Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design

Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design

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Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design Amro Hassanein a,b,c, Freddy Witarsa a, Xiaohui Guo c, Liang Yong d, Stephanie Lansing a, Ling Qiu c,* a

Department of Environmental Science and Technology, University of Maryland, College Park, MD, USA Faculty of Environmental Agricultural Sciences, Arish University, North Sinai, Egypt c College of Mechanical and Electrical Engineering, Northwest Agriculture and Forestry University, Yangling, Shaanxi, China d Guangxi Scientific Experiment Center of Mining, Metallurgy and Environment, Guilin University of Technology, Guilin, China b

article info

abstract

Article history:

This study determined the effect of incorporating a microbial electrolysis cell (MEC) with

Received 10 May 2017

an anaerobic digester (AD) in a single chamber. The study evaluated three treatments: a

Received in revised form

combined AD-MEC operated for 23 days (AD-MEC-23); a combined AD-MEC operated with

30 July 2017

the MEC running for 5 days followed by no MEC for the subsequent 18 days (AD-MEC-5);

Accepted 1 October 2017

and an AD operated for 23 days (AD-only). Food waste was the digestion substrate at an

Available online xxx

inoculum to substrate ratio of 1:1 (VS basis). Cumulative methane and hydrogen during the batch test in AD-MEC-23 (9.4 LCH4, 3.39 LH2, 2.8 LCO2) was higher than AD-MEC-5 (7.6 LCH4, 2.2

Keywords:

LH2, 4.6 LCO2), and AD-only (7.4 LCH4, 0.2 LH2, 5.8 LCO2). The results also showed that using the

AD-MEC

MEC continuously inside the digester (AD-MEC treatment) reduced CO2 concentration to

Hydrogen

approximately 4% at the end of the experimental period, thereby, increasing the useful

Biogas

gases (CH4 and H2) concentrations to a maximum of 95.8%, with an average of 71.9% CH4,

Cathode

17.4% H2 and 10.7% CO2 over the 23-day digestion period in the AD-MEC-23 reactor.

Anode

Additionally, the COD removal in AD-MEC-23 was 12% higher than AD-only. The volu-

