Doctor of Philosophy
Chemical and Biochemical Engineering
Today, microalgae are cultured in large-scale systems such as open ponds and photobioreactors. Open pond systems, while not expensive, have disadvantages, including contamination and high water consumption, resulting in increased cost of harvesting and quick evaporation. Photobioreactors overcome some of the challenges of an open pond system. However, the reactor setup, operation, and microalgae harvesting are relatively more expensive when compared to conventional techniques. In this study, a novel foam-based photobioreactor system was applied to investigate both the cultivation and harvest of microalgae using different surfactant agents, i.e., rhamnolipids, Bovine Serum Albumin (BSA), saponin, and the commercial surfactant Pluronic f127TM.
The study was divided into several phases. In the first phase, the foam stability of different surfactants used and their critical micellar concentration (CMC) were evaluated in water and two other microalgal growth media, namely, Bold’s Basal Medium (BBM) and a garden fertilizer MiracleGroTM. The stable foam was achieved with rhamnolipid concentrations of 1.68 g/L. Similarly, BSA concentrations of 2.24 g/L formed durable foam. The conductivity for rhamnolipids increased with the concentration, with 3810 µS for the highest concentration, 2.18 g/L. Finally, FT-IR analysis showed that rhamnolipids and BSA did not change the outer cell wall of Chlorella vulgaris.
In the second phase, Pluronic f127TM, saponin, BSA, and Rhamnolipids were used as foaming agents to investigate the growth of Chlorella vulgaris on liquid foam in a novel liquid foam-based photobioreactor, where the nutrition uptake was studied. BSA and Pluronic f127TM showed growth of Chlorella vulgaris with a maximum specific growth rate of 0.8/day and 1/ day.
This study's third and final phase investigated BSA, Pluronic f127, saponin, and rhamnolipids as foaming agents for harvesting Chlorella vulgaris. Different concentrations of rhamnolipids, saponin, BSA, and pluronic were used to harvest C. vulgaris cells. For the small-scale study, BSA and saponin showed the highest number of cells collected. Amongst rhamnolipids, BSA, saponin, and Pluronic f127TM, BSA and saponin showed higher harvesting of cells, which was 63% and 57%, respectively, in the large-scale setup. The collection factor was calculated, and it was found that rhamnolipids had no effect in increasing the concentration of microalgae cells. The collection factor ranged from 0.2% to 1 % on the small scale. The collection factor using saponin increased from 27% to 29.5% in two hours, while it decreased for BSA from 27% to 15.4% with an increase in time.
Summary for Lay Audience
Microalgae are microscopic and photosynthetic plants grown in freshwater, salt water, moist rocks, and soil. They require sunlight and carbon dioxide for growth. Many microalgal species are rich in nutrients and bioactive compounds; hence, they have many applications in different fields like food, nutraceuticals, pharmaceuticals, wastewater treatment, carbon capture, etc. The open pond system and photobioreactors are the two methods of large-scale microalgae production. The open pond system and the photobioreactors have advantages and disadvantages. While the photobioreactors overcome most of the drawbacks of the open pond system, there are several challenges in the photobioreactor; hence, a foam-based photobioreactor where foaming agents are added was used to overcome problems faced in photobioreactors.
The first part of the research was to understand the interaction of Chlorella vulgaris microalgae grown in a lab-made growth medium and commercial plant fertilizer called MiracleGroTM with biosurfactants such as BSA and Rhamnolipids. Tests such as zeta potential, conductivity, surface tensions, and FT-IR analysis were done.
The second part of this thesis investigated the growth of Chlorella vulgaris with foaming agents like BSA, Saponin, Pluronic f127, and rhamnolipids. The growth was monitored every day for six consecutive days. The optical density and nutritional uptake, such as nitrate and phosphorus, were observed during that time.
The third part, the final phase of this research, was to separate the cells from the media, which is called harvesting using the same surfactants and biosurfactants. The number of cells collected using foam was measured, and the dry weights were calculated to understand the efficiency and potential of each surfactant and biosurfactant.
Krishnan, Anuradha, "Investigation of Foam based Photobioreactor for the Cultivation of Chlorella vulgaris CPCC 90" (2023). Electronic Thesis and Dissertation Repository. 9811.