Enhanced electricity generation by using algae ...

Science of the Total Environment 456–457 (2013) 91–94

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Short Communication

Enhanced electricity generation by using algae biomass and activated sludge in microbial fuel cell Naim Rashid a, b, Yu-Feng Cui a, Saif Ur Rehman Muhammad a, Jong-In Han a,⁎ a b

Dept. of Civil and Environmental Engineering, Korea Advance Institute of Science and Technology (KAIST), 373-1 Guseong dong, Yuseong-gu, Daejon 305-701, Republic of Korea Dept. of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, 22060, Pakistan

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Electricity is generated using wastes. • Algae biomass and activated sludge are used as substrate in microbial fuel cell. • A dry algae biomass of 5g/L resulted the power output of 1.78W/m2. • Pre-treatment of activated sludge improved electricity production. • Lipid extracted algae is tested to replace whole algae for low cost electricity production.

a r t i c l e

i n f o

Article history: Received 16 January 2013 Received in revised form 13 March 2013 Accepted 17 March 2013 Available online xxxx Keywords: Algae biomass Activated sludge Electricity Microbial fuel cell Pre-treatment

a b s t r a c t Recently, interest is growing to explore low-cost and sustainable means of energy production. In this study, we have exploited the potential of sustainable energy production from wastes. Activated sludge and algae biomass are used as substrates in microbial fuel cell (MFC) to produce electricity. Activated sludge is used at anode as inoculum and nutrient source. Various concentrations (1–5 g/L) of dry algae biomass are tested. Among tested concentrations, 5 g/L (5000 mg COD/L) produced the highest voltage of 0.89 V and power density of 1.78 W/m2 under 1000 Ω electric resistance. Pre-treated algae biomass and activated sludge are also used at anode. They give low power output than without pre-treatment. Spent algae biomass is tested to replace whole (before oil extraction) algae biomass as a substrate, but it gives low power output. This work has proved the concept of using algae biomass in MFC for high energy output. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Currently, the world is confronting with the challenges of high energy demand and escalating fuel prices (Love et al., 2011). To cope ⁎ Corresponding author. Tel.: +82 42350 3629; fax: +82 42350 3610. E-mail address: [email protected] (J.-I. Han). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.03.067

with such challenges, researchers have paid considerable attention to introduce sustainable and cost-effective methods of energy production (Yinghua et al., 2013; Yuri, 1994; Scacchi et al., 2010). In this regard, microbial fuel cell (MFC) has grasped widespread attention (Zhou et al., 2012). MFC offers tremendous potential to produce energy in the form of electricity (Venkata et al., 2008). “MFC is a bioelectrical chemical system for power generation based on the exploitation of

