Bengaluru's Bellandur Lake has long been plagued by the issue of heavy foaming that recurs year after year, particularly after heavy rains. A team of researchers from the Centre for Sustainable Technologies (CST) at the Indian Institute of Science (IISc) that has monitored the foam for the past four years have published a study highlighting three possible reasons behind the foaming phenomenon — untreated sewage entering the lake, heavy rains that bring run-off, and suspended solids containing certain bacteria. The study also indicated that a specific type of surfactant commonly found in household washing powders and shampoos plays a dominant role in driving the foaming phenomenon.

In the study that was recently published in the journal Science of the Total Environment, the researchers highlighted untreated sewage as one of the contributing factors to the foaming and said that due to its large size, the lake takes approximately 10-15 days for sewage to disperse. During this time, organic material present in the sewage undergoes degradation in the absence of oxygen, settling down as sludge, according to the study. As more sewage enters the lake, surfactants in the sewage do not decompose but instead become loosely attached to the settled sludge, leading to a gradual increase in surfactant concentration — sometimes reaching up to 200 times the original levels — the scientists found. 

The second factor is heavy rainfall, which brings substantial amounts of city run-off into the lake overnight. According to the IISc study, this inflow churns up the surfactant-laden sludge, dislodging accumulated surfactants and bringing them back into solution, thereby making them ready to foam. While deep inside the lake, little foam is observed as air bubbles do not form there. However, as the water level rises during rainy periods, the water containing high concentrations of surfactants spills over into the lake's outlet, trapping air bubbles and transforming into foam at depths of up to 25 feet. This phenomenon is vital in converting the surfactant-laden water into foam, as explained by Lakshminarayana Rao, Associate Professor at CST.

In addition to these two factors, the researchers also suggest that suspended solids containing certain bacteria may play a role in foam formation and stability, although further experiments are required to validate this mechanism.

To study the foam formation process, the team collected water samples from the lake, analysed various parameters, and created a laboratory model to track changes in the chemical composition of surfactants across different lake regions and throughout the year. Reshmi Das, a PhD student at CST and one of the authors of the study, collected water and foam samples from the lake every month over the past four years, conducting experiments to gain insights. The researchers collaborated with officials from the Bangalore Water Supply and Sewerage Board (BWSSB) and Bangalore Development Authority (BDA) to gather the necessary samples.

Based on their findings, the authors propose that preventing the entry of untreated sewage into the lake is crucial to avoid the accumulation of surfactants and sludge, thereby preventing their churning and the resulting foaming at the outfall. They also suggest that in cases where immediate prevention is not feasible, removing the accumulated sludge from polluted lakes — especially before the rainy season — and ensuring proper disposal can significantly address this persistent issue.