Plenty of people search for an osmosis reverse water filter. The usual name is reverse osmosis, or RO for short, and mixing up the word order is an easy slip. It means the same thing either way: a compact treatment system, usually under the kitchen sink, that pushes tap water through a very fine membrane and sends the cleaner water to a small faucet of its own. Below, we explain how it works and when it makes sense on Durham Region water.

How it works, in plain terms

The pressure in your water line does the work. It pushes water against a semi-permeable membrane, a thin sheet that lets water molecules through but holds back most dissolved salts and metals. The flow splits in two. Treated water goes on to your faucet. Reject water carries what the membrane held back down the drain. The U.S. EPA calls these streams permeate and concentrate. For the science of osmosis itself, see our older post on how reverse osmosis works.

The four stages, one at a time

An RO membrane rarely works alone. NSF notes that most systems add one or more filters on either side of it. A typical home setup, as described in a University of Nebraska-Lincoln Extension guide, runs like this:

  1. Sediment pre-filter. Catches sand, silt and rust flakes before they can clog the membrane.
  2. Carbon pre-filter. Removes chlorine. Durham disinfects its water with chlorine, and the thin film composite membranes in many units break down in chlorinated water. This stage protects the membrane, and the chlorine taste goes with it.
  3. RO membrane. The membrane itself. Water is forced through, and most dissolved minerals, metals and salts are rinsed away to the drain.
  4. Carbon post-filter. One last pass before the faucet for any leftover taste or odour.

Most units hold treated water in a small pressurized tank so a glass fills quickly. Our guide to the reverse osmosis water system covers that hardware: the tank, the faucet, the drain connection and tankless designs.

What an osmosis reverse water filter removes, and what it misses

What it reduces

The membrane is good at dissolved substances. The Nebraska guide lists lead, arsenic, nitrate, fluoride, sodium, calcium and magnesium among the contaminants household RO units remove. In NSF's guide to certified products, reverse osmosis is the only filter type listed for fluoride and for nitrate.

Models vary, so check the paperwork. Health Canada strongly recommends a device certified to the matching standard, which for reverse osmosis is NSF/ANSI 58. Certification only confirms the claims that standard covers, so the performance data sheet tells you which contaminants were actually tested.

What it does not handle

  • It is not a disinfection system. The Nebraska guide says RO is not recommended for microorganisms and should only be fed coliform-free water, because bacteria can damage the membrane and pinhole leaks can let contamination through.
  • Dissolved gases get through. Hydrogen sulphide, the rotten egg smell in some well water, passes straight through the membrane.
  • So do some pesticides, solvents and volatile organic compounds. The carbon stages catch some of these, which is part of why they are there.
  • It treats one tap. Showers and laundry still get regular water, so it does nothing for hard water or chlorine odour in the rest of the house. That job belongs to a softener or a whole-home filter, and our post on reverse osmosis and water softeners explains the difference.

Is it worth it on Durham tap water?

For most homes on the Lake Ontario supply, RO is optional. The Region of Durham says its water is safe to drink and meets aesthetic quality guidelines, and that installing a softener or filtration system "is a personal preference and not required with the Region's supplied water." Health Canada says much the same: water that meets the national guidelines does not usually need extra treatment for health reasons.

Our view is simple. On Lake Ontario water, RO is mostly about taste and convenience. Our earlier post covers the benefits of reverse osmosis in general, but a few situations make a stronger case. If you buy bottled water anyway, a tap at the sink replaces the jugs. If your home is older, think about lead. The Region says a house built before the mid-1950s may have a lead service connection. Replacing that pipe through the Region's Lead Service Replacement Program is the real fix. Until then, its lead fact sheet suggests a filter certified for lead reduction, and many RO systems carry that certification. And if a well test finds nitrate, RO is the kind of system that can be certified to reduce it.

If chlorine taste is your only complaint, RO is probably more than you need. The Region suggests keeping a jug of water in the fridge, since free chlorine evaporates over time. A carbon filter certified to NSF/ANSI 42 for chlorine reduction also does the job.

Trade-offs to weigh

Start with fluoride. The Region adds it in Courtice, Oshawa, Whitby, Ajax and Pickering to improve oral health, and an RO unit reduces it along with most other dissolved substances.

Some people worry about minerals. RO does remove calcium and magnesium, but Health Canada says water is a minor source of these compared with food. People who eat a reasonably balanced diet do not need supplements.

Then there is the water that goes down the drain. The EPA says a typical under-sink unit sends five or more litres to the drain for every litre of treated water. Models with the EPA's WaterSense label must keep that to 2.3 litres or less, so the label is worth looking for.

If your home is on a well

Many rural homes in Scugog, Uxbridge and Clarington use private wells, and well water needs its own plan. Test for bacteria first. The Region of Durham offers free testing of private well water and recommends checking for harmful bacteria at least three times a year. If a test comes back bad, RO is not the fix. Health Canada notes that UV systems certified to NSF/ANSI 55 Class A are suitable for disinfecting contaminated water.

Groundwater also tends to be harder. Port Perry's municipal wells measure 274 mg/L of hardness, against 125 mg/L for the Lake Ontario supply. The Nebraska guide notes that softening hard water before it reaches the RO unit can extend membrane life. It also advises a sediment pre-filter only on unchlorinated water, since carbon there can encourage bacteria to grow on the membrane. Many rural homes have a septic system as well, and RO reject water adds to its load. Our water softener service covers the hardness side.

Maintenance basics

An osmosis reverse water filter only works as well as its upkeep. Health Canada stresses following the maintenance instructions that come with the device. In practice, that means a few habits:

  • Change the pre- and post-filters on schedule. The right interval depends on how much water you use and what is in it, so go by the manufacturer's.
  • Check the membrane now and then. It outlasts the filters, but a damaged membrane is hard to spot by taste. A total dissolved solids (TDS) reading, which some units display, is one way to check.
  • Drain the storage tank about every two weeks, as the EPA suggests. Many manufacturers also recommend sanitizing it once a year.
  • If water runs to the drain nonstop, the automatic shutoff valve may have failed.

We replace filter cartridges and repair water treatment systems, so call us if something seems off.

Your next step

If you are thinking about an osmosis reverse water filter for your kitchen, we test your water before recommending anything. The answer might be an under-sink RO system, whole-home filtration, a softener or some combination. Our registered plumbing technicians install and service water treatment across Oshawa and Durham Region, and financing and rentals are available. See our water filtration service, call (905) 576-3043 or ask us to test your water.