Reverse osmosis systems are highly effective water filtration systems… but they’re not perfect, and not all water pumped in will make it out of your taps.
What Is Reverse Osmosis?
Put simply, osmosis is the process of water with very few impurities moving to a body of water with high impurities.
So, if you had a container split in half with a semi-permeable membrane (a coffee filter, for example) and you filled one half with fruit juice and the other half with water, the water will migrate through the filter towards the fruit juice.
Reverse osmosis is the opposite. It’s the process of pulling pure, clean water from a source that’s full of impurities, like pulling water from fruit juice back through the coffee filter.
Reversing the process of osmosis is not a naturally occurring phenomenon. It requires energy in the form of a pump to push water through a filter, leaving any impurities behind. The result is clean water in your home, free of approximately 95% of impurities.
But those impurities don’t just vanish…
How Much Water is Wasted?
Instead of those impurities collecting on the filter or gathering in a tank as a solid, they are flushed out in a “reject stream”.
This stream is a very highly concentrated stream of water, full of all the contaminants that couldn’t pass through the filter.
How much water does reverse osmosis waste in the reject stream? It depends on your specific filter and the level of impurities in your water. You’ll need 2 figures to work out how much water is wasted:
- The permeate flow rate – how many gallons of purified water per minute is your filter pumping out?
- The feed flow rate – how many gallons of water per minute are entering the water filter?
So, divide the feed flow rate by the permeate flow rate, then times the sum by 100. This works out your recovery rate.
If your permeate flow rate is 5gpm and your feed flow rate is 4gpm, your recovery rate is 80%. Then you’ll know that 20% of your water is wasted through reverse osmosis.
How to Reduce Reverse Osmosis Water Waste
One solution is to run the “reject stream” back into the filter, but this can cause damage as you’re steadily increasing the impurities in the water. This puts a strain on the pump and filter.
The best way to reduce water waste is to simply put your rejected water to good use. Watering your yard, for example, is one way to use the rejected water without wasting it down the drain.
Finally, ensure that you check the pump regularly. If the pump malfunctions or gradually drops the pressure of the water, you’ll see a great increase in waste as water isn’t being pushed with enough force to make it through the filter.