Another alternate to alum is to use one of the polymer versions of aluminum salts. These include poly-aluminum chloride (PAC) or aluminum chlorhydrate (PCH). Both of these are liquids and so are more easily dispersed. The other advantage is that they do not alter pH like alum, but have the same +3 charge, and because they are polymer chains of aluminum, have more charge density. You should be able to buy either from an industrial chemical supplier in most larger cities.
The cost of PAC or PCH will likely be double that of alum on a per lb. of aluminum, but the dosage will be 25% lower and will disperse better so you will see better results. Jar test to see how much you need, but target 10-15 ppm.

Mike