Operation of Bioelectrochemical System on the Effluent of
a Two-stage Anaerobic Process for Additional Energy
Recovery
K. R. Fradler*, J. R. Kim*, G. Shipley**, J. Massanet-Nicolau**, R. M. Dinsdale**, A. J. Guwy*, G.
C. Premier*
Sustainable Environment Research Centre (SERC)
* Faculty of Advanced Technology, and
(E-mail: [email protected], [email protected], [email protected])
** Faculty of Health Sport and Science, University of Glamorgan, Pontypridd, Mid-Glamorgan,
CF37 1DL, United Kingdom
(E-mail: [email protected], [email protected], [email protected], [email protected])
Abstract
Anaerobic bioenergy production processes such as fermentative biohydrogen (BioH 2), anaerobic
digestion (AD) and bioelectrochemical system are well known for converting municipal or various
biomass feedstocks to useful energy carriers. However the performance of a microbial fuel cell
(MFC) on the effluent of a two stage biogas process has not yet been investigated extensively in
continuous reactor operation on complex substrates. In this study we have determined to what extent
a microbial fuel cell (MFC) can further recover energy from a two-stage biohydrogen and biomethane
system. The performance of a four module tubular MFC at five different organic loadings was
determined in terms of power generation, COD removal efficiency, coulombic efficiency (CE) and
energy conversion efficiency (ECE). A power density of 3.1 W m-3 was observed at the highest OLR
(0.05 g sCOD L-1 d-1), which resulted in the highest CE (60%) and ECE (0.8%). Counter to the CE
and ECE performance, the COD removal efficiency decreased at higher organic loadings (35.1 4.4%).
Keywords
Bioelectrical system; anaerobic digestion; bioelectricity; energy efficiency
INTRODUCTION
The environmental benefits of sustainable waste management with simultaneous energy
recovery has been widely highlighted in the literatures, and drive a significant motivation for
the development of effective bioenergy process. However, the prospect of polishing the
effluents from wastewater treatment/anaerobic bioprocess, and maximizing energy recovery
are essential from the point of view of water reuse and improved energy recovery and
sustainability of the whole bioenergy processes. Anaerobic systems such as anaerobic
digestion (AD) and biohydrogen (BioH2) will generally discharge effluents which are still
burdened with high strength of organic contaminants which contains potential useful energy
vectors, thus it requires secondary recovery and post-processing. The soluble compounds
from these processes will mainly include volatile fatty acids, which are odorous and increase
biological oxygen demand (BOD) on the subsurface environments. These biodegradable
contaminants in the effluent also represent a potential source of energy with development of
appropriate technologies (Guwy et al., 2011). The context (Guwy et al., 2011) and degree to
which VFAs and other components in synthetic wastewater can be removed in multi-modular
MFC system have been considered (Kim et al., 2010), (Kim et al. 2011). However, the
performance and energy recovery of MFC systems using real effluent from a two stage AD
and BioH2 system has not yet been investigated. Several studies have showed increased
energy efficiency by using pre-fermented wastewater for bioelectricity generation in MFCs
(Kannaiah Goud and Venkata Mohan, 2011). Microbial Fuel Cells were linked to a hydrogen
fermenter on food processing wastewater (Oh and Logan, 2005) or vegetable waste
(Mohanakrishna et al., 2010). Recently Sharma and Li combined an anaerobic BioH2
fermenter with a MFC and continuously fed synthetic glucose wastewater (Sharma and Li,
2010). To the authors knowledge the combination of a realistically scalable MFC reactor with
a two stage biogas producing system has not been investigated yet.
In this work the effluent of a fermentative BioH2 and AD was introduced into a BES
(Bioelectrochemical system) for bioelectricity production. A tubular four-module MFC
reactor was fed with the AD effluent at different OLRs. The effect of changing OLRs on the
performance of a tubular four-module MFC was investigated to estimate power production,
coulombic- and COD removal efficiency and energy conversion efficiency on the basis of the
MFCs influent COD concentration and with respect to the degraded COD.
