Integrated drip hydroponics-microbial fuel cell system for wastewater treatment and resource recovery

Bioresource Technology Reports 9 (2020) 100392

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Integrated drip hydroponics-microbial fuel cell system for wastewater treatment and resource recovery Ravi Kumar Yadav1, P. Chiranjeevi1, Sukrampal, Sunil A. Patil

T

⁎

Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Mohali (IISER Mohali), Knowledge City, Sector 81, SAS Nagar, 140306, Punjab, India

ARTICLE INFO

ABSTRACT

Keywords: Drip hydroponics Microbial electrochemical technology Domestic wastewater Microbial fuel cells Nutrients removal Bioelectricity

The development of low-cost, less energetically and chemically-intensive, and easy-to-operate technologies is desired for the efficient management of wastewaters at the point sources. In this context, we tested a novel integrated drip hydroponics-microbial fuel cell system design with domestic wastewater. It consisted of influent and effluent ducts along with ten reactor units. Each unit hosted the cocopeat bed matrix, a graphite electrode assembly, and a lemongrass sapling. After 3 h operation in a batch recirculation mode, the integrated system achieved 72 ± 2.4% COD, 83 ± 1.1% phosphate, and 35 ± 4.2% ammonia removal efficiencies. The efficiencies increased considerably after 12 h operation. It also yielded low levels of power output and plant biomass. The occurrence of various microbial and plant activities, along with adsorption and filtration processes, resulted in the efficient performance by the integrated system. The simple but efficient system design could offer an easy-to-implement approach for wastewater treatment at the household and small community levels.

1. Introduction Due to rapid urbanization and population increase, the natural freshwater reservoirs are depleting at an unprecedented rate. It is also resulting in the generation of large quantities of wastewater and thereby putting enormous pressure on the existing water utility sector as well as the environment. The gap between the wastewater quantity and treatment capacity is rapidly growing. Globally, the average treatment of municipal and industrial wastewaters is around 70, 38, 28, and 8% in high-income, upper-middle-income, lower-middle-income, and low-income countries, respectively (United Nations world water development report, 2017). As per the report, if the situation continues ‘as is,’ more than two-thirds of the global population shall face the water quality and quantity related problems by 2050. Due to the increasing scarcity of resources, the world is also aiming for transitioning the wastewater treatment facilitates into the resource recovery facilities. The existing centralized and decentralized treatment plants i) are mostly costly and energetically or chemically-intensive, ii) are based on the use of multiple treatment units, iii) suffer due to operational complexity, and iv) offer no or minimal resource recovery options (Crini and Lichtfouse, 2020; Wee et al., 2016). Moreover, non-upgradable or complex designs make it difficult to implement or integrate emerging resource recovery and wastewater treatment technologies in the Corresponding author. E-mail address: [email protected] (S.A. Patil). 1 Equal contribution. ⁎

https://doi.org/10.1016/j.biteb.2020.100392 Received 12 January 2020; Accepted 25 January 2020 Available online 30 January 2020 2589-014X/ © 2020 Elsevier Ltd. All rights reserved.

existing infrastructure (Cornejo et al., 2019; Crini and Lichtfouse, 2020). Another critical aspect of the existing wastewater treatment processes is the emissions of greenhouse gases, which have not been paid enough attention to so far (Campos et al., 2016). All these issues are driving the researchers to explore different options i) for either upgrading the existing facilities or developing decentralized technologies that can be implemented at different scales, and ii) to explore passive technology-based and CO2 neutral approaches to achieve both wastewater treatment and resource recovery in a cost-effective and environment-friendly manner. In particular, the development of passive technologies based on ecological principles has been at the forefront of the research activities (Mahmood et al., 2013). Constructed wetlands (CW) and ecological engineered systems and their derivatives are examples of such technologies that are based on the pollutant removal capabilities of some plants along with microorganisms (Todd and Todd, 1994; Mahmood et al., 2013). CW technology has been applied widely for wastewater treatment for decades (Vymazal, 2011). Its components include basins filled with sand and gravel media in specific proportions, plant species, and naturally developed microbes and other invertebrates (Mahmood et al., 2013). The ecological engineered systems mimic the natural cleansing functions of wetlands for wastewater treatment. They consist of diverse biotas, such as aquatic macrophytes, submerged plants,

