Author(s): Carlos Pacheco; Iran Lima Neto

Linked Author(s):

Keywords: Eration; Eutrophication; Chlorophyll-a; Mixing; Turbulence

Abstract: Algal blooms are a worldwide problem, especially in the Brazilian Northeast, a highly-populated semiarid region with more than 200,000 water-supply reservoirs, which has faced human losses due to cyanotoxin poisoning. Bubble plumes have long been used to minimize eutrophication symptoms in lakes and reservoirs by artificial destratification and hypolimnetic oxygenation. However, previous studies have been carried out normally under stratified water conditions in relatively deep lakes and reservoirs. On the other hand, laboratory experiments using different mixing devices in unstratified tanks have suggested that small-scale turbulence can influence algal growth by affecting cellular activities, promoting cell disruption, and/or changing the settling velocity of the cells. Nevertheless, the effect of small-scale turbulence on algal dynamics and metabolic rates is still unclear, and the results obtained from different experimental studies may not be representative of natural environments, such as lakes and reservoirs. In this study, we investigated the impact of bubble plumes on the behavior of algae in a hypereutrophic, shallow and unstratified lake in the Brazilian Northeast. The results indicated that flow turbulence and circulation induced by bubble plumes promoted a faster decay of algae over time, as compared to baseline data obtained outside these plumes. The time-variation of chlorophyll-a, cyanobacteria, and pheophytin-a were adjusted to first-order kinetic models, both inside and outside the bubble plumes. The net reduction rates of chlorophyll-a and cyanobacteria due to induced circulation were well described by a log-curve as a function of a dimensionless parameter that describes bubble plume hydrodynamics. These algal reduction rates were also related to the net growth rate of pheophytin-a, which suggests that algal removal was caused mainly by cellular death. Moreover, the relationship proposed here was used to predict the effect of bubble plume-induced circulation on algal removal in larger unstratified lakes, for different scenarios of air flow rates. The simulations indicated that the use of point-source bubble plumes can be an effective measure to minimize the local impacts of eutrophication in water-supply reservoirs and potentially reduce water treatment costs.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920227

Year: 2022