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biocatalytic reactions with active microbial fuels” (Rosenbaum and Angenent, 2010). MFC is composed of two components, anode and cathode. At anode, organic compounds are oxidized and electrons are liberated (Wen et al., 2011). These electrons move through an external circuit to the cathode. At cathode, electrons combine with an electron acceptor (Rosenbaum and Angenent, 2010; Wang et al., 2010) to generate electricity. Generally, pure organic compounds such as acetate, glucose and cysteine are used as substrate at anode. However, their use is economically unviable (Vlyssides and Karlis, 2004). Alternatively, the mixture of biodegradable materials, such as domestic wastewater, piggery wastewater, and animal wastewater can be used as substrate (Martin et al., 2010; Rodrigo et al., 2007). Such types of substrates have some limitations. For example, the electricity generation from pure substrates is economically unviable (Velasquez-Orta et al., 2011). On the other hand, power output of mixed organic compounds is quite low (Wen et al., 2011). To improve the power output and to make MFC technology cost-effective, new substrates should be introduced (Yuri, 1994). In this perspective, the use of algae biomass as a substrate can provide an opportunity for sustainable and low-cost production of electricity (Velasquez-Orta et al., 2009). Algae are potential pollution vector in lakes and ponds causing eutrophication (Bubrick, 1991). A large amount of algae residues is produced during water treatment process. These residues are discharged into the sewer system without pre-treatment, which increases the pollution load in downstream wastewater plants (Bringmann and Kühn, 1980). Therefore, algae residues have to be removed before being discharged into the environment. Algae residues can be used to produce a wide variety of biofuels such as methane, hydrogen, biodiesel and bioethanol (Karube et al., 1986; Scott et al., 2010). However, the biofuel production from algae biomass is not cost-effective (Lehr and Posten, 2009). To use algae biomass in MFC is a promising approach to generate electricity. This approach serves dual purpose, waste treatment and electricity generation. Certain bacteria such as Alcaligenes faecalis, Enterococcus gallinarum, Pseudomonas aeruginosa, and Shewanella use algae biomass as nutrient and transfer electrons to the electrodes. Activated sludge is reported to contain various types of electricity producing bacteria. Therefore, activated sludge is used as an inoculum at anode. However, the bacteria can't consume algae biomass directly due to its strong cell wall. Pre-treatment is a necessary step to break the cell wall and to make it digestible for the microbes. Various pre-treatment techniques can be used to make algae biomass digestible for the microorganisms. In this study, we have proposed to use algae biomass as a substrate for high power output. A mixture of activated sludge and algae biomass is used as anolyte. Algae biomass, which is composed of proteins, carbohydrates and lipids, serves as nutrient source for the microbes present in activated sludge. Thermal and sonication pre-treatments are employed to improve net power output. 2. Material and methods 2.1. Algae biomass and activated sludge pre-treatment Scenedesmus was selected as algae specie due to its high lipid contents and its use as nutrient source for the microbes, at anode. It was obtained from Korea Research Institute of Bioscience and Biotechnology (KRIBB), Republic of Korea. Scenedesmus was grown in an open pond system. Its biomass was collected after 15 day cultivation and dried at 40 °C until constant weight. For pre-treatment experiments, dry algae biomass (after mixing with the medium, described in Section 2.3.) was sonicated at 20 KHz at maximum power output of 2.2 kW for 2 min (Choi et al., 2011). Two types of pre-treatment were applied to activated sludge: (1) sonication and (2) thermal pre-treatment. The sonication conditions applied for algae biomass were used for activated sludge as well. For thermal pre-treatment, activated sludge was kept at 70 °C for 24 h (Vlyssides and Karlis, 2004).

2.2. MFC design Cubical, dual-chamber reactors made by acrylic with each electrode chamber holding a volume of 112 mL (7 cm × 4 cm × 4 cm) were used in this study. Both, anode and cathode had working volumes of 100 mL. The reactor was separated by cation exchange membrane (CEM, CMI-7000, Membrane International, Inc. USA) with an effective surface area of 16 cm2 (4 cm × 4 cm). A carbon brush of same configuration (L = 2.5 cm, D = 2.5 cm) connected with titanium wire was used as cathode electrode. 0.4 mg/cm2 Pt catalyst (20 wt% Pt/C, Alfa Aesar) was coated on both sides of carbon cloth. All experimental stuff was autoclaved at 121 °C for 20 min (Velasquez-Orta et al., 2009). 2.3. Operation A mixture of activated sludge 50% (w/w), collected from Daejeon Sewage Treatment Plant, Daejeon (Metropolitan City, Republic of Korea) and media 50% (w/w) containing KH2PO4 5356 mg, K2HPO43H2O 164 mg, Na2HPO4 12H2O 11867 mg, MgCl26H2O 100 mg, NaHCO3 1000 mg, CaCl22H2O 66 mg, NH4Cl 500 mg, MnSO46H2O 15 mg, FeSO47H2O 25 mg, CuSO45H2O 5 mg, CoCl25H2O 0.0125 mg, NiSO4 32 mg, ZnCl2 23 mg, and (NH4)6Mo7O24H2O 14 mg (per liter) (Yang et al., 2011) was pumped into the anode compartment. Activated sludge served as anodic inoculum (Patil et al., 2009). Aeration at a flow rate of 50 mL/min was provided in cathode during the MFC start-up operation. The MFC start-up operation was carried out in fed-batch mode at 25 ± 2 °C. After three acclimation cycles, a replacement of total solution only with substrate and medium was conducted and afterwards reproducible voltages were successfully obtained. Subsequently, power densities at an external resistance of 1000 Ω at different concentrations of substrate were measured. Electrochemical properties were monitored with a data acquisition system (Model 2700, Keithley instrument) connected to a personal computer at 10 min intervals. The study-state voltage and current were measured as I = V/R, where V is the measured voltage, R is the external resistance and I is the current. The polarization curve was obtained by plotting voltage vs current density. The power density (P) was calculated from the measured voltage as P = V2/RA, where A is the projected cathode surface area (Yuan et al., 2011). For discharge operation, an external resistance of 1000 Ω was connected to the circuit (Logan and Regan, 2006). All polarization experiments were conducted three times. 2.4. Analysis Chemical oxygen demand (COD), protein, and carbohydrate were measured by standard methods (American Public Health Association, APHA, 1995) (Velasquez-Orta et al., 2009). Total solids (TS) and volatile solids (VS) were measured by standard method (Method 5220, APHA 1995) (Cheng et al., 2006). Samples were filtered through 0.45 μm (pore diameter). The nitrate and phosphate contents of filtrates were measured with reaction kit TNT plus HR (5–35 mg/L NO3-N, TNT 836, Hach) and HR (1.0–100 mg/L, TNT 10127, Hach), respectively. 3. Results and discussion 3.1. Electricity generation using activated sludge Activated sludge was used at anode to generate voltage. The voltage produced by pre-treated and without pretreated activated sludge (as control) was compared. In pre-treatment, thermal and sonication techniques were employed according to the method mentioned in Section 2.1. The basic characteristics of activated sludge are given in Table 1. A voltage of 0.289 V was produced in un-pre-treated, 0.242 V by thermal pre-treated and 0.400 V by sonicated sludge (Fig. 1). This shows the positive effect of sonication on the voltage. The increase in


