Reverse osmosis technology must be mastered:
how does the RO system operate, and how to deal with the pollution of RO film?
Reverse Osmosis technology, also known as RO (Reverse Osmosis), mainly uses the pressure difference on both sides of the membrane as the power to realize the separation and filtration of the membrane, which is a very advanced and effective energy saving membrane separation technology.
Basic principles and advantages of RO
Reverse osmosis membrane is the core component of reverse osmosis technology. It is an artificial semi-permeable membrane with certain characteristics. It is made of polymer material and simulates biological semi-permeable membrane material.
The principle of the technology is higher than the osmotic pressure of the solution, the pressure on one side of the membrane, when the pressure exceeds its osmotic pressure, the solvent will penetrate in the opposite direction, the substance and water away. The solvent obtained on the low pressure side of the membrane is called osmotic solution; The concentrated solution obtained at the high pressure side is called a concentrate.
If seawater is treated by reverse osmosis technology, 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, the operating cost is lower depending on the water pressure as the driving force.
No large amount of waste acid lye discharge, does not pollute the environment;
The system is simple, easy to operate, high degree of automation;
The raw water quality has a large range of adaptation, effluent water quality is stable;
The equipment covers a small area and the maintenance workload is small.
RO water treatment basic technology
First, one stage treatment process. After the liquid enters the membrane assembly, the pure water and the concentration liquid are extracted. Compared with other reverse osmosis water treatment processes, the overall flow of this process is more convenient and simple to operate, but it has higher limitations and cannot meet higher water quality requirements.
Second, the first stage multi - stage processing technology. On the basis of a one-stage treatment process, the liquid was concentrated in multiple steps. Compared with the first-stage treatment process, this process is more complex, can meet the higher water quality requirements, and realize the recycling of water resources.
Third, two-stage and one-stage treatment process. In cases where the actual water quality requirements cannot be achieved by using the primary method, the secondary one-stage treatment process can be used. Compared with the above two first-stage processes, the use of the two-stage and one-stage treatment process can extend 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 municipal sewage
In the advanced treatment of urban water pollution, reverse osmosis technology can increase the recovery rate of sewage and is widely used.
Different materials of reverse osmosis membranes produce different water pollution depth treatment effect. Generally speaking, in the advanced treatment of urban water pollution, after the treatment of urban domestic sewage reaches the standard, the water quality after treatment is required to be higher (such as the reuse of reclaimed water). At this time, cellulose triacetate hollow fiber membrane and spiral coil polyvinyl alcohol composite membrane can play a better role.
Compared with other materials of reverse osmosis membranes, the above two materials of reverse osmosis membranes on fecal coliform bacteria interception rate reached 100%, color is not higher than 1 degree, osmotic liquid in 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) Treatment of heavy metal ions
The reverse osmosis water treatment technology is applied to industrial wastewater treatment, the effect is also very good, in line with the industrial economy and reasonable overall design principle, can reduce energy consumption and operation cost and operation management difficulty.
Reverse osmosis devices used for industrial wastewater treatment are generally internal pressure tube or roll type components, the pressure is generally stable at about 218MPa, in terms of heavy metal ion recovery effect is excellent. The operating pressure of the reverse osmosis unit based on the internal pressure tube component is stable at 217MPa, and 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 content in oily wastewater mainly exists in three forms, including emulsified oil, dispersed oil and floating oil. Comparatively speaking, the treatment method of dispersed oil and floating oil is relatively simple. After mechanical separation, precipitation and activated carbon adsorption treatment, the corresponding oil content can be greatly reduced. However, for emulsified oil, it contains organic matter, which can play the role of surfactant, and the oil content generally exists in the size of micron particles, so it has high stability, and it is difficult to effectively and quickly realize the separation of water and oil.
With the help of reverse osmosis water treatment technology, the emulsion can be concentrated and separated without destroying it. Subsequently, the concentrate can be incinerated and the osmotic solution can be recycled or discharged.
At present, in the treatment of oily wastewater, for the consideration of final treatment effect and effluent quality, reverse osmosis water treatment technology is generally combined with other treatment methods. For example, DEMUL-B1 was used as demulsifier to demulsify high concentration O/W type spinning oil wastewater, and then OSMONICS SE reverse osmosis membrane was used to further treat the demulsified water sample. The results show that the removal rate of COD of the purified water by demulsification and reverse osmosis treatment reaches 99.96%, and the oil content is almost undetectable.
Desalinate brackish water
In the process of desalting brackish water, the introduction of reverse osmosis water treatment technology can effectively inhibit magnesium ions, calcium ions and other inorganic salt ions contained in brackish water, so as to enhance the quality of purified water.
At the present stage, people's requirements on the quality of purified water are increasing, and the original treatment method (adding scale inhibitor in salt water) is difficult to meet people's practical requirements, so reverse osmosis water treatment technology is the inevitable choice.
