How to deal with RO membrane fouling
Membrane fouling refers to the irreversible phenomenon in which particles, colloidal particles, or solute macromolecules in the feed solution in contact with the membrane adsorb and deposit on the membrane surface or pores due to physical or chemical interactions with the membrane, or concentration polarization that causes certain solutes to exceed their solubility and mechanical effects, resulting in a decrease in membrane pore size or blockage, and a significant decrease in membrane flux and separation characteristics.
Microbial contamination
1）Reasons for its formation
Microbial contamination refers to the phenomenon where microorganisms accumulate at the membrane water interface, thereby affecting system performance. These microorganisms multiply and grow using reverse osmosis membranes as carriers and nutrients from the concentrated water section of reverse osmosis, forming a biofilm layer on the surface of the reverse osmosis membrane. This leads to a rapid increase in pressure difference between the inlet and outlet of the reverse osmosis system, a rapid decrease in water production and desalination rate, and pollution of the product water. Biofilms composed of microorganisms can directly (through enzymatic action) or indirectly (through local pH or reducing potential) degrade membrane polymers or other reverse osmosis unit components, resulting in shortened membrane lifespan, damage to membrane structure integrity, and even major system failures.
2) The control method
The control method of biological pollution can be achieved by continuously or intermittently sterilizing and disinfecting the incoming water. Sterilization and dosing devices should be installed for raw water collected from the surface and shallow underground, and chlorine fungicides should be added. The dosage is generally based on the residual chlorine content in the influent being greater than 1mg/L.
Chemical fouling
1) Reasons for its formation
The common causes of chemical pollution are the deposition of carbonate scale inside membrane components, which is mostly due to misoperation, incomplete scale inhibitor dosing system, and interruption of scale inhibitor dosing during operation. If not detected in a timely manner, 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, scaling inside the membrane element will also occur. Mild scaling inside the membrane element can be restored to its function through chemical cleaning, and in severe cases, it can also cause some heavily polluted membrane elements to be scrapped.
2) The control method
The control method to prevent scaling inside the membrane element is to first select a suitable reverse osmosis scale inhibitor for the water source quality of the system, and determine the optimal dosage. Secondly, strengthen the monitoring of the dosing system, closely monitor subtle changes in operating parameters, and promptly identify the cause of any abnormalities. In addition, the high Fe3+content in water is mostly caused by pipeline systems. Therefore, steel lined plastic pipelines should be used as much as possible for system pipelines, including water source pipelines, to reduce Fe3+content.
Suspended particulate matter and colloidal pollution
1) Reasons for its formation
Suspended particles and colloids are the main substances that clog reverse osmosis membranes, and they are also the main reasons for exceeding the effluent SDI (sludge density index). Due to differences in water sources and regions, there are significant variations in the composition of suspended particles and colloids. The main components of unpolluted surface water and shallow groundwater are bacteria, clay, colloidal silica, iron oxides, humic acid products, as well as artificially excessive input of coagulants and coagulants (such as iron salts, aluminum salts, etc.) in the pretreatment system. In addition, the precipitation formed by the combination of positively charged polymers in raw water and negatively charged scale inhibitors in reverse osmosis systems is also one of the causes of such pollution.
2) The control method
When the suspended solids content in the raw water exceeds 70mg/L, coagulation, clarification, and filtration pretreatment methods are usually used; When the suspended solids content in the raw water is less than 70mg/L, coagulation filtration is usually used for pretreatment; When the suspended solids content in the raw water is less than 10mg/L, the direct filtration pretreatment method is usually used. In addition, microfiltration or ultrafiltration is an effective membrane treatment method for turbidity and insoluble organic matter that has emerged recently. It can remove all suspended solids, bacteria, most colloids, and insoluble organic matter, and is an ideal pre-treatment process for reverse osmosis systems.