Reusing RO reject water safely at home

Reverse osmosis (RO) purifiers send part of the water they take in down the drain as 'reject' water. In a dry year, that water is worth catching.

The rules. Following a National Green Tribunal order, the Ministry of Environment, Forest and Climate Change published the Water Purification System (Regulation of Use) Rules, 2023. They cover how reject water should be managed and came into force on 10 November 2024. The BIS standard for RO purifiers, IS 16240:2023, sets a minimum recovery of 40%, meaning at least 40% of the water taken in should come out purified.

Where reject water can go. The Central Pollution Control Board's guidelines under these rules, issued on 23 September 2026 (as summarised by the legal research service TeamLease RegTech), say RO reject water should be reused as much as possible for non-drinking uses such as floor cleaning, vehicle washing, toilet flushing and sprinkling on construction sites. Water that cannot be reused should go into the sewer.

Never for drinking or cooking. CPCB notes reject water contains concentrated salts, dissolved pollutants, minerals and microorganisms.

Easy start. Run the purifier's reject pipe into a bucket rather than the sink, and use that bucket to mop floors or flush the toilet. Empty and rinse the bucket often so it does not get dirty.

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