Waste treatment

metric current was 108.7 A/m3 based on MEC volume and 17.3 A/m3 based on total AD-MEC volume, while the current density was 1.35 A/m2 (cathode surface area). The total energy produced from the AD-MEC-23 (414 kJ) was higher than AD-MEC-5 (325 kJ) and AD-only (295 kJ), with an increase of energy production of over 400% output energy compared to the input energy to power the MEC. The results showed that the novel MEC design incorporated into an AD reactor increased the biogas quality, overall energy production, and waste treatment. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi, 712100, China. E-mail address: [email protected] (L. Qiu). https://doi.org/10.1016/j.ijhydene.2017.10.003 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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Introduction Biomass residue without appropriate treatment can cause substantial environmental problems, but proper treatment and disposal of organic biomass can be expensive [1,2]. Anaerobic digestion (AD) is potential solution to treat organic biomass while producing renewable energy. AD is a collection of processes by which microorganisms break down substrate in the absence of oxygen; the AD pathway is divided into two main stages based on gas production: acidogenic phase, producing hydrogen (H2), and methanogenic phase, producing methane (CH4) [3,4]. AD technology can convert waste into renewable energy in the form of biogas that is comprised of different gases: CH4 (60e75%), carbon dioxide (CO2) (25e30%), and trace amounts of nitrogen (N2), hydrogen sulfide (H2S), and H2 [5,6]. Microbial electrochemical technologies (METs) use microorganisms to catalyze electrochemical reactions. These METs include microbial fuel cells (MFCs) that generate electrical power when oxygen is present at the cathode and microbial electrolysis cells (MECs) that generate hydrogen while consuming a small amount of electrical power, and are promising approaches for capturing the energy in waste biomass [7]. MECs can produce hydrogen gas with smaller voltages (as low as 0.2 V) than those required for electrolytic hydrogen production (1.23 V) [8,9]. Hydrogen can be produced in MECs from different substrates, including acetic acid, butyric acid, lactic acid, glucose, cellulose, and different types of wastewater [10]. Lu et al. [11] reported that MECs can degrade organic matter while producing hydrogen at high yields using effluent from an ethanol-type reactor as the substrate (i.e., reducing sugars, ethanol, acetic acid, propionic acid, butyric acid, and valeric acid). Additionally, a pair of ferric or zero-valent iron electrodes inserted into an up-flow anaerobic blanket (UASB) reactor improved the treatment of azo-dye and other organics in wastewater [12]. Previous research has been conducted to inhibit methanogenesis in MECs to decrease methane yield and increase hydrogen gas content, including exposing cathodes to air, UV irradiation, and electrolytic oxygen production [13e15]. However, the reported techniques may not be cost effective over long-term operation due to additional energy input, as well as inherent difficulties in controlling methanogenesis in MECs [13e15]. In contrast, a separate study [16] reported using nickle metal nanoparticles and granular activated carbon to increase methanogenesis in MECs and showed that MECs inoculated using anaerobic digester sludge increased CH4 production. Similar to this previous research [16], this research detailed a novel MEC-AD design that aimed to increase CH4 production from the hydrogen generated using a MEC, resulting in increased CH4 and reduced CO2 without the need to inhibit methanogenesis. An anaerobic digestion and anodic oxidation from a MEC were combined to improve the wastewater degradation process. Furthermore, rapidly developing bioelectrochemical technology has been proven to be a promising platform for CO2 capture and conversion compared with other methods [11]. With a small voltage supplied to the MEC, electromethanogens can use electrons or hydrogen formed at cathode to convert CO2 into CH4 directly [17e19]. Previous research investigating energy recovery using MEC on various organic compounds (glucose, protein, acetate,

swine wastewater, cattle manure, and several other VFAs) have found that the energy recovered in the H2 gas is more than the amount added as electrical energy from an outside resource [20e25]. MECs have also been tested for their ability to efficiently convert waste organic compounds from sludge fermentative liquid to electrons and H2 [25e28]. There are, however, limited studies that have embedded MEC inside an AD. Cai et al. [29] attempted to integrate an anaerobic reactor with microbial electrolysis and found an increase in CH4 production and increased COD removal compared to AD-only. However, their AD-MEC design required an anion exchange membrane to separate the anode and cathode chamber. A separate study investigated using an independent anode coupled with a 180 mL stainless steel digester that functioned as a cathode [30]. However, the large distance between the electrodes (i.e. the digester body and the anode) can hamper electron flow between the electrodes and result in increased ohmic losses [31,32]. The improved design tested in this study confined the anode and the cathode within a compartment, limiting the distance between the electrodes and thus, reducing the potential for ohmic losses while still allowing liquid exchange between the two compartments. One objective of the study was to improve upon previous designs and present a novel combined MEC-AD design with the following: 1) an MEC containing an anode and a cathode inside a single chamber to allow efficient electron flow between the two electrodes; 2) a MEC chamber containing slits to limit “electron leak” between the MEC and AD liquid but still allowing liquid and gaseous exchange between the MEC and AD.; 3) a mobile MEC chamber that could be easily inserted into an existing AD. In addition, there has not been a previous study that has investigated using a MEC combined with an AD to treat food waste. To the best of our knowledge, only acetate, glucose, waste activated sludge and acetate have been used in previous AD-MEC studies [29,30,33,34]. Moreover, previous studies have not compared the energy production difference when the MEC is located within an AD for only part of the digestion period and compared the results to an AD-MEC operated during the entire digestion period. This study serves to address these research gaps. In this study, a combined AD and MEC was tested by incorporating a MEC into an AD reactor for treating food waste. Three treatments were studied in duplicate: a combined AD-MEC operated for 552 h (23 days) with an applied voltage of 0.9 V (AD-MEC-23); a combined AD-MEC with voltage applied to the MEC (0.9 V) for the first 120 h (5 days) and no voltage applied for the rest (432 h; 18 days) of the experimental duration (AD-MEC5); and an AD-only reactor. The AD-MED-5 reactor was used to determine if the voltage should be applied to the MEC through the whole digestion process or only at the start of the process. All three reactors ran in duplicate for a total of 552 h (23 days).

