Optimisation of red light-emitting diodes irradiance for illuminating mixed microalgal culture to treat municipal wastewater

Abstract

This paper evaluates the effect of variation in red light-emitting diodes (LEDs) irradiance on the growth rate and biomass productivity of a mixed culture of microalgae grown on synthetic municipal wastewater, with and without CO 2 addition. Red LEDs were used to illuminate microalgal culture from the centre of 21-L stirred-tank photobioreactors made of transparent Plexiglas, each reactor having a working volume of 16 L. The reactors were operated in batch mode with pH control, and under continuous illumination for 30 days at ambient temperature. Mixing was achieved through the use of overhead mechanical stirrers operated at 100±1 and 60±1 revolution per minute, before and after the addition of CO 2 , respectively. Three average values of irradiance of 429.9, 582.7 and 730 .8 µmol.s -1 .m -2 were used to illuminate the reactors, with a control reactor operated in the dark. CO 2 addition resulted in about two-fold increase in biomass productivity in all the experimental reactors. The bioreactor with medium irradiance yielded the highest biomass productivity and maximum specific growth rate of 0.034 g.L -1.d -1 and 0.109 d -1 , respectively. The findings in this study show that both microalgal growth rate and biomass productivity are not always directly proportional to irradiance, despite the influence of process and operational parameters. Furthermore, a medium amount of irradiance resulted in optimum growth and productivity of the mixed microalgal culture.

Current advances in microalgae-based treatment of high-strength wastewaters: challenges and opportunities to enhance wastewater treatment performance

“…The most important limitations for the treatment of high-strength wastewaters, especially in open systems such as HRAPs, include relatively large area requirements and high evaporation rates (Acién et al 2016;Garfí et al 2017;Young et al 2017). Other issues directly affect the ability of microalgae to grow in high-strength wastewaters, mainly ammonia inhibition (Azov and Goldman 1982), light blockage by solids (Mohammed et al 2013;Marcilhac et al 2014) and unbalanced macronutrients ratio (Franchino et al 2013). These limitations restrict the presence and growth of microalgae to a few genera, depending on their ability to adapt to the wastewater composition and environmental conditions in photobioreactors.…”

“…in wastewater (González-Camejo et al 2019). PAR availability for the culture broth does not depend on only sunlight intensities but is also influenced by excessive blocking and scattering of light by suspended solids present in high-strength wastewaters (Mohammed et al 2013;Marcilhac et al 2014). Additionally, high turbidity and colour in most digestates also reduce light availability for autotrophic and mixotrophic growth during microalgae cultivation (Marcilhac et al 2014).…”

Influence of light on biomass and lipid production in microalgae cultivation

“…Chlorella vulgaris grown in a closed flat plate bioreactor (25 L working volume) with red and white LED (I 420–520 /I 580–680 ) at increased light intensity obtained faster growth (µ = 0.36 d −1 ) and higher lipid content (22.2%) (Metsoviti et al., 2020). Mixed microalgae cultured in plexiglass photobioreactor (16 L working volume) using red LED has a specific growth rate of 0.109 d −1 (Mohammed et al, 2014). These findings can be used as guidelines on the application of LEDs in large‐scale microalgae cultivation.…”

Microalgae and sustainable wastewater treatment: a Review

“…manifold or serpentine; Tredici, 2004), and position of illumination (e.g. internally or externallyilluminated; Mohammed et al, 2014a;2013;Ogbonna et al, 1999). On the other hand, operational classification of PBR could be based on mixing (e.g.…”