Agronomic and Environmental Performance of Lemnaminor Cultivated on Agricultural Wastewater Streams—A Practical Approach

Lemna minor: Unlocking the Value of This Duckweed for the Food and Feed Industry

Duckweed (Lemna minor L.) is a small floating aquatic plant that has an important economic impact in several industrial areas. With its high biomass production, reasonable protein content, and resilience to several climates, it has been attracting increasing interest for potential use in animal and human food systems. Historically consumed in southwest Asia, this duckweed is now gaining attention as a potential novel food in Europe. This manuscript explores the contributions of duckweed to various food and feed industries, including aquaculture and livestock, while also pointing out the incipient research carried out for human consumption. Most importantly, it highlights the potential of Lemna minor as a vegetable for future human consumption whether eaten whole or through extraction of its nutrients.

On the rise: Development of a multi-tiered, indoor duckweed cultivation system

Duckweed species (Lemnaceae) are suitable for remediation and valorization of agri-feed industry wastewaters and therefore can contribute to a more sustainable, circular economy where waste is a resource. Industrial applications will, however, require space efficient cultivation methods that are not affected by prevailing weather conditions. Here, the development and operation of a multi-tiered duckweed bioreactor is described. The developed prototype bioreactor depicted in this paper is composed of four cultivation layers (1 m2 each) with integrated LED lighting (generating up to 150 μmol m-2  s-1 ), a system of pumps and valves to manage the recirculatory flow (2.5 L min-1 ) of wastewater, and an automatic harvesting system. Using a nutrient poor medium, good growth of the duckweed species Lemna minor was achieved in the bioreactor, and this was matched by strong nutrient depletion from the medium, especially for phosphorus (45-mg total phosphorus [TP] removed per m-2  day-1 ). A fully automatic harvesting arm reliably captured similar amounts of duckweed biomass across multiple harvesting cycles, revealing a future scenario whereby labor and interventions by human operators are minimized. Further developments to advance the system towards fully automated operation will include, for example, the use of specific nutrient sensors to monitor and control medium composition. It is envisaged that multi-tiered, indoor bioreactors can be employed in the agri-feed industry where wastewaters are, in many cases, continuously generated throughout the year and need remediating immediately to avoid costly storage. Given the extensive use of automation technology in conventional wastewater treatment plants, multi-tiered duckweed bioreactors can be realistically integrated within the operating environment of such treatment plants. PRACTITIONER POINTS: Duckweed is suitable for remediation and valorization of agri-feed wastewater. Industrial duckweed applications require space efficient cultivation methods. Development and operation of a multi-tiered duckweed bioreactor is detailed. Flow dynamics and automatic harvesting in the bioreactor are optimized. It is concluded that a multi-tiered bioreactor can be used in industry.

Nutritional Value of Duckweed as Protein Feed for Broiler Chickens-Digestibility of Crude Protein, Amino Acids and Phosphorus

Duckweed is gaining attention in animal nutrition and is considered as a potential alternative protein source for broiler chickens. In order to evaluate the nutritional value of duckweed, three individual batches were investigated. They consisted of a mixture of Lemna minuta and Lemna minor (A, 17.5% crude protein), Spirodela polyrhiza (B, 24.6% crude protein) and Lemna obscura (C, 37.0% crude protein). Treatment diets contained 50% batch A, 50% batch B, and 25, 50 and 75% of batch C. All diets were fed to broiler chickens (Ross 308) from an age of 21 to 27 days. Diets with a share of 50 and 75% of batch C led to decreased feed intake (109.3 and 74.9 g/day, respectively) compared to the control. Standardized ileal digestibility of crude protein and amino acids differed significantly between duckweed batches, at values for methionine between 49.9 and 90.4%. For all amino acids, batch A consistently had the lowest and batch C the highest digestibility. Batches had different tannin contents of 2943, 2890 and 303 mg/kg for batches A, B and C, respectively. The apparent ileal digestibility of phosphorus differed significantly between all batches (50.8-78.9%). Duckweed can be used as a protein feed for broiler chickens. However, a defined and stable biomass composition optimized for the requirements of broiler chickens is needed.

