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.
Highlights
The provision of nutritious food is a challenging endeavour [1,2]
Total nitrogen (TN) removal was expressed per frond surface area, the rate decreased as density increased (one-way ANOVA: F(7) = 9.287, p < 0.001; Figure 2d)
A similar pattern was found for total phosphorous (TP) removal in which the overall removal of TP from synthetic dairy wastewater increased as plant density increased (one-way ANOVA: F(7) = 5.11, p < 0.001; Figure 2e)
Summary
The provision of nutritious food is a challenging endeavour [1,2]. Climate change, a reduction in the per capita availability of arable land, as well as soil erosion, chemical overuse and finite resources have decreased food security [3,4,5,6]. CE principles encourage the adoption of closed-loop production patterns, whereby waste is appropriated as a resource [8], reducing emissions and energy consumption in the process [9]. Large volumes of wastewater are created as a consequence of dairy production and processing. It is estimated that up to 10 L of wastewater is created per litre of milk processed, making dairy processing waste one of the most significant waste streams in the food industry [12]. Dairy processing wastewaters tend to contain high concentrations of organic matter, measured as chemical oxygen demand (COD): 2000–6000 mg L−1 COD [13]; 4420 mg L−1 COD [14]; 55,430–70,150 mg L−1 COD [15]
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