Open thin-layer cascade reactors for saline microalgae production evaluated in a physically simulated Mediterranean summer climate

doi:10.1016/j.algal.2017.06.004

Algal Research

Volume 25, July 2017, Pages 381-390

https://doi.org/10.1016/j.algal.2017.06.004 Get rights and content

Highlights

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An open photobioreactor for mass production of saline microalgae was designed.
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Reactors were evaluated with a realistic physical climate simulation technology.
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Nannochloropsis salina batch cultivation in a Mediterranean summer climate
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Productivity, efficiency, growth rate, and biomass density exceeded literature data.
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Pond liner channels enable inexpensive thin-layer cascade reactors on a large scale.

Abstract

While microalgae hold the promise for conversion of sunlight and CO₂ to a wide variety of products, the economics of algae processes are still debatable. We have designed an open thin-layer cascade photobioreactor for high-cell density cultivation of saline microalgae to advance economic microalgae mass production. Pilot-scale reactors with a surface area of up to 8 m² (cultivation volume 50–140 L) were constructed and evaluated using a dynamic climate simulation technology (light, air temperature and humidity) integrating natural sunlight and multi-color LED arrays for a highly realistic reproduction of the sunlight spectrum. Batch processes with Nannochloropsis salina were performed in these reactors in the physically simulated Mediterranean summer climate of Almería, Spain – an ideal location for outdoor microalgae cultivation. Two reactor variants were examined: one with a smooth but expensive rigid channel made of polyethylene sheets, and one with a more uneven but significantly less expensive channel made of pond liner. Maximal intra-day growth rates of 1.9 d⁻¹ were observed at a cell density of 1–3 g L⁻¹. The maximal cell density of 50 g L⁻¹ was obtained within 25 days. These high growth rates and cell densities markedly exceed literature data. No difference in growth between the channel variants was observed. This suggests that cost-efficient large-scale thin-layer cascade reactors with inexpensive pond liner channels are feasible. The high cell density allows a reduction of harvesting cost. Optimal process conditions were identified by analyzing the batch and daily economic bioprocess metrics: At a cell density of 17 g L⁻¹, an areal biomass productivity of 25 g m⁻² d⁻¹ (volumetric productivity 4 g L⁻¹ d⁻¹) and a photosynthetic conversion efficiency of 4.6% were observed. The reactor design is discussed in detail to encourage further advancement of thin-layer algal cultivation technology.

Graphical abstract

Introduction

Outdoor mass production of microalgae is poised to impact renewable bio-production of food, chemicals and energy [1]. Algae cultivation is sustainable as it does not compete with agricultural activities and can utilize salt- or waste water, thereby circumventing depletion of valuable fresh water resources [2]. At present, only few high-value industrial products such as pigments are generated by outdoor microalgae processes. For low-value bulk chemicals or biofuels, production cost are currently too high to provide economic viability [3].

The choice of a cultivation system has a major effect on production cost. Open cultivation systems are suitable to generate low-value products [4], [5], [6]. The most widely used open cultivation system is the raceway pond. However, with its depth of 15–30 cm, low cell densities of only 1–1.5 g L⁻¹ are achieved [7], [8], [9]. Hence, energy costs for circulation and harvesting of the algae are high and the cultures are susceptible to contamination [10], [11]. A promising alternative is the thin-layer cultivation concept pioneered in Třeboň, Czech Republic [12], [13], [14]. This bioreactor concept provides for a microalgae suspension to flow down a sloped channel in a layer of < 1 cm thickness. At the end of the channel, the suspension is pumped up again to the starting point. High cell densities of 30–50 g L⁻¹ after 2–3 weeks have been reported in outdoor cultivation of Chlorella and Scenedesmus freshwater strains in the Czech Republic and Greece [15], [16], [17]. Moreover, the microalgae production cost in thin-layer systems were estimated to be only 20% of the cost in raceway ponds [11].

Unfortunately, technical information on thin-layer cascade reactors is scarce [11], [12], [18] and a detailed functional evaluation of all reactor parts is not available, hampering the advancement of thin-layer cultivation. Furthermore, a comparison of different reactor setups is difficult because precise intra-day growth rates have not been determined yet. Moreover, no thin-layer cascades for saltwater have been developed yet, although saline cultivation is desirable because of reduced freshwater use and contamination risk as well as improved CO₂ containment in the aqueous phase due to higher pH. Finally, the channels of published thin-layer cascade reactors were made from expensive rigid materials like steel, glass, concrete or fiberglass [19], [20], resulting in a high investment cost compared to other open cultivation systems. Thus, the development of thin-layer cascade reactors addressing these issues appears to be a valuable addition to the growing field of microalgae research.

