espace.library.uq.edu.au · Web viewFerreira-Aparicio et al., 2009). Voltage reversal in fact is a...

MFC-Cascade Stacks maximise COD reduction and avoid voltage reversal

under adverse conditions

Pablo Ledezma1, John Greenman2 and Ioannis Ieropoulos1*

1 Bristol Robotics Laboratory, Universities of Bristol and of the West of England, Frenchay Campus, Bristol BS34 8QZ, U.K. 2 Department of Applied Sciences, University of the West of England, Bristol BS16 1QY, U.K.

* Corresponding author: [email protected]

Telephone: +44 (0) 117 32 86318

Fax: +44 (0) 117 32 83960

Abstract

This study presents a 6-unit continuous flow Microbial Fuel Cell (MFC) stack organised as a

vertical cascade and tested under different electrical configurations. Under parallel

conditions, the stacked-system demonstrated stable operation despite the substrate

imbalances inherent to cascading and higher power and current densities than individual

MFCs (positive stacking effect), whilst the cascading dynamic allowed for a cumulative COD

reduction of >95% in less than 5.7h, equivalent to a very competitive 7.97 kgCOD m-3 d-1.

When changed to a series configuration, the stack exhibited considerable losses (cross-

conductance) unless correct fluidic/electrical insulation of the units was applied; with this

problem resolved, the stack in series also exhibited electrical stability and performance

superior to individual cells. In both electrical configurations, the 6MFC system was

purposefully deprived of feed repeatedly for up to 15 days, and in no case resulting in

significant losses post-starvation. The combined results of 14 months of experimentation

This is a post-print version of the following article: Ledezma, Pablo, Greenman, John and Ieropoulos, Ioannis (2013) MFC-cascade stacks maximise COD reduction and avoid voltage reversal under adverse conditions. Bioresource Technology, 134 158-165. doi:10.1016/j.biortech.2013.01.119

hereby presented, demonstrate that cascade-stacking of small units can result in enhanced

electricity production (vs single large units) and treatment rates without the need for

expensive catalysts. Moreover, it is also demonstrated that MFC operation (even when the

continuous flow is stopped) in series configuration, substrate imbalance and starvation does

not necessarily result in cell-voltage reversal.

Keywords

Microbial fuel cells; stacking; wastewater treatment; voltage reversal

Introduction

Microbial Fuel Cells (MFCs) have been receiving increased interest in the last three decades,

since the production of electricity, from a wide variety of organic wastes, was demonstrated

without the need for artificial mediators (Habermann & Pommer, 1991). MFCs are thus an

attractive alternative for effluent treatment (Clauwaert et al., 2008) and despite the

technological progress, it is clear that current bench top MFCs (particularly individual

systems), cannot cope with the volumes and flow rates needed to viably operate in a

wastewater treatment plant. Scaled-up systems are therefore needed to allow for efficient

and high treatment of such large volumes, but also to be able to produce sufficient

electricity so that their operation for wastewater treatment can be made autonomous

(Clauwaert et al., 2008; Winfield et al., 2011).


However, scaling-up via increasing reactor sizes carries the inherent challenge of increased

internal resistances (Rint), since this is volume-dependent (Dekker et al., 2009; Ieropoulos et

al., 2008). This is probably one of many factors that could explain why large volume MFCs

have not demonstrated high-power output levels as smaller bench systems have. In fact,

power density has been demonstrated to be higher at the smaller scale (Ieropoulos et al.,

2008), meaning that practical (useful) levels of power from MFCs are more likely to be

generated via miniaturisation and multiplication, rather than enlarging single reactors

(Fornero et al., 2010; Ieropoulos et al., 2008; Ieropoulos et al., 2010a; Kim et al., 2011). In

terms of MFC unit stacking of various sizes, there have been numerous reports in the

literature, (Aelterman et al., 2006; Dekker et al., 2009; Ieropoulos et al., 2003; Ieropoulos et

al., 2005; Ieropoulos et al., 2010a; Ieropoulos et al., 2010b; Melhuish et al., 2006; Oh &

Logan, 2007; Shimoyama et al., 2008; Shin et al., 2006; Wang & Han, 2009; Wilkinson, 2000;

Zhuang & Zhou, 2009; Zhuang et al., 2012), however much less attention has been paid on

the cascade element of multiple MFC units in a stack operating under continuous flow

conditions (Aelterman 2006; Ieropoulos 2008, Galvez 2009, Pinto 2010, Winfield 2011) and

how the losses from direct fluidic paths oppose electron flow (Ieropoulos 2008, Zhuang &

Zou 2009; Shuang et al 2012). The aim of this paper is therefore to address the issues of

fluidic conductance in a continuous flow cascade system and demonstrate improved

operation both in terms of power output as well as treatment efficiency.

