Troubled waters: sewage and effluents sink the Cauvery

Along its course, the river Cauvery receives water from multiple tributaries — many of which now function less like rivers and more like drains. Reservoirs such as the Mettur Dam regulate its flow, while hundreds of towns and industrial clusters draw from it for drinking water, irrigation, and manufacturing.

For generations, the river’s rise and retreat has dictated planting seasons, sustained temple towns, and regulated how water was divided. Now, it is increasingly used to absorb the sullage and sewage that cities and industries are unable, or unwilling, to treat.

Only a fraction of the sewage generated across the basin is treated adequately. The rest — domestic waste, industrial effluents, agricultural runoff, and pharmaceutical residues — enters the river largely untreated, often diluted during monsoons and released when flows are high.

It is in this context that farmers, activists, and residents along the Cauvery speak of a river that appears alive but behaves like a sink. Their accounts — of effluents released at night, sewage mixing with irrigation water, and illnesses without obvious cause — are rarely acted upon. But recent scientific studies are beginning to confirm what these communities have long suspected: the Cauvery now carries an invisible load of contaminants that conventional systems neither detect nor remove.

At Mettur, where the Cauvery is first restrained by a massive dam before it begins its long journey across Tamil Nadu, farmers describe a ritual that plays out every flood season. When surplus water is released, sometimes exceeding 10,000 cubic feet per second, industrial effluents are allegedly discharged into the river under the cover of night.

M. Thangaraj, president of the Tamil Nadu Iyarkkai Vivasayigal Munnetra Sangam, says the timing is deliberate. The force of the release dilutes the waste, washing it downstream before it can be detected. Alongside this, untreated sewage and wet waste from the Mettur municipality are also said to enter the river. “In the morning, we see the result,” he says, referring to dead fish floating near the banks. Each time, officials attribute the deaths to low dissolved oxygen levels. But, according to Mr. Thangaraj, oxygen drops because effluents have entered the water.

Mettur Municipal authorities deny these allegations, pointing out that three sewage treatment plants have been operational for the past six years and that sewage is no longer discharged into the Cauvery. The Tamil Nadu Pollution Control Board (TNPCB) maintains that there are no effluent discharge points near the river and that monthly water samples show no pollution.

Erode, meanwhile, represents one of the most acute urban pressure points along the Cauvery. The river flows for about 84 kilometres through the district, but a 17-kilometre stretch within the Erode Corporation limits, between R.N. Pudur and Vendipalayam, remains the most contaminated.

The city hosts over 500 bleaching, dyeing, and tannery units. Wastewater from many of these flows into streams like Pichaikaranpallam, Nochipallam, and Sunnambu, which drain directly into the river.

Erode citygenerates 35 to 40 million litres of sewage every day, but only about 8 MLD is treated. The rest enters the river untreated. In 2022, the Central Pollution Control Board (CPCB) officially classified this stretch of the Cauvery as polluted. Although the TNPCB directed the Corporation to accelerate underground sewerage works, progress remained slow until March 2024, when the State announced new sewage treatment plants at six identified discharge points.

Downstream, the scepticism only deepens. In Namakkal district, the river encounters a different kind of pressure. A. Adhavan, State general secretary of the Lok Janshakti Party and a resident of Pallipalayam, traces the problem back to the Noyyal river crisis in Tiruppur. When dyeing units there faced closure, many simply relocated to the banks of the Cauvery, setting up shop in Komarapalayam, Pallipalayam, and Kokkarayanpettai.

Each dyeing machine, Mr. Adhavan says, can discharge up to two lakh litres of effluents a day. Regulations require this wastewater to be treated and recycled. “But most factories don’t do that,” he alleges.

Instead, land is purchased close to the river or its channels. Borewells pump Cauvery water into the factories illegally, and the same pipelines are used to release effluents back into the river.

Complaints to the TNPCB, Mr. Adhavan says, lead to temporary closures lasting a few days or the registration of token cases. Operations invariably resume soon. Along the riverbanks, fear has altered daily life. “People no longer drink the water supplied by municipalities. They buy packaged water because they are afraid of pollution, and potential risks of cancer,” he alleges.

In Karur, years of farmers’ protests and a landmark Madras High Court ruling forced dyeing units to adopt zero liquid discharge systems(an advanced industrial water treatment process that recovers and reuses nearly all wastewater). This reduced direct effluent release into the Amaravathi, a tributary of Cauvery. Yet, domestic sewage and industrial waste continue to flow unchecked. Effluents once stored at the Orathupalayam dam are now discharged directly downstream, eventually entering the Cauvery at Semangi. “The impact isn’t visible when the river is full,” says R.S. Mugilan of the Tamil Nadu Environmental Protection Movement. “But environmentally, the damage is enormous.”

Further downstream, the crisis becomes structural rather than episodic. Sundara Vimal Nathan, secretary of the Tamil Nadu Cauvery Farmers’ Protection Federation, points to the dismantling of traditional safeguards that once protected the river. Historically, two buffer zones — neer nilai porambokku (waterbody commons) and vella karai porambokku (flood-bank commons) — prevented construction and industry along the river’s edge. These zones absorbed floods and filtered waste before it reached the water.

Today, they have been systematically encroached upon. Commercial establishments, housing layouts, and informal settlements sit directly on the banks, discharging septic waste and raw sewage into the river.

In Thanjavur district, the situation is especially dire around Kumbakonam, a town nestled between the Cauvery and the Arasalar. Its 44 temple tanks, once fed by river channels, now function as repositories for urban waste. Here, municipal solid waste often piles up along the banks of the river, turning the area into a breeding ground for stray animals.

