50 © IWA Publishing 2011 Water Science & Technology | 64.1 | 2011

Submersible microbial fuel cell for electricity production

from sewage sludge

Yifeng Zhang, Lola Gonzalez Olias, Prawit Kongjan and Irini Angelidaki

ABSTRACT

A submersible microbial fuel cell (SMFC) was utilized to treat sewage sludge and simultaneously

generate electricity. Stable power generation (145± 5 mW/m2, 470 Ω) was produced continuously

from raw sewage sludge for 5.5 days. The maximum power density reached 190± 5 mW/m2. The

corresponding total chemical oxygen demand (TCOD) removal efficiency was 78.1± 0.2% with initial

TCOD of 49.7 g/L. The power generation of SMFC was depended on the sludge concentration, while

dilution of the raw sludge resulted in higher power density. The maximum power density was

saturated at sludge concentration of 17 g-TCOD/L, where 290 mW/m2 was achieved. When effluents

from an anaerobic digester that was fed with raw sludge were used as substrate in the SMFC, a

maximum power density of 318 mW/m2, and a final TCOD removal of 71.9± 0.2% were achieved.

These results have practical implications for development of an effective system to treat sewage

sludge and simultaneously recover energy.

doi: 10.2166/wst.2011.678

Yifeng ZhangLola Gonzalez OliasPrawit KongjanIrini Angelidaki (corresponding author)Department of Environmental Engineering,Technical University of Denmark,DK-2800 Lyngby,DenmarkE-mail: ria@er.dtu.dk

Prawit KongjanPresent address:Department of Science,Faculty of Science and Technology,Prince of Songkla University,Pattani,Thailand 94000

Key words | anaerobic digestion, electricity generation, sewage sludge, submersible microbial fuel

cell, TCOD removal efficiency

INTRODUCTION

Sludge management has become a key issue in wastewatertreatment, as it represents 20–60% of the total expensesof conventional wastewater treatment plants (Zabranskaet al. ; Uggetti et al. ). Anaerobic digestion is a typi-

cal technology for sewage sludge treatment due to its lowenergy consumption and potential energy (biogas) recovery(Verstraete et al. ). However, the conventional anaero-

bic digestion processes require a long digestion time (�20days) and excessive energy input (Skiadas et al. ). Analternative strategy is direct conversion of excess sludge to

electrical energy in microbial fuel cells (MFCs).MFCs are devices that use bacteria as the catalysts to

oxidize organic matter and generate electricity (Logan et al.; Min & Angelidaki ; Rabaey & Keller ;

Logan ). In comparison with a conventional anaerobicdigestion process, MFC technology has many specific advan-tages, such as the possibility to be operated at low substrates

concentration and temperatures below 20 WC, under whichconditions the anaerobic digestion is difficult to function.These advantages imply that MFC technology can find appli-

cation niches and be a good complement for anaerobicdigestion technology (Pham et al. ). Moreover, various

organic matters such as simple carbohydrates, lowmolecularweight organic acids, starch, amino acids, chitin, cellulose,domestic wastewaters, food process wastewater, marinesediment organic matter and manure sludge waste have

been successfully utilized for power generation in MFCs(Min & Logan ; Liu et al. ; Logan ; Oh &Logan ; Rismani-Yazdi et al. ; Scott & Murano

; He et al. ; Zhang et al. ). The feasibility ofdirect conversion of sewage sludge into electricity viaMFC has also been proposed in a few studies (Behera &

Ghangrekar ; Jiang et al. ). However, all thesestudies used a two-chambered MFC configuration with achemical cathode (e.g. ferricyanide as electron acceptor),which face important limitations in terms of large-scale appli-

cation (Behera & Ghangrekar ; Jiang et al. ).Therefore, more effort should focus on reactor configuration,power density and waste removal to render the MFC

technology more economically feasible and applicable(Pham et al. ).

Considering the above challenges, a more cost-effective

reactor configuration needs to be developed for sewagesludge treatment. In this study, a submersible MFC

Figure 1 | Schematic diagram of the SMFC.

51 Y. Zhang et al. | Electricity production from sewage sludge Water Science & Technology | 64.1 | 2011

(SMFC) (Min & Angelidaki ), was used for sludge treat-

ment and electricity generation. In the single-chamberedSMFC, a specially designed anode chamber was notneeded, which reduces the construction costs and makes it

easy to apply in natural anaerobic environments or existinganaerobic reactor, for treatment of sludge and recoveringenergy in the form of electricity. Moreover, the treatmentefficiency and electricity generation could be increased

due to the close distance of the anode and cathode elec-trode. The aim was to make a more compact and simplesystem with reduced construction and operation cost.

