Reverse Osmosis Technology: How RO Systems Operate and How to Address RO Membrane Fouling

Basic Principles and Advantages of RO
The reverse osmosis membrane serves as the core component enabling reverse osmosis technology. It is an engineered semi-permeable membrane with specific properties, manufactured from polymeric materials that mimic biological semi-permeable membrane structures.
Reverse osmosis, also known as reverse permeation, is a membrane separation process driven by pressure differences to separate solvent from aqueous solutions, thereby filtering impurities from water. It is termed “reverse” because it operates against the natural direction of osmosis.
The technical principle involves applying pressure to one side of the membrane at a level exceeding the solution's osmotic pressure. When pressure surpasses osmotic pressure, the solvent permeates in the opposite direction, separating substances from water. The solvent obtained on the low-pressure side of the membrane is called permeate; the concentrated solution obtained on the high-pressure side is called concentrate.

When treating seawater with reverse osmosis technology, freshwater is obtained on the low-pressure side of the membrane, and brine is obtained on the high-pressure side. This enables reverse osmosis pressure to achieve the purposes of separation, extraction, purification, and concentration.

Working Principle of Reverse Osmosis Systems
Reverse osmosis is a membrane separation water treatment technology employing cross-flow filtration as a physical method. Its advantages include:
- Low operating costs at ambient temperatures, utilizing water pressure as the driving force;
No discharge of large volumes of waste acid or alkali solutions, preventing environmental pollution;
Simple system design, easy operation, and high automation;
Wide adaptability to raw water quality with stable treated water quality;
Compact equipment footprint and minimal maintenance requirements.

Basic RO Water Treatment Processes
First, Single-Stage Single-Pass Treatment. After entering the membrane module, the liquid is separated into purified water and concentrate. Compared to other RO processes, this method offers a simpler overall workflow and easier operation. However, it has significant limitations and cannot meet higher water quality requirements.
Second, Single-Stage Multi-Pass Treatment. Building upon the single-stage single-pass process, this method involves multiple concentration steps. Compared to the single-stage process, this method is more complex but can meet higher water quality requirements and enable water resource recycling.
Third, Two-Stage Single-Pass Process. When the single-stage method struggles to achieve practical water quality requirements, the two-stage single-pass process can be employed. Compared to the two single-stage processes mentioned above, using the two-stage single-pass process extends the service life of reverse osmosis membranes, requires less manual operation, and reduces corresponding treatment costs.

Applications of RO in Water Treatment
Advanced Municipal Wastewater Treatment
In advanced municipal wastewater treatment, reverse osmosis technology enhances wastewater recovery rates and finds widespread application.
The effectiveness of deep wastewater treatment varies depending on the material of the reverse osmosis membrane. Typically, in urban wastewater treatment, after municipal domestic sewage meets discharge standards, higher-quality treated water is required (e.g., for reclaimed water reuse). In such cases, cellulose triacetate hollow fiber membranes and spiral-wound polyvinyl alcohol composite membranes demonstrate superior performance.
Compared to other membrane materials, these two types achieve 100% retention of fecal coliform bacteria, maintain color below 1 degree, and produce permeate with total dissolved solids (TDS) between 1 mg/L and 2 mg/L. Additionally, they exhibit higher water flux and superior fouling resistance.

Industrial Wastewater Treatment
1) Heavy Metal Ion Treatment
Applying reverse osmosis water treatment technology to industrial wastewater demonstrates excellent results, aligning with the principles of industrial economic rationality in overall design. It reduces energy consumption, operational costs, and operational management complexity.
Reverse osmosis units for industrial wastewater treatment typically employ internal pressure tubular or spiral-wound modules, maintaining a stable operating pressure around 2.8 MPa. They exhibit exceptional performance in heavy metal ion recovery. Specifically, reverse osmosis systems based on internal pressure tubular components operate at a stable pressure of 2.7 MPa. Under these conditions, nickel recovery exceeds 99%, with separation rates ranging from 97.12% to 97.17%.

2) Treatment of Oily Wastewater
Typically, oils in wastewater exist in three primary forms: emulsified oil, dispersed oil, and floating oil. Compared to these, dispersed oil and floating oil are relatively easier to treat. Mechanical separation, sedimentation, and activated carbon adsorption can significantly reduce their concentrations. However, emulsified oil contains organic matter that acts as a surfactant. Oil particles typically exist at the micrometer scale, granting them exceptional stability and making effective, rapid oil-water separation challenging.
Reverse osmosis (RO) water treatment technology enables concentration and separation without disrupting the emulsion. Subsequently, the concentrate undergoes incineration, while the permeate is recycled or discharged.
Currently, in oil-containing wastewater treatment, RO technology is commonly combined with other methods to optimize final treatment outcomes and effluent quality. For instance, a proprietary DEMUL-B1 demulsifier is used to break the emulsion in high-concentration O/W spinning oil wastewater. The demulsified water is then further treated using OSMONICS SE reverse osmosis membranes. Results demonstrate that the “demulsification-reverse osmosis” process achieves a COD removal rate of 99.96%, with oil content becoming virtually undetectable.

