Optimisation of microalgae upstream and downstream processes for sustainable biodiesel production

Okoro, Victor Ikenna (2022) Optimisation of microalgae upstream and downstream processes for sustainable biodiesel production. Doctoral thesis, Northumbria University.

Preview

Text (Doctoral thesis) okoro.victor_phd(16036517).pdf - Submitted Version Download (5MB) | Preview

Abstract

The sustainability of microalgae biodiesel production primarily relies on the choice of algae species, carbon sources utilised and mode of cultivation. Careful choices of these features can stimulate the production of commercial and suitable biodiesel of international standards. Optimisation of the key units such as cultivation, harvesting and cell disruption is necessary to cut down excess inputs that will not only circumvent hike in the cost of production but help identify the overall conditions for optimal biodiesel yields. In this study, firstly, two selected oleaginous microalgae species Chlorella vulgaris (C. vulgaris) and Nannochloropsis oculata (N. oculata) were cultivated in photobioreactor (PBR) and multi-cultivator (MCR) mixotrophically and photoheterotrophically utilising glycans such as β-glucan, β-mannan and xylan, and folic acidas carbon sources using artificially made wastewater and formulated growth media (BG 11). The aim is to investigate their effects on biomass productivity and growth rate, lipids content, fatty acid composition, biodiesel yields and properties. Biodiesel properties were estimated empirically using equations available in literature. Secondly, microalgae samples were harvested via flocculation. Cell wall disruption and lipid extraction were carried out using a mild and low energy device ‘Tissue-lyser II’ after cell wall weakening via osmotic shock. Thirdly, key parameters of three microalgae biodiesel production units comprising of photoheterotrophic cultivation, harvesting by flocculation and cell disruption were optimised using the response surface methodology (RSM).

Results obtained from the preliminary growth experiment conducted using wastewater (mixture of distilled water and miracle gro. fertilizer) showed a limited degree of growth (maximum OD of 0.44). Supplementing the medium with folic acid solution inhibited microalgae growth and eventually terminated the growth phase, signifying that the dissociation of folic acid in culture did not make available the much-needed carbon required for algae growth, and therefore is unsuitable for use as a carbon source. Utilising a properly formulated Blue Green (BG 11) growth medium supplemented with organic carbon sources enhanced growth. Results obtained after a period of 8 cultivation days in BG 11 showed that C. vulgaris outweighs N. oculata in all growth parameters such as biomass concentration, productivity, and growth rate, signifying better nutrient absorption and photosynthetic activities by the former species. Biomass concentration of 0.186, 0.287, 0.267, 0.214 and 0.232 gl-1 were obtained from C. vulgaris and 0.133, 0.223, 0.238, 0.167 and 0.195 gl-1 from N. oculata when cultivated mixotrophically in culture supplemented with control (culture not supplemented with carbon source), glucose, mannose, β-glucan and β-mannan respectively. Comparatively, these results are higher than 0.184 and 0.218 gl-1 obtained from C. vulgaris, and 0.165 and 0.184 gl-1 from N. oculata obtained when cultivated photoheterotrophically, indicating that the act of passing air bubbles into culture (aeration) practiced under mixotrophic mode of cultivation further supplements culture with carbon dioxide (CO2) which resulted in the upsurge in growth parameters observed. The outcome of optimising the photoheterotrophic mode of cultivation clearly showed that growth is dependent on the combination and interaction of both nutrients: organic carbon (glycans) and organic nitrogen sources(urea). More so, utilising β-glucan and β-mannan from different sources (yeast and barley) and structures as carbon sources did not significantly influence growth parameters when compared. Additionally, the growth performance of both microalgae species cultivated in MCR was also influenced by glycans (β-glucan, β-mannan and xylan) when utilised as carbon sources in such a decreasing order of xylan>β-mannan>β-glucan. The algae species and glycans studied are good candidates for biodiesel production, with maximum biomass and lipid yields from the species cultivated in the culture supplemented with xylan.

Thereafter, the two selected algae species which were cultivated under the same conditions were harvested and flocculated at different times in acidic, alkaline, and neutral media, aimed at investigating and comparing how flocculation parameters can influence algae flocculation efficiencies. Results revealed a high flocculation efficiency (over 80 %) of both species, but at different flocculation conditions. It was observed that N. oculata flocculated better than C. vulgaris in alkaline culture (pH of 11) whereas C. vulgaris is better flocculated in acidic culture (pH of 3). At maximum flocculation, it was observed that N. oculata was very resistive to cell loss when compared with C. vulgaris. Cell loss of C. vulgaris is over 3fold higher than that N. oculata.

In another unit process, wet and dry harvested algae species were subjected into cell disruption regime. Results obtained from cell disruption experiments conducted on wet algae samples using the Tissue-lyser showed that owing to the stronger cell wall composition and matrix which amplifies the resistance to shear damage of N. oculata, lower disruption efficiency was observed in N. oculata (55.9±5.7%) than C. vulgaris (84.15±0.95%) under the same disruption conditions. Observably, the effects of treatment time were coherent in the first 20 minutes, thereafter, the rate of disruption declines. Also revealed was that more cells were disrupted at lower specific energy on agitating culture of higher biomass concentration due to the exposure of more biomass to the grinding beads resulting in the disruption of more cells. Cell disruption efficiency increases with increasing specific energy and decreasing biomass concentration. Energy input as a function of time in the disruption regime was relatively low. However, energy utilisation estimated in form of specific energy (energy input per unit mass) is high due low mass of algae biomass treated. The specific energy that resulted in maximum disruption efficiency and lipids yield is 18 MJ/g and can be further reduced by increasing the mass of biomass in the treated culture. Also, lipid was extracted from dry algae samples subjected to bead shaking using the Tissue-lyser II. Maximum lipid yields of 26, 18, 23.5, 17.5 and 11.5 % by weight of dry C. vulgaris, and 23.5, 15, 22.5, 17 and 8.5% by weight of dry N. oculata were obtained from culture grown under mixotrophic cultivation using β-glucan, glucose, β-mannan, mannose and control respectively, indicating that glycans are potential carbon sources for sustainable microalgae lipid yields for biodiesel production. For obvious reason, the specific energy consumption is relatively very low (0.9 MJ/g) which is one-twentieth of the 18 MJ/g obtained on treating wet algae utilising the same device. More so, the influence of various carbon sources on fatty acid composition and biodiesel fuel properties were observed to be significant in this study. The values of key biodiesel properties that can enhance engine performance (cetane number, oxidative stability and viscosity) are within the range of set international standards, signifying that glycans utilised in this study can enhance the volume and quality of biodiesel from microalgae extracts.

Actions (login required)

View Item

Downloads