =120-min
COD removal efciency 99% The combined EC–EO process, it has been
observed that COD removal efciency
increases with time
2018 (Yılmaz Nayır and
Kara, 2018)
Virus mitigation in
drinking water
Batch volume =200-Ml
Steel electrodes (EC), BDD/Si for EO.
Pure Grade 2Ti as an inert cathode.
Effective area =15 cm
2
Experiment Condition =2.1
mM NaHCO
3
pH =7 ~ 8
Total reaction time =5 min
RPM =200
- The improved virus mitigation achieved by
EC-EO in model surface waters
2019 (Heffron et al.,
2019)
Removal of estrogenic
compounds from water
Fe-Steel electrodes
Effective area =18 cm
2
Current =8.88 mA/cm
2
Operation time =8-min
RPM =40. pH =~ 6
DOC =~90% EC-EO system suggested that this process
may be more efcient than AOPs
2020 (Maher et al., 2020)
Dairy wastewater Bipolar connection mode. Titanium
(TiO
2
), Al and Fe.
IED =7.5 mm, pH 6–9
RPM =100
Working volume =500 mL
Conductivity =20.08 mS/ cm
COD removed 99 % in 2 h. Turbidity
97.87 % removed in 40 min.
The optimum COD and turbidity removal
efciencies were obtained at an applied
current 3 A for BP2.
2021 (Turan, 2021)
Cotton Textile Industry
wastewater
Electrodes; 2-Anode for EC Al and for
EO IrO
2
/Ti. Cathode =Al
pH =4, RPM =150
Current density =42 mA/cm
2
,
Electrolysis time =42 min
Reactor volume =500 mL
Working volume =400 mL
Anode area =65 cm
2
Cathode area =48 cm
2
Red 3 BS 150 % reactive dye
conc. 600 mg/L
EC process achieved 89 % color and
76 % COD removal rate. 97 % COD
and color.
Full mineralization was obtained by the
combined EC-EO treatment technology.
2022 (Asfaha et al.,
2022)
Acidied biodiesel
wastewater
Al-Al electrode (EC),
Ti/SnO
2
anode for EO
Current density =0.5 A
Operation time =150-min
pH =6.16
COD, TSS, and oil-grease RE were
98.9, 98.2, and 99.8 %, respectively.
The combined EC-EO process has a
potential to treat biodiesel wastewater
with a high pollutant load.
2022 (Yazici Guvenc
et al., 2022b)
Vegetable oil renery
wastewater
Al and stainless-steel (SS) electrodes,
RPM =500
CD =30 mA cm
2
Operation time =240 min
pH =4,
SEC =2.067 kWh/kg COD
Combining EC with EO resulted in a
total COD RE% of 98.72.
%. and a total energy consumption of
2.067 kWh/kg COD.
2023 (Saeed et al., 2023)
Slaughterhouse
wastewater
Anodes =Ti/IrO
2
-Ta
2
O
5
Cathode: Stainless steel
EC electrodes =Al & Fe
J =20 mA cm
–2
Area =5 cm
2
,
Distance =1 cm
pH =7.18 ±0.1
Removal Efciencies =93 (TOC), 85
(COD), 76 (TP), and 91 % (TN)
Complete discoloration and disinfection of
cattle SHWW were achieved
(Sandoval et al., 2022)
Poultry slaughterhouse
wastewater
Iron electrodes, monopolar
conguration
Treatment time =90 min.
Current density =40 mA
cm
–
2
,
pH =7.1,
Flow rate =0.05 L min
–
1
.
Removal Efciencies COD =
(89.4%).
Peroxy-EC =COD (95.5%)
Polyaluminum chloride (PAC) as a
coagulant-aid enhances the removal
efciency.
(Eryuruk et al., 2018), (
Sandoval and Salazar,
2021)
EC – O
3
Textile Industry
wastewater
Reactor Volume =2 L
Electrodes =11
Area =100 cm
2
pH =11.82
Current =1–10 A
RPM =200
EC-O
3
treatment gives more than
95% color removal
Two-step EC → O
3
treatment was more
economically justied
2019 (Bili´
nska et al.,
2019)
Greywater Al Al and Fe electrodes, Greywater
volume =750 ml, Area =30 cm
2
,
IED =2 cm
pH =7.0, O
3
=47.4 mg/L CD
=15 mA/cm
2
, and
electrolysis time =60 min
85% of COD and 70% of TOC were
removed by EC/O
3
O
3
compared to other chemical oxidants
(H
2
O
2
, PMS and PDS) was more efcacious
in EC/O
3
process.
2019 (Barzegar et al.,
2019)
Distillery spent wash Al–Al electrodes
Effective area =71.50 cm
2
. NaCl =
2000 mg/L
pH =3.21
Current density =9.75 A/
cm
2
, Inter electrode Distance
=3 cm
EC-O
3
COD and color RE 97.27%
and 98.72%, respectively.
Ozone assisted EC was found more
benecial as compared to plain EC.
2020 (Wagh et al., 2020)
Cardboard factory
wastewater
Anode Fe electrode.
Cathode Stainless Steel. Effective
area =62.8 cm
2
. Batch volume =1K
mL
Current =9.6 mA/cm
2
, Time
20 min, and pH 12.
Ozone dosage =1 g/h
EC/O
3
removed 74.7 % and 97.5 %
of COD and color.
EC/O
3
process as a fast and suitable
method for real cardboard wastewater.
