How is Cauvery water supply treated for contamination?

Drawing 1450 million liters of water per day from 150 kms and distributing it to households —how does that work out? How do Bengalureans get safe water?

When you heard on November 22 that effluents from an exploded boiler’s sugar factory entered River Shimsha, did you hear some alarm bells too? There were some fears that the potassium-laced river waters would enter River Cauvery near Shivanasamudra, which would pollute the Bengaluru water supply network. Would they have come true?

“The city’s water supply would not be affected. The polluted waters would get diluted by the time it reaches the city,” explained Vishwanath Srikantaiah, an expert on water-related issues.

The Karnataka State Pollution Control Board officials were camping on the site and sending the water samples for testing. K.L. Savitha, District KSPCB officer, said they have not received the samples back after testing. However, she added, that there was no cause for alarm.

Cauvery water  

Cauvery water supply to the city is sourced from Krishnarajasagara and Kabini dams. The water from T.Narsipur pumping station is drawn to Thorekadanahalli (T.K. Halli), where BWSSB’s treatment and purification plant is stationed.

Potable water in India needs to meet the standards set by the Bureau of Indian Standards (BIS). These were parameters that were published first in 1983, revised in 1991 and again in 2012 to meet international specifications recommended by the World Health Organisation (WHO). The latest version in India is BIS 10500 : 2012. It specifies the physical, chemical and bacteriological factors that indicate potable water.

Nithyanand Kumar, waste water management official from the Bengaluru Water Supply and Sewerage Board (BWSSB) said that currently, Bangalore draws 1450 MLD of water per day from Cauvery for about 8.5 million Bengalureans.

Water treatment plant

Thorekadanahalli (T.K. Halli) water treatment plant has been treating the water for the Cauvery Stages I to IV. Water treatment process has been followed to treat water that is mostly pure after most of the contaminants are removed at the T K Halli plant, said Nithyanand Kumar. The treated water enters the city through steel pipes in order to prevent leakages and then gets supplied to households.

BWSSB’s 550 MLD water purification plant, which is said to be Asia’s biggest water purification plant at TK Halli near Malavalli, Mandya taluk, has five zonal operations. The water undergoes five stages of purification and pumping. The technology used in the plant is both conventional as well as updated. Spread over 25 acres, with just 52 employees, the water purification plant is a capital intensive unit that is operating at different levels and is automated.

Treatment process

The main contaminants in the river water are physical, said B M Manjunath, BWSSB’s Public Relations Officer. These are obvious and can be seen directly if you hold a glass of water against light. They impact simple factors of colour, odour, taste, pH, turbidity and total dissolved solids (TDS). A water becomes turbid if it is hazy due to its suspended particles, rather than chemical contamination.

The water falls in thin sheets over a number of steps in the cascade aerators to absorb oxygen. Oxidation kills the bacteria, according to an official quoted by The New Indian Express.

Chlorination process

This is followed by large amounts of chlorination. They are injected in the form of massive amounts of gas through pipes. However, as the chlorine used is almost 300 kg per day, in order to kill harmful organisms in the water, the staff in the T.K. Halli plant has to take precautions, so that the humans involved do not get impacted by chlorination!

Bacteriological parameters, according to BIS, factor bacteria, ie faecal and total coliform, which should not exist in potable water at all. Yet, faecal coliform, mostly from the faeces of humans or animals, is quite common in water sources in India. Hence even in household treatment systems, people use simple filters with chlorine tablets, or more sophisticated UV filtration systems, in order to remove faecal and total coliform. Biological and bacteriological pollution is treated with 2 ppm chlorination, said Manjunath. It kills new microbes that enter water.

The next step is that of Alum Dosing. Aluminum Sulphate is mixed with the water in order to ensure the right turbidity level. Hence, chemical contamination is mostly low if the municipal water supply is from Cauvery and the water gets treated at the source before it gets piped, explained Manjunath.

Filtration process

Filtration is the next step. Municipal water supply agencies usually leverage sand filters to remove suspended particles. TDS is the measure of total solids dissolved in the water. Sedimentation of water is allowed to take place, by allowing suspended particles to settle. Some chemicals, mostly alum, are added to accelerate sedimentation, even as very fine and colloidal particles are removed through coagulation.

Water is passed through beds of sand filters or other granular materials. The process of rapid filtration helps to remove pathogenic bacteria. The filters could be cleaned through backwashing or ‘forced upward movement’ of wash water and compressed air.

From T K Halli, the water flows to Tataguni, Kanakapura Road, through three pumping stations. First, the water is drawn to Harohalli, pulled for 25 kms by gravity, without the need to be pumped. From this point, underground pipelines of 1,200 mm width take the waters to Tataguni. This stretch works against gravity, with the water pulled up by 1,500 ft.

Manjunath said that after the water gets treated, the BWSSB collects about 2,000 samples from all over the city to get tested in the central as well as the southern BWSSB labs. Cauvery water is mostly found to be free of chemical contaminants, he said.

Groundwater contamination can pollute Cauvery water

Sometimes, salts and minerals dissolved in groundwater might leak into piped water. Houses that depend on water from open wells, borewells, or tankers and not on BWSSB water supply might get contaminants through chemicals that percolate through rocks and soil. Nitrate contamination could be frequent due to untreated sewage and fertilisers in outer regions of the city. Houses there might tend to go in for Reverse Osmosis (RO) purifiers. These are said to be highly energy-intensive. They remove most chemicals and microorganisms speedily, but the run off water is concentrated with pollutants and is hazardous.

However, the water supply from Cauvery project is mostly clean, according to Manjunath. Vishwanath, the water activist, said that it was “an engineering marvel” that met the challenge of not only sourcing and supplying, but also treating and cleaning the water successfully.

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