THE INFLUENCE OF LEMNA-GIBBA L. ON THE DEGRADATION OF ORGANIC MATERIAL IN DUCKWEED-COVERED DOMESTIC WASTE-WATER

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 bubbl ing pumps and a combination of the latter two were compared with a con trol system without duckweed to find out which role duckweed played du ring the degradation of organic material. Removal of COD was significa ntly faster in the presence of duckweed. Removal efficiencies after 3 d were 74-78% in duckweed-covered treatments compared to 52-60% in unc overed controls. DOC levels remained constant and were similar in axen ic and non-axenic duckweed-covered systems, suggesting that heterotrop hic 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 wa s supplied appear important. because the influence of the living duckw eed community could not be simulated satisfactorily by artificial surf aces for bacterial growth, by oxygen pumps or by a combination of both . (C) 1998 Elsevier Science Ltd. All rights reserved.