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Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production a
Adam J. Dassey & Chandra S. Theegala
a
a
Department of Biological and Agricultural Engineering, Louisiana State University & LSU AgCenter, Baton Rouge, LA 70803, USA Published online: 29 Oct 2013.
To cite this article: Adam J. Dassey & Chandra S. Theegala (2014) Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production, Environmental Technology, 35:6, 691-697, DOI: 10.1080/09593330.2013.842602 To link to this article: http://dx.doi.org/10.1080/09593330.2013.842602
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Environmental Technology, 2014 Vol. 35, No. 6, 691–697, http://dx.doi.org/10.1080/09593330.2013.842602
Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production Adam J. Dassey and Chandra S. Theegala∗ Department of Biological and Agricultural Engineering, Louisiana State University & LSU AgCenter, Baton Rouge, LA 70803, USA
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(Received 11 April 2013; final version received 4 September 2013 ) Electrocoagulation has shown potential to be a primary microalgae harvesting technique for biodiesel production. However, methods to reduce energy and electrode costs are still necessary for practical application. Electrocoagulation tests were conducted on Nannochloris sp. and Dunaliella sp. using perforated aluminium and iron electrodes under various charge densities. Aluminium electrodes were shown to be more efficient than iron electrodes when harvesting both algal species. Despite the lower harvesting efficiency, however, the iron electrodes were more energy and cost efficient. Operational costs of less than $0.03/L oil were achieved when harvesting Nannochloris sp. with iron electrodes at 35% harvest efficiency, whereas aluminium electrodes cost $0.75/L oil with 42% harvesting efficiency. Increasing the harvesting efficiencies for both aluminium and iron electrodes also increased the overall cost per litre of oil, therefore lower harvesting efficiencies with lower energy inputs was recommended. Also, increasing the culturing salinity to 2 ppt sodium chloride for freshwater Nannochloris sp. was determined practical to improve the electrocoagulation energy efficiency despite a 25% reduction in cell growth. Keywords: electrocoagulation; algae harvesting; biodiesel
Introduction Microalgae have shown potential to be a fuel source that can alleviate the world’s dependence on petroleum crude. However, the estimated costs for algal oil production span nearly two orders of magnitude from $0.24 to $11.25 L−1 with average prices predicted at $5.10 L−1 .[1] Because of the small cell size (5–50 μm in diameter), low biomass concentrations of micro-algal cultures, and marginal density difference with culture water (average 1020 kg/m3 ), harvesting and dewatering algal biomass consumes large amounts of energy.[2–4] More efficient and economic harvesting technologies need to be developed to enhance the commercial viability of the microalgae biofuels industry.[5] Electrochemical techniques such as electroflotation and electrocoagulation offer the possibility to be an innovative, cheap, and effective method of algae harvesting that requires minimum amounts of chemicals.[6] Electrocoagulation is an electrochemical technique designed to disperse coagulating metal ions from oxidizing electrodes (typically aluminium or iron). To release these coagulating ions, an electric potential between the anode and cathode is needed to drive the forward reaction. The chemical reaction that takes place is made up of two independent half-reactions which describe the changes at each electrode. As seen using aluminium electrodes, oxidation occurs at the anode, producing aluminium ions and free electrons Al → Al3+ + 3e− . ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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Simultaneously, reduction occurs at the cathode, receiving water molecules and free electrons to produce hydroxide ions and hydrogen gas. The final pH will always be higher than the initial pH due to hydroxyl formation at the cathode [7] 3H2 O + 3e− → 3OH− + 1.5H2 .
(2)
The number of electrons released from the electrode is stoichiometrically related to the amounts of reactants consumed or products generated. These electrons are measured as the charge placed across the electrodes. The relationship between charge and the amount of product is expressed by Faraday’s law i∗ t ∗ MW m= , (3) 96485∗ e where m is the mass of the product produce (g); i is the current applied between electrodes (A); t is the time the current was applied (s); MW is the molecular weight of the element in question; and e is the number of electrons produced from the half reaction. The opinions on the accuracy of Faraday’s law, however, vary greatly amongst researchers. Some indicate that only 50% of the predicted mass is produced [8] while others indicated that 200% is actually produced.[9] Sasson et al. recently showed that regardless of pH values and electric currents, at least 80% of the iron in solution complied with Faraday’s laws of oxidation.[10]
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The metal ions then combine with the hydroxyl ions, creating a metal-hydroxide complex, which is a standard coagulating mechanism [11]
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Al3+ + 3H2 0 → Al(OH)3 + 3H+ .
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Coagulation uses positively charged ions to reduce the negative repulsive charges (zeta potential) of colloids so that attractive van der Waals forces can bring cells together and form flocs, which will precipitate faster.[12] Only a minimal reduction in zeta potential is necessary for significant algae removal (>90%).[13] The aggregation process of these particles to form flocs is described as colloidal destabilization. Once floc formation is achieved, harvesting costs are reduced significantly and filtration, sedimentation or flotation can be used for harvesting instead of centrifugation.[14] However, the cost of floc formation must justify the reduced costs for these other harvesting techniques. Electrocoagulation has shown to be an effective water treatment technique for wastewater,[15] phosphate removal,[16] distilleries,[17] municipalities,[18] textiles [19] as well as algae.[20] However, there are not enough studies reported that provide techniques for reducing the energy and electrode consumption.[21] Such techniques are pivotal for electrocoagulation to be an effective algae harvesting technique. The purpose of this research was to determine the economic viability of using electrocoagulation to harvest microalgae for biodiesel. Energy consumption under various salinities and charge densities as well as aluminium and iron electrode efficiencies were the main parameters accounted for in this cost analysis.