Material and methods Substrate and inoculum Food waste collected from the Northwest A&F University restaurant located in Yangling, Shaanxi Province, China

Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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was used as the substrate for the experiment. The food waste was pre-treated by removing the bones and plastic, followed by grinding, and then stored at <4  C for two days. The total solids (TS) and volatile solids (VS) contents of the food waste were 23.6% and 22.1% (of the wet sample), respectively. A substrate to inoculum ratio of 1:1 (by VS) was used for each reactor. The effluent from a previous MEC combined with AD experiment that ran for more than 25 days was used as inoculum. The inoculum had 4.7% TS and 2.8% VS content. The final TS and VS of food waste and inoculum mixture in each reactor were 6.8% and 4.9%, respectively. The total volume of the mixture in each reactor was 900 mL.

Reactor set up A duplicate clear glass bottles (1200 mL) were used as reactors in each treatment: One MEC (Fig. 1) was inserted into each ADMEC reactors, while the AD-only treatment had no electrode. All the digesters were placed in a hot water bath maintained at approximately 35  C. A sampling tube inserted from the top of the reactor into the middle of the mixture was used for collecting liquid samples (Fig. 1).

Microbial electrolysis cell (MEC) Each MEC consisted of a PVC sleeve made of PVC pipe with a diameter of 3.496 cm, length of 15.5 cm, and total liquid volume approximately 135 mL. Eleven holes (approximately 0.35 cm  5.5 cm) were created on the PVC tube to ensure

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continuous liquid exchange between the liquid inside the MEC and the liquid outside the MEC (Fig. 1).

MEC anode and cathode configuration The anode was constructed using eleven graphite plates, with each plate having a dimension of 15 mm  150 mm  1 mm, a purity of 99.95%, and electric resistivity of 1000 mU cm. The graphite plates were separated by rubber bands to create spaces between each plate (Fig. 1). The graphite plates were inserted into a stainless steel cylinder (stainless steel grade 201, with a 185 mm diameter  150 mm length  0.5 mm thickness) that had an electric resistivity of 68.5 mU cm. The combination of graphite and stainless steel was used to ensure proper conductivity between the electrodes, as the electric resistivity of stainless steel is lower than the electric resistivity of graphite and combining the two can reduce the total ohmic resistance of the anode [35e37]. Moreover, the use of a stainless steel cylinder grade 201 in the anode helped to minimize the distance that the electrons need to flow through the graphite plate and provides high conductivity, as it contains approximately 5.5e7.5% manganese. In previous studies, the electrical energy produced increased 10 times when manganese was incorporated into a graphite anode [37,38]. Preliminary experiments showed that the use of stainless steel 201 cylinder increased the current density 4 ± 1 times compared to alternative materials that were tested (stainless steel 301, graphite felt and graphite fiber). Finally, stainless steel 201 has a lower cost compared to most carbon-based material. The cathode was constructed of a stainless steel (grade 201) cylinder (313 mm diameter  150 mm length  0.5 mm

Fig. 1 e Schematic diagram of the experimental design.

Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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thickness). The anode was located within the cathode cylinder, with rubber bands separating the two electrodes to prevent contact. The cathode and anode were connected to the circuit by insulated wires (Fig. 1).

