This is a very common problem and there is no single solution.  Some suspended solids consist of natural organic matter (NOM) or dead bacteria, and can be removed by performing a high pH cleaning.  Other suspended solids (such as limestone, dolomite, phosphorite, metal hydroxides) are acid soluble and can be removed by low pH cleaning.

Suspended solids that are made of crystalline aluminosilicates such as silts and clays are insoluble in water.  The use of an effective high pH cleaning chemical with dispersants can help remove silts and clays as long as they are small enough to pass through the feed channels.  However, if they are large particles that are lodged into the feed channels, then cleaning using reverse flow may be necessary (i.e. circulate in the direction of concentrate to feed).  When cleaning using reverse flow, never exceed 2/3 of the maximum allowable flow recommended by your membrane manufacturer.

One cleaning chemical that has been shown to be extremely effective at dispersing silts and clays in brackish water RO  systems is the AWC C-237. For seawater plants where membrane fouling tends to consist of a combination of heavy biological and organic fouling, in addition to silts and clays, we recommend the AWC C-227.