2021 (Mehralian et al.,
2021)
(continued on next page)
A.A. Shah et al.
Water Research 252 (2024) 121248
9
Table 1 (continued )
Combined processes Wastewater Design and process parameters Combined process operating
parameters
Process efciency Comments Year/References
Tofu wastewater Anode (Al) and cathode (ferrous
plates)
Batch volume =500 mL
pH =3.75
Effective area =0.0135 m
−2
Operation Time =60 min.
Fixed voltage =20V
EC/O
3
highest turbidity removal of
78.3 % and COD 51.9 %.
EC/O
3
is effective in reducing pollutants in
tofu wastewater.
2022 (Oktiawan et al.,
2022)
EC-Electro-Fenton Heavy metal
wastewater
Stainless-steel Cathode, Fe plate
anode.
Time =30 min
H
2
O
2
Conc. =49.4 mM
Current Density =72.92 A/
m
2
. pH =3
Removal efciency of Cu-EDTA
98.2%
Hydroxyl radicals were responsible for the
destruction of Cu-EDTA
2018 (Guan et al., 2018)
Pharmaceutical
wastewater
Electrodes =Fe
–
Fe
Volume =300 mL
Plate area =25.5 cm
2
IED =2 cm
Current =7.8 mA cm
−2
pH =3–11
Operation time =60 min
RPM =800
Achieving 88.5, 90, and 84.5% TOC
removal in the presence of PMS, PS,
and H
2
O
2
, respectively.
The role of H
2
O
2
, PMS, and PS which
decides the formation of free reactive
radicals and oxidative species.
2020 (Govindan et al.,
2020)
Leachate concentrate Fe anode, Carbon-Felt Cathode, DSA
Anode
Working area =48 cm
2
Volume =1000 mL
Current =30 mA/cm
2
Voltage =7 V
IED =1–2 cm
Energy =67.8 kWh/kg COD
The process reduced 57 % of the
COD and 60 % of the NH
4
+
EC-EF like reactions simultaneous removal
of organics and NH
3
in treating LC.
2021 (Ding et al., 2021)
Tannery
wastewater
Boron–doped diamond (BDD) as an
anode. Carbon-Felt Cathode.
Volume =500 mL
Time =2 h (EF) - 5 h (EC)
pH =3.8, RPM =200
Effective Area =50 cm
2
IED =0.3 cm
Color and COD removals were 100
% and 88 %.
Combination avoids environmental issues
related to the disposal of EC sludge.
2022 (Rezgui et al.,
2022)
Heavy metal
wastewater
N-Co/Fe-PC cathode and a graphite
anode.
Volume =200 mL
Current =50 - 150 mA
pH =3 – 7, IED =2 cm
NaCl =10– 80 mg/L
99.69, 96.40, and 83.62% removals
of Cu, Ciprooxacin, and TOC were
obtained.
Heterogeneous electro-Fenton-EC
exhibited excellent performance.
2023 (Sun et al., 2023)
EC-Photo- Fenton
(UV)
Steel wastewater Al electrodes
Volume =1000 mL
IED =3 cm
Na
2
SO
4
=10 mg/L
Phenol =50–200 mg/L
(Fe
2+
/H
2
O
2
=1.5 1.5,
pH =4, Time =25 min
CD =1.5 mA/cm
2
Power =29.15 kWh/kg COD
Phenol and COD removal using EC-
PF are 100 and 97 %, respectively.
The EC-PF process is an effective process
for the removal of organic materials.
2018 (Malakootian and
Heidari, 2018)
Real tannery
wastewater
2 Al electrodes
IED =1 cm
Working Volume =180 m/L
Current =500 mA
pH =11.41, Time =20 min
TDS =62.4 %
COD =50.3 %
Very efcient treatment of high strength
real tannery wastewater.
2019 (Moradi and
Moussavi, 2019)
Tannery wastewater Al electrodes Batch Volume =1 L
IED =30 mm
Current density =12 A/cm
2
pH =10.78
Experiment run =90 mints
EC-UV treatments combined
afforded 94.1 % of COD reduction.
EC and UV treatment effective for the
reduction of the COD.
2020 (Jallouli et al.,
2020)
Raw Slaughterhouse
wastewater
Ti-RuO
2
anode,
Carbon felt cathode.
1-L batch reactor
pH =3, Current density 2.5
mA cm
–
2
,
Inuent treatment time 60,
60, and 80 min,
Removal Efciency of TOC 92, 92,
and 95 %
Removal efciencies of COD, up to 95 %,
using the combination of Fenton & solar
photo-Fenton process
(P´
aramo-Vargas et al.,
2016)
Distillery industrial
wastewater
Electrode =Fe/Fe, Inter–electrode
distance =0.75 cm, pH =7, Reaction
time =4 h
Current =0.175 A dm
−2
Power =6.97 kWh m
−3
UV power =32 W
Combined process removal
efciency for color-100% and COD
−95.63%.
Combined process is effective in treating
industrial efuent and wastewater.
2023 (Asaithambi et al.,
2022b)
EC-Sono Brewery wastewater Electrode =Al/Al
Inter–electrode distance =2 cm
Volume =800 mL
Current =100 A/m
2
pH =7.0
Reaction time =60 min
Surface Area =23 cm
2
Color and COD were obtained as
99.2 and 60.5 %.
Efcient technique for pollutants removal. 2018 (Dizge et al., 2018)
Leachate from
municipal solid waste
Batch Volume =2 L
US power =360 W
Gap =3 cm
Current =18-mA/cm
2
Treatment time =60 min.