Complexity is further increased when considering the peripherals required for the operation

of MFC stacks (Ieropoulos et al., 2009). According to a life-cycle evaluation of MFCs, which

has found that fabrication costs should be brought to a minimum (i.e. no expensive


catalysts) (Pant et al., 2011), it is clear that the inclusion of additional electronic components

in the operation e.g. DC-DC converters (with their inherent losses) can only increase system

costs and thus reduce the competitiveness of the technology. Accordingly, the most

inexpensive way to achieve working voltages and currents is through a combination of series

and parallel configurations. Yet this is not without problems, with cell-voltage reversal being

one of the most commonly reported phenomena for stacks in a series configuration.

Probably observed for the first time in MFCs by Aelterman et al. (2006), this incident was

then further investigated by Oh & Logan (2007), with unbalanced substrate concentrations

and starvation reportedly being the main causes of reversal. Further investigation of MFC

stacks in series has since been largely neglected with a few exceptions (Dekker et al., 2009;

Ieropoulos et al., 2008; Kim et al., 2012; Pinto et al., 2010). During this time the consensus

has been that series connection, under continuous flow, almost always implies cell voltage

reversal (Kim et al., 2011; Lefebvre et al., 2011), despite the fact that in the initial studies,

this was not the case (Ieropoulos et al., 2008; Oh & Logan, 2007). Since wastewater

treatment systems operate in continuous or semi-continuous mode, perhaps there should

be no hesitation in using, at least partially, series connections within the stacks to increase

voltage to usable levels. Moreover, Pinto et al. (2010) have demonstrated that MFCs with

anolyte staging connected fluidically in series, are capable of larger treatment capacity than

the same cells in (fluidic) parallel configuration. Enhanced treatment is of particular

importance, especially since MFCs might find their first commercial application in

wastewater treatment, as currently the market value of the electricity produced at low

performance levels is considerably lower than the added value of the environmental service

of treating the waste (Fornero et al., 2010).


The present investigation follows on from all these observations and previously

demonstrated stable stack operation (Ieropoulos et al., 2010b) and proposes a simplified,

inexpensive solution to stacking for maximising treatment without parasitic cross-

conduction or voltage reversal losses.

Materials and Methods

1. Construction of the MFC-Cascade system


A cascade-type MFC system was modelled using Solid Edge v20.0 UGS PLM Software

(Siemens, Germany). The design incorporated both an upflow siphon-type tubing system

(embedded) that allowed continuous flow whist maintaining a constant internal electrolyte

volume, in addition to a dripping mechanism that created a fluidic isolation and thus

electrical insulation between MFC units of the cascade. In such a system, the fluid flow was

mostly gravity driven, with the need to pump only to the fuel cell at the top.

The MFC units were also designed to allow for continuous flow of nutrients even in the

event of channel blocking or the absence of cells in the cascade (for e.g. maintenance). The

design was then analysed for structural reliability and mouldability with the ProtoQuote®

analysis system (ProtoMold Injection Moulding Service, Shropshire, UK) and modified to

guarantee that the design could be fabricated by Rapid prototyping techniques as well as

Injection Moulding, since the latter can dramatically reduce the costs of fabrication. Once

finalised, the design was used to make, by means of Fused Deposition Modelling (FDM

Titan, Laserlines, Bedford, UK), packs of 3 identical 2-chamber MFCs (each triplet in a

containing unit, see Fig. 1) with 6mL internal volume for each half-cell in ABS (Acrylonitrile

Butadiene Styrene) plastic. The electrodes, both anode and cathode, were made of 180 cm2

of 20 g.m-2 plain carbon fibre veil (PRF Composites, Dorset, UK) without any added precious

metals or catalysts; the current collector for each electrode was made of plain NiCr wire (6

cm long, ø0.45 mm; SWC, Essex, UK). A cation-exchange-membrane with a surface area of 6

cm2 (VWR, Leicestershire, UK) was placed between the two chambers. The original inoculum

was activated sewage sludge provided by the local water utility company (Wessex Water

Services Ltd). Two single MFCs with identical dimensions were set up at the same time and

operated independently as controls.


