Explainer
It’s not the end of the story after you flush the commode.

In November 2015, the city of Chennai woke up to a startling sight. Cooum, a river whose name was almost synonymous to sewage in the state was sparkling in the sun. Following heavy rains in the city, the waterbody that has remained contaminated for several decades was flushed of the sludge that polluted it. Castigating the state government’s many failed attempts to clean-up the Cooum, the National Green Tribunal remarked, “What you couldn't do in a year, the rain did in a day or two.”

But this natural ‘clean-up’ was short-lived. The river which was once an integral part of the socio-economic life of the city, is back to its 'drain' status, with around 30% of the city's sewage dumped in it. 

This disgusting sight together with the unbearable stench in the heart of the city, bring to fore an important question. Where does Chennai's poop really go?

What happens after you flush?

You think it’s the end of the story after you flush the commode to get rid of your poop. But here’s what happens.

The contents along with water are then carried into the sewers with the help of gravity to the section under the manhole nearest to your home. According to officials, a manhole exists every 100 feet in a city and the volume it can hold depends on the locality and nature of houses (an apartment would require more volume) in the area. Your poop is then carried through sewer lines to one of the 250 sewage pumping stations in Chennai.

(Sanitary sewer system)

Breaking down the poop

So, for example, if you live in T Nagar, all your poop goes to a pumping station in Pondy Bazaar. This happens to be the lowest point in the area and serves a system that depends on gravity. These pumping stations operate for 24 hours and have three wells for collection, screening and suction.

While the collection well is the point where the sewage is collected, the screening well is used to remove the solid waste. The remaining liquid is then carried through the suction well to a sewage treatment plant or STP.

What happens at an STP

Chennai is divided into five zones – Kodungaiyur (1 & 2), Koyambedu (3), Nesapakkam (4) and Perungudi (5). Each of these areas have 3 STPs in total, so that makes it a total of 12 STPs.

At these treatment plants, chemical reactions are induced and unwanted particles are made to settle down through a centrifugal effect. A sludge digester that exists in the system converts the solid residue into methane gas.

The methane gas is then used to supply energy for the operations in the treatment plants it is generated in, say officials.

But not all STPs convert the solid residue into methane gas. In others, authorities claim the residue is stored within the plant itself. When questioned further about the space required for decades worth of sludge, one official said, "We have enough space within the STP as of now. But the Tamil Nadu Agriculture University has been roped in to do research on the possible use if this sludge as fertiliser in fields."

Where does the ‘treated water’ wind up?

Meanwhile, the water that’s left behind is treated with chlorine to reduce faecal coliform.

Once the process is complete, the treated water is then released into the city’s waterbodies – the Cooum, the Adyar River or the Buckingham Canal. S Dayalan, a superintending engineer at the Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) insists, “All the chemical contents and even faecal matter is kept in check.”

(Aerial view of Perungudi STP)

But not all parts of Chennai are connected to the sewage system. In these areas, the sewage is collected in septic tanks. These tanks are sponsored, built and maintained by individual residents/owners. The waste collected is then transported through sewer tankers to treatment plants.

While the process may seem sound on paper, the reality is far less utopian. And the reason is rather simple - the infrastructure requirements do not match the development of an expanding city.

Expanding city

To understand Chennai’s sewerage system better, we will have to rewind a little, to 1978. This was the year the city's Metropolitan Water Supply and Sewerage Board was established.

The water supply and sewerage services in the city were taken over by the Board (CMWSSB) from the Corporation of Madras, as it was known then. It was meant to exclusively attend to Madras' growing needs for sewerage services, and thus the Board inherited the underground pipes and sewer systems that were built back in the British era. If you can't hear the creaks from the weight of this responsibility already, here are the unavoidable cracks that followed.

Back in the 1970s, the Chennai Metropolitan Area, comprising the city and its suburbs, that came under the Metro Board's control extended to 176 sq km. But in 2010, this figure more than doubled to 425 sq km to accommodate more municipalities and town panchayats.