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emergent plants, microbes, along with the filter feeders at different levels (Mohan et al., 2010). They are simple and have low energy consumption. A few critical issues related to the large space requirement and slow rate treatment, however, limit their use as stand-alone technologies (Ilyas and Masih, 2017). Several adaptations and modifications have thus been applied for improving the performance of these technologies. The notable intervention has been the integration of microbial electrochemical technology such as microbial fuel cells (MFCs) in these processes to enhance their treatment efficiencies and make them energy efficient (Yadav et al., 2012; Oon et al., 2015; Wei et al., 2015; Xu, 2015; Aguirre-Sierra et al., 2016; Mohan et al., 2011; Chiranjeevi et al., 2013; Yeruva et al., 2018). A technology-based on the successful integration of CW and MFCs is under development and testing at the pilot-scale (http://imetland.eu). It should be noted that in these cases, mostly one or two larger basins or treatment units are used and placed underground. If one or both of these units fail(s), then the continuous wastewater treatment process can get affected. Considering the above-discussed aspects associated with the existing and emerging integrated wastewater treatment processes, we tested a proof-of-concept integrated drip hydroponics-MFC (IHP-MFC) system design for wastewater treatment, energy recovery, and plant cultivation in this study. The individual technologies, Viz., hydroponics, and MFCs are at different stages of development and application. For instance, the hydroponics approach is used for the cultivation of vegetables in soilless but nutritionally rich water media (Sharma, 2019). It is being tested with domestic sewage as the nutrient medium to make it more sustainable (Sutar et al., 2018). The other technology, MFC, is still under development for wastewater treatment (Ieropoulos et al., 2012; Linares et al., 2019; Flimban et al., 2019). Although promising technology concept in terms of less energy consumption and sludge production, it suffers due to limitations such as high capital cost, low power output, and scale up to industrially relevant capacities (Santoro et al., 2017; Chen et al., 2019; Slate et al., 2019). It should be noted here that the plant-MFC technology is a promising and closely related integrated approach but primarily focuses on electricity harvesting by utilizing the unique plant-microbe relationship in the rhizosphere region in the soil and marshy environments (Strik et al., 2008; Nitisoravut and Regmi, 2017). With the proposed IHP-MFC system, we aim to address the issues associated with the individual aerobic, anaerobic, and MFC based wastewater treatment technologies. We also aim to provide an alternate low-cost design concept to the closely-related CW and ecological engineering technologies with a prospect for implementation at the small scales such as households through this study. We hypothesize that the integration of abiotic and biotic components in a single reactor unit can facilitate aerobic, anaerobic, plant nutrient uptake, and rhizosphere and electroactive microbial activities along with the filtration and adsorption processes, which in turn contribute to the removal of various wastewater constituents efficiently.

collection duct and finally to the reservoir tank. Plastic containers with 8 cm diameter at the top, 7 cm at the bottom, and a height of 8 cm, providing an empty bed volume of 250 mL, were used as the reactor units. Each reactor unit housed cocopeat as the support bed material or matrix, the plant sapling, and the electrode assembly, which resulted in about 200 mL working volume. The details of all these components are provided in the following sub-sections. The system with multiple reactor units hosting support bed material, plant sapling, and electrode assembly along with the wastewater distribution manifold and effluent collection duct is referred to as the IHP-MFC system in the manuscript. 2.1.1. Plant selection We used Cymbopogon citratus, which is a C4 monocot plant that belongs to the family Poaceae in this study. It can adapt to different harsh environmental conditions easily and grow well at temperate climates. It is a perennial fast-growing aromatic grass, which can grow up to a height of at least 1 m with numerous stems with stiff leaves arising from short and tuft rhizomatous roots (Srivastava et al., 2013). It bears linear, green leaf-blades which are tapered at both ends. It is commonly known as lemongrass for its characteristic lemon-odor due to the presence of a cyclic monoterpene named as “citral,” which can be extracted in the form of oil. This oil is used extensively in the food industries and Ayurvedic medicines due to its anti-amoebic, anti-bacterial, anti-diarrheal, anti-filarial, anti-fungal, and anti-inflammatory properties (Falah et al., 2015). Because of its robust growing nature, commercial, and economic importance, lemongrass was used as the test plant in the integrated system. Similar size saplings of lemongrass obtained from a nearby nursery (Ajit nursery, Sector 81, Mohali) were used in the experiments. 2.1.2. Support bed material Cocopeat was used as the support bed material in the reactor units. It is an organic, fibrous by-product obtained from the coconut husk. It is an eco-friendly porous media, which is increasingly being used in horticulture as a cultivation substrate due to its stable physicochemical and biological properties. It can resist a wide range of pH, electrical conductivity, and other chemical attributes, which make its use as supportive media attractive in hydroponic systems (Awang, 2016). It has a high porosity and water holding capacity, which limits the airwater relationship, and lowers the aeration within the medium. This condition prohibits oxygen diffusion to the roots of the plants and makes the rhizosphere environment anaerobic (Awang, 2016). The high cation exchange capacity of the material allows the nutrient absorption on its surface and helps in supplementation of nutrients to the plants. The porous material also supports the growth of native bacteria and fungi. Hence, it acts as an excellent growth support media and provides sufficient anchoring for the roots of the plants. After use, the mineralized nutrients present in the cocopeat along with the organic contentment can be used as the organic fertilizer. Cocopeat was also obtained from the nearby nursery (Ajit nursery, Sector 81, Mohali). Before use in the reactor units, it was washed thrice with tap water followed by drying (70 °C overnight) to remove the impurities and dust and finally sieved to get uniform particle size of 1–4 mm. An equal amount of 12 g of cocopeat was placed to fill 3/4th of the volume in each reactor unit. The porosity of cocopeat was estimated to be 47 ± 3% based on the procedure from Kalaivani and Jawaharlal (2019).

