Reverse osmosis is a membrane desalination process in which separation takes place at the level of ions and molecules. Under a pressure exceeding the osmotic one, water is transported through the non-porous active layer of the membrane, while dissolved salts remain in the stream that is rejected. Neither mechanical filtration nor ion exchange achieves that separation.
Principle
Feed water is delivered by a high-pressure pump into the membrane housings and travels along the surface of the membrane elements. Part of the flow passes through the membrane and goes to the user: this stream is called the permeate. The remainder continues along the sheet, taking up the rejected salts, and is discharged to drain as the concentrate. Separation is therefore continuous and produces two streams at once — unlike filtration, where what is retained accumulates within the media and is removed by backwashing.
The driving force of the process is the applied pressure less the osmotic pressure of the solution. Osmotic pressure is proportional to salinity and amounts to roughly 0.7 bar for every thousand milligrams of salts per litre, so the pump has to exceed it with a margin: at insufficient pressure permeate flow stops altogether. Hence the division of the catalogue into series that differ not in output but in design salinity: on fresh water the working pressure is of the order of 10–13 bar, on brackish water it reaches 20–21 bar, on seawater 60 bar. Housings, pipework and pump are designed for their own pressure, and a unit of one series is not a substitute for another.
Cross-flow velocity along the membrane is a parameter of the operating regime in its own right, not a consequence of output. A layer of raised salt concentration forms at the membrane surface: water leaves through the sheet while dissolved matter stays behind. This is known as concentration polarisation; it locally increases osmotic pressure, reduces permeate flow and accelerates scaling. The longitudinal concentrate flow continuously sweeps that layer away, which is why concentrate flow is set by calculation rather than by a wish to save water.
The output of membrane elements depends on feed temperature through viscosity: cold water passes through the active layer less readily. The correction is about 3 % per degree of deviation from the reference 25 °C, so at a winter 8 °C the flow is roughly half of the summer value. A unit sized on summer water without a temperature margin will not reach its design flow in winter, and the shortfall has to be covered by the tank or by the duty cycle.
Rejection is defined as the share of salts retained in the concentrate. What matters is the base it is counted from: the share that passes through the membrane is taken from the feed salinity, so at the same rejection saline water yields more salts in the permeate than fresh water does. For that reason brackish and seawater series are built with more selective elements, and where one stage is not enough a two-stage scheme is used: the permeate of the first stage feeds the second, and total demineralisation reaches 99.9 %.
Principle
The semi-permeable membrane of a reverse osmosis unit is not a sieve. Its active layer, some tenths of a micrometre thick, has no through pores: transport is described by the solution-diffusion model, in which water dissolves into the dense polymer, diffuses through it along a gradient of chemical potential and desorbs on the far side. Separation is governed not by the geometry of an opening but by differences in solubility and mobility within the polymer. The practical rule follows directly: the behaviour of an impurity cannot be predicted from its size — an analysis and a rejection table are required.
Particle size does not decide the outcome. The hydrated sodium ion, one of the smallest in solution, is rejected by 98 % and more: it carries a charge and holds a shell of water molecules around it, which makes entering a non-polar polymer energetically unfavourable. A carbon dioxide molecule is smaller yet electrically neutral, and therefore passes through the active layer almost unimpeded. The same explains the weak rejection of boric acid and of a number of low-molecular uncharged organics.
This property has a direct consequence for the scheme, and one that surfaces only after commissioning. Carbon dioxide passes into the permeate while the bicarbonate ion that was in equilibrium with it stays behind the membrane. The equilibrium shifts, the pH falls, and the permeate turns out markedly more acidic than the feed and aggressive towards carbon steel and concrete. This is corrected by degassing or by alkali dosing downstream of the membrane, but the decision belongs to the design stage: replacing pipework destroyed within the first season costs more than the correction unit.
Practice
Reverse osmosis is used where dissolved matter has to be removed. Suspended solids, colloids, iron and manganese are retained more cheaply and at earlier stages; they reach the membrane solely as a cause of its premature failure. The economics are not in the membrane's favour here: a set of elements is comparable in cost to the pretreatment itself.
The depth of desalination is set by the unit series and the number of stages. A single stage provides demineralisation of the order of 98–99 %, a two-stage scheme up to 99.9 %. The required quality is stated not as "clean water" but as limits on salinity, conductivity and individual parameters, because those are what determine the series and the number of stages.
The membrane retains micro-organisms, yet the permeate cannot be regarded as biologically stable. It is nearly free of salts and therefore weakly buffered, and the surfaces of the tank and the pipework downstream are colonised by biofilm faster than with raw water. For that reason the line after the unit is covered by ultraviolet disinfection and provision is made for sanitising the tank.
Where the water requirements exceed what a single-stage membrane scheme delivers, electrodeionization follows as the next stage. It works specifically on permeate: raw water at full salinity drives it out of its operating regime, so the order of stages here does not tolerate rearrangement.