COD removal was found to be 98 %
in the case of sono-EC.
Sono-EC was found to be efcient in fresh
leachate treatment.
2018 (Afsharnia et al.,
2018)
Landll Leachate
Wastewater
Sono power =100 W
Time =36.05 min.
Surface Area =40 cm
2
Current =2.75 A/dm
2
Power consumption =2.33
kWh/m
3
Gap =0.80 cm
Combined US+EC had a high color
(100 %) and COD (94 %) removal
efciency.
Combined US+EC process, enhances the
efciency of pollutants removal.
2021 (Asaithambi and
Govindarajan, 2021)
Industrial efuent
wastewater treatment
Electrode =Al/Al
Batch Volume =600 mL
Gap =1–3 cm
Current =0.30 to 1 A/dm
2
Electrolysis Time =45 min
Initial efuent pH =4 - 10
The maximum removal of
COD—97.50% and color—100%.
Minimum power consumption—0.55
kWh/m
3
using the combined SEC process.
2022 (Arka et al., 2022)
A.A. Shah et al.
Water Research 252 (2024) 121248
10
processes such as electrootation, sedimentation, and adhesion to
bubbles. The precipitation of metal hydroxides is signicantly inu-
enced by the pH of the wastewater (Ungureanu et al., 2020). The
removal of pollutants in EOx can be attributed to two main mechanisms:
direct anodic oxidation and indirect oxidation. In direct oxidation,
pollutants in the wastewater are mineralized by hydroxyl radicals (
•
OH)
during the anodic electron transfer reaction at the electrode surface, as
shown in Fig. 6 (a) (Aguilar et al., 2018; Sharma and Simsek, 2019). In
indirect oxidation, when chloride is used as the supporting electrolyte,
active chlorine is electrochemically generated, leading to the oxidation
of pollutants. The choice of electrode material plays a crucial role in the
efcacy of electrochemical treatment(Garcia-Segura et al., 2020; Liu
et al., 2019; Salazar-Banda et al., 2021) However, a limitation of EOx is
its reduced effectiveness in treating water and wastewater with high
levels of suspended solids. Therefore, it is necessary to remove sus-
pended solids from the wastewater using alternative techniques before
employing the EOx treatment system. One such approach is the inte-
gration of EOx with other methods, such as the combined process of EC
and EOx (Asfaha et al., 2021).
In recent years, the combination of EC and EOx processes, either in
sequential or simultaneous arrangements, has been employed for
wastewater treatment(Asfaha et al., 2021; ¨
Ozyonar and Korkmaz, 2022)
. For example, Kara et al. (2018) conducted a study on the treatment of
container washing wastewater (CWW) using a combined EC-EOx pro-
cess. CWW is known to contain various organic compounds, including
surfactants from cleaning agents (Yılmaz Nayır and Kara, 2018). The
Fig. 6. (a) Principle of Operation for EO Process in Wastewater Treatment (adopted from (Kaur et al., 2019)). (b) Schematic of the bench-scale EC-EO treatment
process and associated sampling points. Each process was operated as a batch system (Lynn et al., 2019).
A.A. Shah et al.
Water Research 252 (2024) 121248
11
results showed that in the combined EC-EOx process, the removal ef-
ciency of COD improved over time, but the oxidation time had limited
effectiveness based on UV–vis scans. Analysis using gas chromatogra-
phy–mass spectrometry (GC–MS) revealed the formation of complex
chemical compounds during the process. In another study, Heffron et al.
(2019) investigated the mitigation of viruses using EC as a pretreatment
followed by electrooxidation treatment with boron-doped diamond
electrodes (Heffron et al., 2019). The ndings indicated that compared
to conventional treatment methods involving ferric salt coagulant and
free chlorine disinfection, the EC-electrooxidation system demonstrated
lower effectiveness in surface waters but higher effectiveness in
groundwaters.
The results suggest that while the EC-EOx treatment system was not
universally benecial across all water sources, the improved virus
mitigation observed in model surface waters highlights the need for
further investigation. The enhanced virus reduction achieved by EC-EOx
was likely attributed to a combination of physical removal through
coagulation/ltration, ferrous iron-based disinfection, and electro-
oxidation disinfection, as illustrated in Fig. 6 (b).
Isik et al. (2020) conducted a study to investigate the effect of
electrochemical pre-treatment on fungal treatment of pistachio pro-
cessing wastewater (PPW). EC-electrooxidation (EC-EOx) was employed
as electrochemical pre-treatment prior to fungal treatment of PPW (Isik
et al., 2020). The study evaluated the inuence of current density
(50–300 A/m
2
) and operating time (0–240 min) on the removal of COD
and total phenols. The combined electrochemical-assisted fungal treat-
ment process achieved a remarkable removal efciency of 90.1 % for
COD and 88.7 % for total phenols when supported by EOx
pre-treatment. The EOx method exhibited superior performance in
pre-treating PPW compared to the EC method for fungal treatment. The
ndings demonstrated that the proposed combined process yielded
higher pollutant removal compared to individual approaches involving
EC, EOx, and fungal treatment.
Ryan et al. (2021) investigated the combined use of EC and EOx for
water treatment. They found that EOx alone was effective in removing
certain trace organic compounds (TOrCs) in groundwaters but had
limitations for other compounds (Ryan et al., 2021). In surface waters,
EOx showed moderate removal for some TOrCs but was hindered by
dissolved organic carbon (DOC) interference. However, when EC was
used in combination with EOx, it signicantly improved the removal of
TOrCs in surface waters by reducing DOC levels. The sequential EC-EOx
process demonstrated synergistic effects in treating surface waters with
higher DOC levels, while no signicant improvement was observed in
waters without DOC.
