- An integrated pond system, consisting of duckweed and algae ponds, was investigated for duckweed production and for further treatment of anaerobically treated domestic wastewater. The system consisted of 10 ponds in series, arranged in 3 stages of 2 duckweed ponds, 3 algae ponds and 5 duckweed ponds, respectively. Production of duckweed ranged from 7.4–16.4 g/(m2.day) (or 27–60 ton/(ha.year)) dry weight in the first pond, to 2.7–8.2 g/(m2.day) (or 10–30 ton/(ha.year))in the last pond. Regression analysis suggested that the production was positively influenced by the concentration of organic compounds in the pond water. The ammonium concentration in the range of 20–60 mg/l NH4-N did not affect duckweed production. Fifty-six percent of the pond influent nitrogen, mainly ammonium, was removed. Ammonium removal was due to uptake by the duckweed plants (18%), nitrification (3%), sedimentation (8%) and combined volatilization of NH3 and denitrification (73%).