Batch investigation of constructed wetland microbial fuel cell with reverse osmosis (RO) concentrate and wastewater mix as substrate

Batch investigation of constructed wetland microbial fuel cell with reverse osmosis (RO) concentrate and wastewater mix as substrate

Biomass and Bioenergy 122 (2019) 231–237 Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: www.elsevier.com/locate/b...

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Biomass and Bioenergy 122 (2019) 231–237

Contents lists available at ScienceDirect

Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe

Research paper

Batch investigation of constructed wetland microbial fuel cell with reverse osmosis (RO) concentrate and wastewater mix as substrate

T

Bhaskar Dasa,∗, Somil Thakura, M. Sai Chaithanyaa, Pinakpani Biswasb a b

Department of Environmental and Water Resources Engineering, School of Civil and Chemical Engineering (SCE), VIT, Vellore, Tamil Nadu, 632014, India Environment Research Group, R&D, Tata Steel Ltd, Jamshedpur, 831007, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Constructed wetland Microbial fuel cell Graphite electrodes Sewage sludge Reverse osmosis concentrate Bio-electricity

In the present study we have integrated Microbial Fuel Cell with Constructed Wetland (CW) to evaluate its potential to remove Chemical Oxygen Demand (COD) and Total Dissolved Solid (TDS) along with electricity generation from the mixture (1:1) of domestic wastewater (WW) and reverse osmosis (RO) concentrate. Two vertical flow CW-MFCs have been investigated (with plant and without plant) in batch mode for 69 days. The plant species Canana India has been used for its suitability to sustain saline environment. The COD and TDS mass balance is studied for different organic loading and the result is compared with their removal efficiencies and electricity generation in the lab scale microcosms. The result shows that CW-MFC with plant (CW-MFC-1) and CW-MFC without plant (CW-MFC-2) produced maximum cell voltage of 0.86 V and 0.75 V respectively with COD removal of 88.1% and 83.1% and TDS removal of 82.05% and 77.5%. The maximum Normalized Energy Recovery (NER) values in CW-MFC-1 (NERs = 6.415 Wh kg−1 COD and NERv = 1.124 Wh m-3) was also found more than CW-MFC-2 while maximum Columbic Efficiency (CE) was observed in CW-MFC-2 (C.E. = 1.9%). Enrichment of the nutrient from the RO concentrate positively influenced the performance of the CW-MFC with plant synergistically.

1. Introduction Constructed Wetland systems are a combination of physical and biological processes, which are being used effectively for treatment of wastewater from different sources like domestic, industrial, agriculture, mine drainage, sewage dewatering [1,2] due to its low operational cost, economical installation and minimum maintenance required. Since the start of 21st century, researchers have paid a lot of attention in field of microbial fuel cells. A typical MFC consist of 4 basic components, i.e.; anode, cathode, proton exchange membrane (PEM) and an electrical circuit connecting anode and cathode. Anode is kept in an anaerobic chamber filled with organic substrate and act as an electron collector because of its opposite charge [3]; while cathode is kept in aerobic conditions so as to provide sufficient oxygen; which has high redox potential, to act as electron acceptor. When substrate breaks down, electrons and protons are generated in anode chamber. Electrons get accumulated on anode with the help of exoelectrogenic bacteria and are transferred through external circuit to cathode [4]. Protons makes their way to cathode through the fluidized media; that carries them from anode chamber to cathode chamber [5]. These protons (H+) meet electrons at cathode surface. Electricity generated can be measured



with the help of external electric circuit when reduction reaction occurs on cathode in the presence of electron, protons and oxygen (electron acceptor). Major requirement for working of a typical MFC is availability of redox gradient [6], between the anode and cathode chamber, which are maintained artificially but can be achieved with sediment MFC and CWMFC naturally. A CW-MFC can achieve it simply by placing anode inside saturated soil (anaerobic) and connecting it to cathode placed at top of soil surface [7,8] in the plant root zone. To adapt the anoxic condition of the waterlogged root of the macrophytes, used in CW-MFC, they generally develop aerenchyma, to transfer oxygen from shoot to the root zone [3] thus creating naturally aerobic condition in the cathode zone. Preliminary investigations on various designs of constructed wetland incorporating MFC (Fig. 1) have been explored already and hence is a proven technology for reducing COD, simultaneously generating electricity. Some of them are operated unplanted [9,10] having open air cathode and others with plants [11–14] where the cathode are placed in the root zone of the plant. The different plant species, the types of the electrode material, the flow characteristics and the types of substrates are the primary design

Corresponding author. E-mail address: [email protected] (B. Das).

https://doi.org/10.1016/j.biombioe.2019.01.017 Received 22 August 2018; Received in revised form 27 December 2018; Accepted 21 January 2019 0961-9534/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Different setups of constructed wetland-microbial fuel cells: The flow regime of the system (from left to right) is: (a) batch feed; (b) continuous upflow with aeration in cathode area; (c) continuous upflow with different anode cathode spacing; (d) simultaneous upflow-downflow; (e) horizontal flow with effluent recirculation.

