Flow Rate and Water Depth Alters Biomass Production and Phytoremediation Capacity of Lemna minor

Heavy metals/-metalloids (As) phytoremediation with Landoltia punctata and Lemna sp. (duckweeds): coupling with biorefinery prospects for sustainable phytotechnologies

Heavy metals/-metalloids can result in serious human health hazards. Phytoremediation is green bioresource technology for the remediation of heavy metals and arsenic (As). However, there exists a knowledge gap and systematic information on duckweed-based metal phytoremediation in an eco-sustainable way. Therefore, the present review offers a critical discussion on the effective use of duckweeds (genera Landoltia and Lemna)-based phytoremediation to decontaminate metallic contaminants from wastewater. Phytoextraction and rhizofiltration were the major mechanism in 'duckweed bioreactors' that can be dependent on physico-chemical factors and plant-microbe interactions. The biotechnological advances such as gene manipulations can accelerate the duckweed-based phytoremediation process. High starch and protein contents of the metal-loaded duckweed biomass facilitate their use as feedstock in biorefinery. Biorefinery prospects such as bioenergy production, value-added products, and biofertilizers can augment the circular economy approach. Coupling duckweed-based phytoremediation with biorefinery can help achieve Sustainable Development Goals (SDGs) and human well-being.

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.

Duckweed: Research Meets Applications

The Special Issue "Duckweed: Research Meets Applications" of the journal Plants (ISSN 2223-7747) presents a comprehensive update of the current progress in the field [...].

Phytoremediation of diclofenac using the Green Liver System: Macrophyte screening to system optimization

Green Liver Systems employ the ability of macrophytes to take up, detoxify (biotransform), and bioaccumulate pollutants; however, these systems require optimization to target specific pollutants. In the present study, the aim was to test the applicability of the Green Liver System for diclofenac remediation considering the effects of selected variables. As a starting point, 42 macrophyte life forms were evaluated for diclofenac uptake. With the three best performing macrophytes, the system efficiency was evaluated at two diclofenac concentrations, one environmentally relevant and that other significantly higher (10µg/L and 150µg/L) and in two system sizes (60L and 1000L) as well as at three flow rates (3, 7, and 15L/min). The effect of single species and combinations on removal efficiency was also considered. The highest internalization percentage was recorded in Ceratophyllum spp., Myriophyllum spp., and Egeria densa. Phytoremediation efficiency with species combinations was far superior to utilizing only a single macrophyte type. Furthermore, the results indicate that the flow rate significantly affected the removal efficiency of the pharmaceutical tested, with the highest remediation efficiency obtained with the highest flow rate. System size did not significantly affect phytoremediation; however, increase diclofenac concentration reduced the systems performance significantly. When planning the setup of a Green Liver System for wastewater remediation, basic knowledge about the water, i.e., pollutant types and flow, must be utilized during planning to optimize remediation. Various macrophytes show diverse uptake efficiencies for different contaminants and should be selected based on the pollutant composition of the wastewater.

Cultivation of Lemna Minor on Industry-Derived, Anaerobically Digested, Dairy Processing Wastewater

The growth and nutrient uptake capacity of a common duckweed (Lemnaceae) species, Lemna minor "Blarney", on dairy processing wastewater pre-treated by an anaerobic digester (AD-DPW) was explored. L. minor was cultivated in small stationary vessels in a controlled indoor environment, as well as in a semi-outdoor 35 L recirculatory system. The use of AD-DPW as a cultivation medium for L. minor offers a novel approach to dairy wastewater treatment, evolving from the current resource-intensive clean-up of wastewaters to duckweed-based valorisation, simultaneously generating valuable plant biomass and remediating the wastewater.