Materials and methods Salinity of culture medium For electrocoagulation to be an effective algae harvesting technique, sufficient electrolytes must be present within the culture medium to provide efficient conductivity. For brackish or saltwater species, the lack of these electrolytes is not an issue, but many freshwater mediums will need additional ions. A freshwater species, Nannochloris sp. was cultured in 1 L of standard Guillard’s F/2 medium as well as mediums with 1, 2 and 4 g/L of NaCl. Aeration for all 12 flasks was supplied by a 30 L/min air pump (Hailea V-30) which was split through a 24-port manifold each with its own 0.32 cm brass needle valve for individual air flow control. Attached to each needle valve was 30 cm of 0.48 cm I.D. Tygon tubing with 2.54 cm air diffuser (AquaticEco AA1). After seven days of culturing under florescent lighting, the biomass growth was measured as total suspended solids (TSS) to determine if the added electrolytes had a negative impact on the growth. The zeta potential of these cultures was also measured using a Malvern Zetasizer Nano electrophoretic light scattering unit.
Electrocoagulation testing Both Nannochloris sp. and Dunaliella sp. were cultured to concentrations of approximately 100 mg/L for the electrocoagulation testing and assumed to contain 20% lipid content. The most common microalgae (Chlorella, Dunaillea, Isochrysis, Nannochloris, Nannochloropsis, Neochloris, Nitzschia, Phaeodactylum, and Porphyridium) possess oil levels between 20% and 50% (w/w) and exhibit reasonable productivities.[22] The electrochemical cell consisted of 1.3 L acrylic reactor that was 73.5 cm2 by 18 cm tall. Both the anode and cathode comprise two (each) perforated (.3175 cm O.D. holes, accounting for 40% of open area) aluminium (milled alloy 3003) or iron (carbon-milled steel) plates (0.16-cm thick) connected in parallel. Each plate had a submerged surface area of approximately 90 cm2 for a total electrode surface area of 361 cm2 (180.5 cm2 for anode and 180.5 cm2 for cathode). The acrylic reactor was set upon a ceramic stir plate (Barnstead Cimarec, SP131325) and filled with 1 L of algal culture and a 3.8 cm magnetic stir bar. The electrodes were separated about 1.3 cm from their nearest counterpart and designed to rest on the edge of the reactor. The DC power supply (Mastech, HY1803DL), which allowed for amperage and voltage adjustment, was attached to the electrodes by alligator clips. The set-up is represented in Figure 1. The current control was always adjusted to its maximum setting before the start of all testing. The stir plate was turned on (level 7 setting) to initiate mixing and a sample of the uniform culture was collected to serve as a baseline for removal efficiency computations. Due to the turbulence provided by the plate electrodes within the reactor, vortex mixing was not indicated as a problem during testing. The DC power source was turned on and the voltage was adjusted to obtain the desired amperage (0.05–1.0 A). The amperage was maintained for the predetermined electrocoagulation time (1 or 2 min). For all the experiments, irrespective of the electrocoagulation time, only 1 min of flash mixing (velocity gradient, G = 90 s−1 ) was performed on every sample. For experiments with a 1 min electrocoagulation time, the 1 min flash mixing occurred simultaneously for the entire duration of the minute. However, if the testing required longer applications of amperage (e.g. 2 min), then no mixing occurred during the first minute of applied amperage, but was initiated during the final minute of amperage. Varying the flash mix allowed the length of initial coagulation to remain constant throughout the all tests, as well as prevented excessive shear on the algal cells when electrocoagulation lasted longer than 1 min. After the current was removed from the cell, an additional flocculating mix (level 3 setting, G = 18 s−1 ) was initiated for 15 min. An appropriate adjustment of mixing speed improved flocculation of algal cells for more efficient harvesting.[23] After the flocculating mix was completed, the reactors were allowed to rest for 30 min. During this time the flocculated algal cells either floated to the surface
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Environmental Technology
Figure 1.
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The electrocoagulation experimental set-up with dual perforated plate electrodes for batch microalgae harvesting.
or settled to the bottom. These flocculated algal cells were considered ‘harvested’ for this application. An additional sample was taken at the midpoint (about 7.6 cm below the water level) to determine the concentration of any unsettled or non-floating algal cells. The samples taken were quantified by measuring the absorbance at 680 nm with a spectrophotometer (Genesys 20). These values were converted to TSS using a calibration curve correlating the culture absorbance to the culture density of Nannochloris sp. and Dunaliella sp. (mg/L). Results and discussion Salinity of culture medium Algae differ in their adaptability to salinity and based on their tolerance extent, they are grouped as halophilic (salt requiring for optimum growth) and halotolerant (having response mechanism that permits their existence in saline medium).[24] Nannochloris sp., a freshwater algal species, was selected to observe its tolerance to saline medium (Figure 2). This tolerance was an indication of the species suitability for electrocoagulation harvesting. Cora and Hung indicated that additional NaCl was necessary for maximum current efficiency for their electrocoagulation set-up.[25] The additional salt in the medium showed an average decrease in biomass of 25.7% (±3.8%) for 1 ppt NaCl. At 2 ppt no further loss in biomass of significance was seen (p > 0.05). However, at 4 ppt NaCl, the biomass decreased by 53.6% (±11.3%) from the original culture. Other authors have indicated that the reduced biomass was offset by increased lipid concentration, which was likely triggered due to the stress of a saline medium. The total fat content of the alga grown in different salinities varied in the range of 24–28% (w/w), whereas the control was
Figure 2. The columns represent the final biomass concentration in milligram per litre after one week of culturing Nannochloris sp. in various salinities. The dashed line shows the decrease in zeta potential with increased salinity.
