Where engineering meets biology: The Computational Fluid Dynamic analysis of a stacked duckweed bioreactor

The necessity for sustainable farming practices, wastewater valorisation and circular economy applications have prompted increased interest in duckweed cultivation. As floating plants, duckweed species show rapid growth and can be cultured agri-food industry wastewater. Further, the resulting plant biomass is a valuable high-protein livestock feed and a biofuel feedstock. The development of multitiered (i.e., vertically stacked) indoor bioreactors enable reliable, high-capacity growth irrespective of seasons. Here, a Computational Fluid Dynamic (CFD) approach was applied to a pilot-scale duckweed cultivation system to provide insight into wastewater hydrodynamics to support further development and optimisation. CFD modelling and validation indicated that the pilot-scale system behaved non-ideally, with 60.1 % of the volume considered stagnant, and with surface channelling also identified. Analysis of the aspect ratio and inlet/outlet port positions of the cultivation tray enabled a 24.3 % decrease in stagnation, as well as a significant reduction in channelling, when the number of tray inlet/outlet ports was increased from two to three. Thus, the current study highlights the value of in-depth evaluation of the fluid flow using CFD, as a strategy to improve design of duckweed cultivation systems. This strategy can be further expanded to incorporate local and temporal nutrient depletion and predict duckweed growth rates.