Impact of different electrodes, mediators, and microbial cultures on wastewater treatment and power generation in the microbial desalination cell (MDC) Open Access

AEM

anion-exchange membrane

BAS

Bannari Amman Sugars

BESs

bio-electrochemical systems

BOD

biochemical oxygen demand

C6FeK4N6

potassium ferricyanide

CD

current density

CE

coulombic efficiency

CEM

cation-exchange membrane

CMDC

control MDC

CNT-CS

carbon nanotube-chitosan

COD

chemical oxygen demand

DC

desalination chamber

DE

desalination efficiency

DO

dissolved oxygen

ETMs

electron transfer mediators

HRT

hydraulic retention time

IQMDC

individual quadruple MDC

IR

internal resistance

MDC

microbial desalination cell

MFC

microbial fuel cell

mV

millivolt

NaCl

Sodium Chloride

PBS

phosphate buffer solution

PD

power density

PFM

potassium ferricyanide mediator

P_max

maximum power

PQMDC

parallel quadruple MDC

SEM

scanning electron microscope

SQMDC

series quadruple MDC

TDS

total dissolved solids

TEM

transmission electron microscopy

UASBR

up flow anaerobic sludge blanket reactor

UB

United Brewery

USMDC

upflow stacked MDC

The basic foundation of the modern society is water and electricity. However, the world is facing pressure due to shortages in these resources because of overutilization of natural reserves, global warming, and pollution (Salman & Ismail 2020). In today's world, it becomes imperative to explore and adopt renewable and sustainable energy options into the energy mix, to reduce reliance on conventional sources (Santoro et al. 2017a). Microbial desalination cells (MDCs) are the kind of technology which can treat wastewater and saline water and generate energy simultaneously. The general structure of a microbial desalination cell (MDC) consists of separate anode and cathode compartments, along with an intermediary desalination chamber (DC) in the middle (Gadkari et al. 2018; Jafary et al. 2020b). Two ion exchange membranes separate the DC from the anode and cathode chambers and promote the movement of ions towards the opposite compartments (Ping et al. 2016). The microbial communities present in the anode chamber breakdown the organic matter (OM), and produces protons and electrons that move through the external resistance that are later used up in the cathode area, resulting in the generation of bioelectricity (Logan et al. 2019).

Bioenergy derived from organic waste emerges as a promising source of sustainable energy (Jingyu et al. 2017). Cao et al. (2009) introduced an MDC reactor with a brine capacity of 3 mL demonstrating the potential of MDC technology. Mirzaienia et al. (2017) successfully removed nickel and lead from industrial wastewater in Iran using MDCs. Simultaneously, Malakootian et al. (2019) explored MDC's effectiveness in reducing the presence of Cu and Zn in industrial effluent, highlighting its promising role. Pant et al. (2010) reported that carbon-based materials such as carbon felt, carbon paper, and graphite rods, widely recognized for their excellent conductivity, long-lasting stability, and cost-effectiveness, are frequently chosen for utilization in bio-electrochemical systems (BESs).

Research efforts have demonstrated the practical feasibility of using alternative substrate sources like petroleum waste to enhance the real-world implementation of MDCs (Ashwaniy & Perumalsamy 2017). Among the crucial components, the materials used for constructing the anode represent a clear departure from traditional methods and significantly impact the effectiveness of power generation within BESs (Liu et al. 2005). To prevent pH fluctuations that could disrupt bacterial metabolism, Qu et al. (2012) investigated the operational dynamics of interconnected and continuously operating MDCs. Moreover, Ping et al. (2015) demonstrated that integrating MDC with other systems proficiently removes various inorganic ions, including stubborn boron, resulting in water quality suitable for irrigation.

The literature survey showed little information on the electrode, mediator, aeration, and microbial culture used and its impact on the performance of the MDC reactor. Very few studies were conducted for the identification of microbial activities on the electrode surface. Many of the experiments utilized synthetic wastewater, and the reported results showed significant variations. Hence, the aim of the present experimental study was to understand the performance of the MDC system for varying electrode materials and microbial culture on the wastewater treatment, power generation, and the desalination of the saline water. An attempt was also made to check the efficiencies with the presence of mediators and aeration on the MDC reactor with the real industrial effluents. The study aimed to determine the optimal approach for effectively treating brewery and distillery industrial wastewater while maximizing power generation, with the goal of seamless integration into existing treatment facilities.

In the present experimental study, wastewater samples were collected from different sources: distillery industrial wastewater, specifically spent wash from Bannari Amman Sugar industry in Alaganchi Village, Nanjangud, Mysore, Karnataka; and brewery industrial wastewater obtained from United Breweries Ltd, Thandya Industrial Area Chikkayana chatra, Nanjangud, Mysore, Karnataka. Initial sample characterization was carried out at the process and analysis labs of The Department of Environmental Engineering, JSS Science and Technology University, Mysore. The details are provided in Table 1.

The analysis of the distillery wastewater spent wash revealed a BOD of approximately 37,000 mg/L and a COD of 125,000 mg/L, indicating a notably high-strength wastewater. In contrast, the analysis of the brewery wastewater sample indicated a BOD of 745 mg/L and a COD of 3,733 mg/L. These BOD and COD values of the brewery sample indicated a medium-strength wastewater with good biodegradability potential.

The saline water sample was collected from Kundapura, Udupi, located in the state of Karnataka. Kundapura sea water underwent testing at Ganesh Consultancy & Analytical Services in Mysore, Karnataka, and the corresponding results are presented in Table 2. The TDS was measured to be approximately 50,000 mg/L. Further analysis, drawn from a comprehensive review of existing literature, reveals that most experiments primarily employed artificial saline water with concentrations ranging from 10,000 to 25,000 mg/L. Given this context, a range of salt concentrations (specifically 5,000, 10,000, 20,000, and 35,000 mg/L) were used to control various experiments to investigate their influence on desalination efficiencies, as reported across multiple literature sources.

In the present study, the reactor setup made of a 5 mm thickness plexiglass, was fabricated by RR Creation, Mysore, Karnataka, India. Carbon rod electrode of length 90 mm and diameter 15 mm with surface area 45.92 cm² (source: Vijaya laboratories, Mysore) and carbon brush electrode of length 55 mm and diameter 45 mm (source: PSP Instruments, Pune). However, carbon plate electrodes of 50 mm × 30 mm × 5 mm (source: RR creation, Mysore) and carbon cloth electrode of (50 mm × 50 mm × 1 mm) (source: PSP Instruments, Pune) are also used in the present study. The surface of the carbon rod and carbon plate electrodes were roughened and immersed in the deionized water for 24 h prior to the experiment.

