藻株Monoraphidium sp.於不同實際廢水生產生質柴油之比較並添加醋酸增進其油脂含量之研究

Abstract

Since the industrial revolution from the 18th century, the demand of energy has been greatly growing in order to promote a better quality of life. However, fossil fuel is a limited and non-renewable energy source. It is necessary to develop some renewable and alternative energy sources to mitigate the crisis of increasing energy demand. Biodiesel is one kind of bioenergy that is one type of renewable energy source and has been widely used in the present. It is produced by the transesterification process of converting triglycerides to fatty acid methyl esters that is accumulated in plant cells. Recently, some studies demonstrate that using microalgae is a promising technology to produce biodiesel because the footprint to cultivate microalgae is low, microalgae growth do not compete with current cereal, microalgae has a relatively high growth rate, etc. In addition, comparing with some energy crops, it has been elucidating that microalgae can produce even ten-times more biodiesel in the same area. Therefore, the aim of this study is to establish a cultivating strategy that could generate biodiesel from microalgae as much as possible by culturing in wastewater. In this study, a self-isolated microalgae strain Monoraphidium sp. was cultured in both hi-tech industrial and food processing wastewater to observe the individual nutrient removal efficiency and biodiesel production. Besides, acetate as an additional carbon source was supplemented, trying to enhance its lipid content. The microalgae Monoraphidium sp. strain was cultivated in long-tube reactors to optimize its growth factors and then the lipid content was estimated under the optimal conditions in a 5-L reactor. Comparing with synthetic medium and food processing wastewater, the results showed that the Monoraphidium sp. has the highest growth rate (1.46 day-1) and lipid productivity (70 mg/L-d) when being cultured in the hi-tech industrial wastewater. The removal rate of ammonium, nitrate, and phosphate in the end of the batch experiment can be achieved in 100%, 100%, and 81%, respectively. While providing additional carbon sources, including glycerol, acetate, and isopropyl alcohol, in the hi-tech industrial wastewater could increase the lipid productivities of Monoraphidium sp. in 84, 68, and 98 mg/L-d, respectively, which were all higher than that of original hi-tech industrial wastewater (50 mg/L-d) in long-tube reactor. From the result of acetate supply, Monoraphidium sp. could not grow directly in a high concentration of acetate above 20 mM. However, appropriate dosage of acetate supply or using a fed-batch type of acetate supply can dramatically increase the growh rate of Monoraphidium sp. and consequently increase its lipid productivity. It could synthesize more lipids (in the lipid content of 37%) and achieve the highest lipid productivity (109 mg/L-d) under a mixed carbon source with 5% CO2 and 5mM acetate. In addition to acetate supply, the strategy of limiting nitrogen source in two-stage cultivation was conducted, resulting in increasing lipid contents in 6-9% regardless of illumination intensity, phototrophic growth, or heterotrophic growth. These results demonstrated that nitrogen deficiency is still a significant factor to the lipid production in Monoraphidium sp.. On the other hand, the lipid content under the heterotrophic growth was higher than that of the phototrophic growth both with nitrogen deficiency, elucidating that the supplement of acetate was benefit of producing lipid in Monoraphidium sp.. Finally, the cultivation strategy with fed-batch acetate supply and two-stage limiting nitrogen was carried out within the 5 L reactor, achieveing the highest amount of total lipid production in 336 mg/L (high illumination intensity) and 336 mg/L (low illumination intensity) after 14-days cultivation (6 days after nitrogen deficiency). The acetate uptake by Monoraphidium sp. was excluded on day 18 and the COD value was gradually increased during this moment. According to the results from this study, by utilizing the two-stage nitrogen-limiting cultivation strategy combining with adding acetate to enhance the potential of biodiesel production of Monoraphidium sp. using the hi-tech industrial wastewater, we can achieve the goal of treating a mixed wastewater with high amount of ammonia and acetate and generating biodiesel simultaneously.