Table 1 Different plant species utilized in the CW-MFC works are tabulated below. Sl. No

Type of electrode

Mode of operation

Plant species

Type of WW

Voltage (V)

Maximum power density

COD removal efficiency (%)

Reference

1 2 3 4 5 7 8 9 10 12

Graphite mat Graphite disc Graphite disc GAC/SSM GAC/Stainless steel Graphite plate Graphite Grannular Graphite GAC GAC Graphite Pt & Carbon cloth

Batch Continuous Continuous Vertical flow Vertical flow Vertical flow Horizontal flow Vertical flow Vertical flow Vertical flow Horizontal flow Vertical flow

Oriza sativa L Eichhornia crassipes Eichhornia crassipes Ipomoea aquatic Ipomoea aquatic Phragmites australis Phragmites australis Phragmites australis Ipomoea aquatic Ipomoea aquatic Phragmites australis Canna Indica

– Domestic sewage Fermented distillery Synthetic Synthetic Swine Synthetic Swine Synthetic Synthetic Municipal Synthetic

0.708 0.732 0.809 0.201 0.740 0.495 0.748 0.309 0.610 0.500

– – 80.08 mWm−2 55.05 mWm−3 12.42 m Wm−2 0.042 Wm−3 0.094 Wm−3 0.268 Wm−3 0.302 Wm−3 0.852 Wm−3 70 Whm−3 19.60 mWm−3

– 71.82 86.67 95.00 94.80 76.50 95.00 80.00 85.70 85.70 85.1 53.23

[3] [15] [15] [16] [14] [17] [18] [19] [20] [21] [13] [12]

increases. The range between 3000 and 5000 mg L−1 had given maximum voltage. However if the salinity level is kept on increasing; voltage generation shows a declined effect. At a concentration beyond 9000 mg L−1, the wetland plant was damaged. In this context, it is clear that salinity levels play a major role in combined CW-MFC systems. As RO concentrate is rich in inorganic salt concentration, these inorganic salts can be used as feed for microorganisms for their spontaneous growth. Further it enhanced growth of exoelecrogens [26]; Oliver [27]; Lisa [28,29], nanowires and electron shuttles thus reducing the internal resistance and accelerate the process of treatment [30,31] which might leads to better performance of MFC through more stable voltage generation. Hence the RO concentrate may be very potential substrate for MFC which has not been still been investigated. On the basis of the above hypothesis, in our present work we have used the RO concentrate mixed with domestic wastewater in 1:1 ratio (WW-RO mix) as a substrate in CW-MFC. Different studies have been conducted on the treatment of the saline wastewater with different plant species and salinity level, but for simultaneous power generation and treatment of the wastewater the RO concentrate was not been experimented. The salinity level in the WW-RO mix were maintained as per the literature for optimal power generation and root zone

consideration for CW-MFC. The Table 1 provides the summarized results of the above parameters with their treatment efficiency and energy recovery. RO technology was typically designed as per norms to treat brackish water, but is being used for normal household water purification. Generally 15–65% of water sent to RO units becomes waste, thus leading to production of more wastewater with high TDS (A [22]. having limited disposal techniques available. These nutrients present in RO concentrate in the form of TDS are mainly inorganic salts (high and less soluble sodium, potassium and barium salts). Considering the effects of salinity on electrochemical performance of CW-MFC there are two contradictory effects [23]; J. [24]. Studies indicates rise in power generation due to reduction of internal ohmic resistance at increased salinity levels. On the other hand increased salinity level may decrease soil enzyme activity thus inhibiting the microbial performance for wastewater treatment [25]. To find out optimum salinity levels for better performance in CW-MFC, they carried out a study in which twelve plant species were tested under different salinity levels in the domestic wastewater and found adverse effects on microbial activities above 2000 mg L−1. Similarly [24], showed that when salt concentrations are increased continuously until 5000 mg L−1, voltage generation 232

