Quick answer: An industrial RO system pushes water through a semi-permeable membrane to remove dissolved salts and most contaminants. Recovery is typically 50 to 75 percent on a single pass, and the rest leaves as concentrate. Most RO failures are pre-treatment failures: the membrane is only as good as what reaches it.
Specification
| Parameter | Typical | Notes |
|---|
| Principle | Reverse osmosis across a semi-permeable membrane | Removes dissolved salts, not just suspended solids |
| Recovery | Typically 50–75% on a single pass | The remainder leaves as concentrate and must be dealt with |
| Pre-treatment | Mandatory — multimedia filtration, softening or antiscalant, carbon or dechlorination | Skipping it is the single most common cause of early membrane failure |
| Chlorine | Must be removed before the membrane | Free chlorine destroys polyamide membranes permanently |
| Feed pressure | Provided by a high-pressure pump sized to feed salinity | Brackish and seawater duty differ substantially |
| Membrane life | Shortened sharply by fouling and scaling | Track normalised permeate flow and differential pressure, not raw readings |
| Cleaning | Periodic CIP, acid for scale and alkaline for organics | Build the CIP skid in from the start, not after the first failure |
| Configuration | Single pass, double pass, or with UF pre-treatment | Double pass where permeate quality must be very high |
Where RO fits, and the mistake to avoid
RO sits at the polishing end of a process chain, whether the goal is process water, boiler feed, or reusing treated effluent. It does not replace the stages before it. A membrane presented with suspended solids, oil, iron, hardness or free chlorine will foul, scale or oxidise, and the plant that looked cheap at purchase becomes a membrane replacement subscription.
The practical rule: decide the pre-treatment from a full water analysis before choosing the RO itself. Hardness calls for softening or antiscalant, iron and manganese call for oxidation and filtration, organics call for carbon, chlorine must be removed entirely, and any suspended solids must be taken out upstream — often by ultrafiltration when the feed is treated effluent rather than well water.
Water reuse and zero liquid discharge
Reuse projects usually pair ultrafiltration with RO: UF removes the suspended and colloidal load so the RO sees a clean feed, and the RO removes dissolved salts so the permeate can go back into the process. What remains is the concentrate, and how you handle it decides whether the project works economically.
Zero liquid discharge takes this further — the concentrate is evaporated and crystallised so that only solids leave the site. ZLD is technically achievable and energy-intensive; it is normally driven by a discharge permit rather than by economics. Where it is required, the sludge and salt handling at the end of the chain deserves as much attention as the RO itself.
Frequently asked questions
What pre-treatment does an RO system need?
It depends entirely on the feed water analysis. Typically: multimedia or ultrafiltration for suspended solids, oxidation and filtration for iron and manganese, softening or antiscalant dosing for hardness, activated carbon or bisulphite for chlorine removal, and pH adjustment. Free chlorine must be removed completely — it destroys polyamide membranes permanently, and that damage is not recoverable by cleaning.
What recovery rate should I expect?
Typically 50 to 75 percent on a single pass, meaning 50 to 75 percent of the feed becomes permeate and the rest leaves as concentrate. Pushing recovery higher raises scaling risk sharply because the concentrate becomes more saturated. Higher overall recovery normally comes from a second-stage RO on the concentrate, not from over-driving the first stage.
How long do RO membranes last?
It is governed by fouling, scaling and oxidation rather than by calendar age. Well pre-treated systems with disciplined cleaning run for years; poorly pre-treated ones can lose membranes in months. Track normalised permeate flow, salt rejection and differential pressure — normalised, because raw readings move with temperature and feed salinity and will mislead you.
Can RO treat my wastewater for reuse?
Often yes, but only after the wastewater has been treated conventionally first. The usual chain is biological treatment, then ultrafiltration, then RO. RO is a polishing step, not a wastewater treatment plant, and feeding raw or partly treated effluent to a membrane is the fastest way to destroy it.
What do I do with the concentrate?
This is the question that decides whether a reuse project is viable, and it is worth settling before buying the RO. Options are discharge within permit limits, further concentration with a second RO stage, evaporation, or full zero liquid discharge with crystallisation. Each step up costs significantly more energy.
What is the difference between single pass and double pass RO?
Single pass sends the feed through one RO stage. Double pass sends the permeate through a second RO, giving much higher purity for applications such as boiler feed, pharmaceutical or electronics process water. Double pass costs more in capital and energy and lowers overall recovery, so it is specified only where the quality requirement genuinely demands it.
How often does an RO system need cleaning?
When performance indicators say so, not on a fixed calendar. The usual triggers are a 10 to 15 percent fall in normalised permeate flow, a 10 to 15 percent rise in differential pressure, or a measurable fall in salt rejection. Acid cleaning addresses scale; alkaline cleaning addresses organics and biofouling.
What information do you need to quote a system?
A full feed water analysis, the required permeate quality and flow rate, the intended use (process, boiler feed, reuse or discharge), operating hours per day, and any local discharge limits on the concentrate. Without the water analysis any proposal is guesswork, and pre-treatment is where most of the cost difference lies.
Related
Send your feed water analysis, required permeate quality and flow rate and we will propose the pre-treatment and the RO together: request a water treatment proposal.
