How to clean up Bellandur and Varthur lakes

Bellandur lake (900 acres) and Varthur lake (450 acres) are the two biggest lakes out of the 262 surviving water bodies within Bengaluru city limits, being the last in the cascading chain of lakes in the Koramangala-Challaghatta valley, into which all the sewage waste and monsoon flow from south east Bengaluru empties, accounting for 40% of the city’s untreated sewage, estimated to be 450 million litres per day (MLD).

Of this, only around half is treated through a sewage treatment plant (STP). This has choked Bellandur and Varthur lakes, which suffer from the same malaise as 90% of the city’s lakes, only on a much larger scale. Toxic foam with unbearable stench and fire regularly emanate from these two lakes, affecting the locals. The polluted lakes pose a serious health hazard not only to the locals but to all those who consume vegetables grown by farmers in the downstream areas using their water.

According to an Indian Institute of Science (IISc) report, 38%of the city’s lakes are surrounded by slums and 82% have witnessed a loss in the catchment. Rampant encroachment of the lake body, choking of upstream channels, clogged storm water drains, vanishing raja kaluves, dumping of solid municipal waste and discharge of industrial effluents in lake beds and the direct daily inflow of almost 200 mld of untreated sewage into them are the chief reasons for the present condition of these two lakes.

The National Green Tribunal has therefore very rightly taken cognisance of the state of the lakes and is conducting frequent hearings. The government, too, has done well by bringing in all the agencies (BDA, BBMP, KLDCA, PCB, etc) together to carry out de-weeding, desilting, solid and municipal waste removal, upgradation of the STP, construction of engineered foreshore wetland, closing down of polluting industries in the vicinity, etc. A few basic things still remain, however, and the government must address them.

The first and foremost is the twin challenges of arresting the inflow of dry season sewage and keeping the lake full. The approach of the government is to address it by treating all the waste water flowing into the lake. STPs are, however, capable of only treating the water up to the secondary stage (as tertiary treatment is prohibitively costly). This would still leave micro-nutrients in the waterbody allowing the algae bloom to return again and again, choking the lake, thus nullifying all the effort.

This problem could be sorted out to some extent by having an engineered vegetative wetland at the mouth of Bellandur lake (Agara–Koramangala side), which would absorb nitrogen, phosphorus, potassium (NPK) and not let them into the main waterbody, thereby reducing the algae bloom problem. The wetland would also compensate the evapo-transpiration loss from the lake, estimated to be 40 mld.

That will still leave around 200 mld of untreated sewage in Bellandur lake, which has to be by-passed. This could be done by constructing a diversion drain around the lake such that, at the same time, monsoon water is allowed into the waterbody. The monsoon water flowing in would not only dilute the wet sewage, it would also keep the lake a little fresher.

Moreover, this diversion channel would empty itself into Varthur lake, with dry sewage having been cleaned in a natural way to some extent by the channelised flow of water over a long distance. The ingress into Varthur lake would, therefore, be relatively less polluted. All these methods should be repeated for Varthur lake.

Another crucial aspect of the programme would be to clean the drains and channels leading into Bellandur lake from the Koramangala-Challaghatta valley, which is chock-a-block now, thanks to unprecedented urbanisation and industrialisation in the area. Seven main ingress points have been identified as inlets into Bellandur lake and all these must be cleaned up. That involves dissuading upstream farmers and horticulturists from using NPK fertilisers, besides controlling the industrial effluents flowing in.

Unending woes

Two more issues have been dogging Bellandur and Varthur lakes, just as they affect other lakes. One is accumulation of silt, organic sludge and inorganic and other solid waste at the bottom of the lakes over the years. It is estimated that around four feet of silt is accumulated in the two lakes. Many engineering solutions focus on desilting and restoring the lakes.

But for lakes of the size of Bellandur and Varthur, that would prove to be only a misadventure — not only would it involve expensive digging and transportation of the mountain of silt to distant places, but one would also have to find a suitable place to dump it. Moreover, the lake is no more being used for irrigation, so no harm would occur if de-silting is not done at this stage. There are technologies to eat away the organic sludge, but that can be done at a later stage.

A word about the most disturbing feature of Bellandur and Varthur lakes -- the foam and froth formation or ‘pollution snow’ formation, which has been recurring with alarming regularity. This has to be tackled again through an engineering process without lowering the west weir, as suggested in some quarters. Lowering the height of two west weirs (Kempapura and Bellandur side) will not only result in uncontrolled flooding downstream but would also expose the dry lake-bed in non–monsoon period, inviting encroachment, which would gobble up the remaining lake land.

Instead, it would be better to consider smoother outlet formation towards Bellandur side, instead of the present vent which gets choked whenever there is excess inflow, causing the above fog formation, which further spreads during windy days or heavy rains. Smoother outflow has already been achieved to a great extent towards Kempapura side. Besides this, heavy duty aerators on jetties should be erected for continuous aeration of the water body and mechanised weed removal should be continued. This would not only keep the lake surface clean but also allow sunlight to penetrate the lake, thereby oxygenating it.

(The writer is a former principal chief conservator of forests and founder-member secretary, Lake Development Authority)