Toxic Foam In Bengaluru's Bellandur Lake Caused By Surfactants In Detergents, Untreated Sewage: IISc Study

For many years Bellandur Lake in Bengaluru had gained notoriety for being extremely polluted and filled with toxic foam that often gets spilt into the roads and nearby buildings.

Despite it being a phenomenon that recurred every year during the rainy season, so far, scientists had failed to identify what was causing it.

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But a new study by researchers at the Centre for Sustainable Technologies (CST) at the Indian Institute of Science (IISc) has claimed to uncover the mystery.

According to the researchers who studied the water in Bellandur Lake for four years, the foaming has puzzled scientists as it increases only after heavy rains. The phenomenon is counterintuitive as increased water flow is supposed to dilute pollutants in the lake.

The researchers have identified three potential reasons for the toxic foam formation in Bellandur Lake.

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These are untreated sewage, heavy rainfall and suspended solids containing certain bacteria.

Results of the study, ‘Unravelling the reason for seasonality of foaming in sewage-fed urban lakes,’ were published in the journal Science of the Total Environment.

"Because the water body is large, the sewage takes 10-15 days to disperse through the lake. Meanwhile, some of the organic material gets degraded without 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 – in some cases, up to 200 times the original concentration entering the lake," explained Chanakya HN, chief research scientist at CST and one of the authors of the study.

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"Imagine adding one full scoop of washing powder into a bucket of water; it will definitely foam given the right conditions," he explains.

Researchers say heavy rainfall that brings in large quantities of runoff from the city overnight is yet another critical factor.

The heavy rainfall brings 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.

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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 that turn into foam.

"This is an important phenomenon that converts the surfactant-laden water into foam," reports Lakshminarayana Rao, Associate Professor at CST.

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The team 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. A single type of surfactant commonly used in household washing powders and shampoos played a dominant role in causing the foaming.

"The suspended solids containing certain bacteria might be responsible for foam formation and stability. However, the mechanisms involved need to be validated through further experiments," the researchers mention.

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According to the researchers, stopping untreated sewage into the lake is crucial to prevent the build-up of surfactants and sludge, their churning, and the resulting foaming at the outfall. Removing the accumulated sludge in the polluted lakes – at least before the rains – and adequately disposing of it could also help.

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