Phytoremediation performance of Lemna communities in a constructed wetland system for wastewater treatment

Duckweeds are considered suitable plants for bioremediation of wastewaters because they can tolerate and take up diverse pollutants and large quantities of nutrients. They have year-round vegetative growth that can reach very high growth rates using wastewater nutrients that can accumulate intracellularly or be utilized to produce new biomass. The constructed wetland that services the town of Forano (Central Italy) is composed of three interconnected, but distinct, treatment pools (P1, P2, P3). In this study the phytoremediation performance of the whole treatment system was assessed taking into consideration pH, temperature, oxygen, nitrates, phosphates, sulphates, and pathogenic bacteria (Escherichia coli) at the inlet and outlet of each pool once per season over a year. The remediation efficiency of P2 was lower than in P1 and P3, and this was shown to be due to an extended, dense free-floating Lemna mat. The mat was thick enough (18–30 mm) to block light penetration, and heat and gas exchange to the underlying water resulting in dark, cool and near-anaerobic conditions. These conditions also limited aerobic biodecomposition processes and natural control of the E. coli population. The underlying fronds of the mat in P2 were highly necrotic and became a source of nutrients, shown by an increase in nitrates and phosphates from the inlet to the outlet. Lemna use in phytoremediation is often considered highly important, but it is shown here that active management interventions are required and should involve periodic harvesting of Lemna to avoid mat over-development and formation of necromass.