Growth Condition Optimization and Bacterial Control for Mass Production of Microalgae

Abstract

Microalgae are considered as one of the most promising future energy feedstocks because of some advantages, such as the simple cellular structure, short production cycle, high lipid content, and fast growth. However, relatively high production costs due to low lipid productivity, high nutrients demand, and high water/energy consumption are some of the major obstacles impeding commercial production of algal biofuels. Microalgae strain Chlorella sorokiniana DOE1412 was used in this dissertation due to its robust growth and high productivity. This work focuses on two area that can reduce the production costs: optimizing environmental growth conditions, and a bacteria removal strategy for recycled water. Cultivation of microalgae for biofuel production is highly influenced by numerous environmental conditions, including physical conditions (e.g., light and temperature) and chemical conditions (e.g., nutrients, salinity, and pH). These environmental conditions not only affect the accumulation of biomass but also influence the biochemical composition of microalgal biomass. The pH is one of the most critical environmental conditions in microalgal cultivation since it determines the solubility and availability of CO2 and nutrients, and has a significant influence on microalgal metabolism. Here, cell growth and lipid content of Chlorella sorokiniana DOE1412 were first evaluated at different pH values in flask cultivation. Culture pH was manipulated by CO2 addition. The optimal pH for DOE1412 is approximately 6.0 when only accounting for cell growth and lipid production and not considering the CO2 efficiency. A flat panel airlift photobioreactor (PBR) was used for scale-up cultivation at five different pH levels (6.5, 7, 7.5, 8 and 8.5). Data of pH values and CO2 addition was collected by a data logger. Biomass productivity increased with decreasing pH. By taking into account not only the cell growth and lipid production but also CO2 addition, the lowest value of CO2 addition was achieved at pH 8 (2.01 g CO2/g biomass). The fatty acid profiles and biodiesel properties, such as iodine value (IV), saponification value (SV), cetane number (CN), degree of unsaturation (DU), long-chain saturated factor (LCSF), and cold filter plugging point (CFPP), were determined as a function of pH. The calculated CN of biodiesel, which theoretically could be produced from the algae cultivated at pH 6.5, 7 and 7.5, satisfied the US standard ASTM D6751; among them, the pH 6.5 products met the European standard EN 14214. Finally, protein content in microalgal biomass increased with increasing pH, while C/N ratio in cells decreased. Microalgae grown in open systems are prone to biological contaminations, including bacteria, zooplankton, virus, and other algae. Cultivation water recovered from large-scale open raceway ponds contains bacteria that can affect culture health. Developing an inexpensive and effective strategy to control bacterial contamination in recycled cultivation water is essential for making algal production financially and environmentally sustainable. We tested several bacterial deactivation strategies, including ozonation, chlorination, chloramination and the addition of hydrogen peroxide, on artificial recycled water samples containing mixtures of microalgae Chlorella sorokiniana DOE1412 and Escherichia coli strain DH5α. Disinfectant decay, bacterial deactivation and algal survival curves were obtained for each disinfectant at different doses to determine bacteria removal rate, minimum contact time, microalgae survival rate, disinfectant residual, and concentration-time (CT) value. These data were used to compare the efficiency of different bacterial deactivation methods. Results showed that chlorination with an initial dosage of 0.5 mg/L is the most economic and successful method for selectively deactivating bacteria. A 5-log reduction of E. coli cells was achieved after 1 min of contact time, while the microalgae survival rate was 58%. No chlorine was detected after 5 min of exposure. The other methods investigated showed higher CT value or lower microalgae survival rate for 5-log deactivation of E. coli in artificial recycled water samples. The effectiveness of chlorination was also confirmed when tested on the authentic recycled water samples.