Sanni et al. (2022) aimed to strategically integrate EC and EOx
processes for the treatment of highly contaminated wastewater from
industrial container wash water (IWW), as illustrated in Fig. 7 (Sanni
et al., 2022). The wastewater had high levels of COD and phosphorus
concentration. The results showed that the combined EC-EOx process
achieved signicant removal of phosphorus and COD, with 97 % and 95
% removal rates, respectively, meeting the sewer discharge standard.
The combined process of EC-EOx demonstrated superior performance,
resulting in a nal COD concentration of 205 mg/L compared to the EOx
treatment alone, which achieved a nal COD concentration of 720
mg/L. Similarly, Asfaha et al. conducted an evaluation of the combined
EC-EOx process to assess its effectiveness in removing color, TOC, and
COD (Asfaha et al., 2022). The ndings indicated that the combined
EC-EOx process achieved remarkable efciency of 97 % in COD and
color removal. The extent of dye degradation was analyzed using a
combined approach of Fourier-transform infrared spectroscopy (FT-IR)
and proton and carbon nuclear magnetic resonance spectroscopy (1H
and 13C NMR) conrming the complete degradation of the organic
contaminants into carbon dioxide and water. This study demonstrates
that the utilization of mesh IrO
2
/Ti electrodes as a treatment method
holds great promise in meeting the discharge limits for industrial
efuents.
In a recent study by Alvarez et al. (2023), a combined electro-
chemical process was investigated at the laboratory scale for treating
real wastewater generated from the processing of mackerel in an in-
dustrial facility (´
Alvarez et al., 2023). The EC stage, using Al anodes,
effectively removed larger suspended particles, resulting in a COD
removal of ~ 60 % at pH 7.5, which was more efcient than conven-
tional treatment methods. However, the desired level of removal was not
fully achieved, leading to the implementation of electrooxidation as the
subsequent step. EOx, employing a graphite anode and a Ti cathode,
exhibited rst-order oxidation kinetics and successfully achieved a nal
COD value below the discharge limit after 7.5 min of processing at pH 6.
This integrated treatment proved to be efcient in removing high con-
centrations of dissolved organic matter and suspended particles typi-
cally present in such wastewater streams. Bilgin et al. (2023) conducted
a recent study investigating the application of a combined EC-EOx
process for the treatment of mature leachate. The results showed that
the EC process achieved a COD removal of 56.6 %, while the EOx process
achieved a removal of 45.6 %. When the EC-EOx process was employed,
a higher COD removal efciency of 69.6 % was observed (Bilgin et al.,
2023).
The combined EC-electrooxidation processes demonstrate the po-
tential for enhanced wastewater treatment by combining the advantages
of EC and electrooxidation. These studies highlight the effectiveness of
such combined approaches in removing various pollutants and
improving the quality of treated wastewater. Further research is needed
to optimize operating parameters, evaluate the applicability of these
processes to different wastewater types, and assess the scalability and
cost-effectiveness of the combined EC-electrooxidation systems.
Fig. 7. Combined EC-EO setup for COD and Phosphorous removal from IWW (adopted from (Sanni et al., 2022)).
A.A. Shah et al.
Water Research 252 (2024) 121248
12
3.4.2. Combined EC-Ozonation
Ozonation, known as one of the most effective technologies for water
and wastewater treatment, enhances biodegradability by directly
oxidizing various organic and inorganic substances and reducing sludge
production (Rizzo et al., 2020). Ozone (O
3
) generates hydroxyl radicals
(HO
•
), highly potent and non-selective oxidants, enabling the indirect
oxidation of a broad range of pollutants (Ahmed et al., 2021). Due to
these capabilities, O
3
has been extensively studied in pilot and full-scale
applications for wastewater treatment (Nakhate et al., 2019; Sathya
et al., 2019). However, certain pollutants with high molecular weight
may exhibit resistance to ozonation. Therefore, the synergistic combi-
nation of EC and O
3
has emerged as a promising approach to enhance
the removal efciency of these pollutants, while simultaneously
reducing treatment time and sludge production, as illustrated in Fig. 8
(a). The combined EC and O
3
method has garnered considerable interest
from researchers in the treatment of industrial wastewater, showcasing
promising results, particularly in the treatment of wastewater (Ahan-
garnokolaei et al., 2021; Mohammad Ali Ahangarnokolaei et al., 2021;
Bili´
nska et al., 2019).
Barzegar et al. (2019) assessed the performance of the EC-ozonation
process in treating greywater and investigated the inuence of various
parameters on the removal COD and TOC (Barzegar et al., 2019).
Electrodes (Fe, 80 mm ×30 mm ×2 mm) were immersed in the grey-
water, and electrolysis was facilitated by a DC power supply. After 60
min of electrolysis time, under the conditions of pH =7.0, ozone con-
centration of 47.4 mg/L, and current density of 15 mA/cm
2
, a signicant
removal of 85 % for COD and 70 % for TOC was achieved. The combined
EC-ozonation process demonstrated remarkable effectiveness in
removing COD and TOC from greywater, as depicted in Fig. 8(b),
showcasing the working principle of the combined process. The
EC-ozonation system functioned optimally under neutral conditions,
aligning with the pH levels of the treated greywater being close to
neutrality.