MEC voltage and calculations A voltage of 0.9 V was applied across the MEC electrodes using a DC power supply regulator (model YH-305D, Yi-Hua Inc., Guangzhou, China). The power supply had two leads: one positive and the other negative. The negative lead of the power supply was connected to the cathode, with a 10 U resistor serially connected between them, while the positive lead was connected to the anode. The voltage drops across the 10 U resistor was recorded using a data logger every minute and transferred to a computer using the USB wire connection. Ohm's law (I ¼ V/R) was used to calculate the current where V was the voltage drop measured across the 10 U resistor (R). The volumetric current density (IV) (A/m3) was calculated twice based on the average of current over a 24 h period using the total liquid volume (900 mL) and the MEC liquid volume (135 mL). The daily volumetric H2 and CH4 production rates (Q) were normalized to the reactor liquid volume (m3 gas/m3/d). The amount of energy added to the circuit by the power source, adjusted for losses across 10 U resistor (WE), was calculated according to Call and Logan [9]. WE ¼

X  n

 IEap Dt  I2 RDt

where Eap (V) is the voltage applied using the power source, I is the current, Dt(s) is the time increment for n data points measured during the experiment, and R 10 U is the external resistor. The energy of hydrogen (WH2) and methane (WCH4) production were calculated using Gibbs free energy (DGH2 ¼ 237.1 kJ/ mol, DGCH4 ¼ 890.4 kJ/mol), as described previously [39]. The total energy of each treatment (WTre. ¼ WH2 þ WCH4) and total energy gain (WTreWe¼WH2 þ WCH4WE) were calculated every 24 h.

Chemical, and gas analyses Total solids (TS) and volatile solids (VS) of food waste, inoculum, and bioslurry were measured according to Standard Methods [40]. Chemical oxygen demand (COD) was conducted using a colorimeter and Method 410.4 (AQ3700, Thermo Scientific Orion, MA, USA). Biogas production was quantified using precision wet gas meters (model W-NK-0.5, Shinagawa Co., China). Gas samples (60 mL) were taken using a gas tight syringe through a three-way valve connected along the gas line connecting the reactor to gas meter inlet and analyzed using a Gas Chromatograph (GC) (model GC2014C, Shimadzu Corporation, Japan).

Statistical analyses ANOVA and Tukey-Kramer analyses were performed to compare the significant effect of adding the MEC to an AD based on gas production (biogas, CH4, H2, and CO2) and COD concentration during the 23-day experiment. The statistical analyses were conducted using SPSS with an alpha value of 0.05.

Result and discussion COD removal The initial COD concentration in three reactors was 15,640 mg/L. Combining MEC and AD promoted faster and higher COD/organic matter breakdown by the microorganisms. After 8 days of digestion, the AD-MEC-23 had removed 57% of the COD compared to 48% for AD-MEC5 and 43.9% for AD-only treatment. After 23 days, the AD-MEC-23 had the largest COD reduction (82.7 ± 0.1%; 2700 mg/L) after 23 days of digestion compared to 74.6 ± 0.09% COD reduction in the ADMEC-5 reactor (3980 mg/L) and 68.4 ± 0.27% COD reduction in the AD-only reactor (4940 mg/L) (Fig. 2), with all reactors having significantly different final concentrations (pvalue < 0.01) (Fig. 2A). Similar to this study, previous studies [29,41] have also shown that using bio-electrochemical technology increased COD removal by 5e15% compared to the ADonly treatment. Our combined bioelectrochemical technology inside the AD achieved the higher end of this literature values at 14.3% higher COD removal compared to the AD-only treatment. These results show that using a MEC combined with AD reactor increased the ability of the reactor to reduce the COD concentration during digestion (Fig. 2A).

