PhD Thesis Defence Presentations - Polytimi Papavasileiou

The object of this work is to propose an alternative way of utilizing rice hull, an underexploited type of lignocellulosic biomass, aiming to produce valuable microalgae biomass rich in natural antioxidant phenolic compounds. This study offers a unique and fresh look into biomass valorization with microalgae towards the production of value-added products, that can help meet the ever-growing demand from cosmetics and food industry for products of natural origin. On the one hand, rice hull is rich in cellulose, hemicellulose and thus is an excellent candidate for the production of a nutrients-rich hydrolysate that can serve as an economic and sustainable microalgae feed. On the other hand, microalgae biomass is considered as one of the most valuable types of biomass, able to biosynthesize a wide variety of biochemicals with endless applications. To this end, microalgae derived natural phenolic extracts with potent antioxidant activity were the target product of this study. 

The basic steps of the studied fields in this work can be categorized in the following: firstly, study of rice hull hydrolysis, secondly, cultivation of microalgae in rice hull hydrolysate, thirdly, study of the production of phenolic compounds from microalgae and fourthly, study of biomass phenolic extraction techniques. In addition, valuable biosilica recovery from the spent rice hulls was also covered. Finally, a simple technoeconomic analysis took place in the end of the study, to help better understand the feasibility of such a project in real life at a greater scale. 

The first step of this study consists of the thermochemical hydrolysis of rice hull, in which various types of pretreatments were tested, in particular acid and alkaline pretreatments. Also, the effect of various hydrolytic parameters was tested, such as hydrolysis time, solids loading, acid dose, temperature, whereas the importance of mechanical pretreatment (particle size) was also highlighted. The outcome of this study was the obtainment of the optimum hydrolytic conditions which lead to the production of a sugars- and nitrogen-rich liquid hydrolysate suitable for use as microalgae growth medium. 

Next, a microalga strain suitable for phenolics production was selected among several ones, after their cultivation autotrophically under the same conditions. The selected microalga, Botryococcus braunii, was then grown in rice hull hydrolysate aiming to combine high biomass production with optimum phenolics accumulation intracellularly. Various series of cultures in heterotrophic and mixotrophic conditions were carried out to in order to understand the mechanism which favors the production of phenolics of this microalga. What is more, a two-stage cultivation scheme was also tested in which B. braunii was grown first mixotrophically or heterotrophically in rice hull hydrolysate and secondly autotrophically, as a means to further improve its phenolics yields. On top of this, further studies on the autotrophic stage of the two-stage culture were done, with the goal to find adequate culture parameters that can enhance the total phenolic content of the produced biomass. From the above study, the optimum cultivation conditions leading to high phenolics accumulation were selected. 

Furthermore, the extraction of phenolics from microalgae biomass was also greatly investigated during this study. In particular, several solvents and extraction techniques were tested and compared towards their efficiency to effectively extract phenolic compounds, whereas both qualitative and quantitative methods were used for the evaluation of the obtained extracts. 

Finally, the technoeconomic assessment of the proposed process included the design of a complete flowchart for the entire process, the estimation of the total capital costs for each equipment type, the estimation of the total operational costs on a year-round basis, and an investment plan to calculate the revenues and return of investment in a hypothetic scenario that was selected for analysis.

The conclusion of this study is that a multitude of important results were obtained in all the research fields studied, while innovative elements were added in the global effort to implement technologies that adhere to the principles of circular economy. The assessment of this specific study is that the combination of lignocellulosic biomass with microalgae cultivation is not only feasible, but a new technology that opens new avenues for the creation of complex biorefineries of lignocellulosic biomass with microalgae, which will in the future be able to combine the production of energy and value-added products with the utilization of these unexploited but valuable natural resources.