The Many Faces of Osmosis: From Forward to Reverse and Everything In Between

Reverse Osmosis (RO) is perhaps the best known?membrane filtration technology and a water purification process.? It removes contaminants, particles, and dissolved substances from water by forcing it through a semi-permeable membrane.?RO uses the smallest membrane pore size of any filtration technology (less than 0.0001 microns) and requires very high operating pressures of 10 - 90 bar. Its very effective at removing almost all dissolved salts and organic compounds. It is often used for boiler fees supply, waste water recycling and desalination, but what are the principles behind it and how does it differ to the offer types of osmosis technologies?

Unlike other types of membrane technology such as Ultra Filtration (UF) and Micro Filtration (MF), RO membranes operate exclusively as a cross cross flow system.?All of the feed water flows tangential (parallel) to the membrane surface and continues to concentrate solid particles over the membrane length. A proportion of the feed water passes through the membrane as permeate, leaving the progressively more concentrated brine / reject behind. This principle can be seen below.

Osmosis / Forward Osmosis / Reverse Osmosis

To understand reverse osmosis, its good to understand just what is it the reverse of. Basic Osmosis occurs naturally when two solutions with different solute concentrations are separated by a semi-permeable membrane, which allows only the fresh water to pass through. The water moves from the low concentration side to the concentrated side to balance the concentration on both sides. As the water flows into the more concentrated side, pressure builds up in that compartment, which is known as osmotic pressure. Osmosis is central to all life on Earth and some examples of this are as follows:

• Plant roots absorb water from the soil via osmosis, where soil water has a lower solute concentration compared to the plant's root cells.
• Freshwater Fish?live in a hypotonic environment, where the salt concentration is lower outside the cell than inside. They do not need to drink water, but do need to absorb salts through their gills. Water enters their bodies via osmosis, and they excrete dilute urine (with salt traces) to maintain balance and prevent their cells bursting.
• Saltwater Fish?live in a hypertonic environment, where the salt concentration is higher outside the cell than inside. They lose water via osmosis continuously and must drink seawater while excreting excess salts through specialised gills.
• Humans cannot drink seawater as our kidneys produce urine with a lower alt concentration than seawater (unlike salt water fish). This means your kidneys must use more water from your body to excrete the salt than the water you gained from drinking seawater leading to dehydration!

Forward Osmosis (Figure A) is an engineered version of naturally occurring osmosis, with higher salt concentrations provided on the draw side of the membrane to maximise the osmotic pressure and with no differential hydraulic pressure.?ΔP < Δπ

Reverse Osmosis (Figure B) is the process of overcoming this osmotic pressure (π) by applying additional hydraulic pressure (P) to the concentrated side to force water to the lower concentrated side of the membrane. This separates the contaminants from the fresh water and is an extremely valuable technique in water treatment. (P2 - P1) > Δπ

Flux / Osmotic Pressure / Mechanical Pressure Relationship

The driving force of all forms of osmosis is based around the differential between the osmotic pressure (Δπ) and the hydraulic pressure (ΔP) as measured across the membrane. Where?Δπ is larger Forward Osmosis shall prevail and where?ΔP is larger, Reverse Osmosis shall take place. The water permeability coefficient (Lp) is determined by the the membrane material, pore size, fouling & scaling and salinity of the feed. The higher the feed salinity the lower the coefficient of permeability.

Therefore the higher the recovery rate desired, the lower the coefficient and higher the osmotic pressure, resulting in more hydraulic pressure and energy. Higher recovery = Higher energy and cost. The relationship is as follows:

J = Lp x [(Δπ) ? (ΔP)]

Lp: Membrane water permeability coefficient (L/m2·h·bar)

J: Measured water flux (L/m2·h)

Δπ: Osmotic pressure difference across the membrane (bar)

ΔP: Applied hydraulic pressure difference (bar)

Applications for RO

The first RO membranes was developed in the 1950’s at the University of California and the first commercial RO plant was installed in Coalinga, California, in 1965. It was a small-scale plant designed to treat brackish water, providing potable water to the local community. Since these early days RO technology has spread around the world and its estimated that there are close to 20,000 desalination plants operating today.

A feed flow is presented to the membrane, a volume of liquid called permeate will pass through and what does not pass the membrane is called retentate or reject. The disposal of this liquid is a big challenge with RO as its essential a concentrated volume of the contaminants that are being excluded.

