A Usually there are 3 main indexes to observe to determine the necessary cleaning.

1. Differential pressure across the system. If the differential pressure between feeding and rejection is greater than 10-15%, cleaning should be implemented.
2. Normalized product flow. If the normalized product flow decreases 10%+, cleaning should be implemented.
3. Salt rejection. If the salt rejection decreases 10%+, clean the membrane.

A To clean the membrane, we can do both onsite and offsite.
There are general procedures to follow through:
1. A chemical cleaning that is to remove foulants, scales.
2. A biological cleaning to make the membrane free of microorganisms.
Four factors that would determine the efficiency of the cleaning.
1. Type of chemical chosen to use. This determines a membrane’s ability to withstand the action of the chemical cleaners. Some membranes have low tolerance to PH and temperature, and some are sensitive to chlorine.
2. Temperature. In general, the warmer the solution, the more effective is the cleaning process.
3. Flow rate. Higher velocities in pipelines is preferred.
4. Time of cleaning.

Let us know your membrane type and your process in operation. We could advise you on the chemicals for cleaning and the standard of operation to operate. Depending on the size complexity of your operation, it may be hard for you to implement the cleaning by yourself.  

Almost forgot, the most important rule of cleaning membrane is never to let the membrane dry out after contact with the process stream.