Agbebi, TV; (2019) Design and characterization of a miniature photobioreactor for microalgae culture. Masters thesis (M.Phil), UCL (University College London).

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Abstract

Microalgae process design involves several interacting variables which need to be evaluated and optimised for maximal product yield. To optimise the microalgae culture process, high through-put culture systems are required for early-stage investigation of the interacting culture and operating parameters. This work focuses on the design and engineering evaluation of a miniature pneumatic photobioreactor (mPBR) that can be adapted for heterotrophic and phototrophic microalgae cultivation. The initial objective was to design a mPBR which mimics a conventional largescale pneumatic photobioreactor (PBR). This led to the design and fabrication of a prototype 6-well mPBR, with four bubble columns and two airlift columns. The four bubble columns were fitted with two different size (diameter) sintered disc spargers (5 and 12 mm), while that of the airlift (ALR) was 8 mm. Light was supplied by cool LED light with a maximum light intensity of 200 µmol m−2 s−1. Mixing was provided by bubbling gas through the sintered disc with an average pore size of 13 µm. Comparison of the different configurations shows that the flat based bubble column with the largest sintered disc well (FBC, 12 mm) had the highest mass transfer coefficient (kLa) values of 196 h−1, while a pyramidal based bubble column with the smallest sintered disc (PBC, 5 mm), had the lowest kLa values. Mixing time was generally under 5 seconds using flow rate from 0.6 Lh−1 to 2.4 Lh−1, while the lowest evaporation rate of 16 % (v/v) loss over 5days was observed in the PBC. Further characterisation of Chlorella sorokiniana in them PBR showed that the FBC had the best performance in both mixotrophic and phototrophic culture i.e. highest biomass concentration of 1.20 gL−1 in the mixotrophic culture was obtained at a flowrate of 1.8Lh−1 (the highest flowrate used), while phototrophic culture had the highest biomass concentration of 1.5 gL−1 at 200 µmolm−2 s−1 and 0.6 Lh−1. The difference in growth performance in the sintered disc-mPBRs was attributed to the difference in the diameter of the sintered disc: while smaller gas fraction and bigger bubbles in the PBC led to reduced gas hold-up and lower mass transfer, it, however, provided optimal condition for C. sorokiniana growth in the reactor. Hence for optimal growth performance, the bubble size and velocity must be optimised using an appropriate sparger. This led to the use of a single orifice sparger; where a higher biomass, less evaporation, and better reproducibility was observed. Further evaluation of the potential of them PBR as early-stage process development tool was done by determination of the total lipid content followed by FAME analysis. Lipid content of up to 35.86 % (WW −1) DCW and high composition of C16 and C18 fatty acids obtained were comparable to literature. Evaluation of critical growth parameters using a design of experiment (DoE) approach, showed that light intensity and initial culture density are the two main factors that affects biomass concentration in the mPBR. Increase in initial nitrogen concentration led to corresponding increase in the final biomass concentration up till the critical nitrogen concentration where it became inhibitory. Overall, the mPBR was established as a tool for phototrophic, heterotrophic, and mixotrophic culture of microalgae for rapid and early stage evaluation at small scale.

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