That much will go bad before the average desert rafter/water-settler uses it all up. I buy 2 jars of McCormick alum at Safeway for $3.99 each and dissolve it in hot water in a 1L Nalgene bottle. That solution is pretty well saturated and I am set for a season or two of settling dish water, handwash, and filter for drinking water.
I bought my alum at King Soopers on Baseline, look in the spice isle or with the canning supplies (used to make pickles crisp) You need 1-2 teaspoons to treat a 5 gallon bucket on the San Juan. Getting the ratio right is important and takes me several tries as the water chemistry changes season to season.