20%.[24] The effect of salinity on algal lipid production was not investigated during this study. It was also noted, however, that as the salinity increased, the zeta potential of the algal cells decreased. The charge of an algal cell is typically electronegative for pH 4–10, ranging from −10 to −35 mV.[26] The zeta potential for the 0 ppt Nannochloris culture measured −33.6 mV and decreased to −13.9 when the species was cultured in 4 ppt solution. The cultures grown in salinity were more likely to form clusters, as complete charge neutralization was not necessary to initiate cell aggregation.[26] The cell aggregation was a potential cause of the reduced growth rates at higher salinities. The importance of salinity was seen when a current was applied across the electrode set-up depicted in Figure 1. The energy utilized to maintain these amperages was compared across the various salinities used to culture Nannochloris sp. (Figure 3). These concentrations were compared with a
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Figure 3. The correlation of NaCl concentrations as an electrolyte and the energy consumed per cubic metre to maintain various amperages across aluminium electrodes.
Dunaliella sp. medium (15 ppt NaCl) which was considered to provide maximum conductivity. When the current was increased to 1.4 A for 1 min, the energy required to maintain said amperage increased by 0.107 kWh/m3 between 15 and 1 ppt NaCl. More than 18 V was needed to maintain 1.4 A at 0 ppt. This was limited, however, by the capacity of the DC power source. It was postulated by the curve (dotted line in Figure 3) that maintaining the same amperage at 0 ppt required an additional energy of 0.285 kWh/m3 than 1 ppt. As the amperage was reduced to below 0.6, the additional energy requirements between 1 and 15 ppt was 80% less than the projected energy difference at 1.4 A (Figure 3). As evidenced by Ofir et al., adjusting the salinity for electrocoagulation proved beneficial to avoid excessive energy consumption.[27] The actual effects of harvesting Nannochloris sp. with 20% lipids under saline conditions by electrocoagulation can be determined when the results of Figures 2 and 3 are combined to provide a cost per litre value (Figure 4). It was noted that when no salinity was added (0 ppt), the culture not only had the highest biomass production (Figure 2), but also the highest energy consumption for electrocoagulation (Figure 3). Conversely, when 4 ppt of NaCl was added to the culture, the lowest biomass growth resulted (Figure 2) but the electrocoagulation process was also the most efficient (Figure 3). Assuming that 100% of the algal biomass from Figure 2 was harvested by the electrocoagulation energy requirements in Figure 3, a cost value was applied by the following equation: Lculture 864 goil 1 ∗ ∗ ∗ mgalgae %lipids Loil 3 mculture X kWh $0.09 $ ∗ 3 ∗ . = galgae Loil mculture kWh
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Based on the energy requirements to produce 1L of oil, a culture medium salinity of 2 ppt was found to be the most cost effective dosage in terms of biomass produced and conductivity achieved for electrocoagulation. Despite the greater biomass produced under non-saline conditions, the lack of
Figure 4. The cost per litre oil based off of the correlations between the growth densities (Figure 2) and the energy consumption (Figure 3) due to various salinities.
conductivity made harvesting by electrocoagulation uneconomical. Likewise, the conductivity provided by 4 ppt NaCl in the culture medium was offset by the reduction in algal biomass. It should also be noted that the addition of salinity, if necessary, for a full scale freshwater operation would only be required during initial culturing due to a recycling of the effluent. It is also possible that the salinity of these freshwater cultures could increase without the addition of chemical salts due to high evaporation rates often seen in open ponds. Additionally, as the species becomes acclimated with the higher salinity, the reduction in biomass would become less prevalent. Therefore, the potential negative effects to produce extra conductivity are anticipated to be minimal.
Electrocoagulation The main costs associated with electrocoagulation are the energy inputs from current application across the electrodes and the loss of electrodes due to corrosion. Of the two costs, the corrosion of the electrode typically accounts for over 70% of the total costs.[28] Therefore, the most cost effective electrode material must be selected. For harvesting algae, aluminium has often been considered as the more effective electrode material.[18,29,30] However, over the course of 2012, the price of aluminium has averaged $2.187/kg (±$0.12/kg), whereas the price of iron has averaged $0.113/kg (±$0.02/kg). Therefore, both materials were tested to compare the combined effect of efficiency and cost. Applying the desired amount of aluminium or iron through electrocoagulation can be achieved through various combinations of amperages and times. These combinations produce a certain charge density based on the electrode surface area. Numerous authors have had varying opinions on the most effective charge density for electrocoagulation. These values range from 2–5 A/m2 under Jiang et al.[28] to 25–125 A/m2 by Cerqueira et al. [19] and even 182 A/m2 from Kannan et al.[17] However, a high charge
Environmental Technology
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Figure 5. The harvest efficiency of Nannochloris sp. under various charge densities producing similar concentrations of aluminium and iron from the electrodes.
