Will a Poorly Working Sand Filter Cause Scummy Foam on Top of Pool Water?

Foamy Water

• Scummy water can definitely be the sign of a poorly working sand filter. The foam, however, is a different story. The foam in your pool water is usually the result of soft pool water (low calcium hardness) or the overdosage of algaecide. Foam can also occur if the water level is too low and air enters the filter lines. The filter is designed to operate with water. Therefore, when the water drops below the skimmer line from things such as bather overflow and evaporation, the filter will blow foamy, bubbled water from the return jet.

Sand Filters

• Sand filters require maintenance in order to keep them working efficiently. The sand inside the filter that is responsible for trapping the dirt and debris can last for at least eight years; many pool owners experience even longer periods before needing to replace it. Even if it's still capable of trapping dirt, however, the sand must be rinsed about once per month. When the pressure gauge rises 8 to 10 pounds (psi) above its normal operating pressure, which can vary based on filter size, this means the water is having trouble passing through the sand because it's too dirty, thus making the internal pressure of the filter rise. When this happens, backwash the filter.

Scummy Foam

• Combine a dirty filter with any of the foamy water causes and this can result in a foamy, dirty and scummy-looking swimming pool -- a sign of poor pool maintenance that can harbor conditions such as algae growth. Body oils and tanning lotions can leave a filmy residue on the top of the pool water when the filter is not operating enough or is running dirty. In general, a clean, operational filter should be run for 12 hours per day, according to most pool professionals.

True Signs of a Bad Filter

• Signs of a poorly working sand filter include the need to more frequently backwash it. If you find yourself backwashing the filter more than twice per month because the pressure is quickly increasing, it could be the sign of a problem. Also, if the sand is blowing from the return jet, that means a lateral is broken inside the filter. The laterals are a series of screen-covered tubes that extend from a PVC core inside the filter. The screens are porous enough to allow water through but trap out the dirt and debris. If one of these break, sand will enter the pool and disrupt the water chemistry significantly.