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  What is Alum and how does it control Algae?

ALUM (aluminum sulfate) is a nontoxic material commonly used in water treatment plants to clarify drinking water. In lakes alum is used to control algae, not by killing the algal organisms, but by reducing the amount of the nutrient phosphorus in the water. Like most other plants, algae requires phosphorus to live and reproduce. Algal growth is usually limited by the amount of that mineral available in the water.

Phosphorus enters the water either externally, from run-off or ground water, or internally, from the nutrient rich sediments on the bottom of the lake. Phosphorus is released from the sediments under anoxic conditions that occur when the lake stratifies and oxygen is depleted from the lower layer.

Even when external sources of phosphorus have been curtailed by best management practices, the internal recycling of phosphorus can support explosive algal growth. Alum is used primarily to control this internal loading of phosphorus from the sediments of the lake bottom.
On contact with water, alum forms a fluffy aluminum hydroxide precipitate called floc. Aluminum hydroxide (the principle ingredient in common antacids such as Maalox) reacts with phosphorus to form an aluminum phosphate compound. This compound is insoluble in water under most conditions so the phosphorus in it can no longer be used as food by algae organisms.

As the floc slowly settles, some phosphorus is removed from the water. The floc also tends to collect suspended particles in the water and carry them down to the bottom, leaving the lake noticeably clearer.


On the bottom of the lake the floc forms a layer that acts as a phosphorus barrier by combining with phosphorus as it is released from the sediments. The floc is harmless to water creatures and aquatic plants. (Sorry, alum does not control rooted aquatic weeds.)

A sediment alum treatment can last up to ten years, depending on how much alum is applied, and lake conditions such as sedimentation rate and external phosphorus loading. Best results are obtained when steps are first taken to control the external sources of phosphorus . Some of these steps are simple, like encouraging the use of phosphorus free fertilizers and detergents or discouraging large flocks of waterfowl. More costly measures may be necessary such as installing a sewer system, building sedimentation impoundments, and diverting agricultural and urban run-off. Alum can also be used to treat water from a nutrient rich inlet before it enters the lake. Any of these best management practices that are in place before the alum treatment will improve its effectiveness and extend its life.


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