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 °C (wet weight basis), respectively, and were strongly affected by seasonal change. The
temperature-dependent coefficients Θ 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° to 30 °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