Wagh et al. (2020) proposed a combined ozone-assisted EC tech-
nique to enhance the degradation of color and COD in distillery spent
wash (Wagh et al., 2020). A continuous ozone-assisted EC process was
conducted, and the optimized removal efciencies obtained were 97.3
% for COD and 98.7 % for color removal. The combined treatment of
ozone-assisted EC demonstrated enhanced degradation of complex
organic compounds into simpler biodegradable carboxylic acids, signi-
fying the breakdown of melanoidin.
Navas-C´
ardenas et al. (2022) conducted a study proposing a com-
bined approach combining sedimentation, EC, and ozone processes for
the treatment of wastewater in an MDF industry located in Ecuador. The
combined process achieved signicant removal efciencies for COD
(90.9 %), color (~100 %), total solids (TS) (73.7 %), and suspended
solids (SS) (99.7 %), resulting in a 4.5-fold increase in the biodegrad-
ability of the water. This combined process effectively improved the
water quality of the efuent from the MDF panel industry, complying
with the environmental regulations in Ecuador for most of the examined
physicochemical parameters.
The combined EC-ozonation processes demonstrate the potential for
improved wastewater treatment by combining the strengths of EC and
ozonation. These studies highlight the enhanced removal of pollutants
and improved water quality achieved through the synergistic effects of
EC and ozonation. Table 1 showcases an example of a combined EC-
ozonation process from the literature. It presents detailed performance
parameters and characteristics of this specic process. Further research
is needed to optimize operating parameters, evaluate the applicability of
these processes to different wastewater types, and assess the scalability
and cost-effectiveness of the combined EC-ozonation systems.
3.4.3. Combined EC-electro Fenton processes
EF is an advanced oxidation process that eliminates persistent
organic pollutants by generating hydroxyl radicals (
•
OH) through spe-
cic mechanisms (Bello and Raman, 2019). The EF process can be
classied into heterogeneous and homogeneous electro-Fenton-like re-
actions, depending on the mechanism of pollutant degradation. The
homogeneous Fenton reaction involves catalyst dissolution, generation
of •OH radicals, and subsequent oxidation of organic compounds. The
Fenton reagent, consisting of ferrous ions and an oxidant like H
2
O
2
,
produces highly reactive
•
OH radicals with oxidizing solid properties
(Yümün et al., 2022; Jain et al., 2018). This reactivity allows OH
•
to
react with organic compounds, converting them into organic radicals,
which effectively oxidize persistent pollutants. These radicals react with
organic compounds, converting them into organic radicals that effec-
tively oxidize persistent pollutants. The radicals undergo further
oxidation reactions, forming secondary and tertiary metabolites, ulti-
mately converting pollutants into nontoxic products such as CO
2
and
H
2
O, as shown in Eq. (5) (Hussain et al., 2021).
RH +OH
•
→ H
2
O +R
•
→ Oxidation (5)
In contrast to the homogeneous Fenton process, a heterogeneous
Fenton-like reaction occurs when Fe
+2
is substituted with Fe
+3
or other
transition metal ions in the Fenton reagent system. The heterogeneous
Fenton process relies on adsorption as the main mechanism. It involves
the adsorption of organic compounds onto the catalyst surface, in-situ
generation and action of
•
OH radicals on the organics, and subsequent
desorption of oxidation products from the catalyst surface, as presented
in Fig. 9 (Bas¸aran Dindas¸ et al., 2020; Hussain et al., 2021; Sun et al.,
2023). The EF process has demonstrated remarkable efciency in
completely degrading harmful organic substances found in various
Fig. 8. (a) Experimental setup for the combined EC +O
3
process. Ozone supply system: I. gas (O
2
) cylinder, II. gas dryer, III. ozone generator, IV. rotameter, V. ozone
meter, VI. diffuser; EC system: 1. mono-polar anode, 2. mono-polar cathode, 3. set of bi-polar electrodes, 4. electrochemical cell, 5. power supply, 6. magnetic stirrer,
7. circulation pump (adopted from (Bili´
nska et al., 2019)). (b) Combined EC +O
3
process working principal (adopted from (Barzegar et al., 2019)).
A.A. Shah et al.
Water Research 252 (2024) 121248
13
wastewater types, including dyes, heavy metals, landll leachate,
textile, drugs and pharmaceuticals, and electronic efuents (Baiju et al.,
2018; Garcia-Rodriguez et al., 2018; Jiang et al., 2020; P. Kaur et al.,
2019; Titchou et al., 2021; Yang et al., 2019). Challenges associated with
the EF process, such as the need for pH adjustments and associated costs,
can be overcome by integrating it with the EC process. This amalgam-
ation enhances the overall effectiveness of wastewater treatment and
mitigates the drawbacks related to pH adjustment (Nidheesh et al.,
2021b). Researchers have investigated the synergistic effects of
combining EC and Fenton-like reactions to address the limitations of the
EC process and capitalize on the offered benets.
Guan et al. (2018) examined the electrochemical degradation of
Cu-EDTA and subsequent removal of released Cu
2+
ions using a com-
bined EF and EC approach (Guan et al., 2018). Optimal conditions for
highly efcient destruction of Cu-EDTA complexes and removal of
copper ions from wastewater were achieved, and the removal of liber-
ated Cu ions involved processes such as adsorption, coagulation,
co-precipitation, and cathodic reduction.
