{"id":89059,"date":"2026-06-16T20:00:00","date_gmt":"2026-06-16T20:00:00","guid":{"rendered":"https:\/\/www.ampac1.com\/blog\/how-reverse-osmosis-works\/"},"modified":"2026-06-16T20:00:00","modified_gmt":"2026-06-16T20:00:00","slug":"how-reverse-osmosis-works","status":"publish","type":"post","link":"https:\/\/www.ampac1.com\/blog\/how-reverse-osmosis-works\/","title":{"rendered":"How Reverse Osmosis Works: The Complete Technical Guide | AMPAC USA"},"content":{"rendered":"

Reverse osmosis sounds complicated. The underlying principle isn’t. Water is pushed through a membrane so fine it passes water molecules and blocks nearly everything dissolved in them \u2014 salts, metals, nitrates, PFAS, bacteria. That’s it. What makes commercial and industrial RO systems complex is the engineering around that membrane: the pressure required to overcome osmotic pressure, the pre-treatment to protect the membrane, the recovery management to control waste, and the system controls that keep everything running reliably. This page explains how it all works.<\/p>\n

The Osmosis Problem (and How RO Solves It)<\/h2>\n

Osmosis is the natural movement of water through a semi-permeable membrane from a region of lower dissolved solids concentration to a region of higher concentration. Left alone, water moves in the wrong direction for purification \u2014 toward the contaminated side.<\/p>\n

Reverse osmosis applies pressure greater than the osmotic pressure of the feed water, forcing water to move against its natural direction \u2014 through the membrane and away from the dissolved contaminants. The pressure required depends on the TDS of the feed water:<\/p>\n\n\n\n\n\n\n\n\n
Feed Water Type<\/th>\nTypical TDS (ppm)<\/th>\nOsmotic Pressure (PSI)<\/th>\nRequired Operating Pressure<\/th>\n<\/tr>\n<\/thead>\n
Low-TDS municipal water<\/td>\n100\u2013300 ppm<\/td>\n5\u201315 PSI<\/td>\n50\u2013100 PSI<\/td>\n<\/tr>\n
Moderate-TDS municipal or well water<\/td>\n300\u20131,000 ppm<\/td>\n15\u201350 PSI<\/td>\n100\u2013200 PSI<\/td>\n<\/tr>\n
High-TDS brackish groundwater<\/td>\n1,000\u20135,000 ppm<\/td>\n50\u2013250 PSI<\/td>\n200\u2013400 PSI<\/td>\n<\/tr>\n
Seawater<\/td>\n30,000\u201345,000 ppm<\/td>\n350\u2013500 PSI<\/td>\n800\u20131,200 PSI<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Operating pressure must exceed osmotic pressure to produce flow. The margin above osmotic pressure \u2014 called net driving pressure (NDP) \u2014 determines production rate. Lower NDP means lower flow; higher NDP means higher flow up to the membrane’s rated maximum pressure.<\/p>\n

What the RO Membrane Actually Does<\/h2>\n

A commercial RO membrane is a thin-film composite (TFC) element consisting of three layers:<\/p>\n

    \n
  1. Polyester support web<\/strong> (~120 \u00b5m thick) \u2014 structural layer, provides mechanical support<\/li>\n
  2. Microporous polysulfone interlayer<\/strong> (~40 \u00b5m thick) \u2014 transition layer between support and active skin<\/li>\n
  3. Ultrathin polyamide active layer<\/strong> (~0.2 \u00b5m thick) \u2014 the functional rejection layer where separation occurs<\/li>\n<\/ol>\n

    The polyamide active layer is what separates water from contaminants. Water molecules (H\u2082O, 0.275 nm diameter) pass through at a molecular level via a solution-diffusion mechanism \u2014 they dissolve into the membrane material, diffuse through it, and emerge on the permeate side. Contaminants are rejected by a combination of size exclusion (molecules larger than the membrane pore size cannot pass) and charge repulsion (the polyamide surface carries a slight negative charge that repels dissolved anions).<\/p>\n

    This is why RO removes both uncharged molecules (like urea \u2014 too large) and ionic contaminants (like nitrate \u2014 charged repulsion plus size) with very different mechanisms, but high efficiency for both.<\/p>\n

    The Four Streams in an RO System<\/h2>\n\n\n\n\n\n\n\n\n
    Stream<\/th>\nAlso Called<\/th>\nDescription<\/th>\n<\/tr>\n<\/thead>\n
    Feed water<\/strong><\/td>\nInlet water<\/td>\nSource water entering the system after pre-treatment<\/td>\n<\/tr>\n
    Permeate<\/strong><\/td>\nProduct water, RO water<\/td>\nPurified water that has passed through the membrane \u2014 the output you use<\/td>\n<\/tr>\n
    Concentrate<\/strong><\/td>\nReject, brine<\/td>\nFeed water that did not pass the membrane; contains all rejected contaminants at elevated concentration<\/td>\n<\/tr>\n
    Bypass<\/strong><\/td>\nBlend stream<\/td>\nOptional: untreated feed water blended with permeate to achieve a target TDS (used in beverage and some process applications)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Every RO system produces both permeate and concentrate. Concentrate must be disposed of \u2014 typically to drain, sewer, or in some industrial applications, evaporation. The ratio of permeate to feed water is the recovery rate<\/strong>, a critical system design parameter covered in detail in RO Recovery Rate Explained<\/a>.<\/p>\n

