This is a very common ...

This is a very common problem: I presume you have a conventional surface water treatment plant with coagulation/flocculation/sedimentation/rapid sand filtration. It is likely that the flocks are not proper settling before the rapid sand filters. You can check that based on the turbidity: the turbidity after sedimentation should be about 1 NTU and no flocks should be visible. If the turbidity is too high all these not settled flocks are going to the RSF while they are only there for polishing!!!. The reason that the flocks do not settle properly can be: 1) not optimal coagulant dosing 2) algae in the feed water(they rather float) you should try a high chlorine dose at the front of the plant. 3) not optimal fast mixing (should be >1000  4) not optimal slow mixing 5) more water production than the design capacity. 6) not optimal pH