2. Materials and methods 2.1. Configuration of the integrated IHP-MFC system Two identical customized integrated systems were designed and fabricated using different materials as described henceforth. The individual system consisted of three major components, which include, a) polyvinyl chloride (PVC) pipe (Ø 7.5 cm) as the main structure that housed 10 reactor/MFC units and acted as effluent collection duct; b) plastic container (10 L capacity) used as a reservoir with a submersible pump (15 W capacity, maximum 650 L/h flow rate) for pumping and recirculating wastewater; and c) a drip manifold made up of chlorinated PVC pipe (Ø 2 cm) equipped with adjustable drip valves at equal distance for dripping of wastewater to each reactor unit (Fig. 1). One end of the manifold was connected to the submerged pump placed in the reservoir to pump wastewater to each reactor unit. The effluent from the reactor units was allowed to drain through the small holes into the

Porosity (%) = 1

BD PD

× 100

(1)

where, BD is bulk density and PD represents the particle density of cocopeat. 2.1.3. Electrode material Non-catalyzed disc-shaped graphite was used as the anode and cathode electrodes (projected surface area of 12.56 cm2 and total surface area of 31.4 cm2 each) in the reactor units. Before use, the 2

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(A) Drip manifold

Effluent collection duct

Cymbopogon citratus Cathode Cocopeat

Pump Wastewater Reservoir

Reactor/ MFC unit

Anode

(B)

Fig. 1. Schematic (A) and experimental setup (B) of the integrated Drip Hydroponics-Microbial Fuel Cell (IHP-MFC) system.

electrodes were polished with sandpaper (Grit designation: P220) and pre-treated [1 N HCl acid treatment for 2 h followed by soaking in distilled water for 1 h and drying at 60 °C in the oven] (Feng et al., 2010). Insulated copper wires of 1 mm thickness were used as the current collectors and connected with the electrodes using conductive cement. The anode was placed horizontally at a depth of 6 cm in the cocopeat matrix, and the cathode was placed (at a distance of 6 cm from the anode) at the surface of the cocopeat matrix. The later served as an open-air cathode in each reactor unit.

2.3. System operation – control and main experiments Before testing the two integrated IHP-MFC systems (named as IHPMFC-1 and IHP-MFC-2), different control experimental conditions in reactors, namely, without any component (WC), with cocopeat (C), cocopeat with a plant sapling (C + P), and cocopeat with the electrode assembly (C + E) were set up to check the wastewater treatment in each condition. The integrated system comprised of all components, Viz., cocopeat, electrodes, and plant (hereafter referred to as ‘I’), was then evaluated for the wastewater treatment and resource recovery performance. For every change in the reactor condition, the individual units in each system were modified accordingly. When the condition was switched from WC to C, all the reactor units were filled with 12 g of dried cocopeat uniformly. During C + P, only plant saplings were placed in the cocopeat, whereas, in the C + E condition, only electrodes were placed within the cocopeat matrix. Thus, each reactor unit in the C + E case acted as a single MFC unit. The systems were operated indoor and under a similar set of conditions at ambient temperature (22 ± 3 °C), and in a batch-recirculation drip-hydroponics mode for the wastewater feeding. In each experimental run, 10 L clarified domestic sewage (i.e., primary effluent) was fed to the system via the drip manifold pipe. Each reactor unit received the feed at a flow rate of 8 L/

2.2. Wastewater characterization Domestic sewage collected from the sedimentation tank of the primary treatment unit of the sewage treatment plant facility located at IISER Mohali residential campus was used as the wastewater source in this study. The characteristics of the primary effluent collected during the whole study were: pH, 7.5 ± 0.1; Conductivity, 1.06 ± 0.1 mS; COD (chemical oxygen demand), 393 ± 55 mg/L; Phosphates, 3 ± 1 mg/L; Ammonia, 41 ± 12 mg/L; Nitrates, 0.9 ± 1.4 mg/L; BOD (biological oxygen demand), 181 ± 38 mg/L; and Total coliforms, 5.4 × 109 ± 10.4 MPN/100 mL. 3

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cathode surface area (m2).

h. In each experimental condition, systems were acclimatized and stabilized by feeding the fresh wastewater for a minimum of 7 days. The influent and effluent samples were analyzed for at least three times at different reactor conditions (WC, C, C + P, C + E and I) in duplicate systems. The wastewater treatment performance at each condition was evaluated by operating both systems for 3 h, i.e., at a hydraulic retention time (HRT) of 3 h. The performance of the complete integrated systems (I) was also evaluated at longer HRTs of 6 and 12 h for wastewater treatment. During the stabilized performance of the individual MFC units in the I-O condition, polarization tests were conducted to check the electrochemical behavior of single MFC units in the integrated systems. After that, the 10 MFC units were connected in parallel or series modes to determine the maximum open circuit voltage (OCV) of the complete IHP-MFC systems and to conduct the polarization tests. Finally, the integrated systems were operated in parallelly (IP) and serially (I-S) connected closed-circuit modes to compare the wastewater treatment performance with the integrated system under OCV operated conditions (I-O).

