Untreated sewage key to lake foaming: Study

BENGALURU: A four-year study by researchers at Indian Institute of Science (IISc) has come up with some interesting results regarding the likely reasons behind the foaming of Bellandur Lake.

While rainfall, entry of untreated sewage into the lake, and presence of suspended solids containing certain bacteria have been found to be among the reasons leading to the foaming, researchers are most intrigued by a fourth factor: The presence of a type of surfactant (foam-causing agent), commonly used in most home detergent powders and shampoos, in the lake water.

Results of the study, conducted by the Centre for Sustainable Technologies (CST), IISc, have been published in the May 2023 issue of the journal 'Science of the Total Environment'.

One way of preventing the foaming, researchers say, is to stop the flow of untreated sewage into the lake, besides removing the sludge that accumulates before monsoon.

How sludge accumulates

Researchers found that part of the untreated sewage that enters the lake turns into sludge. "Given that the lake is large, untreated sewage that enters the lake takes 10-15 days to disperse. During that time, a part of the organic material gets degraded in the absence of oxygen and settles down as sludge," the IISc study revealed.

Over time, as more sewage flows in, part of it gets loosely attached to the sludge, the concentration of which gradually increases. "In some cases, it is up to 200 times the original concentrate entering the lake," said Chanakya HN, chief research scientist at CST and one of the authors of the study.

With heavy rainfall, the surfactant, acting as a foaming agent, is dislodged from the sludge, making it foam-ready. Chankaya likened the process to adding a scoop of washing powder into a bucket of water, causing foaming.

Lakshminarayana Rao, associate professor at CST, said: "Initially, there is just a little foam deep in the lake, but when it rains, the water, along with a large concentration of surfactants, gushes into the lake that has a depth of 25 feet. With that, the air bubbles get trapped, leading to foaming."