The benefits of restoring urban lakes in the tropics

Lake restoration in developing economies, particularly in the tropics, is a major challenge given the severe levels of pollution from untreated wastewater and the warm climate conducive to microbial and algal growth. Restoration goals are often ill-defined or unachievable. Here we describe the successes that can be achieved through a control, compared with intervention case study of the two urban lakes in Bengaluru, India, one of the world's largest and fastest growing mega-cities. The unrestored control, Bellandur Lake, was severely polluted by 231 million litres per day (MLD) of untreated wastewater. The restoration site, Jakkur Lake, receives 10 MLD of treated wastewater and also receives some tertiary treatment by circulating the effluent through a constructed wetland before it enters the lake. The water quality of Bellandur Lake can only be described as extremely bad. Organic pollution levels in the main inflow were high (BOD₅ of 199 mg/l, faecal coliforms 6.9 Log MPN/100 ml, total suspended solids (TSS) of 285 mg/l) leading to the complete deoxygenation of lake even at the surface. The levels exceeded use-base standards for bathing water and fisheries. The high levels of organic pollution and low oxygen conditions also led to extreme levels of methane emissions that occasionally led to the lake surface catching fire. Total nitrogen (TN) and total phosphorus (TP) concentrations in the lake were extremely high (47 mg/l and 6.3 mg/l) respectively with low Secchi depth (SD). Despite the high nutrient levels, very little phytoplankton growth occurred (chlorophyll-a of 0 mg/l), most likely due to the high TSS loads which restricted light availability. In comparison, the wastewater treatment and wetland at Jakkur Lake markedly reduced organic pollution of the main inflow (BOD₅ of 32 mg/l, faecal coliforms 4.1 Log MPN/100 ml, TSS of 48 mg/l). Levels of coliforms in the lake were above the standards for bathing waters. Total nitrogen (TN) and total phosphorus (TP) concentrations in the lake reduced (10.5 mg/l, 2.4 mg/l) but still classify the lake as extremely hypereutrophic. The lower TSS levels did, however, enable dense phytoplankton blooms to develop (max chlorophyll-a of 600 µg/l) which are in part responsible for the higher levels of dissolved oxygen in the lake water, albeit and as expected with large diurnal fluctuations. The comparison highlights the benefits that standard wastewater treatment provides to restore urban tropical lakes in context of rapidly urbanising catchments, and even though Jakkur Lake is by no means fully restored, it sustains water quality that allow propagation of fisheries and shore-based recreation. It also greatly contributes to greenhouse gas emission reductions. Further restoration measures are likely needed for urban tropical lakes, particularly to tackle pollutant loads in monsoon periods, but restoring community pride in the uses of a lake is an important milestone of the restoration efforts.