We asked a few manufacturers about the standards they adhere to. Here is what they said

Philips Electronics India: Gold seal Class A certified by the US-based Water Quality Association, certified by KIWA Water Research of the Netherlands and BIS certification for UV filters

KENT RO systems: Certificates from Shriram Institute for Industrial Research and Spectro Labs (Pvt) Ltd, New Delhi

Ion Exchange India: Member of Water Quality Association and certificates from many labs

Eureka Forbes: Over 104 certificates from national and international laboratories and universities. Gold seal certificate from Water Quality Association, endorsement of the Indian Medical Association

Hindustan Lever Limited: Meets US Environmental Protection Agency standards for microbiologically safe drinking water. Certificates from London School of Hygiene and Tropical Medicine

Usha Brita did not respond