Thus, the objectives of this study were to (i) test the feasi-bility of the SMFC for direct conversion of sewage sludgeinto electricity in an anaerobic reactor, and (ii) study the

possibility of linking the SMFC with anaerobic digestion totreat sewage sludge and simultaneously recover energy.

MATERIALS AND METHODS

Inoculum and medium

The raw sludge was the sludge mixture collected from theprimary and secondary sedimentation basin at LyngbyWastewater Treatment Plant, Copenhagen, Denmark, andwas utilized as both the inoculum and fuel. Raw sludge

pH was 5.7± 0.3 and had total chemical oxygen demandcontent (TCOD) of 49.3± 2.7 g/L. The suspended solidscontent (SSs) in the sludge sample was 11.3± 0.1 g/L. The

effluent of an anaerobic digester (AD), which was used forbiogas production from the raw sludge, was collected atthe same plant. The effluent pH was 7.0± 0.3 and had a

TCOD of 14.8± 0.2 g/L. The SSs content in the sludgesample was 14.75± 0.1 g/L. The collected sludge and ADeffluent samples were first purged with nitrogen gas and

then were stored at 4 WC prior to use. For testing the effectof TCOD concentration on the electricity generation,the sludge was diluted to the desired concentration (5.0–49.7 g-TCOD/L, as indicated) using tap water.

SMFC configuration and operation

The SMFC used in this study was constructed asdescribed by Min and Angelidaki (Min & Angelidaki

). The schematic of the SMFC is shown in Figure 1.The SMFC was composed of an anode electrode and anonconductive polycarbonate plate making a rectangular

cathode chamber (3 × 3 × 1 cm, 9 cm3), which were sub-mersed in a anaerobic glass reactor (total volume of

1,000 mL, liquid volume of 600 mL). The cathodechamber was connected with two plastic tubes for airsupply (or air out). The anode electrode was a 3 × 3 cm

piece of not wet-proofed carbon paper (Toray carbonpaper, E-TEK division, USA). The cathode electrode wasa 5% wet proofed carbon paper (3 × 3 cm), and one side

of the electrode was coated with Pt (0.5 mg/cm2 with20% pt, E-TEK). The Pt coated side was hot pressedwith a proton exchange membrane (Nafion 117, DuPont

Co., USA) prepared as previously described (Min &Logan ). The distance of the anode and cathode elec-trode was approximately 0.2 cm in the reactor. Electrical

connections and pretreatment of electrodes were done aspreviously described (Zhang et al. ).

The raw sludge was firstly diluted ten times with tapwater and then used as the inoculum and fuel to start-up

the reactor. Following inoculation and stable power gener-ation, the diluted sludge was replaced with the raw sludge.The system was considered to be ready for stable electricity

production when the maximum voltage output of one batchcycle was reproducible after filling the reactor with freshfuel, at least twice. The fuel in the reactor was refilled

when the voltage dropped below 50 mV (resistance of470 Ω). The cathode chamber was continuously flushedwith air at a flow rate of 5 mL/min. Magnetic stirring (300rpm) was used to ensure effective mixing in the reactor.

The reactors were operated at room temperature (25± 3 WC).All electrode transfers and inoculation procedures wereconducted in an anaerobic glove box (Coy Scientific Pro-

ducts). All liquid samples were taken from the middle levelof the anaerobic glass reactor, where the SMFC was sub-mersed (Figure 1). All experiments were carried out in

duplicate.

Figure 2 | Power generation and TCOD removal with time from raw sludge.

52 Y. Zhang et al. | Electricity production from sewage sludge Water Science & Technology | 64.1 | 2011

Analyses and calculations

Standard method (APHA/AWWA/WPCF ) wasemployed to analyze TCOD, soluble chemical oxygen

demand (SCOD), total solid (TS), volatile solids (VS), andvolatile suspended solids (VSS). Conductivity was deter-mined using a CDM83 conductivity meter (Radiometer),and pH was measured with a PHM210 pH meter (Radio-

meter). Volatile Fatty acids (VFAs) were measured by gaschromatography (Agilent 6890) as previously described(Zhang et al. ). Hydrogen and methane in the headspace

of the anaerobic glass reactor (shown in Figure 1) were ana-lyzed using a gas chromatograph (MicroLab, Arhus,Denmark) equipped with a thermal conductivity detector

(TCD) and a s-m stainless column packed with Porapak Q(50/80 mesh). Nitrogen is used as the carrier gas.

