Reverse osmosis system operation and membrane fouling treatment

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15 Dec 2022

Reverse osmosis system operation and membrane fouling treatment

Reverse osmosis technology mainly uses the pressure difference on both sides of the membrane as the power to realize the separation and filtration of the membrane. It is a very advanced and effective energy-saving membrane separation technology.
RO Fundamentals and Advantages

The reverse osmosis membrane is the core component of the reverse osmosis technology. It is an artificial semipermeable membrane with certain characteristics. It is made of polymer materials and simulates biological semipermeable membrane materials.

Reverse osmosis system, also known as reverse osmosis, is a membrane separation operation that uses pressure difference as a driving force to separate solvents from aqueous solutions, and is a process of filtering impurities from water. Because it is opposite to the direction of natural infiltration, it is called reverse osmosis.

The technical principle is to apply pressure to one side of the membrane under the action of higher than the osmotic pressure of the solution. When the pressure exceeds its osmotic pressure, the solvent will permeate in the opposite direction to separate these substances from water. The solvent obtained on the low-pressure side of the membrane is called permeate; the concentrated solution 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. The reverse osmosis pressure can be used to achieve the purpose of separation, extraction, purification and concentration.
Reverse osmosis is a water treatment technology using membrane separation, which belongs to the physical method of cross-flow filtration. Its advantages are as follows:
·At room temperature, relying on the pressure of water as the driving force, the operating cost is low;

·No large amount of waste acid and alkali discharge, no pollution to the environment;

·The system is simple, easy to operate and highly automated;

·It has a large adaptability range to the raw water quality, and the effluent water quality is stable;

·The equipment occupies a small area, and the maintenance workload is small.

RO water treatment basic process
First, one-stage one-stage treatment process. After the liquid enters the membrane module, the pure water and concentrated liquid are drawn out. Compared with other reverse osmosis water treatment processes, the overall process of this process is more convenient and easy to operate, but it has high limitations and cannot meet higher water quality requirements.

Second, one-stage multi-stage treatment process. Based on the one-stage one-stage treatment process, the liquid is concentrated in multiple steps. Compared with the one-stage one-stage treatment process, the complexity of this process is higher, which can meet higher water quality requirements and realize the recycling of water resources.

Third, two-stage one-stage treatment process. In the case where it is difficult to meet the actual water quality requirements using the primary method, the secondary and one-stage treatment process can be used. Compared with the above two first-stage processes, the use of the second-stage one-stage treatment process can prolong the application life of the reverse osmosis membrane, and does not require too much manpower operation, and the corresponding treatment cost is also reduced.

Application of RO in water treatment
Advanced treatment of urban sewage

In the advanced treatment of urban water pollution, reverse osmosis technology can increase the recovery rate of sewage and is widely used.

There are differences in the advanced treatment effects of water pollution produced by reverse osmosis membranes of different materials. Generally speaking, in the advanced treatment of urban water pollution, after the domestic sewage of urban residents has been treated up to the standard, the requirements for the treated water quality are higher (such as reclaimed water). At this time, cellulose triacetate hollow fiber membrane, Spiral-wound polyvinyl alcohol composite film can play a better effect.

Compared with reverse osmosis membranes made of other materials, the reverse osmosis membranes of the above two materials have a retention rate of 100% for fecal coliform bacteria, a chromaticity of no higher than 1 degree, and a permeate of 1mg/L~2mg/L. At the same time, the reverse osmosis membranes of these two materials have higher water flux and stronger anti-pollution ability.

Industrial Wastewater Treatment

1) Dealing with heavy metal ions

Applying reverse osmosis water treatment technology to industrial wastewater treatment has a very good effect, which is in line with the overall design principle of industrial economy and rationality, and can reduce energy consumption, operating costs and difficulty in operation and management.

The reverse osmosis device used for industrial wastewater treatment is generally an internal pressure tube or roll-type component. The pressure is generally stable at about 218MPa, and the effect is excellent in the recovery of heavy metal ions. Among them, the operating pressure of the reverse osmosis device based on internal pressure tubular components is stable at 217MPa. At this time, the recovery rate of nickel is above 99%, and the separation rate of nickel is in the range of 97.12%~97.17%.

2) Treatment of oily wastewater

Generally speaking, oil in oily wastewater mainly exists in three forms, including emulsified oil, dispersed oil, and floating oil. In comparison, the treatment methods of dispersing oil and floating oil are relatively simple. After relying on mechanical separation, precipitation, and activated carbon adsorption, the content of the corresponding oil can be greatly reduced. However, for emulsified oil, it contains organic matter, which can play the role of surfactant, and the oil generally exists in micron-sized particles, so it has extremely high stability, and it is difficult to effectively and quickly realize water-oil separation.

With the support of reverse osmosis water treatment technology, concentration and separation can be achieved without destroying the emulsion, and then the concentrated liquid is incinerated, and the permeate is recycled or discharged.

At this stage, in the treatment of oily wastewater, due to the consideration of the final treatment effect and effluent quality, reverse osmosis water treatment technology is generally used in combination with other treatment methods. For example, self-prepared DEMUL-B1 is used as a demulsifier to demulsify high-concentration O/W spinning finish wastewater, and then the demulsified water sample is further treated with OSMONICS' SE reverse osmosis membrane. The results show that the removal rate of COD reaches 99.96% and the oil content is almost undetectable in the purified water after "demulsification-reverse osmosis" treatment.

Desalinated brackish water

In the process of desalination of brackish water, by introducing reverse osmosis water treatment technology, it can effectively suppress inorganic salt ions such as magnesium ions and calcium ions contained in salt water, and realize the enhancement of pure water quality.

