Bengaluru: Morning walkers around Ulsoor Lake were shocked to see thousands of dead fish that came floating to the lake’s surface on Monday. There were fish of all sizes, shapes and varieties that puzzled the walkers. Some of them were even worried that there could be something in the air and hygiene around the lake, and went back home, cutting short their morning walk. “It’s concentrated human waste that causes this phenomenon in city lakes and not industrial effluents as a lot of people and NGOs think,” said an expert with the Karnataka State Pollution Control Board (KSPCB).
“The fish die when the level of dissolved oxygen in a lake becomes less. When the level of dissolved oxygen goes down, the amount of undigested organic matter goes up in a water body,” explained Dr Vamanacharya, former KSPCB chief.
Thousands of fish were found floating dead on the Ulsoor lake and washed up on its banks Monday morning, shocking morning walkers. Experts blamed the BWSSB for letting untreated sewage water into the lake from the Raja Kaluves (primary drains) at four different points for the fish dying in such large numbers .
“This happens quite frequently in lakes because of sewage being let into them,” said Dr Vamanacharya, former chairman, Karnataka State Pollution Control Board. “At Ulsoor Lake, there are at least four points where sewage directly flows into it from Indiranagar, MG Road, Richmond Road , and their surroundings,” he noted.
“The mixing of concentrated sewage with the lake water makes it rich in nitrogen, leading to growth of algae , which absorb lots of oxygen from the water, competing with the fish for it. Initially, the fungi can survive, but fish cannot due to the lack of oxygen and eventually, they all die," he said. After the fish, other aquatic life too slowly dies, according to him.
“People may blame effluents from industries , but the fish die due to lack of oxygen and not because of poisoning. It’s no way related to industrial waste let into the lake. Firstly, there are no industries around and secondly, industrial waste does not do this type of harm. This phenomenon may not happen every year. But this year due to rise in temperature, there was less water to dilute the concentrated sewage flowing into the lake, leading to more algae formation. Its human waste that did this and not industrial chemicals,” he said.
Treat sewage
The solution, he believes, is treating sewage before it is let into the lake. “We’ve to treat the water to some extent, but not fully as fish cannot live in ultra-filtered water as they need food. We call this secondary treated water and not tertiary treated. Earlier, we had advised BWSSB and BBMP and the government had formed a lake development authority. But it was not given any funds and so could not work. The BWSSB and BBMP should now together install a small treatment plant of at least 1-2 MLD to let in secondary treated water. Any extra sewage should be drained away by a parallel pipe to big treatment stations of the BWSSB,” he suggested.