Heterogeneous anion-exchange membrane and cation-exchange membrane were used for better ion transfer. An anion-exchange membrane (RA LEX^® MEMBRA NE AM(H)-PP) and cation-exchange membrane (RA LEX^® MEMBRA NE CM(H)-PP) were procured from Aquatreat Systems & Engineers, Delhi, India. Both the AEM and CEM were immersed into the deionized water for 12 h before use in the experimental study.

Active anaerobic microbial culture was collected from the United Breweries Ltd and Anheuser Busch InBev India Ltd (Unit: SPR Distilleries Pvt Ltd), Mysore, Karnataka. Microbial cultures are an important part of the MDC, as the microbes will digest the nutrients present in the wastewater and help in transferring the electron to the electrode. Furthermore, potassium ferricyanide, K₃[Fe(CN₆)] and phosphate buffer solution (PBS) were used as mediators to enhance the system performance. External resistance is one of the important factors that affect the performance of MDCs, and was procured from Green complex, Mysore, India.

The SEM analysis (Model: EPMA-1720T) was conducted in the Department of Polymer Science and Technology, JSS Science and Technology University, Mysuru. In this study, the ELICO LI127 pH meter from JSS S&TU Process Lab was used. TDS analysis was conducted using Labtronics microprocessor COND-TDS-SAL meter LT-51. Additionally, the samples were analyzed using a Remino imported TDS Meter. Voltage and current data collection utilized the DT830D LCD multimeter. To capture readings, a Datalogger (CEM DT-175CVS) was employed to log voltage and current measurements every hour.

The possible reactions in the MDC were considered as follows:

In the present study, four different conditions were considered. The impact of different electrodes (carbon rod, carbon plate, carbon cloth and carbon brush), experiments with and without aeration in the cathode chamber, application of different mediators (potassium ferricyanide and PBS), different microbial culture (anaerobic sludge, curd, and mixed anaerobic sludge with curd) on the voltage and current generation were analyzed. Different experiments with varying conditions given above were conducted to determine the removal efficiencies of COD and TDS. The analysis involved identifying current density, power density, and coulombic efficiency to assess the optimal conditions for achieving the highest power generation and treatment efficiencies.

The experimental was started by positioning the reactor and introducing substrate, microbial culture (obtained from an anaerobic reactor) into the anode chamber, electrolyte solution into the DC, and tap water into the cathode chamber. The anode and cathode were connected through an external resistance. Voltage and current measurements were taken hourly using a datalogger. pH and TDS readings were recorded every 4 h for samples from the anode, DC, and cathode. COD analysis was done daily. Collected data were documented under various experimental conditions and employed for analysis and performance assessment. For a morphological examination of the electrode and membrane surfaces and the attached microbial growth, SEM analysis was conducted. This analysis encompassed the new electrode, used anode and cathode electrodes, new AEM, new CEM, used AEM (anode side), used CEM (cathode side), used AEM (DC side), and used CEM (DC side).

R_ext refers to the external resistance (Ω); V refers to the voltage (mV); I refers to the current (mA).

Nutrient_RE refers to the removal efficiency of the nutrient (%); Nutrient_(in) refers to the concentration of the nutrient at inlet (mg/L); Nutrient_(t)refers to the concentration of the nutrient at outlet (mg/L).

C_i refers to the initial salt concentration (mg/L); C_f refers to the final salt concentration (mg/L).

I refers to the calculated current (mA or A); A_sa refers to the surface area of anode electrode (cm²); A_vol refers to the volume of anode chamber (m³).

V refers to the voltage (mV); R_ext refers to the external resistance (Ω); A_sa refers to the surface area of anode electrode (cm²); A_vol refers to the volume of anode chamber (m³).

C_p refers to the coulombic amount; C_th refers to the total amount of electricity that can be theoretically obtained from simulated COD oxidation; M_O2 refers to the molecular weight of oxygen (O₂) = 32 g; I refers to the current (mA); b refers to the number of electrons exchanged for oxygen used = 4; F refers to the Faraday's constant = 96,485 C/mol; A_vol refers to the volume of wastewater in anode chamber (mL); ΔCOD refers to the change in COD concentration (mg/L).

The current generation profile is presented in Figure 2(b). The current generation followed a similar pattern, and the maximum current was generated at 2.16 mA with the presence of a carbon brush electrode. On the other hand, maximum currents of 1.97, 1.7, and 0.76 mA were also recorded in the presence of carbon rod, carbon plate and carbon cloth electrodes, respectively. The maximum current generation was observed with the HRT of 10–40 h. Ma & Hou (2019) conducted studies on the MDC with 3-D porous carbon nanotubes with chitosan sponge anode. The result of which reported an open circuit voltage of 1,017 ± 2.2 mV, with the COD and TDS removal rate of >90% and 16.5 ± 0.09 mg/h. This study also reported the selection and importance of the electrode on the MDC performance.

Carbon brush electrodes had a higher surface area compared to carbon rods, carbon cloth, or carbon plate electrodes. The increased surface area allowed for more microbial attachment and biofilm formation, leading to enhanced electrochemical reactions, increased voltage, and current generation. The carbon cloth and carbon plate electrodes might have a smaller effective surface area, resulting in reduced voltage generation. Additionally, if the surface of the carbon plate was not adequately rough or porous, it might limit the attachment of bacteria and decrease the overall electrochemical activity.

The current was found to be 0.79 mA at the initial stage (24 h), then reached maximum of 2.16 mA (48 h) and further reduced to 1.76 mA (72 h) before finally stabilizing with time. Initially, rapid electrochemical reactions at the electrode–electrolyte interface can lead to high current. However, MDCs are dynamic systems influenced by microbial activity, properties of electrode material, and ion transport mechanism. Fluctuations may occur due to changes in microbial populations, biofilm development, and ion gradients. Over time, as microbial communities stabilize, biofilm matures, and electrochemical reactions become consistent, the current stabilizes, resulting in a steady voltage output. In a similar study, Wang et al. (2020) reported that the current generation by UMDC, USMDC, and SMDC reactors got reduced from 6.91, 5.50, and 3.86 A to 0.39, 0.89, and 0.90 A, respectively. They reported the current generation of SMDC and USMDC was declining at a slower rate than the UMDC, which was mainly due to the Ohmic resistance impact on the UMDC. The present study showed a good agreement with the literature survey.

The COD removal rate was found to be in decreasing order with time (HRT 6 days). The maximum COD removal efficiencies varied between 38, 38, 35, and 22% for carbon brush, carbon rod, carbon plate and carbon cloth electrode, respectively, in the first 24 h. In the present study, the COD removal rate was fluctuated at initial conditions (18–20% per day) and found similar for the carbon rod, carbon plate, and carbon brush electrodes due to the active microbial decomposition of the OM at the anode section. The reason for the higher COD removal for the carbon brush electrodes is due to the high surface area and rough texture of carbon brushes which promoted the attachment and growth of bacteria capable of degrading organic pollutants. This improved microbial activity led to enhanced COD removal efficiency in the MDC reactor.

