Membrane technologies
UF, NF and RO are the same principle at three scales. Understanding what each one retains — and what it does not — is what makes choosing between them an engineering decision rather than a preference.
One family, three very different jobs
All three are barriers. The differences are scale and driving force.
Ultrafiltration, nanofiltration and reverse osmosis are the same idea at three different scales: a pressure differential drives water through a semi-permeable membrane, and whatever is too large to pass is retained. UF retains the largest contaminants, NF the intermediate, RO the dissolved salt.
Because the separation is by size and charge rather than by a phase change, membrane processes do not consume water to work. Their cost is pressure — the energy the feed requires to overcome osmotic pressure and the resistance of the membrane itself — and their performance depends on how clean the feed is kept.
That last point is what links them: they are usually not alternatives to each other but successive barriers in one train. UF protects RO. RO produces the low-salinity water. A cartridge filter protects both.
Ultrafiltration (UF)
UF is a coarse barrier with a genuinely tight pore size, typically in the tens-of-nanometres range. It removes suspended solids, colloids, most microorganisms and large organics. Because those particles are orders of magnitude larger than dissolved salt, UF does not desalinate — the salt passes straight through and the permeate salinity is essentially the feed salinity.
Its value is protection and definition. UF converts an ambiguous feed into a feed with a measurable integrity test, which is what allows an RO stage downstream to be designed against a real feed specification rather than an assumption.
UF is sized on flux: the membrane area is the design flow divided by a design flux, and both are engineer-supplied. EngiMetric calculates that area; it does not choose the flux for you, because flux is a membrane-product and feed-quality decision.
Nanofiltration (NF)
NF sits between UF and RO. Its pores are small enough to retain divalent ions and larger organics while passing much of the monovalent salt, so it removes hardness — calcium, magnesium, sulfate — more effectively than it removes sodium and chloride.
That selectivity is what distinguishes it. NF is the process for softening and sulfate reduction without the full desalination footprint of RO, and it operates at substantially lower pressure because the osmotic pressure it has to overcome is far lower.
The same size-based logic applies: EngiMetric sizes NF membrane area from stated flux and flow, and states the divalent rejection rather than assuming a catalogue value.
Reverse osmosis (RO)
RO is the barrier that removes dissolved salt. It operates against a much higher osmotic pressure than UF or NF, which is why it is the process that consumes real energy and why the pressure is one of the primary design variables rather than a fixed property.
RO is also the only one of the three where the platform performs a full salt balance. Because it rejects the majority of the feed salt, the concentrate is a genuine stream with its own flow and its own quality — and both are derived from conservation rather than stated.
The membrane area follows the same flux relationship as UF and NF, but the constraints around it are much tighter: the recovery you may run is limited by concentrate stability, and the product quality is a consequence of the rejection rather than a target you can specify independently.
See a membrane barrier worked through: the UF + RO worked engineering example follows ultrafiltration as a quantitative pretreatment stage — its 95% filtrate recovery, the 116.4 m³/day solids loss, and the continuity that makes it the RO stage's feed.
How they differ, side by side
The three differ along three axes at once. Scale determines what is retained: suspended solids for UF, divalent ions for NF, dissolved salt for RO. Driving pressure follows: low for UF, moderate for NF, and high enough to overcome seawater osmotic pressure for RO. And consequence follows from that pressure — UF and NF are low-energy barriers used for protection and specific-ion removal, while RO is the energy-intensive step that actually desalts.
This is why they are not competing alternatives for the same job. Choosing between them means deciding which separation the water actually needs. If the objective is a defined, low-turbidity feed for a downstream stage, that is UF. If it is hardness reduction at moderate pressure, that is NF. If it is fresh water from seawater, that is RO.
Where each one sits in a treatment train
In practice the barriers appear in sequence, each protecting the next and removing something the next cannot tolerate. A typical arrangement is coarse screening and media or cartridge filtration to control solids, ultrafiltration where a defined feed specification is needed, then reverse osmosis for the desalination itself, followed by post-treatment for stability and re-mineralisation.