The voltage across an external resistor in the MFC cir-cuit was monitored with 10 min intervals using a

multimeter (Model 2700, keithley Instruments, Inc., Cleve-land, OH, USA) linked to a differential multiplexer (Model7701, Keithley Instruments, Inc.). Except where specially

stated, the external resistor was 470 Ω. The current intensity(I) was calculated according to Ohm’s law, I¼V/R, whereV is the voltage and R is the resistance. Power density

(P¼ IV/A), and Coulombic efficiency (CE) were calculatedas previously described, with the power density normalizedby the projected surface area of the anode (Liu et al. ).In a polarization curve test, the external resistor wasvaried ranging 10 to 10,000 Ω to determine the maximumpower density and internal resistance of SMFC.

RESULTS AND DISCUSSION

Electricity generation from raw sludge

Following the enrichment procedure, a stable voltage withraw sludge (TCOD of 49.7± 2.7 g/L) was generated afterthree additional transfers (approx. 20 days). An example of

one cycle of power generation and TCOD removal effi-ciency in SMFC with the raw sludge is shown in Figure 2.The stable power density of SMFC reached 145± 5 mW/m2 (0.24± 0.05 V) with a period of around 5.5 days

(Figure 2). Accordingly, TCOD of raw sludge graduallydecreased with operation time, from the initial 49.7± 2.7to 10.8± 0.5 g/L after 5.5 days operation, resulting in a

removal efficiency of 78.1± 0.2%. The corresponding cou-lombic efficiency was 3.3± 0.1%, indicating that some

electrons derived from carbon have been consumed by

other mechanisms (e.g. bacterial biomass generation,aerobic oxidation due to oxygen diffusion from cathode,neutral metabolites diffusing to the cathode chamber) than

power generation (He et al. ; Lee et al. ). Themethane and hydrogen were less than 1% in the headspacein any of the experiments (data not shown), which could bedue to the low temperature and short operation time in this

study. However, the TCOD removal efficiency and CEobserved here were still much higher than those of previousstudies, in which excess sewage sludge was treated and

maximum TCOD removal of 46.4% and CE of 0.96% wereobserved in a two-chambered MFCs (Behera & Ghangrekar; Jiang et al. ). The more compact configuration and

relative shorter electrodes distance of the SMFC couldreduce the ohmic loss and increase electron driving force,which would thereby increase CE and accelerate substrate

degradation as observed in this study. The above resultsclearly demonstrate that the SMFC can generate electricityfrom sewage sludge.

Cell voltage and power output as a function ofcurrent density

The polarization curve (Figure 3) showed an open circuitvoltage (OCV) of 0.36 V (0 mA/m2) was first obtained with

raw sludge. However, with the current density increasingto approx. 327 mA/m2, the voltage decreased sharply to0.29 V possibly showing the presence of activation overvol-tage at the lower current generation (Logan et al. ).Then the voltage decreased almost linearly to 0.07 V(1,817 mA/m2) indicating that the ohmic overvoltage gener-ated by membrane resistance and electrolyte resistance

affected the power output. The change of voltage with cur-rent density is in good agreement with results previously

Figure 3 | Variation of power output and voltage with different currents by using

the raw sludge.

Figure 4 | Effect of TCOD concentration of sludge on power density of SMFC.

Figure 5 | Variation of power output with different currents by using the anaerobic

digested sludge.

53 Y. Zhang et al. | Electricity production from sewage sludge Water Science & Technology | 64.1 | 2011

reported (Kim et al. ; Jiang et al. ). At a current den-sity of 980 mA/m2 at 220 Ω, a maximum power density of

190± 5 mW/m2 was obtained. The OCV observed (0.36 V)was lower than that obtained with the SMFC in previousstudy, in which 0.72 V of OCV was achieved with acetate

modified wastewater (Min & Angelidaki ). The lowerOCV could be due to oxygen diffusion through the mem-brane which could increase the anodic potential. It could

also be due to the relatively low pH (5.7) in the reactorwhich may affect the anode potential. According to thepolarization curve, the SMFC can be operated effectivelyat external resistance of 220 Ω with maximum power

output (Logan et al. ). This maximum power outputobtained with raw sludge was little higher than the powerdensities of 158 mW/m2 obtained in two chamber MFCs,

when using sludge as substrates at same pH condition (pH6.0), which could be explained by the close distance ofanode and cathode electrode in the SMFC (Fan et al.). It is clear that additional research on advanced elec-trode materials is needed to improve the electricitygeneration of the SMFC from sewage sludge.

