As Konstantinos Krallis point out, it depends on the type of foulants expected in the water being treated. A common procedure, if biofouling and carbonated scaling are ocurring, includes an alkaline solution (to get rid of the biofouling) followed by an acidic solution to disolve the carbonated scale.
Detailed procedures can be found in the RO design manual published by the major membranes manufacturers such as Filmtec (http://www.dowwaterandprocess.com/support_training/literature_manuals/filmtec_manual.htm) or CSM (http://www.csmfilter.com/) both can be downloded for free.
first try with pure RO water by back washing for a long time. than you go for slightly acidic solution.using more chemicals will lead to clog the membrane .
first you have to clean with pH 10 or 11 depending ont he membrane. You mix NaOH with dodecyl sodium sulfonate, heat up to 30°C.
You need to clean with an alkaline solution first otherwise in the presence of silica it will precipitate.
Then use citric acid or HCl and clean at pH 2. Citric acid is used at pH 4 otherwise it´s chelating effect won´t work due to the pKa. also heat up at 30°C
Alkaline (NaOH) washing followed by acid (HCl) washing is the most commonly used procedure in RO cleaning. In case of DOW filmtec membrane please check on this link,
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_060a/0901b8038060a66f.pdf?filepath=liquidseps/pdfs/noreg/609-23010.pdf&fromPage=GetDoc
Dear Sherif,
You may find the attached publication of interest to you: Optimization of Chemical Cleaning of Organic-Fouled Reverse Osmosis Membranes.
Good luck