3) Desalination of Brackish Water
During brackish water desalination, incorporating reverse osmosis technology effectively suppresses inorganic salt ions like magnesium and calcium, enhancing purified water quality.
Current demands for purified water quality exceed what traditional methods (adding scale inhibitors to brackish water) can deliver, making reverse osmosis an essential solution.
When operating reverse osmosis units for brackish water desalination, it is essential to regularly test the SDI index, strictly control recovery rates, monitor pressure differentials between membrane modules, and continuously measure changes in product water yield and salt rejection rates. In practice, the salt rejection rate of reverse osmosis units remains stable above 96%, with the desalinated water meeting China's drinking water standards.

Addressing RO Membrane Fouling
Membrane fouling refers to irreversible changes where particles, colloidal particles, or solute macromolecules in the feed solution interact physically or chemically with the membrane, or where concentration polarization causes certain solutes to exceed their solubility limits at the membrane surface. This leads to adsorption or deposition on the membrane surface or within pores, reducing pore size or causing blockage, resulting in a significant decline in membrane flux and separation performance.

Microbial Fouling
1) Causes
Microbial fouling refers to the accumulation of microorganisms at the membrane-water interface, impairing system performance.
These microorganisms use the reverse osmosis membrane as a substrate and proliferate using nutrients from the concentrate stream. They form a biofilm layer on the membrane surface, causing a rapid increase in the pressure difference between the feed and permeate streams, a sharp decline in permeate flow rate and salt rejection, and contamination of the product water.
The microbial biofilm can degrade membrane polymers or other reverse osmosis components either directly (via enzymatic action) or indirectly (through localized pH or redox potential effects). This shortens membrane lifespan, compromises membrane structural integrity, and may cause significant system failure.
2) Control Methods
Biological fouling can be controlled through continuous or intermittent disinfection of the feedwater. For raw water sourced from surface or shallow groundwater, a disinfection dosing system should be installed to add chlorine-based disinfectants. The dosage is typically determined by maintaining a residual chlorine level in the feedwater >1 mg/L.

Chemical Contamination
1) Causes
Common chemical contamination involves carbonate scale deposition within membrane elements, often resulting from operational errors, inadequate scale inhibitor dosing systems, or interrupted inhibitor dosing during operation. If undetected, within days this leads to increased operating pressure, elevated pressure differentials, and reduced water production rates. Incompatible scale inhibitors or insufficient dosing can also cause membrane fouling. Mild fouling may be reversed through chemical cleaning, but severe cases may render heavily contaminated membrane elements unusable.
2) Control Methods
To prevent membrane scaling, first select a reverse osmosis scale inhibitor suitable for the system's feedwater quality and determine the optimal dosage. Second, enhance monitoring of the dosing system, closely tracking subtle changes in operating parameters and promptly investigating any anomalies. Additionally, elevated Fe³⁺ levels in water are often introduced through piping systems. Therefore, utilize steel-lined plastic piping for all system lines, including feedwater lines, to minimize Fe³⁺ content.

Suspended Particles and Colloidal Contamination
1) Causes
Suspended particles and colloids are the primary substances causing fouling in reverse osmosis membranes and exceeding the SDI (Silt Density Index) in treated water.
The composition of suspended particles and colloids varies significantly depending on the water source and geographical location. Typically, uncontaminated surface water and shallow groundwater primarily contain: bacteria, clay, colloidal silica, iron oxides, humic acid products, and excessively dosed flocculants or coagulants (such as iron salts, aluminum salts, etc.) from pretreatment systems.
Additionally, positively charged polymers in raw water can form precipitates by binding with negatively charged scale inhibitors in reverse osmosis systems, contributing to this type of contamination.

2) Control Methods
When raw water suspended solids exceed 70 mg/L, pretreatment typically involves coagulation, clarification, and filtration. For raw water suspended solids below 70 mg/L, coagulation-filtration pretreatment is generally used. When suspended solids in raw water are <10 mg/L, direct filtration is typically used as the pretreatment method.
Moreover, microfiltration or ultrafiltration has recently emerged as an effective membrane treatment for turbidity and non-dissolved organic matter. It can remove all suspended solids, bacteria, most colloids, and non-dissolved organic matter, making it an ideal pretreatment process for reverse osmosis systems.

Precautions for RO Application
During water treatment applications of reverse osmosis technology, wastewater must undergo necessary filtration. Filtration forms the foundation for reverse osmosis effectiveness. Strict control of the filtration process is essential to prevent contaminants from entering the reverse osmosis system, thereby protecting permeate membranes and equipment, enhancing water yield, and reducing corrosion risks.
Regular flushing of the reverse osmosis unit is essential, particularly for cleaning scale deposits. This maintains the semipermeable membrane's performance and extends the unit's service life.
When idle, reverse osmosis units are susceptible to microbial growth due to stagnant wastewater accumulation. Therefore, during shutdown periods, the unit must be flushed and disinfected. Additionally, proper temperature settings during downtime are crucial for protecting the reverse osmosis membrane.
Operators must strictly adhere to operational procedures and standards while continuously enhancing their professional competence. Thorough inspections should be conducted before system activation to prevent operator errors that could damage the equipment, ensuring reliable operation and efficient wastewater treatment.