METHODS
Feedstock
Wheatfeed obtained from a flour mill operating in Barry, South Wales (Premier Foods) was
used as substrate. The pellets were partially hydrolyzed using alkali, before being fed into the
fermenters. The pellets were soaked in water overnight in a refrigerator allowing them to
dissociate, then diluted with water and sufficient NaOH to raise the pH to 12. The volatile
solids (VS) content was 50 g L-1. This feed was then transferred to a feed storage tank before
pumping to a 10 L working volume BioH2 fermenter as required. The storage tank was
maintained at 2 – 8 oC to limit microbial growth. In the second stage a 25 L reactor working
at 35 oC was used for methanogenesis.
Microbial Fuel Cell (MFC) construction and operation
A longitudinal tubular reactor with four MFC modules (each module of 0.25 L) as previously
reported (Kim et al., 2010; Kim et al., 2011), was used. The carbon veil anodes (230 x
450 mm per module; PRF Carbon, UK) were wrapped around a perspex cylinder of diameter
1 cm. The membrane electrode assembly was made of a cation exchange membrane (CMI700, Membrane International Inc.; 122 x 192 mm) assembled with a carbon cloth (163 x
82 mm) cathode containing 0.5 mg cm-2 Pt. The modules were separated by ballast and
orifice plates, to maintain separation. The reactor was inoculated with anaerobic digester
sludge (1:10); 40 mM acetate in 50 mM phosphate buffer, vitamins and minerals under 1000
Ω resistance. After batch start-up, the reactor was inclined (30 °) and operated continuously
at five different organic loading rates (OLRs). The effluent from the anaerobic digester was
filtered through a stainless steel sieve to exclude particles larger than 0.21 mm, which clog
the reactor and could not be degraded within four modules. The composition of the filtered
effluent varied on a daily basis and contained on average 3.300 g COD L-1, 2.500 g sCOD L1
, 0.080 g L-1 acetic acid, 0.030 g L-1 butyric acid and exhibits a pH-value of 7.7, as well as a
conductivity of 12.9 mS cm-1. Different OLRs (0.036; 0.053; 0.086; 0.337; 0.572 g sCOD L1 -1
d designated to OLR1 – 5, respectively) were maintained by dilution of the filtered AD
effluent. The lower OLRs were chosen because previous studies showed lower coulombicand COD-removal efficiency at high organic loading rates above 1 g COD L-1 d-1 (Sharma
and Li, 2010), (Nam et al., 2010). The MFC influent was kept refrigerated and introduced at
the lower end of the tubular reactor at a rate of 0.5 mL min-1 through an external peristaltic
pump (Watson and Marlow, Falmouth, UK). The constant flow rate lead to a hydraulic
retention time (HRT) of 33.3 h for the whole reactor or 8.3 h for each module respectively.
All reactors were independently connected in series to R = 1000 Ω resistors and operated at
room temperature (20 ± 4°C).
Analyses
The voltage output across the load of every module was monitored by LabVIEW TM software
(National Instruments) at 10 min intervals. After three HRTs samples were taken with a
syringe from the influent and at the end of every module. The pH value (pH meter Mettler
Toledo; Urdorf, Switzerland) and the conductivity (737 Conductivity meter; Mettler Toledo
Inc., Urdorf, Switzerland) were determined before the liquid samples were acidified (HCl,
12 M) and preserved frozen. Before analysing the sCOD content with a standard method
(Methode 5220, HACH COD system, HACH Co., Loveland, CO, USA) the defrosted
samples were centrifuged at 12,000 rpm for 3 min. Volatile fatty acids (VFA) where assayed
using a gas chromatograph (Perkin Elmer Clarus 500 GC). Power curves were calculated
using a potentiostatic method with Solartron Instruments (Amatek - Solartron Analytical;
1287 Electrochemical Interface, Farnborough, UK). The measurement was carried out after
three HRTs and the potential was changed after a stable value was observed (10 min).
