how alum works

• Alum is the name used for aluminum sulfate, chemical formula Al2(SO4)3. It is also known as soda alum, filter alum, concentrated alum, pearl alum, pickle alum, cake alum, papermaker’s alum and patent alum. Alum’s main function is to clarify or floc the pool water.
• When alum is broadcast into the water (not added to the skimmer of a sand filter as a filter aid) with a pH near 7.0, it forms a gel-like precipitate that bridges or sticks together. The precipitate is aluminium hydroxide, chemical formula Al(OH)3.
• The alum precipitate then forms small bundles (called flocs) that trap suspended particles as they fall through the water.
• This bridging process is known as flocculation. It creates a large amount of sediment on the bottom of the pool made up of both the dirt (suspended particles) and the gel-like alum precipitate.
• The sediment is then usually vacuumed to waste ratherthan filtered out because the amount of sediment is more than most filters can handle. The secret to using alum as a floc is to adjust the pH of the water to near 7.0. For alum to work properly, the pH must be adjusted to 7.0.
• When alum is used as a sand filter aid, the alum initially plugs up some of the “holes” between the sand particles in the filter. If the pH has been adjusted to near 7.0, the alum then begins to form that same gel-like material that can trap dirt particles on their way through the filter. This effectively makes the filter remove smaller particles than it normally is capable of removing.

The big question is whether to use alum or not. This is not an easy question. Liquid, organic polymer water clarifiers are far better, faster and a lot less of a hassle to use than alum. If the purpose is to clarify cloudy or hazy water, it is better to recommend using a water clarifier than alum. If the water is really dirty for instance , if you can only see down into the water an inch or two . In that case then a one-time dose of alum may be better than multiple doses of a water clarifier. If the purpose is to make a sand filter more efficient, then alum may be a better choice. The last reason is that alum has been around for a very long time and some customers like using alum instead of the “new-fangled” polymers.

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