Swimming Pool Blog

How to Use Alum Flocculant in Pools

How to Use Alum Floculant in Pools
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.


  1. Buy Aluminum Sulfate online, or find at a local garden center.
  2. Skim, vacuum and brush the pool to remove large debris.
  3. Balance pool pH, Alkalinity, Calcium, Chlorine and Cyanuric acid levels.
  4. Broadcast 4 lbs of Alum over the pool surface, per 10,000 gallons.
  5. Put Multiport valve on Recirculate and run pump for 2 hrs; Brush pool.
  6. Turn pump off for 12-24 hours, or until the Alum settles to the pool floor.
  7. Place Multiport valve on Waste, and vacuum to waste, the settled Alum.


  • You must be able to vacuum to waste. Cartridge filters or filters with Slide Valves don't normally have this option. Read an earlier post for options on vacuuming to waste, even if you don't have a multiport valve on your filter.
  • When vacuuming to waste, it must be done slowly, to avoid disturbing the layer, which has the consistency of wet toilet paper. Add a hose to the pool to replace water lost to vacuuming to water.
  • If you don't have a multiport valve, don't run the pump after adding the alum, as it will clog your filter fast. Use a pool brush instead to create circulation in the pool, and help distribute the alum.
  • You can use any kind of Aluminum Sulfate (aka Aluminum Sulphate) - food grade, technical grade or garden grade - just be sure not to use Ammonium Sulfate, which is a different chemical.
  • The dosage of 4 lbs per 10,000 gallons is an average amount. Depending on the amount of solids in the pool, effective dosage can be 2 lbs to 8 lbs per 10,000 gallons.
  • If your pool pump is on a timer, remove the 'ON' tripper, so that the pump won't come on in the morning, and clog your pool filter, or disturb the settled alum.
  • If there is algae present, you can add 2-4 lbs of shock per 10,000 gallons, immediately after adding the Alum.
  • Alum is most active in warmer water temperatures of 70-80°, but also works in colder water temperatures.


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.

  1. Balance the chemistry to normal levels.
  2. Raise chlorine levels to 10% of cyanuric acid levels.
  3. Backwash the sand filter for an initial cleaning and pressure gauge reading.
  4. With the filter running, pour in 6 oz. of Alum for each 100 lbs of filter sand in the tank.
  5. Backwash when filter pressure rises 8-10 lbs above the starting pressure.
  6. Repeat treatment if needed.


- Rob