Asian Scientist Magazine (Sept. 09, 2023) — The Bellandur Lake in Bengaluru, a city in Southern India, has a reputation for excessive foaming. At times, it has frothed so much that the foam spilled over to the roads, causing traffic jams. On other occasions, parts of the lake have caught fire because of methane build up from untreated sewage, sending toxic aerosols from the foam far beyond the lake’s boundaries.
These foaming events are seasonal. Bellandur Lake usually foams following pre-monsoon and monsoon rains. This has puzzled researchers as the rains should lower the concentration of foaming agents instead of leading to even more foaming. In a new study published in the journal Science of the Total Environment, researchers from the Indian Institute of Science (IISc) tackle the mystery behind this counterintuitive phenomenon.
“When it rains, surfactants bound to suspended solids dissolve back into the water,” Reshmi Das, the lead author of the study and a graduate researcher at IISc, told Asian Scientist Magazine.
Surfactants are a type of foaming chemicals used in soaps and detergents. On analyzing Bellandur Lake sediments, the team found high concentrations of surfactants. Similar to those used in commercial cleaning agents, these surfactants hinted at anthropogenic pollution as the cause of the high surfactant concentration.
Surfactants are great at removing dirt. This property is also why they bind easily to suspended solids in the lake water. The team observed that the greater the organic matter content of the sediment, the more surfactant was stuck on its surface. In previous work, the researchers found that Bellandur Lake has extremely low dissolved oxygen, with concentrations as low as nil. The lack of oxygen ensures that these surfactants are not broken down and, therefore, persist for a long time.
To test the hypothesis that rains expose these surfactants, the researchers replicated the lake conditions in their lab. To their lake model, they added raw sewage similar to that in Bellandur Lake and placed a sediment layer at the bottom. They then put a lid on top to simulate the low oxygen.
Sodium dodecylbenzene sulphonate, a surfactant used in the study, readily adsorbed to the sediment in the lake model. On diluting a sample of this sediment with water, and a few cycles of shaking in a centrifuge, the surfactant fell off. Free from the sediment, it created foam in the lake model. This worked for both tap water and demineralized water, indicating that the presence of any charged particles in the water played no role in desorption.
Possibly in a similar way, the additional water from rains disrupts the suspended solids in the lake, freeing the surfactants attached to them. Next, the researchers investigated the stability of foam after rain. They aerated wastewater and then diluted it with tap water. As compared to undiluted wastewater, diluted wastewater produced a greater amount of foam and it took longer to dissipate.
In future work, the team plans to look at other factors behind the foaming. For instance, it has been suggested that certain types of bacteria may be better at retaining surfactants. Meanwhile, this study explains the paradox of increased foaming following rains as a symptom of a dying urban lake.
Bellandur Lake receives untreated sewage far beyond its capacity to break it down and has depleted oxygen that keeps surfactants in the system. In a vicious cycle, this problem has worsened over the years. Foaming has also been witnessed in other lakes in the city such as Varthur Lake.
Tackling this threat requires breaching this cycle. “We are planning sustainable water treatment strategies. The first step is to increase the oxygen levels in the lake. You can either add algae that can increase it or add treated water with higher dissolved oxygen,” said Das.
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Source: Indian Institute of Science ; Source: Adobe Stock
The paper can be found at: Unravelling the reason for seasonality of foaming in sewage-fed urban lakes
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Tags: wastewater, lakes, pollution, chemistry