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microbial activities [25]. The Canana Indica has been selected for its higher nutrient uptake capability, and root activity in saline water. The study is conducted in pilot scale batch mode CW-MFC with and without plant. The results can be of great relevance because of their potential applicability to treat wastewater and RO concentrate while simultaneous generation of electricity. For instance, residential/commercial colonies where RO units are operated for providing water and wastewater is generated in large quantity, combined CW-MFC could be an eco-friendly solution. 2. Materials and methods 2.1. CW-MFC setup Two identical CW-MFC systems were fabricated by using Polyvinyl Chloride (PVC) pipe of 2 mm thickness, internal diameter 17.5 cm and length of 63 cm. Both of them were filled with gravel of size 2–4 mm. Canna indica is collected from the university nursery and planted in CWMFC-1 setup. In CW-MFC-2, no plant was used but all the design parameters was similar to CW-MFC-1. The bottom ends of both systems were closed with PVC caps and made water-tight by sealing with epoxy; while the upper ends were left free to atmosphere. An overflowing port was provided at the top of each system for draining off the excess influent; and sampling port was provided for collecting the sample from the both CW-MFC systems at the bottom; which was 63 cm from the top. A glass-wool layer of thickness 1.5 cm was placed above the anodic zone in both the systems, which separates anodic and cathodic zones, helps in maintaining conditions similar to MFC [17]. Two identical graphite electrodes of 10 mm thickness, plate-type were placed; one at root zone at 17 cm from the top to act as air-cathode. Another electrode was placed at a distance of 42 cm from the top in anaerobic condition which acts as anode of MFC. The projected surface area of anode and cathode was 25 cm2 which were placed vertically [32] 24 cm apart. Insulated copper wires were used to connect the cathode and anode through clamps to an external resistor (Rex = 1000 Ω, unless stated otherwise) and connected to multimeter for continuous monitoring of Voltage and current generation. The schematic diagrams of both CWMFCs are shown in the Fig. 2. The systems were kept in open atmosphere to give plant natural conditions for its growth. The environmental condition (Temperature and % Relative humidity) were measured daily which is provided in Supplementary Information I.

Fig. 2. Schematic representation of both the experimental setup.

micro-organisms for their growth and hence was not considered for COD mass balance. Both the wetland systems were operated under batch mode in similar environmental condition and hence subjected to natural evapotranspiration. At different time interval the water loss due to evapotranspiration is replenished with WW-RO mix and the volume is measured. As the evapotranspiration volume is also influenced by the environmental factors like temperature and humidity, the daily evapotranspiration loss is not same. Hence the volume of WW-RO mix is different in every batch. Total ten batches of influents were added to the two systems throughout the experiment. First batch was operated for 6 days, second for 9 days, and subsequent batches for 9, 9, 9, 6, 6, 6, 6, 3 days respectively. The hydraulic retention time for CWe MFC 1 and CW-MFC 2 was 24.33 days and 21.40 days respectively (Supplementary Information I). The %COD and %TDS removal is calculated for every batch individually i.e., The COD and TDS of the influent and effluent in mg L−1 is converted into mg by multiplying with available volume of the solution in the system at that instance and then removal efficiencies are evaluated by subtracting final effluent concentration of that batch from the influent one. The remaining COD and TDS value after every batch is added to the influent COD and TDS of the next batch to maintain mass balance. The detail of calculation is included in Supplementary Information II (for COD) and Supplementary Information III (for TDS). The COD and TDS loading rate is calculated by dividing total COD and TDS by anode volume. The voltage and current were measured using digital multimeter (MASTECH MAS830 series) across a resistance of 1000 Ω for every 8 h. Power is calculated by dividing the square of voltage by resistance. The power density and current density were calculated by dividing the power and current obtained by surface area of anode in respective systems [33]. The NERv (kWh m-3) and NERs (kWh (kgCOD)−1 has been calculated to estimate the efficiency of power generation in respect with the volume of wastewater treated and amount of COD removed. [34]. NER is considered an appropriate parameter for comparison of different MFC's because of its non-dependence on reactor size and consideration of wastewater flow and organic removal efficiency

