What Is Reverse Osmosis? How RO Water Filters Work

How Reverse Osmosis Works

Ordinary osmosis moves water across a membrane toward the side with a higher concentration of dissolved material. Reverse osmosis applies pressure in the opposite direction. Water molecules pass through the membrane while much of the dissolved material remains in the concentrate stream.

The membrane does not work alone. A common home system includes:

The number of stages is not a quality score. A well-tested three-stage system may be more credible than a “seven-stage” product with no accessible performance report.

What Does an RO Filter Remove?

There is no honest universal percentage table for all RO products. Performance changes with the membrane, pressure, temperature, incoming concentration, recovery rate, maintenance and the substance being measured.

NSF/ANSI 58 includes required testing for total-dissolved-solids reduction and optional claims for specified substances. A certification only supports the claims in the exact model’s listing.

• Find the exact brand and model in the certifier’s live database.
• Check the individual reduction claims, not just the standard logo.
• Confirm rated daily production, efficiency and recovery.
• Read the conditions and replacement capacity attached to each claim.
• Ask for a current report when a seller makes an additional claim.

RO is commonly paired with carbon because the technologies do different jobs. The membrane addresses dissolved material; carbon is used for substances and taste compounds it is designed to adsorb. Neither should be assumed to remove “everything”.

Why RO Makes Sense in a Malaysian Home

Malaysia’s treated supply is monitored, but the water reaching a kitchen has also travelled through distribution pipes, a building or household storage tank and internal plumbing. Conditions differ between neighbourhoods, buildings and properties. RO gives the household a final, controlled purification barrier at the point where drinking water is used.

For local context, Air Selangor reported 99.93% compliance with Malaysia’s Ministry of Health drinking-water standards in 2024 for its service area. That figure should not be stretched into a claim about every utility, private source, building tank or internal pipe in Malaysia.

RO is especially compelling for households that want:

• A broad treatment foundation rather than a filter aimed mainly at sediment, taste or odour.
• Lower and more consistent dissolved-solids output despite differences in incoming water.
• A system whose exact contaminant-reduction performance can be tested and documented.
• Purification combined with convenient hot, chilled, ambient or sparkling dispensing.

Do not use a purifier to hide a persistent colour, smell or supply problem. Follow the current notice from your water utility or building management and investigate the cause. If testing is justified, use an appropriate laboratory and select treatment for the result.

The Reject-Water Trade-off

RO creates reject water as part of the process. The amount varies widely by design and operating conditions. The US EPA says a typical point-of-use system can reject five gallons or more for every gallon treated; a WaterSense-labelled system must reject no more than 2.3 gallons per gallon treated and meet independent performance criteria.

Three figures make comparisons clearer:

• Efficiency rating: the percentage of incoming water that becomes available as treated water.
• Recovery rating: a related measure of feed water recovered under specified conditions.
• Waste-to-treated ratio: litres rejected for each litre available to drink.

The rated figure only means something alongside its test conditions. A five-litre source tank and one-litre reject tank do not, by themselves, prove a 4:1 recovery ratio across every cycle.

Countertop systems may collect concentrate in a removable tank, while installed systems often send it to a drain. Some designs recirculate source water. Each format therefore has a different reject-water route and cleaning routine.

Does Removing Minerals Make RO Water Unhealthy?

RO reduces calcium and magnesium together with many of the dissolved substances it is designed to remove. Minerals already dissolved in water can be absorbed by the body, so the right argument is not that they are unusable. The important point is that the amount varies widely by water source and is usually smaller and less dependable than the amount supplied by food.

WHO’s hardness review says food typically provides more than 80% of calcium and magnesium intake, while water commonly contributes about 5–20%. Hard, mineral-rich water can contribute more; soft water may contribute very little. A varied diet is therefore the more reliable way to meet mineral needs.

This is not a simple choice between “minerals” and “clean water”. Carbon and ion-exchange media can reduce selected contaminants, including some PFAS, while ultrafiltration can address particles such as microplastics. The difference is breadth: RO provides a tighter all-round treatment foundation. Velta prioritises that purification performance; essential minerals are obtained far more meaningfully and consistently through a varied diet than through drinking water.

See the worked food-and-water comparison in Does RO Water Remove Important Minerals?.

Why RO Is the More Involved Purification Option

RO can reasonably be positioned as a premium treatment platform because its dense membrane, pressure control, pre-treatment, post-treatment and reject-water management require more engineering than a simple taste-and-odour cartridge. It normally costs more to build, install and maintain.

That investment can provide lower-TDS output and a broad set of potential reduction claims. EPA notes that point-of-use RO can potentially reduce substances including lead, PFAS, arsenic, some volatile organic compounds and microorganisms. But “potentially” matters: the exact model, complete treatment train, certification, source water and maintenance determine performance.

Maintenance Matters

An RO system’s performance depends on maintenance, and the exact model’s schedule matters more than a generic internet estimate. Service can include:

• Replacing sediment and carbon cartridges.
• Replacing the RO membrane at rated life or when validated indicators show decline.
• Sanitising tanks, tubing, faucets and other wetted parts as directed.
• Checking for leaks and drain restrictions.
• Verifying output with the method specified by the manufacturer.

A TDS display is useful for tracking changes in dissolved solids. It does not identify individual contaminants or prove microbiological safety.

RO vs Carbon Filtration

Carbon remains useful inside an RO system because it protects the membrane and refines taste. Used alone, however, it is not the same broad dissolved-substance barrier. For households seeking the most comprehensive all-round option in this comparison, RO is the stronger choice.