Optimization of microalgal production in a shallow outdoor flume

The marine diatom Cyclotella cryptica was grown over a period of 13 months in a 48‐m 2 shallow outdoor flume. The use of foil arrays at intervals of 1.2 m to effect systematic vertical mixing in the flume was found to significantly enhance microalgal production ( p = 0.006). Average photosynthetic efficiencies (based on visible irradiance) with and without the foil arrays in place were 9.6 ± 0.8 and 7.5 ± 0.5% (±95% confidence intervals), respectively. A cost–benefit analysis indicated that the foil arrays were cost‐effective if the value of the algae exceeded about $2.28 kg 1 of ash‐free dry weight (AFDW). Parallel experiments performed in four 9.2‐m 2 flumes showed that production was maximized when the cells were grown on a 2‐day batch cycle between harvests rather than on a 1‐ or 3‐day batch cycle. The optimum initial concentration (immediately after harvesting) of the algae was negatively correlated with the time interval between harvests and ranged from approximately 39 g AFDW/m 3 on a 3‐day cycle to 213 g AFDW/m 3 on a 1‐day cycle. The increase in production resulting from growth on a 2‐day rather than a 1‐day batch cycle was about 19% and was statistically significant at p = 0.0003. Growth of C. cryptica over a total period of 122 days during the 13‐month study in the 48‐m 2 flume under near‐optimal conditions (2‐day batch cycle, initial concentration 155 g AFDW/m 3 ) resulted in an average production rate (±95% confidence interval) of 29.7 ± 2.7 g AFDW/m 2 d.