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

Abstract

Given its high biomass production, phytoremediation capacity and suitability as a feedstock for animal and human nutrition, duckweeds are valuable multipurpose plants that can underpin circular economy applications. In recent years, the use of duckweeds to mitigate environmental pollution and valorise wastewaters through the removal of excess nitrogen and phosphate from wastewaters has gained considerable scientific attention. However, quantitative data on optimisation of duckweed performance in phytoremediation systems remain scant. In particular, a mechanistical understanding of how physical flows affect duckweed growth and remediation capacity within vertical indoor multi-tiered bioreactors is unknown. Here, effects of flow rate (0.5, 1.5 or 3.0 L min−1) and medium depth (25 mm or 50 mm) on Lemna minor biomass production and phytoremediation capacity were investigated. Results show that flow rates and water depths significantly affect both parameters. L. minor grew best at 1.5 L min−1 maintained at 50 mm, corresponding to a flow velocity of 0.0012 m s−1. The data are interpreted to mean that flow velocities should be low enough not to physically disturb duckweed but still allow for adequate nutrient mixing. The data presented will considerably advance the optimisation of large-scale indoor (multi-tiered, stacked), as well as outdoor (pond, lagoon, canal), duckweed-based remediation of high nutrient wastewaters.

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

“…At present, it is not clear to what extent stagnant zones can limit nutrient supply to fast growing duckweed (most likely this is concentration dependent), and it is also not clear to what extent higher water velocities, as well as turbulence, impact on plant growth. Recent work by Coughlan et al (2022) shows that floating duckweed species tolerate only limited hydrodynamic disturbances, with flow velocities of more than 0.28 m min −1 best avoided because of the negative impact on growth. Thus, it is recommended that further developments of new duckweed bioreactors integrate CFD modeling with detailed information of plant requirements and limitations.…”

“…The purpose of the developed multi‐tiered system is phytoremediation of nutrient‐enriched agri‐feed wastewaters. To test operation of the current multi‐tiered prototype, a dilute hydroponics medium was used, which consisted of FloraGrow, 0.25 ml L −1 , and FloraMicro, 0.25 ml L −1 (General Hydroponics: Coughlan et al, 2022). This medium is not optimal for duckweed growth but was selected as representative of agri‐feed waste.…”

“…The cultivation medium is pumped from the sump tank to the top tray (i.e., Tray 1) (Reef Pump 4000 DC Aquarium Pump, Tropical Marine Centre, UK) using a standard, variable speed aquarium pump with a digital controller (12–33 W, min.–max.). Flowrates can also be modulated by diverting excess medium back to the sump prior to it entering Tray 1, thus giving a dual control of flowrate (for the importance of control of the flow rate, see Coughlan et al, 2022, 2022). A gravity‐fed flow then supplies each tray, in turn, with Tray 1 draining into Tray 2, Tray 2 into Tray 3, and Tray 3 into Tray 4.…”

“…The nutrient solution was the same as used for the stock culture. Plants were maintained within the multi‐tiered system at a consistent flow rate (2.5 L min −1 ) and medium depth (50 mm), under an average light intensity of 120 μmol m −2 s −1 PAR, at ~21°C with a 16 h:8 h light:dark photoperiod, conditions that had previously been shown to be suitable for growth (Coughlan et al, 2022).…”

“…For example, the geometry of the cultivation tray will affect the mixing of medium (Maguire et al, 2023), which, in turn, can affect plant growth. Similarly, the flowrate required for nutrient mixing will also affect plants (Coughlan et al, 2022). Biomass harvesting is traditionally achieved through the use of hand‐removal techniques with dip nets, strainers, or drainage‐traps (Skillicorn et al, 1993; Xu et al, 2012), although in recent years, automated systems such as robotic surface skimmers and conveyer‐belt extraction systems have been used for duckweed harvesting from ponds, lagoons, and canals (Timmerman & Hoving, 2016).…”