Govindan et al. (2020) explored the degradation and transformation
mechanism of atenolol using EF, EF-like based on SO
4
•−
, and EC pro-
cesses (Govindan et al., 2020). The EF and EF-like processes demon-
strated higher degradation efciency than EC, with the EF (EC-H
2
O
2
)
system achieving the highest atenolol degradation efciency, while the
SO
4
•−
-based EF-like processes showed superior TOC reduction compared
to the
•
OH-based EF process. The addition of common oxidants
enhanced the efciency of the EC process for the treatment of waste-
water containing atenolol and various co-existing anions.
Ding et al. (2021) proposed and investigated a combined approach of
EC with an electro-Fenton-like process, utilizing a dual-anode system,
for the treatment of leachate concentrate (Ding et al., 2021). The study
examined the impact of pH, electrode gap, electrical charge, and sus-
pended solids on the removal efciency of organics and ammonia. The
results indicated successful removal of ~57 % of organics and 60 % of
ammonia under specic operating conditions. In this investigation, a
dimensionally stable anode electrode was employed, demonstrating the
capability to generate hydroxyl radicals. These radicals play a crucial
role in oxidizing organic materials and other persistent pollutants,
leading to the observed efcient removal as presented in Fig. 10(a).
The combined EC-electro Fenton processes offer potential
advantages for the degradation of persistent organic pollutants in
wastewater. By combining EC with Fenton-like reactions, these pro-
cesses can improve pollutant removal efciency and overcome limita-
tions associated with pH adjustment and reduce cost. Further research is
needed to optimize operating parameters, evaluate the applicability of
these processes to different wastewater types, and assess their scalability
and cost-effectiveness.
In a study by Rezgui et al. (2022), a combined approach combining
EF and EC processes was implemented to treat tannery wastewater
(Rezgui et al., 2022). By optimizing each individual method, the com-
bined process achieved a COD removal rate of (88.1 ±4.8) % and
complete elimination of chromium, meeting the permissible discharge
limits. The estimated global environmental impact of the combined
treatment was ~ 1.7 times lower compared to that of individual pro-
cesses. In a recent study by Sun et al. (2023), as shown in schematic in
Fig. 10(a), a novel approach combining heterogeneous EF (hetero-EF)
and EC processes was proposed for the efcient removal of
Cu-ciprooxacin complexes (Cu-CIP) (Sun et al., 2023). Under specied
conditions, optimal removal efciencies of 99.6 % for Cu, 96.4 % for
CIP, and 83.6 % for TOC were achieved. The primary contributor to the
degradation of Cu-ciprooxacin complexes was
•
O
2
–
, and the complexes
underwent degradation into smaller molecules, releasing Cu
2+
.
In another study by Dai et al. (2022), the mechanism of tetracycline
removal was investigated in an EC system combined with an EF reaction
(Dai et al., 2022)(see Fig. 11). The system utilized a Fe anode and a
carbon nanotube cathode. Optimal conditions for achieving a removal
rate of 97.2 % were determined, and the Fe-CNT system demonstrated
excellent performance in removing TOC. The use of a carbon nanotube
cathode reduced oatation, promoted oc aggregation and growth, and
enhanced adsorption and aggregation of organic molecules. The in-situ
production of hydrogen peroxide and the EF reaction on the cathode
surface facilitated the rapid degradation of tetracycline.
These studies demonstrate the potential of combined EC-electro
Fenton processes for the efcient removal of contaminants from
wastewater. The combination of EC and EF reactions enhance the
treatment efciency and offers advantages such as improved pollutant
removal, compliance with discharge limits, and reduced environmental
impact. Further research is needed to optimize process parameters,
assess the scalability and cost-effectiveness of these combined processes,
Fig. 9. Heterogeneous Fenton-like oxidation process (Hussain et al., 2021).
A.A. Shah et al.
Water Research 252 (2024) 121248
14
and explore their application to different types of wastewaters.
3.4.4. Combined EC-photo (UV) Fenton processes
The standalone use of EC technology for wastewater treatment can
face practical limitations, especially when dealing with highly polluted
or contaminated wastewater. Therefore, there is a need for effective and
cost-efcient treatment techniques. In response to this challenge, several
studies have shown that combining EC with other treatment methods,
such as UV, can enhance its performance (Asaithambi et al., 2022a).
AOPs are highly effective in the degradation of various pollutants due to
their ability to target a wide range of chemical structures (S. Li et al.,
2022; Tufail et al., 2020). UV radiation generates highly reactive radical
species, including hydroxyl radicals (HO
•
), sulfate radicals (SO
4
•−
), and
reactive chlorine species (RCS, e.g., Cl
•
, Cl
2
•
, and ClO
•
) (´
Alvarez et al.,
2020; Guo et al., 2018).
UV photolysis is widely used in wastewater treatment, especially for
disinfection purposes. However, there is a need for suitable technology
to expand its application to other forms of water remediation. Fig. 12(a)
show the experimental setup (Moradi and Moussavi, 2019). In this
process, the energetic radiation from UV strongly inuences the organic
compounds in wastewater, leading to the breaking of chemical bonds,
rearrangements, and redox reactions. As shown in Fig. 12(b), UV radi-
ation can induce degradation of both inorganic and organic molecules
through direct photolysis, which occurs through three main pathways:
homolytic and heterolytic cleavage of molecules, or via photoionization
(Jallouli et al., 2020). By incorporating UV irradiation during the EC
process, a better performance was observed, attributed to the increased
availability of photoactive sites. This improvement can be attributed to
the higher production rate of OH
•
radicals resulting from the
photo-reduction of Fe(OH)
2
+
and the photodecomposition of complexes
formed from Fe
3+
reactions (Aziz et al., 2016).