The Plant Designer models this as a treatment train rather than as separate calculators. You add the barriers that match your source, and the platform sizes the areas and duties each one implies, then balances the feed forward through them. UF and RO use separate bases for exactly this reason — a UF flux is not an RO flux, and borrowing one for the other would produce a plant that cannot be built.
Membrane technologies in the library
Each entry states its own modelling level. Anything EngiMetric does not calculate is marked, rather than implied to be engineered.
- Media Filtration Quantitatively modelled
Granular-bed filtration that removes suspended solids ahead of a membrane, sized here from design flow, filtration rate and bed geometry.
- Feedwater Screening & Treatment Selection Quantitatively modelled
Compares a stated or measured feedwater analysis against the barriers a desalination pretreatment train can actually contain, raising a candidate only from a fact you supplied or the platform derived — and reporting its own confidence rather than choosing for you.
- Ultrafiltration Quantitatively modelled
Low-pressure membrane barrier that removes colloids, bacteria and suspended solids ahead of RO or NF, quantified by membrane area, TMP and flux condition.
- Nanofiltration Quantitatively modelled
Loose-rejection membrane separation between UF and RO, used for hardness reduction and partial demineralisation before an RO stage.
- Ion Exchange Quantitatively modelled
Exchanger-based demineralisation and softening, sized by resin volume, bed area, service flow and regenerant demand.
- Cartridge Filtration Reference only
Disposable or washable depth filters that protect downstream membranes from particles, ahead of any high-pressure stage.
Membrane calculators
Each tool calculates one thing and shows its working. They are the right next step once the principle makes sense.
- RO Recovery Calculator
Calculate RO permeate and concentrate flow from feed flow and system recovery rate.
- RO Salt Rejection Calculator
Calculate membrane salt rejection percentage from feed and permeate TDS concentrations.
- RO Permeate TDS Calculator
Project expected permeate total dissolved solids based on feed water salinity and rejection specs.
- RO Osmotic Pressure Calculator
Estimate RO feed osmotic pressure using a simplified van’t Hoff model from TDS, temperature, and solute properties.
- RO Design Studio
Build a preliminary RO system design with feed inputs, staging, custom membrane data, sequential element estimates, balances, and warnings.
- Filter Sizing Calculator
Size a conventional rapid-gravity multimedia filter from design flow, filtration rate, bed depth and media effective size: required filter area, design area after the engineer's safety factor, and media volume. The unit count needs a stated catalogue unit area, and no clean-water head loss is invented.
- Cartridge Filter Sizing Calculator
Size cartridge and bag filter elements and housings from design flow, feed solids and the manufacturer's element flow rating: hourly solids load, element count and housing count. No element size is assumed and no differential pressure is invented.
- Ultrafiltration (UF) Membrane Engineering
Size UF membrane area from permeate flow and design flux, compute transmembrane pressure from the feed/filtrate/concentrate pressures, and read normalised flux against the reference condition to expose fouling.
- Nanofiltration (NF) Membrane Engineering
Close the NF water and solute balance from a stated recovery and rejection to get permeate and concentrate flow and TDS, then size membrane area from the permeate flux.
- RO Feed Pressure & Specific Energy Calculator
Estimate required RO feed pressure (permeate osmotic gap + net driving pressure) and specific energy consumption with optional energy recovery.
- RO Membrane & Vessel Element Count Calculator
Calculate number of RO membrane elements and pressure vessels from permeate flow, design flux, and element area.
Demo cases with membrane barriers
Each case opens in the Plant Designer as a working, editable design. They are synthetic demonstration bases — not project data.
- Seawater — open intake, single RO stage Quantitative case
- Brackish groundwater — single RO, higher recovery Quantitative case
- Seawater — UF pre-treatment ahead of RO Quantitative case
- Two-Stage SWRO — staged array Topology only
- UF Prologue + Brackish RO Quantitative case
Put it into a plant
Membrane barriers are placed in the treatment train in the Plant Designer, and each carries its own sizing basis. Ultrafiltration and reverse osmosis are sized separately and then balanced forward, so the feed reaching the RO stage is a derived quantity rather than a stated one.