Which phenomenon is a recognized cause of mud accumulation in a rapid sand filter?

Prepare for the American Water Works Water Treatment Plant Operator Test. Study with multiple choice questions, each with hints and explanations. Get ready to excel!

Mud balls are discrete clusters of fine solids—typically clay, silt, and organic matter—that form in a rapid sand filter. When these balls break loose and accumulate, they create a mud layer within the filter bed, clogging pores and increasing head loss, which reduces filtration efficiency and can lead to channeling. This is a recognized cause of mud accumulation because the fine materials coalesce into these mobile balls rather than spreading evenly as a uniform cake.

Preventing mud ball formation involves limiting fines in the influent, ensuring proper coagulation and media conditions, and using effective backwashing (sometimes with air scour) to break up and flush them out. The other factors listed don’t describe this process: negative head pressure relates to pressure conditions rather than producing mud balls; hours in service is just a time metric and doesn’t cause mud accumulation; sand grains cementing would create a different problem—a more rigid, bound bed—rather than the mobile mud balls that form the troublesome mud layer.