After a period of normal operation, the RO membrane components may be contaminated by suspended solids or insoluble salts in the feed water. The most common pollutions are calcium carbonate precipitation, calcium sulfate precipitation, metal oxide precipitation (iron, manganese, copper, nickel, aluminum, etc.), silicon oxide, mixture of inorganic or organic deposits, NOM natural organic materials, synthetic organic compounds (such as antisludging agent / dispersing agent, cationic polyelectrolyte), microbial (such as algae, fungi, mold), etc.
The nature and rate of pollution depend on various factors such as water quality and system recovery rate. The pollution is usually gradual and will damage components of RO membrane in a relatively short period of time if it is not controlled early. It is recommended that membrane components be cleaned when they are confirmed to be contaminated, or clean the components before a prolonged downtime or as a part of regular routine maintenance.
Chemical cleaning or physical flushing is required when the reverse osmosis system or device shows the following symptoms:
Under normal feed water pressure, the water yield decreases by 10-15% from the normal value;
To maintain the normal water yield, the feed water pressure increases by 10-15% after temperature correction;
The yield water quality decreases by 10-15% and the salt penetration rate increased by 10-15%.
The feed water pressure increases by 10-15%;
The pressure difference between the segments of the system increases significantly.
The main operating parameters to maintain the stability are water yield, water yield back pressure, recovery rate, temperature and TDS. If these operating parameters fluctuate, it is recommended to check whether contamination has occurred or check whether the actual operation of reverse osmosis is normal under the premise of the change of key operating parameters.
The basic method to confirm whether the membrane element has been polluted is to monitor the overall performance of the system regularly. The effect of contamination on the membrane component is gradual and the extent of the effect depends on the types of the contamination. The cleaning cycle of the contaminated reverse osmosis membrane depends on the actual situation on site. The normal cleaning cycle is once every 3-12 months.
It is important to clean the membrane component when the membrane component is only mildly contaminated. Heavy pollution will prevent chemical agents from penetrating deeply into the contaminated layer and affect the cleaning effect. If the performance of the membrane component reduces to 30-50% of the normal value, it is impossible to fully recover the initial performance of the membrane component as it leaves the factory lately.
In the design of spiral membrane system, the product water from reverse osmosis can be used to wash the pollutants in the system to reduce the cleaning frequency. Soaking the reverse osmosis membrane components in product water can help to dissolve and peel off dirt, and reduce the frequency of chemical cleaning.
What pollutants to be cleaned and how to clean the pollutants depend on the pollution situation on site. There are many factors involved in choosing appropriate chemical cleaning agents and reasonable cleaning schemes. First of all, it is necessary to contact the equipment manufacturer, membrane manufacturer, or chemical agent service provider. Then determine the main contaminants and select the appropriate chemical cleaning agents. Special chemical cleaning agents used in the manufacture of reverse osmosis membrane agents are sometimes applied to a particular contaminant or contamination condition, and the application shall follow the product performance and the instructions provided by the pharmaceutical supplier. Sometimes the single membrane component that has been contaminated can be removed from the reverse osmosis device for testing and cleaning tests to determine the appropriate chemical agents and cleaning scheme. In order to achieve the best cleaning effect, sometimes a combination of different chemical cleaning agents can be used. For complex cases where several kinds of pollution exist at the same time, low pH and high pH cleaning solution should be used to clean the component alternately (the mineral scale pollutants should be cleaned with low pH cleaning solution firstly, and the organic matter should be cleaned with high pH cleaning solution later).