Reverse Osmosis Process Explained: How It Purifies Water

RO unit performance depends on controlling three major problems: scaling (precipitation when dissolved solids concentrate beyond saturation), fouling (plugging from suspended solids, colloids, microorganisms, or sticky polymer reactions), and chemical attack (oxidation creating membrane holes). Fouling is the largest industry problem, often caused by interactions between cationic coagulants and anionic scale inhibitors forming sticky polymers. Fouling control requires balancing flux rate (permeate per membrane area per day) and cross flow rate—higher flux pushes more materials against membranes while higher cross flow washes them away. Chemical attack prevention involves removing oxidants through activated carbon, sulfite injection, or UV light. Understanding these challenges is essential for maintaining RO system efficiency and membrane lifespan.

Proper pretreatment is essential for protecting reverse osmosis membranes from fouling and scaling; the three main pretreatment methods include filtration (sand or multimedia filters for removing total suspended solids), iron/manganese removal (using oxidation and catalytic media like Birm or Phlox), and softening (ion exchange using resin to remove calcium and magnesium); each method requires specific dimensioning based on water quality parameters such as contaminant concentrations, pH, alkalinity, and flow rates to ensure optimal RO system performance and longevity.

Fouling in RO membranes refers to the deposition of unwanted materials on the membrane surface, which reduces water flow and membrane efficiency. There are four main types of fouling: biological fouling (caused by bacteria and microorganisms), particulate fouling (caused by suspended particles), organic fouling (caused by organic compounds like alcohols and sugars), and inorganic fouling or scaling (caused by mineral deposits like calcium and magnesium carbonates). Scaling specifically occurs when inorganic compounds precipitate out of solution and deposit on the membrane surface, forming hard layers that significantly reduce membrane performance. Prevention strategies include maintaining proper pH levels (6.5-7.5), water softening to remove minerals, effective pre-treatment, and using appropriate membrane materials.

Fouling and scaling are major challenges in reverse osmosis membrane filtration, where fouling refers to the accumulation, deposition, or adsorption of foulants (organic matter, microorganisms, or colloidal particles) onto membrane surfaces or within pores, while scaling involves inorganic crystallization coatings formed when dissolved salt solubility limits are exceeded; both phenomena reduce permeate flow, increase pressure drop, and decrease membrane efficiency, and can be minimized through proper membrane selection based on water analysis, appropriate operating conditions, pre-treatment systems, and regular CIP (clean-in-place) cleaning protocols.

RO systems require proper pre-treatment to extend module life and improve performance, focusing on preventing fouling, controlling scaling, and inhibiting membrane degradation. Standard feed water quality parameters include SDI (3-5, optimal at 3), MFI (around 4), TOC (around 3 mg/L), and COD (around 10 mg/L). Water classification depends on TDS and TOC levels: low TDS/TOC water (desalination, ultra-pure water) has minimal organic content and salinity, while high TDS/TOC water (seawater, leachate) has higher concentrations. Seawater typically has pH 8.1, TDS ~35,000 mg/L. Scaling occurs when concentrate stream concentration exceeds saturation limits. CSM Design Guide provides scaling limits: LSI ≤1.5, Calcium Carbonate ~230 mg/L, Calcium Sulfate ~6,600 mg/L. Scale inhibitors include SHMP, organophosphates, and polyacrylates. Acid injection controls calcium carbonate scaling by dissolving CaCO3 through calcium ion reactions with acid. LSI should be negative for effective scale control.

Fish kidneys and red blood cells contain aquaporin pores that are perfectly shaped and charged to allow water molecules to pass through easily while blocking other substances. Danish company Aquaporin has mimicked this natural membrane structure to create desalination membranes. Unlike energy-intensive reverse osmosis (which pushes water against membranes), Aquaporin membranes use forward osmosis, pulling water through with a fraction of the energy required. These membranes are approximately 100 times more permeable than conventional desalination membranes used in large-scale plants, representing a major advancement in sustainable water treatment technology.

Forward osmosis relies on thin-film composite membranes (100 nm polyamide active layer) achieving 99.5% salt rejection through solution diffusion. Unlike reverse osmosis membranes designed for hydraulic pressure, these membranes enable natural osmotic flow without external pressure. Concentration polarization reduces effective driving force, but advanced designs achieve 20 GFD versus 1 GFD in conventional systems. The technology excels at treating extreme industrial waters like fracking wastewater (2-3x seawater salinity with sand, bacteria, oil) producing water cleaner than municipal tap. Non-pressurized designs use inexpensive plastics, eliminating costly pressure vessels while maintaining self-cleaning properties as foulants simply slough away.

