| Περίληψη: | The present study focuses on the kinetic study of biomass growth and lipid synthesis of the microalga C. vulgaris under heterotrophic conditions in addition to the energy valorization of the produced biomass. The main reason of this strain attracting so much attention is that it constitutes a versatile microorganism. It has the ability to grow fast, in various kinds of media and wastewaters, on extreme conditions such as extreme pH values and temperatures and tolerate the presence of toxic compounds. Moreover, it accumulates intracellular lipids resulting in high lipid content, the composition of which is suitable for biodiesel production.
The goal of the first set of experiments was to investigate the pH range that can support the growth of C. vulgaris, and, more specifically, to identify the optimal pH for the microalga’s growth, under heterotrophic conditions. Furthermore, the effect of pH on accumulation of intracellular lipids was studied. A wide range of pH values was tested using buffer solutions. The optimal pH for biomass growth and lipid accumulation under sulfur starvation was found to be 7.5, resulting in the maximum specific growth rate of 0.541 days-1 and the maximum total lipid content of 53.43% g gDW-1.
The aim of the second set of experiments was to determine the effect of starvation from different nutrients (S, P and N) on biomass growth and lipid accumulation of C. vulgaris. The potential differences in the effect of nutrient starvation could affect the selection of the most appropriate wastewater mixture for microalgal cultivation, as in many cases, in a realistic application of wastewater treatment using microalgae the mixture of various wastewaters is used as a substrate. The nutrient starvation that had the most significant effect on lipid accumulation was that of sulfur.
Moreover, the use of Volatile Fatty Acids as potential carbon sources for C. vulgaris was investigated. VFAs are metabolic products, intermediate or final, of many processes, such as anaerobic digestion, dark fermentation and acidogenesis. The goal was to identify the VFAs that can be used as carbon sources by C. vulgaris, individually or in a combination with glucose, grown without the presence of light. The VFAs tested were acetic, propionic, butyric and isobutyric acid. C. vulgaris failed to grow on propionic, butyric and isobutyric acid as sole carbon sources. However it was able to grow on acetic acid, resulting in a maximum specific growth rate of 0.429 days-1. Moreover, the combination of the VFAs mentioned above in addition to glucose was used as carbon source, simulating the case in which the produced effluent streams of these processes contain VFAs in addition to other organic compounds. The microalga was again unable to assimilate propionic, butyric and isobutyric acid. The maximum specific growth rate obtained from the assimilation of glucose and acetic acid was 0.534 days-1.
In the following part of the present study, phenolic compounds were tested as potential carbon sources for the microalga. Before carrying out the experiments with different phenols used individually as sole carbon source, an acclimation stage took place. The phenolic compounds tested as potential carbon sources for C. vulgaris were vanillic, ferulic, syringic, gallic, p-hydroxybenzoic acid and catechol, in a concentration of 0.5 g L-1. Catechol inhibited the growth of the microalga from the second stage of the acclimation process, in a concentration of 100 mg L-1, verifying the highly toxic effect of the specific phenolic compound. C. vulgaris failed to grow on vanillic, ferulic and p-hydroxybenzoic acid as sole carbon sources in a cultivation period of 15 days. However, gallic and syringic acid were degraded by C. vulgaris in the respective experiments, resulting in low μmax (0.063 and 0.103 days-1 respectively).
The biotreatment of Olive Mill Wastewater using C. vulgaris was subsequently investigated. OMW is a wastewater with complex composition with the most distinctive features being the high concentration of phenolic compounds, and the low nitrogen content, all present in the form of organic nitrogen, which make OMW a non-easily degradable substrate for microalgae. The microalga was unable to grow on OMW, even after the regulation of pH at the optimal value and the addition of a sulfur and nitrogen source. However, the growth of the microalga in a medium containing 10% v v-1 OMW supplemented with glucose and BG-11 broth showed that it can tolerate the phenols present (in a concentration of approximately 0.5 g L-1), and that the inhibiting factor in OMW is the lack of nutrient bioavailability.
The next part of the present study concerns the energy valorization of the produced C. vulgaris biomass. Biodiesel production from microalgal biomass apart from harvesting and drying also requires the additional steps of transesterification and purification. These steps augment the cost of biofuel production, thus making it an economically unviable solution. In an effort to explore the potentials of microalgal biomass exploitation, the energy valorization of C. vulgaris biomass through direct combustion and anaerobic digestion were also investigated. The specific calorific value of C. vulgaris dry biomass was found to be 24,525 ± 182 kJ kg-1 or 5,861 ± 44 kcal kg-1. The methane potential of C. vulgaris biomass was evaluated by conducting BMP assays in mesophilic and thermophilic conditions, with and without pretreatment (ultrasonication). The maximum yield was obtained in mesophilic conditions without pretreatment, presenting the value of 389.07 ml gVS-1added. Finally, by comparing the energy valorization methods, the highest energy productivity (20.331 kJ Lreactor-1 day-1) was obtained from the direct combustion of C. vulgaris biomass.
Last but not least, the modeling of the behavior of C. vulgaris in pH 7.5 under sulfur stress took place, resulting in satisfying data fitting and parameter estimation.
|
|---|
