Scum or foam at filter bed - From the Water and Wastewater.com Help Forum

Water and Wastewater.com Help Forum - Scum or foam at filter bed

obviously you have not only turbidity as a problem in your raw water.
Traces of lime or aluminum salts do not cause foaming!
Therefore you have to identify and remove the foaming substances (natural, like degradation products of resin or manmade).
By the way chlorination will produce byproducts with these compounds leading to deterioration of taste and quality.
Removal might be achieved by ion exchange (if ionic contaminants), adsorption (granular activated carbons), oxidation ...