Citation
Vandamme D., Pontes SCV, Goiris K, Foubert I, Pinoy LJJ, Muylaert K.
Evaluation of electro-coagulation-flocculation for harvesting marine and
freshwater microalgae
Biotechnology and bioengineering, vol 108 (10), 2320-2329.
Archived version
Author manuscript: the content is identical to the content of the published
paper, but without the final typesetting by the publisher
Published version
http://onlinelibrary.wiley.com/doi/10.1002/bit.23199/abstract
Journal homepage
http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-0290
Author contact
Dries.vandamme@kuleuven-kulak.be
your phone number + 32 (0)56 6041
IR
https://lirias.kuleuven.be/handle/123456789/306325
(article begins on next page)
Evaluation of electro-coagulation-flocculation for harvesting
marine and freshwater microalgae
Dries Vandamme
1*
, Sandra Cláudia Vieira Pontes
2
, Koen Goiris
3
, Imogen Foubert
1
, Luc Jozef
Jan Pinoy
2,4
, Koenraad Muylaert
1
.
1
Laboratory Aquatic Biology, K.U.Leuven Campus Kortrijk,
E. Sabbelaan 53,
8500 Kortrijk, Belgium
2
Department of Industrial Engineering, Laboratory for Chemical Process Technology, KaHo
St.-Lieven, Associated to K.U.Leuven, as Faculty of Industrial Sciences, Technologie
campus, Gebroeders Desmetstraat 1, B-9000 Gent, Belgium
3
Department of Microbial and Molecular Systems (M2S), Leuven Food Science and
Nutrition Research Centre (LFoRCe), Laboratory of Enzyme, Fermentation and Brewing
Technology, KaHo St.-Lieven, Associated to K.U.Leuven as Faculty of Industrial Sciences,
Technologie campus, Gebroeders Desmetstraat 1, B-9000 Gent, Belgium
4
Department of Chemical Engineering, K.U.Leuven, Willem de Croylaan 46, B-3001
Leuven, Belgium
*Corresponding author: Email: Dries.Vandamme@kuleuven-kortrijk.be
Tel: +32 56 246257
Fax: +32 56 246999
Keywords
coagulation / microalgae / dewatering / aluminum / electrodes / electrolytic flocculation
Running Title
Evaluation of ECF for harvesting microalgae
Abstract
Although microalgae are considered as a promising feedstock for biofuels, the energy
efficiency of the production process needs to be significantly improved. Due to their small
size and low concentration in the culture medium, cost efficient harvesting of microalgae is a
major challenge. In this study, the use of electro-coagulation-flocculation (ECF) as a method
for harvesting a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum)
microalgal species is evaluated. ECF was shown to be more efficient using an aluminum
anode than using an iron anode. Furthermore it could be concluded that the efficiency of the
ECF process can be substantially improved by reducing the initial pH and by increasing the
turbulence in the microalgal suspension. Although higher current densities resulted in a more
rapid flocculation of the microalgal suspension, power consumption, expressed per kg of
microalgae harvested, and release of aluminum were lower when a lower current density was
used. The aluminum content of the harvested microalgal biomass was less than 1 % while the
aluminum concentration in the process water was below 2 mg l
-1
. Under optimal conditions,
power consumption of the ECF process was around 2 kWh kg
-1
of microalgal biomass
harvested for Chlorella vulgaris and ca. 0.3 kWh kg
-1
for Phaeodactylum tricornutum.
Compared to centrifugation, ECF is thus more energy efficient. Because of the lower power
consumption of ECF in seawater, ECF is a particularly attractive method for harvesting
marine microalgae.
Introduction
Due to the combination of a high areal productivity, a high lipid content and limited
competition with food crops for arable land, microalgal biomass is an attractive feedstock for
the production of biofuels. At present, however, microalgae are only produced on a limited
scale for high-value products such as food supplements, natural pigments and poly-
unsaturated fatty acids (Cardozo et al., 2007; Raja et al., 2008; Spolaore et al., 2006). Energy
inputs during the production of microalgal biomass are very high and often exceed the energy
content of the microalgal biomass (Pienkos and Darzins, 2009; Wijffels and Barbosa, 2010).
To use microalgal biomass as a feedstock for biofuels, the cost and energy efficiency of the
process needs to be improved dramatically (Greenwell et al., 2010; Tredici, 2010).
Because of their small size (typically a few micrometer) and low concentration in the culture
medium (0.5 – 2 g/l), harvesting microalgal biomass is a major challenge. Most existing
microalgal production systems use energy intensive centrifuges to harvest microalgae
(Heasman et al., 2000). Consequently, harvesting represents a major fraction of the total
energy demand of the production process (Grima et al., 2003; Uduman et al., 2010). If the
microalgae could be preconcentrated 30-50 times by coagulation-flocculation and gravity
sedimentation prior to centrifugation, the energy demand for harvesting could be strongly
reduced (Harun et al., 2010; Tredici, 2010; Uduman et al., 2010).
Microalgae can easily be flocculated using metal coagulants such as Fe
3+
or Al
3+
salts (Ahmad
et al., 2006; Bernhardt and Clasen, 1991; Papazi et al., 2009). In wastewater treatment,
electro-coagulation-flocculation (ECF) has been proposed as an alternative for chemical
coagulants (Mollah et al., 2001; Mollah et al., 2004). In ECF, iron or aluminum ions are
released from a sacrificial anode through electrolytic oxidation. Compared to coagulation-
flocculation with Fe
3+
or Al
3+
salts, ECF has the advantage that no anions such as chlorine and
sulphate are introduced in the process water. The electrolytic oxidation of the sacrificial
anode, however, requires electricity.
During ECF, the following reactions occur at the anode:
Using an aluminum anode :
Al Al
3+
+ 3 e
-
x Al
3+
+ y OH
-
Al
x
(OH)
y
z+
The speciation of the aluminum hydroxides formed during ECF is highly variable and is
strongly influenced by pH (Mouedhen et al., 2008).
Using an iron anode :
Fe Fe
2+
+ 2 e
-
Fe
2+
+ 2 OH
-
Fe(OH)
2
or
Fe Fe
3+
+ 3 e
-
Fe
3+
+ 3 OH
-
Fe(OH)
3
It is not clear whether ferrous or ferric ions are formed during ECF (Sasson et al., 2009).
Moreover, Fe
2+
can be rapidly oxidized in solution to Fe
3+
in the presence of oxygen. Release
of Fe
2+
during ECF leads to green hydroxide precipitates, while Fe
3+
ions results in yellow
hydroxide precipitates.