Short CommunicationStacked optical waveguide photobioreactor for high density algal cultures
Introduction
Biofuels derived from algae represent a promising source of alternative fuel that could help meet ever increasing energy demands and address rising concerns with regards to carbon emissions leading to global warming (Chisti, 2007). Microalgal systems are an attractive feedstock for biofuel due to their independence from soil fertility (i.e. they do not compete with arable land area or forest ecosystems for their development) (Chisti, 2008, Stephens et al., 2010), relative independence from seasonal cycles allowing for year round production, high oil content (as percentage of biomass) (Chisti, 2007), and significantly higher productivity rates as compared to oilseed crops (Brennan and Owende, 2010, Chisti, 2008).
Currently, the most popular systems for commercial production of algal biomass for biofuels are so-called open systems – including natural, circular, or raceway ponds (Chen et al., 2011, Zittelli et al., 2013). The relatively low capital cost associated with the pond bioreactors, adds to their popularity, but they do suffer from a number of operational and control problems related to evaporation, temperature fluctuations, sensitivity to culture contamination, and other environmental factors such as rainfall (Zittelli et al., 2013). These issues have led to development of closed PBR systems, which provide a more controlled environment. A number of closed PBRs have been developed including flat, tubular, manifold, and biofilm bioreactors (Chen et al., 2011, Zittelli et al., 2013). Generally speaking the increased capital costs of these reactors are offset by the operational advantages outlined above (Chisti and Yan, 2011). In all these bioreactors, adequate light supply and distribution across the bulk of the algal culture is a key factor affecting productivity (Chisti, 2007). An overabundance of light, typically at the illuminated surface, can lead to photoinhibition (Brennan and Owende, 2010, Janssen et al., 2003), while insufficient penetration of light into the culture (e.g. due to self-shading effects) leads to optically dark regions incapable of supporting optimal growth. To avoid this, PBRs require high surface to volume ratios allowing light to be distributed to as large a fraction of the culture as possible. Most PBR designers attempt to resolve the illumination problem by actively mixing the culture volume to expose the algae, on an average, to sufficient number of photons (Janssen et al., 2000, Molina Grima et al., 1999). However, these active mixing mechanisms tend to be energetically demanding and lead to higher operational costs (Chisti, 2007, Molina Grima et al., 1999). This is one of the reasons that the energy return on investment (EROI) – ratio of energy produced to energy input to the system – for algal biosystems is relatively low compared to other fuel sources, and in some cases has been estimated to be less than one (Beal et al., 2012b). Development of PBRs that can provide optimal light delivery with a reduced requirement for active mixing would, therefore, be of significant interest (Chisti and Yan, 2011).
To mitigate the effect of poor light distribution, PBRs with internal light distribution and guiding structures have been developed. Techniques that have been demonstrated include the use of: surface plasmon based light back scattering (Torkamani et al., 2010), LED array panels (Choi et al., 2013), optical fibers (Chen et al., 2006), and planar waveguides (Dye et al., 2011). Most of the studies were applied to relatively low density algae cultures (∼OD 3), whereas high production systems necessitate high-density cultures. Jung et al. (2012) recently demonstrated single layer slab-waveguide systems that used near surface evanescent fields for algal growth and characterized the spatial–temporal growth patterns. The “stackable” nature of these single-layer systems is attractive in that it allows for increased productivity on a limited land area. However, the shallow depth of the evanescent field near the waveguide surface limits overall achievable biomass accumulation, thereby requiring a large number of stacks, which significantly increases capital costs.
Leveraging the advantages of short-light path design and eliminating the limitations of evanescent field illumination, a 10-stack PBR with integrated slab waveguides was developed. Light was allowed to escape the waveguide surface via scattering from an etched surface and penetrate deep inside the bioreactor. The performance of the bioreactor is primarily quantified in two ways: biomass accumulation evaluated by measurement of surface coverage and optical density (OD) of the bacterial colonies and volumetric ethylene production rates from a genetically engineered strain (Ungerer et al., 2012).
Section snippets
Fabrication of the 10-stack photobioreactor and light scattering waveguides
The bioreactor consisted of a frame to hold 10 slab waveguides for delivering light to microalgae, and a photomask to block uncoupled light from entering the bioreactor (Fig. S1(a) and (b)). A 3D printer (Connex 500, Objet Geometries Inc.) was used to print a frame of the 10 stack PBR. The frame was printed using a photocurable resin (VeroClear, Objet Geometries Inc.) and then coated by parylene C to prevent gas and liquid leakage. The photomask was also printed using the same photocurable
Increased biomass accumulation inside the photobioreactor
To demonstrate the spatial uniformity of biomass accumulation in the bioreactor (indicative of even light distribution), algae were cultured in the 10-stack PBR for 3 weeks. Red LEDs (630 nm) were coupled into the ends of waveguides from both sides of the PBR. Growth was monitored for 3 weeks by measuring the fluorescent intensity of the Synechocystis colonies on the top layer of the PBR via a fluorescent microscope (Materials and Methods, Fig. S2). The initial base line surface coverage density
Conclusions
A 3D stackable waveguide photobioreactor that aims to alleviate poor distribution of light within conventional photobioreactors, was demonstrated. The approach of using closely stacked slab waveguides enabled uniform distribution of light within the bioreactor leading to an eightfold increase in biomass growth compared to a control bioreactor. The ten-stack photobioreactor was also capable of supporting consistent production of ethylene over 45 days, demonstrating its suitability for biofuel
Acknowledgements
This work was supported by the Advanced Research Project Agency – Energy (DE-AR0000312). We thank Jianping Yu, PhD (NREL, Golden, CO) for providing the Synechocystis sp. PCC 6803 2× EFE algal strain.
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Both authors contributed equally to this work.