have you tried adding a ...

have you tried adding a polymer to your coagulant?  Also we have one site where we use an up-flow "clarifier of sorts".  It is basically filled with resin beads (about 6 feet thick of resin beads) and the resin beads are charged with a coagulation to make them positive as silica sand typically has a negative charge naturally and then after the water flow up through the positively charged resin beads it gently overflows into a multimedia filter comprised of anthracite, granular activated carbon on top of a gravel bed.  The gravel bed then of course sits on an under-drain system (we have had huge success with the Phoenix under-drain system) and then flows into our baffled clearwell where we inject a disinfectant (Cl) before we distribute (0.1 ppm free or 0.5 ppm total)