density (75 A/m2 ) in the presence of chloride ions could inactivate all algae cells in suspension, rendering them useless for further culturing when recycling the effluent.[31] After numerous preliminary tests, 4 sets of charge densities (12 total) were applied by varying the amperage (0.05– 1.0 A) and the time of amperage application (1 and 2 min), which would produce the same theoretical concentration of aluminium and iron ions in solution (Figure 5). The charge densities varied between 1.39–8.31 A/m2 for iron and 2.77–27.7 A/m2 for aluminium. Based on the four sets of charge densities used, it was determined that the charge density did not significantly (p > 0.05) impact the harvesting efficiency within each metal group. For example, applying 0.6 amps for 1 min or 0.3 amps for 2 min across the aluminium electrodes resulted in approximately 77% harvesting efficiencies by producing similar concentrations of aluminium ions as determined by Faraday’s law. Similarly, applying 0.2 amps for 1 min or 0.1 amps for 2 min across the iron electrodes resulted in approximately 55% harvesting efficiencies by producing similar Faradaic concentrations of iron ions. Supplemental testing with reduced amperages applied across the metal electrodes for as long as 10 min showed similar results. As other researchers have indicated,[30,32] the aluminium electrodes were more effective than the iron electrodes. The aluminium electrodes harvested nearly 20% more biomass throughout the testing. However, there were noteworthy variations in the energy required to maintain the currents applied in Figure 5. The voltage needed to maintain higher currents for shorter amounts of time consumed more energy than maintaining smaller currents for longer periods of time.[10,33] Gao et al.[20] saw similar increases in energy consumption when the charge density was increased in the range of 0.5–5.0 mA/cm2 . When treating large volumes of water, these differences in energy consumption can have a huge impact on the cost of the final product. Figure 6 shows the energy consumed per cubic metre for the aluminium and iron electrodes under the various charge densities.
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Figure 6. The energy requirements to produce aluminium and iron ions under various charge densities.
Figure 7. The cost (electrode and energy) per litre of oil as a function of the Nannochloris sp. harvesting efficiencies by aluminium and iron electrodes under multiple charge densities.
The differences in energy usage were greater when larger concentrations of aluminium were dissolved from the electrode, indicating that a lower charge density would be more economical in terms of harvesting. While the aluminium electrodes proved to be more efficient at harvesting, the iron electrodes showed better conductivity. From an economic standpoint, the ability of the iron electrodes to maintain currents at lower voltages will likely compensate for the lower harvesting efficiency experienced in Figure 5. The final cost to harvest microalgae with 20% lipid content by electrocoagulation was determined by the harvest efficiency shown in Figure 5, the energy consumption in Figure 6, and the cost of the electrode material. The division of harvesting costs was plotted against the metal ion concentration produced by 1 and 2 min current applications from the electrodes (Figure 7). As seen from Figure 7, the electrodes accounted for 50–85% of costs in electrocoagulation, depending on the electrode material and charge density. Even though the aluminium electrodes showed better harvesting efficiencies, the material costs were nearly 20 times that of iron electrodes. The 20% higher harvesting efficiency of aluminium did not offset the difference in metal prices. This trend was further confirmed (Figure 8) by harvesting Dunaliella
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A.J. Dassey and C.S. Theegala the iron electrodes. However, the iron electrodes showed better conductivity when equivalent currents were applied. This energy efficiency, paired with the cost of iron at approximately 20 times less than aluminium, exemplified iron as the more suitable electrode for algal harvesting. By using the appropriate strategy, the pre-concentration of microalgae for biodiesel production by electrocoagulation could see less than $0.03/L oil.
Acknowledgement Figure 8. The cost per litre of oil as a function of the Dunaliella sp. harvesting efficiencies by aluminium and iron electrodes under multiple charge densities.
We would also like to thank Evan Terrell for his assistance with experimentation and data collection.
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Funding sp., a brackish water species (15 ppt NaCl), by the said electrocoagulation process at the 2-min electrocoagulation time. When harvesting Dunaliella, the electrodes accounted for 70–85% of the costs for electrocoagulation depending on the electrode material and charge density. The electrodes accounted for a greater percentage of the costs in comparison with Nannochloris because of the higher salinity in which Dunaliella was cultured resulted in more efficient energy consumption. At times the aluminium electrodes were shown to harvest 30% more biomass than the iron electrodes under similar conditions, but this still could not compensate for the difference in metal prices. It was also determined that despite the lower harvesting efficiency achieved at approximately 1.4 mg/L of either aluminium or iron ions, the economics were more favourable at 1.4 mg/L than the higher harvesting efficiencies achieved at greater metal concentrations (3.42–5.41 mg/L). This strategy of sacrificing harvesting efficiency for improved energy and cost consumptions was confirmed by Dassey and Theegala as the preferred harvesting goal through algae harvesting by centrifugation.[34] Therefore, while Matos et al. were able to achieve over 97% efficiency with their similar electrocoagulation algal harvesting set-up, their current density of 83 A/cm2 for 10 min was more costly overall.[35]
Conclusion Electrocoagulation proved to be an effective algae preconcentration step for both Nannochloris sp. and Dunaliella sp. Culturing Nannochloris sp. in 2 ppt NaCl decreased the biomass production by 25.7% but was found to be the most cost effective dosage in terms of biomass produced and conductivity during electrocoagulation batch tests. It was anticipated that as the culture became acclimated with the increased salinity, the algal productivity would return to its original growth rates while maintaining the improved conductivity. When comparing iron and aluminium electrodes, aluminium consistently harvested 20% more biomass than
Funding for the graduate’s work was received from the Louisiana State University’s Economic Development Assistantship.
