Impact of light spectrum on outdoors tubular photobioreactors used for microalgae-based wastewater treatment

Microalgae-based wastewater treatment involves various factors, including algal ecosystems and environmental conditions. Light spectral composition, particularly wavelength and intensity, is a key but underexplored under out-of-laboratory conditions. Therefore, this study investigates the effects of different light wavelengths on microalgae in wastewater treatment systems using filtered sunlight. It examines critical physiological and biochemical processes in microalgae, including pigment synthesis, carbon fixation, and nutrient uptake. A tubular photobioreactor (PBR) setup with two diameters, 4-inch (10 . 16 cm) and 6-inch (15 . 24 cm), was tested outdoors for three months using blue, green, and red filters. These filters transmitted radiation and were transparent to infrared light, ranging from 750 nm to blue-green-infrared light (peaking at 450 nm), green-infrared light (peaking at 520 nm), and red-infrared light (peaking at 600 nm). The study analyzed 56 absorption curves, finding that light color influences microalgae physiology even under the same radiation levels. Pigment production was enhanced with only 10 % of total solar radiation, avoiding photoinhibition. Chlorophyll production was highest in the 4-inch PBR with a green filter, while blue light filters promoted pigment accumulation in 6-inch PBRs. However, the 6-inch reactors exhibited a lower growth rate compared to the 4-inch reactors, with significant effects of light color seen only at extreme irradiance levels. Larger diameter PBRs also increased the nitrification process, regardless of the filter used. These results have important implications for biofuel production, wastewater treatment, and bioremediation, underscoring the role of filtered light in enhancing microalgae performance in environments requiring deeper sunlight penetration. • Microalgal PBRs were tested outdoors with different diameters and color films. • Chlorophyll and irradiance showed significant correlations across all PBRs. • Green and blue filters produced more pigments with lower radiation at thinner PBRs. • Nitrification was only observed in the thicker PBRs regardless of the color filters. • Thicker PBRs showed a lower growth rate in response to irradiance than thicker ones.