Aluminum sulfate, also called alum, aids swimming pool filters to maintain clean and clear water. You can also spread it on the pool's surface. It attracts dirt particles and coagulates them so you can clean them out of the pool with ease. 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 straight to the sand filter or pool's surface.

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Alum helps clarify dirty pool water.

Added to Sand Filter

Step 1

Test the pool's pH level with a kit available at pool supply stores. Dip the included litmus paper into the pool water and read the results according to package instructions.

Step 2

Apply pH increaser or pH decreaser to the water to achieve a pH level in your pool of 7.0 to 7.2. These products are readily available at pool supply stores.

Step 3

Turn on the pool's circulation system.

Step 4

Add 1 pound of alum for every square foot of filter area through the skimmer. Check the manufacturer's label on the filter to find the square footage.

Step 5

Vacuum any sediment, or floc, that escapes the filter.

Step 6

Readjust the pH level of the pool to its normal level.

Spread Over Surface

Step 1

Adjust the pool's pH level to 7.0 to 7.2 using a pH increaser or decreaser and a pool pH test kit available at pool supply stores.

Step 2

Turn on the pool's circulation system.

Step 3

Add between 4 and 8 pounds of alum per 10,000 gallons of water. Use 8 pounds per 10,000 gallons for extremely dirty water.

Step 4

Turn off the circulation system after four hours or when the pressure gauge on the filter reaches the limit set by the manufacturer.

Step 5

Wait six to 12 hours for the sediment to settle on the bottom of the pool.

Step 6

Vacuum the sediment from the pool, taking special care not to disturb the water more than necessary. Excessive motion in the water will cause the sediment to break apart, making it more difficult to vacuum.

Step 7

Replace the water you removed while vacuuming to achieve the desired water level in the pool.

Step 8

Adjust the pH level back to its normal level.