When to Use a Flock for a Swimming Pool

Swimming pool owners have an important goal when it comes to their pools: maintaining crystal-clear, sanitary water. Most pool owners sanitize water by adding a disinfectant--usually chlorine--but sometimes it's necessary to take steps to address cloudiness by adding a clarifier. When cloudiness becomes severe, or there is an algae problem, it may be necessary to add a flocculent. Flocculants are similar to clarifiers, but they coagulate contaminants into large particles that sink to the bottom of the pool rather than getting sucked through the filter. From there, you can vacuum them out of the pool.

Cool pool in the summer sun.
credit: GregoryButler
Use flock occasionally to return cloudy water to a crystal-clear and sparkling state.

When Do You Need to Flock?

Typically, you need clarifiers and flocculants when swimming pool water is cloudy, yet chemical levels are correct and filtration units are working fine. Often, fine particulate matter that leads to cloudy pool water ends up in a pool after heavy winds and rain. Also, shocking the pool often leaves a residue of fine particulate matter that needs to be removed. When you shock the pool to get rid of algae or other organic matter, the residue includes dead algae, as well as other particulate matter that escapes filtration.

Compared to adding a clarifier, flocking is an extreme treatment--and it involves more work. If the water is only mildly cloudy, it's better to stick with the clarifier, which coagulates contaminants into smaller particles that the pool filter can remove.

How to Flock

Most commercial pool flocculants contain aluminum sulfate, or alum, which has been used as a water clarifier for centuries. The typical dose of alum needed to address a pool's cloudy water is approximately 4 pounds per 10,000 gallons. Alum fell into a period of disrepute in the 1970s, when it was suspected as a cause of Alzheimer's. This suspicion has been debunked, but as a result, some products use polymers instead of alum. If you choose one of these, consult the container for the proper amount to use. Be careful not to add too much -- excessive flocculant can actually make the water cloudier.

Before you add flocculant, adjust the chemical balance of the pool water, paying particular attention to pH, total alkalinity and water hardness. It's also important for the water to have sufficient chlorine (minimum, 1 ppm). Backwash the pool filters before flocking, then broadcast the floc over the water. Run the circulation pump for two hours, occasionally brushing down the sides of the pool. Then turn the pump off, and leave it off for 12 to 24 hours. This allows the sediment to fall to the bottom of the pool.

The sediment should be visible as a cloudy film on the bottom of the pool. Vacuum this film away, using the pool vacuum set to waste. You'll probably lose a significant amount of water during this process, so run a garden hose into the pool to replace it.

Check the Pool Filters

Large particles of sediment that you can vacuum away will sink to the bottom of the swimming pool. Some of the sediment, however, won't be large enough to sink, and may end up in the pool's filtration unit. It can clog the filter, but you can prevent his by backwashing, or reversing, the filter as needed.