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Lemnaceae are being exploited to remediate a variety of different wastewaters. Dairy processing waste is produced in large amounts, and contains a range of valuable plant nutrients, for example, nitrate, ammonium, phosphate, iron and calcium. Our aim was to remediate dairy processing waste with the duckweed plant Lemna minor. However, initial trials failed to establish growth of L. minor on this medium. A lack of growth can be due to a lack of essential plant micro-and macro-nutrients, or the presence of phytotoxic ingredients. In this study we show that not just nutrient concentrations, but also the ratios between them can be important in facilitating growth. Using lab-scale experiments in which L. minor were grown on 100 mL of synthetic wastewater, we demonstrated that the skewed Ca:Mg ratio in synthetic dairy industry wastewater is a key obstacle to good growth. Experiments showed that a ratio which favours magnesium over calcium negatively affects L. minor growth and photosynthetic yield, leading to RGRs as low as 0.05 day -1 . A change in this ratio to favour calcium, through the addition of calcium sulphate, leads to RGRs of 0.2 -0.3 day -1 . Experiments lead us to conclude that a Ca:Mg ratio of 1:1.6 (by molar concentration) or greater is necessary for Lemna minor growth, and therefore phytoremediation of dairy industry processing wastewater.

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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.