2.4.3. Plant growth Plant saplings were allowed to grow with the wastewater feed. The plant biomass was estimated after running the integrated systems for one month by the gravimetric method. The newly grown leafy biomass within this interval was harvested and dried at 60 °C until a constant weight was attained for each sample. 3. Results and discussion 3.1. Wastewater treatment performance of the integrated systems The consolidated data of various parameters analyzed to evaluate the performance of two independent but identical systems, namely, IHP-MFC-1, and IHP-MFC-2, operated at 3 h of HRT, and in an open circuit mode are presented in the e-supplementary material. Based on these data, the removal of the major wastewater components is presented and discussed in the following sub-sections.

2.4. Analyses and calculations

3.1.1. COD removal The total organic carbon removal is reported in terms of COD removal. The maximum COD removal efficiency of 72 ± 2.4% was achieved by the integrated systems (I-O condition) at 3 h HRT. It was followed by the C + P (70 ± 1.6%), C + E (64 ± 2.5%), C (58 ± 1.2%), and WC (20 ± 3.7%) conditions (Fig. 2a). A similar trend was also observed for the BOD removal in all experimental conditions. The maximum BOD removal efficiency of 81.5 ± 2.5% was achieved with the integrated system at 3 h HRT. These data suggest that with the addition of each component in the reactor units, the COD removal efficiency increased significantly. For instance, the incorporation of the cocopeat (C) in the reactor alone showed a 38% enhanced COD removal efficiency than the reactor without cocopeat (WC). Incorporating electrodes and plant components further increased the COD removal by 52% compared to the WC condition. These results indicate that the presence of different microenvironments facilitated by various reactor components, Viz., cocopeat, plant, and electrode assembly, led to the maximum COD removal in the case of the integrated systems. The microenvironments include physicochemical (due to the porous and fibrous cocopeat bed matrix), aerobic, anoxic, along with the rhizosphere and electrochemical conditions. Whereas WC experimental condition lacked these different microenvironments except the aerobic condition, and thus showed very less COD removal efficiency during the same experimental duration. In this case, carbon removal occurred only through the activity of aerobic microorganisms. In the integrated systems, due to multi-porous structure and water holding capacity, cocopeat acts as an adsorbent of the organics, which, in turn, can facilitate and favor the growth of microorganisms. Aerobic and microaerophilic conditions prevail at the surface and subsurface of the cocopeat matrix, which allows the growth of aerobic and facultative microbes and thereby contributes to the aerobic treatment process in the system. It should be noted that the continuous mixing of wastewater because of recirculation, which is a part of the system design, also facilitates aerobic treatment in the reservoir in each experimental condition. Whereas, anoxic conditions at the lower zones in the reactor units facilitate further mineralization of the organics by anaerobic microorganisms. The anode placed at the bottom of the cocopeat may act as an additional electron acceptor under the anaerobic conditions, thereby contributing to organic matter removal under the soluble electron acceptor depleted conditions. Rhizoremediation can also be the contributing factor in this case. The excretion of root exudates, which are the major source of carbon, can stimulate the survival and action of rhizobacteria, which results in more efficient degradation of recalcitrant organic pollutants in the rhizosphere region (Salgado et al., 2017). The removal of organic matter in each reactor unit and the

2.4.1. Characterization of wastewater influent and effluent parameters The characterization of the influent and effluent samples was done by monitoring pH, conductivity (Oakton PC2700 pH/ Conductivity meter), Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD5) (Oxitop BOD, incubation at 20 °C), ammonia, nitrate, phosphate, and total coliforms (by Most probable number - MPN test) according to the standard protocols from “Standard Methods for the Examination of Water and Wastewater” (Rice et al., 2012). The wastewater treatment performance was evaluated by calculating the removal efficiencies, according to Eq. (2).

Removal efficiency =

Ci

Cs Ci

× 100

(2)

where, Ci represents the initial concentration of the specific constituent in the influent (mg/L), and Cs denotes the concentration of the same constituent (mg/L) in the effluent. The removal efficiencies were calculated by considering the influent concentration during the specific period of experimental condition. The data presented are averages along with uncertainties based on at least three experimental runs for each condition. 2.4.2. Electrochemical parameters The MFC units in the C + E and complete integrated (I) systems were operated initially in an open circuit mode (denoted as ‘I-O’ systems). OCV was recorded using the auto-range digital multimeter and datalogger (Keithley 2400). The polarization tests were performed to know the cell design point of the systems by using a variable circuit resistance method at 200 KΩ to 4 Ω external resistors. Each resistor was left connected until a stable voltage value was attained. It ranged from < 1 min at high resistances to 30 min at low resistances. Before switching to the next resistor, the electrodes were left unloaded to regain the maximum OCV. The slope of the polarization curve represents the internal resistance of the system, which was calculated using Eq. (3).