In the operation of brinkish water desalination using reverse osmosis unit, it is necessary to test the SDI index regularly, strictly control the recovery rate, pay attention to the pressure difference between membrane components, and measure the change of water yield and desalting rate in real time. In practice, the desalting rate of reverse osmosis device is stable at more than 96%, and the water quality after desalting meets the standard of drinking water.
How to deal with RO membrane contamination
Membrane pollution refers to the particles, colloidal particles or solute macromolecules in the material liquid in contact with the membrane due to physical and chemical interaction with the membrane or due to concentration polarization of some solute on the surface of the membrane concentration exceeds its solubility and mechanical action caused by adsorption and deposition in the membrane surface or membrane hole resulting in the membrane aperture becomes smaller or blocked, so that the membrane flux and separation characteristics decreased significantly irreversible change phenomenon.
Microbial contamination
1) Formation reasons
Microbial contamination refers to the accumulation of microorganisms at the membrane-water interface, which affects the performance of the system.
These microorganisms multiply and grow with the help of nutrient salts in the concentrated water section of reverse osmosis membrane as the carrier, forming 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, a rapid decrease in water yield and desalting rate, and pollution of the product water.
Biofilms composed of microorganisms can degrade membrane polymers or other reverse osmosis unit components directly (through enzyme action) or indirectly (through local pH or reduction potential action), resulting in shortened membrane life, damage to membrane structural integrity, and even major system failures.
2) Control method
Biological contamination can be controlled by continuous or intermittent sterilization of influent water. Sterilization and dosing devices should be set for raw water collected from the surface and shallow underground, and chlorine fungicides should be added. The dosage is generally subject to the residual chlorine content of water > 1mg/L.
Chemical pollution
1) Formation reasons
The common chemical pollution is the deposition of carbonate scale in membrane elements, which is caused by misoperation in most cases, imperfect scale inhibitor dosing system, and interruption of scale inhibitor dosing during operation. If it is not found in time, the phenomenon of increased operating pressure, increased pressure difference and decreased water production rate will occur within a few days. If the selected scale inhibitor does not match the water quality or the dosage is insufficient, the phenomenon of scaling in the membrane elements will occur. The scaling in the lighter membrane elements can be restored by chemical cleaning, and the serious situation will also cause the scrap of some seriously polluted membrane elements.
2) Control method
To prevent scaling in membrane elements, firstly select the reverse osmosis scale inhibitor suitable for the water quality of the system, and determine the best dosage. Secondly, we should strengthen the monitoring of dosing system, pay close attention to the subtle changes of running parameters, and find out the cause of anomalies in time. In addition, the reason for the high Fe3+ content in water is mostly brought by the pipeline system. Therefore, the system pipeline, including the water source pipeline, should adopt steel-lined plastic pipe as far as possible to reduce the Fe3+ content.
Suspended particulate matter and colloidal pollution
1) Formation reasons
Suspended particles and colloids are the main substances for fouling reverse osmosis membranes, and also the main reason for exceeding effluent SDI (sludge density index).
Because of the different water sources and regions, the composition of suspended particles and colloids also have great differences. The main components of surface water and shallow groundwater that are usually not contaminated are: bacteria, clay, colloidal silicon, iron oxide, humic acid products, and flocculant and coagulant aid (such as iron salt, aluminum salt, etc.) that are artificially overinvested in the pretreatment system.
In addition, positively charged polymers in raw water and negatively charged scale inhibitors in reverse osmosis system combine to form precipitation, which is also one of the causes of such pollution.
2) Control method
When the content of suspended matter in raw water is more than 70mg/L, the pretreatment methods of coagulation, clarification and filtration are usually adopted. When the content of suspended matter in raw water is less than 70mg/L, the pretreatment method of coagulation filtration is usually adopted. When the content of suspended matter in raw water is less than 10mg/L, the pretreatment method of direct filtration is usually adopted.
In addition, microfiltration or ultrafiltration is an effective way of membrane treatment of turbidity and insoluble organic matter, which is emerging recently. It can remove all suspended matter, bacteria, most colloid and insoluble organic matter, and is a relatively ideal pretreatment process of reverse osmosis system.
Notes for RO application
In the application of reverse osmosis technology in water treatment, sewage should be filtered. Filtration is the basis of reverse osmosis technology to play a role, to strictly control the filtration process, avoid impurities mixed in water into the reverse osmosis system, in order to protect the osmosis membrane and equipment, improve the water yield, reduce the possibility of corrosion.
The reverse osmosis device should be washed regularly, especially for dirt cleaning, maintain the good performance of the semi-permeable film, and extend the service life of the device.
The reverse osmosis device will be affected by the surrounding sewage when not in use, which will breed microorganisms. Therefore, during the outage of the device, it needs to be washed and disinfected, and the temperature during outage is well set to protect the reverse osmosis membrane.
Operators should strictly abide by the operating procedures and norms, constantly improve their professional quality, and carefully inspect the device before use to avoid damage to the device due to the operator's mistakes, so as to ensure the normal operation of the device and smooth implementation of sewage treatment work.