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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production a
Adam J. Dassey & Chandra S. Theegala
a
a
Department of Biological and Agricultural Engineering, Louisiana State University & LSU AgCenter, Baton Rouge, LA 70803, USA Published online: 29 Oct 2013.
To cite this article: Adam J. Dassey & Chandra S. Theegala (2014) Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production, Environmental Technology, 35:6, 691-697, DOI: 10.1080/09593330.2013.842602 To link to this article: http://dx.doi.org/10.1080/09593330.2013.842602
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Environmental Technology, 2014 Vol. 35, No. 6, 691–697, http://dx.doi.org/10.1080/09593330.2013.842602
Reducing electrocoagulation harvesting costs for practical microalgal biodiesel production Adam J. Dassey and Chandra S. Theegala∗ Department of Biological and Agricultural Engineering, Louisiana State University & LSU AgCenter, Baton Rouge, LA 70803, USA
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(Received 11 April 2013; final version received 4 September 2013 ) Electrocoagulation has shown potential to be a primary microalgae harvesting technique for biodiesel production. However, methods to reduce energy and electrode costs are still necessary for practical application. Electrocoagulation tests were conducted on Nannochloris sp. and Dunaliella sp. using perforated aluminium and iron electrodes under various charge densities. Aluminium electrodes were shown to be more efficient than iron electrodes when harvesting both algal species. Despite the lower harvesting efficiency, however, the iron electrodes were more energy and cost efficient. Operational costs of less than $0.03/L oil were achieved when harvesting Nannochloris sp. with iron electrodes at 35% harvest efficiency, whereas aluminium electrodes cost $0.75/L oil with 42% harvesting efficiency. Increasing the harvesting efficiencies for both aluminium and iron electrodes also increased the overall cost per litre of oil, therefore lower harvesting efficiencies with lower energy inputs was recommended. Also, increasing the culturing salinity to 2 ppt sodium chloride for freshwater Nannochloris sp. was determined practical to improve the electrocoagulation energy efficiency despite a 25% reduction in cell growth. Keywords: electrocoagulation; algae harvesting; biodiesel
Introduction Microalgae have shown potential to be a fuel source that can alleviate the world’s dependence on petroleum crude. However, the estimated costs for algal oil production span nearly two orders of magnitude from $0.24 to $11.25 L−1 with average prices predicted at $5.10 L−1 .[1] Because of the small cell size (5–50 μm in diameter), low biomass concentrations of micro-algal cultures, and marginal density difference with culture water (average 1020 kg/m3 ), harvesting and dewatering algal biomass consumes large amounts of energy.[2–4] More efficient and economic harvesting technologies need to be developed to enhance the commercial viability of the microalgae biofuels industry.[5] Electrochemical techniques such as electroflotation and electrocoagulation offer the possibility to be an innovative, cheap, and effective method of algae harvesting that requires minimum amounts of chemicals.[6] Electrocoagulation is an electrochemical technique designed to disperse coagulating metal ions from oxidizing electrodes (typically aluminium or iron). To release these coagulating ions, an electric potential between the anode and cathode is needed to drive the forward reaction. The chemical reaction that takes place is made up of two independent half-reactions which describe the changes at each electrode. As seen using aluminium electrodes, oxidation occurs at the anode, producing aluminium ions and free electrons Al → Al3+ + 3e− . ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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Simultaneously, reduction occurs at the cathode, receiving water molecules and free electrons to produce hydroxide ions and hydrogen gas. The final pH will always be higher than the initial pH due to hydroxyl formation at the cathode [7] 3H2 O + 3e− → 3OH− + 1.5H2 .
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The number of electrons released from the electrode is stoichiometrically related to the amounts of reactants consumed or products generated. These electrons are measured as the charge placed across the electrodes. The relationship between charge and the amount of product is expressed by Faraday’s law i∗ t ∗ MW m= , (3) 96485∗ e where m is the mass of the product produce (g); i is the current applied between electrodes (A); t is the time the current was applied (s); MW is the molecular weight of the element in question; and e is the number of electrons produced from the half reaction. The opinions on the accuracy of Faraday’s law, however, vary greatly amongst researchers. Some indicate that only 50% of the predicted mass is produced [8] while others indicated that 200% is actually produced.[9] Sasson et al. recently showed that regardless of pH values and electric currents, at least 80% of the iron in solution complied with Faraday’s laws of oxidation.[10]
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The metal ions then combine with the hydroxyl ions, creating a metal-hydroxide complex, which is a standard coagulating mechanism [11]
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Al3+ + 3H2 0 → Al(OH)3 + 3H+ .
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Coagulation uses positively charged ions to reduce the negative repulsive charges (zeta potential) of colloids so that attractive van der Waals forces can bring cells together and form flocs, which will precipitate faster.[12] Only a minimal reduction in zeta potential is necessary for significant algae removal (>90%).[13] The aggregation process of these particles to form flocs is described as colloidal destabilization. Once floc formation is achieved, harvesting costs are reduced significantly and filtration, sedimentation or flotation can be used for harvesting instead of centrifugation.[14] However, the cost of floc formation must justify the reduced costs for these other harvesting techniques. Electrocoagulation has shown to be an effective water treatment technique for wastewater,[15] phosphate removal,[16] distilleries,[17] municipalities,[18] textiles [19] as well as algae.[20] However, there are not enough studies reported that provide techniques for reducing the energy and electrode consumption.[21] Such techniques are pivotal for electrocoagulation to be an effective algae harvesting technique. The purpose of this research was to determine the economic viability of using electrocoagulation to harvest microalgae for biodiesel. Energy consumption under various salinities and charge densities as well as aluminium and iron electrode efficiencies were the main parameters accounted for in this cost analysis.
