Though private sector thermal plants in the country perform better than government-owned ones, there is "immense scope for improvement" in almost all units so that they can pollute less and generate more electricity with efficient use of available resources.

The study behind the ratings, done by experts at the Centre for Science and Environment(CSE), also noted that Delhi is home to one of the most polluting power plants in the country- NTPC's Badarpur Thermal Power Plant- which has contributed in turning the capital into the most polluted city in the world.

The study, done under CSE's Green Rating Project (GRP), analysed and rated 47 coal-based thermal power plants from across the country on a variety of environmental and energy parameters. About half of all plants operating in 2011-12 were selected for the rating.

"The objective of the study was to give a clear picture of the environmental performance of the sector. Our finding is that in India, where demand for power is increasing, power plants are performing way below the global benchmarks", said Sunita Narain ahead of the study's release on Saturday.

She said, "Given the rapid increase in coal-based power projected by the government, stress on precious resources like water and land will increase and air and water pollution will worsen unless corrective measures are taken by the industry and policy-makers".

The study was released jointly by father of the green revolution M S Swaminathan, environment secretary Ashok Lavasa and chief economic advisor Arvind Subramanian in a function here, organized to award the greenest power plants.

Three top power plants (CSES- Budge Budge, JSWEL-Toranagallu and Tata-Trombay) were awarded for their overall environmental performance, while two others received awards for their efficient use of resources such as energy and water.

Speaking about the rating programme, CSE's deputy director general Chandra Bhushan said, "The Green Rating Project is one of the very few public-disclosure projects in the world in which a non-governmental, non-industry organization rates the environmental performance of industries and makes the results public. We follow a robust and transparent process and the outcomes of our ratings (of other sectors) have been used by companies as well as policymakers to improve policies and practices."

The study found that the country's thermal power plants are estimated to draw around 22 billion cubic meter of water, which is over half of India's domestic water need. It also noted that 55% of the units were violating air pollution standards which are already extremely lax.

"The performance of NTPC, the largest coal-power producing company in India, was found to be below par. NTPC did not disclose its data, and hence was rated based on a primary survey and publicly available information", said lead author of the study Priyavrat Bhati.

The six plants of NTPC that were rated received scored poorly on almost all parameters. The worst of the lot was Delhi's Badarpur plant.

Speaking about methodology of the rating study, Bhati said, "The project selected a diverse group of plants from all regions, of various vintages, sizes and technologies and owned by all major companies, including state and central ones, to ensure as wide a representation as possible. The GRP is a participatory process -- companies voluntarily disclose data and permit the GRP team to independently scrutinize the plants and their records".

The plants were rated on around 60 parameters covering everything from coal and water use and plant efficiency to air and water pollution and ash management.

Local community views were given due weightage along with the plants' compliance record and environment policies. The ratings involved comparing performance of the plants against global best practices.

Bhati, programme director of CSE's sustainable industrialization team, said: "Some of the plants did not want to participate. Yet, we assessed them on the basis of field-level surveys and publicly available data. We were encouraged by the transparency showed by a number of state-owned plants that voluntarily disclosed data despite being inefficient and highly polluting."