The Impact of Salt Accumulation on the Growth of Duckweed in a Continuous System for Pig Manure Treatment

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.

Integrated Multitrophic Aquaculture; Analysing Contributions of Different Biological Compartments to Nutrient Removal in a Duckweed-Based Water Remediation System

Duckweed (Lemnaceae) can support the development of freshwater aquaculture if used as extractive species in Integrated MultiTrophic Aquaculture (IMTA) systems. These aquatic plants have the advantage of producing protein-rich biomass that has several potential uses. On the contrary, other biological compartments, such as microalgae and bacteria, present in the water and competing with duckweed for light and nutrients cannot be harvested easily from the water. Moreover, as phytoplankton cannot easily be harvested, nutrients are eventually re-released; hence, this compartment does not contribute to the overall water remediation process. In the present study, a mesocosm experiment was designed to quantify the portion of nutrients effectively removed by duckweed in a duckweed-based aquaculture wastewater remediation system. Three tanks were buried next to a pilot-scale IMTA system used for the production of rainbow trout and perch. The tanks received aquaculture effluents from the adjacent system, and 50% of their surface was covered by duckweed. Daily water analyses of samples at the inlet and outlet of the mesocosm allowed quantification of the amount of nutrients removed in total. The portion removed by duckweed was determined by examining the nutrient content in the initial and final biomass. The portion of nutrients removed by other compartments was similarly estimated. The results show that duckweed is responsible for the removal of 31% and 29% of N and P, respectively. Phytoplankton removed 33% and 38% of N and P, respectively, while the biofilm played no major role in nutrient removal. The remainder of the removed nutrients were probably assimilated by bacteria or sedimented. It is speculated that a higher initial duckweed density can limit phytoplankton growth and, therefore, increase the portion of nutrients removed by the duckweed compartment.

Biogas Potential Assessment of the Composite Mixture from Duckweed Biomass

The article presents the research results of anaerobic digestion processes in bioreactors of composite mixtures based on initial and residual biomass of Lemna minor duckweed and additives: inoculum (manure), food waste, and spent sorbents to determine biogas potential (biogas volume, methane content). Duckweed Lemna minor, which is widespread in freshwater reservoirs, is one of the promising aquatic vegetation species for energy use. Residual biomass is obtained by chemically extracting valuable components from the primary product. The purpose of the research was to evaluate the possibility of the energy potential of residual biomass of Lemna minor to reduce the consumption of fossil fuels and reduce greenhouse gas emissions. This is in line with the International Energy Agency (IEA) scenarios for the reduction of environmental impact. The obtained results confirm the feasibility of using this type of waste for biogas/biomethane production. The recommendations on the optimal composition of the mixture based on the residual biomass of Lemna minor, which will allow for an increase in biogas production, are given. The obtained data can be used in the design of bioreactors.

Lemna minor Cultivation for Treating Swine Manure and Providing Micronutrients for Animal Feed

The potential of Lemna minor to valorise agricultural wastewater into a protein-rich feed component to meet the growing demand for animal feed protein and reduce the excess of nutrients in certain European regions was investigated. Three pilot-scale systems were monitored for nine weeks under outdoor conditions in Flanders. The systems were fed with a mixture of the liquid fraction and the biological effluent of a swine manure treatment system diluted with rainwater in order that the weekly N and P addition was equal to the N and P removal by the system. The design tested the accumulation of elements in a continuous recirculation system. Potassium, Cl, S, Ca, and Mg were abundantly available in the swine manure wastewaters and tended to accumulate, being a possible cause of concern for long-operating recirculation systems. The harvested duckweed was characterised for its mineral composition and protein content. In animal husbandry, trace elements are specifically added to animal feed as micronutrients and, thus, feedstuffs biofortified with essential trace elements can provide added value. Duckweed grown on the tested mixture of swine manure waste streams could be considered as a source of Mn, Zn, and Fe for swine feed, while it is not a source of Cu for swine feed. Moreover, it was observed that As, Cd, and Pb content were below the limits of the feed Directive 2002/32/EC in the duckweed grown on the tested medium. Overall, these results demonstrate that duckweed can effectively remove nutrients from agriculture wastewaters in a recirculated system while producing a feed source with a protein content of 35% DM.