An underground system that covers only half of Chennai

The city now has been divided into 15 areas overall and 5 sewerage systems have been marked to cover its expanse. Of these 15 areas, only eight have a completely functioning underground system to treat sewage, Chennai Metrowater Board admits.

As a result, some reports suggest that only one-third of the 1,800-2000 million litres of sewage produced by the city every day is even treated. This means that your poop could go into waterbodies that connect to rivers and lakes that you eventually source drinking water from.

An official in CMWSSB admitted that while the capacity of the sewage treatment plants is 764 million litres per day (MLD), they only receive 550 MLD of sewage water in a day.

The areas that continue to lack a complete and functioning sewage system are North Chennai’s Tiruvottiyor, Manali, Madhavaram, West Chennai’s Ambattur, Valasaravakkam, and South Chennai’s Alandur, Perungudi and Sholinganallur.

Watch what you drink

In Ambattur, close to 3,000 residents of Thiruvengadam Nagar have begun a battle against 400 apartments in the area. These occupants are yet to link their sewage channels to the underground lines that are freshly laid in the area by the Metrowater.

So where does the sewage go?

The apartment dwellers, according to locals, have been draining their waste, completely untreated, into the Ambattur Lake, a vital source of drinking water for the city.

This move by the residents, is reportedly to avoid a one-time payment of Rs 20,000 to set up the line. "It is with great difficulty that we brought the drainage system. But those in the apartments did not take the connection. They are polluting the lake which is now full of hyacinths and garbage," said Manoharan Johnson, president of the association told Times of India.

Metrowater shifts blame

S Dayalan, the CMWSSB superintending engineer says the illegal release of sewage through storm water drains is responsible for the contamination of water bodies. “They try to save money and not install septic tanks or connect to the sewer pipes and thus this problem arises,” he claims.

According to authorities, the STPs in Nesapakkam and Perungudi receive 500 tanks (each with a capacity of 9000 litres) of sewage a day from septic tanks. A number that clearly doesn’t match the city’s demographic, activists claim. Overall, according to the official, the city only generates 550 MLD of sewage, all of which is “completely treated”.

But past reports paint a different picture.  An audit conducted by the Arappor Iyakkam in 2016, an anti-corruption organisation suggested that Metrowater has been illegally dumping sewage into waterbodies for years.

The stinking truth?

“The first big lie here is that we generate only 550 MLD of sewage. Delhi, which is roughly twice the size of Chennai generated 3800 MLD of sewage. So, Chennai must at least generate 1000 MLD,” argues Jayaram Venkatesan of the Arappor Iyakkam.  “Since the treatment plants don’t have the capacity to treat this additional waste, they dump the raw sewage into water bodies,” he alleges.

This illegal dumping has allegedly been happening across 3,600-odd rivers, lakes, canals, storm water drains and other waterways. “If at least 10 of our waterbodies had been maintained properly, then we wouldn’t be facing the water crisis that we are today. The Korattur Lake for instance was handed over to Metrowater to use for drinking purposes. But the contamination of sewage in it rendered it useless for the purpose,” says Jayaram.

When asked about these allegations, however, the superintending engineer smiles, before saying, “The Tamil Nadu Pollution Control Board keeps track of the quality of water we release into the waterbodies. They will stop our operations if it is not up to the mark.”

As far as the Cooum is concerned, authorities place the blame for its state, solely on those residing near the river. "They use storm water drains to release sewage into the river. We have already started restoration work and in the next two years we will plug all these drains and ensure the river is clean again," the engineer assures. 

Rs 2000 crore has been set aside by the state government for this project but it seems money alone will not be enough to achieve the set goals. What we also need is for people to care about where their poop goes. 

So, where does your poop go? If you live in an area with a functional sewage treatment system, part of the poop becomes methane gas or remains stored in the treatment plants, and the rest gets treated and is released into waterbodies. Otherwise, it goes straight into a nearby water body, and it could eventually make its way back to your home as domestic water.

Also Read: Where does all of Bengaluru's poop go?