Forest Lake Alum Treatment

Alum (aluminum sulfate) is a nontoxic compound that many people have in their kitchen cupboard. It is used as an additive in pickling and canning, and used by many municipalities for drinking water clarification. When applied to a lake by a specialized service provider, liquid alum forms a white, cotton-ball looking substance called flock. The flock sinks to the bottom of the lake and forms a one-inch thick layer on the bottom. The alum flock attracts and binds phosphorus in the water. Once it has sunk to the lake bottom, it compresses and creates a seal that prevents phosphorus in the sediment from re-circulating. On average, a typical alum treatment is capable of binding 70-90% of the internal phosphorus load in a lake. The watershed district targets phosphorus, because in excess amounts, it causes algae blooms and lowers water quality.

The CLFLWD has worked with many partners to construct water quality projects for more than a decade. These projects have led to a significant decline in phosphorus entering the lake. While less phosphorus is entering the lake, a project is still needed to address the phosphorus that is already in the water. Alum treatments specifically address the phosphorus that is already in the water and as such are often the final step in a multi-year effort to reduce phosphorus in our lakes. The CLFLWD has performed alum treatments on both Shields Lake and Moody Lake.

Yes. Alum is a non-toxic water quality treatment that has been used to improve lake water quality for decades. District staff and engineers will be actively monitoring the lake during the application to ensure the proper dosing is being applied and that the pH of the water remains stable.

Read the alum treatment FAQ sheet for answers to more commonly asked questions