The present study can be compared with study conducted by Xu et al. (2020), where an MDC reactor equipped with carbon cloth electrodes was used. Their findings revealed a gradual increase in COD removal rate, rising from 225.5 ± 1.2 mg/L/day in the sixth cycle to 264.0 ± 1.5 mg/L/day in the 11th cycle at an initial salinity of 30 g/L. Additionally, they observed that at a DC salinity of 15 g/L, the COD removal rate steadily rose from 251.5 ± 2.0 mg/L/day in the 13th cycle to 265.0 ± 0.9 mg/L/day in the 15th cycle during the stable phase. The same was also explained by Ragab et al. (2019) who used carbon cloth electrode for the synthetic wastewater treatment in the MDC reactor and reported 91–89% COD reduction with maximum COD influent concentration of 3,000 mg/L. In the present study, better results were obtained with the carbon brush electrode having more surface area in comparison to the carbon cloth electrode.

On the other hand, in the present study, carbon cloth and carbon plate electrodes might exhibit lower COD removal due to lesser area contact between the electrode surface and the wastewater. In the case of carbon rods that can support bacterial growth and biodegradation, they might not provide as efficient contact and diffusion of wastewater across the electrode surface compared to other electrode types, thus leading to moderate COD removal.

A similar study conducted by Wang et al. (2020) showed the performance of the USMDC, SMDC, and UMDC after 36 h was 90.3, 76.9, and 60.6%, respectively. The performance of the USMDC was better when compared with the other two MDC's due to its design, high charge transfer efficiency and smaller Ohmic resistance. In the present study, the salt reduction was found to be 18–20% per day within 24 h, which further 12–14% after 72 h and finally observed to be 9–11% after 96 h. This initial efficiency was due to the rapid establishment of salinity gradient driving the movement of ions from the DC to the anode and cathode chambers. However, the gradual decrease in salt removal percentage over time might be due to the microorganism's colonization and formation of biofilms on the membrane surface, hindering the ion transfer and leading to reduced efficiency.

In the present experimental study, the desalination ratio with time is presented in Figure 4(b). A maximum salt reduction of 60, 57, 54, and 48% was achieved for carbon brush, carbon plate, carbon rod, and carbon cloth electrodes, respectively. The daily salt removal was found to vary from 20 to 28% within 48–72 h and then reduced to 6–15% after 72 h. The TDS removal in the system was found to be consistent with time. The TDS reduction in all the experimental conditions at the initial stage was found to be fluctuating. This might be due to the adaptation of the microorganism in the system.

In the same way, Jafary et al. (2020a) showed that with the reduction of the salt concentration in the DC, there was an increase in the internal resistance, a decrease in the current generation and a decrease in the desalination rate. On the other hand, carbon cloth and carbon plate electrodes might have shown lower TDS removal due to limited electrochemical reactions and ion exchange capacity. Carbon rods can facilitate some ion adsorption and reduction, their surface area and electrochemically active sites might not be as extensive as those provided by carbon brush electrodes, resulting in moderate TDS removal.

The pH was found to be frequently fluctuating in the case of carbon brush and carbon rod electrodes. This might be due to the microbial activities and metabolisms at the anode which was generating the H⁺ and OH⁻ ions. Similarly, the pH of the DC was also found to be increased from 7.37, 7.81, 7.42, 7.37 to 7.82, 7.94, 7.6, and 7.82 in the case of carbon rod, carbon cloth, carbon plate, and carbon brush electrode, respectively (Figure 5(b)).

In the present study, an increase in the cathode pH was also observed. It was found that pH was increased from 6.89 to 8.08 for the carbon brush electrode (Figure 5(c)). Safwat et al. (2022) manipulated the influent pH in the MDC system and observed distinct voltage variations. In cycle 1, they noted a notably higher maximum voltage at anode pH 8 compared to pH 6 (410 vs. 272 mV). However, in cycle 2, the maximum voltages at pH 8 and 6 showed a closer similarity, measuring 69 and 71 mV, respectively. These findings indicated significant influence of pH on MDC voltage. In the present study, the reason for the increased pH in the cathode chamber is mainly due to the reduction of oxygen that sometimes generated hydroxide ions (OH⁻) which could migrate to the DC and react with sodium ions (Na⁺) present in the saline solution, forming sodium hydroxide (NaOH).

It was also observed that, the anode TDS increased with time from 3,400 to 4,546 mg/L for the carbon brush electrode. In the cathode chamber, the TDS was increased from 1,059 to 3,318 mg/L and from 4,614 to 6,500 mg/L for carbon brush and carbon rod electrode, respectively. The TDS gradually increased from 3,400 to 3,767 mg/L in 48 h, 4,014 mg/L in 96 h. A similar trend was also observed for the cathode chamber 1,059 to 1,822 mg/L in 48 h, then 2,590 mg/L in 96 h.

Bacteria in the anode chamber metabolize OM through oxidation reactions. As a result, organic compounds are broken down into simpler substances, which can contribute to the increase in TDS concentration. In comparison the research study by Wang et al. (2020) reported that, the pH of the anode decreased with time due to ion migration and this resulted in the deviation of the bacteria to work in the optimal condition. Microbial activity can result in the release of acidic metabolic by-products, leading to a decrease in pH. Hydroxide ions (OH–) are generated as part of the water reduction process and accumulation of hydroxide ions contributes to the observed increase in pH over time.

Initially, the PFM and aeration reported a lower voltage of 540 mV likely indicating the time required for microbial communities to adapt and establish efficient electron transfer pathways. The subsequent increase to 992 mV after 24 h suggested microbial growth and biofilm development, enhancing mediated electron transfer. Further, decline to 538 mV at 72 h might result from diminishing organic substrates or changes in mediator concentration. Finally, the drop to 296 mV may be influenced by factors such as substrate depletion, or biofilm detachment.

The high voltage generation with the PFM with aeration system can be attributed to the enhanced electron transfer capabilities. Potassium ferricyanide acted as an electron mediator, facilitating the transfer of electrons between the microorganisms and the electrode surface. Oxygen is the key component for the electrochemical reactions that occur in the cell. By providing a continuous supply of oxygen, the system ensured that the cathode could efficiently facilitate the reduction reactions, leading to enhanced current generation. The low voltage generation with only PBS was attributed to the absence of an effective electron mediator. Phosphate buffer alone did not provide an extensive suitable means for efficient electron transfer resulting in limited voltage output, when compared with the presence of aeration.

The observations of the present study can be corroborated by experiments conducted by Ebrahimi et al. (2023) studied constructed wetland-MFCs and operated under different aeration regimes to demonstrate varying voltage outputs. The cell operated under intermittent aeration achieved the highest voltage of 0.450 V. Following closely were the cells under continuous aeration regimes, with voltages measuring 0.426 V (CA-II) and 0.419 V (CA-I) respectively. In contrast, they also reported the cell without any aeration only managed to attain an output voltage of 0.355 V.