At this stage, people's requirements for the quality of pure water are increasing, and the original treatment method (adding antiscalant to salt water) is difficult to meet people's actual requirements, and the introduction of reverse osmosis water treatment technology is an inevitable choice.

In the desalination operation of brackish water using reverse osmosis devices, it is necessary to regularly test the SDI index, strictly control the recovery rate, pay attention to the pressure difference between the membrane modules, and measure the changes in water production and desalination rate in real time. In practice, the desalination rate of the reverse osmosis device is stable above 96%, and the water quality after desalination meets the domestic drinking water standard.

How to deal with RO membrane fouling
Membrane fouling refers to the particles, colloidal particles or solute macromolecules in the feed liquid in contact with the membrane, which is caused by physical and chemical interactions with the membrane or concentration polarization so that the concentration of certain solutes on the membrane surface exceeds its solubility and mechanical action. Adsorption and deposition on the membrane surface or in the membrane pores cause the membrane pore size to become smaller or clogged, resulting in an irreversible change phenomenon that significantly reduces the membrane flux and separation characteristics.
Microbial contamination

1) Causes

Microbial fouling refers to the phenomenon that microorganisms accumulate on the membrane-water interface, thereby affecting the performance of the system.

These microorganisms use the reverse osmosis membrane as a carrier, rely on the nutrients in the concentrated water section of the reverse osmosis to reproduce and grow, and form a biofilm layer on the surface of the reverse osmosis membrane, resulting in a rapid increase in the pressure difference between the inlet and outlet water of the reverse osmosis system. rapid decline while contaminating the product water.

The biofilm composed of microorganisms can directly (through the action of enzymes) or indirectly (through the action of local pH or reduction potential) degrade membrane polymers or other reverse osmosis unit components, resulting in shortened membrane life, damage to the integrity of the membrane structure, and even cause major system failure.

2) Control method

Biological contamination can be controlled by continuous or intermittent disinfection of influent water. Sterilization and dosing devices should be installed for raw water collected from the surface and shallow underground, and chlorine-based fungicides should be added. The dosage is generally based on the residual chlorine content of the influent > 1mg/L.

Chemical pollution 

1) Causes

The common chemical pollution is the deposition of carbonate scale in the membrane element, most of which are misoperation, imperfect scale inhibitor dosing system, interruption of scale inhibitor dosing during operation, etc. If it is not discovered in time, the operating pressure will increase, the pressure difference will increase, and the water production rate will decrease within a few days. If the selected scale inhibitor does not match the water quality or the dosage is insufficient, the membrane Scaling phenomenon in the element, light fouling in the membrane element can restore its function through chemical cleaning, and in severe cases, it will also cause some seriously polluted membrane elements to be scrapped.

2) Control method

To prevent fouling in the membrane elements, firstly select the reverse osmosis antiscalant suitable for the water quality of the system water source, and determine the optimal dosing amount. Secondly, strengthen the monitoring of the dosing system, pay close attention to the subtle changes in operating parameters, and find out the reasons for abnormalities in time. In addition, most of the reasons for the high Fe3+ content in water are caused by the pipeline system. Therefore, the system pipelines, including water source pipelines, use steel-lined plastic pipelines as much as possible to reduce the Fe3+ content.

Suspended particulate matter and colloidal pollution

1) Causes

Suspended particles and colloids are the main substances that foul reverse osmosis membranes, and are also the main cause of excessive effluent SDI (sludge density index).

Due to the different water sources and regions, the composition of suspended particles and colloids is also quite different. Generally, the main components of unpolluted surface water and shallow groundwater are: bacteria, clay, colloidal silicon, iron oxides, humic acid products, and artificially excessive flocculants and coagulants (such as iron salts) in the pretreatment system , aluminum salts, etc.) etc.

In addition, the combination of positively charged polymers in raw water and negatively charged antiscalants in reverse osmosis systems to form precipitates is also one of the causes of this type of pollution.

2) Control method

When the content of suspended solids in raw water is more than 70mg/L, the pretreatment methods of coagulation, clarification and filtration are usually used; when the content of suspended solids in raw water is less than 70mg/L, the pretreatment method of coagulation and filtration is usually used; When <10mg/L, the pretreatment method of direct filtration is usually used.

In addition, microfiltration or ultrafiltration is an effective method for membrane treatment of turbidity and non-dissolved organic matter that has emerged recently. It can remove all suspended solids, bacteria, most colloids and non-dissolved organic matter. It is an ideal pretreatment process for reverse osmosis systems. .

Precautions when using RO

During the application of reverse osmosis technology in water treatment, necessary filtration of sewage should be carried out. Filtration is the basis for reverse osmosis technology to play a role. The filtration process must be strictly controlled to prevent impurities from entering the reverse osmosis system in the water, so as to protect the permeable membrane and equipment, increase water output, and reduce the possibility of corrosion.

The reverse osmosis device should be flushed regularly, especially to clean the scale, maintain the good performance of the semi-permeable membrane, and prolong the service life of the device.

When the reverse osmosis device is not in use, it will be affected by the confining sewage, thereby breeding microorganisms. Therefore, during the shutdown period of the device, it needs to be washed and disinfected, and the temperature during the shutdown period should be set well to ensure Protect the reverse osmosis membrane.

Operators should strictly abide by the operating procedures and operating specifications, continuously improve their professional quality, and carefully check the device before use to avoid damage to the device due to operator mistakes, ensure that the device can operate normally, and carry out sewage treatment work smoothly.

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