Conclusions

Based on the results of this work, the following are the main conclusions:

1. N. salina and C. vulgaris grown in full strength seawater media proved to have the

Chapter 5 Summary and Conclusions

173 2. A lipid productivity of 23.9r0.6 mg L1 d1 for C. vulgaris grown in full strength

seawater media was comparable to a lipid productivity of 22.4r1.2 mg L1 d1 for the same alga grown in freshwater media.

3. All microalgae could be effectively harvested by flocculationsedimentation using either aluminum sulfate or ferric chloride.

4. In nearly all cases, aluminum sulfate was a somewhat superior flocculant than ferric chloride.

5. The optimal flocculant dosage (mg L1) for 95% recovery of the biomass following a standardized treatment increased linearly with increasing concentration of the microalgal biomass in the broth. In other words, the flocculant dosage for a given level of biomass recovery under standardized processing conditions increased with an increase in the cell specific surface area in the range of 26450 Pm2 cell1. Therefore, the main mechanism of the flocculation appeared to be cell surface charge neutralization.

6. The efficiency of microalgal biomass recovery by flocculation depended on the following factors: the type of flocculant; the flocculant dose; the biomass concentration in the broth; the microalgal species; and the ionic strength of the culture broth. The microalgal species appeared to be the most important variable.

7. In a continuous flocculationsedimentation process designed for N. salina, a maximum

recovery efficiency of 86% was attained using aluminum sulfate (229 mg L1) and a residence time of about 148 min. Potentially this flocculation efficiency may be enhanced by a redesign of the sedimentation tank, a subject of possible study in future work outside this thesis.

Chapter 5 Summary and Conclusions

174

8. The flocculant adhering to the algal biomass paste did not interfere with extraction of the oils via a modified Bligh and Dyer (1959) method.

9. The optimal extraction conditions for total recovery the lipids from 1 g (dry basis) of N. salina biomass paste were as follows: solvent volume of 33 mL; a solvent composition of

5.7:3:1 by volume of chloroform, methanol and water; a temperature of 25qC; and an extraction time of 2 h. These extraction conditions could recover 96.1% of the oil present in the biomass. Compared to the standard extraction method (modified Bligh and Dyer (1959) method), the single step optimized extraction method developed in this work reduced the solvent mixture volume to ~52% and the extraction time to ~22%.

175

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