Rob Cox January 10, 2017
Aluminum Sulfate, commonly called "Alum", has been used for thousands of years as a water clarifier. Ancient Egyptian hieroglyphics describe the use of "aluminum earth" and first century Greek and Chinese documents tell of mining and use of alum for clearing water. By the 16th century, Alum was in widespread use across England, for treating municipal water supplies. Its use continues to this day in water treatment facilities, as a flocculant to clear turbid water, and remove phosphates, pathogens and other undesirable particles.
"Floccing" a swimming pool with aluminum sulfate was a popular way to clear extremely cloudy or swampy pools, up until the creation of polymer pool clarifiers. Or perhaps it was the now debunked theory in the 60's and 70's, of a connection between Alzheimer's disease and aluminum. Whatever the cause, bottles of alum have been absent from pool store shelves (including our own), for over 30 years.
Alum Floc has been making a quiet comeback however, as a quick way to turn around neglected or abandoned pools, without having to drain the water in the pool. Vinyl and fiberglass pools are not supposed to be drained fully, the liner can relax, or a fiberglass shell can buckle or shift. And for areas with water restrictions, many pool owners are prohibited from refilling pools from the garden hose, and forced to pay for expensive water delivery.
Aluminum Sulfate is a double salt with a strong positive charge. Particulate matter in solution have negative charges. Over the period of several hours, the alum molecule attracts suspended particles. Eventually it grows to a large size and heavy weight, until it slowly sinks to the pool floor. It works best with well balanced pool water, and in water temperatures of at least 70° F. The tricky part of using alum is that the aluminum hydroxide precipitate must be vacuumed to waste the following day. Alum is considered a flocculant, because it sinks material to the floor - as opposed to clarifiers, which enlarge particles only slightly, for trapping within your pool filter.
Alum can be used in small quantities in a sand filter, to improve filtration temporarily. It should not be used in D.E. or Cartridge filters, as it can clog the small pores of the fabric, and ruin the filter media. Sand filters however, have large enough spaces between the sand grains, that most of the alum flushes out easily during backwashing. D.E. powder can also be used in small quantities as a filter aid for sand pool filters.