Materials and methods Salinity of culture medium For electrocoagulation to be an effective algae harvesting technique, sufficient electrolytes must be present within the culture medium to provide efficient conductivity. For brackish or saltwater species, the lack of these electrolytes is not an issue, but many freshwater mediums will need additional ions. A freshwater species, Nannochloris sp. was cultured in 1 L of standard Guillard’s F/2 medium as well as mediums with 1, 2 and 4 g/L of NaCl. Aeration for all 12 flasks was supplied by a 30 L/min air pump (Hailea V-30) which was split through a 24-port manifold each with its own 0.32 cm brass needle valve for individual air flow control. Attached to each needle valve was 30 cm of 0.48 cm I.D. Tygon tubing with 2.54 cm air diffuser (AquaticEco AA1). After seven days of culturing under florescent lighting, the biomass growth was measured as total suspended solids (TSS) to determine if the added electrolytes had a negative impact on the growth. The zeta potential of these cultures was also measured using a Malvern Zetasizer Nano electrophoretic light scattering unit.
Electrocoagulation testing Both Nannochloris sp. and Dunaliella sp. were cultured to concentrations of approximately 100 mg/L for the electrocoagulation testing and assumed to contain 20% lipid content. The most common microalgae (Chlorella, Dunaillea, Isochrysis, Nannochloris, Nannochloropsis, Neochloris, Nitzschia, Phaeodactylum, and Porphyridium) possess oil levels between 20% and 50% (w/w) and exhibit reasonable productivities.[22] The electrochemical cell consisted of 1.3 L acrylic reactor that was 73.5 cm2 by 18 cm tall. Both the anode and cathode comprise two (each) perforated (.3175 cm O.D. holes, accounting for 40% of open area) aluminium (milled alloy 3003) or iron (carbon-milled steel) plates (0.16-cm thick) connected in parallel. Each plate had a submerged surface area of approximately 90 cm2 for a total electrode surface area of 361 cm2 (180.5 cm2 for anode and 180.5 cm2 for cathode). The acrylic reactor was set upon a ceramic stir plate (Barnstead Cimarec, SP131325) and filled with 1 L of algal culture and a 3.8 cm magnetic stir bar. The electrodes were separated about 1.3 cm from their nearest counterpart and designed to rest on the edge of the reactor. The DC power supply (Mastech, HY1803DL), which allowed for amperage and voltage adjustment, was attached to the electrodes by alligator clips. The set-up is represented in Figure 1. The current control was always adjusted to its maximum setting before the start of all testing. The stir plate was turned on (level 7 setting) to initiate mixing and a sample of the uniform culture was collected to serve as a baseline for removal efficiency computations. Due to the turbulence provided by the plate electrodes within the reactor, vortex mixing was not indicated as a problem during testing. The DC power source was turned on and the voltage was adjusted to obtain the desired amperage (0.05–1.0 A). The amperage was maintained for the predetermined electrocoagulation time (1 or 2 min). For all the experiments, irrespective of the electrocoagulation time, only 1 min of flash mixing (velocity gradient, G = 90 s−1 ) was performed on every sample. For experiments with a 1 min electrocoagulation time, the 1 min flash mixing occurred simultaneously for the entire duration of the minute. However, if the testing required longer applications of amperage (e.g. 2 min), then no mixing occurred during the first minute of applied amperage, but was initiated during the final minute of amperage. Varying the flash mix allowed the length of initial coagulation to remain constant throughout the all tests, as well as prevented excessive shear on the algal cells when electrocoagulation lasted longer than 1 min. After the current was removed from the cell, an additional flocculating mix (level 3 setting, G = 18 s−1 ) was initiated for 15 min. An appropriate adjustment of mixing speed improved flocculation of algal cells for more efficient harvesting.[23] After the flocculating mix was completed, the reactors were allowed to rest for 30 min. During this time the flocculated algal cells either floated to the surface
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Figure 1.
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The electrocoagulation experimental set-up with dual perforated plate electrodes for batch microalgae harvesting.
or settled to the bottom. These flocculated algal cells were considered ‘harvested’ for this application. An additional sample was taken at the midpoint (about 7.6 cm below the water level) to determine the concentration of any unsettled or non-floating algal cells. The samples taken were quantified by measuring the absorbance at 680 nm with a spectrophotometer (Genesys 20). These values were converted to TSS using a calibration curve correlating the culture absorbance to the culture density of Nannochloris sp. and Dunaliella sp. (mg/L). Results and discussion Salinity of culture medium Algae differ in their adaptability to salinity and based on their tolerance extent, they are grouped as halophilic (salt requiring for optimum growth) and halotolerant (having response mechanism that permits their existence in saline medium).[24] Nannochloris sp., a freshwater algal species, was selected to observe its tolerance to saline medium (Figure 2). This tolerance was an indication of the species suitability for electrocoagulation harvesting. Cora and Hung indicated that additional NaCl was necessary for maximum current efficiency for their electrocoagulation set-up.[25] The additional salt in the medium showed an average decrease in biomass of 25.7% (±3.8%) for 1 ppt NaCl. At 2 ppt no further loss in biomass of significance was seen (p > 0.05). However, at 4 ppt NaCl, the biomass decreased by 53.6% (±11.3%) from the original culture. Other authors have indicated that the reduced biomass was offset by increased lipid concentration, which was likely triggered due to the stress of a saline medium. The total fat content of the alga grown in different salinities varied in the range of 24–28% (w/w), whereas the control was
Figure 2. The columns represent the final biomass concentration in milligram per litre after one week of culturing Nannochloris sp. in various salinities. The dashed line shows the decrease in zeta potential with increased salinity.
