A reverse osmosis system removes a broad range of dissolved contaminants from drinking water by forcing water under pressure through a semipermeable membrane that blocks particles as small as individual ions. For most Denver-area homeowners, reverse osmosis is the most effective point-of-use treatment available — capable of reducing lead, nitrates, arsenic, fluoride, dissolved minerals, chlorine, and many other contaminants to very low levels.

How Reverse Osmosis Works

Water passes through several filtration stages in a reverse osmosis system. A sediment pre-filter removes particulates like silt and rust that would clog or damage the membrane. An activated carbon pre-filter reduces chlorine and chloramines, which would degrade the membrane if allowed through. The RO membrane itself is the core of the system — it allows water molecules to pass while blocking dissolved salts, metals, and most chemical compounds. A post-carbon filter then polishes the water before it reaches the tap.

The result is water that is significantly purer than what enters the system. Most RO membranes reject 90 to 99 percent of the contaminants they are designed to address, though rejection rates vary by contaminant and membrane quality.

Contaminants That Reverse Osmosis Effectively Reduces

RO is particularly well-suited to removing contaminants that other filtration methods struggle with. Lead is one of the most important — RO removes it effectively, which matters in older Denver homes where lead solder or lead service lines may be present. Nitrates, which can come from agricultural runoff in Front Range water supplies, are also well-addressed by RO. Arsenic, which occurs naturally in some Colorado groundwater, is significantly reduced by quality membranes.

Fluoride — added to municipal water supplies — passes through carbon filters but is largely rejected by an RO membrane, making RO the primary choice for homeowners who want to reduce fluoride in their drinking water. Dissolved minerals including calcium, magnesium, and sodium are also reduced, which means RO water is essentially soft at the drinking tap regardless of whole-house hardness levels.

  • Lead — critical for older Denver homes with pre-1980s plumbing
  • Nitrates and nitrites from agricultural runoff
  • Arsenic — occurs naturally in some Colorado groundwater
  • Fluoride — not removed by carbon filters but reduced by RO membrane
  • Dissolved hardness minerals (calcium, magnesium, sodium)
  • Chromium, barium, radium, and many pharmaceutical compounds

What Reverse Osmosis Does Not Remove

RO is not effective against all contaminants. Some volatile organic compounds and dissolved gases — including radon — can pass through the membrane. Bacteria and viruses are generally blocked by a quality RO membrane, but RO is not certified as a standalone disinfection method for biologically contaminated water. For well water with potential bacterial contamination, RO should be paired with UV disinfection rather than relied upon alone.

Pesticide rejection rates also vary depending on molecular size and polarity — a carbon pre-filter handles most pesticide reduction rather than the membrane itself.

Practical Considerations for Colorado Homeowners

RO systems produce a concentrate stream — sometimes called wastewater — that carries rejected contaminants to the drain. Efficiency ratios vary; higher-efficiency systems waste less water per gallon produced, which matters in water-conscious Colorado. RO systems require periodic maintenance: sediment and carbon pre-filters typically need replacement every six to twelve months, and the RO membrane every two to three years. Colorado’s hard water shortens pre-filter life because of the mineral load the system processes.

A whole-house water softener paired with an under-sink RO unit is a common and effective combination — the softener protects plumbing and appliances from scale while the RO provides purified drinking and cooking water at the kitchen tap. Done’s team can help you design the right system for your home’s water. Learn more on our water quality and treatment page, or explore our full range of plumbing services.