2.2. Reverse osmosis concentrate and WW collection and analysis WW and Reverse osmosis concentrate were collected from WW treatment plant and RO treatment plant in VIT University, Vellore Campus and mixed with 1:1 ratio as the substrate used in the experiment. The ratio is kept as 1:1 to ensure that the salinity level would be within 2000–5000 mg L−1. Immediately after the mixing of water, the raw WW, RO concentrate and WW-RO concentrate mix was analysed for the physiochemical parameters mentioned in Table 2. This analyses was carried out for every batch before adding in the CW-MFC throughout the experiment. All the parameters are analysed as per the APHA standard [43]. The average COD and TDS concentration of all the batches of WW-RO mix samples was found to be 3040 mg L−1 (range 2560–3520 mg L−1) and 2650 mg L−1 (range 2638–2978 mg L−1) respectively. 2.3. CW-MFC operation and batch addition Raw WW of 250 ml was collected and mixed with the gravel in the anodic zone to acclimatize both the CW-MFC systems prior to the initiation of the experiment for 2 days; as the raw WW contain several micro-organisms which will adhere to the wetland media and electrodes; stimulates electricity generation. The COD contribution for this small amount of WW was negligible as this will be consumed by the 233

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Table 2 Physiochemical characteristics of influent wastewater, RO concentrate and WW-RO mix samples. S.No

Parameter

Raw sewage water

Reverse Osmosis reject water

Raw sewage and Reverse Osmosis reject water Mix sample (1:1 ratio)

1 3 4 5 6 7 8 10

pH Electrical conductivity (μScm−1) TDS (mgL−1) TSS (mgL−1) Alkalinity (mgL−1) Chlorides (mgL−1) Hardness (mgL−1) COD (mgL−1)

7.2 160 1080 740 465 280 730 3020

7.7 5750 3970 285 820 950 1140 3250

7.3 3130 2650 565 680 700 980 3040

Table 3 Equations for calculation of parameters. Sl. No.

Parameters

Equation

Variables

1

COD Removal Efficiency (%)

Ci − Ce Ci

2

CE(%)

M ∫0t Idt Fbva ΔCOD

Ci = Initial COD (mgL−1) Ce = Final COD (mgL−1) M = Molecular weight of oxygen(32) I = Average current during time dt F = Faraday's constant Va = Volume of Anode ΔCOD = COD Removed, b = 4 (number of electrons exchanged per mole of oxygen) ΔCOD = COD Removed Vww = Volume of Wastewater Pavg = Average Power d = Time(days) Ecell = Voltage Rext = External Resistance Aan = Area Of Anode

3 NER ( 4

Wh m3

and

Wh ⎞ ⎟ kg BOD



Power (mW) Power density (mW/m2)

× 100

NERs =

× 100

Pavg × d ⎤ NERv [1000 × Vww ⎥



=

Pavg × d [1000 × ΔCOD]

2 2 Ecell Ecell Rext Aan Rext

for its calculation. Units used are “kWh” in place of “KJ”, so that to have a proper understanding between academic and industrial field. Usage of this parameter enhances the quantitative understanding of energy performance in MFC [35]. Equations that were used in calculation of various parameters are in Table 3.

in CW and found that Canna Indica has higher bio mass, which contributes surface area for microbial growth and high average root activity (Oxidation activity- α-NA oxidation capacity higher than 80 μg (g.h)−1 for diffusing Oxygen in to the substrate. In both the CW-MFC-1 and CW-MFC-2 there is a gradual increase of % COD removal rate till 5th batch (42 days) and after that it has been decreased (Fig. 3) though the COD loading rate is fairly constant. This signifies that the CW with plant and without plant have similar trend to COD removal. It is to be noted that COD is evaluated for both the systems for every three days and systems are not drained completely until the end of the experimental run. Hence, along with the progression of experiment, the digested biomass might get clogged in the cathode zone leading to reduction of oxygen levels and formation of anaerobic environment resulting reduction of %COD removal. This phenomenon affects both the CW with and without plant similarly. Before adding every batch, the untreated COD from the previous batch was also available. The COD from the fresh batch and remaining COD of the previous batch collectively is considered as the cumulative COD [18,37]. The Fig. 4 shows the cumulative %COD removal for both the setups; which is 88.1% and 83.1% in CW-MFC-1 and CW-MFC-2 respectively. The calculation of the cumulative %COD removal is

3. Results and discussion 3.1. COD removal The COD removal efficiencies along with the COD loading rates were compared for both the systems in each batch (Fig. 3). The highest %COD removal in CW-MFC-1 and CW-MFC-2 are 70.18% which is in 3rd batch (24 days) and 55.56% which is in 2nd batch (15 days) respectively. The removal efficiency of CW-MFC-1 is found to be more when compared to CW-MFC-2. The plant root of Canna Indiaca helps in creating micro-aerobic zones at the root level which leads to more degradation of organic matter [15,36]. In the study Feng [25] demonstrated the impact of saline environments on different plant species

Fig. 3. The % COD removal along with COD loading rate for CW-MFC-1 and CW-MFC-2 in each batch.

