Optimizing Light Requirements and Dilution Rates for Maximizing Algal Cell Growth and Lipid Content for Biofuel Feedstock Production

Abstract

There is a rapidly growing interest in the potential of microalgae as feedstock for the next generation of biofuels. The current study was designed to meet the following objectives: (1) Investigate the interdependence among cell density, biomass and lipid productivity to maximize feedstock production from outdoor cultures; (2) Investigate the effect of cycling high-intensity lights to identify optimum Dark:Light (D/L) ratios, which can be effectively used by researchers to maximize aerial biomass productivities; (3) Develop a deterministic model to simulate the biomass productivity in an continuous-flow bioreactor at various hydraulic retention times. Results indicated that variation in HRTs has a major bearing on biomass and lipid productivities. In general, higher net biomass productivity was achieved at shorter HRTs. However, the maximum net lipid productivity was achieved at longest retention time for all algal species studied. Upon stopping the media flow, the lipid concentrations increased at the expense of proteins and to a lesser degree on carbohydrates. The study also indicated that cultures closer to their physiological cell density limit divert the incident energy more towards lipid production. Results from experiments targeted at the second objective indicated that enhancement of the photosynthetic utilization of high intensity light due to light modulation can be achieved. Both growth rates and cell concentrations reached their maximum values under medium light cycle ratio (4/1 D/L) and medium frequencies (6.4-25.6 Hz), and progressively decreased with further increase in D/L ratio and frequency. Light efficiency measurements based on chlorophyll content were interrelated with the cell concentrations and growth rates. Outdoor culture-cycling studies suggested that aerial productivities could be significantly maximized by increasing the culture volume or depth for a given light exposed area if an optimum D/L ratio is maintained. A deterministic mass balance model was developed to simulate microalgal productivities in outdoor continuous-flow systems. The productivity models were comparable to actual experimental data and statistically validated. Higher biomass cell concentrations was achieved at longer HRTs and declined with shortening the HRTs. Maximum volumetric and areal production was achieved at the shorter, 6 h HRT, producing 125 g/m3∙day and 43 g/m2∙day, respectively. Growth variations due to fluctuations in available average light intensities and biomass concentrations were also simulated, concluding that productivity is directly dependent on light availability and biomass concentrations in algal bioreactors.