Duckweed (Lemna) is a possible solution for the treatment of aqueous waste streams and the simultaneous provision of protein-rich biomass. Nitrification-Denitrification effluent (NDNE) from pig manure treatment has been previously used as a growing medium for duckweed. This study investigated the use of a continuous duckweed cultivation system to treat NDNE as a stand-alone technology. For this purpose, a system with a continuous supply of waste streams from the pig manure treatment, continuous biomass production, and continuous discharge that meets the legal standards in Flanders (Belgium) was simulated for a 175-day growing season. In this simulation, salt accumulation was taken into account. To prevent accumulating salts from reaching a toxic concentration and consequently inhibiting growth, the cultivation system must be buffered, which can be achieved by altering the depth of the system. To determine the minimum depth of such a system, a tray experiment was set up. For that, salt accumulation data obtained from previous research were used for simulating systems with different pond depths. It was found that a depth of at least 1 m is needed to prevent a significant relative growth inhibition at the end of the growing season compared to the start. This implies a high water consumption (5–10 times more than maize). As a response, a second cultivation system was investigated for the use of more concentrated NDNE. For this purpose, salt tolerance experiments were conducted on synthetic and biological media. Surprisingly, it was observed that duckweed grows better on diluted NDNE (to 75% NDNE, or EC of 8 mS/cm) than on a synthetic medium (EC of 1.5 mS/cm), indicating the potential of such a system.

Livestock-intensive regions in Europe face dual challenges: nutrient surpluses and a high dependency on import of high-protein feedstocks. This study proposes duckweed (Lemnaceae) as a potential solution by recovering nutrients from manure-derived waste streams while producing protein-rich biomass. This study evaluated the performance of duckweed treatment systems at a pig manure processing facility in Belgium. Three outdoor systems were monitored over a full growing season under temperate climate conditions. Duckweed cultivated on constructed wetland effluent showed die-off and low protein content, while systems supplied with diluted liquid fraction and nitrification–denitrification effluent achieved consistent growth, yielding 8 tonnes of dry biomass/ha/year and 2.8 tonnes of protein/ha/year. Average removal rates were 1.2 g N/m²/day and 0.13 g P/m²/day. Growth ceased after approximately 100–120 days, likely due to rising pH and electrical conductivity, suggesting ammonia toxicity and salt stress. Harvested duckweed had a high protein content and a total amino acid profile suitable for broilers, though potentially limiting in histidine and methionine for pigs or cattle. Additionally, promising energy and protein values for ruminants were measured. Although high ash and fibre contents may limit use in monogastric animals, duckweed remains suitable as part of a balanced feed. Its broad mineral profile further supports its use as a circular, locally sourced feed supplement.

Duckweed (Lemnaceae) is a promising alternative feed crop, particularly in regions with nutrient surpluses and protein deficits, as it grows efficiently on nutrient-rich agricultural wastewater and provides protein-rich biomass. However, its high moisture content and rapid post-harvest spoilage pose major storage challenges. This study evaluated (co-)ensiling as a cost-effective preservation strategy for duckweed. Three separate experiments were conducted to assess the ensilability of duckweed alone and in combination with various agricultural co-substrates and additives, including corn silage, beet pulp, grass silage, hemp shives, hay, molasses, sun-dried duckweed and CaCO₃. Duckweed alone could not be successfully ensiled due to excessive moisture, resulting in poor acidification and high levels of undesirable fermentation products. During the long-term co-ensiling test, a duckweed–corn silage mixture containing 29% fresh duckweed and 71% corn silage showed the most stable fermentation profile, with low pH, limited fermentation losses, and no detectable butyric acid. A duckweed–grass silage mixture containing 51% fresh duckweed and 49% grass silage allowed higher duckweed inclusion and retained the highest level of apparent pepsin-digestible protein after storage, but showed elevated acetic acid and ethanol concentrations. A duckweed–beet pulp mixture containing 74% fresh duckweed and 26% beet pulp enabled the highest duckweed inclusion rate, but showed signs of clostridial fermentation, likely due to excess moisture. Microbiological analysis of this beet pulp mixture showed reduced Enterobacteriaceae after ensiling, but also increased clostridial counts. Oxalic acid concentrations were low in all duckweed-based silages, with the largest reduction observed in the duckweed–grass mixture. Overall, the results show that duckweed co-ensiling is feasible but highly dependent on co-substrate selection and moisture control. Further formulation optimisation is required, particularly for high-duckweed mixtures, to reduce the risk of clostridial fermentation and improve practical applicability as a storable feed ingredient.