20%.[24] The effect of salinity on algal lipid production was not investigated during this study. It was also noted, however, that as the salinity increased, the zeta potential of the algal cells decreased. The charge of an algal cell is typically electronegative for pH 4–10, ranging from −10 to −35 mV.[26] The zeta potential for the 0 ppt Nannochloris culture measured −33.6 mV and decreased to −13.9 when the species was cultured in 4 ppt solution. The cultures grown in salinity were more likely to form clusters, as complete charge neutralization was not necessary to initiate cell aggregation.[26] The cell aggregation was a potential cause of the reduced growth rates at higher salinities. The importance of salinity was seen when a current was applied across the electrode set-up depicted in Figure 1. The energy utilized to maintain these amperages was compared across the various salinities used to culture Nannochloris sp. (Figure 3). These concentrations were compared with a
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Figure 3. The correlation of NaCl concentrations as an electrolyte and the energy consumed per cubic metre to maintain various amperages across aluminium electrodes.
Dunaliella sp. medium (15 ppt NaCl) which was considered to provide maximum conductivity. When the current was increased to 1.4 A for 1 min, the energy required to maintain said amperage increased by 0.107 kWh/m3 between 15 and 1 ppt NaCl. More than 18 V was needed to maintain 1.4 A at 0 ppt. This was limited, however, by the capacity of the DC power source. It was postulated by the curve (dotted line in Figure 3) that maintaining the same amperage at 0 ppt required an additional energy of 0.285 kWh/m3 than 1 ppt. As the amperage was reduced to below 0.6, the additional energy requirements between 1 and 15 ppt was 80% less than the projected energy difference at 1.4 A (Figure 3). As evidenced by Ofir et al., adjusting the salinity for electrocoagulation proved beneficial to avoid excessive energy consumption.[27] The actual effects of harvesting Nannochloris sp. with 20% lipids under saline conditions by electrocoagulation can be determined when the results of Figures 2 and 3 are combined to provide a cost per litre value (Figure 4). It was noted that when no salinity was added (0 ppt), the culture not only had the highest biomass production (Figure 2), but also the highest energy consumption for electrocoagulation (Figure 3). Conversely, when 4 ppt of NaCl was added to the culture, the lowest biomass growth resulted (Figure 2) but the electrocoagulation process was also the most efficient (Figure 3). Assuming that 100% of the algal biomass from Figure 2 was harvested by the electrocoagulation energy requirements in Figure 3, a cost value was applied by the following equation: Lculture 864 goil 1 ∗ ∗ ∗ mgalgae %lipids Loil 3 mculture X kWh $0.09 $ ∗ 3 ∗ . = galgae Loil mculture kWh
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Based on the energy requirements to produce 1L of oil, a culture medium salinity of 2 ppt was found to be the most cost effective dosage in terms of biomass produced and conductivity achieved for electrocoagulation. Despite the greater biomass produced under non-saline conditions, the lack of
Figure 4. The cost per litre oil based off of the correlations between the growth densities (Figure 2) and the energy consumption (Figure 3) due to various salinities.
conductivity made harvesting by electrocoagulation uneconomical. Likewise, the conductivity provided by 4 ppt NaCl in the culture medium was offset by the reduction in algal biomass. It should also be noted that the addition of salinity, if necessary, for a full scale freshwater operation would only be required during initial culturing due to a recycling of the effluent. It is also possible that the salinity of these freshwater cultures could increase without the addition of chemical salts due to high evaporation rates often seen in open ponds. Additionally, as the species becomes acclimated with the higher salinity, the reduction in biomass would become less prevalent. Therefore, the potential negative effects to produce extra conductivity are anticipated to be minimal.
Electrocoagulation The main costs associated with electrocoagulation are the energy inputs from current application across the electrodes and the loss of electrodes due to corrosion. Of the two costs, the corrosion of the electrode typically accounts for over 70% of the total costs.[28] Therefore, the most cost effective electrode material must be selected. For harvesting algae, aluminium has often been considered as the more effective electrode material.[18,29,30] However, over the course of 2012, the price of aluminium has averaged $2.187/kg (±$0.12/kg), whereas the price of iron has averaged $0.113/kg (±$0.02/kg). Therefore, both materials were tested to compare the combined effect of efficiency and cost. Applying the desired amount of aluminium or iron through electrocoagulation can be achieved through various combinations of amperages and times. These combinations produce a certain charge density based on the electrode surface area. Numerous authors have had varying opinions on the most effective charge density for electrocoagulation. These values range from 2–5 A/m2 under Jiang et al.[28] to 25–125 A/m2 by Cerqueira et al. [19] and even 182 A/m2 from Kannan et al.[17] However, a high charge
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Figure 5. The harvest efficiency of Nannochloris sp. under various charge densities producing similar concentrations of aluminium and iron from the electrodes.
