I am going to have to disagree on settling out glacial silt. Much of the glacial "silt" is not really silt, but finer particles in suspension and cannot be settled out. Settling the water will be helpful for your filter, but it will still get clogged with suspended solids. Same goes for organics that are in suspension. Coagulants make those non-settling particles glom together and become big enough to settle out or to attach to the coagulant chemically. That is why coagulation works.