The current generation with and without aeration conditions of MDC in the system is presented in Figure 6(b). It was observed that the current generation fluctuated at the initial stage (0.59–1.88 mA) and then it got maximized (1.96 mA) with time and finally declined (0.58 mA). The maximum generated currents were observed to be 1.96 mA in the presence of aeration and C₆FeK₄N₆ mediator. However, a maximum current of 1.792, 1.786m, and 1.676 mA were found in the case of only with PFM mediator, aeration with PBS and only with PBS, respectively.

It was also reported that, the presence of DO within the cathode chamber played a pivotal role as the final recipient of electrons. Conversely, a reduction in DO led to a corresponding decrease in the final electron reception, consequently leading to reduced electricity generation (Hedbavna et al. 2016; Malakootian et al. 2018, 2019).

Regarding COD removal, Quan et al. (2012) previously documented that in the anaerobic control of the anode, the MFC immediately generated a maximum voltage of approximately 400 mV upon introducing a new substrate. By the end of each cycle, which lasted about 120 h, the percentage of COD removal reached above 89%.

The COD removal rates were found to be 30–40% in 24 h, 20–30% in 72 h and 10–20% in 72–96 h. The COD reduction efficiencies were found to be gradually decreasing with time. The reason for the reduction in the COD removal efficiencies was a reduction in the organic compounds present in the wastewater and microbial degradation. The reason for the high removal rate at the initial stage of the experiment was due to the availability of the substrate in the solution. Malakootian et al. (2018) demonstrated that elevating the DO concentration from 0 to 6 ppm within the cathode segment resulted in a notable enhancement in arsenic removal efficiency, escalating it from 30 to 76%. The present study was also in good agreement with the results found in the literature.

Although the cathodic aeration primarily affects the cathode compartment, it indirectly influences the anode section. By enhancing the cathodic reduction reactions, the process made more electron available for acceptance in the reactor. This affinity enhanced the microbial activity in the anode section where they can participate in various oxidation reactions of the organic compounds and contributes to high COD reduction in the anode section. The PBS mediator with aeration helped enhance the overall performance of the MDC by improving electron transfer efficiency, promoting microbial activity, and facilitating the oxidation of organic compounds in the anode section.

Moreno et al. (2019) found that despite the lower thermodynamic potential associated with ferrocyanide reduction in the cathode compartment, its fast kinetics offer greater available potential in MDC systems. Their research indicated that inferior desalination performance was observed when using oxygen reduction as the cathodic reaction stemmed from the limited potential available. In the present study, the removal rate of TDS at the initial stage was slow and then gradually increased and found to be consistent. The trend was found to be similar in all the experiments however the studies which were conducted with the presence of aeration, enhanced the reactions, and provided a better result in TDS reduction.

The cumulative TDS reduction is presented in the Figure 8(b). The daily TDS removal was found to be varying from 15–20% within 48–72 h and then reduced to 6–15% after 72 h. The TDS removal in the system was found to be consistent with time. The reason might be due to the concentration gradient that initiated the process at the start of the experimental studies. The cathodic aeration system did not directly impact the DC of the MDC. The desalination process in the MDC was primarily driven by ion transport across ion exchange membranes.

The study conducted by Clauwaert et al. (2007) reported the generation of efficient electricity through a biocathode within a microbial fuel cell under ambient conditions in Australia. They concluded that the presence of DO within the cathode, posed a significant bottleneck for MFC performance. However, in the present study, results represented the opposite approaches of enhanced performance with mediator + aeration. The PBS mediator with aeration helped microorganisms in the anode compartment to oxidize OM, releasing electrons in the process. These electrons were transferred to the anode electrode with the help of mediator, which acted as a redox mediator.

Hydroxide ions (OH⁻) generated in the cathode chamber may migrate through the ion exchange membrane into the desalination chamber. As the concentration of hydroxide ions increased, the pH of the DC rose. It was also observed that the cathode pH was also increased from 7 to 8 and 8.5 for all the experimental studies (Figure 9(c)). The pH of the different experiments was found to be in similar trends and have distinct differences based on the initial pH of the solution. The possible reason could be water electrolysis which occurred at the cathode and produced hydroxide ions (OH⁻) through the reduction of water molecules. It was observed that TDS of the anode chamber also got increased in all the experimental studies similar to the trend observed for anode and DC. TDS increased with time from 6,905 to 7,998 mg/L in case of aeration with PFM. In the cathode chamber TDS increased from 6,523 to 7,932 mg/L for PBS and PFM. Wang et al. (2020) reported that the pH of the DC was decreased by 0.3–0.7 due to the transfer of the proton through an ion exchange membrane and consumption of the protons in the anode and cathode chamber. This result was found to be opposite to what was observed in the present study.

During the degradation of complex organic compounds, intermediate products may be generated. These products could have contributed to TDS concentrations in addition to the original OM, resulting in an increase in the anode chamber's TDS. In the cathode chamber, oxygen reduction occurred, leading to the formation of hydroxide ions (OH⁻) and other ions increasing the TDS concentration. However, Bergel et al. (2005) explored the reduction of oxygen levels within a proton exchange membrane fuel cell using a seawater biofilm. Their findings indicated a clear correlation by finding that with reduction of DO concentration, the level of generated current also decreased. In the DC, salt ions (e.g., sodium, chloride) are transported across ion exchange membranes driven by the electrical potential generated by the microbial and electrochemical processes. As positively charged ions (cations) move toward the cathode and negatively charged ions (anions) move toward the anode, this migration of ions can influence the local pH due to changes in ionic composition.

Initially, when a mediator is introduced, it may enhance the electron transfer between microorganisms and the electrode, leading to an initial surge in voltage. However, over time microbial communities adapt to the presence of the mediator, altering their metabolic pathways and electron transfer mechanisms. The research conducted by Li et al. (2020) indicated that the application of voltage could effectively elevate desalination efficiencies. Specifically, electrical conductivities were documented at 837 and 744.5 mS/cm for 1.5 and 2 V, respectively. The present study reported a similar voltage without applying any external voltage to the MDC reactor. Najafgholi et al. (2015) demonstrated that elevated power density correlated with increased electrolyte conductivity. This rise in conductivity was linked to a reduction in internal resistance, subsequently contributing to enhanced power generation. The study suggested that augmenting conductivity through the use of NaCl could represent an economical method to boost power generation from SMFCs.

