AN INTEGRATED KINETIC MODEL FOR WATER HYACINTH PONDS USED FOR WASTEWATER TREATMENT

Abstract

Water hyacinth ( Eichhornia crassipes) is a floating aquatic plant which has been employed for wastewater treatment in many parts of the world. By planting water hyacinth in a wastewater pond, part of the gaseous oxygen produced by photosynthetic activity of the green leaves is translocated to the stems and roots and to the water body; this oxygen is used by the aerobic and facultative bacteria in biodegrading organic matter contained in the wastewater. Two groups of bacteria normally exists in a water hyacinth pond (WHP), namely the suspended bacteria which are present in the liquid portion and the biofilm bacteria which are attached on the surfaces of the roots of the water hyacinth plants and on the side walls and bottom layers of the pond itself. Current design criteria for WHPs do not emphasize the roles of these two groups of bacteria, but they are based on either empirical relationships or first-order reaction rate and complete-mix flow condition. This study attempts to show the significance of both the suspended and biofilm bacteria and the flow hydraulics (based on dispersion number) in the reduction of organic matter in the WHP system. Kinetic coefficient of the suspended bacteria was calculated from a first-order reaction rate including the effect of organic loading rate. For the biofilm bacteria, the reaction rate was also first-order and based on the substrate flux into the biofilm. The integrated kinetic model proposed for the WHP incorporates the activities of both the suspended and biofilm bacteria and the hydraulic dispersion number. The model was found to be satisfactory in predicting biochemical oxygen demand (BOD) removal in a full-scale WHP treating an anaerobic pond effluent. Design guidelines and an operating strategy to achieve improvement in BOD removal efficiency in WHP are given. © 1998 Elsevier Science Ltd. All rights reserved