Two tubular undulating row photobioreactors (TURPs) with a very high illuminated surface/volume ratio (400 m(-1)) were designed and constructed for the growth of photosynthetic micro-organisms. Experiments were conducted under outdoor conditions; and Arthrospira recycling was performed with airlifts (one for each row). The rows in each reactor faced east-west and consisted of a flexible polyvinyl chloride pipe (22 m long, 0.01 m bore) arranged in a sinusoidal shape. We studied the hydraulic performance of the sine-shaped photobioreactor rows during culture recycling in the TURPs at a very high Reynolds number (4200), when Arthrospira showed Newtonian fluid behavior. The sinusoidal pipe arrangement imposed a sine waveform on the culture, which led to better light utilization. During summer, a volumetric productivity of 7.2 g l(-1) day(-1) was reached in the TURP-5r (5 rows m(-2)), whereas an area productivity of 35 g(-2) m(-2) day(-1) was obtained in the TURP-10r (10 rows m(-2)). This was due to more light being available in the TURF-5r, because its rows were more spaced out and the photic ratio (Rf) was low (3.0). In the TURP-10r, the closer rows caused a dilution of the sunlight, but gave a better light distribution inside the Arthrospira culture and improved the light utilization. This was attributed to the high R-f (6.0) of this reactor.