The role of algae in a deep wastewater self-regeneration pond Original Research

The present investigation was designed to gain information on the role of algae in a deep wastewater self-regeneration pond. The experimental pond was continuously fed with secondary effluent from a municipal wastewater treatment plant. Biological, physical and chemical profiles were recorded throughout the water column during the period July 1996 to December 1997. The removal efficiencies of several chemical and sanitary indicators were calculated. The time course of species biomass was used to evaluate phytoplankton dynamics and its relationship with the performance of the deep pond. The efficiency of the system seemed to be conditioned by temperature and phytoplankton biomass. Improved performance was observed during periods of mixing. Chlorococcales and Volvocales grew extensively during stratification periods. Euglenophyceae dominated during the mixing. When the total phytoplankton biomass exceeded 15,000 mgC m−3, the removal efficiencies of the chemical oxygen demand (COD) and the suspended solids acquired negative values. These phytoplankton blooms were associated with an intense increase in pH and, consequently, in NH3 levels, in the epilimnion, during the stratification. When such a bloom occurred, considerable decreases in zooplankton and bacterial biomasses were observed, which temporarily destabilized the trophic structure of the pond.