Treating Sewage with Plants


Lake pollution is common in developing countries. It is a common sight to see domestic wastewater discharge in the lake. In Pakistan, in one case, industrial wastewater is discharged into the lake. Cost of treating lake’s water is simply exorbitant and prohibitive.

Within this context, it was refreshing to see a post in the India Water Portal, according to which, “a radical new method is fast emerging as an effective and sustainable solution to increasing pollution in urban lakes. Called floating treatment wetlands (FTW), they are artificial islands with plants that stay afloat on the lake. The plants clean the lake through the hydroponics system, resulting in a cleaner, beautiful lake and an improved habitat for creatures that depend on it. We have already heard of this being successfully implemented in Hauz Khas lake in Delhi. The latest one to benefit from this treatment method is Neknampur Lake on the outskirts of Hyderabad city." (www.indiawaterportal.org/articles/treati...21e83bc1bb-231038001)

"Inaugurated on February 2, 2018, on World Wetlands Day, the Neknampur FTW has been recognised as the largest of its kind in the country by The India Book of Records. It’s the creation of Dhruvansh, an NGO, that along with the Hyderabad Metropolitan Development Authority (HMDA) and district authorities, has been working to revive the lake since 2016.”

As the post rightly says, diverting sewage is not a solution as you will end up polluting another water body. A sewage treatment plant is costly to build and maintain and it does not remove pathogens and the metallic content. The sponsors chose this biological sewage treatment method based on hydroponics technique that cleans the lake by absorbing nutrients dissolved in the water and thereby supporting living species inside the lake.

According to the post, “The FTW is an ingenious bamboo raft with its sides made of thermocol blocks and plastic bottles which are chemically inert. A layer of gunny sacking stretches across the raft’s bottom to create a tray that holds a two-centimetre layer of gravel. Saplings have been planted in the soil with their roots reaching into the water. This 3000 sq. ft. raft supports around 3500 saplings of different varieties. These include wetland plants, mosquito repellents and ornamental plants like cattails, bulrush, citronella, canna, hibiscus, fountain grass, flowering herbs, tulsi and ashwagandha. The plants absorb the high levels of phosphorus and nitrogen in the sewage water entering the lake. The FTW costs a fraction of what a conventional sewage-treatment plant would cost. The first FTW, with 140 saplings, was introduced in May 2017 in a small area of the lake and it showed remarkable results. The team then increased its size to hold 3500 saplings.

Another unique feature of the restoration process is the way water hyacinth is used to clean the lake. Water hyacinth is usually seen as a problem because it chokes water bodies and destroys other aquatic life. “If we can use hyacinth in a creative way, it can play a major role in cleaning the lake by absorbing the pollutants in the sewage. We have cleared 80 percent of the hyacinth in the lake but have let the remaining 20 percent to grow. We are using the cleared weed as an organic manure by mixing it with cow dung to grow plants both in the FTW and in the surroundings.”
Besides the FTW, Dhruvansh is also using aerators to oxygenate the water. They have also planted more than 5000 shrubs around the lake. The combination of all these measures has had a significant impact on the lake. The biological oxygen demand (BOD) in the water has fallen from 28 to 3.2, nitrates from 27 to 8, phosphates from 7 to 0.5 while dissolved oxygen levels have increased from 0 to 4.5, according to the Telangana pollution control board. “The lake has so much biodiversity, with 178 species of birds, 41 species of reptiles, pythons, mongoose, mammals and it is increasing by the day,”

The results - BOD in the water has fallen from 28 to 3.2, nitrates from 27 to 8, phosphates from 7 to 0.5, DO increased from 0 to 4.5 – are simply fascinating.

F H Mughal