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Marrying mechanised Sewage Treatment plants (STPs) with natural treatment systems can be a cost effective option to prevent dumping of untreated wastewater into rivers, a researcher at the Indian Institute of Technology (IIT) Bombay has said.
Such hybrid treatment systems will also help to meet the stringent wastewater treatment norms put in place by the National Green Tribunal in 2019, said Pradip Kalbar, an assistant professor at Centre for Urban Science and Engineering at IIT-Bombay in a paper published recently in the journal Clean Technologies and Environmental Policy.
As much as 80 per cent of India’s surface water is contaminated with a whopping 40,527 million litres of untreated wastewater discharged into rivers and lakes every day. Currently, STPs treat only 44 per cent of wastewater and even this treatment is not sufficient to meet the current discharge standards issued by the NGT, Kalbar said.
Stringent norms
Before the tribunal’s order, the treatment plants were required to ensure that the treated water had a biochemical oxygen demand (BOD) of less than 30 mg/L. BOD represents the amount of organic matter in the wastewater. The revised standard is less than 10 mg/L, Kalbar said.
To meet these norms, apart from upgrading the existing STPs, there is a need to install new systems to treat the rest 56 per cent of the wastewater, which is discharged without any treatment. The hybrid model that he has proposed aims to tap the merits of both mechanised treatment system and natural system.
Attaining this new standard with the current system would be very unsustainable with either of the methods, he said. Mechanised systems are feasible until a BOD of 30 mg/L. Any further purification would increase the costs multifold as this requires advanced treatment technologies. Natural systems such as the constructed wetlands, on the other hand, would need extensive land to meet the requirements since land requirement grows in multiples for every additional BOD to be removed.
Combo system
To solve this problem, Kalbar proposes a novel system called the hybrid treatment system, which pairs both mechanised systems and natural systems to achieve the discharge requirements. First, the wastewater (typically having a BOD of about 300 mg/L) is treated with a mechanised system, and it is brought to a BOD of 30 mg/L. The water can then be treated with a natural system until a BOD of 10 mg/L is attained.
During the typical lifetime of 30 years for a treatment plant, a mechanised system would accumulate annual costs that would exceed the initial land investment required for a hybrid system. The use of hybrid systems in place of high-demand mechanised systems for treatment beyond 30 mg/L significantly reduces costs and resources, Kalbar suggested.
As per his calculation, deploying such a hybrid systems throughout the country would ultimately lead to an annual savings of 540 GWh, enough to power all of Mumbai for half a year.
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