Taking advantage of the precision in the fabrication technique, along with the predictable

hygroscopic swelling of the parts (avg. +0.399% according to the manufacturer), the MFCs

were watertight without the need for screws, fixtures or sealants thus reducing assembly

time, complexity, cost and weight of the stacks. Consequently, the utilised design conveyed

highly practical “plug-and-play” modularity, whereby each MFC could be easily added to a

stack or removed for e.g. maintenance, both in a matter of seconds and without affecting

the downstream flow for the rest of the stack.

2. Operation

The stacks and control MFCs were continuously supplied with sterile 5mM Acetate TYE

anolyte (5mM C2H3NaO2, 0.1% w/v Tryptone, 0.05% w/v Yeast Extract; pH 6.8 unbuffered,

680 ± 20 μS.cm-1) at 120 µL.min-1 by a multi-channel peristaltic pump (Watson-Marlow

Pumps Group, Cornwall, UK), equivalent to an organic loading rate (OLR) of 8.366

kgCOD.m3.day-1 and a hydraulic retention time (HRT) of 0.63h (per MFC in the cascade). In

additional experiments, the anolyte was replaced by an artificial wastewater solution as

formulated by Aldrovandi et al. (2009), without buffering. The catholyte was fresh tap water

in all cases.

The external load (Rext) connected to each MFC or Stack was varied over time in an attempt

to achieve impedance-matching (Rext = Rint ), which according to Jacobi’s law, is the condition

under which maximum power is transferred to the external load, a.k.a the Maximum Power

Transfer (MPT) point. By repeatedly testing the MFCs/Stacks using polarisation — standard

technique with a range of external resistors (30kΩ to 50Ω) and 5min steps with a variable This is a post-print version of the following article: Ledezma, Pablo, Greenman, John and Ieropoulos, Ioannis (2013) MFC-cascade stacks maximise COD reduction and avoid voltage reversal under adverse conditions. Bioresource Technology, 134 158-165. doi:10.1016/j.biortech.2013.01.119

resistor box (ELC, Annecy, France) — it was possible to observe which Rext values produced

MPT. These same values (which decreased over time, indicating a reduction of R int as

previously described by Ieropoulos et al. 2010b) were then utilised as the load in the normal

operation of the cells, leading to continuous MPT over time.

Additional experiments involved the connection of the MFCs in series and parallel

configurations; the resulting stacks were tested for stability by different cycles of starvation

brought about by stopping and re-starting the anolyte pump for different periods of time.

All tests were carried out at ambient temperature in an attempt to simulate a pragmatic

natural environment for a period of 14 months.

Polarisation and power curves for stacks and single units were generated under the same

continuous flow by connecting the MFCs to load values ranging from 50000 to 5Ω (at 3min

intervals, found sufficient to allow the MFCs to reach quasi-steady-state for each resistance

value).

3. Data collection and analysis

3. 1 Electrical output

Electrical output was measured in real time in the form of voltage with a PicoLog ADC-16

interface (Pico Technology, Cambridgeshire, UK) connected to a computer. Current

production was calculated using Ohm’s law by which the current I = V/R , where V is the

measured voltage and R is the load. Electrical power production was determined using the

derivation of Joule’s law where the power P = V x I. Power and current densities were


normalised based on total anodic compartment volume (6cm3, multiplied by the number of

MFCs when stacked).

3. 2 COD measurements

The total COD of samples was determined according to the potassium dichromate oxidation

method with a kit from VWR (COD MR test system, Lutterworth, UK) and using a compatible

photometer (Aquagem, Jenway, Chelmsford, UK) and interference filter module (COD MR

IM, Jenway, Chelmsford, UK). Samples were collected at the outlet port of each MFC and

compared with the supplied sterile input anolyte (Acetate TYE or Artificial wastewater).

