About Desalination — Reverse Osmosis Simulator
This simulation models reverse osmosis (RO) desalination — the process by which pressurized saline water is forced through a semi-permeable membrane that blocks dissolved salts while allowing water molecules to pass. The model calculates osmotic pressure using the Van't Hoff equation (pi = iMRT), net driving pressure, permeate flux, salt rejection, and specific energy consumption (SEC in kWh/m³) in real time as you adjust operating conditions.
Reverse osmosis is the dominant technology for large-scale seawater desalination today, supplying fresh water to hundreds of millions of people worldwide — from the megaplants of Saudi Arabia and the UAE to smaller brackish-water systems used in agriculture and municipal supply across arid regions.
Frequently Asked Questions
What is reverse osmosis and how does it differ from natural osmosis?
In natural osmosis, water moves spontaneously across a semi-permeable membrane from a region of low salt concentration toward a region of high concentration until osmotic pressure equilibrium is reached. Reverse osmosis applies external pressure greater than the osmotic pressure on the saline side, forcing water molecules to travel in the opposite direction — from the salty feed water through the membrane and out as purified permeate. The membrane rejects dissolved salts, typically achieving 99% or higher salt rejection in modern RO systems.
How do I use this simulation and what do the controls do?
Select the feed water type (seawater at 35 g/L or brackish water at 2 g/L), then adjust operating pressure (ΔP), membrane area, recovery rate, and membrane permeability using the sliders. The RO module diagram updates live to show feed, reject, and permeate salinity. The energy chart plots specific energy consumption (SEC) against operating pressure so you can find the optimal operating point, shown as an orange dot. The green dashed line marks the thermodynamic minimum SEC — the theoretical lower bound no real system can beat.
What is osmotic pressure and why does it set the minimum operating pressure?
Osmotic pressure (pi) is the pressure difference required to prevent water flow across a membrane separating solutions of different concentration. For seawater at 35 g/L, osmotic pressure is approximately 27 bar; for brackish water at 2 g/L it is roughly 1.5 bar. An RO system must apply operating pressure greater than the osmotic pressure to achieve any net permeate flow — operating exactly at osmotic pressure produces zero flux. Real seawater RO plants typically operate between 55 and 80 bar to overcome concentration polarization and maintain commercially useful flux rates.
What equations govern permeate flux and salt rejection in this model?
Permeate flux (Jw) follows the solution-diffusion model: Jw = Am × (ΔP − Δpi), where Am is the membrane water permeability coefficient (L/m²/h/bar), ΔP is the applied pressure, and Δpi is the average osmotic pressure difference accounting for concentration polarization. Salt passage is modeled with a salt permeability coefficient B (fixed at 0.05 L/m²/h), a concentration polarization factor of 1.2, giving Cp = B × Cf × CP / (Jw + B). Salt rejection R = (1 − Cp/Cf) × 100%. As flux increases with higher pressure, salt rejection improves because the faster water flow dilutes salt diffusing through the membrane.
How does specific energy consumption (SEC) relate to operating pressure and recovery?
SEC (kWh per cubic metre of permeate) is the key economic metric for desalination. It is calculated as SEC = ΔP × Qf / (eta × Qp), where Qf is feed flow, Qp is permeate flow, and eta is pump efficiency (75% in this model). Higher operating pressure increases SEC because more energy is consumed per unit of feed; however, very low pressure near osmotic pressure also increases SEC because flux collapses and recovery falls. The optimum pressure for seawater RO is typically around 55–65 bar. The thermodynamic minimum SEC = pi_feed × ln(1/(1−r)) and represents the irreducible energy cost of separating the saline solution at a given recovery rate, set by the second law of thermodynamics.
What is concentration polarization and why does it matter?
Concentration polarization occurs when rejected salts accumulate in a thin boundary layer on the high-pressure side of the membrane surface, raising local osmotic pressure above the bulk feed value. This effectively reduces net driving pressure and can cause scaling or membrane fouling over time. In this simulation a concentration polarization factor of 1.2 is applied — meaning the membrane surface sees 20% higher salt concentration than the bulk feed. In real systems, turbulence promoters, spacers, and cross-flow velocity are engineered to minimise this effect. Severe concentration polarization is a major challenge in high-recovery brackish water systems.
When was reverse osmosis desalination first developed and commercialised?
The semi-permeable membrane effect was understood from the 18th century, but practical RO membranes were first demonstrated by Sidney Loeb and Srinivasa Sourirajan at UCLA in 1959–1960, who developed the first asymmetric cellulose acetate membrane capable of rejecting salt at useful flux rates. The first commercial seawater RO plant opened in Jeddah, Saudi Arabia in 1978. Thin-film composite (TFC) polyamide membranes, introduced in the 1980s and still dominant today, dramatically improved permeability and salt rejection over cellulose acetate. Modern plants like the Sorek B facility in Israel (2023) produce over 200 million m³ per year at SEC approaching 3 kWh/m³.
What other phenomena and simulations are closely related to RO desalination?
RO desalination connects to several areas modelled elsewhere on this site. Fluid dynamics governs cross-flow behaviour and pressure drop along the membrane module. Heat transfer is relevant to thermal desalination alternatives such as multi-stage flash (MSF) and multi-effect distillation (MED), which dominated before RO became cost-competitive. Membrane filtration (ultrafiltration, nanofiltration) operates on the same solution-diffusion principle at lower pressures and is widely used as RO pretreatment. Climate tipping points are linked to desalination because freshwater scarcity, driven by changing precipitation and glacial retreat, is one of the strongest pressures driving expansion of desalination capacity globally.
How is RO desalination used in engineering and industry today?
RO desalination produced roughly 65% of all desalinated water worldwide as of 2024, with global installed capacity exceeding 100 million m³/day. Beyond municipal water supply, RO is used in semiconductor fabrication (ultrapure water), pharmaceutical manufacturing, food and beverage processing, boiler feedwater treatment, and offshore oil platforms. Energy recovery devices (ERDs) such as pressure exchangers recover kinetic energy from the high-pressure reject stream, reducing net SEC from around 8–10 kWh/m³ in 1990s plants to 2.5–3.5 kWh/m³ in modern facilities. Brackish water RO systems treating groundwater can achieve SEC below 1 kWh/m³.
Is it true that desalination can solve the global freshwater crisis?
Desalination can significantly augment freshwater supply in coastal and arid regions, but it is not a universal solution. Its main limitations are energy cost (still several times higher than conventional water treatment), brine disposal (the concentrated reject stream must be managed to avoid harming marine ecosystems), and geography — landlocked regions or high-altitude areas cannot easily benefit from seawater RO. The technology works best as one component of a diversified water portfolio alongside conservation, wastewater recycling, and demand management. Solar-powered RO, currently under active development, could address the energy challenge in sun-rich regions and potentially reduce the carbon footprint of desalination substantially.
What are the current research frontiers in RO membrane technology?
Active research areas include aquaporin-based biomimetic membranes that mimic the ultra-high water permeability of biological cell channels, graphene oxide and carbon nanotube membranes with sub-nanometre pore precision, and advanced TFC membranes with nanoparticle additives (zeolites, MOFs) to enhance both flux and rejection simultaneously. Researchers are also working on closed-circuit RO and batch RO configurations that operate closer to thermodynamic reversibility, potentially reducing SEC to near the theoretical minimum. Anti-fouling surface chemistry to extend membrane lifetime and reduce cleaning chemical consumption is another major priority, since membrane replacement accounts for a significant fraction of plant operating costs.