Dirty water: Thenpennai river carries Bengaluru’s sewage to TN

CHENNAI: Thenpennai River may be the lifeline for north Tamil Nadu. But for the ‘cosmopolitan’ Bengaluru, the river is just a sewage outlet.

An analysis of the Thenpennai water conducted by the Tamil Nadu Pollution Control Board (TNPCB) confirms that the river is carrying sewage as tests show the presence of high concentration of faecal coliforms and total coliforms. What’s more, you cannot even take a dip!

Frothing of Thenpennai river in Kelavarapalli Dam near Hosur in Krishnagiri has become an annual event during monsoon seasons. Many claim that the frothing is due to the sewage it carries from Bengaluru, the city which the river crosses before reaching Tamil Nadu.

According to a report submitted by the pollution control board to the southern bench of National Green Tribunal (NGT), the TNPCB and Central Pollution Control Board (CPCB) collected water samples at the Chokkarasanapalli entry point, Kodiyalam Village, upstream of Kelavarapalli Dam (Bagalur village), middle point near second shutter of the dam and downstream of the dam near Marasandiram village road in October 2024.

The result of analysis (RoA) revealed that the total coliform level fluctuated from around 2.42 lakh MPN/100ml (most probable number per 100ml of water) to 19,863 MPN/100ml between Chokkarasanapalli entry point and downstream of the dam. Also, faecal coliform level fluctuated from around 98,040 lakh MPN/100ml to 10,462 MPN/100ml between the two points. As per CPCB standards, faecal coliform holds the permissible limit of 2,500 MPN/100 ml for bathing standards. The RoA reveals that Thenpennai water is not good even for bathing.

“From the (RoA) report, the total and faecal coliform at Chokkarasanapalli entry point reveals that there is a discharge of sewage/waste water in Thenpennai river from Karnataka,” the report said.

Similarly, dissolved oxygen (DO) levels in the sample points were found to be less than 5mg/l in four of the five spots. DO levels below 5 mg/l indicate domestic or industrial pollution. As against the 3 mg/l standard for biological oxygen demand (BOD), the Thenpennai water showed BOD levels between 32 and 20 mg/l. Similarly, chemical oxygen demand (COD) levels were between 112 mg/l and 72 mg/l as against the bathing standard of 10 mg/l.

It may be noted that the NGT has issued orders in July 2022 to Tamil Nadu and Karnataka governments to ensure proper sewage and solid waste management in the villages adjoining the river up till Kelavarapalli and apart from regularly monitoring the water quality in important locations. However, the NGT reopened the case following recent frothing-up incidents in Kelavarapalli.

Based on the analysis report, R Kannan, member secretary of the TNPCB, wrote to the CPCB a few days ago, requesting it to take measures to curtail sewage discharge from Bengaluru through the Bellandur and Varthur lake systems and industries located in the neighbouring state. In another development, the TNPCB has decided to seek Anna University to study the reasons behind frothing incidents in the Thenpennai and Palar rivers.

An expert attached to the Board explained that traditionally, TNPCB would collect samples of river water and test it for total dissolved solids (TDS), biological oxygen demand (BOD), chemical oxygen demand (COD), heavy metals and other parameters. “But, those tests will not give results as far as frothing incidents are concerned. Only an-ion and cat-ion tests will provide composition of pollutants in the water,” he opined.