Results and Discussion

1. Individual operation

Originally, a single triplet (3MFCs; see yellow triplet in Fig.1) and two single reference MFCs

were inoculated and operated until a steady-state (based on voltage output) was observed

for a period of at least 7 days; this took approximately 3 weeks in the case of the triplet. It

was soon realised that 3MFCs were not sufficient for full nutrient utilisation/treatment (data

not shown). Hence, a second triplet was placed immediately below thus making a cascade of

6 MFCs, but still fed only from the drip at the top of the cascade. However, the new triplet

was not inoculated with activated sludge but just left to be colonised by the cells shedding

off from the MFCs placed above them.


In less than 24h, a clear electrical potential rose in these newly added MFCs (data not

shown). The system was tested continuously for a further 3 weeks until steady states were

reached for all 6 MFCs. The power curves obtained at the end of this period (see Fig. 2)

indicate a pattern, whereby the fuel cell placed at the top of the cascade (MFC1) produced

the maximum power and current densities, and the fuel cells downstream produced lower

values, in a linear relationship with their position in the cascade; a significant correlation

was actually found between the maximum power density (MPD) points for each MFC (see

insert table on ) and the corresponding current (CCD) with regards to the actual positioning

(linear regression: r2= 0.9296; p= 0.0019). Similarly, the cells presented differential open

circuit voltages (OCVs) in linear proportionality (r2= 0.9472; p= 0.0011), with MFC1’s OCV =

584.76 mV and 576.70, 567.22, 528.47, 511.97 and 503.32 mV respectively for MFCs 2-6.

Nevertheless, MPT for all cells was found to occur at very similar values (avg. 3.27 ± 0.21 kΩ)

for all cells, indicating near-identical Rint. The latter could be expected because all MFCs were

of identical in construction, so the differences in OCV can be explained by the cumulative

degradation of the nutrient supply, brought about by feeding the stack only at the top cell;

as the anolyte flows from one cell to the other, electrons are extracted by the bacteria, thus

changing the anolyte’s redox potential with direct consequences for the anodic potential of

the cells. Furthermore, but to a lesser extent, it is also possible that a cumulative reduction

in the catholyte’s dissolved oxygen also contributed to this effect.

2. Stacked operation

2.1 Parallel connections: positive stacking-effect


After estimating the individual performance of each cell in the stack, the cells were

connected in parallel and left to stabilise; as expected the OCVs for both the stack and the

individual MFCs were very similar (ca. 560mV). After a week of stacked operation, a

polarisation sweep was performed, with the results shown on Table 1a. As can be seen, the

6MFC stack was capable of producing approx. 7 times more power than individual MFCs,

implying a superior power density from the pack. In a similar fashion, the MCP was 8.33-fold

larger than the single units’ production. After another ~8 months of operation in parallel

(see Table 1b), both individual and stacked systems exhibited improvement, but the

superior performance of the stack was maintained with respect to the reference cells (7.46

W m-3 and 91.11 A m-3 vs 6.83 W m-3 and 85 A m-3), implying that this effect is permanently

maintained throughout biofilm development stages. The benefits of stacking – when this is

done optimally – are thought to be a consequence of impedance matching (see above),

whereby the plurality of units tends to uniformity in terms of Rint. If the majority of the cells

perform well then the negative effects of the underperforming minority are compensated

for, and in the longer term their performance is “assisted” by, the stronger majority. If the

opposite case occurs, then the overall performance is brought down (Ieropoulos et al.,

2010c). The importance of Rint balancing is further explored in section 2.5.

2.2 Parallel connections: cumulative COD reduction to high standards

2.2.1 COD reduction for Acetate-based anolyte

The COD-reduction was measured both for the individual reference cells and for each cell of

the stack operating in parallel, with the latter being the preferred configuration due to


higher currents obtained. The ΔCOD levels achieved were based on the COD of the sterile

Acetate TYE anolyte (1982.5 ± 2.12 mg L-1). Determinations were performed until the COD-

reduction values became stable (approx. 3 months after connection in parallel; see Fig. 3

“5mM Acetate + TYE 1”). A second set of determinations were performed one month later

(see Fig. 3 “5mM Acetate + TYE 2”), but no significant difference was found. The average

ΔCOD for the top-placed MFC1 (cascade) was 591 mg L-1, whereas the ΔCOD for a non-

stacked MFC was on average 542 mgCOD L-1. The difference observed can be explained by

the positive stacking-effect previously discussed: operation at higher current densities

means enhanced abstraction of electrons from the carbon source, naturally translating into

better COD reduction rates than in individual units.