Fig. 4. Cumulative % COD and TDS removal in CW-MFC-1 and CW-MFC-2. 234

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Table 4 Influent COD and TDS concentration and mass of each batch sample of WW-RO mix added along with day and volume (L). CW-MFC-1 Volume Added (L) Batch

Day

1 2 3 4 5 6 7 8 9 10

0 6 15 24 33 42 48 54 60 66

CW-MFC-2 Influent COD Conc. (mgL

5.5 0.9 1.3 1.4 1.1 1.2 1.3 1.1 1 0.8

3200 3200 2880 3200 3360 3520 3040 3082 3040 2560

−1

Influent TDS )

Mass (mg)

Conc. (mgL

17600 2880 3744 4480 3696 4224 3952 3390 3040 2048

2790 2807 2890 2916 2978 2772 2796 2671 2638 2681

−1

Volume Added (L) )

−1

Mass (mg) 15345 2526 3757 4082 3276 3326 3635 2938 2638 2145

Influent COD Conc. (mgL

5.5 1.0 2.0 1.5 2.0 1.5 1.0 1.3 1.0 1.0

3200 3200 2880 3200 3360 3520 3040 3082 3040 2560

Influent TDS )

Mass (mg)

Conc. (mgL−1)

Mass (mg)

17600 3200 5760 4800 6720 5280 3040 4007 3040 2560

2790 2807 2890 2916 2978 2772 2796 2671 2638 2681

15345 2807 5780 4374 5956 4158 2796 3472 2638 2681

decrease rather gradually. This implies that the salt saturation level after the 6th batch (42 days) may affect the TDS removal efficiency of the plant but does not affect significantly for the CW without plant. The Fig. 4 shows the cumulative %TDS removal for both the setups; which is 82.05% and 77.53% in CW-MFC-1 and CW-MFC-2 respectively. The cumulative TDS removal efficiency with plant (CW-MFC-1) is more than the one without plant (CW-MFC-2. Nutrient removal is one of the major uptake mechanisms by plants in CW-MFC's; facilitating TDS removal by absorbing the nutrients in the influents [39–41]. The TDS removal in CW-MFC-1 is more which is due to the presence of plant; i.e., roots of the plants have ability to act as filters when they grow; thus facilitating reduction of dissolved solids; coupled by nutrient uptake of the plant. This can be relatable with the study by Mohannad [30] where the nutrient uptake and dissolved solids removal by Basilicum plant was observed in similar environment. Also, it is observed that even though overall TDS removal efficiencies are good for both setups, batch wise TDS removal efficiencies are reduced considerably with time in experimental run. This may be probably due to the TDS which may choke the soil pores, and also Iit is hypothesized that this may happen due to clogging problem due to the prolonged usage of wetland systems. It is to be noted that the presence of TDS up to some extent helped in reducing the internal resistance. Studies done by Ref. [27] indicate that there is significant drop in internal resistance; further boosted power generation to some extent; when salinity is not exceeding 20 gL−1. In present study it is found that salinity of the influent is approximately between 1.5 and 2 gL−1; thus may be concluded that salinity present in the RO concentrate aided in power generation; which needs further exploration in varying doses of salinity. In support to this, Canna Indica showed better growth in saline environments which adds advantage for sustainability of CW-MFC-1. Moreover the excessive presence of salinity affected the microbial groups. The studies of [24] on salinity effects on performance of CW-MFC specifies that salinity concentrations such as 3 gL−1 affected the anode biological processes. So, the presence of TDS may help the system to increase the voltage differences but needs further modifications such as selection of adoptive plant and microbial species for further increase in the efficient functioning of the system.

