New smog tower

Source: By Abhinaya Harigovind: The Indian Express

Ahead of its infamous smog season, Delhi on 23 August 2021 got a ‘smog tower’, a technological aid to help combat air pollution. How does a ‘smog tower’ like the one inaugurated by Chief Minister Arvind Kejriwal behind Shivaji Stadium Metro station work?

The structure is 24 m high, about as much as an 8-storey building — an 18-metre concrete tower, topped by a 6-metre-high canopy. At its base are 40 fans, 10 on each side.

Each fan can discharge 25 cubic metres per second of air, adding up to 1,000 cubic metres per second for the tower as a whole. Inside the tower in two layers are 5,000 filters. The filters and fans have been imported from the United States.

The tower uses a ‘downdraft air cleaning system’ developed by the University of Minnesota, said Anwar Ali Khan, senior environmental engineer, Delhi Pollution Control Committee, who was in charge of the project.

IIT-Bombay has collaborated with the American university to replicate the technology, which has been implemented by the commercial arm of Tata Projects Limited.

Polluted air is sucked in at a height of 24 m, and filtered air is released at the bottom of the tower, at a height of about 10 m from the ground. When the fans at the bottom of the tower operate, the negative pressure created sucks in air from the top. The ‘macro’ layer in the filter traps particles of 10 microns and larger, while the ‘micro’ layer filters smaller particles of around 0.3 microns.

The downdraft method is different from the system used in China, where a 60-metre smog tower in Xian city uses an ‘updraft’ system — air is sucked in from near the ground, and is propelled upwards by heating and convection. Filtered air is released at the top of the tower.

Computational fluid dynamics modelling by IIT-Bombay suggests the tower could have an impact on the air quality up to 1 km from the tower. The actual impact will be assessed by IIT-Bombay and IIT-Delhi in a two-year pilot study that will also determine how the tower functions under different weather conditions, and how levels of PM2.5 vary with the flow of air.

An automated Supervisory Control and Data Acquisition (SCADA) system in the tower will monitor air quality. Levls of PM2.5 and PM10, besides temperature and humidity, will be measured constantly, and will be displayed on a board atop the tower.

Monitors will soon be installed at various distances from the tower to determine its impact at these distances. The project aims to provide purified air in a “localised” area, officials said.

In 2019, the Supreme Court directed the Central Pollution Control Board (CPCB) and the Delhi government to come up with a plan to install smog towers to combat air pollution. The court was hearing a matter related to air pollution in the national capital due to stubble-burning in Punjab, Haryana and Uttar Pradesh. IIT-Bombay then submitted a proposal for the towers to the CPCB. In January 2020, the Supreme Court directed that two towers should be installed by April as a pilot project.

The smog tower at Connaught Place is the first of these towers. The second tower, being constructed at Anand Vihar in east Delhi with CPCB as the nodal agency, is nearing completion.

Since 2009, an increase of 258% to 335% has been observed in the concentration of PM10 in Delhi, a 2016 report by the CPCB noted. But the most prominent pollutant in Delhi and neighbouring areas is PM2.5, the report said.

This is the first experiment with a large-scale outdoor air-purification system in India. Small smog towers have been raised in Netherlands and South Korea; larger ones have been set up in China. Experts said there isn’t enough evidence that smog towers work.

“We haven’t come across any clear data that has shown that smog towers have helped to improve the outdoor ambient air quality of a city, either in India or other parts of the world. How do you filter air in a dynamic scenario, when it is not a confined area?” Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment, said.

Dipankar Saha, former additional director, CPCB, and former head of the air quality monitoring division in Delhi, too said there were no proven efficiency calculations on such installations. “We would have to control emissions at the ground level, not create emissions and then try to clean it,” he said.

Delhi has three smaller air purifiers (about 12 feet tall) installed by the Gautam Gambhir Foundation in Krishna Nagar, Gandhi Nagar, and Lajpat Nagar — essentially big versions of indoor air purifiers.