Why does RO purifier discard water?

Reverse-Osmosis or RO water purifiers are widely considered to be the most effective water purifiers we can get for home use. These purifiers are quite advanced, using a system of pumps, layered filters, UV light, and more, to deliver the purest drinking water possible. These purifiers effectively convert hard water to soft water, remove harmful chemicals and excess dissolved salts and minerals, kill and filter out microbes, and of course, filter out suspended particulate matter.

However, the very thing that makes them so effective – the layered filtration system – also results in significant water wastage. In the simplest terms, an RO filter works by forcing water under high pressure through an ultra-fine membrane (with 0.001 micron holes in some cases). This catches virtually any particle that’s not a water molecule, but these filters also get dirty very quickly and need to be flushed frequently to stay effective. As a result, water is wasted.

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By some estimates, the wastage rate is as high as 75 per cent. To put it another way, for every glass of water you drink, two to three glasses of water are wasted by the filtration process. This discarded water is high in dissolved salts and impurities, and likely even contains a large amount of dead organic matter as a result of the filtration process. It’s not safe to use this for bathing, cooking, or drinking.

However, waste RO water need not be thrown away. The rich mineral content of the water makes it a great source of nutrients for plants, for example. The waste RO water is usually collected in a separate storage tank installed alongside the RO unit and can be extracted quite easily.

Waste RO water can also be used for the first wash of a laundry cycle – provided your water source isn’t packed with harmful chemicals – and the same applies for washing cars or mopping your floor. For any kind of cleaning activity, we’d recommend filtering the RO water with a simple cloth filter, and diluting it with regular water so the high salt concentration does no damage to your surfaces.

Given the state of our water sources in many areas, there’s no escaping the use of RO filters. They are inefficient at saving water, but when they’re your only reliable source of pure drinking water you don’t have an option. If you must use an RO filter, at least try and minimise wastage by reusing the discarded water in some manner.