How to Use Aluminum Sulfate in Your Pool

Written by Michael Dean
February 28, 2024

aluminum sulfate next to a swimming pool

Aluminum sulfate has been used for hundreds of years as a water clarifier, and today, it remains a useful and popular chemical for a variety of purposes. In a swimming pool, it can be used as a flocculant, helping to clear up extremely murky swimming pools. But if you’re new to this chemical, you may wonder how to use it.

This article will cover everything there is to know about aluminum sulfate, including how to use it, the pros and cons of using it, and much more! Let’s get into it!

Main Takeaways 

  • Aluminum sulfate, or alum, is a coagulant called a flocculant, which you can use to clear cloudy pool water.
  • You can also use alum in sand filters to temporarily improve filtration.
  • You should be careful with how liberally you use aluminum sulfate, as it can lower the pH of your pool water.
  • Alum is an inexpensive pool treatment compared to other methods of clearing cloudy water.

What Is Aluminum Sulfate?

Aluminum sulfate, also known as alum, is a chemical compound. It is a versatile substance—white, odorless, and crystalline—and dissolves very easily in water. Alum comprises aluminum, sulfur, and oxygen.

It has various uses in a variety of different settings:

  • Water purification – used as a coagulant to remove impurities and clarify water
  • Papermaking – helps bind fibers
  • Textiles – dyeing fabrics
  • Medicine – various purposes, including use as a coagulating agent to treat minor cuts and abrasions, deodorant, and more

Of course, in a pool, alum is used for water purification purposes.

How Does Alum Work in Swimming Pools?

Since alum can act as a coagulant or a binding agent, you can use it in a swimming pool to clarify the water. It works by acting as a flocculant, a substance that helps clump together small suspended particulate matter that makes the water cloudy. This is because alum carries a positive charge, while most particulate matter contains a negative charge. Since opposite charges attract, the positive charge in alum pulls the negatively charged particles in the water toward it. When alum binds to these particles, large matter clumps together and sinks to the bottom of the pool.

Finally, once this happens, you can easily remove the clumps – effortlessly clearing a cloudy pool with microscopic matter. With the help of alum, say hello to a beautifully blue and sparkly pool once again!

How to Use Aluminum Sulfate in Your Pool

You can use aluminum sulfate to deal with various pool issues, including algae and cloudy water. Here’s my step-by-step process on how to use alum in your pool.

Step One: Test the Water

Before you proceed, it’s best to test the water to ensure everything is properly balanced. Alum works best in well-balanced pool water and temperatures between 70 to 75 degrees Fahrenheit. Additionally, your pH must be at the ideal levels as well (7.2 to 7.8). The alum will not work correctly if the pH is too high or too low. Furthermore, check your chlorine, calcium hardness, and total alkalinity.

Step Two: Clean the Pool

To help your alum work as efficiently as possible, clean your pool. Skim, vacuum, and brush the pool to remove any larger debris. By cleaning the pool beforehand, you allow the alum to work on the smaller particles you cannot personally clean up.

Step Three: Dissolve the Alum

Fill a 5-gallon bucket with fresh, clean water and slowly add the alum according to the manufacturer’s instructions, then stir until the alum is completely dissolved. Then, distribute the alum evenly around the pool.

Step Four: Circulate the Water

Run the pool pump for 2 hours to allow the alum to get to work. This will allow the alum to bind to the tiny particles and form larger clumps. At this point, you should also brush the pool to dislodge any other small particles and debris.

Step Five: Settle the Water

After your pump has thoroughly mixed the water, leave the pool alone to let the water settle for at least 12 to 24 hours. This will allow the particles clumped together to settle at the bottom of the pool.

Step Six: Vacuum the Pool

Using a pool vacuum, remove the clumps that have settled at the bottom of the pool. Set the vacuum to “waste” mode to avoid sending the clumps back into the filter.

Step Seven: Test the Water Again

Check the water again. If dealing with an algae issue, follow it up with a dose of pool shock to eradicate any lingering spores. If dealing with cloudy water, simply measure your pH, TA, and chlorine to ensure they’re within the proper levels.

How to Use Alum in a Sand Filter

Alum has traditionally been used in sand filters in small quantities to temporarily improve filtration. Do not go overboard with the quantity, and do not use it with cartridge or D.E. filters—it’ll clog the small pores of the fabric and spoil the filter. Sand filters, however, can be flushed free of the alum during backwashing since there are large enough spaces between the sand grains.

