Do Reverse Osmosis Systems Waste a Lot of Water?

Do reverse osmosis systems waste a lot of water? Well, it depends on what you mean by a lot! And it also has to be looked at in the context of the purity of water produced by RO systems compared to other technologies that may not waste any water.

At the heart of every reverse osmosis system is a spiral wound RO Membrane.  This membrane is a highly-engineered sheet of semi-permeable materials. In other words it allows water to pass through - but with some difficulty. By making the membrane not-too-permeable this also makes it resist the passage of contaminants. As pure water passes through the membrane, contaminants accumulate on the other side. These contaminants need to be flushed off the membrane or they eventually clog the pores and prevent the pure water from being pushed through. So the "waste" stream of an RO system serves the very vital role of preserving the function of the membrane and permitting pure water to be continually produced.

Water Ratios vs Absolute Volumes

The amount of water that used by a reverse osmosis as it creates pure water has historically been expressed as a ratio. So if a system created 1 gallon of purified water and wasted 3 gallons, the ratio would be expressed as 1:3. The knee-jerk reaction to these ratios is usually the realization that the RO system is wasting more water than it's creating, which is true, but it's important to look at the absolute volume of water that is being used in the process because it's the volume of water (not the ratio) that will tell us the cost of this water and how the overall operating cost of the system stacks up against other technologies (hint: it's very favourable). When you look at the actual volume of water being used to create the pure RO water, the cost of this water is very low because it isn't really very much, especially when viewed in the context of a home's total water usage.

Some RO's Waste A Lot More Water Than Others

As technology has advanced so too has the science of reverse osmosis membranes. Historically the ratio of product water to waste water ranged from 1:3 to 1:6. In other words, much more water was wasted than was produced. The unfavourable ratio was as side effect of the fact that a higher wastewater velocity meant a cleaner membrane for longer.

But newer membrane technologies have been recently developed that don't accumulate waster materials as readily and consequently a lower water velocity is adequate to "sweep" the rejected contaminants to drain. Aquatell is proud to offer Pentair's GRO technology in our RO systems that use the most water-efficient membranes you can buy. GRO membranes achieve a 1:1 ratio of purified water to waste water.

Other Ways to Evaluate System Operating Costs

There are really only two water purification technologies that will remove pretty much everything from water - distillation and reverse osmosis. A good example is sodium in water.  Sodium is a pretty common water contaminant.  Only a reverse osmosis system or a water distiller will remove sodium from water. Carbon won't do it and particulate/sediment filtration won't either.

A distiller does not waste a single drop of water while an RO system will use water to make water. While that makes the distiller much more water efficient it fails miserably in other efficiency metrics. For instance, a distiller typically operates at about 800W of electrical power and usually it takes about 1 hour of run time to generate 4L (about 1 gallon) of purified water. The electricity input costs here dramatically outweigh the water-waste costs of even a not-so-efficient RO system.