A Comparison of Nannochloropsis salina Growth Performance in Two Outdoor Pond Designs: Conventional Raceways versus the ARID Pond with Superior Temperature Management

The present study examines how climatic conditions and pond design affect the growth performance of microalgae. From January to April of 2011, outdoor batch cultures of Nannochloropsis salina were grown in three replicate 780？L conventional raceways, as well as in an experimental 7500？L algae raceway integrated design (ARID) pond. The ARID culture system utilizes a series of 8–20？cm deep basins and a 1.5？m deep canal to enhance light exposure and mitigate temperature variations and extremes. The ARID culture reached the stationary phase 27 days earlier than the conventional raceways, which can be attributed to its superior temperature management and shallower basins. On a night when the air temperature dropped to ？9°C, the water temperature was 18°C higher in the ARID pond than in the conventional raceways. Lipid and fatty acid content ranged from 16 to 25% and from 5 to15%, respectively, as a percentage of AFDW. Palmitic, palmitoleic, and eicosapentaenoic acids comprised the majority of fatty acids. While the ARID culture system achieved nearly double the volumetric productivity relative to the conventional raceways (0.023 versus 0.013？g？L？1day？1), areal biomass productivities were of similar magnitude in both pond systems (3.47 versus 3.34？g？m？2day？1), suggesting that the ARID pond design has to be further optimized, most likely by increasing the culture depth or operating at higher cell densities while maintaining adequate mixing. 1. Introduction Raceway ponds have been commonly used as microalgae production systems since the 1970s [1]. Their simplicity and low costs have led to their adoption in commercial microalgae production systems [2]. The conventional design utilizes a motor driven paddle wheel to propel the culture around a circular track. Many variations on this basic pond design have been explored, and there have been efforts to combine conventional raceways with photobioreactors in a two-stage cultivation process [3–5]. Given that microalgae-derived biofuels have yet to be proven economically feasible on an industrial scale, all aspects of the production chain require further optimization. Cultivation system optimization could lead to more favorable economics by raising annual biomass yields and reducing capital and operating costs. In an outdoor production environment, both conventional raceways and photobioreactors are subject to inclement weather that can create suboptimal growth conditions. Any attempt to control or mitigate temperature extreme can decrease the net energy output of the production system and increase capital costs.