An industrial unit is supplied skid-mounted and comprises membrane housings with elements, a high-pressure pump, cartridge prefiltration, instrumentation and controls. The site has to provide a feed line of the design flow, a concentrate discharge line, power supply and a controlled room temperature: freezing damages membrane elements irreversibly.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
Reverse osmosis is not applied as a stage in its own right. Mechanical treatment ahead of it is mandatory, and the rest of the pretreatment follows from the analysis: iron removal and aeration where iron and manganese are present, sorption or reducing-agent dosing where residual chlorine is, softening or antiscalant dosing to prevent carbonate and sulphate scaling.
Requirements for the water entering the membrane apparatus are stated numerically: a silt density index of no more than 5, iron and manganese within hundredths of a milligram per litre, no free chlorine. Those figures determine the pretreatment train, not a general impression of the source.
A storage tank is provided downstream: the membrane apparatus runs in cycles by level and does not deliver instantaneous draw-off. The tank is covered by ultraviolet disinfection, and where the permeate is aggressive a pH correction or mineralisation unit is introduced, depending on the intended use of the water.
The concentrate line is treated as part of the process scheme rather than as an auxiliary drain. Its flow and salinity are calculated, and the discharge point and limits are agreed with the water utility or the local regulator; coastal sites may carry additional requirements.
Scheme
Operation
Energy
Specific energy consumption is governed by working pressure and therefore by feed salinity. For fresh water it amounts to some 0.3–0.8 kWh per cubic metre of permeate, for brackish water 0.8–2 kWh, for seawater 3–5 kWh even with energy recovery devices. The remaining operating costs — chemicals, cartridges, servicing — usually total less than the drive of the high-pressure pump, which makes the operating regime as economically significant as the price of the unit.
Consumables
What goes to drain
The concentrate holds all the rejected salts in a reduced volume of water, so its salinity is several times that of the feed. At 50 % recovery it roughly doubles; at 75 % it increases fourfold. On fresh water a quarter to a half of the feed goes to drain, on brackish and seawater the share is higher. Discharge is agreed in advance: the limits are set by the water utility or the local regulator and differ from site to site. In individual cases the concentrate is subject to further treatment or evaporation, and that is a separate item of the project.
Recovery
Recovery is the share of feed water delivered to the user as permeate. For the units in the catalogue it reaches 75 % on fresh water and 65 % on brackish water. The figure is not chosen freely: its upper bound follows from the saturation indices of sparingly soluble salts in the concentrate, and exceeding it produces scaling on the last elements along the flow path. Flow has to be calculated at the inlet rather than at the permeate: at 50 % recovery every cubic metre of treated water requires two cubic metres of feed, and that requirement falls on the source, not on the unit.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
On the basis of flow alone only the series and an order of cost can be named. The series is determined by feed salinity rather than by output: at the same permeate flow, a fresh-water unit and a brackish-water unit differ in the number of membrane elements, the pump and the working pressure. The analysis also determines the pretreatment train, on which membrane life depends directly, so selecting without a report shifts the risk to the operating stage.
On fresh water a quarter to a half of the feed goes to drain; on brackish and seawater the share is higher. That water is not lost through carelessness: the rejected salts leave with it, and reducing its share means raising the concentration in the concentrate path. The limit follows from the saturation indices of sparingly soluble salts; beyond it scaling begins, and higher recovery is paid for with unscheduled cleaning and shortened element life.
With working pretreatment and an observed flushing schedule the service life is 3–5 years. It is determined by operating conditions rather than by the calendar: free chlorine, iron, elevated recovery and long shutdowns with concentrate left in the housings cut it to months. The approach of the end of life is visible in advance from the rising differential pressure and from permeate conductivity at an unchanged regime.
Yes, and this is one of the principal uses of the technology, though water leaving the membrane requires finishing. Permeate is weakly mineralised and practically without buffering capacity, so for drinking use mineralisation and pH correction are provided, and the line after the unit is covered by ultraviolet disinfection: in water free of salts biofilm develops faster.
Sodium-form ion exchange reduces hardness by replacing calcium and magnesium with sodium; total salinity is retained and the sodium figure rises somewhat. Reverse osmosis reduces salinity as a whole, sodium, nitrates and sulphates included, but requires pretreatment, electrical energy and a concentrate discharge. If the task is limited to scale and no sodium limit applies, softening is the more economical answer; with brackish water or a nitrate exceedance it does not address the task at all.
No. The series differ not in a margin of strength but in design pressure and configuration: the housings, valves and pump of a seawater unit are rated for 60 bar, whereas on fresh water the working pressure is 10–13 bar. Operating such equipment on fresh water means running away from the design point, with excess energy consumption and unjustified capital cost; margin is built in through output and duty cycle, not by changing the series.
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