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.