On the other hand, the maximum current generation was found between 20 and 40 h. The current generation trend showed a gradual reduction (1.7–0.49 mA) with time (Figure 10(b)). The maximum current was recorded to be 1.976, 1.79, and 1.68 mA in the presence of normal tap water, PFM, and PBS. Initially, the generation of the current was slow and with time (20–40 h) it increased and finally reduced due to various factors. The reason for the initial fluctuation (0.6–1.8 mA) of the current generation might be due to the microbial stabilization in the reactor. Once stabilized, degradation of OM and electron transfer commenced. The current generation with time was found to be decreasing (from 1.8 to 1.0 mA) which might be due to lower availability of substrate in the system which reduced microbial degradation.

The mediator PFM had a higher redox potential compared to the phosphate buffer. PFM can readily accept electrons from the microbial community and transfer them to the anode electrode. This might have resulted in improved overall performance and power output. Sevda & Sreekrishnan (2012) previously used neutral red (NR) as an electron mediator in their study. They observed a maximum voltage of 0.730 V and a current of 0.4 mA at a concentration of 0.01 mM. In the control group where no mediator was used, they observed a maximum voltage of 0.339 V and a maximum current of 0.25 mA.

On the other hand, in the present study when water was used as a cathode mediator, it acted as a conductor, allowing for the flow of electrons between the anode and cathode. The movement of electrons generated from microbial metabolism can occur directly through water, facilitating efficient electron transfer and resulting in higher current generation.

Santoro et al. (2017b) conducted experiments on efficient platinum group metal free cathode catalysts and reported a maximum COD removal efficiency of 73–83% in the presence of a 0.023M mediator. The present study also represented the application of the mediator on COD removal when compared with normal water. The COD removal rates were observed to be 30–40% in 48 h, 20–30% in 72 h and 5–15% post 72 h. The COD of the solutions gradually decreased with time.

The reason for higher COD removal efficiencies (82%) for the PFM, is as it is an electron acceptor in the MDC reactor. This could accept electrons released during microbial metabolism, and enhance the oxidation reactions at the anode when compared with only water or PBS, which lack the ability to efficiently accept electrons. The reason for the low COD removal (78%) was water alone did not have the ability to accept electrons released during microbial metabolism. This limits the oxidation reactions at the anode and subsequently reduces the efficiency of COD removal, when no mediator was used. Phosphate buffer might offer some conductivity to the system, but it lacked the ability to effectively accept electrons when compared with PFM.

Lin et al. (2014) investigated the impact of mediators on toluene degradation and power generation. They added varying amounts of NR (100, 200, and 300 μM) to the MFC supplied with 11.09 mg/L of toluene in the anode. The average time taken for toluene consumption in the MFCs containing 100–300 μM of NR was measured at 34.1 ± 0.05 h, which exceeded the time observed in unmediated MFCs (16.2 h). Malakootian et al. (2018) conducted experiments on desalination, COD and arsenic removal, the result of which showed a maximum COD removal of 32% where normal water was used in the cathode and synthetic wastewater was used in the anode chamber. The present study reported a higher COD removal; compared to their studies. The COD removal efficiencies were found to be identical and no such impact of the mediator on the COD removal was observed. COD removal was primarily driven by microbial activity and the biological oxidation of organic compounds by the microorganisms present. Mediators, on the other hand, were primarily involved in enhancing electron transfer between the microbial community and the electrode, promoting electricity generation.

Luo et al. (2012) showed that the performance of MDC with microbial consortium generated two times higher conductivity, four times higher power density 52% COD removal and 131% coulombic efficiency. The result was more effective when compared with the controlled MDC. The present study also showed better results with mediator and microbial culture. The reason for the initial variation (till 20 h) of the TDS was due to the concentration gradient, driving the reaction. TDS removal was found to be gradually increasing and similar in the case of aeration conditions. The removal efficiency was found to be stable after 85 h of the reactor run.

In the MDC reactor, TDS removal occurred primarily through the migration of ions from the anode to the cathode. Water, being a neutral mediator, did not interfere with the ion migration process and enables efficient TDS removal when compared with PBS or PFM, which might introduce additional ions or impede ion migration. When PBS or PFM was used as mediators, they could introduce additional ions or molecules into the system. This could affect the ion migration process and hinder the efficiency of TDS removal. The study conducted by Salman & Ismail (2020) revealed an initial fluctuation in the removal of TDS and this variability could be attributed to a lag time required for the microorganisms present in the reactor to become active. However, after a 20-day operational period, the active degradation efficiencies were reported to be 93.7%.

It was observed that certain electron transfer mediators (ETMs) used in MDCs had alkaline properties and these ETMs accumulate in the cathode chamber, they react with protons, consuming them and leading to an increase in pH. The DC pH also increased from 7 to 8 in all the experimental conditions (Figure 13(b)). The reason might be if the ion exchange membrane gets clogged, it restricts the ion movement. In such cases of AEM, accumulation of the anions in the DC occurs over time. The increased concentration of anion contributes to the increase in pH.

The cathode pH increased from 8.76 to 8.85, 6.9 to 7.77 and 7.66 to 8.46 for the experiments with tap water, phosphate buffer and potassium ferricyanide used as mediator, respectively (Figure 13(c)). The pH of different experiments was found to be in a similar trend as in the present case. However, this could also be due to the ion migration into the cathode and reduction reaction. The TDS of anode increased from 4,055 to 5,394, 6,523 to 7,926 and 6,523 to 7,932 for tap water, phosphate buffer and potassium ferricyanide used as a mediator, respectively. In the cathode chamber TDS increased from 4,614, 890, and 920 mg/L to 6,500, 2,151 and 2,376 mg/L, respectively for the same experiments. Small ions, such as sodium (Na+), chloride (Cl-), and potassium (K+), migrate from the DC through the ion exchange membrane into the anode and cathode chamber, which eventually increases the TDS.

Halim et al. (2021) conducted studies involving variations in the inlet pH and observed an initial increase in output during the early days of operation, followed by a subsequent decrease over time. The maximum output, characterized by a power density of 1,459.02 mW·m⁻², current density of 1,288.9 mA·m⁻², and voltage of 1,132 mV, was identified when utilizing Bhairab river water as the feedstock at a pH of 8. In the present study, the ion migration process can influence the concentration of hydrogen ions (H+) and hydroxide ions (OH–) in the DC. As cations are removed, there can be a reduction in the concentration of H + ions, leading to a localized increase in pH. Similar to the anode chamber, the cathode chamber may also contain buffering systems that can help regulate pH changes.

Anaerobic sludge is a complex mixture of various microorganisms, including bacteria, archaea, and fungi, which collectively form a diverse microbial community. The presence of various microorganisms in anaerobic sludge can lead to synergistic interactions among them, potentially facilitating enhanced electron transfer and metabolic activity. On the other hand, curd is rich in OM and nutrients that can serve as substrates for microbial metabolism. This can support the growth and activity of electrogenic microorganisms, contributing to enhanced electron transfer and power generation. Based on these scenarios, in the present study anaerobic sludge and curd were utilized as microbial sources.