included in Supplementary Information II. The cumulative COD removal efficiency with plant (CW-MFC-1) is more than the one without plant (CW-MFC-2). In general, the removal efficiencies of wetlands are calculated without considering evaporation and evapotranspiration; which are not necessarily negligible and can be significant [38]; hence volume losses are estimated in order to maintain mass balance. The loss of solution in both the systems batch wise is listed in the Table 4 along with influent COD and TDS concentrations. It has been stated earlier that the volume of WW-RO mix in every batch is depended on the evapotranspiration loss of the two system. Hence the amount of COD (in mg) and TDS (in mg) added in each batch is depended not only their concentration (mg L−1) but also the evapotranspiration loss of both the system. 3.2. TDS removal One of the main objectives of this study is also to remove the TDS from the RO concentrate. The %TDS removal and TDS loading rate (kg TDS m−3) is considered in every batch and plotted in Fig. 5. The TDS loading rate is found to be within 3.35–4.80 kg TDS m−3. The TDS loading rate is fairly constant in all the batches except the first one. The %TDS removal calculations are performed similar to the %COD removal efficiency as mentioned in section 3.1 and is included in Supplementary Information III.). The highest %TDS removal in CWMFC-1 and CW-MFC-2 are 51.83% which is in 4th batch (33 days) and 40.47% which is in 1st (6 days) respectively. Similar to %COD removal, the %TDS removal also more in CW-MFC-1 which signifies plant's contribution in removing the TDS. The %TDS removal in CW-MFC-1 decreases after the 6th batch (42 days) in a fairly constant TDS loading. Under almost similar TDS loading the %TDS removal in CW-MFC-2

3.3. Voltage generation and power density Voltage is monitored continuously by connecting a multimeter to both the CW-MFC systems. The peak voltage observed in CW-MFC-1 and CW-MFC-2 was 0.86 V and 0.75 V respectively. Fig. 6 shows the cumulative voltage generation against time and each batch for a span of 69 days. It is believed that wetland with plant increased the concentration of oxygen at the root zone creating greater redox potential gradient between anode and cathode thus enhancing the electron transfer rate through the external circuit. Moreover as the upper part of

Fig. 5. The % TDS removal along with TDS loading rate for CW-MFC-1 and CWMFC-2 in each batch. 235

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the COD and TDS from the WW-RO mix and power generation than CWMFC without plant. However the TDS and COD loading is important because improper loading may antagonistically affect the effectiveness. The COD loading in our study is 2–6 kg COD m−3 which we found sufficient to remove 88.1% COD in CW-MFC with plant, which is yet to be optimized. Apart from that, Nutrients in dissolved form, present in RO concentrate seems to help the operation of CW-MFC with plant. Canna Indica employed in this study showed no deterioration in health, sustained in the salinity levels and contributed to enhanced removal efficiency as well as voltage generation. Further study on batch and continuous upflow model in different TDS and COD loading rate is required for optimizing the relationship of combined COD and TDS loading for treatment and electricity generation. Acknowledgements Fig. 6. Cumulative voltage generation with respect to time in each batch.

