Beyond using alum for cyanuric acid reduction, alum is a fantastic flocculant and water clarifier and has been used in this capacity for thousands of years.
The ancient Egyptians knew how to use alum (potassium aluminum sulfate) to coagulate material out of cloudy river water, allowing it to settle to the bottom for ease of filtration. It is the first chemical water treatment known. Use of alum as a coagulant by the Romans was mentioned in around 77 AD. By 1757, alum was being used for coagulation in municipal water treatment in England.
Alum is aluminum sulfate, a natural salt compound and a great water clarifier. When alum and water combine, aluminum hydroxide is formed. Added to a pool, this compound tends to stick to itself, trapping small dirt particles in the process as it sinks to the floor.
Plenty of service technicians still use alum today, recognizing it as a tried-andtrue water clarifier. It can involve a bit of elbow grease. To begin, the pH should be adjusted to 7.0. Most recommend using 4 pounds of alum per 10,000 gallons of water, although more can be added, depending on the amount of suspended water particulates. It should be premixed into a slurry and added around the perimeter of the pool. Allow the water to circulate until the filter pressure gauge goes up by 8 psi, and then allow the floc to settle. Finally, vacuum the floc to waste, and readjust the water balance.
Using alum can be a hassle, so many service technicians prefer to use synthetic organic polymer clarifiers to get the job done.
The polymers used for clarifying swimming pools are long bundles of repeating carbon units with positive charges all along their surfaces. When added to water, they form long strings with positive surface areas.
A lot of debris tends to be negatively charged. When the negatively charged dirt encounters the positively charged polymers, the opposites attract, and the dirt is drawn to the polymer. Groups of these polymers collect together and form masses until are bundles of flocs. The flocs are large enough that they can then be vacuumed or filtered out of the water by the pools filtration system.
There are hundreds of formulations of polymers for water clarification that have been designed for different applications or circumstances. Some clarifiers are formulated for spas, with higher resistance to floc breakdown from the severe turbulence. Some clarifiers are more suitable than others for specific temperatures. Some polymers can handle different pH ranges than others. And some can withstand higher chlorine levels.
One big warning before adding a polymer clarifier: be aware if the pool water also contains a metal sequestering agent or scale remover. The polymer is positively charged while the chelating agent of the sequestering agent is negative. When the two molecules encounter one another, they bond, and the cloudy result can be almost disastrous.
Generally speaking, clarifiers and flocculants are useful chemicals in a service technician’s arsenal.
They are chemicals that cause fine suspended particles to combine into larger clumps that can then be filtered. These tiny dirt particles are difficult to remove by normal filtration or automatic pool cleaners because they are so small, they remain suspended in pool water. But even as tiny as they are, the particles can really affect the water clarity when enough of them are floating in the pool.
That is because the dust particles in water reflect light, causing the water to appear cloudy. These dust particles are negatively charged, repelling each other, so that under normal circumstances, they will never agglomerate to sizes that can be filtered.
There are many reasons for water to appear cloudy, however, and it is not a good idea to assume that a clarifier will solve the problem.
Water can be cloudy due to early algae growth, insufficient chlorine, high total alkalinity, calcium hardness, or pH, bad filtration, and more. Therefore, before adding a clarifier, it is a good idea to make sure that the water is in balance and sanitized, and ensure that the filtration system is working properly.
It’s better to solve these problems than blindly pouring a clarifier into the water.
For example, if the pH is too high, then calcium carbonate will precipitate out, making the water cloudy. Adding clarifier to water like this will not fix the underlying problem.
And if the filter has holes or cracks, adding a clarifier is not going to help much. Sand filters can develop channels that allow some of the debris to escape back into the pool.
