It can but it does not bind up the P in the soil. There are possible sewer treatment products that may do that but I have not heard of anyone doing that to a pond bottom and watershed. Have you had the soil tested ? If there has been no fertilization then I assume the P source is the dirt. I also am not sure about the long term reabsobshion of the initially bound P out of the alum/P bond by the more acid water due to treatment.

I would want to know for sure if the P is high from a soil test before I started spending $ on alum and what the long term effect would be.