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Nov 30, 2018·9 min read
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Commercial Reverse OsmosisResidential Water Filtration SystemsReverse OsmosisWater Treatment

How Does Reverse Osmosis Work In The Water Purification Process?

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Sammy FaragAMPAC USA — November 30, 2018
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Quick Answer: Reverse osmosis works by applying hydraulic pressure (40-80 PSI for home systems, 800-1200 PSI for seawater) to overcome osmotic pressure and force water molecules through a semi-permeable polyamide membrane with pores of approximately 0.0001 microns — too small for dissolved salts, heavy metals, bacteria, and most contaminants to pass through. Clean water (permeate) passes through; concentrated contaminants are rejected in a concentrate stream.

One of the many questions you may have asked yourself over the years is how does reverse osmosis fit into the realm of water purification? We hear a lot on the word especially its abbreviation RO. The filter sells itself if you attach the word RO to it. Keeping the branding aspect of it aside, we decided to answer some questions like why people tend to trust more on the word, what is the role of reverse osmosis in the water purification process.

To answer the first question, millions of people in the U.S. trust reverse osmosis for their water purification needs because:

  • It is effective and safe
  • Provides amazing taste
  • Comes at a reasonable price
  • And the systems are fully automated.

And the way reverse osmosis in itself fits into all of the water solutions is actually pretty interesting.

Reverse osmosis is a process when tap water is pushed through a semi-permeable membrane by inducing water pressure and what comes out in the other side is water without contaminants.

How does it work?
Water from the tap when pushed through a semi-permeable membrane, leaves a trail of contaminants on the first side of it. This membrane is customized in each filter model based on the water quality of the source. The membrane is specially designed so that molecules other than water are not able to pass through. This brings us an unadulterated water output on the other side.

 

What about the contaminants that are removed?
RO is part of an elaborate 3 stage process on an average. The first part, which can be further detailed based on the water quality, is responsible to subsequently remove bigger contaminant molecules from the water. Impurities like sand particles, dirt, leaves, stones etc. are filtered out in the pre-filtration stage. At times water softeners are used to dissolve hard chemicals like lead and potassium that can spoil the membrane. The contaminants that remain back in the membrane after the final stage, are removed during scheduled maintenance dates which are usually 6 months apart.

 

What are the basic components of an RO system?
It basically comprises of 3 stages that are elaborated up to 7 stages in case of bad water quality. But the main are pre-filter stage, reverse osmosis and post filter stage.

  • Processes in the pre-filter stage, which can be more than 2 even, are meant to remove contaminants before the actual filtration. This is to ensure that the membrane does not suffer a setback by generating a mountain of contaminants blocking the first side. To save the quality of the membrane and the effectiveness of the RO, certain chemicals and physical impurities are eliminated beforehand in this pre-filter stage.
  • In the RO stage, the actual filtration takes place. Pre-filtered or softened water is passed through the semi-permeable membrane by applying pressure and what comes out the other side is absolutely pure drinking water.
  • The last stage which can be up to 2 processes, is meant for the water taste. Minerals are added using mineralizing filters to offer health benefits and improve the taste, the odor of the water.

What could be the best option for an RO filter?
based on our extensive 28-year research, we feel you must first know what is your water quality index. It determines the number of impurities in every unit of water. But our best sellers over the years have been 3 and 4 Stage Reverse Osmosis Systems. Many folks also prefer the Under-sink water filters which are not visible immediately to maintain the interiors of their houses. One of the most popular choices is also our wide range of counter top filters.

What is the typical lifespan of an RO membrane?

High-quality RO membranes last 2–5 years depending on feed water quality and maintenance frequency. AMPAC USA systems use thin-film composite (TFC) membranes rated for extended service life. Regular pre-filter replacement and periodic membrane cleaning significantly extend operational longevity.

How much water does an RO system waste?

Standard RO systems recover 50–75% of feed water as permeate (purified output), with the remainder discharged as concentrate. AMPAC USA's high-recovery commercial systems achieve up to 85% recovery using energy recovery devices and optimized flow design, reducing operational costs substantially.

What pressure is required for a reverse osmosis system?

Brackish water RO systems typically operate at 150–600 PSI, while seawater systems require 800–1,200 PSI. AMPAC USA designs each system to match source water salinity and desired flow rate, incorporating energy-efficient high-pressure pumps with variable frequency drives (VFDs) to minimize power consumption.

Can reverse osmosis remove viruses and bacteria?

Yes. RO membranes provide absolute removal of bacteria (>99.9999%) and viruses (>99.99%), making them one of the most effective water purification technologies. AMPAC USA systems exceed NSF/ANSI 58 standards and include pre-treatment stages to protect membrane integrity.

