Two closed-coil photobioreactors operating at high Dean number in dense algal cultures

Arthrospira platensis was grown from low to high biomass concentrations in two closed-coil photobioreactors (1.0 and 6.0 m-long). Both had an incorporated mandrel to control culture temperature. The aim of this study was to verify if an appropriate Dean number (De = 3950) could be used to maintain high photosynthetic efficiency (PE) when Arthrospira platensis is grown at very high biomass concentrations (>= 12.0 g l-1). The highest PE (7.45%) was attained using the 1.0 m-long closed-coil photobioreactor (CCP1). In CCP1 the Arthrospira culture was recycled by an airlift system; air was injected at the bottom of the three risers. The relationship between airflow rate (AFR) and Dean number (De) was investigated using tap water. The De increased very quickly with an airflow rate that was lower than 1.0 l min-1. To achieve De > 3000, we operated with an airflow rate of 2.0 l min-1, which rose to 6.0 l min-1 to obtain De = 3950. Further increases in AFR (till 8.0 l min-1) did not show any significant rise in the De. Three different orifice diameters (bores of air-sparger) were investigated (0.6, 1.2, and 1.8 mm) and consequently, three different bubble velocities (vb) were achieved at the sparger (118, 29.5 and 13.1 m s-1 respectively). Operating with an airflow rate of 6.0 l min-1, Arthrosrira trichomes were quickly fragmented when the orifice diameter was 0.6 mm (vb = 118 m s-1). Fragmentation was reduced to 32% when the orifice diameter was 1.2 mm (vb = 29.5 m s-1). The phenomena was further reduced when Od = 1.8 mm (vb = 13.1 m s-1). Culture grown inside the 6.0 m-long closed-coil photobioreactor (CCP6) was recycled using a pump system: the high De of 3950 was not reached using the airlift system. In this reactor, the highest PE was 6.21%.