Alum, also known as aluminum sulfate, doesn't kill algae, but it can help reduce the growth of algae by removing phosphorus

 Alum, also known as aluminum sulfate, doesn't kill algae, but it can help reduce the growth of algae by removing phosphorus: 

How it works

Alum turns into a flaky floc that coats the bottom of a body of water, binding phosphorus and preventing it from re-entering the water. This starves the algae, which needs phosphorus as a food source. 

How long it takes

After an alum treatment, the algae will eventually settle to the bottom of the water, but it can take weeks. 

Other considerations

The pH of the water affects how alum works. When the pH is between 4 and 5, alum is usually in the form of positive ions. 

Other ways to remove algae include:

Physical harvesting: Methods like centrifugation, sedimentation, filtration, and flotation can be used to remove algae. However, these methods can be expensive and energy-intensive.

Chemical methods: Organic, inorganic, and electroflocculation are chemical methods that can be used to remove algae.

Bioflocculation: Microalgae or bacteria can be used to bioflocculate algae.

Flocculation by pH adjustment: Adjusting the pH of the water can be used to flocculate algae.

Magnetic nanocomposite based microalgal harvesting: This is a state-of-the-art harvesting technique that uses magnetic nanocomposites to harvest algae.