Alum Treatment Update

Capitol Region Watershed District (CRWD) is committed to improving water quality in Como Lake, a beloved water resource in Saint Paul. Despite efforts to address the water quality in the lake over the years, decades of stormwater runoff and pollution have made a new Como Lake plan necessary. The Como Lake plan identifies strategies to address the water quality issues, which includes treatment of the water over the course of several years, beginning this spring.

Learn more about the Herbicide and Alum Treatments

Change is coming to Como Lake

This spring two treatments to address water quality will occur, an alum treatment in mid-May and an herbicide application that happened in early April. Alum (aluminum sulfate) will be applied to Como Lake to reduce phosphorus being recycled inside of the lake. Phosphorus, a pollutant carried to the lake through runoff in storm drains, comes from decaying organic matter like leaves, grass clippings, pet waste, and soil. Over time, high levels of phosphorus in Como Lake — three times higher than Minnesota standards — have led to algae blooms that choke the oxygen from the lake, leading to fish kills and strong odors mid-summer. CRWD and its partners have worked to reduce phosphorus flowing to the lake in stormwater runoff by 20% over the past two decades. Como Lake’s water quality remains poor due to the high levels of phosphorus that have built up in the lake over time as well as stormwater runoff. Excess phosphorous in the lake must be addressed to reach water quality goals. Alum is commonly used in drinking water treatment and has been a safe lake management tool for decades. The treatment is expected to dramatically reduce algae growth and improve water clarity and quality.


On May 18, weather permitting, a licensed contractor will begin applying alum by boat to Como Lake. The application will take 3-5 days.  During this time there will be large tanks in the east parking lot. Signs will be posted around the lake to inform visitors during the application period.

Alum is safe for humans, pets and wildlife and there are no water contact restrictions during the application. Water quality monitoring will occur throughout the treatment to ensure it is being applied safely and correctly for the best results.

Visitors can expect an immediate change in Como Lake with clearer water and fewer algae blooms.

What is an alum treatment?

An alum treatment consists of applying liquid alum (aluminum sulfate) below the surface of the water from a boat. When the alum meets the water, it turns into a non-toxic substance, or floc, called aluminum hydroxide. As the floc settles, it binds to phosphorus in the water and also forms a barrier on the bottom sediments that prevents phosphorus from being consumed by algae. The alum treatment is expected to be effective at reducing phosphorus in Como Lake for several years.


Images below: Boat applying alum to Como Lake.