The authors acknowledge the help of VIT, Vellore, India for the financial support provided under Seed Money for Research (RGEMS) for FY 2017-2018 to carry out this research work. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biombioe.2019.01.017. References [1] Ghrabi Ahmed, Latifa Bousselmi, Fabio Masi, Martin Regelsberger, Constructed wetland as a low cost and sustainable solution for wastewater treatment adapted to rural settlements: the Chorfech wastewater treatment pilot plant, Water Sci. Technol. 63 (2011) 12. [2] B.E. Logan, Microbial Fuel Cells, John Wiley & Sons Inc, Hoboken, New Jersey, 2008. [3] Zheng Chen, Yan-Chao Huang, Jian-hong Liang, Feng Zhao, Yong-guan Zhu, A novel sediment microbial fuel cell with a biocathode in the rice rhizosphere, Bioresour. Technol. 108 (2012) 55–59. [4] Lu Lu, Defeng Xing, Zhiyong Jason Ren, Microbial community structure accompanied with electricity production in a constructed wetland plant microbial fuel cell, Bioresour. Technol. 195 (2015) 115–121. [5] Hong Liu, Ramanathan Ramnarayan, Bruce E. Logan, Production of electricity during wastewater treatment using a single chamber MFC, Environ sci techno 38 (2004) 2281–2285. [6] Andrzej Bialowiec, Laura Davies, Antonio Albuquerque, Peter F. Randerson, Nitrogen removal from landfill leachate in constructed wetlands with reed and willow: redox potential in root zone, J. Environ. Manag. 97 (2012) 22–27. [7] C.E. Reimers, L.M. Tender, S. Fertig, W. Wang, Harvesting energy from the marine sediment-water interface, Environ. Sci. Technol. 35 (2001) 192–195. [8] L.M. Tender, C.E. Reimers, H.A. Stecher, D.E. Holmes, D.R. Bond, Harnessing microbially generated power on the seafloor, Nat. Biotechnol. 20 (2002) 821–825. [9] Clara Corbella, Jaume Puigagut, Improving domestic wastewater treatment efficiency with constructed wetland microbial fuel cells: influence of anode material and external resistance, Sci. Total Environ. 631–632 (2018) 1406–1414. [10] Lei Xu, Bodi Wang, Xiuhua Liu, Wenzheng Yu, Yaqian Zhao, Maximizing the energy harvest from a microbial fuel cell embedded in a constructed wetland, Appl. Energy 214 (2018) 83–91. [11] Pratiksha Srivastava, A.K. Yadav, B.K. Mishra, The effects of microbial fuel cell integration into constructed wetland on the performance of constructed wetland, Bioresour. Technol. 195 (2015) 223–230. [12] Pratiksha Srivastavaa, Saurabh Dwivedi, Naresh Kumar, Rouzbeh Abbassi, Vikram Garaniy, Asheesh Yadav, Performance assessment of aeration and radial oxygen loss assisted cathode based integrated constructed wetland-microbial fuel cell systems, Bioresour. Technol. 244 (2017) 1178–1182. [13] Clara Corbella, Jaume Puigagut, Marianna Garfí, Life cycle assessment of constructed wetland systems for wastewater treatment coupled with microbial fuel cells, Sci. Total Environ. 584–585 (15) (2017) 355–362. [14] Shentan Liu, Hailiang Song, Xianning Li, Fei Yang, Power generation enhancement by utilizing plant photosynthate in microbial fuel cell coupled constructed wetland system, Int. J. Photoenergy 2013 (2013). [15] S. Venkata Mohan, G. MohanaKrishna, P. Chiranjeevi, Sustainable power generation from floating macrophytes based ecological microenvironment through embedded fuel cells along with simultaneous wastewater treatment, Bioresour. Technol. 102 (2011) 7036–7042. [16] Shentan Liu, Hailiang Song, Size Wei, Fei Yang, Xianning Li, Bio-cathode materials evaluation and configuration optimization for power output of vertical subsurface flow constructed wetland-Microbial fuel cell systems, Bioresour. Technol. 166 (2014) 575–583. [17] Yaqian Zhao, Sean Collum, Mark Phelan, Tristan Goodbody, Liam Doherty,

Fig. 7. Columbic efficiency (CE) and NER (Normalized energy recovery) of CWMFC-1 and CW-MFC-2.

the wetland is exposed to atmosphere also contributes to some extent in maintaining aerobic conditions [18]; and the higher availability of O2 can amplify the cathodic potential. Hence CW-MFC-1 produced more voltage than CW-MFC-2. It is to be noted that the voltage is peaked for a maximum time after the addition of fresh solution which may be due to the higher breakdown of organic matter in the influent. But the similar trend is not observed throughout the experiment which may be due to several problems such as clogging in the cathodic region, deterioration of the cathode [42], the addition of fresh solution to the system in the dried condition. Also, the presence of salinity in the influent supported in reducing the internal resistance, giving rise to more voltage needs further exploration of optimum dosage of salinity. Power densities are calculated for respective voltages; percentiles for voltage and power densities were estimated and plotted as a graph (Supplementary Information IV). It is observed that CW-MFC-1 and, CW-MFC-2 produced 0.4 V for more than 50% of the total operational time. Though CW-MFC-1 is having higher peak voltage than CW-MFC-2 the later shows more consistency voltage generation and stable power density. The volatility in CW-MFC-1 may be probably due to the incorporation of plant; indirectly contributing to the ionic changes as the plant may have its own ionic properties; needs further studies to testify. In Fig. 7, it can be clearly seen that with increase in columbic efficiency, The NERs values are also increasing. Whenever we added new batch COD loading use to increase and part of it was being transported to cathode zone with the downflow, that is why though COD reduction was more, CE and NER observed were comparatively less. Maximum NERs with plant (NERs = 6.415 Wh kg−1 COD and NERv = 1.124 Wh m−3) was more than NERs without plant and the maximum CE without plant (C.E. = 1.9%) is higher than that of with plant. 4. Conclusions Selection of appropriate plant species can be effective in reducing 236

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