Recognition of key factors on attached microalgae growth from the internal sight of biofilm

Microalgae cultivation with wastewater could realize the advanced water treatment and pollutant conversion to biomass. Attached microalgae cultivation mode, that can avoid the high-cost and energy-extensive consumption process of biomass recovery from water in suspended cultivation mode, is getting increasing attention. During the attached cultivation, light and nutrient concentration in the internal biofilm, play a direct and crucial role in regulating the growth of microalgae. Hence, the distribution of light and nutrients at different depths of biofilm were first explored in this study together with the change rules of its internal distribution under different external nutrient levels. It demonstrated that the gross growth rate was enhanced by increasing the external nutrient level. Seen from the internal sight of biofilm, the internal nutrient level had a positive response to the external nutrient change. Nutrients (especially nitrogen) distributed homogeneously through the biofilm, and no serious nutrient starvation occurred at the surface layer of biofilm. Photosynthesis rate linearly decreased along the depth of microalgae biofilm (10-120 μm). In conclusion, light, rather than nutrient, would be the key influencing factor on attached microalgae growth. How to optimize the internal light distribution would determine the wastewater purification efficiency based on attached microalgae cultivation.