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Title
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Process optimisation for the reuse of growth medium in the cultivation of marine microalgae in a closed photobioreactor
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Author
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Abstract
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| Microalgae have a wide range of applications in industries such as feed and aquaculture, functional food, chemicals, pharmaceuticals, cosmetics and biofuels. Despite their high potential, widespread large-scale cultivation of microalgae especially for low-value applications struggles to get off the ground. To increase the economic viability of bulk cultivation, production costs need to be reduced. Recycling growth medium not only allows salvaging the unused nutrients remaining in the water after harvesting, but also permits to reduce water demand and to avoid or reduce waste water treatment costs. To be able to implement a recycling method in a fully automated production system of microalgae, several challenges such as cleaning of spent medium, replenishment of nutrients and comprehending positive or negative effects on microalgal growth needed to be addressed. The present study explored a method to rejuvenate centrifuge supernatant (centrate) in a robust and inexpensive way, mapped nutrient consumptions and formulated dosing strategies for an automated replenishment, and gained understanding in the effect of medium recirculation on the cultivation of Nannochloropsis sp. and Tisochrysis lutea, separately and across species borders. In Chapter 2, a polishing treatment was sought for centrate originating from centrifugation of high density cultures of Nannochloropsis sp. to feed into a microfiltration setup generating contaminant-free growth medium. High pH-induced coagulation/flocculation (with NaOH or Ca(OH)2) and ‘rapid’ sand filtration (start exhaust flow rate of 9 m³·m-2·h-1) were investigated separately, and linked at optimal settings to reduce membrane fouling and increase microfiltration performance. Stand-alone sand filtration provided a meagre improvement in centrate filterability; ‘modified fouling index’ decreased 21 ± 21% after 55 min. High pH induced coagulation/flocculation with NaOH and Ca(OH)2 at pH 11.5 reached flocculation efficiencies of 90 ± 1% and 92 ± 4% respectively. When combining both techniques, the obtained flocculated centrates, however, resulted in rapid clogging of the sand bed. Consequently, only the combination of NaOH coagulation/flocculation at pH 10.5 and subsequent sand filtration was further investigated. This combination removed 78 ± 18% of particles and improved ‘modified fouling indices’ with 75 ± 19%. After 60 minutes filtration runs, the sand bed was backwashed with (acidified) water during 5 minutes to regenerate the sand bed. The results indicated that there is room for improvement by optimizing washing regimes of the sand filter. In any case, the suggested strategy will greatly decrease membrane fouling thus allowing prolonged recirculation of growth medium. The impact of medium recycling on a continuous monoculture of Nannochloropsis sp. cultivated at high density was assessed in Chapter 3. It was observed at two different scales (1.3 L laboratory-scale reactors and a 24 m³ outdoor pilot setup) that it is feasible to cultivate Nannochloropsis sp. with reused, microfiltered and replenished medium. Average yields on light of 0.8 ± 0.2 g·mol-1 PAR and 0.26 ± 0.08 g·mol-1 PAR were obtained at indoor laboratory and outdoor pilot scale setups, respectively. Additionally, nutrient consumption rates were documented in order to guide the replenishment of reused media. It turned out that, due to a disproportionate consumption of nutrients, the replenishment strategy had to be adjusted to limit accumulation or depletion of nutrients in the medium upon repeated medium recycling. The N/P feed concentration was therefore altered from 37/1.3 to 24.7/1.75 (mM/mM). For each recycle round, an important decrease in nutrient (56%), salt (66%) and water (80%) consumption was obtained. Unfiltered centrate too was applied successfully as reused growth medium. However, if one wishes to prevent unwanted contamination in the production of multiple monocultures on a large scale facility, the use of this unfiltered media source is, while cost-efficient, not recommended. When multiple monocultures are cultivated in a large-scale installation, mixing of filtered harvest broths sourced from these cultures will occur. While reusing this medium, allelopathic effects may appear and induce favourable growth responses for some genera but not others. In Chapter 4, the potential of reusing medium from two algae species (Nannochloropsis sp. and Tisochrysis lutea) for growing monocultures of the opposite algal strain was evaluated at three successive cultivation setups with increasing volume; 400 mL in turbidostat mode, 2.6 L and 220 L in semi-continuous mode. Whereas small scale tests were principally used to detect major differences in growth characteristics when using recycled media, pilot scale experiments provided important information on nutrient consumption and generated productivity data that can be extrapolated to large scale cultivation systems. No contamination or negative effect on productivity occurred during cultivation of either species with reused, microfiltered and replenished medium derived from the other species. Microfiltration and balanced nutrient supplementation of the recycled medium contributed to this result. Average yields on light of 1.6 ± 0.5 and 0.6 ± 0.2 g·mol-1 PAR were obtained for Nannochloropsis sp. and T. lutea respectively. By reusing medium, a significant reduction in water footprint (77%) and salt expenditure (71%) was obtained. At pilot scale, a total nutrient cost reduction of 68% was achieved. In conclusion, the results of this thesis led to the implementation of a medium recycling method in a fully automated production system of microalgae, which in turn contributes to the economic viability of a large-scale cultivation of microalgae. Overall, compared to no medium recirculation, a reduction of 67.5% in medium cost and 34.0% in total variable production cost, or € 7.3 kg-1algae, is accomplished when 80% of the medium is reused on a 0.38 ha, 108 m³ microalgae cultivation plant. |
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Language
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English
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Publication
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Ghent
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Ghent University, Faculty of Sciences, Department of biology
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2019
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Volume/pages
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159 p.
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Note
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Diels, Ludo [Supervisor]
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Roef, Luc [Supervisor]
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Tavernier, Serge [Supervisor]
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Vyverman, Wim [Supervisor]
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Full text (publisher's version - intranet only)
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