Clear drains, fill our lakes

Bengaluru city receives 64,000 crore litres of rainwater each year. Very little of this water is harvested, and most of this water is allowed to leave the city. The city also produces 37,100 crore litres of sewage. Only a part of this sewage is treated in the Sewage Treatment Plants (STPs) operated privately or by the Bangalore Water Supply and Sewerage Board (BWSSB).

Also read: Urgently needed: A lake planning policy

The rest of the sewage is raw. In addition, there is an unspecified quantum of industrial effluents, which are mostly untreated.

Flow patterns

Thus our drains and lakes receive a mix of rainwater, partly treated sewage, raw sewage and industrial effluents. There is a vast difference in their flow patterns.

The rainwater comes only on 60 wet days in a year, which are spread over seven months. On average, a wet day adds 10,670 Million Litres (ML) of water to the city’s water system.

The sewage flow is perennial, and does not fluctuate much. It adds 1,400 ML of mixed sewage to the city’s water system every day (on average).

The industrial effluents are typically released periodically, but their flow pattern or toxicity levels are not known.

Since the industrial effluents can only be treated in Effluent Treatment Plants (ETPs), the rest of this article will not discuss this factor. Suffice it to say that they need to be isolated and treated separately.

Quality fluctuations

There is a large fluctuation in the quality and quantity of the water that flows into our lakes:

During 305 days of the year, there is a steady flow of 1,400 MLD of mixed sewage. During 60 wet days (spread over seven months), the flow swells by 10,670 ML of rainwater.

Also read: Poor water retention in lakes: An eco-imbalance

In other words, the rainwater dilutes the mixed sewage by a factor of 7.6; and the quantity of water increases almost nine-fold.

Mitigation challenges

This huge fluctuation in both quality and quantity of water makes it difficult to manage and harness the water, as explained below:

The most economical strategy is to treat the raw sewage separately. But when it gets mixed with treated sewage and rainwater, we have to treat the whole mix once again. This becomes economically unviable.

It is difficult to set up an STP if the incoming water quantity shoots up nine times for 60 days in a year.

During the wet days, our drains are required to carry nine times the normal water quantity, which causes flash floods in the city. Note that the flood waters have raw sewage!

Since most of our lakes are silted up, they cannot hold enough water for our needs.

Since the water is mixed, we cannot fill up our lakes with it, unless we treat it first.

Bypassing a lake is not a good option, because all our lakes form a chain. So if we bypass one lake, the sewage goes to the next lake. Thus we will end up bypassing all lakes, leaving all lakes dry!

Urgent steps

All things considered, we must take the following steps urgently:

The industrial effluents must be intercepted at the industry itself, and treated fully. (at most, a Common Effluent Treatment Plant, CETP, can be set up for the whole cluster.)

Where the drains are properly isolated from sewage, they can be connected to recharge wells. This calls for creation of a large number of recharge wells.

The entire drainage system was never designed to carry any treated sewage. We must re-evaluate the entire drainage system for this purpose; and ensure that it is capable of carrying 10 times the daily flow.

We must also identify the encroached/clogged drains that cause the flooding, and clear up these drains.

All residential zones must be scanned once again to ensure that all households are connected to local STPs or BWSSB STPs (no household is allowed to discharge raw sewage in drains).

All STPs must be re-examined to ensure that they treat water to meet the Karnataka State Pollution Control Board (KSPCB) norms.

All major drains must have debris-traps and silt-traps at regular intervals. BBMP must set up logistics to dispose of all debris and silt collected in these traps.

All lakes must be desilted to the original water holding capacity, starting from the highest lake in the chain. (Lakes cannot be desilted in random order)

Once the chains of lakes are able to store good quality water, the water can be recycled after further treatment. It would be best to take the water to a large reservoir outside the city where final treatment is provided. After that, the water can be mixed with the Cauvery water supply.

To mitigate the flash floods, we need to identify our floodplains, and designate certain lakes as flood-mitigation lakes. Then these lakes must be modified and managed to buffer the floodwaters whenever the valley receives heavy rains.

(Ramprasad V is co-founder of Friends of Lakes; Nagesh Aras is a member)