Estimation of the potential of Lemna minor for effluent remediation in integrated multi-trophic aquaculture using newly developed synthetic aquaculture wastewater

Aquaculture is an important source of animal protein and a key contributor to global food security. However, aquaculture can exert a negative effect on the aquatic environment due to the release of effluents containing high nutrient levels. In integrated multi-trophic aquaculture (IMTA), the waste produced by one species is the input for another, referred to as extractive species (ES). Potential ES include plants. In the present study, it was explored whether Lemna minor can be used to remove nitrogen and phosphorus from aquaculture wastewater. A representative synthetic wastewater was designed based on the composition of aquaculture effluents found in the literature. Synthetic wastewater was found to be a suitable medium for growth of Lemna minor, and plants readily took up NH4+-N, NO3-N and PO43--P. In particular, NH4+-N concentrations rapidly decreased. The highest removal rates per square meter of water surface, calculated for NH4+-N, NO3-N and PO43--P, were, respectively, 158, 206 and 32 mg·m-2·day-1, and these rates were achieved at a plant surface density of 80%. As removal of nutrients is essentially a surface area–related process, the effect of plant density on nutrient uptake was determined. Uptake of nutrients per square meter of surface area was highest at the highest plant density. Yet, when uptake rates were calculated per square meter of water area covered by Lemna fronds, the highest removal rates were found at the lowest plant density, and this is likely to be associated with a reduced intraspecific competition. The present work enables the calculation of potential nutrient uptake by Lemna minor and lays the foundation for a more scientific approach to the design of duckweed-based aquaculture wastewater treatment systems.