The lakes of Bengaluru once popular for their beauty, are now famous for producing toxic foam, fire, foul odour and dead fishes. For centuries, these lakes have met the household and agricultural needs of the local inhabitants. In fact, many of these lakes are a natural part of city’s geography. Bengaluru has an undulating terrain and is marked by a series of valleys radiating from a ridge that forms three major watersheds, the Hebbal valley, Vrishabhavathi valley and the Koramangala & Challaghatta valleys.
The lakes dotting each of these watersheds are interconnected through a vast drainage network that was meant to carry storm water from one water body to another. Now this network mostly carries untreated sewage water and partially treated industrial & domestic wastewater. About 1258 MLD (million litres of sewage per day) is generated in the city daily and almost 65% of it remains untreated. This unchecked discharge of polluted water has proved to be devastating for the city’s lakes.
However, local bodies comprising of the government, residents and researchers have now seen some success in restoring, a few of these lakes using research-backed methods. For example, Jakkur lake in the north of Bengaluru has a sewage management system that consists of a sewage treatment plant (STP) and a constructed wetland. This lake receives about 10 MLD of treated water from Jakkur STP and 0.5 MLD of raw sewage from an open storm water drain.
Treated water coming out of the Jakkur STP passes through a constructed wetland before entering the lake. This wetland is highly aerobic and colonised by algae and native macrophyte species Daphnia and Rotifera. The macrophytes help to remove fine particulate matter and algae perform nutrient remediation by taking up carbon, nitrogen, phosphorus and heavy metals.
The scientists studying this sewage management system have found it to be effective in cleaning up the lake water. Priyanka Jamwal, a research fellow Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore, tried to measure the efficiency of this sewage management system and found that the level of ammonia reduced from~25mg/l at the inlet to less than 5 mg/l at the outlet of the lake.
In another lake, Puttenahalli, located near JP Nagar in South Bengaluru artificial floating islands were created to filter out sewage remains released into the water body. The structure of the island is made using polyvinyl chloride (PVC) pipes, fishing nets and discarded 1 litre PET bottles. Plants like vetiver, canna and colocasia then anchor themselves to this structure. The roots of the plants float in water and feed on the nutrient rich water thereby removing the pollutants.
After the installing the floating islands, the level of dissolved oxygen (DO) in in the lake water shot up from 1.3 mg/litre in July 2016 to 3.8 mg/litre in a span of a year. As the results are promising and cost of making an artificial island low, the citizen-driven Puttenahalli Neighbourhood Lake Improvement Trust (PNLIT), who conceived the project, plans to expand it.
“All untreated sewage should be treated first, and then let into constructed wetlands to remove the nitrates and phosphates that remain after secondary STP treatment. More plants need to be built by BWSSB with all the sewerage networks linked to the plants and unauthorised release of sewage into storm water drains has to be prevented”, said, Sharad Lele Senior Fellow at ATREE and a member of Bellandur lake committee formed by the government of Karnataka
Several citizens’ groups have signed MOUs with the municipality (BBMP) to protect and rejuvenate the lakes. The lake groups spend their own funds and time to maintain, patrol, report on illegal sewage dumping and pressuring BWSSB to maintain the quality of the STP effluent as in the case of Jakkur lake. To create the feeling of ownership, recreational activity at the lakes like bird watching, drawing contests, music events, tree planting, walks are conducted. Other renovated city lakes are Sankey tank, Madiwala Lake and Nagavara lakes.
Though things have started improving for several lakes, the outlook for the largest lakes in the city- Bellandur and Varthur– is still looking grim. Bellandur lake spreads across an area of 365 ha. It is situated 5 km, upstream of Varthur Lake which spreads over an area of 220 ha. Both lie at the end of the lake series as per city’s topography and receive entire untreated sewage and industrial effluent from other upstream lakes.
Around 600 million litres per day (MLD) of sewage is diverted to their sewage treatment plants but only 200-220 MLD gets treated and the rest flows into the lakes. A recent study carried out by TV Ramachandran, Professor at Indian Institute of Science (IISc), Bangalore, has now shown that the wetlands − margins along the lakes– have the potential to naturally degrade the domestic and industrial waste dumped in it. Wetlands help to recharge the groundwater by functioning in tandem as anaerobic-aerobic ponds. The high influx of effluent endows the lake with most tolerant micro flora like bacteria, algae and hardy aquatic plants.
Bacteria degrade the organic biomolecules such as polysaccharides, protein and lipids under anaerobic condition. Next, the algal systems reduce carbon, absorb inorganic nutrients and increase pH during aerobic condition. Along with the biotic community, the seasonal variations in temperature, wind speed and rainfall significantly modify the physical, chemical and biological process in the lakes.
For example, the ability to process pollutant varies greatly from pre-monsoon to monsoon. During the pre-monsoon season, water hyacinth blooms grow exponentially to cover 75% of the lake. They form a permanent cover on the lake surface, making the lakes suffer a lack of oxygen. Also, the overgrowth lowers light penetration and hampers algal growth. However, during monsoon in Varthur lake, the dense water hyacinth is washed out, by the high wind velocity and the large quantum of fresh water runoff. This helps to create open aerobic algal ponds with optimum light penetration.
What is also important to understand is that the lakes natural ability for bio-remediation is limited. Optimum bio remediation happens when the amount of pollutants entering the lake is below its threshold treatment capability.
The efficiency can be enhanced though, by regular clearing of the floating macrophytes, de-silting lakes for increasing the water residence time and regulating the concentration of carbon in the incoming sewage.
There might be no easy and quick solutions for reviving the unhealthy lakes but long-term solutions encompassing technological, ecological and social process are likely to improve the lake environment as already seen in some of the lakes.