The average ΔCOD for the 6MFC-cascade stack was found to be 1753.5 mg L-1 or 88.45%,

producing an effluent of 229 ± 8.48 mgCOD L-1. The latter, although favourable, was

nevertheless of insufficient quality, particularly with reference to the European Union urban

wastewater treatment directive (EU WWTD), which requires COD reduction to a minimum

of 125 mgCOD L-1 (EEC, 1991). As ΔCOD was found to be cumulative in the cascade system, it

became clear that adding further units below the stacked system would assist in reaching

acceptable levels. However, due to the impracticality of setting up a second 6-MFC stack —

with comparable levels of biofilm maturity — within a reasonable timeframe, the addition of

a second 6MFC stack immediately below was not immediately feasible. Recirculation of the

effluent was also not suitable due to the long HRTs employed (degradation of the substrate

by planktonic bacteria would have continued regardless, so the feedstock would have been

inconsistent, with continuously decreasing COD over time). Further degradation capabilities


were thus ‘simulated’ i.e. estimated by switching the feedstock, at the top of the cascade, to

a concentration of ~230 mgCOD L-1 (obtained by the dilution of 116mL of 5mM Acetate TYE

into 1L of distilled water). This may have not been the most ideal simulation scenario, since

it eliminated the bacterial component from the feed, however it was one practical way of

guaranteeing constant COD feeding for prolonged periods of time at the levels exhibited by

the original stack’s effluent. The combined results of COD degradation for the 6MFCs at ~2

gCOD.L-1 and then 6MFCs at ~230 mgCOD L-1 are presented in Fig. 3.

With the ‘simulated’ addition of 3 more MFCs, ΔCOD reached 95.3%, while no significant

difference (in percentage terms) was found when the anolyte passed through more cells

(see Fig. 3: MFCs 10, 11 and 12). With the presented operating conditions and OLR, a

cascade of 9 fuel cells would have thus been sufficient to obtain an effluent of acceptable

quality in terms of COD (effluent: 92.5 ± 6.36 mgCOD L-1) and a very competitive removal

rate of 7.97 kgCOD m-3 day-1 (Tchobanoglous et al., 2003).

2.2.2 COD reduction for Artificial wastewater

The artificial wastewater displayed a COD value of 164 mg L-1 for an OLR of 1.04 kgCOD m-3

d-1. Despite the lower concentrations, a wastewater of such levels would still be above EU

WWTD regulations therefore requiring treatment before release. When this artificial

wastewater was used as the anolyte feed, reduction was found to be 39.1% for the 6MFC

stack (ΔCOD 64± 3.61 mgCOD L-1), resulting in an effluent with ~100 mgCOD L-1 (see Fig. 3).

This is satisfactory in terms of COD, and in fact it was found that for such low-concentration


effluents, a 3MFC-staging was sufficient to reach adequate levels (ΔCOD: 31.7%, effluent:

112±5.0 mgCOD L-1).

2.3. Series connection: the need for electrical insulation

After the parallel configuration (see Fig. 4a) experiment was concluded, the stack’s electrical

connections were switched to a series configuration, with all the other experimental

conditions remaining the same. The OCV of the stack was found to be ~2,270 mV, which

was considerably less than expected (3275.44 mV based on multiplying by 6 individual

OCVs). This drop has also been observed in other investigations (Aelterman et al., 2006;

Ieropoulos et al., 2008; Wang & Han, 2009; Winfield et al., 2011; Zhuang & Zhou, 2009;

Zhuang et al., 2012) and linked to ionic cross-conduction between cells’ anodes (conductive

electrolytes create an electric bridge between anodes, but these are also linked via external

cables to cathodes due to series configuration), creating “antagonistic paths” that negatively

affect electrical performance. In the current study this effect was not caused by the anolyte

(fluidically/electrically insulated by air gaps) but by the catholyte, which was not flowing in

an isolated manner; this is illustrated in Fig. 4b. The tap water conductivity was ~680 μS cm-

1 compared to ~350 μS cm-1 for the anolyte, so the effects of cross-conductance could be

anticipated to be more deleterious than in the case of anolyte cross-conduction.