    How a Commercial RO System Is Arranged<\/h2>\n

    A commercial RO system is not just a membrane in a housing. The full system typically includes:<\/p>\n

      \n
    1. Pre-treatment train<\/strong> \u2014 protects the membrane from fouling and damage. Minimum for municipal water: sediment pre-filter + carbon block. Well water typically requires additional pre-treatment for iron, hardness, and bacteria. See Commercial RO Buyer’s Guide<\/a> for pre-treatment requirements by feed water type.<\/li>\n
    2. High-pressure pump<\/strong> \u2014 raises feed water pressure to the operating level required to overcome osmotic pressure and produce flow. In commercial systems, this is typically a multi-stage centrifugal pump or a positive displacement pump rated for continuous-duty operation.<\/li>\n
    3. Pressure vessel(s)<\/strong> \u2014 fiberglass or stainless steel housings, each containing one or more spiral-wound membrane elements in series. Commercial systems may have multiple pressure vessels arranged in arrays.<\/li>\n
    4. Membrane elements<\/strong> \u2014 typically 4-inch (residential\/light commercial) or 8-inch (commercial\/industrial) spiral-wound TFC elements. Each element is a flat membrane sheet wound around a central permeate collection tube, with feed spacers between membrane layers to maintain flow channels.<\/li>\n
    5. Instrumentation<\/strong> \u2014 feed pressure, permeate pressure, concentrate pressure gauges; TDS monitors on feed and permeate; flow meters; optional: conductivity sensor with automatic divert to drain for out-of-spec water.<\/li>\n
    6. Control system<\/strong> \u2014 auto-shutoff on low feed pressure (pump protection), high-pressure shutoff, optional programmable auto-flush to clear concentrate from membrane surface during standby periods.<\/li>\n
    7. Post-treatment (application-dependent)<\/strong> \u2014 UV disinfection, remineralization, pH adjustment, storage tank, booster pump, or polishing stages (EDI, mixed-bed DI) for high-purity applications.<\/li>\n<\/ol>\n

      What RO Removes (and Doesn’t Remove)<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
      Contaminant<\/th>\nTypical RO Rejection<\/th>\nMechanism<\/th>\n<\/tr>\n<\/thead>\n
      Total dissolved solids (TDS)<\/td>\n90\u201399%<\/td>\nSize exclusion + charge repulsion<\/td>\n<\/tr>\n
      Calcium, magnesium (hardness)<\/td>\n95\u201399%<\/td>\nSize + charge<\/td>\n<\/tr>\n
      Nitrates (NO\u2083\u207b)<\/td>\n85\u201395%<\/td>\nCharge repulsion (monovalent anion, lowest rejection of common ions)<\/td>\n<\/tr>\n
      Arsenic (As(V))<\/td>\n92\u201396%<\/td>\nSize + charge (As(III) lower: 40\u201370% \u2014 oxidize to As(V) before RO)<\/td>\n<\/tr>\n
      Lead<\/td>\n95\u201399%<\/td>\nSize + charge<\/td>\n<\/tr>\n
      PFAS (PFOA, PFOS)<\/td>\n90\u201399%<\/td>\nSize exclusion (large molecules)<\/td>\n<\/tr>\n
      Fluoride<\/td>\n90\u201396%<\/td>\nSize + charge<\/td>\n<\/tr>\n
      Chromium-6 (Cr(VI))<\/td>\n92\u201397%<\/td>\nCharge repulsion<\/td>\n<\/tr>\n
      Bacteria<\/td>\n>99.9%<\/td>\nSize exclusion (cells far too large for membrane pores)<\/td>\n<\/tr>\n
      Viruses<\/td>\n99\u201399.9%<\/td>\nSize exclusion<\/td>\n<\/tr>\n
      Chlorine<\/td>\nLow (<50%)<\/td>\nCarbon pre-filter removes chlorine before membrane \u2014 not an RO function<\/td>\n<\/tr>\n
      Dissolved gases (CO\u2082, H\u2082S, radon)<\/td>\nLow<\/td>\nSmall uncharged molecules pass through membrane; degassing equipment required<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      Why Operating Conditions Change RO Performance<\/h2>\n

      RO system output and rejection are not fixed values \u2014 they change with operating conditions:<\/p>\n