Effect of circulation and aeration on wastewater treatment by floating aquatic plants

Laboratory tests were performed in order to examine growth characteristics of floating plants in the presence of urban sewage. The aquatic plants selected for this study included Eichhornia crassipes (water hyacinth), Pistia stratiotes (water lettuce) and Salvinia natans (floating fern). Circulation and aeration of the pre-settled sewage enhanced the kinetics of the process, as compared to the control systems. However prolonged application of circulation or aeration may have a different effect. It is shown that in the presence of Eichhornia and Pistia plants, that are supplemented with circulation or aeration, the local standards of biochemical oxygen demand (BOD) -20 mg L-1 and chemical oxygen demand (COD) -70 mg L-1 were reached after 2 and 4.5 days of treatment, while 7-8 days were required to this end without circulation or aeration. Aeration and circulation (without plants) enhanced the ammonium removal capacity from 30.4 to 66.5%, in the control, up to 93.4-100%. In the presence of aquatic plants, aeration and circulation, there was additional 5-35% decrease in the water quality indicators. In this context, the best results were obtained when the action of Eichhornia crassipes and Pistia stratiotes plants, was combined with that of aeration and circulation. In contrast to common practice, the kinetic of BOD COD and turbidity reduction was found to follow a second order, rather than first-order modified reaction model. The new results can be used for design calculations regarding expected removal of pollutants by aquatic floating plants. (C) 2009 Elsevier B.V. All rights reserved.