  1. Before anything, backwash the sand filter and take a pressure gauge reading.
  2. Test and rebalance water chemistry to the right levels, then increase the chlorine levels to around 10% of CYA (cyanuric acid).
  3. Pour the alum into the skimmer. The dosage should be around 6 ounces of alum for 100 pounds of filter sand.
  4. Backwash the sand filter again when filter pressure rises 7-10 pounds above the initial pressure. If needed, repeat the process.

When to Use Aluminum Sulfate in Your Pool

Alum may not be one of the most common pool chemicals, but it can definitely come in handy in certain situations. Here are some circumstances in which you will want to use aluminum sulfate in your pool.

Cloudy, Hazy Water

Aluminum sulfate flocculant will be your best friend if your water looks hazy or cloudy. The cloudiness might result from tiny particles such as dust, pollen, and body lotion. If this is the case, use alum to clump up these tiny particles and remove them with a vacuum.

Recent Heavy Rain

Rain carries a lot of contamination, and heavy rain can introduce a lot of dirt, debris, chemical contamination, and various metals into the pool, which can cause cloudiness. Aluminum sulfate can help clear this up quickly.

Algae Bloom

Alum can temporarily clump some algae, but I recommend using it along with standard algae infestation treatments, such as algaecide, pool shock, and a heavy round of pool cleaning to remove all the algae thoroughly.

How Much Aluminum Sulfate to Use

The amount of alum you’d need for your pool depends on factors such as the size of your pool and the severity of the issue you’re dealing with. A general rule of thumb is 1 to 8 pounds of alum per 10,000 gallons of water.

For instance, if a pool is 10,000 gallons and mildly cloudy, you may only need around 1 pound of alum. However, if the same pool is moderately cloudy, you may need 5 pounds of alum.

For a better idea of how much aluminum sulfate to use, consult the manufacturer’s instructions.

Pros and Cons of Aluminum Sulfate

Aluminum sulfate can be a handy chemical for pool maintenance, but what are the pros and cons of using this chemical over any others?


  • Improves filtration efficiency: Alum can prevent the filter from clogging, prolonging its lifespan, by removing small particles before they reach the filter.
  • Reduces phosphate levels: Phosphates are pesky organic matter and can contribute to algae growth. Alum helps bind and remove these particles from the water.
  • Cost-effective: Aluminum sulfate is excellent for those on a budget since it’s a pretty inexpensive pool treatment compared to other methods of clearing cloudy water.


  • Possible pH imbalance: Alum lowers your pool’s pH, making it acidic. Apart from making it a chore to readjust, lower pH in the water will also irritate the skin, eyes, and lungs and may corrode pool equipment. 
  • Large amounts required: Since how much aluminum sulfate you use highly depends on how cloudy or dirty your water is, you may need to use a large amount of alum to see any results.

Brand Recommendations for Aluminum Sulfate

When using alum, you can choose from either liquid (easier to dissolve into your pool but more expensive) or powdered alum (more economical but requires careful dissolution to avoid clumping). Once you’ve decided on what kind of alum you want, you’ll also need to pick the right product. I recommend brands like Leslie’s and Alliance Chemical when looking for aluminum sulfate for your pool.

But whatever product you ultimately choose, always read the product’s packaging carefully for instructions on how to use and store it.

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Frequently Asked Questions

Does aluminum sulfate lower cyanuric acid in the pool?

While aluminum sulfate is not generally used to lower cyanuric acid levels, it can eliminate some CYA in the water during coagulation.

What are the disadvantages of alum in water?

Alum in water may improve water clarity quickly, but it could lead to a possible pH imbalance, and you may need to use quite a lot of it if you have a particularly cloudy pool.

Does aluminum sulfate lower pH in water?

Yes, aluminum sulfate can lower the pH of the water. Upon dissolving in water, it becomes acidic, which, in turn, makes the water more acidic.

Is aluminum sulfate safe?

Yes, absolutely! Aluminum sulfate is completely safe and widely used in water treatment plants to clear cloudy water. That said, as with any other pool chemical, handle it with caution, use protective gear, and don’t overuse it.

Alum: Inexpensive, Useful and Effective for Pools

Aluminum sulfate can be a helpful tool for clearing cloudy pool water, but it has pros and cons. Remember, unlike clarifiers, you’ll need to vacuum the clumped-up waste after the chemical has done its job! And you’ll be enjoying a beautifully clear pool in no time!

Any more questions about alum or other types of pool chemicals? Drop me a message!

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