Thesis (M.Eng.)--Chulalongkorn University, 2013

Chaetoceros gracilis was cultivated in the batch cultivation system using 5 L airlift photobioreactor. The initial cell concentration was 5 x 105 cells mL-1 and Light was supplied at 135 µmol photon m-2 s-1. This research is divided into four sections. Firstly, the cultivation of C. gracilis with fresh sea water which was cultivated using modified F/2 medium and the reused medium which was cultivated again with the medium after cell separation. A maximum cell density obtained from the fresh medium was approximately 10.73 ± 0.35 x 106 cell mL-1. The maximum cell density of reused medium was 50% lower than that from the fresh medium. Regarding the biochemical composition, total lipid obtained from the reused medium experiment was higher than that from the fresh medium, and the opposite was found for carbohydrate, whereas the level of protein remained unaltered regardless of the management of the medium. Secondly, the culture with varying macronutrient was examined, i.e. silicate, nitrate and phosphate concentration. For the case of 50% nutrient (daily added equally for 4 days) the system exhibited similar performance with that of the control, fresh medium. Carbohydrate was higher with the 50% nutrient experiment whereas lipid seemed to be a major component in the control experiment. Thirdly, the cultivation of C. gracilis with reused 50% macronutrients. The maximum cell densities from the cultivation with 50%silicate, 50% nitrate and 50%phosphate were approximately at the same level, i.e. 11.07 ± 0.78 x106, 11.64 ± 0.11 x106and 9.83 ± 0.12 x106 cell mL-1, respectively. The maximum cell density of reused medium was 50% lower than that from the control experiments. Finally, economics demonstrates that the reuse of medium did not contribute to the cost effectiveness as it affected the growth rate and therefore the system with reused medium requires more inputs to give a comparative performance with the control experiment.