Desalination in not the only use for RO technology and a summary of applications and brine composition are as follows;

Desalination of Seawater and Brackish Water:

• Application: Production of drinking water by removing salts for municipal towns, cities as well as for offshore ships and oil rigs.
• Brine Composition: High salinity, with dissolved salts and potential pretreatment chemicals (e.g., anti scalants, biocides).
• Brine Disposal: Marine discharge is common but can seriously harm ecosystems unless ensuring dilution and dispersion to minimise environmental impacts. Inject brine into geologically suitable underground formations far from groundwater aquifers. Zero-Liquid Discharge (ZLD) with?thermal or evaporation systems to recover water and convert salts into solids.

Industrial Water Treatment:

• Application:?Production of high-purity water for a range of industrial applications where even trace amounts of minerals or impurities can affect processes such as boiler feed and cooling systems.
• Brine Composition: May contain process specific contaminants, such as heavy metals, chemicals, or organic compounds.
• Brine Disposal: Chemical Precipitation to remove harmful constituents like heavy metals before discharge. Repurpose / reuse the brine for cooling, cleaning, or other low-sensitivity industrial processes. Remaining concentrate often blended for sewer discharge.

Food and Beverage Production:

• Application:?RO is used to purify and concentrate liquids in food and beverage processing.?Used to concentrate juices, milk, and whey by removing water, enhancing flavors and nutritional content without using heat.
• Brine Composition: May include high organic content such as compounds, dissolved salts, sugars, and food-grade chemicals.
• Brine Disposal: Organic rich brines can be processed in an anaerobic reactor to generate biogas and reduce waste volume. Discharge brine to municipal wastewater treatment plants, if permissible. Extract valuable compounds (e.g., salt, sugar, or dairy solids) for reuse in production.

Pharmaceutical and Medical Applications:

• Application: RO produces ultra-pure water necessary for pharmaceuticals and medical processes where water purity is critical. RO is used in hospitals and clinics to provide safe, purified water for dialysis machines, removing harmful ions and bacteria.
• Brine Composition: Low salinity but often has high toxicity with trace amounts of chemicals or pharmaceutical residues.
• Brine Disposal: Depending on the brine’s quality, it can be treated as part of municipal wastewater. Neutralise any pharmaceutical contaminants chemically before disposal.

Wastewater Treatment and Reuse:

• Application:?RO is used to treat wastewater and enable water reuse by removing contaminants and reducing environmental impact.?Treats effluent from industries such as mining, textiles, and oil and gas, enabling safe discharge or reuse.?Produces reclaimed water for irrigation, landscaping, and non-potable uses in areas facing water scarcity.?RO is a key component in Zero Liquid Discharge (ZLD) processes, reducing wastewater volume to minimise disposal needs.
• Brine Composition: Highly variable, depending on the wastewater source, with potential organic and inorganic contaminant.
• Brine Disposal: For areas with high sunlight, evaporation systems can reduce brine volume effectively.

Residential and Commercial Water Purification:

• Application:? RO provides high-quality drinking water for households and businesses by removing contaminants, improving taste, and ensuring safety. Installed in restaurants, hotels, and offices for consistent, high-quality water that enhances taste and appearance in food and beverages.
• Brine Composition: Low salinity, with minimal additional contaminants.
• Brine Disposal: Small-scale brine from households and commercial buildings can typically be discharged into municipal wastewater systems. Use brine for non-potable purposes like toilet flushing or landscaping in small systems

Permeate / Water Quality

RO systems can be set up in different ways with stages and passes, which change how the water flows through membrane modules, impacting the system's efficiency, quality, and recovery rate. Understanding the distinction between these two configurations helps in designing and optimising RO systems for various applications such as desalination, water purification, and wastewater treatment.

RO Stages

Stages in RO systems focus on maximising total system recovery, using reject water from previous stages to generate more permeate.?This is particularly useful in desalination plants or zero-liquid discharge (ZLD) systems to minimise brine discharge. The risk of scaling increases with each stage as the concentration factor and additional pressure is needed to pass the reject through the second stage. As additional stages are added, the reject volume is reduced, but becomes more concentrated resulting in a more difficult disposal.?

The 2 stage RO system displayed below is a 2:1 array which means that the concentrate (or reject) of the first 2 RO modules is fed to the next module.

RO Passes

Passes in RO systems aim to improve water purity, with the permeate from one pass going through further filtration to meet strict quality standards. Multi-pass systems are ideal for industries that need water with very low TDS or chemical contaminants, such as pharmaceutical manufacturing or electronics fabrication.

