Exploration of microalgal culture using a semi-closed thin layer cascade photobioreactor for biodiesel synthesis and protein production

Tan, Chung Hong (2020) Exploration of microalgal culture using a semi-closed thin layer cascade photobioreactor for biodiesel synthesis and protein production. PhD thesis, University of Nottingham Malaysia.

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The expanding aquaculture industry increases the prices of fishmeal, the main protein source in fish diet. A promising alternative is microalgal protein. Therefore, the protein production capacities of two green microalgae species, Chlorella sorokiniana CY1 and Chlorella vulgaris ESP-31, were investigated. After optimization, the maximum biomass and protein productivities of Chlorella sorokiniana CY1 reached high values of 4.35 ± 0.09 and 0.856 ± 0.025 g/L/d, while that of Chlorella vulgaris ESP-31 also reached high values of 4.636 ± 0.10 and 0.946 ± 0.065 g/L/d. The cultivation time for both species was only 2 days, wherein Chlorella sorokiniana CY1 and Chlorella vulgaris ESP-31 amassed moderate protein contents of 25.9 ± 1.3% and 26.8 ± 1.3%. The optimum conditions for both species were 50% initial nitrate concentration of Basal medium, 5% CO2 aeration, and 750 µmol/m2/s light intensity. The high biomass and protein productivities of both species indicated their capability as potential protein sources.

After studying protein production from microalgae, a second microalgae-derived commodity that received global attention was microalgal biofuels, in particular biodiesel. Microalgae is widely regarded as the most promising source of green and sustainable fuel for the future, with microalgal biodiesel widely seen as a replacement for diesel fuel. Compared to biodiesel from terrestrial crops such as palm oil and rapeseed, the benefits of microalgal biodiesel include rapid growth rate of microalgal cells, higher carbohydrate and lipid content, higher productivity per unit land area, and utilisation of wastewater for growth. Therefore, five indigenous microalgae species from Taiwan were selected to determine their potential for lipid production, namely Chlamydomonas sp. Tai-01, Tai-03 and Pin-01, as well as Scenedesmus sp. ESP-05 and ESP-07. Chlamydomonas sp. Tai-03 proved to be the best strain, achieving biomass growth and productivity of 3.48 ± 0.04 g/L and 0.43 ± 0.01 g/L/d, accompanied by an oil content and oil productivity of 28.6 ± 1.41% and 124.1 ± 7.57 mg/L/d. This was attained by inoculating 0.12 g/L biomass into BG-11 medium with 25% initial nitrate concentration and light-emitting diode (LED) light intensity of 200 µmol/m2/s. The fatty acid methyl esters (FAMEs) obtained from the Chlamydomonas sp. Tai-03 strain consisted mainly of palmitic acid, oleic acid and linoleic acid, making this microalga a suitable feedstock for biodiesel synthesis.

The microalga species Chlamydomonas sp. Tai-03, which was previously optimised for maximal lipid production for biodiesel generation, received further optimization by investigating the effects of different CO2 concentration and different medium replacement strategy using semi-batch operation. The optimum conditions were found as 5% CO2 concentration and semi-batch operation with 25% medium replacement ratio, which further enhanced the biomass growth and productivity to 4.15 ± 0.12 g/L and 1.23 ± 0.02 g/L/d, with lipid content and productivity of 19.4 ± 2.0% and 239.6 ± 24.8 mg/L/d. Under the medium replacement strategy, 25% of the microalgal biomass could be replaced/harvested daily after the culture entered the medium replacement phase of the cultivation process (which was 8 days after the start of cultivation). Upon analysis, the major fatty acid methyl esters (FAMEs) were palmitic acid (C16:0), oleic acid (C18:1), and linoleic acid (C18:2). These short-chain FAMEs combined with high growth rate make Chlamydomonas sp. Tai-03 a suitable candidate for biodiesel synthesis.

Although microalgae can be turned into numerous valuable commodities, their large-scale cultivation still poses a great challenge to the current microalgal industries. Open systems are cheap to install and operate, easy to clean and maintain, and have high surface area to volume ratio, suitable for microalgal culture. But they are susceptible to contamination, evaporative losses, and unfavourable weather conditions. In contrast, closed photobioreactor systems have high productivity, control of growth parameters, and minimum risk of contamination. But photobioreactors are expensive to install and difficult to operate and maintain. Therefore, an integration of thin layer cascade photobioreactors with a semi-closed setup was proposed that could be used to cultivate greater volumes of different microalgal species and generate different products such as proteins and lipids. This semi-closed photobioreactor system would have the benefits of both open systems and closed systems. An improved concept design had been illustrated and computational fluid dynamics (CFD) analysis using a simulation program (ANSYS) had also been performed to visualise the functionality and mechanism of the concept design. The CFD analysis revealed that the improved concept design of thin layer cascade semi-closed photobioreactor performed satisfactorily and was suitable for the cultivation of different microalgal species to produce proteins for fish feed and lipids for biodiesel.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Show, Pau Loke
Lam, Hon Loong
Keywords: aquaculture industry, microalgal protein, biomass, biodiesel, photobioreactor system
Subjects: T Technology > TP Chemical technology
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Engineering > Department of Chemical and Environmental Engineering
Item ID: 59762
Depositing User: TAN, Chung
Date Deposited: 22 Feb 2020 04:40
Last Modified: 21 Feb 2022 04:30
URI: https://eprints.nottingham.ac.uk/id/eprint/59762

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