Pressure must overcome the osmotic difference
Reverse osmosis uses a selectively permeable membrane to produce a lower-salinity permeate and a more concentrated reject stream. Applied pressure must overcome the opposing osmotic pressure difference and hydraulic losses. Treating RO as a fine sieve is misleading: transport through the selective layer and interactions with different solutes influence rejection.
Seawater, brackish groundwater and treated wastewater impose different salinity, fouling and contaminant challenges. Do not transfer a recovery, pressure or pretreatment arrangement from one feed to another without assessing the new conditions. Temperature, membrane condition and the target product quality also affect performance.
Keep recovery, rejection and flux separate
f, p and c denote feed, permeate and concentrate. Q is flow; C is concentration. Jw is water flux per membrane area, and A is water permeability in units consistent with pressure and flux.
Recovery describes the fraction of feed water recovered as permeate. Rejection describes the reduction in a specified solute concentration across the membrane system under defined conditions. High salt rejection does not imply high recovery, and neither number directly describes the rejection of every trace organic contaminant.
The simplified flux relation uses an effective osmotic-pressure difference. Concentration at the membrane surface can be higher than in the bulk feed because of concentration polarisation. Fouling, pressure loss and changing salinity through the array further complicate the calculation. Use the relationship to understand trends, then use validated design and pilot methods for sizing.
Work a complete mass balance
These steady-state equations neglect accumulation and other inflows or losses. Use mass rather than volume concentration if density differences are material.
For a simplified example, feed flow is 100 m³/hour at 35 kg/m³ dissolved salt. Permeate flow is 45 m³/hour at 0.20 kg/m³. Recovery is 45%, and concentrate flow is 55 m³/hour. The feed carries 3,500 kg/hour of salt, while permeate carries 9 kg/hour. The concentrate therefore carries 3,491 kg/hour, giving approximately 63.47 kg/m³ on this volume basis.
Observed salt rejection is 1 − 0.20/35, or about 99.43%. Notice that a high rejection accompanies a concentrated waste stream. For rigorous seawater design, account for density, individual ions and any precipitation or chemical additions rather than relying solely on this simplified teaching balance.
Stages and passes solve different problems
In a staged array, concentrate from one stage feeds the next stage. This can increase overall recovery while managing flow through successive membrane elements. Salinity and scaling potential rise along the concentrate path, and hydraulic conditions must remain acceptable.
In a two-pass arrangement, permeate from the first pass feeds another RO pass for additional purification. The second pass may target a particular product-quality requirement. Its recovery and reject handling affect the whole-plant balance. A system can have multiple stages within a pass and more than one pass, so these terms should not be used interchangeably.
If two consecutive processes each recover 90% of their feed and there is no recycle, the combined recovery is 81%, not 90% and not 180%. Recycle changes the equations and requires a clear boundary around the complete system.
Pretreatment protects dependable output
| Mechanism | Engineering response to investigate |
|---|---|
| Particulate or colloidal fouling | Intake quality, clarification or membrane pretreatment, filtration and verified feed-quality indicators. |
| Scaling | Concentration at the intended recovery, relevant ion chemistry, precipitation potential and compatible control methods. |
| Organic or biological fouling | Source conditions, pretreatment effectiveness, cleaning strategy and suitable monitoring. |
| Membrane chemical damage | Oxidant exposure, material compatibility, chemical dosing and upset response. |
Track normalised performance so temperature and feed changes do not masquerade as irreversible fouling. Cleaning frequency, chemical compatibility, element replacement and unit isolation affect availability. A high nominal production rate is of limited value if adverse intake conditions repeatedly force shutdown. Include power, pretreatment and residuals systems in the availability assessment.
Energy is more than a membrane-pressure number
Use pressure in Pa and flow in m³/s to obtain watts. Define which pump and stream the calculation represents.
At 60 bar and 100 m³/hour, gross hydraulic power is about 166.7 kW. At 85% pump efficiency, the simplified electrical input is about 196.1 kW. Dividing by 45 m³/hour of product gives 4.36 kWh/m³ for this deliberately simplified pumping calculation. It excludes energy recovery and other plant loads and is not a benchmark for a modern desalination plant.
An energy-recovery device can recover pressure energy from concentrate, materially changing the balance. Intake pumping, pretreatment, post-treatment and transfer to the network add loads. Compare options at the same delivery point and product quality, with consistent assumptions about power prices, operating hours and emissions.
Finish the product and manage the concentrate
Low-mineral permeate generally needs stabilisation and an appropriate disinfection strategy before entering a drinking-water system. Assess alkalinity, pH, corrosivity, blending and compatibility with existing materials and residual disinfectants. Meeting a salinity objective does not establish that the product is stable in the network.
Concentrate disposal depends on the receiving environment and approvals. Marine discharge requires site-specific assessment of dispersion, salinity, chemicals and ecological effects. Inland concentrate can be especially difficult to manage. These are core feasibility questions, not minor details to resolve after choosing membrane capacity.
Use desalination as a system option
Compare dependable output, activation time, minimum production constraints, maintenance, intake vulnerability, transfer capacity and energy dependence. A climate-independent feed does not create an infrastructure-independent supply. Consider common power or transfer failures with other sources.
For planning, retain the feed envelope, net recovery, product specification, whole-plant energy boundary, residuals route and operating assumptions. Continue to portfolio resilience and the role of RO in water reuse.
Sources & further reading
- Australian Drinking Water Guidelines ↗NHMRC and NRMMC · Living guidelines; publication and draft status checked 25 September 2026
External source · Checked 25 September 2026 - Australian Guidelines for Water Recycling ↗Australian Government Water Quality portal · Phase 1, 2006; drinking-water augmentation, 2008
External source · Checked 25 September 2026
Source findings are distinguished from editorial interpretation. Apply current local criteria and project evidence when making engineering decisions.