Reverse osmosis pushes water through a semi-permeable membrane under pressure, rejecting the large majority of dissolved salts. It is the workhorse technology for brackish-water treatment, boiler feed, beverage production and wastewater reuse.
An RO plant is much more than its membranes. Pretreatment, the high-pressure pump, instrumentation, the clean-in-place (CIP) system and post-treatment decide how long the membranes last and what the plant costs to run. This guide covers brackish and fresh-water RO; seawater systems are covered in the desalination guide.
What a complete RO plant includes
- Pretreatment
- Typically media filtration, dechlorination (activated carbon or sodium metabisulphite dosing), antiscalant dosing and cartridge filters, often rated at 5 µm, directly ahead of the high-pressure pump. UF is increasingly used for difficult feeds.
- High-pressure pump
- Usually a multistage centrifugal pump, often with a variable-frequency drive so pressure can be raised as membranes age or water temperature falls.
- Membrane array
- Spiral-wound elements, commonly 4-inch or 8-inch diameter, housed in pressure vessels and arranged in stages (for example a 2:1 array).
- Instrumentation and controls
- Flow, pressure and conductivity on feed, permeate and concentrate, with low- and high-pressure protection and automatic flushing on shutdown.
- Clean-in-place (CIP) system
- A tank, pump, heater and cartridge filter for chemically cleaning membranes in situ. Without one, fouled membranes have to be removed or replaced.
- Post-treatment
- pH correction, remineralisation, UV disinfection or polishing, depending on what the permeate is used for.
Specifications to compare before you request quotes
- Permeate flow at your temperature
- Membrane output falls as water gets colder. Ask what temperature the design is based on (25 °C is common in brochures) and what the plant produces at your lowest expected feed temperature.
- Recovery
- The share of feed water that becomes permeate. Brackish systems commonly run somewhere between 50% and 80%, limited by scaling risk. Higher recovery means less waste water but less margin against scale.
- Permeate quality guarantee
- Guaranteed permeate TDS or conductivity at stated conditions, and whether that guarantee holds for aged membranes or only new ones.
- Membrane projection
- A credible supplier runs the membrane manufacturer's design software for your water and can share the report: number of elements, array, flux, feed pressure and scaling indices on the concentrate.
- Design flux
- Permeate flow per unit of membrane area. Membrane manufacturers publish conservative flux guidelines by feed source; surface water and wastewater need lower flux than well water to slow fouling.
- Materials of construction
- Low-pressure pipework is usually PVC-U; the high-pressure side is normally stainless steel. Check pump wetted materials against the chloride level of your feed.
- Specific energy
- kWh per m³ of permeate. It lets you compare pump efficiency and design pressure between offers.
- Concentrate volume and quality
- How much reject water the plant produces and its expected composition, since it has to go somewhere legally.
Pretreatment decides membrane life
Most RO problems come from fouling and scaling, and both are pretreatment problems. Membrane manufacturers typically require a feed Silt Density Index (SDI) below 5 and prefer below 3. Ask suppliers how their pretreatment will achieve that on your source, especially if it is surface water.
Thin-film composite polyamide membranes, the most common type, are damaged by free chlorine. If your feed is chlorinated, dechlorination is not optional, and it is worth asking how the design protects the membranes if dechlorination fails.
Information to send suppliers with your enquiry
- A full water analysis including silica, barium, strontium, iron, manganese, alkalinity, pH, TOC and turbidity or SDI. Barium, strontium and silica often set the recovery limit.
- Minimum and maximum feed water temperature.
- Required permeate flow and quality, and what the permeate is used for.
- Where the concentrate will be discharged and any limits that apply there.
- Operating hours and whether you need redundancy, for example two trains at 50% each.
- Power supply, available space and site climate.
Common sourcing mistakes
- Comparing quotes designed at different recoveries or temperatures.
- Treating RO as a fix for poor pretreatment, then replacing membranes every year.
- Buying without a CIP system to save capital cost.
- Discovering after installation that the concentrate cannot be discharged where planned.