Development of operational strategies to remove carbon dioxide in photobioreactors
Introduction
Global atmospheric concentration of carbon dioxide increased markedly as a result of human activities [1]. Carbon dioxide is the most important anthropogenic greenhouse gas (GHG) and its concentration has increased from a pre-industrial value of about 280–379 ppm in 2005 [2]. The first regulations aimed at controlling atmospheric pollutants have already been implanted; the signing of the Kyoto Protocol in December 1997 is an historic step in reversing the increase in these emissions. The primary achievement of the Protocol is the commitment of countries referred in the Annex I to reduce their emission some 5% below their country specific 1990 level, in the period 2008–2012 with penalization clauses in case of non-compliance [3].
Biological carbon sequestration using technologies such as controlled photosynthetic reactions may help to alleviate GHG problems, by carrying out reactions in which the CO2 is transferred to the aqueous phase of the system where microbial conversion occurs, resulting in the production of oxygen, biomass, soluble biopolymers, carbonate and bicarbonate and volatile organic compounds [4], [5], [6].
At this moment, the economic return for the operation of these systems may become feasible through the carbon credits [7] and by using the photobioreactor technology to produce biomass. The biochemical composition of the microalgal cells may be of commercial interest, possessing significant proportions of proteins, lipids, carbohydrates, pigments and nucleic acids, and could therefore be used as ingredients in foods destined for human consumption, animal feeds, extraction of biomolecules and in the production of biofuels [8], [9], [10].
In previous studies [11], we demonstrated that CO2 removal by the cyanobacteria Aphanothece microscopica Nägeli in a bubble column photobioreactor was described by a first order kinetic model. In this study was determined that a 15% (v/v) content of CO2 in the air inlet optimized CO2 uptake performance. Furthermore, Jacob-Lopes et al. [12] showed that this inlet CO2 concentration favoured the cyanobacterial growth when compared to a wide range of concentrations (3, 15, 25, 50 and 62%). In both studies, inlet airstreams with 15% CO2 (v/v) were considered the best conditions for biomass growth and carbon dioxide removal, however, this condition leads to substantial losses of underutilized CO2. So, operational strategies should be developed for improve the performance of the CO2 utilization in photobioreactors.
Design and scale-up methodologies for photobioreactors have not been extensively described. Irrespective of the specific reactor configuration and operational mode employed, several essential issues need addressing: (i) effective and efficient provision of light; (ii) supply of CO2 while minimizing desorption; (iii) selection of strains with high growth rate, tolerance to CO2 and temperature; (iv) analysis and definition of operational conditions and (v) scalable photobioreactor technology [13], [14], [15].
Thus the objective of the present study was to evaluate the capacity of the cyanobacteria, Aphanothece microscopica Nägeli to treat air containing 15% carbon dioxide in two types of photobioreactors, bubble column and airlift, under three different operational conditions: simple operation, air recirculation and two sequential reactors.
Section snippets
Microorganism and culture medium
Axenic cultures of Aphanothece microscopica Nägeli (RSMan92) were originally isolated from the Patos Lagoon estuary, Rio Grande do Sul State, Brazil (32°01′S–52°05′W). Stock cultures were propagated and maintained on synthetic BGN medium [16]. The incubation conditions used were 25 °C, photon flux density of 15 μmol m−2 s−1 and a photoperiod of 12 h.
Photobioreactors
The diagram of the experimental apparatus used is shown in Fig. 1. The photobioreactors were constructed in 4 mm thick glass with similar geometry,
Batch reactors with simple operation
Photobioreactors are multi-phase physical–chemical–biological systems with numerous interactions between the process variables and the dynamic alterations between the gas–liquid–solid parts of the system [13]. The use of this type of reactor to eliminate CO2 is considered a promising alternative, since carbon can be fixed by different mechanisms [20]. The kinetic data for the BCR with simple operation are expressed in Fig. 2.
The kinetic data for CO2 removal expressed in terms of the EC and RE
Conclusions
The mitigation of greenhouse gas emissions, especially CO2, represents an important aspect in the sustainable development of industrial activities. CO2 sequestration by photosynthetic reactions may be an adequate strategy from this point of view, since it can be transformed into various products which can be reused in different ways. In the present study, the development of operational strategies and the project of reactors for use in the biological conversion of CO2 by the cyanobacteria
Acknowledgements
Funding for this research was provided by the Fundação de Amparo a Pesquisa no Estado de São Paulo—FAPESP (Brazil) and by the ALFA Programme, II-0259-FA-FC—POLYLIFE (European Union). The authors are grateful to Dr. Maria Isabel Queiroz (Federal University of Rio Grande—Brazil) for providing the microalgal cultures.
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