The coulombic efficiency (CE) and COD removal efficiency were calculated according to the
previous report (Logan, 2008). The energy production per COD consumed (ECOD [Wh g
sCOD-1]) was calculated at each OLR and was based on the voltage and COD consumption
measured after three retention times (99.9h).
𝒕
𝑬𝑪𝑶𝑫 =
∑𝒏
𝒏=𝟏 ∫𝟎 𝑼 ∗ 𝑰 𝒅𝒕
∆ 𝑪𝑶𝑫 ∗ 𝑽𝒓𝒆𝒂𝒄.
(1)
Where n is 4 for four module reactor tested in the report, U is the observed voltage [V] after
three retention times, I is the current [A] integrated over the operation time of one retention
time (8.3 h), V is the whole reactor volume (1 L) and ΔCOD is the difference between the
influent and the effluent (4th module).
The recovered energy from MFC modules (ER) is
𝒕
𝑬𝑹 =
∑𝒏
𝒏=𝟏 ∫𝟎 𝑼 ∗ 𝑰 𝒅𝒕
(2)
𝑽𝒓𝒆𝒂𝒄.
ER. [J L-1] over the entire reactor lengths was calculated by using the voltage Umax and current
Imax at the maximum power production, determined in the polarisation test. The energy
conversion efficiency (ECE) was calculated on the basis of the total influent energy content
(ECEtotal COD) and on the basis of the energy content of the consumed COD (ECEΔCOD). The
published heat of combustion values for wastewater varies due to different composition and
measurement methods in the range of 13.89 to 23.25 MJ kg COD-1 (Owen, 1982), (Heidrich
et al., 2010), (Shizas, 2004). Therefore the energy conversion efficiencies (ECEtotal COD,
ECEΔCOD) were estimated with two different values; a = 13.89 MJ kgCOD-1 (Owen, 1982)
and a = 23.25 MJ kgCOD-1, which represents the average of two heat of combustion values
for different wastewasters calculated from the COD (Heidrich et al., 2010).
𝑬𝑪𝑬𝒕𝒐𝒕𝒂𝒍 𝑪𝑶𝑫 =
𝑬𝑪𝑬∆𝑪𝑶𝑫 =
𝒕
𝑬𝑹
𝑬𝑰𝒏𝒇.
𝑬𝑹
𝑬∆𝑪𝑶𝑫
=
∑𝒏
𝒏=𝟏 ∫𝟎 𝑼 ∗ 𝑰 𝒅𝒕
𝒂 ∗ 𝑪𝑶𝑫𝑰𝒏𝒇
∗ 𝟏𝟎𝟎
(3)
𝒕
=
∑𝒏
𝒏=𝟏 ∫𝟎 𝑼 ∗ 𝑰 𝒅𝒕
𝒂 ∗ ∆ 𝑪𝑶𝑫
∗ 𝟏𝟎𝟎
(4)
RESULTS AND DISCUSSION
Voltage development at different organic loading rates
Figure 1: Voltage generation from each module at different OLRs (0.036 - 0.572 g sCOD L1 -1
d )
Fig. 1 shows the response of voltage generation in each of the four modules to changes in the
OLRs (0.036 – 0.572 g sCOD L-1 d-1) under a fixed resistance (R = 1000 Ω). The voltage
sequentially increased within one retention time from module 1 to 4 in response to the
increasing OLR. The average voltage output decreased as reactor lengths increased due to the
lower organic concentration reaching these modules. The two lowest OLRs (0.036 and 0.053
g sCOD L-1 d-1) resulted in no significant changes in the voltage output, apart from module
four. The fact that the voltage output in this module was zero during the operation of OLR1
indicates a lack of easily degradable substrate (e.g. VFAs) and/or mass transport limitations.