1 0.9 Unit

Value

TS VS Total COD Soluble COD Total carbohydrate (soluble) Total protein Total nitrogen Total phosphate pH

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L –

7980 6380 10110 15.3 32.4 93.06 11.23 5 6.2

± ± ± ± ± ± ± ± ±

50 50 460 5.3 0.26 3.06 3.66 1 0.05

voltage is probably because sonication disintegrates the activated sludge and increases the dissolved carbohydrate concentration and chemical oxygen demand (COD) in the electrolyte (Choi et al., 2011). Denaturing Gradient Gel Electrophoresis (DGGE) analysis of anolyte showed that Desulfovibrio sp., P. aeruginosa, Cytophaga xylanolytica, Dechloromonas sp., Thiomonas perometabolis, and Cytophaga sp. were dominant bacteria to produce electricity (data not shown). In our experiment, the soluble COD was increased from 15.3 ± 5.3 in un-pretreated to 500 ± 5 in pre-treated activated sludge mg/L (data not shown). Increased carbohydrate and COD allowed the bacteria to grow promptly and generate high voltage (Min et al., 2005). Decrease in voltage in thermal pre-treated activated sludge could be due to the suppression of bacterial activity at high temperature. Similar fashion of voltage decrease was shown by Min et al. (2005). 3.2. Electricity generation using algae biomass as substrate A mixture of activated sludge (as an inoculum), medium (as nutrient source) and algae biomass (as substrate) was first fed to the system, for conditioning purpose. Different concentrations (1, 2 and 5 g/L) of algae biomass were used to find the optimal concentration (Fig. 2). A fixed concentration of sludge (50% w/w) and the medium (50% w/w) was used. Open circuit voltage was quantified at each concentration. The initial voltage was accompanied with biomass concentration. The highest initial voltage of 0.89 V was quantified at 5 g/L (5000 mg COD/L) concentration followed by 0.86 V at 2 g/L (2000 mg COD/L) and 0.84 V at 1 g/L (1000 mg COD/L), respectively. The increased voltage at high biomass concentration is because the potential to release electrons is accompanied with biomass concentration. The voltage increased when the microbes had enough food to use in the form of algae biomass. The maximum power density was 1.78 W/m 2 under 1000 Ω using 5 g/L of algae biomass. The power densities were 0.85 W/m2 and 0.56 W/m2 at algae biomass of 2 g/L and 1 g/L. The cell voltage dropped to zero at 1.93 A/m2 and 1 g/L biomass, 2.27 A/m2 at 2 g/L biomass and 4.6 A/m2

0.45 0.4 Sonicate pretreated sludge

Cell voltage, V

Thermal pretreated sludge

1.8 1.6

0.7

1.4

0.6

1.2

0.5

1

0.4

0.8

0.3

0.6

0.2

0.4

0.1

0.2

0

0 0

1

2

3

4

5

Current density, A/m2 Fig. 2. Voltage, current and power output as a function of algae biomass in MFC operation.

