Ojo, EO; (2015) Photobioreactor Technologies for High-throughput Microalgae Cultivation. Doctoral thesis , UCL (University College London).

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Abstract

The evaluation and optimisation of microalgae cultivation process for biomass, lipid and high value chemicals production requires experimental investigation of several interacting variables. This thesis addresses the development of a range of small-scale photobioreactor technologies and shows how they can be applied for rapid, early stage evaluation and scale-up of microalgae cultivation processes. In particular, the work focuses on the engineering evaluation of a novel shaken miniature photobioreactor (mPBr) and a single-use photobioreactor (SUPBr) that can be adapted for both phototrophic and heterotrophic cultivation. A prototype twin-well mPBr was initially designed and fabricated with light provided from cool white light emitting diodes (LED). This was scaled-out to a 24-well mPBr system (4 mL working volume) on a novel shaken platform. High power warm white LEDs provided a maximum light intensity of 2000 µmolm‾²s‾¹. In both systems, surface aeration (via a semipermeable membrane) and mixing were provided by orbital shaking. Real-time control of temperature, relative humidity and CO2 levels was achieved via incubator level control. Amongst the tested geometries of the mPBr, round base and pyramid base gave the best performance. The mass transfer coefficient (kLa) values in the 24-well were measured between 20 – 88 h‾¹ and visual observation of fluid hydrodynamics showed an increase in total surface area with increased shaking frequency. Negligible evaporation was observed at 90% relative humidity for light intensity of < 400 µmolm‾²s‾¹ and at 32 °C, while light intensity variation across the platform is in the range ± 20 µmolm‾²s‾¹. Evaluation of phototrophic culture kinetics of Chlorella sorokiniana in both mPBr designs showed good reproducibility between wells. The best culture performance occurred at 380 µmolm‾²s‾¹, 300 rpm and 5% CO2, where final biomass concentration and total lipid concentration achieved were 9 ± 0.2 gL‾¹ and 55% w/w respectively. The SUPBr comprised a transparent polymeric CultiBagTM operated on the illuminated rotary shaken platform described above. Mixing time values were determined over the range 40 - 220 rpm and were generally less than 40 s. Hydrodynamic studies showed three distinct flow regimes at various shaking frequencies: in-phase, transitional and out-of-phase. Under optimal flow regime, the highest cell concentrations achieved was 6.7 gL‾¹ ± 0.3. Doubling the total working volume resulted in 35 - 40% reduction in biomass concentration due to an increase in the light path length. Phototrophic scale-up criteria from mPBr to SUPBr was successfully achieved based on light–path length and kLa values. Comparison of final biomass concentrations showed similar performance of 6 ± 0.2 gL‾¹ and comparable total lipid production of 25 – 30% by weight at a light intensity of 180 ± 20 µmolm‾²s‾¹. Furthermore, application of the shaken 24-well system for heterotrophic cultivation of microalgae and scale-up to a 7.5 L stirred tank bioreactor was also shown. Cells were cultured in 24 parallel wells, shake flasks and a 7.5 L bioreactor with working volumes of 4 mL, 100 mL and 4000 mL respectively using glucose (10 gL‾¹) as the main carbon source. Constant k(L)a was chosen as scale-up criteria and the values range between 30 – 60 h‾¹. Final biomass concentrations showed good agreement in the range of 4.5 ± 0.5 gL‾¹ and total lipid production of 43 – 50% by weight for the three systems. Overall, the results show the utility of the mPBr and SUPBr technologies for the rapid evaluation and scale-up of both phototrophic and heterotrophic microalgae cultivation conditions.

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