What is the key operational difference between solids-contact clarifiers and conventional clarifiers that leads to improved flocculation?

The key operational difference between solids-contact clarifiers and conventional clarifiers is the recycling of previously formed sludge (solids) in solids-contact clarifiers, which promotes enhanced flocculation. Conventional clarifiers typically involve separate stages of coagulation, flocculation, and sedimentation, with each stage occurring in distinct tanks or zones. In contrast, solids-contact clarifiers combine these processes into a single unit. A crucial element of solids-contact clarification is the recirculation of a portion of the settled sludge back into the influent stream. This recycled sludge provides a high concentration of seed particles, which act as nuclei for floc formation. As the incoming raw water mixes with the recycled sludge, the destabilized particles from coagulation readily attach to these seed particles, leading to rapid and efficient floc growth. This intimate contact between the raw water and previously formed solids significantly enhances the flocculation process compared to conventional clarifiers where flocculation relies solely on the collision of destabilized particles. This results in larger, denser, and more readily settleable flocs, leading to improved water clarity and reduced solids loading on downstream processes like filtration. The amount of sludge recycled is a key operational parameter that is continuously adjusted to optimize performance.