Rint =

dE/dI

(3)

where, Rint is the internal resistance (Ω), E represents potential (V), and I represents current (A). When the reactors were operated in a closed-circuit mode, the current (I) was calculated using the Ohm's law, i.e.,

V = IR

(4)

Power (mW) was derived from the P = IV equation. Current (mA/ m2) and power densities (mW/m2) were calculated by normalizing the respective absolute current and power values with the projected 4

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3.1.2. Phosphate removal The phosphate (P) removal efficiencies achieved in different experimental conditions at 3 h HRT are 1.4 ± 0.2%, 11.6 ± 1.8%, 54.3 ± 3.6%, 73.2 ± 2.9%, and 83.2 ± 1.1%, for WC, C, C + P, C + E, and I-O, respectively (Fig. 2b). The WC condition showed a meager amount of phosphate removal. It can be attributed to the absence of specific microbial activity or any other process for P removal in the aerobic environment. This observation is in agreement with the inefficiency of the aerobic treatment processes for P removal. Experimental condition C showed higher P removal than the WC condition but comparatively much lesser P removal than the C + E condition. Also, in the case of C + P, less P removal than the C + E condition was observed. Enhanced P removal in the case of C and C + P conditions than the WC condition can be attributed to the activity of phosphate-accumulating microorganisms (PAOs) and uptake by plants, respectively. The higher P removal efficiency in the C + E condition can be due to electrocoagulation of negatively charged phosphates at the anode (Tian et al., 2017) and the activity of PAOs. The PAOs proliferate during the cycling of anaerobic and aerobic phases (Oldham, 1986), which are prevalent in the IHP-MFC systems. These observations also correlate well with another comparable system, e.g., CW (Vymazal, 2007; Kadlec, 2016). The removal of P from wastewaters can be performed by using physicochemical or biological processes or through the combination of both processes (Bunce et al., 2018). Based on these observations, the maximum P removal in the I-O condition can be attributed to the contributions from microbial and plant uptake processes in addition to the physical adsorption at the cocopeat matrix and electrochemical reaction at the electrode. 3.1.3. Ammonia removal Ammoniacal‑nitrogen (NH4+-N) is one of the major components of raw domestic wastewater. In this study, the primary effluent contained high ammonia and low nitrate concentrations (41 ± 12 mg/L and 0.9 ± 1.4 mg/L, respectively). Ammonia removal followed a similar trend as that of phosphate removal in the systems operated at different experimental conditions except that for C + P condition at 3 h HRT. Integrated reactor condition (I-O) of both the systems showed maximum ammonia removal efficiency of 35.1 ± 2.4%, followed by C + P (32.3 ± 2), C + E (27.2 ± 3.2), C (4.7 ± 1.2%) and WC (0.5 ± 0.1%) reactor conditions (Fig. 2c). The processes like nitrification and denitrification can occur due to the availability of multiple microenvironments in cocopeat. It is thus clear that the absence of cocopeat in the WC experiment condition resulted in negligible ammonia removal. The ammonia removal efficiency is much higher in C + E than the C condition. It could be because of the microbial ammonia oxidation process at the anode (He et al., 2009) or the recently reported electrode-dependent anaerobic ammonia oxidation (anammox) process (Srivastava et al., 2020a). In the C + P condition, the rhizobacteria activity and uptake by plants are the main factors contributing to the enhanced ammonia removal. Compared to COD and P removal, ammonia removal was, however, not efficient at 3 h HRT in the integrated systems as well as other reactor conditions. A significant increase in the nitrate concentration was observed in the effluents in all reactor conditions (WC: 1.41 ± 0.1 to 1.54 ± 0.03 mg/L; C: 0 to 0.7 ± 0.06 mg/L; C + E: 0 to 5.85 ± 0.12 mg/L; C + P: 0.74 ± 0.04 to 6.1 ± 0.85; I-O: 0.28 ± 0.13 to 5.45 ± 0.48 mg/L). The biological nitrification process involves ammonia (NH3) oxidation to nitrite (NO2−) and nitrate (NO3−) by aerobic bacteria. Irrespective of the operating reactor condition, the pH of the effluent was 7.85 ± 0.03. It should be noted here that optimal biological nitrification process rates occur within a range of 7.5 to 9.0 pH (Amatya and Kansakar, 2011). Among different forms of nitrogen, ammonia is less mobile and amenable biologically than nitrate, which can easily assimilate into the microbial and plant biomass (Caselles-osorio et al., 2011; Mohan et al., 2010). Thus the remaining nitrate was most likely taken up by the plants and microbes as

Fig. 2. A) COD, B) phosphate, C) ammonia removal efficiencies by the IHP-MFC systems at different experimental conditions.

complete system can thus be attributed to all these conditions, multiple bioprocesses, and synergistic microbial interactions in different microenvironments. 5

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Table 1 Wastewater treatment performance of the integrated systems at different HRTs. Parameters

pH Conductivity (mS) COD (mg/L); removal efficiency (%)b Phosphates (mg/L); (%) b Ammonia (mg/L); (%) b Nitratec (mg/L) a b c