In all three conditions, the generated voltage was observed to be stable with time. This was due to consistency in the degradation of OM. On the other hand, the current generation profile is presented in Figure 14(b). It was observed that the current generation initially increased and became stable for a period and then started declining with time. The maximum current generation was observed to be 1.469 mA with the anaerobic sludge from the UASBR reactor, followed by 1.19 and 0.837 mA with the mixed culture and only with curd.

The high voltage generation with anaerobic sludge was due to the presence of electrochemically active microorganisms capable of performing anaerobic respiration. These microorganisms, such as bacteria were capable of transferring electrons to the electrode surface, resulting in increased voltage output. Anaerobic sludge contains a diverse microbial community with specialized microorganisms that excel in the electron transfer process, enhancing the overall voltage and current generation in the MDC reactor. Zamanpour et al. (2016) conducted studies by using different microbial cultures and found that microalgae biocathode MFC had taken 18 days for stabilization and the maximum voltage was found to be 313 mV on the 29th day. The present study also took 24 h to stabilize, however, it took a comparatively lesser time period as the active microorganisms were used.

On the other hand, curd, being a dairy product, may not contain a significant population of electrochemically active microorganisms that are capable of efficient electron transfer. The absence of such microorganisms could lead to lower current and voltage generation compared to anaerobic sludge. The presence of curd in the anaerobic sludge, i.e., mixed culture can dilute the concentration of electrochemically active microorganisms from the anaerobic sludge, potentially reducing the overall electrochemical activity and voltage and current generation in the MDC reactor. The adaptability of both the microbial species was not so suitable for the active degradation of the OM.

Salman & Ismail (2020) conducted investigations involving domestic sewage and the outcomes demonstrated significant COD removal rates of 100 and 84%. These results were attributed to the favourable conditions that supported the mixed microbial cultures responsible for breaking down the domestic wastewater. The present study also provided real wastewater where the microbes were developed for maximum degradation. The maximum daily COD removal efficiency of 30–40% was observed with the anaerobic sludge and 20–30% with the other two conditions. The COD removal efficiencies declined and varied between 8 and 15% after the 72 h of the reactor operation.

High COD removal with anaerobic sludge was due to the presence of microorganisms capable of anaerobic digestion and OM degradation. Anaerobic sludge contains specialized microorganisms that can metabolize a wide range of organic compounds, efficiently breaking them down into simpler compounds and reducing COD levels in the wastewater. A similar result was reported by Zuo et al. (2017) where the combined microbial species, Proteobacteria was the dominant and ranged from 37 to 48%. These communities were found in the MFCs also. They also reported the higher organic loading and higher salt concentration had little impact on the electrically active microbial species.

Curd might not have a diverse microbial community capable of effective OM degradation compared to anaerobic sludge. The limited microbial activity and metabolic capabilities could result in reduced COD removal. The addition of curd to the anaerobic sludge could potentially alter the microbial community dynamics and reduce the efficiency of OM degradation, leading to lower COD removal compared to anaerobic sludge alone.

The TDS reduction rates varied from 10 to 15% in the initial stage of the experiment and reduced to 6–10% after 72 h of reactor operation. The high TDS removal with anaerobic sludge was due to the ability of the microbial community to promote ion exchange and electrochemical reactions. The microorganisms in anaerobic sludge facilitated the reduction and adsorption of dissolved salts, aiding the removal of ions from the wastewater and leading to higher TDS removal. Curd might not have a significant population of microorganisms capable of efficient ion exchange and electrochemical reactions compared to anaerobic sludge. The limited microbial activity and metabolic capabilities can result in reduced TDS removal.

Kalleary et al. (2014) conducted experiments on the removal of 0.1% yeast extract with Malachite green dye by conventional MDC system, where Bacillus subtilis moh3 was used as a microbial source. They have reported a desalination efficiency of 62.2 ± 0.4% along with 0.15 ± 0.05 W/m³ power output. Luo et al. (2012) studied municipal wastewater treated in a conventional MDC reactor with the presence of Biofilm predominantly Actinobacteria. A maximum COD removal of 52% and desalination efficiency of 66% were reported in their study. The present study also highlights the impact of the microbial culture, and the salt concentration on improving the desalination efficiency.

Over time, as more protons are produced and accumulated, the pH in the vicinity of the anode may drop. The pH of the DC initially decreased and then increased. The pH varied from 7.2 to 7.5 for all the experimental studies (Figure 17(b)). The pH of the DC was found to vary from 7.02 to 7.85 for the study where UASBR sludge was used as microbial inoculum.

The electrochemical reactions occurring in the MDC can influence the chemistry of the DC. For example, the production of hydroxide ions at the cathode can contribute to the increase in pH in the DC, especially if there is ion exchange across the membranes. A similar observation was also made for the cathode and is presented in Figure 17(c). The pH increased to 8.88, 8.39 and 8.83 for the anaerobic culture, curd and the mixed microbial culture, respectively.

Similarly, Li et al. (2020) conducted studies on MDCs and explored the influence of externally applied voltage on MDC performance. Their findings indicated that the removal rates of ammonium and phosphate ions were accelerated when compared to free diffusion. Furthermore, the study by Iskander et al. (2018) demonstrated a direct influence of the current generation on the pace of desalination, where the increased current production in the second case corresponded to a higher desalination rate. They reported that pH, COD, and TDS fluctuations depended upon microbial activities.

Migration of cations from the anode to the cathode chamber could occur, creating a local increase in pH at the cathode. The anode and cathode TDS were found to be increasing with time. The anode TDS increased by 20–100% and the cathode TDS increased by 70–170% in the present experimental studies. Bacterial metabolism at the anode produced by-products such as organic acids, alcohols, and other compounds. These by-products can contribute to TDS concentration. The reason for the increase in the TDS of the cathode chamber was due to the ion migration from the DC to the cathode chamber.

The comparison of current density values for different electrodes is presented in Table 3. The carbon brush electrode reported with the highest current density (0.04 mA/cm²). This result could be justified by the unique structure of the carbon brush, which offers an increased surface area for bacterial attachment and efficient electron transfer. In comparison, the carbon plate and carbon rod electrodes also performed well, with current densities of 0.03 and 0.02 mA/cm², respectively. These electrodes had relatively good conductivity and moderate surface areas, enabling decent electron transfer and bacterial colonization. The carbon cloth electrode lagged behind with a current density of 0.01 mA/cm². This lower performance might stem from issues with bacterial attachment and electron transfer.

In the case of aeration and mediator, the best current density was achieved using the baseline PBS solution without aeration, with a value of 0.027 mA/cm². This indicated that in this specific setup, the microbial community's inherent electron transfer capabilities were more effective than introducing an electron mediator or aeration. Both the aeration conditions (PBS with aeration and PFM with aeration) resulted in reduced current densities of 0.02 mA/cm². This suggests that aeration was not additional support in this MDC configuration all the time, specifically when the system was under the impact of a concentration gradient.