density (75 A/m2 ) in the presence of chloride ions could inactivate all algae cells in suspension, rendering them useless for further culturing when recycling the effluent.[31] After numerous preliminary tests, 4 sets of charge densities (12 total) were applied by varying the amperage (0.05– 1.0 A) and the time of amperage application (1 and 2 min), which would produce the same theoretical concentration of aluminium and iron ions in solution (Figure 5). The charge densities varied between 1.39–8.31 A/m2 for iron and 2.77–27.7 A/m2 for aluminium. Based on the four sets of charge densities used, it was determined that the charge density did not significantly (p > 0.05) impact the harvesting efficiency within each metal group. For example, applying 0.6 amps for 1 min or 0.3 amps for 2 min across the aluminium electrodes resulted in approximately 77% harvesting efficiencies by producing similar concentrations of aluminium ions as determined by Faraday’s law. Similarly, applying 0.2 amps for 1 min or 0.1 amps for 2 min across the iron electrodes resulted in approximately 55% harvesting efficiencies by producing similar Faradaic concentrations of iron ions. Supplemental testing with reduced amperages applied across the metal electrodes for as long as 10 min showed similar results. As other researchers have indicated,[30,32] the aluminium electrodes were more effective than the iron electrodes. The aluminium electrodes harvested nearly 20% more biomass throughout the testing. However, there were noteworthy variations in the energy required to maintain the currents applied in Figure 5. The voltage needed to maintain higher currents for shorter amounts of time consumed more energy than maintaining smaller currents for longer periods of time.[10,33] Gao et al.[20] saw similar increases in energy consumption when the charge density was increased in the range of 0.5–5.0 mA/cm2 . When treating large volumes of water, these differences in energy consumption can have a huge impact on the cost of the final product. Figure 6 shows the energy consumed per cubic metre for the aluminium and iron electrodes under the various charge densities.
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Figure 6. The energy requirements to produce aluminium and iron ions under various charge densities.
Figure 7. The cost (electrode and energy) per litre of oil as a function of the Nannochloris sp. harvesting efficiencies by aluminium and iron electrodes under multiple charge densities.
The differences in energy usage were greater when larger concentrations of aluminium were dissolved from the electrode, indicating that a lower charge density would be more economical in terms of harvesting. While the aluminium electrodes proved to be more efficient at harvesting, the iron electrodes showed better conductivity. From an economic standpoint, the ability of the iron electrodes to maintain currents at lower voltages will likely compensate for the lower harvesting efficiency experienced in Figure 5. The final cost to harvest microalgae with 20% lipid content by electrocoagulation was determined by the harvest efficiency shown in Figure 5, the energy consumption in Figure 6, and the cost of the electrode material. The division of harvesting costs was plotted against the metal ion concentration produced by 1 and 2 min current applications from the electrodes (Figure 7). As seen from Figure 7, the electrodes accounted for 50–85% of costs in electrocoagulation, depending on the electrode material and charge density. Even though the aluminium electrodes showed better harvesting efficiencies, the material costs were nearly 20 times that of iron electrodes. The 20% higher harvesting efficiency of aluminium did not offset the difference in metal prices. This trend was further confirmed (Figure 8) by harvesting Dunaliella
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A.J. Dassey and C.S. Theegala the iron electrodes. However, the iron electrodes showed better conductivity when equivalent currents were applied. This energy efficiency, paired with the cost of iron at approximately 20 times less than aluminium, exemplified iron as the more suitable electrode for algal harvesting. By using the appropriate strategy, the pre-concentration of microalgae for biodiesel production by electrocoagulation could see less than $0.03/L oil.
Acknowledgement Figure 8. The cost per litre of oil as a function of the Dunaliella sp. harvesting efficiencies by aluminium and iron electrodes under multiple charge densities.
We would also like to thank Evan Terrell for his assistance with experimentation and data collection.
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Funding sp., a brackish water species (15 ppt NaCl), by the said electrocoagulation process at the 2-min electrocoagulation time. When harvesting Dunaliella, the electrodes accounted for 70–85% of the costs for electrocoagulation depending on the electrode material and charge density. The electrodes accounted for a greater percentage of the costs in comparison with Nannochloris because of the higher salinity in which Dunaliella was cultured resulted in more efficient energy consumption. At times the aluminium electrodes were shown to harvest 30% more biomass than the iron electrodes under similar conditions, but this still could not compensate for the difference in metal prices. It was also determined that despite the lower harvesting efficiency achieved at approximately 1.4 mg/L of either aluminium or iron ions, the economics were more favourable at 1.4 mg/L than the higher harvesting efficiencies achieved at greater metal concentrations (3.42–5.41 mg/L). This strategy of sacrificing harvesting efficiency for improved energy and cost consumptions was confirmed by Dassey and Theegala as the preferred harvesting goal through algae harvesting by centrifugation.[34] Therefore, while Matos et al. were able to achieve over 97% efficiency with their similar electrocoagulation algal harvesting set-up, their current density of 83 A/cm2 for 10 min was more costly overall.[35]
Conclusion Electrocoagulation proved to be an effective algae preconcentration step for both Nannochloris sp. and Dunaliella sp. Culturing Nannochloris sp. in 2 ppt NaCl decreased the biomass production by 25.7% but was found to be the most cost effective dosage in terms of biomass produced and conductivity during electrocoagulation batch tests. It was anticipated that as the culture became acclimated with the increased salinity, the algal productivity would return to its original growth rates while maintaining the improved conductivity. When comparing iron and aluminium electrodes, aluminium consistently harvested 20% more biomass than
Funding for the graduate’s work was received from the Louisiana State University’s Economic Development Assistantship.
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