The complex process of restoring lakes in Bengaluru Premium

Policymakers are setting unrealistic expectations for lake restoration efforts

A recent report by the Karnataka State Pollution Control Board (KSPCB) stated that not one lake of the 110 lakes it studied in Bengaluru meets potable water quality standards.

Potable water is water that is safe for human consumption — i.e., water that can be used for drinking or cooking. Safe water means it contains no toxins, carcinogens, pathogenic microorganisms, or other health hazards.

Achieving potable water quality in lakes means that the water must meet the highest quality standards. Rainwater, often considered the purest form of water, becomes contaminated once it contacts the atmosphere. As it moves across natural and man-made surfaces, especially in urban areas, it picks up minerals, chemicals, and pollutants. Even if wastewater is prevented from entering water bodies and only runoff is allowed, the lakes will still not meet potable water quality standards. Additionally, the stagnant nature of lake water leads to biochemical processes that further degrade its quality.

Lakes in Bengaluru receive water from three main sources: treated/partially treated wastewater from sewage treatment plants; rainwater mixed with sewage, also known as combined sewer overflows (CSOs) via storm water drains; and raw sewage from open storm water drains. Given the nature of the inflows, is it reasonable to expect potable water in lakes?

Reducing pollutants in wastewater and storm water runoff comes at a significant cost. Treating 1 million litres of wastewater to meet secondary treatment standards costs approximately ₹1 crore with reoccurring operation and maintenance costs. Most restoration projects focus on deploying sewage treatment plants and supplementing treatment by channelling secondary treated effluent into constructed wetlands. Additionally, during the rainy season, CSOs are managed through diversion channels that allow excess water to flow into the lakes. To further improve the quality, sedimentation ponds are constructed near the CSO outlets.

The KSPCB study revealed that all the 110 lakes fall only under the categories of D (lake water suitable for wildlife propagation and fisheries) and E (water suitable for irrigation, industrial cooling, or controlled waste disposal). In simpler terms, none of them are fit for swimming (B), nor can they be used as a source of potable water, either with treatment (C) or without (A).

One crucial point to note is that the biological oxygen demand (BOD) level required to move from category E or D to category C is 3mg/l. This means that if the water quality falls under categories D or E, any incremental reduction in BOD levels as a result of restoration efforts may not be reflected in the broad ranges used to categorise lake quality. For instance, if the BOD level in a lake is 30 mg/l and restoration efforts reduce it to 15 mg/l, the lake will still be in category D or E, which misleadingly suggests no progress. Additionally, the lack of ambient standards for nutrients, such as nitrogen, further limits the ability to evaluate the impact of interventions on surface water quality.