BENGALURU: Thanks to apathy of successive governments, stormwater drains (SWDs) have turned into sewage carriers, a major source of pollution of city lakes.
Wile authorities are now relying heavily on sewage treatment plants (STPs) to ensure lakes get clean water inflow, experts say STPs aren’t efficient in removing nutrients from the sewage entering waterbodies.
Ramprasad V, an expert on lakes, said, “Although we understand that the best way to bring water into lakes is through SWDs, that doesn’t seem to be the solution right now. As of now, mostly secondary treated water is being let into a few lakes from the existing STPs, but the right way to do it would be to allow only tertiary treated water after the elimination of nutrients to prevent increased growth of algae and water hyacinth.”
Experts say algae and water hyacinth are a clear indication of the presence of nutrients in water. Once the lake is covered with hyacinth and algae, they start affecting the waterbody’s ecosystem, depriving life beneath of oxygen.
Nagesh Aras, another expert, said, “Nitrates, phosphorus and potassium are micro-nutrients and are usually not removed at a secondary treatment plant. If they have to be removed, it has to be done through bioremediation, which simply means passing the water through the roots of various plants which absorb all these nutrients. When that doesn’t happen, the hyacinth in the waterbody tends to do the same thing. So hyacinth and algae are part of the solution, but they eventually cover the entire lake and don’t allow sunlight to penetrate, which in turn doesn’t allow the submerged variety of plants to go through the cycle of photosynthesis. This leads to a drop in the level of dissolved oxygen, causing a huge imbalance to the ecosystem.”
Therefore, raw sewage is directly harmful, but secondary treated water is harmful indirectly.
Diversion drains
Shankar Reddy, special commissioner for lakes, said STPs are likely to remain a permanent solution.
“Having said that, it would be impossible to eliminate SWDs as they play a vital role. It might take a while for SWDs to bring in fresh water into the lakes as there is heavy sewage inflow in the city’s SWDs. However, what we can do from our end to ensure that sewage doesn’t enter lakes is to construct diversion drains near them,” he said. “
According to BWSSB, its STPs have tertiary cleaning technology which eliminates nutrients in the water. An official said, “We have been removing the nutrients and only then allowing water into the lakes. We have received clearance from KSPCB (Karnataka State Pollution Control Board) and CPCB (Central Pollution Control Board) for this.”
The city has over 800km of SWDs, which are desilted often. BBMP, however, still struggles with eliminating sewage from the drains.
Manoj Jain, special commissioner for SWDs, said, “Sewage being present or absent in SWDs completely depends on BWSSB. Bengaluru is a growing city and BWSSB needs to keep an eye on its existing network of drains to prevent sewage from flowing into SWDs.”