Industrial pollution compounds the problem. Near Thirumandankudi, untreated effluents from sugar factories and distilleries are released into the Pazhavar drain, contaminating water used by farmers downstream.

In Tiruvarur district, P.S. Masilamani of the Tamil Nadu Vivasayigal Sangam (CPI) describes irrigation canals so polluted they can no longer support agriculture. In places such as Kottapattu, the water has become unusable. The absence of proper waste management in small towns means sewage routinely mixes with irrigation water.

Mr. Masilamani calls for a special scheme to create dedicated drainage and treatment systems to prevent the mixing of sewage with irrigation water. He points to the textile belts of Karur, Erode, and Namakkal as major contributors where existing treatment systems are failing. The damage, he warns, is not always visible in the water itself. Instead, it shows up on the bodies of farmers — persistent itching, recurring skin conditions — signs of a health crisis that remains largely unacknowledged in the delta region.

As per The Hindu’s findings, municipal administrations in towns along the Cauvery in the delta region, including Tiruvaiyaru, Kumbakonam and Mayiladuthurai, had largely curtailed the direct discharge of untreated domestic sewage into the river, but the impact of sewage discharge was increasingly felt in the irrigation channels branching off from the Cauvery.

An action plan drafted by the State government, following a direction from the National Green Tribunal (NGT), recommended district authorities to upgrade their Sewage Treatment Plants to meet the discharge standards prescribed under the Environment (Protection) Amendment Rules, 2017. It also directed local bodies along the Cauvery river stretch in Mayiladuthurai to remove all municipal solid waste dumped near the riverbanks and strictly comply with the Municipal Solid Waste Management Rules, 2016.

In Mayiladuthurai municipality, the release of domestic sewage directly into the Cauvery had been largely stopped within town limits. However, the entire sewage load was diverted into Sathyavanan Vaikkal, an irrigation channel flowing through Vilanagar, Arupathi, Sembanarkoil and Akkur. Residents said the channel, which had been extensively used for agriculture until about 15 years ago, was now in a pathetic state.

The 5.85 million litres per day (MLD) sewage treatment plant on Sathyavanan Vaikkal at Mannampandal, which operated on the outdated Water Stabilisation Pond technique, had become defunct, resulting in untreated sewage flowing directly back into the channel. The State government had issued orders to revamp the facility using Sequence Batch Reactor technology and enhance its capacity to 9.49 MLD.

In Mayiladuthurai, the Water Resources Department had also planned to provide concrete lining to the Old Cauvery channel, locally known as Pazhancauvery, at a cost of ₹36 crore to prevent the inflow of untreated sewage. This was expected to help curb eutrophication, check weed growth, and ensure smooth water flow. Steps had also been initiated to line the Kuthalam channel to prevent the mixing of domestic sewage, for which ₹7 crore had been sanctioned.

In Sirkazhi, untreated sewage had been discharged into Kazhumalayar, a distributary of the Pudhu Manniyar. Steps had only recently been initiated to install a sewage treatment plant.

Cauvery Dhanapalan, general secretary of the Cauvery Farmers Protection Association, locates the problem of pollution even further upstream. According to him, contamination begins in Kodagu, where fertiliser runoff from coffee plantations settles as toxic silt in the Mettur reservoir. From there, the river accumulates industrial waste at Mettur, dyeing effluents from Erode and Karur via the Noyyal and Amaravathi, cement factory discharge in Namakkal, and untreated sewage in Tiruchi.

A 2021 study published in Science of the Total Environment examined the presence of pharmaceutically active compounds, heavy metals, and physicochemical parameters in 22 locations along the entire stretch of the Cauvery and its tributaries.

Researchers detected substances such as caffeine, carbamazepine, diclofenac, ciprofloxacin, and triclosan across most sampling locations, particularly near wastewater discharge points and during the post-monsoon season. These compounds, originating from untreated sewage, pharmaceutical waste, and human excretion, are not removed by conventional treatment systems.

By the time the water reaches the tail-end regions, the agricultural consequences are severe. In Nagapattinam, nearly half of the 1.72 lakh acres of cultivable land is facing a collapse in yields. High chemical concentrations cause the soil to silt up and harden, preventing roots from penetrating and steadily degrading crop quality, says Mr. Dhanapalan.

Experts pointed out that rampant sand mining has also played a significant role in the Cauvery losing its self-purification capacity. In the lower delta, particularly east of Kabisthalam, reduced sand deposition in the Cauvery and its distributaries had altered the riverbed, making it increasingly clayey.

In an official communication issued a few months earlier, the State government had flagged concerns over severe pollution affecting waterbodies due to the discharge of untreated domestic sewage and industrial effluents, as well as the dumping of solid waste, including plastics and garbage, leading to further contamination.

The State government had decided to enforce stringent provisions under the Tamil Nadu Farmers’ Management of Irrigation Systems Act, 2000, against the disposal of sewage and industrial waste into waterbodies, besides undertaking public awareness campaigns and regular environmental monitoring to address the issue.

Minister for Environment Thangam Thennarasu said the government had sought funds from the Union government under Nadanthai Vaazhi Cauvery scheme to prevent pollution in the Cauvery. “The government is setting up effluent treatment plants wherever SIPCOT industrial estates are present. Our objective is to plug wherever sewage is flowing into the rivers,” he said.

(Compiled by Geetha Srimathi with inputs from M. Sabari in Salem, S.P. Saravanan in Erode, C. Jaisankar in Karur, Nacchinarkkiniyan M. in Tiruchi, and N. Sai Charan and B. Kolappan in Chennai.)