Fe(OH)
2+
+hv → Fe
2+
+OH
•
(6)
R(CO
2
) - Fe
3+
+hv → R(
•
CO
2
) - Fe(II) →
•
R +CO
2
(7)
Both EC and ultraviolet (UV) treatments are characterized by their
ease of operation, simple equipment requirements, and low mainte-
nance needs. However, the combined EC-UV process has received rela-
tively less attention in research studies thus far but has been gaining
increasing attention in recent times.
Malakootian et al. (2018) conducted a study on the application of the
EC–photo-Fenton (EC-PF) process for the removal of phenol from
wastewater generated by the steel industry (Malakootian and Heidari,
2018). The optimal conditions for this process were found to be
Fe
2+
/H
2
O
2
ratio of 1.5, pH of 4, current density of 1.5 mA/cm
2
, and a
duration of 25 min. Under these conditions, the EC-PF process achieved
a remarkable removal efciency of ~100 % for phenol and 97% for
chemical oxygen demand (COD). The power requirements for the EC-PF
Fig. 10. (a) schematic diagram of possible reactions occurred in the EC/EF-like process (adopted from (Ding et al., 2021)). (b) The removal mechanism of Cu-CIP
complexes in the combined hetero-EF and EC process is depicted (adopted from (Sun et al., 2023)).
A.A. Shah et al.
Water Research 252 (2024) 121248
15
process were found to be 29.1 kWh/kg COD, demonstrating its superior
removal efciency and cost-effectiveness compared to alternative
treatment methods.
Moradi et al. (2019) investigated a combination of EC and ultraviolet
(UV) treatment to improve the removal efciency of COD, Cr, and sul-
de in tertiary treated wastewater. Optimal results were achieved with
an applied current of 500 mA and an electrolysis time of 20 min. The
combined EC-UV/ process exhibited enhanced oxidation of oxidizable
inorganic (S
2
−
) and organic (COD) components, as well as a reduction in
Cr(VI) content (Moradi and Moussavi, 2019). Jallouli et al. (2020)
explored an effective and sustainable approach for treating tannery
wastewater using a sequential EC-UV photolytic process (Jallouli et al.,
2020). The combined EC-UV treatment resulted in a signicant COD
reduction of 94.1 %, surpassing the reductions achieved by individual
treatments of EC and UV alone. The nal COD value of 428.7 mg/L was
signicantly below the permissible limit for wastewater discharge into
the Tunisian public sewerage network.
Asaithambi et al. (2022b) developed a method that combines ultra-
sound (US), ultraviolet (UV) radiation, and EC for the removal of color
COD from distillery wastewater. The combined application of UV irra-
diation and EC (UV-EC) at low current densities exhibited a synergistic
effect in reducing turbidity and enhancing the disinfection rate. The UV
+US +EC process achieved a nearly complete removal of color (~ 100
%) and a signicant reduction in COD (95.6%) from distillery
Fig. 11. The schematic diagram illustrates the mechanism of TC removal in the Fe-CNT electrochemical system (adopted from (Dai et al., 2022)).
Fig. 12. (a) The schematic diagram of experimental setup (adopted from (Moradi and Moussavi, 2019)). (b) Mechanism of UV decomposition of chemical com-
pounds (adopted from (Jallouli et al., 2020)).
A.A. Shah et al.
Water Research 252 (2024) 121248
16
wastewater. The combination of EC and UV treatments offers advan-
tages such as enhanced oxidation, reduced energy consumption, and
improved cost-effectiveness. Further research is needed to optimize
process parameters, evaluate the scalability of these combined pro-
cesses, and explore their application in different wastewater treatment
scenarios.
3.4.5. Combined EC-sono Fenton processes
The standalone use of EC technology for wastewater treatment has
certain challenges, such as the formation of a passive lm on the elec-
trode surface, leading to decreased pollutant removal efciency and
increased energy consumption (Afsharnia et al., 2018). Ultrasound
irradiation, known for generating, expanding, and collapsing small
bubbles, can create localized reaction zones and produce radicals as
depicted in Eq. (8), the triple parentheses ")))" represent the application
of ultrasonic irradiation (Moradi et al., 2021) (See Fig. 13):
H
2
O +))) → HO
•
+H
•
(8)
Employing ultrasound in combination with EC shows promise in
mitigating the formation of the passive lm, as it enhances the pro-
duction of radicals and improves pollutant degradation capabilities (Rad
et al., 2020). Sono-EC operates by dissolving metal ions with positive
charges from the electrode surface, neutralizing repulsive forces be-
tween particles. This facilitates the attraction and formation of occu-
lants with particles of opposite charge. Subsequently, these neutral
occulants can be effectively removed from the solution through pro-
cesses like otation or sedimentation. At the cathode, the reactions
generate hydroxyl radicals and H
2
gas, as depicted in Fig. 13 (Darvishi
Cheshmeh Soltani et al., 2020; Moradi et al., 2021; Moussa et al., 2017).
The strict standards for wastewater reuse in water and wastewater
treatment have led to the increasing consideration of sono-EC in recent
years. To assess its effectiveness in eliminating pollutants like phenol,
dye, oil, heavy metals, COD, BOD, and TDS, and to compare it with the
EC method, extensive research has been conducted.
