Reverse osmosis water filtration pushes water through a membrane that holds back the dissolved salts that carbon filters and softeners leave in. That makes it thorough. It does not make it the right fix for every complaint. This guide covers what certified RO systems are tested to reduce, how RO compares with carbon filtration and with softening, and what the Region of Durham's own water reports say comes out of your tap. For the basic science, start with how reverse osmosis works.
What reverse osmosis water filtration is certified to reduce
Health Canada strongly recommends buying a treatment device certified to the matching NSF/ANSI standard. For a point-of-use RO system, the kind that feeds one drinking water tap, that standard is NSF/ANSI 58. Certification means the unit adds nothing harmful to the water and that the claims the standard covers have been tested.
NSF/ANSI 58 has one required claim: reduction of total dissolved solids (TDS), the minerals and salts dissolved in water. Every other claim is optional and tested on its own. The optional claims include:
- lead, copper, cadmium, barium and chromium;
- arsenic, selenium and radium;
- fluoride, nitrate and nitrite;
- volatile organic compounds (VOCs), perchlorate and asbestos;
- cysts and turbidity.
So "certified to NSF/ANSI 58" on a box only tells you the unit passed the TDS test. It does not tell you it reduces lead or fluoride. Read the performance data sheet for the contaminant you care about.
Also know this: RO is not a disinfection system. For bacteria and viruses, NSF's contaminant reduction claims guide lists ultraviolet and microbiological treatment systems, not RO. The everyday upsides, such as taste, are in our post on reverse osmosis benefits.
How RO compares with carbon filtration and softening
According to NSF's summary of the standards, carbon filters are certified under NSF/ANSI 42 for aesthetic issues such as chlorine taste and odour, and under NSF/ANSI 53 for contaminants with a health effect. Softeners fall under NSF/ANSI 44. Their resin swaps the calcium and magnesium that make water hard for sodium or potassium.
| Concern | Carbon filter | Reverse osmosis | Softener |
|---|---|---|---|
| Chlorine taste and odour | Yes (NSF/ANSI 42) | Yes, through a carbon stage | No |
| Trihalomethanes and other VOCs | Can be certified (NSF/ANSI 53) | Optional claim | No |
| Lead | Can be certified (NSF/ANSI 53) | Optional claim | No |
| Fluoride | None in NSF's listings | Optional claim | No |
| Nitrate and nitrite | None in NSF's listings | Optional claim | No |
| Sodium and other dissolved solids | No | Required TDS claim | A sodium-based softener adds sodium |
| Hardness and scale | No | Only at its own tap | Yes, whole house (NSF/ANSI 44) |
| Bacteria and viruses | Not certified | Not certified | Not certified |
If chlorine taste is your only complaint, a carbon filter certified to NSF/ANSI 42 may be all you need. It works under the sink or for the whole home, and sends no water down the drain. Inside an RO unit, the carbon stage has a second job. DuPont's FilmTec membrane manual gives a free chlorine tolerance below 0.1 ppm and says to remove chlorine before water reaches the membrane. Durham's tap water carries more chlorine than that, so a spent carbon stage leaves the membrane exposed.
If hardness is the problem, RO only treats the water at its own faucet. Your water heater and shower still get hard water. That is a softener's job, and our water softener vs water filter guide covers the choice.
What Durham's water reports show, item by item
The figures below come from the Region of Durham's 2025 annual report for the Oshawa system, which supplies Oshawa, Courtice, Whitby, Brooklin, Ajax and Pickering. For each item, the question is whether reverse osmosis water filtration changes anything.
Chlorine
The Region disinfects with free chlorine, not chloramine. Free chlorine in the Oshawa distribution system ranged from 0.30 to 3.60 mg/L in 2025. RO removes it with its carbon stage, and a certified carbon filter does the same. The Region's own tip works too: free chlorine evaporates from a jug of tap water left in the fridge.
Disinfection by-products
Chlorination leaves by-products called trihalomethanes (THMs) and haloacetic acids (HAAs). The 2025 annual averages were 31.1 µg/L for THMs, against a limit of 100, and 18.1 µg/L for HAAs, against a limit of 80. An RO unit only reduces THMs if it carries the optional VOC claim. Carbon filters can be certified for THMs under NSF/ANSI 53.
Fluoride
Fluoride is added in Courtice, Oshawa, Whitby, Ajax and Pickering for dental health. Readings at the Oshawa plant ran from 0.153 to 0.770 mg/L, under the 1.5 mg/L limit. An RO system certified for fluoride reduction will lower it. NSF's claims guide lists only RO systems for fluoride, so a carbon filter will not. Decide if you want the fluoride gone before you choose.
Sodium
Sodium ranged from 14 to 24.3 mg/L. That is far below Health Canada's taste-based objective of 200 mg/L, but Health Canada advises under 20 mg/L for people on strict sodium-reduced diets. Sodium is part of the dissolved solids every NSF/ANSI 58 system is tested to reduce, so RO is the only one of the three that lowers it.
Lead
All 106 plumbing samples tested for lead in 2025 were under Ontario's standard. The highest, 0.0032 mg/L, was also under Health Canada's tighter guideline of 0.005 mg/L. The Region says lead, when it shows up, usually comes from a lead service connection, which a house built before the mid-1950s may have. Health Canada calls full replacement of that line the best fix and says treatment devices are also effective. RO units and NSF/ANSI 53 carbon filters can both be certified for lead.
Hardness
The Region lists hardness at 125 mg/L for the Lake Ontario systems, 186 mg/L in Uxbridge and 274 mg/L for Port Perry's wells. Health Canada calls water over 200 mg/L poor, though hardness is not a health concern. RO lowers hardness at one tap. A softener treats the whole house, and Health Canada recommends keeping a separate unsoftened supply for drinking and cooking if you install one.
Nitrate
Nitrate ranged from not detected to 0.57 mg/L, against a limit of 10. That is no issue on city water. On a rural well it can be, which is why Health Canada tells well owners to test for it. In NSF's listings, only RO systems are certified for nitrate reduction.
The Region says its water is safe and meets the aesthetic guidelines, and that a softener or filter is a personal preference. On city water, RO is a choice you make for taste or to cut sodium or fluoride. In an older home that may still have a lead service line, it is also a reasonable extra barrier.
Test first, especially on a well
Private wells are common in rural Scugog, Uxbridge and Clarington. The Region's Health Department offers free bacteriological testing for private wells and recommends testing at least three times a year. Sample bottles are picked up and dropped off at Regional headquarters in Whitby and at locations in Port Perry, Bowmanville and Uxbridge.
That free test is for bacteria only. Nitrate, hardness and iron need a separate test. If bacteria turn up, RO is not the answer. The Region advises well owners to consider well disinfection or long-term treatment and to check any UV light already installed. Health Canada says UV systems certified as Class A under NSF/ANSI 55 are suitable for contaminated water.
On city water, a test still shows what is behind the complaint and whether it starts in your own plumbing. Hayes tests your water before recommending anything. The answer might be a carbon filter, a softener, an under-sink RO unit or a combination. If you are thinking about reverse osmosis water filtration for every tap, read our guide to a whole house reverse osmosis system and get the water tested before you decide.
Next step
Call (905) 576-3043 or request a water test. Our water filtration service covers under-sink RO and whole-home filtration, and our water softener service covers hardness. The 2026 Oshawa plumbing cost guide puts water filtration at $300 to $800 for an under-sink system and $1,000 to $3,000 for a whole-house system. Financing and rentals are available, and registered plumbing technicians do the installation.