Effect of Artificial Circulation on the Removal Kinetics of Cyanobacteria in a Hypereutrophic Shallow Lake

Abstract

This study consisted of two parts. First, pre-aeration tests were conducted to assess the space-time variability of water quality in a shallow lake. Chlorophyll-a concentration and cyanobacteria cell density varied from 40 to 80 μg L−1 and 0.5–2.0×106 cells/mL, respectively. In the second and major part of the study, aeration tests were carried out to investigate the influence of artificial circulation on the flow hydrodynamics and water quality. Comparison of the measurements taken inside and outside the bubble plumes showed that artificial circulation promotes a faster decay of chlorophyll-a concentration over time. First-order kinetics fitted well the time-variation of depth-averaged chlorophyll-a, cyanobacteria, and pheophytin-a. The net reduction rate of chlorophyll-a and cyanobacteria could be described as a function of a dimensionless parameter β that controls bubble plume hydrodynamics or, alternatively, as a function of the rate of dissipation of turbulent kinetic energy ϵ. This net reduction rate of chlorophyll-a could also be linearly related to the net growth rate of pheophytin-a. This suggests that algal removal was caused mainly by cellular death. Finally, the relationships obtained in this study were used to predict the impact of artificial circulation on algal removal in water-supply reservoirs.