The influence of Lemna gibba L. on the degradation of organic material in duckweed-covered domestic wastewater

Laboratory scale experiments on duckweed-covered domestic sewage were carried out to determine whether removal of organic material is faster in the presence of duckweed. Performance of systems containing axenic and non-axenic Lemna gibba L., artificial plastic duckweed, air bubbling pumps and a combination of the latter two were compared with a control system without duckweed to find out which role duckweed played during the degradation of organic material. Removal of COD was significantly faster in the presence of duckweed. Removal efficiencies after 3 d were 74-78% in duckweed-covered treatments compared to 52-60% in uncovered controls. DOC levels remained constant and were similar in axenic and non-axenic duckweed-covered systems, suggesting that heterotrophic uptake of smaller organic compounds by duckweed was not important. Degradation of organic material was enhanced by duckweed through both additional oxygen supply and additional surface for bacterial growth. The structure of attached bacterial communities and the way oxygen was supplied appear important. because the influence of the living duckweed community could not be simulated satisfactorily by artificial surfaces for bacterial growth, by oxygen pumps or by a combination of both. (C) 1998 Elsevier Science Ltd. All rights reserved.