Gas production and composition Cumulative biogas production during the batch test in the ADMEC-23 treatment was 19% higher than the AD-only treatment (p-value ¼ 0.027) (Fig. 3). In addition, the AD-MEC-23 treatment produced 26.9% more CH4 and 1680% more H2 than the AD-only treatment (p-value ¼ 0.008 and pvalue ¼ 0.003, respectively) (Fig. 3). Carbon dioxide production by the AD-MEC-23 was also significantly lower (by 51.4%) than the AD-only treatment (p-value < 0.001). The results showed that using MEC combined with AD in the same reactor could increase the total useful gas, with the AD-MEC-23 having 74.9% more daily CH4 and H2 production (0.59 m3/m3/day, based on the average over 23 days) compared to the AD-only treatment (0.34 m3/m3/day, based on the average over 23 days). Moreover, the MEC-AD-23 decreased the cumulative CO2 production by 106% (0.125 m3/m3/day, based on the average over 23 days) compared to the AD-only treatment (averaging 0.258 m3/m3/day over 23 days). Compared to the AD-only treatment, running the MEC for only the initial five days (AD-MEC-5) did increase the cumulative H2 by 910.8% (p-value ¼ 0.015) and reduced cumulative CO2 by 20.2% (p-value ¼ 0.003), although not as much as the AD-MEC-23 treatment, with 76.1% lower H2 (p-value ¼ 0.024) and 39% higher CO2 (p-value ¼ 0.001). Moreover, the AD-MEC-5 treatment had 23.5% lower CH4 production (p-value ¼ 0.011) and 11.6% lower biogas production than the AD-MEC-23 treatment (p-value ¼ 0.027), with no significant differences in CH4 and biogas production between the AD-MEC-5 and the AD-only treatments (p-value ¼ 0.719 and p-value ¼ 0.132, respectively) (Fig. 3). The results showed that using the ADMEC-5 performed better than the AD-only treatment, with increased H2 and reduced CO2 production, but did not

Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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Fig. 2 e Chemical oxygen demand (COD) removal (A) and change in COD concentration in the MEC-23, AD-MEC-5, and ADonly treatments (B).

statistically increase biogas and CH4 production, as was seen in the AD-MEC-23 treatment with the MEC operating for the entire 23-day digestion period (Fig. 3B). The results also showed that using the MEC continuously inside the digester (AD-MEC treatment) reduced CO2 concentration to approximately 4% at the end of the experimental period, thereby, increasing the useful gases (CH4 and H2) concentrations to a maximum of 95.8%, with an average of 71.9% CH4, 17.4% H2 and 10.7% CO2 over the 23-day digestion period in the AD-MEC-23 reactor. The results illustrate how a MEC can decrease CO2 production while increasing useful gas production (H2 and CH4) (Fig. 4a). The H2 production rate increased by using the MEC (AD-MEC-23) during the first 24 h to a maximum of 1.3 ± 0.1 m3/m3/day, with a cumulative H2 production of 3.9 ± 0.3 m3/m3 for AD-MEC-23 compared to 0.2 ± 0.04 m3/m3 for the AD-only treatment. As the experiment progressed, the hydrogen concentration decreased as CH4 increased and CO2 decreased (Fig. 4a). The

results showed that the H2 produced by the MEC was combined with CO2 to form CH4, which resulted in reduced CO2 and increased CH4 concentrations within the biogas as compared to the AD-only treatment. Indeed, it has been observed that in most MEC studies that H2 generation in the first couple of days is high, with additional H2 produced by the MEC after the initial period combined with CO2 to produce methane through hydrogenotrophic methanogenesis [24,29,34,39,42,43]. Recent research has also shown that hydrogenotrophic methanogens could accept electrons from MEC electrodes [44] or from hydrogen [43]. Hydrogenotrophic methanogenesis was considered as a key process to capture hydrogen (or electrons) to enhance CH4 production [45]. The results showed that adding a MEC to an AD can not only increase the amount of useful gases (H2 and CH4), but also increase the quality of biogas by reducing the CO2 concentration to 4.2% and increase the useful gas concentration to 95.8%.

Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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Fig. 3 e Cumulative biogas over the 23-day experiment (A), with cumulative biogas, CH4, H2, and CO2 shown in (B). Significant differences were analyzed using Tukey-Kramer analysis with an alpha value of 0.05.