This video introduces aquaporin water filtration technology, a revolutionary breakthrough that harnesses Nobel prize-winning biomimicry. The system uses aquaporin proteins—found in all living cells including humans, animals, and plants—to filter water. These proteins naturally transport only water while blocking bacteria, viruses, and contaminants. After over a decade of research, the technology incorporates these proteins into a stable membrane that allows only clean water to pass through. One gram of aquaporin can filter up to 700 liters per second, making it far more efficient than man-made filters. The system removes heavy metals, PFAS, nitrates, and sulfates while preserving taste. The A2O Pure model features three filters (pre-filter, activated carbon, and aquaporin membrane), leak detection, automatic flushing, and space-saving design. The technology was tested to its absolute limits, including space testing, demonstrating its reliability and durability.

This presentation by Rong Wang from Singapore Membrane Technology Center (SMTc) at Nanyang Technological University demonstrates how membrane technology is being advanced for water reuse applications, including forward osmosis membranes for beverage concentration, biomimetic membranes incorporating aquaporin proteins to enhance water permeability while reducing operating pressure, and low-pressure nanofiltration membranes achieving 90% water recovery in wastewater treatment. The research emphasizes the critical importance of scaling up laboratory discoveries to higher technology readiness levels and commercializing advanced membrane solutions to address global water scarcity challenges.

The commercial viability of forward osmosis required breakthroughs in membrane science and chemistry that were impossible just 30 years ago. Key advancements include: (1) Thinning the membrane support layer from RO-like thicknesses to approximately 1/5th, dramatically reducing hydraulic resistance and enabling practical flux rates of 28-32 LMH; (2) Incorporating aquaporin proteins—biological water channels from plants—that create pinhole-sized pores allowing only water passage while rejecting all contaminants, achieving unprecedented selectivity; (3) Developing closed-loop draw solution chemistry using trimethylamine bicarbonate, which is large enough to be retained by the membrane yet provides sufficient osmotic driving force. These innovations transformed FO from laboratory curiosity to industrial reality, enabling systems that can achieve water recovery rates of 95% while consuming only 27-43 kWh/m³ of energy (potentially halved when utilizing waste heat). The combination of these technological advances created a viable alternative to thermal evaporators for treating challenging industrial wastewaters.

Zero Liquid Discharge (ZLD) systems enable industrial water circularity by recovering up to 98% of water from wastewater through integrated technologies including reverse osmosis, two-stage evaporation, and decanter centrifugation, converting the remaining concentrated pollutants into reusable salt crystals while returning clean water to industrial processes.

Zero Liquid Discharge (ZLD) represents a leading environmental technology guaranteeing water body restoration. Industrial ZLD implementation involves membrane bioreactors for pre-treatment, reverse osmosis for concentration, and crystallization for final separation. Modern systems achieve evaporation with only 10-30 kWh per cubic meter, compared to 680 kWh for direct evaporation. ZLD enables recovery of valuable elements (lithium, magnesium, potassium) from wastewater, transforming waste management into resource recovery. The automotive painting industry demonstrates ZLD effectiveness, achieving 95% water reuse with only 5% loss through evaporation and sludge formation. The Interpipe steel plant in Dnipro operates without discharging any wastewater to the Dnieper River. A Dnipro thermal power plant achieved payback within 11 months by collecting and treating industrial and stormwater for reuse. The UN's ESG framework (Environment, Social, Governance) provides comprehensive water management approach integrating environmental protection with social responsibility. ISO 14001 certification and EU Green Deal requirements drive systematic environmental management. The Rhine River restoration demonstrates that strict regulatory enforcement can achieve complete ecosystem recovery within 10 years. Israel's approach involves treating municipal wastewater through membrane bioreactors, then recharging treated water into aquifers. Reverse osmosis systems producing 45 cubic meters per hour of fully desalinated water can serve 400-450 people, operating 20 hours daily to provide clean water for 9,000 people.

Zero Liquid Discharge (ZLD) is a sustainable water treatment technology that eliminates liquid water waste discharge from industrial processes by recovering virtually all water through pre-treatment, membrane separation (reverse osmosis, nanofiltration, ultrafiltration), and thermal evaporation/crystallization processes, enabling water reuse, resource recovery of salts and minerals, and environmental protection.

Zero Liquid Discharge (ZLD) systems minimize or eliminate liquid discharge in industrial processes by combining evaporation and crystallization technologies; these systems recover water for reuse, reduce waste disposal costs and environmental impact, and recover valuable solids from wastewater streams, with Alfa Laval's plate evaporation technology featuring highly efficient corrugated plate heat exchangers that generate turbulence to enhance heat transfer and reduce fouling, followed by decanter centrifuges that separate solids from the concentrated brine to produce dry, reusable salt products.