Influenta

7.51 ± 0.08 1.11 ± 0.05 410 ± 16 3.8 ± 0.2 41.5 ± 2.1 0.28 ± 0.13

Effluent characteristics at different HRTs 3h

6h

12 h

7.88 ± 0.09 1.08 ± 0.05 115 ± 10 (72 ± 2.4) 0.64 ± 0.1; (83.2 ± 1.1) 27 ± 2.3 (35.1 ± 2.4) 5.4 ± 0.7

7.91 ± 0.01 1.04 ± 0.03 76 ± 6 (81.5 ± 0.9) 0.6 ± 0.1; (84.3 ± 0.4) 20 ± 1.1 (51.7 ± 1.2) 6.4 ± 2.6

7.89 ± 0.01 1.01 ± 0.03 59 ± 3 (85.7 ± 0.6) 0.54 ± 0.1 (85.8 ± 0.6) 9.7 ± 1 (76.6 ± 1) 8.1 ± 1.1

Note: Influent concentration values are during the integrated system (I-O) condition period. The data presented in parentheses are the removal efficiencies in %. For nitrate, the values are for the increase in its concentration.

well as converted to N2 via denitrification or probably via electrode dependent ammonia oxidation process (Srivastava et al., 2020a). In denitrification, nitrate is reduced to nitrogen gas (N2) under anoxic conditions. The aerobic and anoxic environments are prevalent in all reactor conditions except WC, and therefore primarily responsible for the removal of ammonia.

0.195 ± 0.081 V in IHP-MFC-1 and IHP-MFC-2, respectively, was observed. The plants translocate excess fixed carbon through the roots as exudates in the rhizosphere region (Strik et al., 2008). These exudates or rhizodeposition of the plant provides additional supplementation of the carbon required for the propagation of electroactive microorganisms. In addition to the above reason, the placement of electrode assembly in the rhizosphere region further aids in the enhanced electricity generation (Chiranjeevi et al., 2012). After more than a month-long operation, the maximum OCV of the individual reactor units was stabilized at 0.155 ± 0.077 V. The individual MFC units were then connected in parallel and series modes to determine the maximum OCV of the complete IHP-MFC systems. For instance, the IHP-MFC-1 showed maximum OCV of 1.49 ± 0.091 V and 0.158 ± 0.005 V in series and parallel mode of connections, respectively. These data demonstrated the possibility of designed IHP-MFC systems in harnessing bioelectricity while treating domestic sewage.

3.1.4. Wastewater treatment evaluation at the longer hydraulic retention times (HRTs) The integrated systems were also operated at longer HRTs of 6 and 12 h to assess the maximum wastewater treatment efficiency at extended periods. As expected, improved performance in terms of the removal of the major pollutants was observed at the longer HRTs (Table 1). At an HRT of 12 h, > 85% removal in both COD and phosphate was observed. The ammonia removal was also enhanced to > 75% at 12 h HRT. These results suggest that the integrated systems can achieve efficient removal of the major pollutants from wastewater at extended HRTs. The total coliforms in the influent were estimated to be 5.4 × 109 ± 10.4 MPN/100 mL. No significant reduction in the coliforms count (3.9 × 109 ± 10.4 MPN/100 mL) was observed in the integrated systems. These results suggest that the system is not useful for the removal of coliforms from domestic wastewater. The polishing of effluent, in particular, for the removal of coliforms is therefore necessary before it is used for any reuse purpose.

3.2.2. Polarization behavior Polarization tests were conducted by connecting individual reactors or MFC units in parallel and series modes to determine the cell design point that is critical to harvest usable electric power from the whole system. The representative polarization curves are shown for the IHPMFC-1 system (Fig. 3a). A negligible current generation was recorded at a higher resistance of 200 kΩ. It is because negligible electron flow occurs in the fuel cell circuit at the higher resistances. The low resistance allows more electron flow in the fuel cell circuit, which results in the voltage drop. The polarization curves are almost linear, i.e., the voltage drops linearly with the increase in current. The linear behavior of the polarization curve signifies that the ohmic losses are dominant in the system, which is primarily due to a less conductive zone or matrix between the electrodes. The maximum power densities and corresponding current densities were observed at the external electrical resistor of 20 kΩ (31.88 mW/ m2 and 35.63 mA/m2) and 0.120 kΩ (31.58 mW/m2 and 457.8 mA/m2) in the series and parallel mode of connections, respectively. The point at which maximum power density is observed in the polarization curve is generally considered as a cell design point (voltage change region) of that particular fuel cell system. It is usual to operate the cell to the left side of the power density peak and at a high voltage or low current density regions. The low performance in terms of OCV as well as power densities of the individual reactor/MFC units and the integrated systems is mainly due to high Rint. The Rint was calculated by using Eq. (3) and also based on the resistor producing the maximum power. The average Rint of the individual MFC units was about 1.83 ± 0.36 kΩ according to Eq. (3) and 1.86 ± 0.37 kΩ based on the power curve. In the case of integrated systems with MFC units connected in series and parallel modes, the Rint values were about 20 kΩ and 0.120 kΩ based on the power curve, respectively. The electrode material and connection resistances were low in the range of 2–5 Ω. Thus, the high resistance of the MFC units is mainly attributed to the non-conductive cocopeat