In the AD-MEC-5 reactor, after the MEC was stopped (five days after the 23-day experiment began), the reactors operated only as an AD system and thus the total H2 production was significantly lower than AD-MEC-23. In AD-MEC-5, the initial H2 production that occurred during the first five days did result in higher cumulative H2 concentration than the ADonly treatment. Moreover, the CO2 gas concentration was also reduced in the AD-MEC-5 compared to AD-only, likely due to H2/CO2 conversion to CH4, but not to the same extent as the continuous AD-MEC-23 reactor (Fig. 4).

Energy recovery The maximum current achieved for the AD-MEC-23 reactors was 34.3 ± 0.2 mA (achieved in the first 24 h), with an average current of 11.1 mA. The volumetric current was 108.7 A/m3 (based on the MEC volume) and 17.3 A/m3 (based on the total AD-MEC volume). The current density was observed to be 1.35 A/m2 based on the cathode surface area. The total energy produced from the AD-MEC-23 treatment (WAD-MEC-23 ¼ 414 kJ) was higher than the total energy produced by the AD-MEC-5

Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003

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Fig. 4 e Daily CH4 and H2 gas production rates (m3 gas/m3 AD volume), and daily concentrations of CH4, CO2, and H2 for ADMEC-23 (A), AD-MEC-5 (B), and AD-only treatment (C) over the 23-day experiment.

treatment (WAD-MEC-5 ¼ 325 kJ) and the AD-only treatment (WAD-only ¼ 295 kJ) (Fig. 5). In the AD-MEC-23 treatment, the total energy (electricity) that was needed to run the MEC was 20.1 kJ, which was 5% of the total energy that was

produced from the AD-MEC-23 treatment, or 17% of the additional energy produced by the AD-MEC-23 compared to the AD-only treatment (i.e. WAD-MEC-23 e WAD-only). Accounting for only the additional energy produced by the AD-MEC-23

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acknowledges support from the China Scholarship Council (CSC).

references

Fig. 5 e Total energy production for the AD-MEC-23 (W), AD-MEC-5 (W), and AD-only (W) treatments. W-e refer to net energy produced by the two treatments (AD-MEC-23 and AD-MEC-5) after subtracting the total energy produced by the two treatments by the electrical energy used for running the MEC. (WAD-MEC-23 e WAD-only), the overall efficiency of AD-MEC-23 over the electrical input energy for running the MEC exceeded 400%. Thus, while there is a need for input energy to run the MEC, the return from this input energy is much greater, with an increase of percent CH4 in the biogas resulting in less energy use in any subsequent CO2 removal scrubbing technology utilization.

Conclusions A novel MEC design was incorporated into AD system and was shown to be a promising technology to increase biogas and useful gas (CH4 and H2) production, while decreasing CO2 content of the biogas. Moreover, the mobility of the MEC design allows it to be incorporated into any existing digester. The total biogas generated from AD-MEC-23 (17.9 m3/m3) was higher than AD-MEC-5 (16.2 m3/m3) and AD-only treatment (15 m3/ m3), with total useful gas (CH4 and H2) of 12.79 L for the ADMEC-23 treatment compared to 9.8 L for AD-MEC-5 and 7.6 L for AD-only treatment. The study also found that combining AD with MEC could increase useful gases (CH4 and H2) concentrations in biogas to a maximum of 95.8%. Furthermore, there was increased COD reduction in AD-MEC-23 treatment (82.7%) compared to AD-MEC-5 (74.6%) and AD-only (68.4%). AD-MEC-23 treatment increased energy production from waste by 40%, with overall efficiency of AD-MEC exceeded 400% over input energy to power the MEC. A number of future studies can be conducted to optimize energy production in AD-MEC systems, including the use of solar energy to power the MEC and changes in MEC to AD volume ratio.

Acknowledgements This research was supported by the Chinese National Natural Science Foundation (Project IDs: 51576167), The first author

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Please cite this article in press as: Hassanein A, et al., Next generation digestion: Complementing anaerobic digestion (AD) with a novel microbial electrolysis cell (MEC) design, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.003