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
Aluminum, iron, or calcium salts can inactivate phosphorus in lake sediments. Lake projects typically use aluminum sulfate (alum) to inactivate phosphorus. Alum may also be applied in small doses for precipitation of water column phosphorus. When applied to water, alum forms a fluffy aluminum hydroxide precipitate called a floc. As the floc settles, it removes phosphorus and particulates (including algae) from the water column (precipitation). The floc settles on the sediment where it forms a layer that acts as barrier to phosphorus. Phosphorus,released from the sediments, combines with the alum and is not released into the water to fuel algae blooms (inactivation). Algal levels decline after alum treatment because phosphorus levels in the water are reduced.
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).