3.2. Bioelectrochemical and wastewater treatment performance of the systems The activity of electroactive microorganisms that can grow by utilizing the anode as the terminal electron acceptor under anaerobic conditions was monitored by measuring the OCV of the individual reactor units in the integrated systems. The data is presented as the average OCV of the reactor units in each integrated system. The performance of the integrated system was further evaluated by connecting the individual MFC reactor units in series and parallel modes. 3.2.1. Open circuit voltage (OCV) The OCV generated by the reactors was found to be dependent on the operating conditions. Comparatively high OCV was observed during the operation of the I-O condition than the only C + E reactor condition in both systems. Under the C + E reactor condition, initial OCV of the individual reactors in systems IHP-MFC-1 and IHP-MFC-2 was 0.061 ± 0.05 and 0.055 ± 0.026 V, respectively, which increased further and stabilized at 0.109 ± 0.068 and 0.095 ± 0.051 V levels. It is due to the transfer of the electrons produced by microorganisms to the anode, and then to the cathode via an external circuit (RamírezVargas et al., 2019). When the plant was introduced to the system, i.e., in the integrated condition (I-O), further increase in the OCV of the individual reactor units to the level of 0.181 ± 0.082 V and 6

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3.2.3. Wastewater treatment performance of the integrated systems in series and parallel electrical connection modes of the reactor units Both the systems connected electrically in series (I-S) and parallel (IP) modes were evaluated for the wastewater treatment performance at 3 h HRT (Fig. 3b). There was no significant difference in the removal efficiencies under the different electric connection modes by these systems. However, a minor decrease of ~2 to 3% in the removal efficiencies of nutrients was observed in I-S and I-P conditions compared to the integrated system operated in open circuit mode (I-O). These observations suggest that irrespective of the connections modes, efficient wastewater treatment can be achieved by the integrated systems. 3.3. Plant biomass The lemongrass saplings showed considerable growth in terms of leafy biomass (45 ± 15 cm long leaf blades) after one month of their plantation in the reactor units of both IHP-MFC systems. The regular feeding of fresh domestic wastewater to the systems ensured the availability of nutrients for the plants. The dry weight of leafy biomass per plant was approximately 216 ± 39 mg/month. The low amount of plant biomass is most likely due to the limited exposure to the sunlight since experiments were conducted under the indoor environment. Apart from the uptake of nutrients from wastewater, plants also sequester the atmospheric CO2 as well as CO2 generated from microbial degradation of organic carbon and fix it in the form of biomass. It can thus contribute to making the wastewater treatment process CO2 neutral to some extent and is an additional advantage of the system. 3.4. Comparative performance evaluation of the IHP-MFC system with the closely related technologies The IHP-MFC system reported in this paper is closely related to CWMFC, ecological engineered systems, and hydroponics systems in terms of either design or components or operational aspects. Table 2 summarizes their comparative evaluation in terms of the materials used, operational conditions, wastewater treatment, and resource recovery performance, as applicable based on the representative studies primarily with the real wastewaters. The data suggest that the IHP-MFC system performed either at par or even better than the comparable systems for the COD removal. In particular, at short HRT condition, the IHP-MFC system is highly efficient than the other technologies. The ammonia and phosphate removal efficiencies are also at par or better than other technologies in most of the cases. However, the power output of the integrated system is mostly lower than other systems. It is most likely due to ohmic and activation losses, higher Rint, the use of the non-catalyzed air-cathode, and non-optimized set of material and operating conditions. Further research on optimizing different process parameters like feed rate and HRT, reactor parameters such as design components, support bed materials, catalyzed electrode materials, and plants with better growth rates and nutrient uptake capabilities is warranted to improve both wastewater treatment and resource recovery performance of the IHP-MFC system. In addition to the coliform removal through linking of IHP-MFC with other processes, the removal of emerging pollutants such as heavy metals and pharmaceutical residues should also be monitored to evaluate its complete wastewater treatment capabilities. The long-term tests should also be conducted with the optimized materials and process parameters to prove its practical applicability. The understanding of the complex biological interactions occurring in various microenvironments involved in the removal of different pollutants could further help to improve the overall wastewater treatment and resource recovery processes.

Fig. 3. A) Polarization and power density curves of IHP-MFC-1 in the series (top panel) and parallel (middle panel) mode of connections; B) Wastewater treatment performance of the integrated systems in the series (I-S) and parallel (I-P) modes of electric connections.

matrix present between the anode and cathode electrodes as well as the possible poor movement of ions in the reactor. Future research should consider addressing these aspects so as to improve the electricity production performance of the integrated system.

