This will give an overview of an alum treatment. It explains how the alum works so we can all have a general feel for it. I did not write the article. Ken Dillenburg
The length of treatment effectiveness varies with the amount of alum applied and the depth of the lake. Alum treatment in shallow lakes for phosphorus inactivation may last for eight or more years. In deeper lakes, alum treatment may last far longer.
Some lake managers use alum to precipitate phosphorus from the water column by continuously injecting small amounts of alum during the summer months (micro-floc alum injection).