Gravity separation of solids from liquid, producing a clarified overflow and thickened solids underflow has long been used in water and wastewater treatment. Clarification equipment categorized as conventional clarification and solids contact clarification. Conventional clarification utilizes separate basins for coagulation and flocculation ahead of the settling chamber. Solids contact equipment uses the treated water flowing through solids or sludge to produce faster settling particles.
Clarifiers are usually designed to keep the turbidity going to the filters between 5-10 NTU. High turbid water causes rapid head loss or frequent filter backwash. Typical raw water clarification applications include removal of substances such as suspended solids, turbidity, color and silica. Typical wastewater clarification applications include removal of suspended solids, turbidity, color and biomass.
Clarifier consists of essentially four zones; inlet zone, outlet zone, settling zone and sludge zone. If the suspended solids are not readily settleable then coagulation and flocculation steps precede the calrifier inlet zone. Solids size greatly impacts the settling rate. Silt may settle in reasonable time. Many other types of solids require longer hydraulic retention times. Conventional clarifiers are typically designed for 3 hour solids retention time where as solids contact clarifiers are designed for 1-2 hour retention time. Higher retention times generate septic conditions, odour and floating solids due to anaerobic bacteria activity.