4. Conclusions The integration of various components in one reactor unit facilitated 7

Submerged and emergent macrophytes ecosystem; Microbial activity; Rhizospheric activity; Sediment fuel cells

Filtration by gravel; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity Filtration by quartz sand; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity Aeration; filtration by gravel; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity Filtration by gravel; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity Filtration by gravel; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity

Ecological engineered system + MFC

Vertical flow CW + MFC

8

Filtration by quartz sand; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity

Coke biofilter; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity; Heterotrophic and Electroactive Biofilm communities No carrier; Rhizospheric activity; Microbial activity; Plant nutrient uptake; Recirculation Cocopeat as plant support; Rhizospheric activity; Microbial activity; Plant nutrient uptake; Recirculation Cocopeat to form various zones and adsorption and filtration, Rhizospheric activity; Aerobic and anerobic microbial activity; Plant nutrient uptake; MFC; Recirculation

CW + MFC

Microbial Electrochemicalbased Constructed Wetland (METland)

Cymbopogon citratus

Spinach and mint

Digitalis lanata Digitalis purpurea

Juncus effuses

Juncus effuses Typha orientalis Scirpus validus

Phragmites australis

Canna indica

Canna indica

Typha latifolia

Canna indica

Bryophyllum pinnatum, Solanum lycopersicum, Oriza sativa, Lycopodium and Adiantum, Hydrilla verticillata, Myriophyllum Canna indica

Plant type

Domestic wastewater

Sewage wastewater

Municipal wastewater

Pig manure supplemented with starch and molasses

Domestic wastewater

Domestic wastewater

Synthetic wastewater

Synthetic wastewater

Synthetic wastewater

Synthetic wastewater

Synthetic wastewater

Domestic wastewater

Feed

Graphite disc

NA

NA

Coke bed

Granular graphite anode and Ptcoated carbon cloth cathode Graphite rod wrapped by Stainless Steel mesh Carbon fiber felt

Graphite

Carbon felt

Graphite felt

Graphite

Graphite discs

Electrode materials

Batch recirculation 10 L HRT - 0.125 day HRT - 0.5 day

Batch recirculation30 L HRT - 2 day Recirculation at 20 L/min HRT - 10 days

Continuous – 5 L HRT - 0.5 days

Batch HRT - 2 days

Continuous 21 L/day HRT - 2.6 days

Batch - 1.8 L HRT - 1 day

Batch - 65 L HRT - 6.5 day

Batch HRT - 1 day

Batch - 28.8 L HRT - 1 day

Batch - 2.33 L HRT - 4 days

Continuous 20 L/day

Operation mode flow rate and/or volume and HRT

35 77

86

NR

65 95

46

NR

58

NR

NR

91

36.2

NR

NR

NH4+

72

43

79 84

90

77 79.5 80

61

82.2

99

100

69.9

75

87.9

COD

86

83

NR

33 33

86

NR

NR

NR

NR

NR

NR

NR

NR

PO43−

Removal efficiency %

(Yan et al., 2018) (Oon et al., 2015) (Srivastava et al., 2020b) (Srivastava et al., 2015) (Corbella et al., 2015) (Wang et al., 2017)

4.21 mW m-2 (1000 Ω) 6.1 mW m-2 (1000 Ω) 11.67 mW m-3 (1.2 kΩ) 320.8 mW m-3 (NR) 36 mW m-2 (NR) 2.8 mW m-2 21.5 mW m-2 14 mW m-2 (1000 Ω) NA

(Sutar et al., 2018) This study

31.9 mW m-2 (20 kΩ)

(Condret and Hitmi, 2004) NA

NA

(Yadav et al., 2012)

15.7 mW m-2 (NR)

(Ramírez-Vargas et al., 2019)

(Chiranjeevi et al., 2013)

Ref.

128.1 mW m-2 (100 Ω)

Maximum power output (resistor)

NR: Not Reported; NA: Not Applicable; CW: Constructed wetland; MFC: Microbial fuel cell; NFT: Nutrient Film Technology; HRT: Hydraulic Retention Time; COD: Chemical Oxygen Demand; IHP-MFC: Integrated Hydroponics-Microbial fuel cell.

IHP-MFC

Wastewater NFT Hydroponics

Wastewater NFT Hydroponics

Filtration by gravel; Plant nutrient uptake; MFC; Microbial activity; Rhizospheric activity

Horizontal subsurface flow CW + MFC

Vertical flow CW + MFC

Horizontal subsurface flow CW + MFC

Up-flow CW + MFC

CW + MFC

Design aspects and key contributing processes

Technology

Table 2 An overview of the closely related technologies to the IHP-MFC for wastewater treatment and resource recovery.

R.K. Yadav, et al.

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different processes thereby resulting in efficient wastewater treatment even at a short HRT along with the production of low electric power and plant biomass in the IHP-MFC system. The system design based on the use of multiple small reactors is efficient for pollutant removal, offers easy operation and maintenance, produces minimal sludge, and consumes less energy. Its other features include low cost, no chemicals use, no foul odor, and partial CO2 sequestration by plants. The limitations include no coliform removal and possible clogging of the cocopeat matrix in long-term operation.

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