Reverse Osmosis Technology: How Does an RO System Operate, and How is RO Membrane Fouling Treated?

Reverse osmosis (RO) technology primarily utilizes the pressure difference across a membrane to achieve separation and filtration. It is a highly advanced and efficient energy-saving membrane separation technology.

RO Basic Principles and Advantages: The reverse osmosis membrane is the core component of reverse osmosis technology. It is an artificial semi-permeable membrane with specific characteristics, made of polymer materials that mimic biological semi-permeable membrane materials.

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation that uses pressure difference as the driving force to separate the solvent from an aqueous solution. It is the process of filtering impurities from water. Because it is in the opposite direction to natural osmosis, it is called reverse osmosis. The technical principle is that pressure is applied to one side of the membrane at a pressure higher than the osmotic pressure of the solution. When the pressure exceeds its osmotic pressure, the solvent will permeate in the opposite direction, separating these substances from the 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. If reverse osmosis technology is used to treat seawater, fresh water is obtained on the low-pressure side of the membrane, and brine is obtained on the high-pressure side. Reverse osmosis can achieve separation, extraction, purification, and concentration by applying pressure.

Reverse osmosis is a water treatment technology that uses membrane separation and belongs to the physical method of crossflow filtration. Its advantages are as follows:
* Lower operating costs due to water pressure as the driving force at room temperature;
* No large amounts of waste acid or alkali solutions are discharged, thus avoiding environmental pollution;
* Simple system, easy operation, and high degree of automation;
* Wide adaptability to raw water quality, with stable effluent quality;
* Small equipment footprint and minimal maintenance workload.

Basic RO Water Treatment Processes:
* First, single-stage treatment process. After the liquid enters the membrane module, pure water and concentrated solutions are extracted. Compared to other reverse osmosis water treatment processes, this process is more convenient and simpler to operate, but it has significant limitations and cannot meet higher water quality requirements.

* Second, multi-stage treatment process. Based on the single-stage treatment process, the liquid is concentrated in multiple steps. Compared to the single-stage treatment process, this process is more complex, can meet higher water quality requirements, and achieves water resource recycling.
* Third, two-stage single-stage treatment process. When primary methods fail to meet actual water quality requirements, a two-stage, single-phase treatment process can be used. Compared to the two primary processes mentioned above, the two-stage, single-phase treatment process extends the service life of reverse osmosis membranes, requires less manual operation, and correspondingly reduces treatment costs.

RO Applications in Water Treatment

Advanced Urban Wastewater Treatment
In advanced urban water pollution treatment, reverse osmosis technology can increase wastewater recovery rates and is widely used. Different reverse osmosis membrane materials produce varying effects in advanced water pollution treatment. Generally, in advanced urban water pollution treatment, after the treatment of urban residential wastewater meets standards, the requirements for the treated water quality are higher (e.g., for greywater reuse). In this case, cellulose triacetate hollow fiber membranes and spiral wound polyvinyl alcohol composite membranes can perform better. Compared to other reverse osmosis membrane materials, the two materials mentioned above achieve 100% retention rates for fecal coliform bacteria, a color level not exceeding 1 degree, and a permeate concentration of 1 mg/L to 2 mg/L. Meanwhile, both types of reverse osmosis membranes offer higher water flux and stronger resistance to fouling.

Industrial Wastewater Treatment

1) Treatment of Heavy Metal Ions
Applying reverse osmosis water treatment technology to industrial wastewater treatment has proven highly effective, aligning with the overall design principles of industrial economics and rationality. It reduces energy consumption, operating costs, and operational management complexity. Reverse osmosis units used for industrial wastewater treatment are typically internal pressure tubular or spiral wound components, with pressure generally stable at around 2.8 MPa, exhibiting excellent performance in heavy metal ion recovery. Specifically, reverse osmosis units based on internal pressure tubular components operate at a stable pressure of 2.7 MPa, at which point nickel recovery rates exceed 99%, and nickel separation rates range from 97.12% to 97.17%.

2) Treatment of Oily Wastewater
Generally, oil in oily wastewater exists in three main forms: emulsified oil, dispersed oil, and floating oil. In comparison, the treatment methods for dispersed oil and floating oil are relatively simple. Mechanical separation, sedimentation, and activated carbon adsorption can significantly reduce the oil content. However, emulsified oil contains organic matter, which can act as surfactants. Furthermore, the oil particles are generally on the order of micrometers in size, exhibiting extremely high stability and making effective and rapid water-oil separation difficult. With the support of reverse osmosis water treatment technology, concentration and separation can be achieved without damaging the emulsion. Subsequently, the concentrate can be incinerated, and the permeate can be recycled or discharged. Currently, in the treatment of oily wastewater, reverse osmosis water treatment technology is generally combined with other treatment methods to consider the final treatment effect and effluent quality. For example, a self-prepared DEMUL-B1 demulsifier is used to demulsify high-concentration O/W type spinning oil wastewater, and then the demulsified water sample is further treated using an OSMONICS SE reverse osmosis membrane. The results showed that the COD removal rate of the purified water after "demulsification-reverse osmosis" treatment reached 99.96%, and the oil content was almost undetectable.

