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May  2016


Lake Riley Alum Treatment Ended

The Riley-Purgatory-Bluff Creek Watershed District treated Lake Riley with aluminium sulfate (Alum) the week of May 9th.

 The alum treatment will provide safe, effective and long-term control of the amount of algae in our lakes. The result? Cleaner, clearer water and a more pleasurable environment for recreation on and around the lakes. 

Alum (aluminum sulfate) is a compound derived from aluminum, the earth’s most abundant metal. Alum has been used in water purification and wastewater treatment for centuries and in lake restoration for decades. 

 Alum reduces the growth of algae by trapping the nutrient phosphorus—algae’s food source— in sediments. Like most other plants, algae require phosphorus to grow and reproduce. Algal growth is directly dependent on the amount of phosphorus available in the water. Without available phosphorus, algae cannot continue to grow and reproduce. 

  Alum is injected into water several feet below the surface. On contact with water, alum becomes aluminum hydroxide (the principal ingredient in common antacids such as Maalox). This fluffy substance called floc, settles to the bottom of the lake. 

On the way down, it interacts with phosphorus to form an aluminum phosphate compound that is insoluble in water. The result? Phosphorus in the water is trapped as aluminum phosphate and can no longer be used as food by algae. 

An added bonus: As the floc settles downward through the water, it also collects other suspended particles in the water, carrying them down to the bottom and leaving the lake noticeably clearer. 

For more information on the Alum Treatment, check our  Alum Fact Sheet

For more information on Lake Riley, check out our Water Quality Fact Sheet 

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