It is likely that available substrates like acetate and butyrate were already depleted in the
preceding modules. Even though the effluent from the 4th module was 32 mg L-1 sCOD at
OLR1 this organic content might not have enough available organic compounds for
bioelectricity generation under the tested operation conditions. This result indicates that the
effluent of the preceding biogas process contains non-biodegradable organic compounds (e.g.
cellulose and particulate COD from wheatfeed). The remaining organic material might get
partially available within consecutive modules due to further degradation of more complex
organic compounds or by increasing flow rate. The voltage generation in the 4th module
during the OLR2 indicates, that an influent concentration over 74 mg L-1 COD allows
available substrate to enter into the last module.
Power production in the 4-module longitudinal tubular reactor
Table 1: MFC parameters during the continuous operation
pH
[-]
OLR
[gsCOD L-1 d-1]
conductivity
[mS cm-1]
Inf.
Eff.
Inf.
Eff.
max. vol. power density (MVPD)
[W m-3]
st
nd
1
2
3rd
4th
module module module module
OLR1
0.036
7.48
7.46
0.44
0.44
0.018
0.010
0.015
OLR2
0.053
7.55
7.46
0.74
0.78
0.020
0.014
0.017
0.004
OLR3
0.081
7.84
7.58
1.45
1.57
0.175
0.136
0.053
0.031
OLR4
0.337
8.45
7.65
2.68
2.25
2.370
2.820
0.365
0.128
OLR5
0.572
7.87
7.45
3.98
3.66
3.316
3.383
3.388
2.307
Table 1 illustrates the volumetric power density obtained after three retention times. It can be
clearly seen that the maximum volumetric power density (MVPD) correlates with increasing
OLR and decreasing according to reactor lengths. The highest MVPD per module was
reached with OLR5 (2.3 - 3.4 W m-3). Furthermore the power curves measured during
feeding the undiluted AD-effluent (data not presented) demonstrate that the MVPD of every
module can be further increased to 5.5 W m-3. A similar two module reactor reached a power
of 1.35 mW in the 1st module when fed with a synthetic sucrose wastewater at
0.41 g sCOD L-1 d-1 (Kim et al., 2010). This results indicates, that the operation with AD
effluent causes a moderate reduction in power, as an OLR of 0.57 g sCOD L-1 d-1 enabled the
generation of 0.82 mW.
Table 1 also shows that the power production in the final module increases disproportionally
if the OLR is increased from 0.34 to 0.57g sCOD L-1d-1, due to the enhanced availability of
utilizable substrate. In addition, the higher influent conductivity, which increases in
proportion to the OLRs (0.44 to 3.98 mS cm-1), contributes to the enhancement of power
output. This is also reflected in the internal resistance (data not presented), which contains
both ohmic losses due to the conductivity as well as possible concentration losses caused by
mass transfer limitations of utilizable substrate.
It should be noted that if the effluent from the proceeding process is treated in an scaled up
multi-modular MFC reactor without dilution, the conductivity will not undergo such
significant variations, simulated in this study by changing the OLR via dilution. Therefore, a
stack multi-modular system has the potential to achieve higher power densities than those
reported here, because of lower ohmic resistance.
Coulombic efficiency and COD removal efficiency of the bioelectrochemical system
Fig 2 Coulombic efficiency and COD removal efficiency at each OLR
Fig. 2 shows the CE and the COD removal efficiency in the longitudinal tubular MFC and
their dependence on the organic influent loading. It can be seen, that the COD removal
efficiency tends to decrease with the increase reactor OLR, whereas the CE shows an
opposite trend with using biogas process effluent. At the lowest OLR (0.036 g sCOD L-1 d-1)
the COD removal was reduced by 35.1% through the four module reactor system. Even
though the volumetric COD removal is increasing in absolute terms, only 4.4% (37 mg
sCOD) were removed at the highest OLR. The same reactor fed with synthetic sucrose
wastewater at 0.08 - 0.8 g COD L-1d-1 reached COD removal efficiencies of 93 - 43% (Kim et
al., 2011). The trend of lower removal efficiencies at higher organic loading was observed in
several studies if fed with domestic wastewater (You, 2006), swine wastewater (Zhuang et
al., 2012), or the effluent of an hydrogen biofermenter (Sharma and Li, 2010). These results
indicate that a tubular reactor fed with AD effluent requires more modules for effluent
polishing, than the same reactor fed with sucrose, especially at high COD influent
concentrations.