Experiments under electrical load showed that the power production was 97.6% lower than

that produced under parallel-connection mode (MPD: 0.18 W m-3; MCD: 0.55 A m-3). The

system was left to operate for one further month under these conditions, but no


improvement was observed (MPD: 0.13 W m-3; MCD: 0.55 A m-3), emphasising that this was

not a transient effect i.e. change of anodic potentials affecting biofilms.

To resolve this cross-conduction problem, other investigators (Kim et al., 2012; Winfield et

al., 2011; Zhuang & Zhou, 2009) have focussed on increasing the separation distance in

fluidically linked MFCs. In both cases, a larger separation seemed to reduce, although not

completely eliminate, the negative effects of anolyte cross-conductance. The separation

distance (originally 15 cm) in the present study was accordingly increased up to 250cm but

no significant improvement was observed (data not shown).

When a true air-gap system was introduced for the cathodes identical to the one for the

anolyte system (see Fig. 4c), power output increased more than 27-fold (MPD: 4.86 W m-3),

demonstrating the positive effect of fluidically/electrically insulating the MFCs in the

cascade. This improved performance was nevertheless inferior to the average production of

reference individual cells (MPD: 6.83 W m-3). Following two months of continuous

operation, the performance of the stack surpassed that of the reference cells,

demonstrating that fluidic isolation in a series connected cascade is fundamental for

improved operation. As shown in Fig. 5, the air-gap isolated Cascade-stack produced similar

levels of power output as when connected in parallel (in accordance with the laws of Joule

and Ohm), but in fact achieved an overall maximum power density of 9.29 W m-3 (vs 6.83 W

m-3 for individual cells) and a current production of 15.55 A m-3 (higher than the expected

MCD 85 A m-3/6MFCs = 14.17 A m-3) with an identical OCV (3253.12 mV vs 3275.44 mV) to

the mathematical sum of the individual OCVs presented in part 1.


These results clearly demonstrate that it is possible to operate stacks in series without the

voltage losses observed by others and with the added advantage of the positive stacking-

effect (higher power and current densities than for single cells; see Fig. 5), provided that

correct electrical insulation is employed (Fig. 4c). The latter was critical in the present study

for optimum performance, which is in contrast to Dekker et al. (2009) who stated that no

electrical insulation is required for adequate stack performance.

2.4. Series connection: no reversal under substrate imbalance

The cascade-MFC system presented herewith operated under substrate imbalance

conditions by default, since each of the cells in the stream receive a completely different

concentration of nutrients, as shown in Fig. 3. Even under these adverse conditions of non-

uniformity, in a period of >6 months of continuous operation in series, no voltage reversal

was observed (even when the stack was subject to the electrical shorting described in 2.3).

These results are in agreement with Zhuang et al. (2012) , whereby a 5-stage MFC stack

connected in series and with imbalanced substrate concentrations never showed reversal.

Both investigations demonstrate that substrate imbalance is not the cause for reversal in

MFCs. The electrical reasons for cell reversal are discussed in the next section.

2.5. Series and parallel connections: resilience to starvation

The results of the starvation cycles introduced in-between the several months of testing are

depicted in Fig. 6. In parallel configuration, the stack showed the capability to fully recover This is a post-print version of the following article: Ledezma, Pablo, Greenman, John and Ieropoulos, Ioannis (2013) MFC-cascade stacks maximise COD reduction and avoid voltage reversal under adverse conditions. Bioresource Technology, 134 158-165. doi:10.1016/j.biortech.2013.01.119

to pre-starvation current production levels in single-dip (see B, Fig. 6a), double-dip (see A,

