No abstract

Efficient utilization of solar radiation for the photoautotrophic production of cyanobacterium biomass was achieved, using small pipes (ID = 0.01 m) arranged in rows in two photobioreactors facing south-north. A high Arthrospira yield of 47.7 g m(-2) (installation area) d(-1) was attained under outdoor conditions in a tubular undulating row photobioreactor (TURP-10r). During the summer, under a semicontinuous culture regime, the optimal biomass concentration (OBC) in TURP-5r was 6.0 g L(-1): it was 5.0 g L(-1) in TURP-10r. These OBCs made it possible to produce a biomass output rate of 2.7 +/- 0.2 g L(-1) d(-1) in the former and 2.1 +/- 0.1 g L(-1) d(-1) in the latter. When Arthrospira was grown at a preset dilution rate (0.3 d(-1)), sunrise cell density (SrCD) variations were not proportional to the drop of solar radiation. The SrCD was comparatively high at high solar radiation and decreased abruptly with decreasing solar radiation. There was a tendency to stabilize at low solar radiation. In both photobioreactors, the chlorophyll content of the Arthrospira biomass (% of the dry weight) was higher at sunrise than at sunset. A comparison of the chlorophyll biomass content in the TURPs showed no significant differences. Night biomass losses were very high (> 30% of the daylight productivity) when the culture temperature was kept constant at 31 +/- 1.0 degrees C: these losses fell to < 20% of the daylight productivity, when the night temperature of the cultures decreased according to the environmental temperature. Dilution of solar radiation was carried out using two quasi-laminated bioreactors. The rows of S-N facing bioreactors showed a very high growth yield in TURP-10r [about 2.1g (d.w.) MJ(-1)]. In TURP-10r, the high photic ratio (R(f) = 6), the high surface-to-volume ratio (S(ill)/V = 400 m(-1)) and the S-N facing of the rows (better than an E-W orientation) allowed for good results.

A photobioreactor in the form of a 245-m-long loop made of plexiglass tubes having an inner diameter of 2.6 cm was designed and constructed for outdoor culture of Spirulina. The loop was arranged in two planes, with 15 8-m-long tubes in each plane. In the upper plane, the tubes were placed in the vacant space between the ones of the lower plane. The culture recycle was performed either with two airlifts, one per plane, or with two peristaltic pumps. The power required for water recycle in the tubular photobioreactor, with a Reynolds number of 4000, was 3.93 x 10(-2) W m(-2). The photobioreactor contained 145 L of culture and covered an overall area of 7.8 m(2). The photobioreactor operation was computer controlled. Viscosity measurements performed on Spirulina cultures having different biomass concentrations showed non-Newtonian behavior displaying decreasing viscosity with an increasing shear rate. The performance of the two-plane photobioreactor was tested under the climatic conditions of central Italy (latitude 43.8 degrees N, longitude 11.3 degrees E). A biomass concentration of 3.5 g L(-1) was found to be adequate for outdoor culture of Spirulina. With a biomass concentration of 6.3 g L(-1), the biomass output rate significantly decreased. The net biomass output rate reached a mean value of 27.8 g m(-2) d(-1) in July; this corresponded to a net photosynthetic efficiency of 6.6% (based on visible irradiance).