The Grand Anicut or Kallanai is a structure dated to the times of Karikala Chola. Here the Cauvery splits into the Vennar and the Cauvery itself. A branch from the anicut flows into the Kollidam and another is a canal to irrigate the vast deltas beyond. In June the bed of the Cauvery is dry. One could walk across the vast riverbed without getting the ankle wet. As the Cuvery is a closed basin, farmers struggle to access water in the rice bowl of India.
Upstream of the Kallanai is the city of Tiruchi. Here is a waste stabilisation pond located next to the Koraiyar, a small tributary of the Cauvery. The waste stabilization pond or oxidation pond is the simplest of waste-water treatment systems, ideal for tropical climates where the treated wastewater is meant for agricultural use.
The plant here is designed to treat 80 million litres per day but currently is operating at around 40 million litres per day. Wastewater collected from the city in sewage lines is pumped through wet-wells into this pond. Here the wastewater is screened to remove plastics and large objects, taken through a process of anaerobic digestion and then led into a series of ponds.
The waste stabilisation ponds are divided into three sets:
an anaerobic pond, usually deep to allow for digestion of carbonaceous material in the absence of oxygen. The waters stay here for a day.
- a facultative pond for algae to grow and infuse oxygen into the waters, the waters stay for 4 days and
- a polishing pond, shallowest of the three and allowing for oxygenation and for sunlight to disinfect and kill bacteria. The waters are allowed to remain here for 3 days before being released to the river or for agriculture.
Farming and treated wastewater: Next to the treatment unit is the river and across the river is a vast stretch of paddy growing. A talk with the farmer reveals the value of wastewater. He says that paddy seedling grow twice as fast in the water from the polishing pond, thanks to the nutrition content in it. He mentions that farmers are drawing the water and pumping it to a distance of a kilometre for irrigating their paddy fields. The paddy itself grows early and ripens early with full panicles. A quick back of the envelope calculation reveals that in current conditions the wastewater can irrigate nearly 1,500 hectares of paddy. At full load of 80 mld the irrigation potential will be 3,000 hectares.
The treated waste-water when led into the river bed which acts as wetland further cleans up the waters. Filtered through the sand bed, fish is being reared as are ducks. The water is clean enough for washermen to do their laundry.
The future: Tiruchi can expand its sewage network to cover new areas and repair leaks in the existing network to capture all the waste-water it generates. It can then focus on maintaining the waste stabilisation ponds well. A partnership with farmers can see the city facilitating agriculture and allied livelihoods. The whole waste stabilisation system should be opened up to the city as an educational centre to let citizens become water and waste-water literate.
As the NITI Aayog report says, India is running out of water. The Cauvery is one of the first large river basins to run out of it too. Tiruchi can demonstrate how cities and agriculture can be synergistic, climate resilient and water efficient. The time is now and that would be water wisdom.