The CE was below 10% at the three lower OLRs and increased up to 42% and 60% at OLR4
and 5. Hence the CE is therefore higher than in the sucrose fed longitudinal tubular reactor
when an effluent from an anaerobic digester using wheatfeed as feedstock is introduced into
the same system; as the highest CE obtained from 0.24 g sCOD L-1 d-1 was 38-49% (Kim et
al., 2010). An inverse correlation between CE and OLR was observed when a 100 mL glass
bottle MFC was continuously fed with the effluent of an hydrogen producing biofermenter,
where the CE decreased from 5.3 to 1.5% at organic loadings between 1.33 - 6.5 gCODL-1d-1
(Sharma and Li, 2010). Also a wastewater from hydrogen fermentation of coffee processing
wastewater resulted in lower CE (0.98 - 0.3%) at higher OLRs of 1.92 - 4.8 gL-1d-1 (Nam et
al., 2010). Other studies demonstrated the dependence of CE on the substrate availability for
electrogenesis. Therefore the CE dropped from 65% with acetate as substrate to 14% with a
more fermentable glucose substrate under continuous operation (Min and Logan, 2004). This
result implies that the energy recovery by electrogenesis can be enhanced at higher
concentration of electrogenically favourable substrates (e.g. VFAs) at higher organic loadings,
thus further improvement of systems and material can increase treatment capacity and
recovery from high strength of organic wastewater. The high conductivity and VFA content
of the AD effluent is an especially noticeable advantage in terms of power production and CE
compared to domestic and other types of wastewater.
The effect of the OLR on the energy efficiency of a tubular MFC
Table 2: Energy production and efficiency in MFC
OLR
ECOD
ErR
EInfl.
ECEtotal COD
ECEΔCOD
[g sCOD L-1 d-1]
[Wh gsCOD-1
consumed]
[J L-1]
[J L-1]
[%]
[%]
OLR1
0.036
0.001
0.33
686 – 1147
0.03 – 0.05
0.08 – 0.14
OLR2
0.053
0.003
0.41
1028 – 1721
0.02 – 0.04
0.12 – 0.20
OLR3
0.081
0.004
2.96
1570 – 2627
0.11 – 0.19
0.43 – 0.73
OLR4
0.337
0.166
42.62
6994 – 11703
0.36 – 0.61
5.29 – 8.85
OLR5
0.572
0.276
92.95
11543 – 19313
0.48 – 0.81
10.90 – 18.24
Table 2 estimated the energy recovery in a tubular reactor, energy recovered per COD
consumption (ECOD) and energy conversion efficiencies (ECE). As expected ECOD is
increasing with the OLR, due to the direct dependence on the CE. It reaches the highest value
of 0.276 Wh gsCOD-1 consumed at an organic loading rate of 0.572 g sCOD L-1 d-1.
The energy produced or recovered (ER.) in the four module reactor (V = 1L) ranged from
0.33 J to 92.95 J, if every module operated at a max. power output for one entire HRT (8.3h).
(Sharma and Li, 2010) achieved a higher energy recovery of 259 to 337 J L-1 at higher OLRs
of 0.61 to 2.35 g COD L-1d-1. Whereby in contrast to our operation a synthetic glucose feed
was used and the two stage process operated in a significant smaller scale as the volume of
the biofermenter and MFC were 2 L and 0.1 L respectively. However a further increase in
the organic loading resulted also in a lower energy recovery.
