Algal based wastewater treatment (WWT) technologies are attracting renewed attention because they couple energy-efficient sustainable treatment with carbon capture, and reduce the carbon footprint of the process. A low-cost energy-efficient mixed microalgal culture-based pilot WWT system, coupled with carbon dioxide (CO2) sequestration, was investigated. The 21 L stirred-tank photobioreactors (STPBR) used light-emitting diodes as the light source, resulting in substantially reduced operational costs. The STPBR were operated at average optimal light intensity of 582.7 μmol.s(-1).m(-2), treating synthetic municipal wastewater containing approximately 250, 90 and 10 mg.L(-1) of soluble chemical oxygen demand (SCOD), ammonium (NH4-N), and phosphate, respectively. The STPBR were maintained for 64 days without oxygen supplementation, but had a supply of CO2 (25 mL.min(-1), 25% v/v in N2). Relatively high SCOD removal efficiency (>70%) was achieved in all STPBR. Low operational cost was achieved by eliminating the need for mechanical aeration, with microalgal photosynthesis providing all oxygenation. The STPBR achieved an energy saving of up to 95%, compared to the conventional AS system. This study demonstrates that microalgal photobioreactors can provide effective WWT and carbon capture, simultaneously, in a system with potential for scaling-up to municipal WWT plants.
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.
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