Among the different microbial sources used in the experimental studies, anaerobic sludge resulted in the highest current density of 0.023 mA/cm². This signifies the strength of the anaerobic microbial community in promoting efficient electron transfer. Anaerobic microorganisms are well suited for MDCs due to their ability to function in low-oxygen environments and their adaptation to various electron transfer mechanisms. Curd, on the other hand, provided a lower current density of 0.014 mA/cm². This suggested that the microbial community in curd might not be as effective in electron transfer under the MDC conditions. The combination of anaerobic sludge and curd resulted in a current density of 0.019 mA/cm², indicating an improvement compared to curd alone. The presence of anaerobic sludge likely introduces a more diverse and efficient microbial population that enhances the overall electrochemical activity.

The study of Jafary et al. (2020a) showed that, the result of the IQMDC1 (10.39 ± 0.25 mA) and IQMDC2 (9.67 ± 0.15 mA) were found to be better than the results obtained by the CMDC (8.98 ± 0.34 mA) and SQMDC (5.19 ± 0.18 mA) due to the symmetric reactor design. Li et al. (2020) worked on wastewater treatment and nutrient recovery with MDC technology. They have reported an increase in current density with the increase of applied voltage. Current was found to be 1.5 ± 0.15 mA at 0.8 V and 2.4 ± 0.32 mA at 1.5 V applied external voltage. However, in our present study, no external voltage was used.

Among the tested configurations, the carbon brush electrode exhibited the highest power density of 22.20 mW/cm². This could be attributed to its porous structure, which promoted bacterial growth, and its high surface area, which enhanced the number of electron transfer sites. These factors contributed to increased power generation. The relatively moderate power density achieved with carbon rods could be attributed to their geometry and surface area. Carbon rods typically had a smaller surface area compared to other configurations, leading to a less electroactive surface for bacterial growth and electron transfer.

Carbon plates tend to have a higher surface area than rods and potentially better contact with the surrounding solution. This could facilitate enhanced bacterial growth and electron exchange. The combination of potassium ferricyanide and aeration yielded the highest power density of 15.56 mW/cm². This indicated a synergistic effect between the electron acceptor (ferricyanide) and oxygen availability (aeration), leading to improved overall performance in terms of power generation.

Salman & Ismail (2020) documented that during the operation of MDC using synthetic saline water, a peak power density of 373 mW/m² was observed. However, this power density decreased during steady-state conditions when the sample was switched to actual saline water from the wetland environment. The present study was conducted using real wastewater from a brewery and distillery which helps to simulate the actual scenario enabling effective treatment. Aeration could impact the reduction reaction in the cathode and increase the affinity towards the electron which eventually increases the demand and improves the reaction rate.

The result of the PBS with aeration showed a power density of 13.17 mW/cm². The decrease in power density could be due to the competition for available OM between microbial growth and electron generation. Anaerobic sludge provided the highest power density of 9.57 mW/cm². This resulted in the microbial community present in anaerobic sludge being more effective at utilizing the OM and generating electrons in the MDC system. The lower power densities observed with curd and the combination of anaerobic sludge and curd might be due to less favourable microbial activity or metabolic interactions.

Anusha et al. (2018) reported that the MDC1 reactor showed an average output voltage of 139 ± 27 mV, surpassing that of MDC2, which reported results of 78 ± 15 mV. When compared to an MDC lacking a catalyst on the cathode (57.9% and 37.5 mW/m2, respectively), the Ag-SnO2 coated cathode-equipped MDC exhibited a desalination efficiency of 72.6 ± 3.0% alongside a markedly improved power recovery of 62.3 mW/m², representing a 1.67 times enhancement in comparison to the present study with mediated and increased surface area of the electrode that provided maximum results. The results were found to be in good agreement with the literature survey.

The carbon brush electrode exhibited a slightly higher coulombic efficiency of 1.7%. This might be attributed to the brush's porous and interconnected structure, which could provide more favourable conditions for microbial attachment and electron transfer. However, even with these conditions, the overall efficiency is still relatively low. The carbon rod coulombic efficiency of 1.4% suggested that only a small proportion of the electrons generated by microbial metabolism are contributing to the desired electrochemical reactions in the MDC.

The coulombic efficiency obtained with potassium ferricyanide (CE = 1.55%) suggested a slightly higher utilization of electrons compared to PBS. Potassium ferricyanide acted as an electron acceptor, potentially facilitating more efficient electron transfer pathways, and contributing to slightly better coulombic efficiency. The coulombic efficiency achieved with PBS (1.48%) indicated that a small portion of the electrons generated by the microbial community was being effectively captured and utilized for the desired electrochemical reactions. This relatively low efficiency might result from various factors, including inefficient electron transfer pathways, and electron losses through side reactions.

In a similar domain, Li et al. (2020) conducted experiments on an MDC system to treat municipal wastewater and reported 75.5% COD removal, 8.5% coulombic efficiency with recovery of nitrogen and phosphate 66 and 66.7%, respectively, when 3 ion exchange membranes were used. In our present study, maximum results were obtained in the presence of the mediator. Anusha et al. (2018) used a silver-tin dioxide composite catalyst in a cathode to improve the MDC performance and the result of which found coulombic efficiency of 14.4% with catalyst and 9.5% without catalyst. Luo et al. (2012) showed the performance of the MDC with the microbial consortium generated two times higher conductivity, four times higher power density and 52% COD removal and 131% coulombic efficiency.

Among the tested microbial sources, the highest coulombic efficiency of 1.2% was achieved with anaerobic sludge. This implied that the microbial community in anaerobic sludge was more effective at generating and utilizing electrons for the desired electrochemical reactions compared to curd and the mixed source of anaerobic sludge with curd. Anaerobic sludge contained microorganisms adapted to low-oxygen environments, which might favour the generation of electrons through anaerobic metabolic pathways. However, the relatively low coulombic efficiency suggested that there may be inefficiencies in electron transfer or competing pathways diverting electron flow.

On the other hand, the anode electrode presented a contrasting picture, attached to microbial activity. A noticeable biofilm of microorganisms covered its surface. This biofilm, a complex setup of microorganisms, plays a crucial role in the MDC's operation by aiding electron transfer after OM degradation in the anode. This connection between microbial growth and electron transfer emphasized the complex interactions within the MDC, where microbial communities drove the essential reactions, contributing to energy generation and desalination.

Interestingly, the surface of the cathode electrode displayed a small deposit, but not the microbial film. This observation suggested diverse interactions within the MDC. The cathode, where reduction reactions occur, might foster distinct microbial relationships, or employ a different deposition mechanism. With regards to the morphology, Ma & Hou (2019) documented an even dispersion of wire-like CNTs within the chitosan matrix, spanning both the external and internal surfaces of the sponge. This uniform distribution significantly contributed to the creation of an extensive surface area conducive to bacterial proliferation and reactivity.

