Microalgae Production in a Biofilm Photobioreactor
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Microalgae production in a biofilm photobioreactor Ward Blanken Microalgae production in a biofilm photobioreactor Ward Blanken Thesis committee Promotor Prof. Dr R.H. Wijffels Professor of Bioprocess Engineering Wageningen University Co-promotor Dr M.G.J. Janssen Assistant professor, Bioprocess Engineering Wageningen University Other members Dr B. Podola, University of Cologne, Germany Prof. Dr M.C.M. van Loosdrecht, Delft University of Technology Prof. Dr J. Hugenholtz, University of Amsterdam Prof. Dr H.H.M. Rijnaarts, Wageningen University This research was conducted under the auspices of the Graduate School VLAG (Advanced studies in Food Technology, Agrobiotechnology Nutrition and Health Sciences). Microalgae production in a biofilm photobioreactor Ward Blanken Thesis submitted in fulfilment of the requirement for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. Dr A.P.J. Mol, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Friday 2 September 2016 at 4 p.m. in the Aula. W. Blanken Microalgae production in a biofilm photobioreactor 234 pages. PhD thesis, Wageningen University, Wageningen, NL (2016) With references, with summary in English ISBN 978-94-6257-842-5 DOI: http://dx.doi.org/10.18174/384908 Contents Chapter 1 Introduction 9 Chapter 2 Cultivation of microalgae on artificial light comes at a cost 19 Chapter 3 Biofilm growth of Chlorella sorokiniana in a rotating biological contactor based photobioreactor 45 Chapter 4 Predicting microalgae growth 69 Chapter 5 Microalgal biofilm growth under day-night cycles 117 Chapter 6 Optimizing carbon dioxide utilization for microalgae biofilm cultivation 149 Chapter 7 General discussion 175 References 197 Summary 219 Dankwoord 225 About the author 229 List of publications 231 Overview of completed training activities 233 Chapter 1 Introduction Introduction 10 Chapter 1 1.1 Introduction Microalgae are the single cell ancestors to plants that employ light energy to convert carbon dioxide and water into sugar by photosynthesis. Because many microalgal species contain valuable products, large-scale microalgae cultivation could add to traditional agriculture and horticulture. The advantages of microalgae are that they do not require arable land, that many species can be cultivated in salt water, and that high productivity can be obtained by photobioreactor design and operation (Davis et al. 2011; Wijffels et al. 2010). Currently photobioreactor design is mainly focused on suspension cultivation. In addition to suspended life, most microalgae are able to grow in a biofilm. Within a biofilm microalgae live in densely packed slimy layers of numerous microalgae together with other microorganisms that attach themselves to solid surfaces (e.g. slippery rocks in shorelines). In this thesis the potential of microalgal cultivation in biofilm photobioreactors will be evaluated. 1.2 Current microalgae production Microalgae have the potential to be employed for the production of high value products (Del Campo et al. 2007) as well as bulk products (Davis et al. 2011; Wijffels et al. 2010). On a commercial scale only high value products such as, pigments, high value fatty acids, and specific food and feed supplements are produced by cultivating microalgae in suspension (Del Campo et al. 2007). While some companies are investigating the production of bulk products such as, biofuels or ethanol with microalgae, the consensus is that current commercial production costs are too high to compete with comparable agricultural products and oil drilling and refining (Acien et al. 2012; Draaisma et al. 2012). However, it is expected that the production costs of microalgae will drop such that they can compete with agriculture specifically when all biomass components are utilized according to a biorefinery concept (Draaisma et al. 2012; Vanthoor-Koopmans et al. 2013; Wijffels et al. 2010). Based on techno-economic analyses published in the scientific literature the main cost reduction should be achieved by scale up of current technology. The main constraints of large scale microalgal production facilities are photosynthetic efficiency of product formation, nutrient costs, energy required for gas transfer and 11 Introduction mixing of large liquid volumes, and energy costs and capital expensed related to downstream processing (Acien et al. 2012; Norsker et al. 2011). The photosynthetic efficiency of microalgae varies between strains and culture conditions (Breuer et al. 2015). Typical strategies to accumulate high product concentrations in the microalgal cells are nutrient starvation. However, while most microalgae can obtain high photosynthetic efficiencies during growth under nutrient rich