Don't.
storing commas separated strings of ids is a bad idea: it makes them a nightmare to use as SQL is not good at string manipulations.
Have separate table which "links" the records: the id of the row you want to associate with the id list, and the id you want to associate it with. So for your string, you would have 13 rows in the association table. It may seem like hard work, but think about it: which is easiest:
1) Removing id "21" from your string without affecting anythign else
Or
2) This:
DELETE FROM AssociationTable WHERE BaseId=1 AND AssocId = 21