Power output at different initial TCOD concentration

In order to investigate the effect of initial TCOD on thepower generation, raw sludge was diluted with tap water

to reach different TCOD concentrations ranging from 5.0to 49.7 g/L (Figure 4). The power density was firstincreased with initial TCOD concentrations, and the maxi-mum value (290± 5 mW/m2) could be obtained at initial

TOCD of 17 g/L. Further improving the initial TCOD con-centration to 49.7 g/L (raw sludge), the power density wasdecreased to 190± 5 mW/m2, which might be due to the

substrate diffusion gradient caused by the high solidcontent at high TCOD concentration. Mass transport

limitation in the poorly mixed sludge might have limitedthe organic matters flux to the anodic biofilm, thusincreased the concentration losses and also internal resist-

ance of the SMFC (Behera & Ghangrekar ). This masstransport limitation might be reduced by diluting the rawsludge with wastewater before feeding for electricity gener-ation, which needs further study.

Electricity generation using the AD effluent

Additional tests were conducted using the AD effluent asfuel in the SMFC. When the AD effluent was added to thereactor the voltage began to increase after 25 h, and then

it stabilized at 0.33 V (258± 5 mW/m2) over the next 167h (data not shown). The polarization curve (Figure 5)taken following stable power generation indicated a maxi-

mum power density of 318± 5 mW/m2 (220 Ω).Concomitant with power generation, TCOD removal rateof AD effluent was increased with operation time, and

achieved 71.9± 0.2% at the end of the electricity generation(<0.05 V) (Figure 6). Considering the fermentation processin the anaerobic digester, more than 90% of raw sludge

Figure 6 | TCOD removal with time by using the anaerobic digested sludge.

54 Y. Zhang et al. | Electricity production from sewage sludge Water Science & Technology | 64.1 | 2011

(TCOD) was removed by linking biogas production withelectricity generation. The COD removal efficiency inan MFC is determined by the total biofilm surface area(Oh & Logan ). Increasing the surface area of the

electrodes per volume of sludge could accelerate the degra-dation of sludge. The maximum power density generatedfrom AD effluent was much higher than that of raw sludge

powered SMFC. The main possible explanation of theimproved performance, could be the higher content ofreadily organic matter (e.g. VFAs), in the AD effluents.

Thus, it was demonstrated that biogas production can besuccessfully linked with electricity generation in a MFCtreatment of sludge. The maximum power density producedhere with the AD effluent was less than that obtained with

glucose or acetate in single chamber MFCs (Liu & Logan; Liu et al. ). However, this maximum power den-sity was much higher than the power achieved using

acetate modified domestic wastewater in the same system(204 mW/m2) (Min & Angelidaki ). These differentpower densities indicated that the type and composition of

organic matter can affect the maximum power densityachievable in an SMFC. Further study of the energy recoverfrom anaerobic digestion-SMFC processes is required and

currently under investigation.

CONCLUSION

Based on the experiments conducted here, the followingconclusions can be drawn:

(1) The SMFC was successfully used for electricity pro-duction from raw sludge and simultaneously degradedthe organic matters. Stable power generation of 145±5 mW/m2 was produced continuously from raw sludgefor 5.5 days, while the maximum power density was

190± 5 mW/m2. The corresponding TCOD removal effi-

ciency was 78.1± 0.2% with initial TCOD of 49.7 g/L.(2) Dilution of the raw sludge resulted in increase of the

power density. The power density was saturated at

TCOD concentration of 17 g/L, while maximum powerdensity of 290± 5 mW/m2 was achieved.

(3) Electricity generation was shown to be possible fromAD effluent of an anaerobic digester fed with raw

sludge. A maximum power density of 318± 5 mW/m2

was produced by the SMFC, while the TCOD removalrate achieved 71.9± 0.2% at the end of the electricity

generation. It is possible to link an MFC to anaerobicdigestion to recover energy from sewage sludge, provid-ing a new method to offset wastewater treatment plant

operating costs.

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First received 10 February 2011; accepted in revised form 6 May 2011