Residents near Foreshore Estate in Chennai, near the Adyar Estuary were in for a shock on Friday, when the sea water began to froth as it reached the shore. Similar to scenes observed on the lakes of Bengaluru, what seemed to be foam, flew onto the beach from the sea, leaving a bed of white pollutants behind.

But even as the city's residents expressed shock on social media over the occurrence, environmentalists tell TNM that this is a yearly phenomenon during the monsoons. The frothing occurs during the rainy season due to the presence of non-biodegradable and detergent-like effluents coming from municipal and industrial wastes.

"When there are rains, there is increased inflow into sewage treatment plants (STP) as the stormwater also enters the sewage network," explains Pooja Kumar of the Coastal Resource Centre. "Let's say there is an STP that can treat 100 litres an hour, and it receives 100 litres of sewage plus 100 litres of rainwater. The STP is capable of only handling half that volume. So what it released is essentially untreated sewage or partially treated sewage. Our sewage contains detergents which tend to foam up when the water is agitated. When there is sudden flow in a waterbody with many blockages, there are plenty of places of turbulence which increases the agitation and creates the conditions for creating foam,” says Pooja.

“It is not necessary that there is an excess discharge of sewage. This phenomenon is a result of a volumetric increase of sewage as a result of dilution with rainwater. Rainwater also washes out various contaminants from the surfaces it touches, including roads, roof-tops and open earth," she adds.

Last year too, there were multiple reports of foam forming thick blankets on the city's coast from Ennore to Tiruvanmiyur. While the Tamil Nadu Pollution Control Board

(TNPCB) has collected samples from the frothing water every year, they are yet to release any information on the pollutants in the water and suggest an action plan to prevent it.

Environmentalists, however, have pointed out in the past that the froth is a result of illegal outfalls along the Adyar river which are discharging sewage.

However, the TNPCB alleges that agricultural activity is responsible for the pollution.

"This is happening at the mouth of Adyar river and we see it every year at this time, due to increase in agricultural activities," says M Malayandi, Joint Chief Environmental Engineer. All the excess fertilisers and pesticides used on the fields is getting washed into the stormwater drains and making its way to the sea finally. We are currently here at the spot to study the situation. An immediate solution would be to switch to organic fertilisers. "