The stringent standards for wastewater reuse have led to increased
interest in sono-electrocoagulation (SEC) in recent years. Extensive
research has been conducted to evaluate its effectiveness in eliminating
pollutants such as phenol, dye, oil, heavy metals, COD, BOD, and TDS
and to compare it with EC. Dizge et al. (2018) examined the treatment
efciency of EC, ultrasonication (US), and SEC processes for brewery
wastewater (Dizge et al., 2018). Using the electrode combination of
Al/Al, a current density of 100 A/m
2
, pH of 7.0, and a reaction time of 60
min, the SEC process achieved the highest removal efciencies of 99.2 %
for color and 60.5 % for COD. The enhanced efciency of SEC can be
attributed to the regeneration of a new electrode surface through dy-
namic cavitation and/or micro-streaming effects during sonication, as
well as increased mixing efciency leading to stronger bond formation
between coagulants and pollutants.
Afsharnia et al. (2018) investigated the application of SEC with Fe
and Cu electrodes for the treatment of fresh leachate from municipal
solid waste (Afsharnia et al., 2018). The combined sonication and EC
process effectively removed 98 % of COD and ~ 68 % of TSS from the
fresh leachate when operated at 30 V for 60 min. Longer reaction times
and higher voltages improved the removal efciency by enhancing the
release rate of coagulants. Temperature variations during the reaction
affected the physicochemical properties of the solution, further
enhancing the Sono-EC process. Asaithambi and Govindarajan (2021)
conducted a study on the application of a combined SEC process for
landll leachate wastewater treatment, optimizing the process using a
central composite design approach. The combined approach combining
US and EC achieved exceptional removal efciencies of 100 % for color
and 94% for COD while consuming less power (4.50 kWh/m
3
) compared
to standalone EC and US processes.
Arka et al. (2022) focused on treating wastewater through a com-
bined SEC process, investigating the impact of various operating pa-
rameters on COD removal, color removal, and power consumption (Arka
et al., 2022). The combined SEC process was optimized to achieve
maximum COD removal (97.5 %) and maximum color removal (100 %)
while minimizing power consumption. Optimal conditions involved a
current density of 1 A/dm
2
, efuent pH of 5, electrolyte concentration of
6 g/L, inter-electrode distance of 1 cm, and a reaction time of 45 min.
These combined processes exhibit superior treatment efciency
Fig. 13. Schematic ow diagram of the process and corresponding mechanisms of pollutants removal (degradation via reactive radicals and precipitation) in the
sono-EC processes (Moradi et al., 2021).
A.A. Shah et al.
Water Research 252 (2024) 121248
17
compared to conventional methods, making them well-suited for the
treatment of residential efuent.
In conclusion, the combination of EC and sono-EC (SEC) processes
offers a promising solution for treating highly polluted wastewater. The
incorporation of ultrasound irradiation in EC mitigates the formation of
a passive lm on the electrode surface, enhances radical production, and
improves pollutant degradation capabilities. Extensive research has
demonstrated the effectiveness of SEC in removing various pollutants,
such as phenol, dye, oil, heavy metals, COD, BOD, and TDS, with high
removal efciencies. These combined processes exhibit superior treat-
ment efciency compared to conventional methods, making them suit-
able for the treatment of residential efuent and contributing to
sustainable water and wastewater management.
4. Concluding remarks and future perspectives
Developing scalable combined electrochemical (EC) processes is
pivotal for successful industrial implementation. Pilot-scale studies are
imperative to validate their efcacy across diverse wastewater sources,
necessitating the exploration of factors like electrode materials, coating
materials, integration of a third process, and renewable energy
utilization.
Combining AOPs with EC holds signicant potential due to the
generation of radicals that facilitate organic oxidation. Future research
should focus on developing specialized electrodes to generate radicals
more effectively, improving process efciency.
In recent years, combined EC processes for water treatment have
gained traction due to their potential benets, including increased water
volume and quality, reduced membrane fouling, lower environmental
impact, and operational cost savings. However, specic challenges must
be addressed before commercialization.
Additional research is required to reduce electrode consumption and
enhance removal efciency. Pilot-scale investigations must assess
different combined electrocoagulation processes, especially for indus-
trial wastewater. Comprehensive long-term energy evaluations are
crucial, conducted at the pilot scale in continuous ow mode to replicate
commercial conditions. Moreover, dedicated research on electrode
materials is essential to expedite technology commercialization.
Addressing these research needs and challenges will signicantly
advance the eld of combined electrochemical processes, offering sus-
tainable and cost-effective solutions for water and wastewater treat-
ment. The ongoing exploration and optimization of these processes will
contribute to developing efcient and environmentally friendly tech-
nologies, meeting the growing global demand for clean water resources.
CRediT authorship contribution statement
Aatif Ali Shah: Conceptualization, Investigation, Methodology, Re-
sources, Visualization, Writing – original draft, Writing – review &
editing. Sunil Walia: Software, Writing – review & editing. Hossein
Kazemian: Formal analysis, Funding acquisition, Project administra-
tion, Resources, Supervision, Validation, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgment
We gratefully acknowledge the support provided by the Water Life
System Ltd., and particularly Mitacs Elevate program, specically
through awards IT2877 and IT28780. This funding was crucial in
facilitating the hiring of postdoctoral researchers (A.A. Shah and S.
Walia) whose efforts have signicantly contributed to the development
of this review.
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