No abstract

As part of a circular economy (CE) approach to food production systems, Lemnaceae, i.e., duckweed species, can be used to remediate wastewater due to rapid nutrient assimilation and tolerance of non-optimal growing conditions. Further, given rapid growth rates and high protein content, duckweed species are a valuable biomass. An important consideration for duckweed-mediated remediation is the density at which the plants grow on the surface of the wastewater, i.e., how much of the surface of the medium they cover. Higher duckweed density is known to have a negative effect on duckweed growth, which has implications for the development of duckweed-based remediation systems. In the present study, the effects of density (10–80% plant surface coverage) on Lemna minor growth, chlorophyll fluorescence and nutrient remediation of synthetic dairy processing wastewater were assessed in stationary (100 mL) and re-circulating non-axenic (11.7 L) remediation systems. Overall, L. minor growth, and TN and TP removal rates decreased as density increased. However, in the stationary system, absolute TN and TP removal were greater at higher densities (50–80% coverage). The exact cause of density related growth reduction in duckweed is unclear, especially at densities well below 100% surface coverage. A further experiment comparing duckweed grown at ‘low’ and ‘high’ density conditions with the same biomass and media volume conditions, showed that photosynthetic yield, Y(II), is reduced at high density despite the same nutrient availability at both densities, and arguably similar shading. The results demonstrate a negative effect of high density on duckweed growth and nutrient uptake, and point towards signals from neighbouring duckweed colonies as the possible cause.

Given its high biomass production, phytoremediation capacity and suitability as a feedstock for animal and human nutrition, duckweeds are valuable multipurpose plants that can underpin circular economy applications. In recent years, the use of duckweeds to mitigate environmental pollution and valorise wastewaters through the removal of excess nitrogen and phosphate from wastewaters has gained considerable scientific attention. However, quantitative data on optimisation of duckweed performance in phytoremediation systems remain scant. In particular, a mechanistical understanding of how physical flows affect duckweed growth and remediation capacity within vertical indoor multi-tiered bioreactors is unknown. Here, effects of flow rate (0.5, 1.5 or 3.0 L min−1) and medium depth (25 mm or 50 mm) on Lemna minor biomass production and phytoremediation capacity were investigated. Results show that flow rates and water depths significantly affect both parameters. L. minor grew best at 1.5 L min−1 maintained at 50 mm, corresponding to a flow velocity of 0.0012 m s−1. The data are interpreted to mean that flow velocities should be low enough not to physically disturb duckweed but still allow for adequate nutrient mixing. The data presented will considerably advance the optimisation of large-scale indoor (multi-tiered, stacked), as well as outdoor (pond, lagoon, canal), duckweed-based remediation of high nutrient wastewaters.

1Effects of depth and turbidity on the in situ pumping activity of the Mediterranean sponge Chondrosia 1 2 reniformis (Nardo, 1847) were characterized by measuring osculum diameter, oscular outflow velocity, 1 3 osculum density per sponge and sponge surface area at different locations around the Bodrum peninsula 1 4 (Turkey). Outflow velocity was measured using a new method based on video analysis of neutrally buoyant 1 5 particles moving in the exhalant stream of sponge oscula, which yielded results that were in good comparison 1 6to other studies. Using the new method, it was shown that for C. reniformis, oscular outflow had a location-1 7 dependent, in most cases positive relationship with oscular size: bigger oscules process more water per cm 2 1 8 of osculum surface. Turbidity and depth both affected sponge pumping in a negative way, but for the 1 9 locations tested, the effect of depth was more profound than the effect of turbidity. Depth affected all 2 0 parameters investigated except sponge size, whereas turbidity only affected specific pumping rates 2 1 normalized to sponge surface area. Deep water sponges had clearly smaller oscula than shallow water 2 2 sponges, but partially compensated for this lower pumping potential by showing a higher osculum density. 3Both increasing turbidity and increasing depth considerably decreased volumetric pumping rates of C. 2 4 reniformis. These findings have important implications for selecting sites for mariculture of this species. 2 5 2 6