Depth optimization of inclined thin layer photobioreactor for efficient microalgae cultivation in high turbidity digestate

Abstract

Turbidity is a problem with microalgal growth in anaerobic digestate (AD) due to increased light attenuation, consequently resulting in reduced biomass yields. In this study, Chlorella sp. MUR 268 was cultivated in semi-continuous mode using turbid, high NH3 concentrated (160 ± 10 mg L−1) AD of food waste, reaching biomass yield of 4.319 ± 0.18 g L−1 in a inclined thin layer photobioreactor occupying a surface area of 11 m2. Depth optimizations were performed by incrementally increasing the depth of the culture on the surface of the inclined thin layer photobioreactor through 0.005, 008, 0.011, 0.0145 and 0.02 m. The kinetics of electron flow around photosystem II of the microalgae in culture were used as descriptives for light utilization and limitations of the optimizations, via variables including relative electron transfer rate, rETR, and maximum quantum yield, Fv/Fm, and derived parameters including functional relative electron transfer rate (FrETR) and functional relative electron transfer rate ratio (FrETR-ratio). After optimizing the depth of the reactor, areal productivity was increased by approximately 105%, from 10 g m−2 d−1 to 21.134 ± 1.83 g m−2 d−1 (when depth increased from the 0.005 m to 0.011 m at an operational culture volume of 140 L). The most important parameters affecting growth rates and productivity were the mean irradiance inside the culture and the FrETR of photons for phytochemistry. Compared to previous study using digestate of similar turbidity, 9.5 times higher areal productivity was attained.