Running a two pass RO system will lower the recovery rate, lower the concentration factor and therefore the risks of scaling.

What is Forward Osmosis (FO) for?

FO is an emerging technology for desalination and concentrating feed waters that leverages the principle of natural osmosis. FO does not require an external pressure and is therefore is very efficient in comparison to RO.?Due to the lower operating pressure requirements, FO membranes are much less prone to fouling than RO and this is a key advantage. This can be utilised by first being a pre treatment stage to an RO. The FO can also remove the organic and biological load in advance of the RO. These loads would foul an RO and need to be removed as pre treatment. The applications for FO are as follows:

• Dilution Mode: Water moves naturally from a low osmotic pressure feed solution to dilute a high osmotic pressure draw solution. This can be used to dilute a fruit concentrate to a bigger volume.

• Concentration Mode: Water moves naturally from a low osmotic pressure feed solution to concentrate it by removing water into a high osmotic-pressure draw solution. This can be used to concentrate drinks or to concentrate a waste stream for easier disposal.

This is very counter intuitive at first, but remember it is the opposite of what we strive to achieve with RO.

FO is an established technology with food and beverage applications and is now showing promise with applications requiring zero liquid discharge or with dealing with high?

Pressure Assisted Osmosis (PAO): Combines osmotic pressure gradients with moderate hydraulic pressure to enhance water flux across a membrane. Lower energy consumption compared to traditional Reverse Osmosis (RO) at high salinities.

Pressure-Retarded Osmosis (PRO): Harnesses osmotic pressure gradients between freshwater and saline water to generate hydraulic energy via water movement across a semi-permeable membrane.

Osmotically Assisted Reverse Osmosis (OARO)

OARO is a promising advanced membrane technology designed to handle extremely high salinity brines efficiently by combining principles of RO and osmotic pressure gradients. Unlike conventional RO, which relies solely on hydraulic pressure to overcome osmotic pressure, OARO strategically uses osmotic assistance to reduce the energy required for water transport across the membrane. This is achieved by lowering the osmotic gradient by recirculating a saline solution to the permeate side of the membrane. This allows OARO to operate at lower pressures than traditional RO, which is important for treating brines with high concentrations.

By balancing hydraulic pressure (ΔP) and osmotic pressure (Δπ), OARO achieves higher water recovery rates and can handle brine concentrations exceeding 200,000 ppm TDS.? This makes OARO particularly effective for applications such as Zero Liquid Discharge (ZLD) systems, industrial wastewater treatment, and desalination brine management. Its hybrid approach optimises water flux, minimises scaling risks, and offers a more energy-efficient solution for tackling the challenges of extremely saline streams.

Summary

Reverse osmosis technology is a key stone of water treatment, water recycling, water reuse and has a diverse range of applications. Effective pretreatment is crucial to prevent membrane fouling and scaling, which can impair system performance. This includes removing suspended solids, adjusting pH, and adding anti scalants.?Regular monitoring and maintenance are essential to manage fouling, scaling, and membrane degradation, ensuring consistent performance and extending membrane lifespan.

While RO provides access to clean water, the disposal of concentrated brine and energy consumption are environmental challenges that need to be addressed. New technologies of FO and OARO can operate as part of a larger process with RO to work towards a zero or minimal liquid discharge.

RO:

• Hydraulic pressure must exceed Δπ for positive water flux.
• Limited by hydraulic pressure constraints and scaling risks.
• Typical brine Total Dissolved Solids (TDS) concentration 75,000 mg/L.
• Effective at seawater desalination and Industrial process water purification.

FO:

• Water flux depends solely on Δπ (no applied hydraulic pressure unless PAO).
• Limited by osmotic gradient efficiency and draw solution regeneration.?Water flux reduces as osmotic gradients decrease at very high salinities.
• Typical draw solution TDS concentration >100,000 mg/L.
• Effective at concentrating highly saline water or extracting clean water from challenging feed streams.

OARO:

• Bridges the gap between RO and FO by combining hydraulic and osmotic driving forces to handle extremely saline streams efficiently.
• Precise management of osmotic gradients and hydraulic pressure. Membrane scaling risks at extreme concentrations.
• Typical brine TDS concentration >200,000 mg/L.
• Effective extreme salinity (near crystallisation limits) for challenging waste waters.