Foaming in Bengaluru’s Bellandur lake caused by continuous entry of untreated sewage: IISc
As more and more sewage flows through the lake, surfactants in it do not decompose, but get attached to the sludge, gradually increasing in concentration, a research scientist said.
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One of the three reasons highlighted by the team, on Tuesday, is the entry of sewage into the lake which takes 10-15 days to spread in the water body. (Express Photo) A team from the Centre for Sustainable Technologies (CST), Indian Institute of Science (IISc), Bengaluru has been monitoring the cause behind the formation of foam in Bellandur lake for the last four years. One of the three reasons highlighted by the team, on Tuesday, is the entry of sewage into the lake which takes 10-15 days to spread in the water body.
In a release, Chanakya H N, chief research scientist at CST and one of the authors of the study, said, “The first reason is untreated sewage that enters the lake. Since the lake is large, the sewage takes 10-15 days to disperse through the lake; during this time, a part of the organic material gets degraded in the absence of oxygen and settles down as sludge. As more and more sewage flows through the lake, surfactants in the sewage do not decompose and instead get loosely attached to the settled sludge, gradually increasing in concentration.”
Surfactants reduce the surface tension of water. The major source of surfactants are cleaning detergents, shampoos and soaps. The analysis by the team suggested that a single type of surfactant commonly used in most household washing powders and shampoos plays a dominant role in driving this foaming.
Heavy rain another factor
The foaming has puzzled researchers because it counterintuitively increases only after heavy rains, which are supposed to dilute pollutants in the lake that may be causing the foaming.
The second factor is heavy rainfall that brings in large quantities of run-off from the city into the lake overnight. “This high-volume inflow appears to churn up the surfactant-laden sludge, dislodge the accumulated surfactant from the sludge, and bring it back into solution, making it ready to foam. Deep inside the lake itself, there is little foam, because air bubbles do not form. However, as the water level in the lake rises due to rains, the excess water containing large concentrations of the surfactants spills over into the lake’s outlet to depths as high as 25 feet, trapping air bubbles which turn into foam. This is an important phenomenon that converts the surfactant-laden water into foam,” said Lakshminarayana Rao, associate professor at CST and one of the authors.
In addition to these two factors, the researchers suggested that suspended solids containing certain bacteria might be responsible for foam formation.
To study the foam formation, the researchers collected water samples from the lake, analysed various parameters and recreated a lab model to track the changes in the chemical composition of the surfactants across different regions of the lake as well as at different times of the year.
“I had to go to the lake every month over the years to collect water and foam samples, and conduct experiments on them,” said Reshmi Das, PhD student at CST. She took the help of officials from the Bangalore Water Supply and Sewerage Board (BWSSB) and Bangalore Development Authority (BDA) to collect the samples.
The authors proposed in the study that stopping the entry of untreated sewage into the lake is crucial to preventing the build-up of surfactants and sludge, their churning, and the resulting foaming. They also suggested that wherever this is not immediately possible, removing the accumulated sludge in the polluted lakes – at least before the rains – as well as proper disposal of it can help address this burning issue.
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