at 5 g/L of algae biomass (Fig. 2). A positive correlation between current density and open circuit voltage was observed. A negative relationship between the cell voltage and current density was found. The power density decreased with increase in current density. Both, the power density and current density were also the function of algae biomass concentration. High voltage using algae biomass as substrate proves its potential to be used in MFC. A number of studies have been carried out showing the potential of various substrates in MFC; however, the use of algae biomass seems the best choice because of its ability to produce high voltage (Min and Logan, 2004; Powell et al., 2010; Strik et al., 2008; Bucksch and Egeback, 1999; Muhammad et al., 2013; Mehwish et al., 2013). Table 2 shows a comparison of voltage produced by various substrates. 3.3. Electricity generation using pre-treated algae biomass We tested the effect of algae biomass pre-treatment on the voltage. An optimized concentration (5 g/L) of algae biomass was mixed together with activated sludge (50% w/w) and medium. The mixture was pretreated via sonication. The maximum voltage of 0.604 V was produced in un-pre-treated algae biomass and 0.290 V in pre-treated mixture (Fig. 3). It shows the negative effect of sonication. The plausible reason for this phenomenon could not be known yet. We also investigated to replace whole algae (without lipid extraction) with spent algae (lipid extracted) to reduce the overall cost of bioelectricity production as lipid extracted algae contains protein (32.4%), lipid (6.5%), carbohydrate (24.7%) and ash (10%) (Yang et al., 2011). These retaining nutrients could be used by electricity generating bacteria. The voltage produced by lipid extracted algae was 0.021 V only (Fig. 3). Based upon these results, it would be premature to conclude

Untreated sludge

0.25

Table 2 A comparison of power densities using different substrates in MFC.

0.2 0.15 0.1 0.05 0

0.8

Cell voltage, V

Parameters

0.3

2 1g/L 2g/L 5 g/L Voltage Power density

Power density, W/m2

Table 1 Characteristics of activated sludge.

0.35

93

0

10

20

30

40

Time, hrs Fig. 1. Voltage profile of un-treated and pre-treated sludge in MFC operation.

Substrate

Power density, mW/m2

References

Glucose Acetate Acetate Butyrate Butyrate Dextran Starch Swine wastewater Dry algae biomass

212 286 661 220 349 150 242 225 1780

Min et al. (2004) Min et al. (2004) Min et al. (2005) Min et al. (2004) Min et al. (2005) Min et al. (2004) Min et al. (2004) Min et al. (2005) This study

94


0.7 0.6

Cell voltage, V

0.5 0.4

Algae biomass (w/o pretreatment)

0.3

Spent algae biomass

0.2

Sonicated algae biomass

0.1 0 0 -0.1

5

10

15

20

Time, hrs

Fig. 3. Voltage produced by pre-treated, un-treated and spent algae biomass in MFC.

that lipid extracted algae can't be used as substrate to generate bioelectricity. It is likely that toxic chemical retains in lipid extracted algae biomass while extracting lipids, which inhibit the growth of microbes in MFC. Therefore, we suggest further exploring the chemical toxicity effect during lipid extraction and its subsequent use in MFC. Recognizing the potential of lipid extracted algae as MFC substrate would be a meaningful contribution in MFC field. 4. Conclusions This work verifies the concept of using algae biomass as a substrate in MFC. It produces much higher power density than other reported substrates. The use of algae biomass with activated sludge serves dual purpose, the waste mitigation and electricity generation. To introduce algae biomass as a substrate, further developments are required to ensure its cost-effectiveness. Investigating the potential of lipid extracted algae and by exploiting pre-treatment techniques would warrant high energy output at low cost. Acknowledgments This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government Ministry of Education, Science and Technology (MEST) (NRF2012M1A2A2026587). References Bringmann G, Kühn R. Comparison of the toxicity thresholds of water pollutants to bacteria, algae, and protozoa in the cell multiplication inhibition test. Water Res 1980;14:231–41. Bubrick P. Production of astaxanthin from Haematococcus. Bioresour Technol 1991;38: 237–9. Bucksch S, Egeback KE. The Swedish program for investigations concerning biofuels. Sci Total Environ 1999;235:293–303. Cheng S, Liu H, Logan BE. Increased power generation in a continuous flow MFC with advective flow through the porous anode and reduced electrode spacing. Environ Sci Technol 2006;40:2426–32. Choi J-A, Hwang J-H, Dempsey BA, Abou-Shanab RAI, Min B, Song H, et al. Enhancement of fermentative bioenergy (ethanol/hydrogen) production using ultrasonication of

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