Fig. 6a; both of ~100 h) and long-term deprivation cycles (~15 days for cycle C). In series

configuration (see cycles D-G, Fig. 6b), full recovery of power levels was also achieved after

every cycle of starvation, independent of the length of time. Nutrient depletion was shown

to have no adverse effects on the performance and in addition it did not affect the

continuous improvement of the stack’s output (2.60 to 7.25 W m-3) after the electrical

shorting described in 2.3. These results demonstrate that starvation does not necessarily

lead to long-term deleterious effects as previously reported (Oh & Logan, 2007). It has long

been known that nutrient-deprived bacteria switch reversibly to a state of metabolic arrest

(Amy & Morita, 1983). This is also the case for biofilms, but the latter tend to be more

resistant to starvation than planktonic cultures, amongst other adverse conditions (Lappin-

Scott & Costerton, 2003). Starved biofilms have been shown to express starvation-specific

genes, which can bring cells to a state where little or no growth occurs (Chávez de Paz et al.,

2008). Since near-identical levels of production were found post-starvation for all cycles

(See Fig. 6), and recovery was observed in a period of time equivalent to <3 volumetric

replacements (~8 h in all cases except for C; 3.79h for a full volumetric replacement at set

flow rate), it is hypothesised that the grand majority of the cells in the biofilms remained

viable during the deprivation cycles but were dormant, as recently shown in simulations by

Ayati & Klapper (2012). Further research is needed to verify whether this is the case and

also to understand why the recovery in cycle C took considerably longer (ca. 47 h, perhaps

involving recolonisation/replacement of dead cells) but still resulted in levelled performance

(12.52 A m-3 pre vs 12.40 A m-3 post-starvation).


It should be noted that the Cascade-MFC design allows for synchronous effects in starvation:

when the feed is stopped at the top, all of the cells are starved at the same time, which

allows for a “balanced” drop in bacterial activity that is thought to help maintain all cells’

potentials positive. Localised (“imbalanced”) starvation has not been deeply studied in

MFCs, but on PEMFC stacks it has been shown to cause reversal and even chemical damage

to anodes (Ferreira-Aparicio et al., 2009). Voltage reversal in fact is a well understood

phenomenon in electronic circuit and battery theories, linked to an imbalanced increase in

Rint of a cell that is part of a stack (Ieropoulos et al., 2010a). Although the reasons of Rint

increase can be varied, including electrolyte depletion or membrane fouling, the ultimate

cause for reversal is the increase of internal resistance and not substrate imbalance or

starvation per se. A thorough explanation particularly oriented to MFCs of this

phenomenon has been previously detailed by Greenman et al. (2011) .

Further research is needed to fully understand what the actual effects of starvation are on

MFC biofilms and stackability, but undoubtedly, starvation should not necessarily be

avoided. Selectively depriving MFCs could result in the necessary robustness to operate with

e.g. household wastewaters, where the presence of antimicrobial agents is ever increasing.

Conclusion

This investigation showcases MFC-Cascades as an uncomplicated and effective way to

reduce >96% COD from high-load effluents to acceptable environmental standards with


concomitant electricity production. In terms of MFC stacking, it is demonstrated that a

beneficial effect occurs in the form of superior performance (higher power and current

densities) when multiple units are combined together in comparison to single units,

provided correct conditions are set, particularly fluidic/electrical isolation when in series

configuration. The results also tackle the idea that voltage reversal, and thus unstable

performance, are an obligated occurrence when MFC stacks are operated in series or

starved.

Acknowledgements

The authors would like to thank Sam Coupland of the Bristol Robotics Laboratory for his help

in the design and fabrication of the MFC parts. Pablo Ledezma is supported by the National

Science and Technology Council of Mexico (CONACYT) Ref. 206298. Ioannis Ieropoulos is

supported by the Engineering and Physical Sciences Research Council of the UK (EPSRC) CAF

EP/I004653/1.


Figures

Fig. 1. 3D CAD model of the stack; MFCs 1 to 6 are numbered according to their position in

the cascade.

0 10 20 30 40 500

1

2

3

4

MFC1MFC2MFC3MFC4MFC5MFC6

Cell MPD [W.m -3] CCD [A.m -3]MFC1 3.69 14.32MFC2 3.23 13.39MFC3 3.12 13.16MFC4 2.87 10.43MFC5 2.47 7.67MFC6 2.19 7.22

Current density [A.m -3 ]

Powe

r den

sity [

W.m

-3]

Fig. 2. Polarisation experiment of 6 MFCs in cascade with a single feed at the top – individual

operation. Inset table: Maximum Power Density (MPD) points achieved during the

polarisation for each MFC (normalised); the number indicates the position on the cascade

(see Fig. 1). CCD: Corresponding Current Density (normalised) for the MPPs.


