Aluminum sulfate, also called alum, can be a helpful substance to use when your swimming pool water becomes cloudy or greenish. It works as a so-called "flocculant," which attracts dirt particles, phosphates, pathogens, and other undesirable contaminants, causing them to clump together so you can easily vacuum them out of your pool. Alum was once a standard treatment for swimming pools until the advent of polymer pool clarifiers, but it is now making a come-back as an easy way to clear turbid water.

People sitting pool-side.
Alum helps clarify dirty pool water.

Alum comes in powder form and is readily available anywhere that sells swimming pool supplies. You can combine the powder with water to make a slurry to add to the water, but the quickest and easiest method is to add it the straight to the sand filter or to the pool's surface.

How to Use Alum in a Sand Filter

Although alum should not be used in a DE (diatomaceous earth) or cartridge filter, it can be added directly into a sand filter, since the contaminants that clump together are easily back-washed out of the filter for collection by vacuuming.

For alum to do its job in clarifying murky water, the pH level and chemistry in the water must be at the correct level. The first step, then, is to test and adjust the pool's pH level and chemistry.

  1. Test the pool water and 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 every 100 lbs. of filter sand in the tank. Add the alum directly into the filter tank.
  5. Backwash when the filter pressure rises 8 to 10 lbs. above the starting pressure. The rise in pressure occurs because the contaminants have clumped together and are beginning to clog the filter.
  6. Vacuum the pool to collect the clumped contaminants (floc) that have escaped the filter. You should now notice a marked improvement in the clarity of your water.
  7. Test the water chemistry again, and adjust as necessary.

Using Alum on the Pool Surface

It's important that the pool water's pH and chemical levels are correct in order for alum to successfully clarify the water. The first step will be to test and adjust the water's chemistry as needed.

  1. Skim, vacuum, and brush the pool to remove large debris.
  2. Test the pool water and balance its pH and other chemical levels, as needed.
  3. Spread 4 lbs. of alum over the pool surface for each 10,000 gallons of capacity in the pool.
  4. Set the filter pump's multiport valve to "recirculate" and run the pump for two hours. During this water circulation, contaminants will be clumping together due to the effect of the alum.
  5. Brush the sides and bottom of the pool as the water circulates to loosen any clumped particles.
  6. Turn pump off for 12 to 24 hours, or until the clumped contaminants (floc) settles to the pool floor.
  7. Vacuum the pool to collect the settled contaminants. Use care during this step to not vacuum too vigorously, which can break apart the clumped particles. You should now notice a marked improvement in the water's clarity.
  8. Test the water pH and chemistry again, and adjust as needed.