The ECE was calculated with respect to the total energy content of the influent (ECEtotal COD)
as well as in terms of energy content of the consumed COD (ECEΔCOD). The energy recovery
based on the difference in COD [g L-1] between influent and effluent (ECEΔCOD), reached
values between 0.08 - 0.14% at OLR1 and 10.9 - 18.24% at OLR5. This lower energy
recovery is probably influenced by non-electrogenic side reactions expressed by the CE.
(Schroeder, 2007) reported that only 54% of the total Gibbs Free Energy can be recovered
from the oxidation of glucose, indicating a high degree of energy dissipation. In general all
conversion processes involves dissipation of energy, and therefore also the conversion of the
substrate into electricity, as one portion is used to carry out the metabolic reactions in the
bacteria. (McCarty et al., 2011) reported that the losses caused by the microbes to carry out
the conversion reaction are 7%. Further losses derive from the potential efficiency, so cause
the difference between the max. possible thermodynamic potential and the real electricity
producing potential, which is affected by activation, ohmic and mass transfer losses in
bioelectrochemical systems (Lee et al., 2008).
All this reasons as well as the chosen calculation base of the heat of combustion, which
includes also the non-biodegradable fraction of the wheatfeed, significantly affect the ECE.
More accurate values can be obtained if the wastewater energy content of different feed
stocks is determined using bomb calorimetry. The total energy recovery (ECEtotal COD) lies
between 0.03 - 0.05% and 0.48 -0.81%, and the results in the Table 2 indicate that this value
can be increased at higher OLRs. As the energy conversion efficiency depends on the
biodegradability of the substrate, the values reported in the Table 2 are lower than less
complex synthetic wastewater. Previous investigations clearly result in a much higher ECE of
acetate (43%) compared to glucose (3%) in batch mode (Lee et al., 2008) or 17% and 3% in
continuous mode (Min and Logan, 2004).
Higher ECE can be expected if the tubular reactor is extended by the addition of further
modules, as the values reported here refer only to a four module unit. In such a multi-modular
reactor more complex organic matter can be further degraded to a more utilizable substrate
for electrochemically active bacteria. Additionally the conductivity in the tubular system will
not significantly change as the organic content and therefore the ohmic losses will be
significantly lower and can have a positive impact on the ECE. Further investigation such as
the development of electrode materials, module combination, and electrical circuit control
will improve energy recovery, and simultaneously achieve effluent discharge limits.
CONCLUSION
The energy recovery from wheatfeed through longitudinal tubular fuel cell reactor was
investigated in a three stage anaerobic bioenergy recovery process. The influence of the
OLRs (0.036 - 0.574 g sCOD L-1 d-1) on the performance of a continuously operated tubular
four module MFC was investigated in terms of voltage development, maximum volumetric
power density, COD removal and CE as well as energy conversion efficiency.



The voltage development and power production in the MFC increased with the OLR
and decreased within the four module reactor according to reactor length. The highest
organic loading resulted in a power density of 3.1 W m-3, due to the higher
concentration of utilizable substrate and influent conductivity.
The COD removal efficiency lay between 35% at the lowest OLR and 4.4% at 0.572
gCODL-1d-1, where as the CE followed the opposite trend and increased from 4 to
60%.
The energy efficiency of the MFC was calculated for four modules with respect to the
COD consumption (ΔCOD). The highest efficiency (EΔCOD) was in the range of 10.9 18.24% (OLR5) and is expected to be higher than the energy recovery based on the
whole influent energy content (11.5 - 19.3 kJ L-1), which resulted in 0.48 - 0.81%
energy recovery.
Acknowledgements
This research was funded by the RCUK Energy Programme, SUPERGEN Biological Fuel
Cell project (EP/D047943/1) and SUPERGEN (SHEC) project (grant numbers
GR/526965/01 and EP/E040071/1). The Energy Programme is an RCUK cross council
initiative led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC.
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