0 1 2 3 4 5 6 7 8 9 10 11 120

500

1000

1500

2000

5mM Acetate + TYE 15mM Acetate + TYE 2

125

Artificial wastewater

EU WWTD

96%

88.4%

31.7% 95

.8%95.6%95.3%

64.1%

Number of MFCs in the cascade

COD [

mg.L

-1]

Fig. 3. Combined COD reduction over number of MFCs for 5 mM Acetate TYE (average of

two independent repeats marked with and symbols) and artificial wastewater (avg. in

pink ). The dashed curves indicate the overall tendency in COD reduction. The horizontal

limit at 125 mg.L-1 (in green) marks the minimum acceptable levels of COD for release into

the environment according to the EU WWTD. Inset boxes indicate the amount of COD

reduction in percentage, relative to the original COD of the feed.


Fig. 4. Different stack configurations (simplified to 2MFCs). a. Connections in parallel with

the original fluidic configuration: the flow of electrons (indicated by arrows) is concurrent

with the cross-conduction (2-way arrow) between fluidically-linked (see blue tube)

cathodes, resulting in no observable losses. b. Change to series configuration: electric

cables (black) now link the anode (bot. left; yellow) to the top cathode (top right; pink);

however, due to cross-conductance between cathodes (fluidic bridge in blue), some of the

electrons produced at the bottom anode end up at the bottom cathode (bot. right) without

passing through the external load (see red curved arrow), resulting in a direct short-circuit

or ‘shunt losses’ which negatively affect the performance measured at Rext . c. With the

introduction of fluidic insulation between cathodes (mirror image of the anode structure),

‘shunt’ losses are avoided and the stack exhibits a performance equivalent to the theoretical

prediction.


0 20 40 60 80 1000

2

4

6

8

10

6MFC stack in series6MFC stack in parallel

Individual MFCs (avg.)

Current density [A.m -3 ]

Powe

r den

sity [

W.m

-3]

Fig. 5. Comparison of power curves between reference single MFC units and a 6MFC stack

connected in parallel or series configurations, showcasing the positive stacking effect.


0 200 400 600 8000

5

10

15

20

A B C

Time [hours]

Curre

nt de

nsity

[A.m

-3]

0 100 200 300 4000

2

4

6

8

D E

F G

Time [hours]

Powe

r den

sity [

W.m

-3]


Fig. 6. Starvation cycles of a 6MFC cascade stack connected in a: parallel and b: series

configuration. All cycles are presented to scale, but were carried out during the several

months of testing (discontinuity is marked by ). : Start of a cycle (anolyte pump off). :

restart of the feeding (pump on). The length and chronological separation of the cycles are

as follows: A: 110.6 h; 3 weeks after joining the 6MFCs in parallel. B: 106.6 h; 5 weeks later.

C: 355 h; 5 weeks later. D: 51.9 h; 1 week after connecting the stack with true insulation

between feeds. E: 78 h; 2 weeks later. F: 43 h; 10 weeks after E. G: 50.5 h; 4 weeks after F.


Tables

Table 1.

a b

6MFC StackIndividual

MFCs (avg.)6MFC Stack

Individual

MFCs (avg.)

MPP (µW) 120.9 17.3 268.36 40.99

Equiv. MPP per MFC (µW) 20.15 17.3 44.73 40.99

MPD (W.m-3) 3.35 2.88 7.46 6.83

MCP (mA) 1.39 0.17 2.63 0.41

Equiv. MCP per MFC (mA) 0.23 0.17 3.28 0.41

MCD (A.m-3) 38.61 28.33 91.11 85

Table captions

Table 1. Comparative results of polarisation sweeps between a 6MFC stack and single individual cells. a: after one week in parallel configuration. b: after 8 months in parallel mode. MPP: Maximum power point. MPD: Maximum power density. MCP: Maximum current point. MPD: Maximum power density.


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