3) Desalination of Brackish Water In the process of desalinating brackish water, the introduction of reverse osmosis water treatment technology can effectively inhibit inorganic salt ions such as magnesium and calcium ions in the brackish water, thereby enhancing the quality of purified water. Currently, people's requirements for purified water quality are increasing, and the original treatment method (adding scale inhibitors to brackish water) is no longer sufficient to meet practical needs. Introducing reverse osmosis water treatment technology is an inevitable choice. When using a reverse osmosis unit for brackish water desalination, it is necessary to regularly test the SDI index, strictly control the recovery rate, monitor the pressure difference between membrane modules, and measure the changes in permeate flow and desalination rate in real time. In practice, the desalination rate of the reverse osmosis unit is stable at over 96%, and the desalinated water quality meets my country's drinking water standards.

How to Handle RO Membrane Fouling

Membrane fouling refers to the irreversible phenomenon where microparticles, colloidal particles, or large solute molecules in the feed solution that come into contact with the membrane are adsorbed and deposited on the membrane surface or within the pores due to physical and chemical interactions with the membrane, concentration polarization causing certain solutes to exceed their solubility, and mechanical effects. This results in reduced pore size or blockage, leading to a significant decrease in membrane flux and separation characteristics.

Microbial Fouling

1) Causes: Microbial fouling refers to the accumulation of microorganisms at the membrane-water interface, affecting system performance. These microorganisms use the reverse osmosis membrane as a carrier and reproduce with the help of nutrients in the concentrate section of the reverse osmosis system, forming a biofilm layer on the membrane surface. This leads to a rapid increase in the pressure difference between the influent and effluent of the reverse osmosis system, a rapid decrease in permeate flow and desalination rate, and contamination of the product water. The biofilm composed of microorganisms can directly (through enzymatic action) or indirectly (through local pH or reduction potential) degrade membrane polymers or other reverse osmosis unit components, shortening membrane life, damaging the integrity of the membrane structure, and even causing major system failures. 2) Control Methods: Biological contamination can be controlled by continuous or intermittent disinfection of the influent. For raw water from surface and shallow groundwater, a disinfection and dosing device should be installed, and chlorine-based disinfectants should be added. The dosage is generally based on an influent residual chlorine content >1 mg/L.

Chemical Contamination:
1) Causes: Common chemical contamination is the deposition of carbonate scale inside the membrane elements. In most cases, this is due to misoperation, an imperfect scale inhibitor dosing system, or interruption of scale inhibitor dosing during operation. If not detected in time, increased operating pressure, increased pressure differential, and decreased permeate rate will appear within a few days. Scale formation inside the membrane elements can also occur if the selected scale inhibitor is incompatible with the water quality or if the dosage is insufficient. Mild scale formation can be restored to its function through chemical cleaning, but severe cases can lead to the failure of some heavily contaminated membrane elements.

2) Control Methods: To prevent scale formation inside the membrane elements, firstly, select a reverse osmosis scale inhibitor suitable for the system's source water quality and determine the optimal dosage. Secondly, strengthen the monitoring of the dosing system, closely monitor subtle changes in operating parameters, and promptly investigate the cause of any abnormalities. Additionally, high Fe3+ content in water is mostly caused by the piping system; therefore, steel-lined plastic pipes should be used as much as possible in the system piping, including the water source pipes, to reduce Fe3+ content.

Suspended Particulate Matter and Colloidal Fouling

1) Causes

Suspended particles and colloids are the main substances that clog reverse osmosis membranes and are also the main cause of excessive SDI (sludge density index) in the effluent. Due to differences in water sources and regions, the composition of suspended particles and colloids varies considerably. The main components of uncontaminated surface water and shallow groundwater are usually: bacteria, clay, colloidal silica, iron oxides, humic acid products, and excessive amounts of flocculants and coagulants (such as iron salts and aluminum salts) added artificially in the pretreatment system. In addition, the combination of positively charged polymers in the raw water with negatively charged scale inhibitors in the reverse osmosis system to form precipitates is also one of the causes of this type of fouling.

2) Control Methods: When the suspended solids content in the raw water is >70mg/L, coagulation, clarification, and filtration are typically used as pretreatment methods; when the suspended solids content is <70mg/L, coagulation and filtration are typically used; when the suspended solids content is <10mg/L, direct filtration is typically used. In addition, microfiltration or ultrafiltration are recently emerging effective methods for membrane treatment of turbidity and non-dissolved organic matter. They can remove all suspended solids, bacteria, most colloids, and non-dissolved organic matter, making them ideal pretreatment processes for reverse osmosis systems.


Precautions for RO Application: During the application of reverse osmosis technology in water treatment, wastewater must be filtered as necessary. Filtration is the foundation for the effective functioning of reverse osmosis technology. The filtration process must be strictly controlled to prevent impurities from entering the reverse osmosis system, thus protecting the membrane and equipment, increasing the output water, and reducing the possibility of corrosion. The reverse osmosis unit should be flushed regularly, especially to clean scale, maintain the good performance of the semi-permeable membrane, and extend the service life of the unit. When not in use, reverse osmosis units are susceptible to microbial growth due to accumulated wastewater. Therefore, during shutdown periods, they require flushing and disinfection, and proper temperature control is essential to protect the reverse osmosis membranes.

Operators must strictly adhere to operating procedures and specifications, continuously improving their professional skills. A thorough inspection of the unit before use is crucial to prevent damage caused by operator error and ensure normal operation and successful wastewater treatment.