Opportunity wasted to generate power at STPs

Digestors in three BWSSB STPs left unused for two decades

Opportunity wasted to generate power at STPs

For more than two decades, digestors in the city’s three main Sewage Treatment Plants (STPs) have been left utterly unused. This is a shocking lapse that has robbed Bengaluru of a genuine model to generate power and cooking gas from millions of litres of sewage that now pollutes the city’s lakes.

The digestor is what digests the sludge from primary and secondary clarifiers in the STPs. Through methanisation, the methane gas of high calorific value is recovered. Once Hydrogen Sulphide is removed, the product can eventually be used as cooking gas and energy, as Thippeswamy, former Engineer-in-Chief of Bangalore Water Supply and Sewerage Board (BWSSB) explains.

If the digestors were used as originally designed, more than 60% of each STP’s own energy requirement could have been met. These critical components now lie unused at the STPs of Koramangala-Challaghatta valley (218 Million Litres Daily - MLD capacity) off Old Airport Road, Vrushabhavathi valley near Rajarajeswari Layout (180 MLD) and Hebbal valley (stream of Nagavara tank, 60 MLD).

High cost repairs
In the mid-1990S, several crores of rupees were spent to rehabilitate the digestors in the K-C Valley and Vrushabhavathi Valley STPs in the 1990, recalls Thippeswamy. “But till now, they have not been operated,” he points out.

So, is it too late now? “To generate energy in all these three STPs, the digestors need to be made leak-proof for possible gas escape. Add three more units, a gas holder, Hydrogen Sulphide scrubber and gas engine to generate power. By introducing these units, BWSSB can both save energy cost and reduce green house gas emission.”

Thippesway estimates that the Board can generate about 5-6 MW of power from the STPs. There is already a Chennai model to emulate, he says: Four similar waste water treatment plants in Chennai, he informs, have been generating more than 6 MW of power for the last six years.

Citing leakage issues, the sludge is now being fed directly into the drying beds, bypassing the digestors. The residue is virtually being taken away, generating zero revenue for the Board. Using the right method, the sludge can be recovered and sold as bio-solids/pellets for fertiliser use.

Phosphorous recovery
Wastewater typically contains a large amount of nutrients -- particularly phosphorus -- that can pose a harmful threat to infrastructure and the environment. But, as Thippeswamy says, the STPs can extract usable phosphorus from the sludge. “It is an increasingly scare natural resource. India imports it at a very high cost. In the last 10 years, its cost has gone up by 500 times.”

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