Assessing outdoor algal cultivation in panel and raceway photobioreactors for biomass and lipid productivity

Over the past decade, there has been a revival in applied algal research and attempts at commercialization. However, the main limitation in algal commercialization is the process of cultivation, which is one of the main cost and energy burdens in producing biomass that is economically feasible for different products. There are several parameters that must be considered when growing algae, including the type of growth system and operating mode, preferred organism(s), and many other criteria that affect the process of algal cultivation. The purpose of this dissertation was to assess key variables that affect algal productivity and to improve outdoor algal cultivation procedures. The effect of reducing or eliminating aeration of algal cultures at night, in flat panel photobioreactors (panels), was investigated to assess the reduction of energy consumption at night. The lack of aeration at night resulted in anoxic conditions, which significantly reduced lipid accumulation and productivity, but did not affect log phase biomass productivity. In addition, the reduction in aeration resulted in lower pH values, which prevented ammonia volatility and toxicity. Raceways are operated at deeper cultivation depths, which limit culture density and light exposure. Experimentation was accomplished to determine the effects of decreasing cultivation depth, which resulted in increased lipid accumulation and lipid productivity, but did not significantly affect biomass productivity. A comparison of semi-continuous cultivation of algae in raceways and panels in side-by-side experiments showed that panels provided better temperature control and higher levels of mixing, which resulted in higher biomass productivity. In addition, sub-optimal morning temperatures in raceways compared to panels were a significant factor in reducing algae biomass productivity. The results from this research indicate that increasing lipid productivity and biomass productivity cannot be completed simultaneously. Therefore, the desired product will determine if lipid or biomass productivity is more crucial, which also dictates whether the system should be operated in batch mode to either allow lipid accumulation or in semi-continuous mode to allow high biomass productivity. This work is a critical step in improving algal cultivation by understanding key variables that limit biomass and lipid productivity.