The Role of Turbulent Coherent Structures on Microalgal Mixing for Nutrient Removal in Jet and Paddlewheel Raceway Ponds

Outdoor studies were conducted on microalgae cultures in two raceway ponds (kept in constant motion with either jet or paddlewheel) with a flatbed to treat anaerobic digestion piggery effluent and to observe the characteristics of turbulence on microalgal mixing and growth. Acoustic Doppler Velocimeters (ADV) were deployed to record the instantaneous velocity components and acoustic backscatter as a substitution of microalgae concentration. The present research on microalgal mixing considers the effect of event-based turbulent features such as the widely known ‘turbulent bursting’ phenomenon. This is an important aspect, as turbulent coherent structures can result in microalgal mixing, which can lead to significant changes in microalgal growth. The experimental results presented in this paper of two contrasting environments of jet- and paddlewheel-driven ponds suggested that: (1) turbulent bursting events significantly contributed to microalgal mixing when paddlewheels and jets were used; (2) among four type of turbulent bursting events, ejections and sweeps contributed more to the total microalgal mixing; and, (3) a correlation was revealed using wavelet transform between the momentum and microalgal mixing flux when either jet or paddlewheel were used. Such similarities in jet and paddlewheel raceway ponds highlight the need to introduce turbulent coherent structures as an essential parameter for microalgal mixing studies.