Harmful effects of un-ionised ammonia on the zooplankton community in a deep waste treatment pond

The harmful effects of NH 3 on the zooplankton community in a deep waste treatment pond were evaluated under natural conditions. The pond, supplied with secondary effluent from a conventional urban wastewater treatment plant, was designed to improve water quality for agricultural reuse. The aim of the study was to verify the hypothesis suggested in Arauzo et al. (Water Environ. Res. 34(14), 3666) that during phytoplankton blooms in the stratification periods high un-ionised ammonia content values, due to an intense photosynthetic activity and high related pH, lead to a decrease in zooplankton biomass and, thus, to a collapse of the treatment process efficiency. Empirical models were developed to determine relationships between phytoplankton biomass, pH and NH 3 levels. They provided an easy and quick method of detecting when the system was liable to collapse due to the NH 3 effect on the zooplankton community and offered the possibility of adopting measures to guarantee water quality at the effluent. A significant decrease in zooplankton community biomass was observed at un-ionised ammonia levels over 2.5 mg L −1.