Review of folklore in design and operation of secondary clarifiers

Dick published a classic paper in the mid-1970s illuminating popular final settling tank beliefs as folklore with little factual or logical support. Unfortunately, some of that folklore persists in an evolved form and new folklore developed along with technology development. Secondary clarifier folklore examined in this paper include: (1) scraper mechanisms outperform suction pickup mechanisms; (2) surface overflow rate accurately dimensions secondary clarifiers; (3) secondary clarifiers should not be used for sludge storage under any operating conditions: (4) solids flux theory and state point analysis does not predict secondary clarifier failure; (5) dye studies always indicate secondary clarifier deficiencies: (6) rectangular clarifiers outperform circular clarifiers: (7) flocculator center wells encourage density currents; and (8) the sludge volume index must be high to produce low effluent suspended solids.