Kai Mallari's Notes'

Reverse Osmosis Wastes 3-4 Litres Per Litre Produced on Municipal Water (Home Systems)

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Reverse Osmosis Wastes 3-4 Litres Per Litre Produced on Municipal Water

idea

Claim: A traditional under-sink reverse osmosis system connected to a standard municipal supply wastes approximately 3–4 litres of water for every 1 litre of purified water it produces. This is a real operating cost (added water consumption on a metered supply) and an environmental consideration that should be weighed before choosing RO over a simpler carbon-block filter.

Mechanism

Reverse osmosis works by forcing pressurised water across a semi-permeable membrane. The membrane passes only water molecules; everything dissolved (dissolved solids, heavy metals, nitrates, fluoride, PFAS, and most pharmaceuticals) is rejected and swept away as concentrate (the “reject stream” or “brine”).

The ratio of filtered water produced to reject water flushed away is the wastewater ratio:

  • Traditional residential RO (pre-2015 designs): 4:1 to 10:1 — four to ten litres of water rejected per litre of purified output. The US EPA’s WaterSense programme cited 5:1 as typical for common point-of-use RO units.1
  • Modern standard residential RO: approximately 3:1 to 4:1 on a standard municipal supply with adequate pressure (50+ PSI).
  • High-efficiency / permeate-pump systems: approximately 1:1 to 2:1 — these add a small pump that pressurises the reject back through the membrane for a second pass. They cost 300 more than standard systems.

Why Metro Vancouver matters here: Metro Vancouver water is already very low in TDS (30–40 mg/L), soft (0–1 gpg), and safe to drink from the tap. Installing a traditional RO system wastes 3–4 litres of water per litre produced to remove dissolved solids that are already minimal. This is an unusual case where the problem RO solves barely exists, while the water-waste cost is real and ongoing.

The practical numbers: A household that drinks 4 litres of RO water per day generates 12–16 litres of wastewater per day from an average system — approximately 4,400–5,800 litres of additional water consumption per year. At Metro Vancouver residential water rates, this is a modest dollar cost (Metro Vancouver water rates are among the lowest in Canada), but it is measurable on a metered supply.

Scope

This applies to:

  • Standard tank-style under-sink RO systems
  • Tankless under-sink RO systems (same membrane physics; efficiency varies by design)

This does not apply to:

  • Activated-carbon point-of-use systems (no reject stream; no water waste)
  • UV disinfection systems (no water waste)
  • Whole-home RO systems (typically designed for higher efficiency; professional sizing required)

The waste ratio also improves in homes with higher municipal water pressure and worsens at lower pressures (below 40 PSI, reject ratios can worsen significantly). A pressure gauge reading before purchasing is worthwhile.

Idea Compass

North: Where this comes from

  • water-filtration (Home Systems) — the parent note where this idea surfaces as a tradeoff
  • RO membrane physics — the fundamental mechanism of pressure-driven semi-permeable membrane separation creates a reject stream by design; this is not a manufacturing defect

East: Tensions / failure

South: Where this leads

West: What’s similar

  • Air conditioner energy use vs. alternatives — same tradeoff structure: a powerful tool that solves a problem has an operating cost worth weighing against a simpler alternative when the problem is mild
  • Hot water heater standby losses — energy consumed to maintain readiness for a resource that may not be needed at that level

Sources

Footnotes

  1. American Home Water & Air, a water treatment company — RO wastewater ratios: traditional systems 3–5:1 on municipal supply; EPA WaterSense reports 5:1 as typical for common POU RO units; high-efficiency permeate-pump systems achieve near 1:1; inefficient models up to 10:1 — https://americanhomewater.com/the-truth-about-reverse-osmosis-waste-water/ ↩

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