Why Does My RO Purifier Reject So Much Water?

If you've noticed a constant dripping or stream of water from a pipe going into your kitchen drain, that's your RO purifier's reject line. For many families, this drain water can add up to 3–4 litres for every litre of purified water — a significant amount in water-conscious cities like Pune.

The Science Behind RO Waste Water

Reverse Osmosis works by pushing water through a membrane under pressure. Pure water molecules pass through; dissolved salts, heavy metals, and contaminants cannot. These contaminants are flushed away with a portion of the water to keep the membrane clean and prevent scaling. This is the reject or drain water.

Typical Rejection Ratios

A basic 50 GPD (gallons per day) RO system typically has a 3:1 or 4:1 reject ratio — meaning for every 1 litre purified, 3–4 litres go down the drain. Higher quality systems with permeate pumps can reduce this to 1:1.

How to Reduce RO Waste

• Collect the reject water in a bucket and use it for mopping, watering plants, or flushing toilets.
• Install an auto flush valve to reduce membrane scaling and improve efficiency.
• Upgrade to a high-efficiency RO system with a built-in permeate pump.
• Service your filters on time — a clogged pre-filter increases rejection ratio.

The reject water is not toxic. In fact, it's similar to normal tap water in quality — just more concentrated in minerals. Plants that prefer hard water actually thrive with it.