How much Does Reverse Osmosis Wasted Water Cost?

One of the criticisms of Reverse Osmosis filtration is that it increases your water usage and therefore your water bills.

It's true that RO systems produce reject water (often referred to as waste water). In a typical domestic system, around 70% to 75% as much reject water is produced as purified water, although more efficient systems can reduce this significantly.

But let's look into this in detail:

This only affects the relatively small amount of drinking and cooking water you use – not all the water in your home.

Only a small proportion of the water used in a typical UK home is for drinking and cooking.

So it's not quite as bad as it first seems.

But it is a factor.

Let's Check Out the Financial Cost:

In the UK at the time of writing, one cubic metre (1,000 litres) of water costs around £3.00.

That's approximately 0.3 pence per litre.

You should drink around 2L of water a day. Let's also allow another 2L for cooking. That's 4L a day.

To keep the calculation conservative, let's assume an average of 6L of purified water per person per day for drinking and cooking.

Now let's assume the RO system produces reject water equal to around 75% of the purified water produced, which is typical of many standard domestic RO systems (more efficient systems produce considerably less).

So the calculation is:

6L × 75% = 4.5L of reject water per day

At approximately 0.3p per litre:

4.5L × 0.3p = 1.35p per day

Over a month that's approximately 135L of reject water, costing around 40p per person per month.

Bear in mind these calculations deliberately overestimate typical usage. In many households, drinking and cooking water is closer to 4L per person per day, so the actual cost is often noticeably lower.

The Environmental / Carbon Cost:

The reject water from an RO system is not sewage. It is simply the portion of the incoming water carrying away the concentrated minerals and contaminants removed by the membrane. It enters the normal wastewater system, where it is treated by the local water company before being returned to the environment as part of the natural water cycle.

Overall, using a Reverse Osmosis system instead of regularly buying bottled water can significantly reduce plastic waste and the emissions associated with manufacturing and transporting bottled water, while also providing exceptionally high-quality drinking water.

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