Testing bioreactors and processes on a larger scale usually requires outdoor experiments. However, achieving a thorough understanding of an outdoor process is complicated by randomly changing environmental conditions. A solution to this is physical and dynamic day and night climate simulation – the realistic indoor reproduction of outdoor environmental conditions [8], [21]. With the recent advent of light-emitting diode (LED) technology in microalgae research [22], faithful sunlight simulation has become possible. Experiments under controlled environmental conditions allow informed decisions on worthwhile improvements. The development process is accelerated since the experiments are independent of outdoor weather conditions. As a result, the investment for a large-scale outdoor microalgae production facility will more likely be profitable if the process has been thoroughly tested and optimized indoors under the environmental conditions of the envisaged outdoor site.

This paper reports on the design, construction, operation, and evaluation of open thin-layer cascade photobioreactors at TUM AlgaeTec Center (Technical University of Munich, Germany), a microalgae research facility enabling physical and dynamic day and night climate simulation on a pilot scale. The LED-supported climate simulation system is presented and the thin-layer cascade reactor design is discussed. The saline microalga Nannochloropsis salina was chosen as a model to evaluate batch process performance in open thin-layer cascade photobioreactors with a surface area of up to 8 m². In these processes, the climate simulation reproduced the environmental conditions of a Mediterranean summer in Almería, Spain, a suitable location for a large-scale outdoor microalgae cultivation site.

Section snippets

Open thin-layer cascade photobioreactor

The thin-layer cascade reactor design developed and used in this study (Fig. 1) consisted of five modular reactor parts: inlet module, upper channel, flow reversal module, lower channel, and retention tank (parts given in order of the flow of the algae suspension). A detailed technical description of the reactor parts is given in the Supplementary materials.

An off-the-shelf centrifugal pump was used to lift the algae suspension from the retention tank to the inlet module, reducing investment

Growth rates, channel variants, and reactor comparability

Parallel batch processes with the saline microalga Nannochloropsis salina were performed in four thin-layer cascade reactors R1–R4 to evaluate the maximal growth rates in the simulated Mediterranean summer climate and the reactor comparability (Fig. 6). Reactors R1 and R2 were equipped with rigid channels, reactors R3 and R4 were equipped with pond liner channels. The four reactors were situated in the same hall and hence experienced the same environmental conditions (Fig. 6A). The air

Conclusion

Climate simulation – the physical indoor simulation of dynamic outdoor environmental conditions – is an emerging tool in the development of photobioreactors and bioprocesses. In this study, novel open photobioreactors for saline microalgae were designed and evaluated using a globally unique realistic climate simulation technology. In the simulated Mediterranean summer climate of Almería (Spain) – a suitable location for a large-scale outdoor microalgae cultivation site – high growth rates,

Author's contributions

All authors contributed to the conception and design of the study, the interpretation of the data as well as the critical revision and final approval of the manuscript; AA, CP, NB, NM, JG, JS performed the experiments and analyzed the experimental data; AA, DWB wrote the manuscript; TB, DWB obtained the funding.

Acknowledgments

Funding (grant number LaBay74A) provided by the Bavarian State Ministry for Economic Affairs and the Media, Energy and Technology (Munich, Germany), the Bavarian State Ministry of Education, Science and the Arts (Munich, Germany) and Airbus Group (Leiden, The Netherlands) is gratefully acknowledged. Andreas Apel and Christina Pfaffinger were supported by the TUM Graduate School (Technical University of Munich, Germany). The funding sources did not influence the reported research. Stefan Wilbert

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View full text

Open thin-layer cascade reactors for saline microalgae production evaluated in a physically simulated Mediterranean summer climate

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Open thin-layer cascade photobioreactor

Growth rates, channel variants, and reactor comparability

Conclusion

Author's contributions

Acknowledgments

Algal Res.

Algal Res.

Bioresour. Technol.

Aquaculture

Algal Res.

Biomass

Chem. Eng. Sci.

Algal Res.

Bioresour. Technol.

Enzym. Microb. Technol.

Biochem. Eng. J.

Bioresour. Technol.

Algal Research

Products from microalgae: an overview

Second generation biofuels: high-efficiency microalgae for biodiesel production

Bioenergy Res.

A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel From Algae

The promise and challenges of microalgal-derived biofuels

Biofuels Bioprod. Biorefin.

A Realistic Technology and Engineering Assessment of Algae Biofuel Production: Energy Biosciences Institute Report

From laboratory to commercial production: a case study of a Spirulina (Arthrospira) facility in Musina, South Africa

J. Appl. Physiol.

High Density Outdoor Microalgal Culture

Novel outdoor thin-layer high density microalgal culture system: productivity and operational parameters

Arch. Hydrobiol. Algol. Stud.

Dual purpose open circulation units for large scale culture of algae in temperate zones. I. Basic design considerations and scheme of a pilot plant

Arch. Hydrobiol. Algol. Stud.

Ivan Šetlík (1928–2009)

J. Appl. Phycol.