conditions and low light the efficiency typically drops during the product accumulation phase (Breuer et al. 2015). Improvement of the photosynthetic efficiency during the product accumulation phase could therefore increase the economic potential of large scale algae cultivation. The high costs of mixing large liquid volumes and downstream processing are related to the low algae concentration in the photobioreactors. Typically microalgae concentrations are in the range of 0.05% to 0.5% dry solids (Acien et al. 2012; Pahl et al. 2013), and could be further increased to above 1% by choosing a short light path photobioreactor design (Kliphuis et al. 2011b). Because of the low microalgae concentration large liquid volumes have to be handled during microalgae cultivation. This increases the costs of mixing liquid for both cultivation and medium preparation, and additionally it is costly to separate the biomass from the liquid. 1.3 Advantages of microalgal biofilms Phototropic biofilms can improve microalgae cultivation economics because concentrated microalgal paste can be directly harvested (Berner et al. 2014) and the hydraulic retention time can be uncoupled from the microalgal retention time. The latter allows to decrease the liquid volume or to employ dilute waste streams. Therefore cultivation of microalgae biofilms can be cheaper than microalgae cultivation in suspended culture. To successfully employ biofilms it is required that microalgal biofilms can be cultivated at high photosynthetic efficiency. In literature reported photosynthetic efficiencies of microalgal biofilm cultivations vary greatly between studies, but these include photosynthetic efficiencies normally obtained in suspension cultures (Berner et al. 2014; Gross et al. 2015). However a detailed 12 Chapter 1 comparison of photosynthetic efficiencies between suspension and biofilms is still missing. An additional advantage of a phototropic biofilm based production system is that carbon dioxide and oxygen is directly interchanged over the biofilm-gas interphase, minimizing the pressure drop required for gassing and thus reducing the energy costs. Although it is recognized that concentrated CO2 streams are required to ascertain maximal productivity (Ji et al. 2013a; Schultze et al. 2015), the limitations in the CO2 supply are not identified yet. 1.4 Growing a biofilm Phototrophic biofilms are densely packed layers of microalgae that grow attached to a solid surface (Figure 1.1). The phototropic biofilm should be illuminated and should be frequently exposed to water containing nutrients including nitrogen, phosphate and trace elements. Supply of water to the biofilm can be ascertained by (intermittently) submerging the biofilm (e.g. rotating the biofilm between gas and liquid), or by cultivating the biofilm on a water permeable membrane. The advantage of both design principles is that a large fraction of the biofilm surface is in direct contact with the gas phase, allowing for transfer of CO2 into the biofilm and O2 out of the biofilm. Due to the dense packing of the biofilm light is absorbed and attenuated rapidly and nutrients have to be transported by diffusion processes. These gradients of light and nutrients make microalgal biofilm cultivation more complex compared to suspension cultivation. 13 Introduction Figure 1.1. Processes influencing biofilm growth. 1.5 Thesis outline To investigate the potential of microalgal biofilm photobioreactors we employed a rotating biofilm contactor based design (Figure 1.2). This photobioreactor design consists of a vertical orientated disk situated in a liquid container. The disk containing the microalgal biofilm rotates such that it alternates between the gas and liquid phase. Because the liquid phase is kept dark we actively selected for biofilm growth on the disk and alternate the biomass on the disk between light and dark to supply the cells both with light and dissolved nutrients. While employing this reactor design the aim of this thesis study was to optimize the productivity of microalgal biomass in phototrophic biofilms. 14 Chapter 1 Figure 1.2. The Algadisk reactor. The disk rotates continuously, carbon dioxide is supplied via the gas phase and the other nutrients are dissolved in the water phase, which is kept dark. In Chapter 2 illumination of the microalgae culture by artificial light and sunlight is compared. Sunlight is free and abundant but is variable over the day and seasons due to diurnal light cycles, weather conditions, and seasonal changes. These fluctuations in irradiance can be prevented by applying artificial lighting. By means of a techno- economic analysis it was assessed whether the use of artificial light would be an alternative for the utilization of sunlight to drive microalgae production.