The growth of duckweed (Spirodela polyrrhiza) was investigated in an outdoor batch system under 15 different conductivities ranging from 200 μS/cm to 3000 μS/cm with average 110 µmol/m2 daylight intensity. The growth was performed in an anaerobically treated domestic wastewater using an initial plants population of 50 fronds. Increase in Relative Growth Rate—RGR—(in terms of fresh weight and number of fronds) had a significant non-linear (polynomial) relationship with the conductivity. The maximum RGR related to fresh weight of 0.176 was observed at the conductivity of 1200 µS/cm while the maximum RGR related to fronds number was 0.193 at the conductivity of 800 µS/cm. The optimum range for duckweeds growth was found to be between conductivities of 600 µS/cm and 1400 µS/cm. The maximal rates of removal of COD of 14.5 mg/day, 2 PO4 of 0.65 mg/day, NTK of 15.68 mg/day, faecal coliforms of 100% and turbidity of 80.8% were observed in this range. The COD and 2 PO4 removal rates were highly correlated to the growth rate, with the correlation factor of up to 0.783 and, then to the conductivity. The NTK reduction was positively related to the conductivity.