Impact of mixing duration on growth and nutrient removal efficiency of Scenedesmus sp. in a novel raceway pond system

Raceway ponds are regarded as a popular and cost-effective method for microalgae cultivation; however, their performance is strongly influenced by hydrodynamic conditions. Conventional paddlewheel driven systems are restricted to low operating velocities to avoid culture spilling out which often leads to reduced mixing and stagnant zone formation. In this study, a raceway pond was designed with the inclusion of curved slits at the bent zones and submersible pump as an alternative mixing device to prevent culture overflow, improve flow stability and minimize dead zones. This novel integration of structural modifications and pump-based mixing represents a significant advancement over traditional paddlewheel systems by providing higher velocities, enhanced circulation and more uniform algal growth conditions. Experiments were conducted to evaluate the effect of mixing durations in 6 L raceway ponds under identical environmental conditions. The raceway systems permitted a broader velocity range (0.10-0.45 m s^-1) without spillage. The system with continuous 24 h mixing compared to 20 and 16 h mixing resulted in the highest biomass productivity of 1.01 g L^-1 d^-1 and maximum nutrient removal rates of 5.18 mg L^-1 d^-1 and 3.41 mg L^-1 d^-1 for NO₃^- and PO₄^3-, respectively. Submersible-pump configured open raceway pond achieved comparable or higher biomass yield, lower energy consumption with a net energy efficiency of 62%, demonstrating its practicality and cost-effectiveness as a viable alternative to conventional paddlewheel driven systems for large-scale Scenedesmus sp. cultivation.