Reverse Osmosis Technology for Wastewater Treatment: Understanding How RO Systems Operate

Reverse Osmosis Technology for Wastewater Treatment: Understanding How RO Systems Operate

Reverse osmosis technology, also known as RO (Reverse Osmosis), primarily utilizes the pressure difference across a membrane as the driving force to achieve separation and filtration. It is an advanced and highly effective energy-saving membrane separation technology.

01 Basic Principles and Advantages of RO

The reverse osmosis membrane is the core component enabling reverse osmosis technology. It is a semi-permeable membrane engineered with specific properties, manufactured from polymeric materials that mimic biological semi-permeable membranes.

Reverse osmosis, also known as reverse permeation, is a membrane separation process driven by pressure differences to separate solvents 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 this pressure surpasses the osmotic pressure, the solvent permeates in the opposite direction, separating these 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. Reverse osmosis is a membrane separation water treatment technology that belongs to the physical method of cross-flow filtration. Its advantages are as follows:

Operates at ambient temperature using water pressure as the driving force, resulting in low operating costs;

Generates no significant waste acid or alkali discharge, minimizing environmental pollution;

Features a simple system design with user-friendly operation and high automation levels;

Demonstrates broad adaptability to raw water quality while delivering stable treated water quality;

Requires minimal floor space and involves low maintenance and repair demands.

02 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 fails to achieve practical water quality requirements, the two-stage single-pass process can be employed. Compared to the two single-stage processes mentioned above, the two-stage single-pass process extends the service life of reverse osmosis membranes, requires less manual operation, and reduces corresponding treatment costs.

Application of RO in Water Treatment

In municipal wastewater deep treatment, reverse osmosis technology enhances wastewater recovery rates and finds extensive application. The effectiveness of deep wastewater treatment varies depending on the material of the reverse osmosis membrane. Typically, in urban deep 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, these membranes 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 yields 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 pressure of approximately 218 MPa, demonstrating exceptional effectiveness in heavy metal ion recovery. Specifically, reverse osmosis systems based on internal pressure tubular components operate at a stable pressure of 217 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 high 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 is typically 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, followed by further treatment of the demulsified water 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.

Desalination of Brackish Water

During brackish water desalination, incorporating reverse osmosis water treatment technology effectively suppresses inorganic salt ions like magnesium and calcium ions in saline water, enhancing purified water quality.

Current demands for purified water quality have risen, rendering traditional methods (adding scale inhibitors to saline water) inadequate. Adopting reverse osmosis technology has become an inevitable choice. During brackish water desalination using reverse osmosis systems, 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 systems remains stable above 96%, with the desalinated water meeting China's drinking water standards.

 

04 How to Address RO Membrane Fouling

Membrane fouling refers to the irreversible phenomenon where particles, colloidal particles, or solute macromolecules in the feed solution come into contact with the membrane. Due to physical and chemical interactions with the membrane, concentration polarization causing certain solutes to exceed their solubility at the membrane surface, or mechanical action, these substances adsorb or deposit on the membrane surface or within the pores. This reduces pore size or causes blockage, significantly decreasing 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, forming a biofilm layer on the membrane surface. This leads to a rapid increase in the pressure difference between the feed and permeate streams, a sharp decline in permeate flow rate and salt rejection, and simultaneous contamination of the product water. Microbial biofilms can degrade membrane polymers or other RO components either directly (via enzymatic action) or indirectly (through localized pH or redox potential effects). This shortens membrane lifespan, compromises structural integrity, and may cause major system failures.

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 to maintain 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 pipes for all system piping, 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. Their composition 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 artificially excessive flocculants/coagulants (e.g., iron salts, aluminum salts) introduced during pretreatment. 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 <10mg/L, direct filtration is typically used as pretreatment. Furthermore, 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.

05 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 the unit is idle, accumulated wastewater can foster microbial growth. Therefore, during shutdown periods, flushing and disinfection are required, along with proper temperature control to protect the reverse osmosis membrane. Operators must strictly adhere to operational procedures and standards while continuously enhancing their professional competence. Thorough pre-operation inspections are essential to prevent equipment damage from operator error, ensuring reliable system performance and efficient wastewater treatment operations.