In this paper, a combined aquatic treatment process coupling waste stabilization ponds (WSPs) with water hyacinth ponds (WHPs) was investigated as means to upgrade secondary effluent from a waste water treatment plant (WWTP). Naturally-occurring nitrification and denitrification phenomena were monitored and evaluated on a quantitative basis. The WSP supplied oxygen to the post process WHP, while the inside of the WHP provided a unique denitrification environment caused by respiration and biodegradation of the algae separated by hyacinth plant roots. The nitrification and denitrification rates were 0.04 and 0.02 g/kg day at 20 degrees C (wet weight basis), respectively, and were strongly affected by seasonal change. The temperature-dependent coefficients theta were 1.06, which were approximately similar to frequently reported values from other biological nitrogen removal studies. Nitrification and denitrification were expected to occur as the water temperature was maintained between 20 degrees to 30 degrees C. As plant density increased, their rates were also enhanced. Alkalinity balance corresponded fairly well with nitrogen behaviour during most of the operational period. Oxygen balance test results validated that the water hyacinth was crucial not only for separating algal particles from the WSP. but also for biological nitrogen reduction. (C) 2009 Elsevier B.V. All rights reserved.