The implications of deposition on the membrane extended beyond its visual presence. The growth of microbes and chemical reaction on the AEM surface may introduce a subtle yet profound impact on the ion transfer mechanisms within the MDC. As ions traverse through the AEM, the biofilm matrix can introduce resistance, thereby altering the overall electrochemical behaviour of the MDC. This delicate balance between enhanced microbial activity and altered ion transfer dynamics underscored the need for comprehensive understanding and optimization in the MDC design.

Anusha et al. (2018) demonstrated that the TEM image depicting Ag-SnO₂ showcased a well-distributed arrangement of spherical Ag nanoparticles encompassing the SnO₂ particles. This visual confirmation reinforces the notion of a robust intermolecular electrostatic interaction prevailing between Ag and SnO₂. Consequently, the distinctive 3D CNT-CS sponge provides ample pore dimensions and surface area, facilitating the diffusion and establishment of bacteria, while also serving as an efficient conduit for the movement of substrates and protons, as highlighted in prior studies (Wang et al. 2013; Yong et al. 2012).

The AEM on the DC facing had no sign of bio fouling. However, it was observed to have a deposition on it, which indicated the reduction of the ion transfer towards the anode chamber. This unique occurrence highlights a different interaction between microbial growth and the DC environment. This requirement of membrane cleaning might be required for effective ion transfer in these cases. Additionally, similar findings were noted in the case of the CEM for the DC side. There was no deposition of microbial growth found for CEM on the cathode side, implying the clogging or blocking of the membranes. There were also observations made of the colour change of the membrane with time, this might be due to the chemical reaction during the experimental conditions.

In the present study, various factors were considered to control the process of sludge formation. Carbon electrodes were selected, and these materials were resistant to corrosion and degradation. This helped minimize the release of particulate matter into the MDC and reduced sedimentation. The ion exchange membranes (AEM and CEM) that effectively separate the DC from the anode and cathode chambers helped prevent the movement of undesirable particles, and leakages and minimized the sediment formation. Again, the feedwater entering the MDC was ensured to have minimum particulate matter and suspended solids. In a practical scenario, if an MDC is being installed at the site, a clarifier is preferable as a part of pre-treatment to control the incoming solids.

On the other hand, regular removal of excess sludge from the bottom of the reactor could prevent the accumulation of solids. In most cases, maintaining a balanced nutrient ratio within the reactor optimizes microbial activity. Adequate nutrient levels helped in the breakdown of OM and reduced sludge production. Design of the reactor with an optimized flow velocity can help a better degradation and reduce sludge deposition.

Meng et al. (2019) reported the degradation of abundant organics in dehydrated sludge sustained the approximate 200-day operation of two-phase MDC. During this period, the OM content decreased from 64.34 ± 1.64% to 43.3 ± 1.41%. They also reported that the OM removal rate of 32.70 ± 1.67% in two-phase MDC exceeded the 25.71 ± 0.15% observed in the MDC experiment using dehydrated sludge as an anodic substrate, as conducted by Meng et al. (2014). The study conducted by Song et al. (2015) demonstrated maximum removal efficiencies of total organic carbon (TOC) across three depths (0–8, 8–18, and 18–28 cm) in the sediment MFC. They observed an increase of efficiency, i.e., 18.9, 20.9, and 20.8%, respectively, indicating enhanced TOC removal.

The presence of a diverse microbial community can lead to competition among different species for resources and space, affecting the overall efficiency of the MDC. Incompatibility or unfavourable interactions between certain microbial species might hinder electron transfer processes, reducing the rate of OM breakdown and electron production. This can subsequently impact the voltage and current generation, limiting the MDC's performance.

The utilization of carbon cloth electrodes might introduce operational interference leading to low voltage and current generation in MDCs. While carbon cloth is commonly chosen for its conductivity and stability, its surface characteristics or specific properties could affect its performance in facilitating electron transfer. Properties such as insufficient surface area for microbial colonization, or limitations in the electrode's architecture might impede efficient electron flow.

Membranes in MDCs play a crucial role in separating different chambers, facilitating ion transport, and preventing cross-contamination. With continuous run time of MDC, the accumulation of OM, particulates, or biofilm formation on the membrane surfaces can lead to clogging. This impedes ion movement between compartments, disrupts the flow of electrons, and hinders desalination efficiency.

This microbial analysis provided valuable insights into the microbial community's composition and functioning, contributing to the broader understanding of MDC performance. While efforts were made to categorize the predominant microbial culture, the study might have been more comprehensive by employing advanced molecular techniques to identify specific microbial species and understand their individual contributions.

The choice of carbon materials for electrodes was a deliberate decision, considering their widely recognized conductivity, stability, and cost-effectiveness. The incorporation of different types of electrodes, such as metal-based or composite materials, could have provided a comparative assessment of their performance, shedding light on potential improvements or optimizations in MDC design.

The present study focused on the application of different electrodes, mediators, microbial cultures, and aeration on the performance of the MDC reactions, voltage, and current generation. The application of different electrodes resulted in a maximum voltage and current of 702 mV and 2.16 mA, respectively. The same experimental results showed 83% of COD removal and 60% of TDS removal. The reason might be due to the unique structure, offering a significantly larger surface area and a highly porous, three-dimensional matrix. This study signifies the selection of optimized electrodes for the application based on the reactor and the wastewater, which will aid in providing the maximum power output and organic removal.

The results of the experimental study of the application of different experimental conditions showed a maximum voltage of 992 mV and current of 1.96 mA with potassium ferricyanide mediator and aeration condition. A maximum COD and TDS removal of 87 and 59%, respectively was reported for the same reaction conditions.

The application of a mediator resulted in the highest voltage of 697 mV, 1.98 mA with potassium ferricyanide mediator and normal water as mediator. COD and TDS removal efficiency of 82 and 53% was reported with potassium ferricyanide mediator. This acted as an electron shuttle, facilitating efficient electron transfer between microorganisms and the electrode. The applicability will be based on the conditions of the setup and plant equipment automation and availability which may reduce the HRT and provide desired output. It was also observed that maximum COD and TDS reductions of 82 and 51%, respectively, were achieved with the application of different microbial cultures. Anaerobic sludge contains a diverse consortium of microorganisms adapted to function in oxygen-deprived environments.

The results signified the increase of electrode surface area, application of mediator, and anaerobic-mixed microbial culture that has a great impact on the CE. The present experiment study provided a detailed insight into the selection of the electrodes and microbial culture along with the application of mediator on the performance of MDC reactors. It also helped to select the materials based on the requirement and HRT for the experimental conditions to generate the maximum output.

The authors are thankful to Karnataka State Pollution Control Board (KSPCB) for providing the permission to collect the samples form selected industries.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.