Bioremediation at Guwahati's Silsako Beel: What is it?
A model of the proposed Silsako lake in Guwahati.

Guwahati: The bioremediation of the Silsako Beel, a wetland lake situated at the heart of Guwahati, was inaugurated by Assam cabinet minister Ashok Singhal at an event on Wednesday. At the same event, the diversion of the Bahini river into the Silsako was also demonstrated.

The Minister of Housing and Urban Affairs, Ashok Singhal, tweeted: “The bioremediation process of the Silsako Beel will enhance the retention capacity & water quality of the Beel through excavations, aeration, desilting, cleaning & various scientific remedial measures.”

What is ‘bioremediation’ and how does it help in restoring the wetland ecosystem? How does it ensure that all excess flood water from Guwahati, having washed away the city’s waste, gets diverted into the Silsako to be then released into the Brahmaputra?

When the bioremediation project of the Silsako started in 2020, the level of pollution in the water compared to now was found to be at Category 1, a state that needed immediate intervention.

Water bodies prioritised as Category 1 are those that display such extreme levels of pollution that the government needs to attend to them for pollution control. The maximum range of pollutants that is considered acceptable for a water body is around 4-5 but in 2020, the Silsako was running at roughly around 36-37, a range that is approximately seven hundred times more than the acceptable limit.

Following the start of the bioremediation process, the lake is now in Category 4 according to official reports. GMDA sources claim that this was a result of authorities taking action by building silt traps that cause pollutants to sediment and purify the water.

GMDA CEO Kausar Hilaly explains, “After GMC separates solid waste from the flowing water, the sewage water with mud, silt, grease etc., flows into the separate channel and hits a wall we call silt traps.”

The GMDA further elaborated that they created a separate channel (“mathauri” in local terms) around half a kilometre in length for the sewage water to traverse along the entire course of the lake, flowing through many silt traps before it flows into the main lake. Basically, water comes in with a force, hits a wall and then sedimentation happens. The polluted water halts and flows continuously in the increased surface area of the separate channels equipped with silt traps. The cleaner water, bereft of most of the pollutants, then enters the lake.

Hilaly added, “This is a mechanical process. We have to keep removing the collected sludge regularly. This is something we have to have a system in place for. We dump the removed sludge where we can mix it with normal soil.”

GMDA said that following the in-situ bioremediation, the process of engaging plants and microbes to feed on pollutants to clean the water starts. For this stage of the process, GMDA has roped in a company named JM Enviro. The CEO of GMDA claims, “By the time the water filtered out of the silt traps enters the lake, it becomes better quality and clearer. We already have pictures of migratory birds visiting the lake.”

Around 200 trees have already been planted around the lake and all of the trees are geo-tagged. The plan is to plant 7000 trees in total. A plantation of riparian grass and other plantations are also on the table to create a rejuvenated ecosystem.

Regarding the diversion of the Bahini river into the Silsako, GMDA adds, “We have connected all drain water channels to the Bahini which will flow out via Silsako into the Brahmaputra.”

They further explained there are three outfalls for rainwater entering Guwahati into the Brahmaputra. One is the Silsako-Bondajan channel, second is Bharalu channel, and third is from Pamohi via Deepor Beel to the Brahmaputra.

The GMDA added, “We have identified the drains that will carry the water from catchments to the Silsako reservoir so that it flows into the Brahmaputra.”

In 2008, the GMDA commissioned an Israeli company, Tahal, to do a complete survey of the entire drainage system of Guwahati. The GMDA stated, “We have excavated channels as per the report from Tahal and planned for water management. That is a part of the Sponge City project.”

Regarding the Sponge City project which the restoration of the Silsako Beel is a part of, the idea behind “sponge city” is to create sponges like the Silsako beel. The idea is to build a city that is resilient and has infrastructure to do away with excess water. It is a solution for urban flooding.

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Sponges like the Silsako are created and expanded for runoff water to run into them. They enhance the retention capacity of the lakes. Whatever extra runoff water comes in from the hills because of extra rains can be released into those lakes for prompt water management. So primarily, the lakes are excavated.

The CEO of GMDA said, “We use pavers, which are porous instead of concrete. Water seeps into the ground to improve the ground water table. The government has considered us as fit to execute the Sponge City project in Nagaon, Silchar, and Dibrugarh, in addition to Guwahati.”

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