The Osmosis Principle in Reverse
In natural osmosis, water moves through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration — equalising the chemical potential on both sides. Reverse osmosis applies mechanical pressure greater than the osmotic pressure of the feed water, forcing water molecules through the membrane in the opposite direction. The membrane's pores are small enough (around 0.0001 microns) to block dissolved salts, metals, and most organic molecules while allowing water to pass.
The rejected material — concentrated minerals and contaminants — is flushed away as a waste stream, typically called brine or concentrate. A household RO unit produces a volume of this waste water for every volume of purified water it delivers, a ratio that varies by membrane design and feed water pressure.
Typical Four-Stage Configuration
Most residential RO systems sold in Canada use a four or five-stage sequence:
- Sediment pre-filter (5 micron): Removes sand, silt, and rust particles that would clog or abrade the membrane surface.
- Carbon block pre-filter: Reduces chlorine and chloramines, which degrade polyamide membranes over time. Also removes some organic compounds.
- RO membrane: The primary filtration stage. A thin-film composite (TFC) membrane is the current standard in residential units.
- Post-carbon polish filter: Removes any residual taste or odour compounds from the storage tank before water reaches the faucet.
Some systems add a fifth stage — a remineralisation cartridge that adds calcium and magnesium back to the permeate, raising the pH and addressing the flat taste of very low-TDS water.
What RO Removes
When tested and certified to NSF/ANSI 58, a residential RO system is verified to reduce specific contaminants to defined levels. Commonly certified reductions include:
- Lead (Pb) — typically 95–98% reduction
- Arsenic (As, pentavalent form) — typically 94–97% reduction
- Nitrates and nitrites — typically 83–92% reduction
- Fluoride — typically 85–92% reduction
- Total dissolved solids (TDS) — typically 90–96% reduction
- Radium 226/228 — varies by membrane and feed chemistry
RO membranes do not reliably remove all volatile organic compounds (VOCs) with low molecular weight, which is why the carbon pre-filter stage is essential rather than optional. Chlorine removal must happen before the membrane to protect membrane life.
Health Canada's Guidelines for Canadian Drinking Water Quality set maximum acceptable concentrations (MACs) for contaminants including lead (0.005 mg/L) and arsenic (0.010 mg/L). A certified RO system can reduce both well below these thresholds in most feed water conditions. The full guidelines are available at canada.ca.
Water Efficiency and the Waste Ratio
Older residential RO membranes produced roughly three to five litres of brine for every one litre of purified water — a waste ratio of 3:1 to 5:1. More recent membrane designs and permeate pumps (which increase effective pressure without electricity) have brought typical household ratios to approximately 1:1 to 2:1.
Water efficiency matters in Canadian municipal contexts where water billing is volumetric. Households on well systems face a different concern: high hardness or mineral content can accelerate membrane fouling, requiring more frequent replacement.
Pressure Requirements
Most residential TFC membranes require a minimum feed pressure of around 40–60 psi (275–415 kPa) to produce adequate flow. Canadian municipal water mains typically deliver 50–80 psi at the meter, which is adequate. In buildings with pressure-reducing valves set below 40 psi, a booster pump is needed. Well systems vary and should be tested before sizing a membrane.
NSF/ANSI 58 Certification
NSF International's Standard 58 covers point-of-use reverse osmosis systems. Certification under this standard requires third-party testing of both structural integrity (materials in contact with water must not leach harmful substances) and contaminant reduction claims. When a product listing shows "NSF/ANSI 58 certified for lead reduction," it means that specific contaminant has been verified at the listed reduction percentage under standardised test conditions.
The NSF certified product listings database can be searched at nsf.org.
Maintenance Schedule
Filter cartridge life varies by feed water quality. Typical intervals under moderate municipal water conditions:
- Sediment pre-filter: every 6–12 months
- Carbon pre-filter: every 6–12 months
- RO membrane: every 2–4 years depending on TDS and usage volume
- Post-carbon polish: every 12 months
A TDS meter provides a practical way to monitor membrane performance. If the permeate TDS rises significantly relative to feed water TDS, membrane replacement or inspection is warranted.