Alum, short for aluminum sulfate, is a highly versatile and widelyused compound in swimming pool maintenance. While its primary role is as a flocculant, it is also useful in applications like phosphate and cyanuric acid reduction. Alum is known for its ability to bind with particles in water, which makes it invaluable for clearing up cloudiness and improving water filtration efficiency.
1. Alum as a Flocculant
The most common use of alum in swimming pools is as a flocculant. Cloudy or murky water in pools often results from suspended particles, such as dirt, organic matter, or microscopic debris that are too small to be captured by the pool’s filter. Adding alum to the pool water causes a chemical reaction that forms a gelatinous substance, which acts like a net to capture and bind these small particles into larger clumps, known as “floc.”
When alum is added to water, it dissociates into aluminum ions, which are positively charged. These ions attract the negatively charged particles in the water, causing them to bind together and create larger particles. These particles sink to the bottom of the pool, where they can then be vacuumed out or filtered. Pool techs generally leave the filter off for a few hours to allow the floc to settle and then carefully vacuum out the accumulated particles.
Using alum as a flocculant offers several benefits. First, it improves water clarity by removing the fine particles that cause cloudiness. Second, it reduces chlorine demand,
Florida Pool Pro Rudy Stankowitz discusses ALUM on his Talking Pools Podcast. as fewer contaminants are available to react with chlorine, which helps stabilize the water's chemistry. Alum also provides a more efficient alternative to other chemical clarifiers, as it can tackle even severe water turbidity in a single treatment.
2. Alum in Algae Prevention and Treatment
Alum can indirectly help prevent and treat algae in pools by lowering phosphates, which serve as a food source for algae. By reducing phosphate levels, alum makes the pool environment less hospitable to algae, reducing the chances of an algae bloom and, consequently, the need for additional chemical treatments. Alum can also be used as part of a treatment protocol to clear up the water after an algae bloom, by flocculating the suspended particles left behind.
For algae treatment, alum is typically added to the pool in larger amounts to facilitate flocculation. After the algae bloom is addressed with algicides and the algae are killed off, alum is added to bind and clump any remaining particles, creating a floc that can be vacuumed out. This process helps clear up green or cloudy water after an algae outbreak.
Using alum in algae prevention and treatment helps reduce reliance on algicides and heavy chlorine doses. This not only keeps the water more comfortable for swimmers but also reduces chemical costs over time. For pools in warmer climates or in areas with frequent organic debris, alum offers a valuable tool for longterm algae control.
3. Alum as a Cyanuric Acid Reducer
For years, the pool and spa industry has been searching for a way to reduce cyanuric acid concentrations without draining and refilling the pool, but many of the choices offered are expensive.
So it should come as great news to the industry that alum — a commonly used, inexpensive flocculant — has been shown to measurably reduce cyanuric acid levels.
As developed and promoted by Florida Pool Pro Rudy Stankowitz, this method is based on the flocculation properties of aluminum sulfate and the purported complexing of cyanurate with aluminum ions. As provided by Stankowitz, the method is as follows: 1. Adjust pH to 7.0 (Critical). 2. Adjust Total Alkalinity to 80 to 120 ppm, Calcium Hardness to 200400 ppm, Water Temperature to 70 °F to 90 °F (Critical).
3. Ensure the pool is free of heavy debris that could stir the ‘floc’ upon vacuum to waste.
4. Test Cyanuric Acid level. 5. Filter Preparation: a. Sand & DE Filters: With the pump off, manipulate the multiport valve to ‘Recirculate’ position. b. Cartridge Filters: With the pump off, remove the filter element and then reassemble without it.
6. Broadcast Aluminum Sulfate at a rate of 8.33 pounds per 10,000 gallons of water.
7. Set timeclock to run the pump for two hours and then shut off. Remove the ‘ON’ tripper from the time clock to avoid unintentional circulation.
8. Allow the pump to remain off for a minimum of 12 hours. Do NOT allow the pump to run again until the entire process is complete and elements replaced with valves in the correct run positions.
9. With a portable vacuum system, vacuum the floc to waste at a pace slow enough not to stir or ‘break’ the floc.
10. Test residual aluminum level (should be < 0.2 ppm).
11. Test Cyanuric Acid level. The pool industry consulting group onBalance evaluated this method in their lab. They were able to achieve approximately 15 to 20 percent CYA reduction, measured by turbidity testing. When evaluated in the residential pool environment, where the process included floc and water removal/replacement in the “vac-to-waste” process, they were able to achieve a 25-percent reduction in cyanuric acid.
Stankowitz and others have reported higher reductions, including up to 50 percent. Environmental and chemical factors may have varied in those pools.
Strengths: It’s fast (about a 24-hour process), and it’s simple.
Weaknesses: It involves the use of equipment in the vac-to-waste process. The reaction is chemistry and temperature dependent. There is some water loss.
