Alum, short for aluminum sulfate, is a highly versatile and widelyused compound in swimming pool maintenance. While its primary role is as a flocculant, it is also useful in applications like phosphate and cyanuric acid reduction. Alum is known for its ability to bind with particles in water, which makes it invaluable for clearing up cloudiness and improving water filtration efficiency.

1. Alum as a Flocculant

The most common use of alum in swimming pools is as a flocculant. Cloudy or murky water in pools often results from suspended particles, such as dirt, organic matter, or microscopic debris that are too small to be captured by the pool’s filter. Adding alum to the pool water causes a chemical reaction that forms a gelatinous substance, which acts like a net to capture and bind these small particles into larger clumps, known as “floc.”

When alum is added to water, it dissociates into aluminum ions, which are positively charged. These ions attract the negatively charged particles in the water, causing them to bind together and create larger particles. These particles sink to the bottom of the pool, where they can then be vacuumed out or filtered. Pool techs generally leave the filter off for a few hours to allow the floc to settle and then carefully vacuum out the accumulated particles.

Using alum as a flocculant offers several benefits. First, it improves water clarity by removing the fine particles that cause cloudiness. Second, it reduces chlorine demand,

Florida Pool Pro Rudy Stankowitz discusses ALUM on his Talking Pools Podcast.