Comparison between jet and paddlewheel mixing for the cultivation of microalgae in anaerobic digestate of piggery effluent (ADPE)

There is great need to improve turbulent mixing of microalgae cultures grown in turbid wastewater to ensure efficient use of light and nutrients for higher biomass productivity and nutrient removal rates. In this outdoor study, we compared the turbulent mixing and nutrient removal efficiency of conventional paddlewheel driven raceway ponds (PWP) with customized jet-nozzle raceway pond (JNP) on microalgae grown in undiluted anaerobic digestate of piggery effluent (ADPE). Overall, the concentration of microalgae consisting mainly of Cyanobacteria and Chlorella sp. trended higher in the JNP than the PWP with notable absence of diatoms in JNP. The average percentage of ammonium removal rates were found to be significantly higher in the JNP (36.8% ± 3.93) than the PWP (23.5% ± 4.42). The measured amount of turbulent kinetic energy (TKE), as an indicator of algal movement at eight distinct locations of both ponds, trended higher in the JNP than the PWP, suggesting improved mixing performance with higher shear stress on cultures in the JNP. Based on the higher ammonium removal rates and turbulence mixing, JNP was found to be more efficient for the cultivation of microalgae in ADPE than PWP.