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Conclusion

This post explored how reverse osmosis technology delivers high-purity water across a wide range of residential, commercial, and industrial applications. For businesses and organizations requiring reliable RO purification, AMPAC USA engineers custom systems tailored to your specific water quality requirements and flow demands. Contact our team at info@ampac1.com or (909) 548-4900 to discuss your water treatment needs.

The Science of Reverse Osmosis: How Membranes Purify Water

Reverse osmosis is based on the principle of osmosis — a natural process where water moves across a semi-permeable membrane from a region of lower solute concentration to higher concentration, attempting to equalize concentrations on both sides. In reverse osmosis, applied hydraulic pressure overcomes this natural osmotic drive, forcing water molecules through the membrane against the concentration gradient from the concentrated feed side to the dilute permeate side. The “reverse” in reverse osmosis refers to this reversal of the natural osmotic flow direction.

The key component is the thin-film composite (TFC) polyamide membrane, consisting of three layers: a top aromatic polyamide selective layer approximately 0.2 microns thick that performs the actual separation; a microporous polysulfone support layer approximately 40 microns thick providing mechanical strength; and a polyester non-woven backing for structural support. The polyamide layer has an effective pore size of approximately 0.0001 microns (0.1 nanometers) — small enough to reject hydrated ions (sodium and chloride ions are approximately 0.2-0.4 nm in hydrated form), heavy metal cations, and essentially all dissolved organic molecules above molecular weight 150-200 Daltons. Water molecules (0.3 nm, uncharged, able to diffuse through the polymer matrix) pass through via a solution-diffusion mechanism.

Membrane performance is characterized by two key metrics: salt rejection (the percentage of total dissolved solids rejected, typically 95-99%+ for quality TFC membranes) and permeate flux (the volume of water produced per unit membrane area per unit time, expressed as liters per square meter per hour or GFD — gallons per square foot per day). Both metrics are temperature-dependent: lower water temperatures reduce both flux and rejection, while higher temperatures increase flux but may slightly reduce rejection. AMPAC USA system designs account for worst-case winter temperature feed water conditions to ensure adequate production throughout the year.

Frequently Asked Questions

Q: What is the difference between osmosis and reverse osmosis?

A: Osmosis is the natural movement of water through a semi-permeable membrane from low-concentration to high-concentration (equalizing concentrations). Reverse osmosis applies hydraulic pressure to overcome osmotic pressure and force water from high-concentration through the membrane to low-concentration — opposite to the natural direction.

Q: What is osmotic pressure and how does it relate to RO systems?

A: Osmotic pressure is the pressure required to stop osmotic flow across a semi-permeable membrane. For fresh water (500 mg/L TDS), osmotic pressure is approximately 5-10 PSI. For brackish water (5,000 mg/L TDS), approximately 50-80 PSI. For seawater (35,000 mg/L TDS), approximately 395 PSI. RO operating pressure must exceed osmotic pressure to produce permeate.

Q: What is salt rejection in an RO system?

A: Salt rejection is the percentage of total dissolved solids (TDS) that are rejected by the RO membrane. Quality residential RO membranes achieve 96-99% rejection. A system with 97% rejection treating 500 mg/L feed water produces permeate with approximately 15 mg/L TDS.

Q: How do I know if my RO membrane is working properly?

A: Monitor product water TDS with an inexpensive TDS meter. Compare to feed water TDS — rejection should be 90%+ for a residential system. A new membrane typically achieves 96-99% rejection; when rejection falls below 85-90%, consider membrane replacement or inspection for damage.

Q: Why does an RO system have a drain line?

A: As feed water is concentrated by the RO membrane (rejected ions accumulate on the feed side), a concentrate stream carrying rejected contaminants at 3-4x feed water concentration must be continuously flushed to drain. This prevents scale formation on the membrane surface and maintains performance.

Q: Can bacteria grow in an RO system?

A: Bacteria can colonize pre-filter housings, storage tanks, and faucets if systems are not maintained properly. The RO membrane itself effectively rejects bacteria, but post-membrane biofilm in storage tanks and faucet lines is possible. Annual sanitization of the entire system prevents this and is included in standard maintenance recommendations.

Q: Does temperature affect RO system performance?

A: Yes significantly. Cold water (10 degrees C / 50 degrees F) reduces permeate flux by 30-50% compared to room temperature water. RO systems sized for warm climates may need booster pumps in cold-climate installations to maintain adequate production in winter. Temperature also slightly affects membrane rejection rates.

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